8. Light Sources and Recording Methods

8. LIGHT SOURCES AND RECORDING METHODS* 8.1. Light Sources 8.1.l.Introduction High speed photographic recording is one of the most important techniques for the study of transient phenomena in the field of technical engineering and scientific research. The development of light sources adapted to the special uses to be described in this part requires attention to spectral distribution, luminous flux, and pulse duration. About one hundred years ago, sparks were used by E. Mach to take pictures of bullets in flight. Since that time, a rapid evolution of the science has provided intense short duration sources with controlled light output. The production of short pulses needs electric energy stored in a high voltage capacitor which can be released suddenly across air gaps, or across gas filled tubes, or into explosive heating and vaporization of thin metal wires. In the case of explosive flashes, luminous energy is obtained from shock waves initiated by chemical reactions. To complete the list of the so-called thermal sources, we should add the plasma focus source and the laser produced plasmas. Neither was developed or investigated for the purpose of high speed photography, but their application is suitable because a relatively large amount of energy is converted to radiation in the visible spectral range. Since 1960, a second group of light sources has become increasingly important: lasers. Their basic properties are: they are monochromatic, of high intensity, are capable of generating short pulses, and have a high degree of spatial and temporal coherence. Solid state lasers (ruby and neodymium lasers) are well suited for diagnostic applications such as photography, interferometry, and holography. Pulse durations in the nanoand picosecond range can be realized by the techniques of Q-switching and mode locking, respectively. Thereby, peak powers of some tens to hundreds of megawatts are easily available. Flash lamp pumped or laser pumped dye lasers provide tunable sources with narrow spectral band* Part 8 is by M. Hugenschmidt and K. Vollrath. 687 METHODS O F EXPERIMENTAL PHYSICS, VOL. I8B

Copyright @ 1981 by Academic Pre%. lnc. All rights of reproduction in any form reserved.

ISBN 0-12-475956-1

688

8.

LIGHT SOURCES A N D RECORDING METHODS

spectral A energy distribution (relative units)

(a)

I

II

0.1 I

1 dve -Lasers

tpml

FIG.1 . Spectral distribution of the radiation of (a) thermal- (xenon flash lamp) and (b) laser-light sources.

widths which cover the whole range from the uv to the near ir. In some cases, molecular infrared lasers such as CO,(A = 10.6 pm) or HCN(A = 330 p m ) lasers may give a marked increase in sensitivity. Tunability in the infrared is achieved by spin-flip Raman techniques or by rotational line broadening and overlap of electron beam or uv sustained high pressure devices. For the visualization of the ir information, however, rather complicated image converters have to be employed. In Fig. 1 some spectral characteristics of the two groups of light sources are shown. Above, we see the spectral irradiance curve of a xenon flash lamp, a source chosen as an example from among numerous thermal sources. This is characterized by a relatively broad continuum with superimposed strong lines. Most of the laser sources, on the other hand, show narrow lines or relatively narrow bands corresponding to the transitions of the excited and inverted population of energy levels. Only some of the most important types of lasers have been indicated. 8.1.2. Thermal Sources 8.1.2.1. Physical and Photometric Aspects of Light Sources. The part of the spectrum between about 200 to 300 nm, and the far infrared up to several hundred microns which is used for photographic recording occupies only a small portion of the whole electromagnetic spectrum. In the visible range the human eye is the most important detector, so the radiation properties are often described in terms of photometric quantities which are closely related to the spectral response of the eye. These are

8.1.

689

LIGHT SOURCES

TABLEI. Physical and Photometric Quantities

/

of the detector d A 2

surface element of the source d A 1 of the praJected area dA, cos

I,

physical quantities

9

radiation power

Q'

radiant intensity

I* =$

radiance

daQe Le=dQdA,cos

spectral radiance irradiance

photometric quantities

[WI

dt

Le,,=dLe/d

A

€,-d~~,/dA,

[-$-I

~, [SJ

I&-[

dt

I

Luminous intensity Luminance spectral luminance

[%]

ev.*

luminous f l u x

illummonce

L

[lml

-&

"-dIl

- d2e~ v - dQ dA, cas c,

-*I * [ dL dh

L v , = L E

v - dAI

also used for other sensors such as electrooptical detectors or photographic material. It would be more accurate, however, to use physical quantities such as radiant energy, power, emittance, and irradiance throughout the entire spectrum. Both groups of units are briefly reviewed in Table I . In the case of the physical quantities, all units are related to power, measured in watts. The radiant intensity is the power per unit solid angle; the radiance is the power per unit solid angle and per unit projected area. The unit of luminous flux on the other side is the lumen (lm). This corresponds to & of the luminous flux of a blackbody radiator from an area of 1 cm2 which is at a temperature of 2042 K (the solidification temperature of Pt). The luminous intensity, corresponding to the radiant intensity is the flux per unit solid angle measured in candela (1 cd = 1 Im sr-I). Relating this to the unit projected area yields the luminance (cd m+ = Im m-2 sr-l) which is also often called the brightness. The factor VA describing the physiological influence, connects the luminous flux to the radiant power. (Qv = 6821; V,Q,(A) dh.) The maximum of luminosity of 682 Im W-' occurs at a wavelength of 556 nm (V, = I), @,,(A = 556 nm)[lm] = 682, @,(A = 556 nm)[W]. It should be mentioned that differentiation should be made between two eye response curves, depending upon the sensitivities of the two different receptors, the rods and the cones, for which different V , curves are valid.' F. A . Barnes, "RCA Electro-Optics Handbook." RCA Commercial Engineering, Harrison, New Jersey, 1968.

690

8. LIGHT

SOURCES AND RECORDING METHODS

8.1.2.2. Blackbody Radiation. A blackbody absorbs all the incident radiation of the electromagnetic spectrum. The emission is described by Planck’s law giving the temperature and wavelength dependence of the spectral radiance.

(8.1.1) A is the wavelength, c the velocity of light in vacuum, k the Boltzmann constant, h the Planck constant, and T the temperature (h = 6.6252 X 10-34 J S, k = 1.3846 x 1 0 4 3 J K-1). Following Wien’s displacement law, the maximum power per unit wavelength occurs at A,, which is determined by the relation

A,,,T

= 2898

pm K.

(8.1.2)

Blackbody radiation distributions for three different temperatures are indicated in Fig. 2a. More detailed values are given in the literature.2 The overall radiance is obtained by integrating the spectral radiance over all wavelengths, thus yielding the law of Stefan-Boltzmann [Le(T)lhlackhody

(8.1.3)

=

rn = 56.8 nW mP2K-4 is the Stefan-Boltzmann constant.

8.1.2.3. Real Sources of Radiation. The methods applied for the generation of light include heating of solids, electric discharges in gases at low pressures, and high pressure electric discharges. The distributions of the spectral energy densities of such sources are often approximated by a least square fit to a curve of blackbody radiation distribution. By this method, the spectral luminance may be characterized quantitatively by some temperature. A similar situation occurs for the light of natural sources, e.g., the sun, the moon or the sky. Defining the absorptance a as the ratio of absorbed to incident energy, power or radiance, the real spectral radiance of a source as compared to that of an idealized blackbody radiator is given by Kirchoff s law.3 Lek(A9

T ) = a(A, T ) [Le,(A,

T)lblackhody,

where a depends on A and T. The spectral energy distributions of some typical lamps are shown in Fig. 2b-2d. The tungsten filament lamp for example has a continuum with a maximum near 900 nm at 2854 K. Xenon short-arc lamps are also characterized by a strong continuum with a great number of lines espe-

* M . PivonSky and M. R. Nagel, “Blackbody Radiation Functions.” Macmillan, New York, 1961. W. Elenbaas, “Light Sources,” Philips Technical Library, Eindhoven 1972.

’

8.1.

69 1

LIGHT SOURCES

1000 y

0.5

w 02

1

0 5 - i

2

5

Ipml

2

FIG.2. Spectral output of a blackbody radiator and of different types of lamp sources. (a) Blackbody, (b) tungsten filament lamp, (c) xenon short arc lamp, and (d) mercury lamp.

cially in the near ir between 800 and 1000 nm. Mercury lamps show strong line emission from the uv throughout the visible range. For further types of lamps and details, such as electrical parameters, influence of gas-halogenide or alkali-metal additives, fluorescent screens or others on lifetime, spectral distribution of the radiation, luminous flux and efficiency, reference is made to the l i t e r a t ~ r e . ~ , ~ The luminous efficiency 77 relates the effectiveness of a source to produce a luminous flux to the effectiveness of a monochromatic source of the same power in watt, radiating at the wavelength of the maximum of the eye-response curve ( h = 556 nm, assuming a photopic curve). Following the notations of Table I, (8.1.4)

P. Schulz and H . Strub, Lichrruchnih 10, 364 (1958). A . Bauer, Lichffechnih 16, 118 (1964).

692

8.

LIGHT SOURCES A N D RECORDING METHODS

V his again the spectral sensitivity curve of the human eye. Typical values for a tungsten filament lamp with an input power of 1000 W are q = 16.3 lm W-’, whereas with mercury short-arc lamps (for example 200-W lamps) values of about 47.5 Im W-’ can be achieved.

8.1.2.4. Flash Lamps. Flash lamps are capable of emitting intense, short-duration light pulses. Today portable, small size electronic flash sources are available from a great number of manufacturers. Some of them use rechargeable batteries as a power supply, or they are directly operated from the ac line. The flash lamps of these systems are usually operated with xenon. Typical flash durations are about 200 ps to 1 ms, thus giving exposure times which even in the case of simple nature photography of an amateur photographer, may lead to blurred images. Scientific applications of flash lamps include high speed photography, photochemical studies, solid state, and dye-laser excitation. Lamps can be used for single flash and for stroboscopic applications up to several thousands of hertz. Their spectral emission may resemble daylight or may be more intense in the blue so that they can be adapted to the special type of application. Tubes are in use with krypton, xenon, and argon, the luminous flux of which is dependant upon the gases as well as upon electrical circuitry and energy storage capacitor. Light efficiencies of flash tubes can be increased by choosing gases of high atomic mass, e.g., xenon. Thereby, total efficiencies, as defined by the ratio of light energy integrated over all wavelengths to electrical energy of the storage capacitor, of about 40 percent and more have been achieved. The spectral output of a typical xenon flash, the color distribution of which resembles that of daylight has been indicated in Fig. 1. The maximum radiance occurs at wavelengths of about 4500 A. The distribution of the radiation can be approximated by blackbody distributions corresponding to temperatures of 7000-9000 K. A marked increase in intensity as compared to the blackbody radiance is observed in the near infrared due to the intense lines around 8000 A. The spectral output and the flash durations are strongly depending on electric parameters such as the capacitor C , the inductance L , and the discharge resistance R . To obtain short exposure times with flash duration of less than 1 ps, the discharge circuit must have a small overall inductance, comprising the inductance of the discharge, the inductance of the capacitor, and that of the external circuit. The inductance also determines the peak current which induces strong mechanical stresses so that the limitation is given by the explosion point of the tube. In order to get a damped discharge, the resistance R has to be greater than about 4(L/C)1’2. This condition may be difficult to be met, because R varys in time achiev-

8.1. LIGHT SOURCES

693

ing minimal values of even some tenths of an ohm. When the required condition is valid, the pulse half-width is roughly proportional to C . In other cases the current can be inverted, so that several light maxima following the current oscillations are obtained. The lower part of Fig. 3 shows electric circuit diagrams which are commonly used. The voltage is applied continuously to the electrodes. Triggering can be obtained by ionization due to pulses of some tens of kilovolts which can be applied directly to the electrodes or which are applied to a wire which is attached externally to the tube. The trigger pulses yield corona-like filamentary discharges which initiate the main discharge. This initiation may be influenced by the pressure of the gas (which is normally some 100 mmHg up to 1 atm), by the type of electrodes, the thickness of the lamp walls, the high voltage pulse rise-time, gaseous impurities, or sputtered metal. The maximum energy that can be dissipated by a flash tube is as already mentioned determined by the mechanical forces which are built up when the current is switched on, depending thus on the current rise-time. The energy density that flash tubes may tolerate is of the order of 0.2 J/mm3. A great variety of different types of flashlamps are commercially available. Tubes in linear, helical, or other geometric form are made of hard glass or quartz. Diameters range from about 1.2 to 20 mm with lengths of the interelectrode gap from 3 to 300 mm. Electric storage energies can be supplied up to several thousands of joules with voltages from some hundred volts to some kilovolts. Most of the lamps are convectively

arc length

635mm

50mm

orc diameter

l2mm

Lmm

typicol energy

(001-1) J

(ZOO-9OO)J ( 0 5-21 kV

voltage

( 0 A-2) kV

capacitance

-05pF

typical pulsedurat ion

- 1 ps

(02-2)ms

( 0 7 - 2 ) kV

11

-,lo0 pF

5 ~ 2 2 0 0pF

~ 1 5 ps 0

(1- 2) ms

150mm

; ;o;

165 mm

300mm

13mm

17mm

10,000 J

(8000-19,OOO) J

(1-3) kV

(1.9-4)kV

ry+y+,,n rrLyl -I ~

mode of operation

pwer supply, pulse Po forming network

flash tube

trigger-pulse generator

FIG.3. Technical data of some commercially available types of flash lamps.

694

8.

LIGHT SOURCES A N D RECORDING METHODS

cooled by air, but other cooling techniques can be applied as well. Experiments indicated that in the case of high loading or intermittent operation such as stroboscopic application an overheating of the lamp walls, the electrodes or the lead-in seals can be avoided if the specific load is smaller than 5 W/cm2 for free convection air circulation, 40 W/cm2 for forced air cooling, and about 300 W/cm2 for liquid cooling. Typical data for a few types of xenon flash lamps are summarized in Fig. 3. For a more complete description, the reader is referred to the reference^.^" Because of the small discharge volume with a length of 6.35 mm and a diameter of 1.2 mm, the small size xenon flash tubes approximate a point light source and will be especially useful for applications in optical systems (as they are used for flow visualization by means of shadowgraph and interferometric techniques). According to the different capacitor values (0.01 p F to 1 pF) the flash duration can be varied from 150 ns to 3 ps. High frequency stroboscopic systems can be equipped with such lamps. They can be operated up to 1000 Hz. The duration of such a train of pulses is limited to about a tenth of a second also depending upon the value of the energy storing capacitor. The other types of lamps listed in Fig. 3 are essentially used for laser excitation. Their flash duration is adapted to the radiation life time of the ions or molecules around 1 ms for ruby lasers and some tens to hundreds of microseconds for Nd lasers. These standard linear tubes allow for higher energies to be dissipated. Recent developments led to the construction of coaxial capillary flashtubes designed for the excitation of dye lasers. Using low inductive circuits, light pulses with a rise-time of 50- 100 ns have been obtained with energies of the storage capacitor up to 100 J. Using tubes filled with krypton, it is possible to optimize the spectral radiance for specific laser pumping which can be effective to raise for example the efficiency of Nd-YAG lasers. Pulsed mode of operation has been obtained with high pressure mercury capillary lamps which originally have been designed for cw operation. By this, a considerable increase of brightness can be achieved. As it was shown by Dal Pozo er ~ l .the . ~spectral emission under pulse excitation shows high efficiency in the blue and in the near uv part of the spectrum, thus providing an effective pumping source for dye lasers emitting in the blue. F. Friingel, “High-speed Pulse Technology,” Vols. I , 2, and 3. Academic Press, New York, 1965, 1965, and 1976, resp. ’ H . E. Edgerton, “Electronic Flash, Strobe.” McGraw-Hill, New York, 1970. P. Dal Pozo, R . Polloni, and 0. Svelto, Appl. Phys. 6, 341, 381 (1975).

8.1.

LIGHT SOURCES

695

Another type of very intense short-duration pulsed lamp has been described by F e r r a ~who , ~ uses tubes of quartz that are continuously evacuated, whereby a small air leak provides a residual gas pressure of a few torr. The discharges are primarily occurring in the vapors ablated from the quartz tube walls. No significant difference in light output can be observed if other gases such as N2,Ar, He, or C02 are used. Intense light pulses of several microseconds duration have thus been obtained with electric energies in the range of 10 J to about 1000 J. 8.1.2.5. Spark Light Sources. Sparks are convenient flexible light sources for high speed photographic recording.’O They are obtained by the rapid discharge of electrical energy stored in a capacitor. They provide the possibility to obtain extremely short exposure times which are necessary for the investigation of fast events with velocities up to several 10 km/s or even higher. Sparks in air have been the earliest sources to study such phenomena. The volume of these discharges can be made very small, so that they are especially useful to take shadowgraph pictures. As the sharpness of the images depends on the diameter of the source, the light of the spark is sometimes collected by an objective lens and imaged to a small pinhole which then serves as the actual point source. Another application of sparks is the use in schlieren systems. This was first described by Foucault who visualized rapidly changing disturbances, causing refractive index changes in the atmosphere. Toepler continued the development and application of these methods to investigate changes in refractive index in liquids, gases and even in flames. Further, schlieren and interferometric techniques using spark sources have been proposed and described by Schardin” for the purpose of photographing rapidly varying processes. Such systems are now standard equipment in wind tunnel investigations and for aerodynamic studies of sub- or supersonic flows. 8.1.2.5.1. SPARK-FORMATION, ELECTRICAL A N D HYDRODYNAMIC PARAMETERS OF SPARKS. The mechanisms leading to the electric breakdown of the gas between the electrodes can be described by the Townsend and the canal (streamer) models.12 The Townsend mechanism holds for small overvoltages as compared to the static breakdown voltage and C. M. Ferrar, Re\,. Sci. Instrum. 40, 1436 (1969). K . Vollrath and G. Thomer, eds., “Kurzzeitphysik.” Springer-Verlag, Berlin and New York, 1967. I I H. Schardin, ErRph. E x u k f . Nuturwiss. 20, 303 (1942). I* P. Schulz, “Elektronische Vorgange in Gasen und Festkorpern,” 2nd ed. Verlag G. Braun, Karlsruhe, 1974.

696

8.

LIGHT SOURCES A N D RECORDING METHODS

for low values of the electron density where space charge effects can be neglected. The generation of new electrons, the so-called successors, then occurs essentially by the photo effect at the cathode and is described by Townsend's first coefficient a . The canal mechanism requires higher overvoltages. If no is the number of starting electrons, the avalanche achieves a critical value, if the following conditions holds

In no

+ ad

>

= 20.

(8. I .5)

In distance dCritfrom the starting point, the electron avalanche then causes the formation of anode- and cathode-directed streamers which are strongly influenced by their own space charge. The radius of the streamer discharges are of the order of the diffusion radius of the avalanche. After the streamers have arrived at the electrodes, fast luminous fronts, so-called ionizing waves are generated moving in both directions through the gap, achieving velocities in the order of 108-109cm/s. In a homogeneous electric field these conditions have been studied extensively by Koppitz,I3 who used fast framing and streak image converter cameras to detect the phenomena of streamers and ionizing waves. The velocities of the anode- and cathode-directed streamers have also been investigated by Timm. In his experiments a starting electron cloud of lo6 to loBelectrons has been produced in the interspace of the electrodes by multiphoton processes by a focused giant pulse of a Q-switched ruby laser.I4 The electrical properties of sparks are determined by the charging voltage, the capacitor C , the inductance L , and the resistances R , both of the channel and the discharge circuit. L and R are depending on time. The energy W , dissipated in the spark can be calculated from the following equation^:'^

(8.1.6) with

Assuming the inductivities L to be constant, yields us

J. Koppitz, Z.

N u r i c t j h c h . , Teil A 26, 700 (1971).

*'U . Timm, Dissertation, Universitat Hamburg (1972). R . Grunberg, 2. Nufurforsch.. TeilA 20, 202 (1965).

8.1.

697

LIGHT SOURCES

C = 5500 pF

iIkAl

L =lOnH+LK(t)

0

80

40

120'hsl

t

L la)

L =15nH theory of Braginskii

FIG.4. (a) Electrical and (b) gas dynamic parameters of open spark discharges in air. [Electrical parameters from Andreev and Vanyukov;16hydrodynamic parameters from Andreev el d .I T ]

C UR(t)

UOC

-

$ i dt - LC

1 di = Uo - - $ i dt - L - - ,

C

dt

dt

(8.1.7)

Figure 4 shows a typical example as given by Andreev et u / . ' ~ * ' 'concerning the electrical and hydrodynamic parameters of spark discharges. S. I. Andreev and M. P . Vanyukov, Sov. Phys. -Tech. Phys. (EngI. Trunsl.) 6, 700 (1962). l 7 S. I. Andreev, M. P. Vanyukov, and A . B. Kotolov, Sov. Phys.-Tech. PhyA. (EngI. Truns/.) 7, 37 (1962).

698

8.

LIGHT SOURCES A N D RECORDING METHODS

By measuring the electric current, the voltages uR, uL, u c , the resistance R and the dissipated power or energy, respectively, can be calculated following the above mentioned equations. The value of R is thereby strongly influenced by the temporal variation of the radius p ( t ) of the discharge channel. Apart from the initial stages during the streamer state, the growth of the high-current channel can be described with adequate accuracy by hydrodynamic models as they have been worked out by Drabkinala and Braginskii.lQ Thereby, it was assumed that pressures, temperatures and densities are constant over the cross section of the channel. Starting from shockwave theory, these authors derived expressions for the radius of the spark channel, the velocity of the shockwave and the pressures built up as a function of the electrical energy and as a function of time. The radius of the shockwave reveals to be proportional to Ell4 . P2,the velocity to E1/4t-1’2, and the pressure proportional to E / p 2 . E is the energy liberated per centimeter length of the discharge, and p is the shockwave radius depending on time. The lower part of Fig. 4 shows some further results of Andreev ef al., giving the time development of the current, the radius of the shockwave produced by the discharge and the radius of the channel. As compared to the experimental data, it can be seen that the model of Braginskii shows a good agreement, whereas the model of Drabkina gives reliable values only during the first 30-40 ns of the discharge. 8.1.2.5.2. TEMPERATURE A N D DENSITY DISTRIBUTIONS. Temperature distributions in electric sparks have been determined by various authors.20*21Depending upon the gases, this can be done spectroscopically by measuring for example the absolute intensity of a line, the oscillator strength of which must be known, the relative intensities of two or more spectral lines or the intensity of continuum radiation. Measurements performed by Egerova (Fig. 5a) show the distribution of spark temperature in air. First quantitative informations on gas-density profiles have been obtained by Vollrath,22measuring the absorption of soft x rays. Figure 5b shows some results of the density in a spark in argon. Further parameters, for example electron densities, can be measured interferometrically. Some values are also indicated in Fig. 5c as obtained by ir laserS. 1. Drabkina, Z h . Eksp. Teor. Fiz. 21, 473 (1951). S . I. Braginskii, Sov. Phyb. -JETP (Engl. Trunsl.) 7 , 1068 (1958). 2o L. Krauss and H, Krempl, Z . A n g r w . Phys. 16a, 243 (1963). 21 V. F. Egerova, V. J . Isaenko, and A . A. Mak, Sov. Phys. -Tech. Phys. (Engl. Trans/.) 7, 242 (1962). 22 K . Vollrath, Proc. l n t . Congr. High-Sprrd Photogr., 5 t h , Wushington, D . C . . 1960, p. 179. SMITE, New York, 1962. In

l8

8.1.

699

LIGHT SOURCES

-83 ps

0.5

5 (0

1

10 (b)

15imrn1 (Cl

FIG.5. Some fundamental physical properties of electric spark discharges. (a) Radial distribution of temperature of an air spark. [After Egerova ef a/.z1] (b) Radial dependence of the density of an Ar spark (no density at ambient pressure and temperature). [After Vollrathzz] (c) Radial profile of the electron density of an air spark. [After Hugenschmidt and V ~ l l r a t h . ~ ~ ]

interferometric methodsz3using a TEA-C02 laser as infrared source and a liquid crystal-display as an image converter to convert the infrared radiation towards the visible (see Section 2.8.5.2). 8.1.2.5.3. SPECTRAL EMISSION. The optical properties are determined by temperatures and densities in the channel during the discharge. The time dependence of emission can be measured by mutipliers, fast cameras, and spectrometer^.^^ The spectral luminance, thereby, is strongly dependent upon the current density. In most cases, the radiation distribution can be approximated by blackbody radiation. Ar discharges, for example (p = 4 atm) 100 ns after breakdown, resemble a blackbody at a temperature of 31,000 K in the wavelength interval from 4000-9000 A. During the following 300 ns the temperature decays to about 18,000 K.l0 The maximum obtainable values of spectral luminance LvA depend upon the velocity of the energy accumulation which is proportional to dildt. As shown experimentally, LvA is limited because of saturation effects first in the red and subsequently in the violet range of the spectrum. Increasing the electric current leads to higher expansion velocities of the spark channel but not to higher luminance. Table I1 shows some photometric and corresponding electrical data of spark discharges in different gases as given by Vanyukov and Mak.25 8.1.2.5.4. DIFFERENT TYPESOF SPARK DISCHARGES. According to different applications, a great variety of spark sources has been developed. z3 M. Hugenschmidt and K. Vollrath, Proc. Int. Congr. High-Speed P h o f . , IOrh, Nice, 1972 p. 515. Assoc. Nat. Rech. Tech., Paris, 1972. 24 G . Glaser, 2. Naturjbrsch., Teil A 6 , 706 (1951). 25 M. P. Vanyukov and A . A . Mak, Proc. I n f . Congr. High-speed Photog. 5rh, Wushingion, D. C . , 1960, p. 41. SMPTE, New York, 1962.

700

8.

LIGHT SOURCES A N D RECORDING METHODS

TABLE11. Photometric and Electrical Parameters of Spark Discharges in Different Gases" p (kPa)

di/di ( A d ' )

C (pF)

T (K)

L V ( G c d m-*)

lo'=

0.2

70000

370

~~

He

3100

N,

300

1.1

.lo'*

0.1

62000

320

Ar

300

0.3

.lo"

0.2

46000

220

air

100

0.1

40000

I70

0.05~10"

" Data from Vanyukov and Mak.25

Most common is the free spark in air under normal atmospheric conditions. As already mentioned, the duration of the light pulses depends on electrical and geometrical parameters. Low inductivities can be realized, if the capacitor is formed by a circular plate condensor. Plates of Ba-titanate ceramics, for example, have been used by Stenzel for the construction of short duration point-light sources. The ceramic plate has a central hole containing an electrically isolated trigger electrode allowing the main discharge to be initiated at any moment desired. The light is coupled out through a small hole of 1-mm diameter in one electrode. With a capacitor value of 40 n F and a voltage of 10 kV, intense short duration pulses of 0.1 ps duration have been achieved. A more recent development using ceramic 3.3-nF capacitors is shown in Fig. 6.2e Exposure times in the range of a few nanoseconds have been obtained by FischerZ7by using coaxial capacitor discharges. The capacitor is formed by a copper line, isolated externally by a thin Teflon sheet on top with a copperplating which connects into a cap towards the spark gap which is formed between the center electrode and the outer narrow center hole with a short pin. This setup represents thus a very short low inductance transmission line with a distributed capacitance C. The inductance is given by the contributions of the line of about 0.2 nH, of the connecting cap (0.4 nH), and of the gap (0.3 nH). With these nanolites, which are commercially available, light pulses with a rise time of 2 ns and a halfwidth of 8 ns are obtainable with electric energies of 0.01 J. Current densities exceed lo7 A/cmZ and the brightness obtained was measured to be of the order of lo7 cd/cm2. 26 A . Stenzel, LRSL-Note Tech. 17a/57. Dtsch.-Franz. Forschungsinst. St.-Louis, 1957; Pro(,. Int. Congr. High-speed Photog., 8ih, Stockholm, 1968, p. 153, Wiley, New York, 1968. z7 H . Fischer, J . Opt. Soc. Am. 51, 543 (1961).

8.1. LIGHT SOURCES

70 1

With guided sparks (sliding sparks) it is possible to increase the spark length about to the 20-fold of the value of the length of a free spark at the same voltage. With an increase in the length of the interelectrode gap, the spark resistance will also increase, so that a much better adaption of the load to the discharge circuit can be achieved, thus providing a better electro-optic conversion ratio. The electrodes may be connected with an isolating material such as glass, quartz, or glimmer plates o r tubes, or with electrolytic conductive surfaces, such as porous ceramic material drenched with an electrolyte. The pulse durations normally obtained with sliding sparks are in the range of 1 ps. Nearly the double value of intensity with constant pulse half-width can be achieved as compared to free sparks. With a capacitor of C = 0.05 pF and a voltage of 18 kV, Tredwell** has realized light pulses of 0.4 ps duration and a luminous intensity of 5 x lo6 cd. The light output can be increased by increasing the pressure of the gas or by application of rare gases instead of air. Measurements have been performed by FUnferz9 using krypton, argon, and xenon achieving a five-fold light increase. Constricting the discharge by walls as in the case of capillary sparks, higher energy densities are obtained. At the same energies, these discharges are reaching higher temperatures. The optimum diameters of the capillary are depending on the energy and the nature of gas. Spark resistance is determined by the cross section of the bore and by the temperature-dependent electrical conductivity of the plasma. As the rapid cooling of the plasma channel by the gas-dynamic expansion is prevented by the walls, it is not possible to obtain shorter exposure times than with freely expanding sparks. The capillary sparks are spatially well defined so that they are especially suited for schlieren photographic applications. In a certain sense, capillary sparks can be considered as special case of flash-lamp discharges which have already been treated in the preceding section. High light output and short pulse durations are obtained by large area spark discharges, as developed by Schwertl and S t e n ~ e l . ~ A ' schematic diagram of the setup and the temporal shape of the light pulses are both included in Fig. 6 . The radially distributed discharge takes place between a central electrode and a circular concentric electrode in the small volume determined by two glass or quartz plates. Because of its high efficiency, xenon is used as filling-gas at pressures of about 150 Torr. Normally, the discharge is split up in a great number of spokelike filaments. In spite of J. Tredwell, MS Thesis, Elec. Eng. Dept., MIT, Cambridge Massachusetts, 1960.

zs E. Fiinfer, 2. Angew. Phys. 1, 295 (1949).

M. Schwertl and A . Stenzel, Tech. Ber. 12/73. Dtsch.-Franz. Forschungsinst. StLouis, 1973; Proc. I n t . Congr. High-speed Photogr.. lOth, Nice, 1972, p. 335, Assoc. Nat. Rech. Tech., Paris, 1972.

702

8.

LIGHT SOURCES A N D RECORDING METHODS

electrodes pin

3300 pF capacitor

outer coating

Fc electrodes

metal support

I

(b)

0 200 COO 600

lnsl

F

discharge volume A n

\

g electrode

cent& electrode (C)

F I G .6. Different types of spark sources. (a) Spark point source. [After StenzeLZ6] (b) Nanolite. [After Fi~cher.~'](c) Large area spark source. [After Schwertl and S t e n ~ e l . ~ " ]

the fact that the location of the individual spokes is not well defined, the light output shows an excellent reproducibility. Using capacitors of C = 0.15 pF and energy values of E = 12 3, pulses of 0.2-ps duration, and a maximum luminous intensity of 2 x lo7 cd have been realized. 8.1.2.5.5. ELECTRICAL A N D OPTICAL TRIGGERING OF SPARKS (JITTER). Triggered spark gaps are often used for example for fast switching of electric circuits or for the generation of high voltage pulses with short rise time. As we are concerned with light sources, these aspects and applications shall not be considered in the present article. To obtain light pulses at any desired moment, the spark source itself can be used as a triggered spark-gap. One reliable system, which is often used, has a central bore in.one electrode which contains, electrically isolated, a trigger electrode. The main discharge can be initiated by applying a steep high voltage pulse to this trigger electrode, thus providing a change of the electrical potential distribution and an uv preionization due to hard photons produced by the

8.1.

LIGHT SOURCES

703

starting trigger discharge. The voltage applied to the electrodes should not be less than about 80 percent of the static breakdown voltage. The time delay between the initiating high voltage pulse and the instant of breakdown depends on the gas, the pressure, geometric form of the electrodes, gap length, and voltage. With typical triggered spark gaps of this type, time delays of a few lo-* s with a jitter of some s can be realized. Jitter can be minimized by preionizing the gap with radioactive material. Another possibility to obtain short delay times and small jitter is to use three- or four-electrode sparks, where usually the first gap is also preionized. Since the first demonstration of laser triggered spark gaps in 1963 by Guenther and Griffin,31numerous investigations on the problem of optical triggering have been carried out. Giant pulses of ruby and neodymium lasers have been focused to one of the electrodes, thus providing target-induced plasmas which are completely ionized. The uv emission from these plasmas was shown to ionize the neutral surrounding gas. Nitrogen lasers, emitting in the near uv at a wavelength of 3371 8, are inexpensive and especially suited for the initiation of spark discharges because of the short pulse durations of only a few nanoseconds. Recently, experiments have been carried out by Dewhurst at nl.32with single picosecond pulses gated out from the mode-locked train of a Nd laser. The characteristics are very similar to those using conventional Q-switched pulses. For gaps up to a few centimeters and pressures of some bars, subnanosecond jitter has been obtained.

8.1.2.6. Miscellaneous. Before the advent of lasers the sparks and flashlamps described in the foregoing sections represented 'the most important group of light sources designed for the widespread range of photographic recording of rapid events. The intensities, spectral distribution of the radiance, and pulse duration can largely be adapted to the problems to be investigated. Because of their simplicity and relative low cost of operation, they are still in use today and will be used furtheron. For completeness we shall describe in this section some processes in which emission of intense radiation in the visible or near visible spectral range can also be obtained. Large conversion ratios of stored energy to light output have been achieved with exploding wires, explosive flashes, plasma foci and laser-produced plasmas which have been studied extensively and used mainly with respect to other scientific or technical applications. 8.1.2.6.1. EXPLODING WIRES.Exploding wires, for example, can be used to switch off overloaded electrical circuits, to generate strong cylin31

32

A . H . Guenther and R. J. Bettis, J . Phys. D : Appl. Phys. 11, 1577 (1978). R. J . Dewhurst, G . J. Pert, and S. A. Ramsden, J . Phys. D 5,97 (1972).

704

8. LIGHT

SOURCES AND RECORDING METHODS

drical shock waves or to initiate explosive^.^^ These processes are followed by an intense burst of light. The experimental set up simply consists of a thin metal wire with diameters of several 100 pm through which the energy stored in a high voltage capacitor is discharged in an inductively loaded electrical circuit. High current densities of more than lo5- lo6 A/mm2 are thereby generated causing a rapid increase in temperature and an explosive fusion and evaporation of the heated material within a few ps. The transition from the solid to the liquid phase of the material can be observed experimentally by the sudden variation of the electric current due to the change in resistivity. In the following development, the current will even be switched off completely because the metallic vapours are not conducting in their initial phase. High overvoltages L di/dt are thereby built up which can cause reinitiation and breakdown of the gap by the formation of bright, highly conducting arcs. If the overvoltage is not sufficiently high, reignition can occur after a longer timedelay by thermal ionization processes. Measurements reported by Kruge?4 have been carried out by using 20-40 mm long, 0.1-0.2-mm diameter, Cu-Mg wires. The electrical parameters are: C = 40 pF, U = 10 kV, the oscillation period, 66 ps. The plasmas obtained proved to be optically thick during the first some tens of microseconds. Radial temperature and electron-density profile measurements have only been possible, therefore, in the later stages of the development. Both profiles have a cylindrical shape with a minimum central value, the maximum occurring at a distance r of several millimeters, depending on time. Maximum temperature values 90 ps after the initiation are still as high as 12,000 K, electron densities are in the order of some lo1?cmP3. Further, measurements have been performed using electrolytic liquid jets (for example BaNz solutions). In this case, temperatures of 62,000 K have been determined 15 ps after initiation, which also dropped to about 12,000 K in 100 ps. The duration of the light pulses can greatly be influenced by the amount of electrical energy, the material and the dimensions of the wires and the material surrounding the wire (gases at various pressures, liquids or solids). The light output is typically characterized by a first short peak of low intensity at the end of the vaporization period. The beginning explosion is marked by a dark pause, whereas the main light emission occurs during and after the period of reignition. As an example for the application of exploding wires in optical systems, one could point out the investigations reported by Hornung and S a n d e m a r ~who , ~ ~ studied hyper% W. G. Chaces and H. K . Moore, "Exploding Wires," Vols. 1 , 2, 3, and 4. Plenum, New York, 1959, 1962, 1964, and 1968, resp. 34 R . Kriiger, Z . A n g e w . P h y s . 25, 283 (1968). 35 H . G. Hornung and R . J . Sandeman, J . Phys. D 7, 920 (1974).

8.1.

LIGHT SOURCES

705

sonic flows of argon over blunt bodies by means of multiple-wavelength interferometry. 8.1.2.6.2. EXPLOSIVE FLASH.As already mentioned, short duration light pulses can also be obtained by using chemically stored energy of explosives creating rapidly expanding shock waves propagating in rare gases such as argon. In the most simple case, a milky transparent balloon filled with argon attached to a detonator.of some tens of grams of explosive can be used. The shock waves thus produced are characterized by extremely high luminosity which can be further increased by intersecting two waves initiated from two sides which are producing considerably higher pressures in this plane. The pulse widths of typical explosive flash sources are depending upon the length of the rare-gas-filled volume. They are of the order of some 10-100 ps. These bright sources are especially useful for recording in reflected light using high speed rotating mirror cameras where individual framing exposure times are varying from about 10 ns to several rnicro~econds.~~ 8.1.2.6.3. PLASMAFOCUS. Plasmafocus experiments have been performed for the generation of hydrogen or deuterium plasmas with extremely high electron densities and temperatures. The experimental setup uses coaxial cylindrical electrode configurations in which at one side the energy of a storage capacitor is discharged concentrically. The uniform concentric plasma layer between the outer and the inner electrode is then axially accelerated in z-direction by the Lorentz force K = j x B ( j = current density, B = magnetic inductance). At the other end of the inner electrode which is shorter than the outer one, radial magnetic forces cause a rapid compression of the plasma. Measurements of the plasma parameters yielded values of the electron density of several lo2*cm-3 and electron temperatures from 1 to 8 keV (corresponding to several lo7 K). These values are observed in a focus volume of about 5 x cm in diameter and 1 cm in length. The main application of the plasmafocus would, therefore, be that of a pulsed neutron source. Great efforts are undertaken for high speed neutrographic re~ording.~?Simultaneously x-ray emission and strong light emission from the uv up to the medium ir is also obtained. The exact values depend upon the storage-capacitor energy, ranging in typical experiments from some tens of joules to some tens to hundreds of kilojoules, and upon geometric configurations. The lifetime of the plasma in its compression phase is about 100-200 ns. When

Z. Pressmann, Proc,. I n t . Congr. High-Speed Phohof., 5 t h . Wushingmn. D . C . , 1960, p. 56. SMPTE, New York, 1962. 37 D. Ruffner, Ber. IPF 74-3. Inst. Plasmaforsch., Stuttgart, 1974.

706

8.

LIGHT SOURCES A N D RECORDING METHODS

D2is used as a filling gas, strong emission of neutrons can be observed

10” neutrons per pulse with energies in the range of 2.5 MeV).38*3e Measurements ranging from A = 0.2 pm to 8.6 pm have been carried out by Schmidt et ~ 1using . a~deuterium-filled ~ 30-kJ focus. The intensity drops down first proportional to I/AZ, then proportional to l/A4. Investigations of a small 50-J focus, operated with hydrogen, show spectral luminance corresponding to thermal radiation of a blackbody temperature of 80,000 K. The applicability of the plasma focus as an intense light source in a large spectral range including vacuum uv and x-ray range has been pointed out by some investigators. Direct application to photographic recording of flow or plasma phenomena, however, has not yet been published. 8.1.2.6.4. LASERPRODUCED PLASMAS. The high degree of spatial coherence of lasers to be discussed in the following sections allows for high energy densities to be obtained in the focal plane of an objective lens. Laser induced breakdown in gases, for example, giving optically dense plasmas, can be obtained by starting from multiphoton absorption and inverse bremsstrahlungs absorption processes. Threshold values of the power densities depend on the wavelength of the laser radiation as well as on the type of gas and its initial pressure. Lower thresholds are observed by irradiation of solid state targets. The characteristics of the plasma parameters, the temperatures, pressures, and specific internabenergies have been studied extensively. For details, the reader is referred to the l i t e r a t ~ r e . ~ ’The * ~ ~great effort in this field of research has been stimulated by two facts. The first one was due to the possibility of applying high intensity focused laser beams for material processing, thereby causing fusion or evaporation of the material so that laser technology can be used for cutting or drilling purposes. The main fact, however, was to study laser produced plasmas in thermonuclear fusion experiments. The basic concept involves compression and heating of a mixture of deuterium and tritium. The densities which are necessary to approach the breakeven point, where thermonuclear energy equals the laser input energy, are about 1000 g/cm3, ignition temperatures are of the order of 30-40 x lo6 K. ( 1Oln to

L. Michel, K. H. Schonbach, and H. Fischer, Appl. Phys. Lett. 24, 13 (1974). H . Rapp, P h y s . Lett. A 43, 420 (1973). 4u H. Schmidt and H. Conrdds, Verhandlungen der Friihjahrstagung der DPG, Stuttgart, 1974, Phys. Verlag, Weinheim, 1974. J. Schwarz and H. Hora, “Laser Interaction and Related Plasma Phenomena,” Vols. 1, 2, 3A, and 3B. Plenum, New York, 1971, 1972, 1974, and 1974, resp. $ ’ J . F. Ready, “Effects of High-Power-Laser Radiation.” Academic Press, New York, 1971. 38 30

8.1.

LIGHT SOURCES

707

Estimates are showing that those values of symmetric compression of a material should be possible with laser pulse energies of 104-105 J . Experiments have been carried out with ruby and neodymium lasers, providing pulses in the nano- or picosecond range with energies up to some kilojoules. High pulse energies can also be achieved with COz lasers, one of the most powerful systems of 10 kJ being under construction at the Los Alamos Scientific Laboratories. 8.1.3. Laser Light Sources 8.1.3.1. Fundamental Properties. Lasers are generating and amplifying electromagnetic waves at optical frequencies scanning a broad spectral range from the vacuum uv to the ir. As compared to thermal sources, lasers are normally characterized by extremely narrow bandwidths. Laser oscillation has been observed and reported for a large number of neutral and ionized atoms or m 0 1 e c u l e s . ~ ~ -Great ~ ~ efforts are stimulated to extend the band of wavelengths towards the soft x-ray range. The basic principles of lasers, the interaction of radiation fields and photons, respectively, with atomic systems, are theoretically described by quantum-mechanical formalisms. As we are particularly concerned with optical methods, the spatial and temporal coherence, the speckle properties, the contrast or the visibility of fringes and interference patterns are of importance. These topics can largely be treated by the semiclassical or even the classical theory. Starting from a simplified model with a two-energy level material, the radiative energy balance can be set up by a rate equation in which induced emission is considered to be a negative absorption. Using the definition of a net-absorption coefficient, the derivation exactly leads to Kirchhoff s law, which then proves to be consistent with the laser principle.46 Since induced transition probabilities are equal both for absorption and emission, an amplification can only occur as the number density N z of particles excited in the upper energy level E2 is greater than the density N , in the lower state El, whereby the statistical weights have to be taken into account. The activation of the laser material necessary for obtaining inversion ( N z > N , ) can be achieved by different pumping mechanisms depending on the type of the laser used. Optical excitation by the intense light of flash lamps is used in a D. Ross, “Laser, Lichtverstarker und Oszillatoren.” Akad. Verlagsges., Frankfurt, 1966. 44 W. Kleen and R. Miiller, “Laser.” Springer-Verlag, Berlin and New York, 1969. 45 G . Herziger and H. Weber, “Laser, Grundlagen und Anwendungen.” Physik-Verlag, Weinheim, 1972. A. Bauer, Optik 29, 179 (1969). @

708

8.

LIGHT SOURCES A N D RECORDING METHODS

the case of dielectric solid state lasers, dye lasers, and photo-dissociation lasers. Gas lasers and semiconductor lasers can be excited directly by electric currents. Other excitation mechanisms have been successfully applied including electron-beam techniques and gas-dynamic methods for high power gas lasers. The primary process in an “inverted” laser material is the amplification of an incident flux of light or of spontaneously emitted photons. Providing a suitable feedback, this amplification can exceed the losses (absorption, diffraction, mirror losses, etc.) thus producing self oscillation. The feedback can 8.1.3.1.1. MONOCHROMASY A N D MODE SPECTRUM. be realized by a Fabry-Perot-type or ring-type resonator. The common characteristic of nearly all laser resonators is that they are open resonators that do not require side walls. The actual wavelengths of the laser lines are determined both by the fluorescence profile of the considered transition of the laser medium and by the eigenfrequencies of the resonator modes. The spectral line shape is thereby influenced by different broadening mechanisms. In gas lasers, the Doppler effect or pressure broadening is mainly acting on the line width. In solid state lasers there are the statistical Stark fields of thermal vibrating crystal lattice, inhomogeneities, and impurities. The largest spectral widths are observed with dye lasers due to the strong interaction of the dye molecules with their solvents. The amplitudes and phases can undergo irregular fluctuations. These changes are relatively slow, however, they are depending upon the effective spectral width Av. As Av is much smaller than the central laser oscillation frequency vo (Av << vo), the light output can be described by quasi-monochromatic waves. Figure 7 schematically shows the spectral distribution of the laser emission. The frequency dependence of the transition, which is usually approximated by a Gaussian or Lorentzian profile is characterized by the half-width Sv. This frequency bandwidth is actually reduced by stimulated emission, depending on the laser threshold condition G(R1R2)1’2. L a 1, where G represents the gain, R 1 , R2 the two cavity mirror reflectivities, and L the additional losses. According to the Huygens principle, the modes of the open resonators can be considered to be eigenfunctions of a Fredholm integral equation. Assuming linearly polarized waves this equation can be written in a scalar form. Numerical calculations, as for example performed by Fox and Li4’ and K ~ g e l n i k yield , ~ ~ the spatial structure of the distribution of the electromagnetic field inside the laser cavity. As the field strengths are mainly ” A.

G. Fox and T. Li, Bell S y s t . Tech. J . 40, 453 (1961).

*’ H . Kogelnik and T. Li, Proc. IEEE

54, 1312 (1966).

8.1.

709

LIGHT SOURCES

cavity with plane-parollel Fabry - Perot square aperture

B5G TEM01

L Fresnel number

F

= aZ/(L.X)

resonances

(1, = (El

transverse mode distribution

v ( x , ~ )=

mode

A(+)

2

2

Separation

2 +

TEMll

(7) 2

+

(n+l)*y0 sin - sin 20

=Ti: 1 [(s,-s,)

+=(mi LX

- m:+

n:

- n: 11

FIG.7. Longitudinal and transverse mode spectra.

transverse to the direction of propagation, the modes are termed transverse electromagnetic modes (TEM,,,-modes), where m and n are giving the number of intensity nodes in the mirror planes in (x, y)- or (r, 4)directions, r e s p e ~ t i v e l y . Some ~ ~ typical equations concerning the resonances, the longitudinal and transverse mode distribution and the mode separation are also included in Fig. 7. For simplicity, a plane parallel Fabry -Perot square aperture resonator has been chosen. For large Fresnel numbers F , the modes thus described can thereby be considered to be in good approximation with the real modes. Similar but more complicated expressions including Bessel functions can be derived for circular apertures or for curved mirror configurations. As q are large integer numbers ( L >> A,,,), this index characterizing the longitudinal modes is usually omitted. In most cases transverse fundamental mode of operation (m = n = 0) is preferable. This can be achieved by adequate cavity design. Under normal conditions, the emission will be longitudinally multimode with randomly distributed initial phases, however single longitudinal G. Grau, Optische Resonatoren und Ausbreitungsgesetze fur Laserstrahlen. I n “Laser” (W. Kleen and R. Muller, eds.), p. 49. Springer-Verlag, Berlin and New York, 1969.

710

8.

LIGHT SOURCES A N D RECORDING METHODS

mode operation can be realized. This proves to be important for holographic applications. Simultaneous oscillation of a great number of longitudinal modes with strongly coupled phases leads to the generation of trains of ultrashort pulses with ps duration of the individual pulses. Both cases, i.e., the single-mode and mode-locked operation will be discussed more in detail in the following sections. It should be mentioned that besides the use of stable resonators for laser sources as applied to photography or spectroscopy, lasers can also be operated with unstable resonators which in the case of high power lasers allow the energy in the fundamental mode to be extracted from large volumes.5o 8.1.3.1.2. COHERENCE PROPERTIES. Coherence properties of lasers can be described by second or higher order correlation effects. As already mentioned, it is possible to select the transverse fundamental mode TEMoo by suitable resonator configurations. That means that the phases of the emitted waves over the whole diameter of the beam are well correlated. Such a radiation field is termed as spatially coherent. Experimental evidence can be shown by the visualization of the interference pattern obtained for example in Young’s experiment as indicated in Fig. 8, using two pinholes, separated by a variable distance. Fundamental mode of operation is favorable for a great number of applications because it allows for propagation over long distances with minimum angular divergence and for production of high power densities in the focal plane of an objective lens. Due to this fact, it is possible in optical systems to generate nearly exactly diffraction limited point-light sources which are important for shadowgraph techniques. Furthermore, Fig. 8 shows schematically the relationship between the temporal pulse shapes and their spectral distributions for two different wavetrains, the envelopes of which have been chosen arbitrarily to have a rectangular or a Gaussian form, respectively. The spectra are calculated by Fourier transforms from which the bandwidths 6 v are determined. In both cases S v shows to be proportional to the inverse pulse length A t . If V(r,t ) is the complex analytical signal of the light-field amplitude in a more general way,51 it can be written in the quasi-monochromatic approach in the following form

v(,., t ) = p(,., f ) e - j ( 2 n u o t - ~ ( r , t ) )

51

(8.1.8)

A . E. Siegman, Laser Focus May, p. 42 (1971). M . Born and E. Wolf, “Principles of Optics,” 4th ed. Pergamon, Oxford, 1970.

8.1. LIGHT

SOURCES

71 I

1

coherence time

AT 2 coherence length AL = c . A ? typical measuring devices far temporal coherence spatial coherence

J----fql loser detector

gl-$

Michelson interferometer Young's experiment

FIG.8. Illustrations of the coherence properties of lasers

V ( r , 1) and +(r, t ) are both functions of the space coordinates r and the time C. vo is the central frequency. By means of a Fourier transform it will be stated again that $' and may be considered to be nearly constant during a time AT which is smaller than the inverse spectral width 6v. This time is called the coherence time which is defined by

+

AT

3

1/4dv.

(8.1.9)

Experimentally, the temporal coherence can be measured in a two-beam interference experiment using, for example, a Michelson interferometer (Fig. 8). By increasing the mirror spacing s2 in one arm of the interferometer, the interference fringe visibility which is directly correlated to the coherence length A L = CAT is decreased, so that A L can be determined in this way. During AT, the amplitudes and phases of the wavetrain at different times are linearly correlated. As compared to the monochromatic light of

712

8.

LIGHT SOURCES A N D RECORDING METHODS

s, lasers allow to obtain values up thermal sources where AT is at best to lo-* s. Mathematically, these coherence properties can be described by the mutual coherence function r12(7) which in the case of two waves (V,(rl , t ) and V2(r2,t ) ) can be written as

=

( vi(ril t -t 7)

*

v,*(rz

9

t)),

(8.1.10)

where the bracket notation is used to replace the more complex integral relation. The asterix denotes the complex conjugate. It is more convenient, however, to use the normalized coherence function y&) which is related to r12(T) by the following equation: (8.1.11) y12(7)is also a complex function. As already mentioned, its absolute

value can be measured easily by interferometric techniques. The classical theory of coherence can be extended to higher order correlation effects or even to quantum-mechanical formalism as pointed out by G l a ~ b e r . ~ ~ 8 . 1 . 3 . 1 . 3 . SPECKLES. It is a well known fact that laser photographic recordings such as visual observations are characterized by a high contrast granulation pattern which is superimposed to the image information and which forms a background noise. In fact, this is an interference phenomenon due to the coherence properties of laser sources. It is always obtained when laser light is diffusely reflected or transmitted. Figure 9 shows the main features for the case of a simplified experimental set up, where the scattered light is transmitted through a diffusor screen. In the observation or image-plane, respectively, the light distribution reveals the randomly distributed granulation. The grain size depends strongly upon the free aperture of the beam that is upon the number of scattering centers. This can be evaluated by calculating the intensity correlation function for a given intensity distribution in the diffusor plane where the individual scattering centers are located.53 The multiple interferences including their statistical properties can be described by a two-dimensional autocorrelation-function C(a, 6) of the intensity in the (x, y)-image plane. Using again the already mentioned bracket notation, this can be expressed in terms of 52 53

R. J. Glauber, Phys. Rev. 130, 2529 (1963). J. C. Dainty, Opt. Actu 17, 761 (1970).

8.1.

713

LIGHT SOURCES

initial intensity I [ $ , ? ]

image plane

IIx,yl

-jj+ Laser

speckle distribution

centers /Path

&

rr

L

< d ) % . hLb

-1

system,

&

I

(d)nX

-f = X .F DL

Fici. 9. Speckle formation in laser photographic systems.

C(a, 6) = ( I ( & y ) . I*(x

+ u , y + b))

(8.1.12)

(the asterix again denotes the complex conjugate). Averaging over all the phases of the light waves emanating from all the randomly distributed scattering centers in the 5-7 plane yields that an alternative part e t a , b) can be split off the mean value This alternative part is finally responsible for the spatial intensity fluctuations. As the calculation shows, is proportional to the square absolute value of the Fourier transform of the intensity distribution in the scattering plane. In determining the first zero values of this function, one obtains the speckle size which is then only depending on the geometrical form and dimension of the beam aperture in the scattering plane. For rough estimates the values of the mean diameters of the speckles ( d ) are given in Fig. 9. For an optical imaging system using a lens of focal length fand a lens aperture DL,the parameter ( d ) is only limited by the F value F = f/DL. The shape and diameter of the speckle pattern can thus largely be influenced by a suitable choice of these parameters. Rectangular diaphragms are producing long shaped speckle patterns. In normal photographic applications these speckles are disturbing and limiting the high resolution obtainable with lasers. The speckles can be used, however, in photography for the measurements of small-scale distortions of objects

c.

714

8.

LIGHT SOURCES A N D RECORDING METHODS

undergoing mechanical loads, or generally for the possibility of separating a great number of different photographs which are superimposed on a single photographic plate. The photographs have to be taken with different diaphragms so that each picture is characterized by its own speckle pattern. The evaluation and reconstruction of the individual photographs are then simply to be performed by optical spatial filtering t e c h n i q ~ e s . ~ ~ 8.1.3.2. Spectral Ranges Covered by the Most Important Types of Lasers Applied to Fluid Dynamic Research. Table I11 shows a schematic classification for the different groups of lasers. In each group only one or two of the most important characteristic lasers are indicated. The neutral gas lasers, for example, include laser oscillation in 29 ele~~ metal vapors. The ments on about 450 identified t r a n s i t i o n ~including most important laser of this group is the He-Ne laser, the strongest lines of which are in the red at 0.6328 p m and in the ir at 3.39 pm. Molecular lasers which are most effective for high power generation are classified to constitute another group of lasers. These lasers can be excited by electrical, optical, chemical or gas-dynamic pumping. Among these lasers we find the COz laser which can be operated in continuous or pulsed mode and which is emitting a large number of rotational vibrational lines ranging from 9.2. to about 1 1 p m . Single-line operation and tuning over the different lines can be obtained using a dispersive element inside the resonator. These lasers have been used successfully for ir recording which requires special techniques such as evaporation of thin films or liquid

In the case of ionized gas lasers, the transitions are originating from energy levels in the ionized state of atoms (or molecules) in gas discharges. The noble gas ion lasers such as the Ar I1 laser belong to this group. As already pointed out, the largest number of experiments conducted in the past in the field of laser photography have been performed particularly with the group of dielectric solid state lasers, with the chromium-doped ruby laser emitting at X = 0.6943 p m or with neodymium-doped glass or YAG lasers emitting in the near ir at 1.06 pm. Together with frequency doubling, this group of lasers provides pico- or nanosecond duration pulses in the ir, visible, and uv part of the spectrum. Rare-earth ions such as neodymium can also be imbedded in several organic components such as chelates. Most of these chelates are soluble in organic solvents. Historically, it is perhaps interesting to remember that Eu chelate dis54 55

U . Kdpf, Siemens Forsch. Entwicklunyshrr. 2, 277 (1973). R . J . Pressley, “Handbook of Lasers.” Chem. Rubber Publ. Co., Cleveland, Ohio,

1971. as

F. Keilmann, Ber. IPP IV/4. Inst. Plasmaphys. Garching, 1970.

8.1.

LIGHT SOURCES

715

solved in alcohol has been the first liquid material exhibiting laser actione5' The emission of semiconductor lasers occurs mainly in the near infrared. In the case of GaAs, the emitted wavelengths are in the range of 0.83-0.9 pm. These values can be shiftet by various dopants, even towards the visible. Semiconductor lasers can easily be modulated so that they are especially useful for optical communication systems, optical radar or for range finding; they are less important for the purpose discussed in this presentation. While the previously mentioned lasers operate at discrete frequencies, the last group of lasers to be discussed in this section, the dye lasers, is characterized by a large band of fluorescence so that the emitted wavelength can be tuned in this range. These lasers have been studied extenThe tunability makes them an attracsively during the last few tive tool for spectroscopists. For photographic recording these lasers are also very suitable. The lasers are optically excited by means of flash lamps or other laser light sources. Pulse durations of a few nanoseconds to some 10-100 p s can be obtained. Such long pulses are used for the monochromatic background illumination for rotating-mirror framing or streak cameras. In a large part of the visible and near ir spectrum mode-locked pulses have been achieved. Using laser pumping, cw operation can be obtained as well. Among the increasing number of dyes, only those belonging to the group of the Rhodamines and Coumarines have been mentioned in Table 111. Rhodamine 6G, for example, shows high quantum efficiency and a large tuning range. By means of other dyes, including frequency-doubling and Raman-type oscillation, the whole wavelength range from about 0.2 pm to the far ir can be covered. It is important to note that the spectral width 6v of these lasers can be made less than 0.01 A by using etalons and gratings or prisms so that great coherence lengths can be realized causing dye lasers to become also excellent sources for holographic recording.

8.1.3.3. Time-Dependent Emission Characteristics. The fundamental characteristics outlined in the previous sections make the laser an attractive light source for photography. Light output can, thereby, be obtained continuously or in different pulsed modes. 8.1.3.3.1. CONTINUOUSEMISSION. The first laser with which continuous-wave (cw) operation has been obtained is the He-Ne laser emitting at 6328 A. Nowadays, such lasers are commercially available with powers up to some tens of milliwatts. Higher powers can be ST 58

A . Lempicki and H. Samelson, Phys. Lerr. 4, 133 (1963).

F. P. Schafer, "Dye Lasers." Springer-Verlag, Berlin and New York, 1973.

716

8 . LIGHT SOURCES A N D RECORDING METHODS

TABLE111. Spectral Characteristics of Several Types of Lasers" ~~

Group of losers

Type

Typical representolive

Strongest tronsitians (pm)

Neutral gas lasers

Losers

e p .He-Ns laser

mcillaiing o i Molecular gas losers

Ah

06326

(nm)

0 001

3 39

discrete frequencies

Llnwldth.

'

''

loser

wntered around

low pressure

0 037

10 6

high prrrwra

I5 ~

Ionized gas losers

e g..Ar

U

loser

0 5145 0.4880

m0.01

0.4579 Rore earih liquid losera

0 . 9 , Nd-doped chdab

Dielectric Bolid siaie

e.g., Ruby laser.

1.06

laair

Naadymium glass

lasers

0 6943

I

.sm

losar

-

0.001

-0.1 10

----__---------------------------------------------Semiconductor lasers Dye losers

Go-As loser

Tunable lasers

(I

Rhodarnlm ( Carmarine I

0.83 0.9

WI

0.54- o m

*I

0.45-054

W l

Linewidths at room temperature

achieved with noble gas ion lasers, the Ar-ion laser of which is best known. The emission of this laser occurs on several lines in the blue and in the green spectral range. Among the solid state lasers, cw emission has been obtained with ruby- and with neodymium-doped YAG crystals. These YAG lasers are favorable because lasing threshold and pumping requirements are relatively low. Its radiation (A = 1.06 pm) can easily be converted to the visible range by frequency doubling using nonlinear optical crystals (KDP, ADP) outside or even inside the resonator. Both, argon and YAG lasers, are frequently used to generate continuous wave tunable emission. The argon lines are mainly used for the pumping of dye lasers,59whereas YAG lasers are often used to obtain parametric oscillation in crystals, the first cw operation having been achieved by Boyd and Ashkin.60 Large tunable ranges can be obtained by controlling the temperature or by angle tuning of the nonlinear crystals (for example KH2P04,NH4H2P04,LiI03, LiNbO,, or BazNaNb5015). 8 . 1 . 3 . 3 . 2 . RELAXATION PULSES.Among pulsed lasers emitting in the 5s

0. G . Peterson, S. A. Tuccio, and B. B. Snavely, Appl. Phys. Leti. 17, 245 (1970). D. Boyd and A. Ashkin, Phys. Rev. 146, 187 (1966).

WJ G.

8.1.

LIGHT SOURCES

717

visible range, solid state lasers, excited by the thermal light of intense flash lamps are of great importance. The pumping pulse duration has to be adapted to the lifetime of the metastable laser levels (1-2 ms in the case of ruby and some hundreds of ,us in the case of neodymium lasers). The laser oscillations are obtained after the inversion threshold has been achieved. Under normal conditions, a large number of modes with different initial phases are appearing; these are further subject to thermally induced changes of the resonance conditions so that the time shapes of such pulses are randomly distributed sets of spikes which last until the end of the pumping pulse. By the use of longitudinal and transverse mode selection, regularly shaped, periodically oscillating, decaying pulses can be obtained. Because of the strong intensity fluctuations and the rather long overall duration of the order of some hundreds of microseconds to 1 ms, however, both types of pulses, the randomly distributed spikes and the periodically modulated pulses, are not well suited for photographic applications. 8.1.3.3.3. GIANTPULSES.The best and most reproducible laser pulses can be obtained by Q-switching the laser cavity. This technique employs some type of (mechanical, electro-optical or intensity-dependent transmitting dye solution) shutters to suppress cavity feedback during the pumping process. Nearly all the stored energy can then be delivered to a single pulse by rapidly establishing a high Q-value of the resonator (Q = quality) so that short duration pulses of some tens of nanoseconds with peak-powers in the range of some 10 to some 100 of megawatts can easily be obtained. Spectrally, the emission of such giant-pulse lasers, under normal conditions, proves to be multimode. Figure 10 shows schematically the experimental setup which allows for the generation of single mode Q-switched laser pulses.61 The actual shutter is formed by a Kerr or a Pockels cell which allows for time synchronization. Longitudinal mode selection is provided both by a saturable absorber (e.g., cryptocyanine in methanol) and by the Fabry -Perot etalon out-coupling mirror. Transverse fundamental-mode operation is obtained by a near confocal resonator configuration supported by a small aperture diaphragm which produces additional losses for the higher order transverse modes. The time dependence of the pulse as measured by a fast photodiode and oscilloscope (TK 5 19) and the spectral output measured by a Fabry-Perot interferometer are also included in Fig. 10. The frequency spacing from one order of interference to the next higher one has been chosen to be 1.5 GHz. This proves to be sufficient to resolve longitudinal modes which are separated by Av = c/2L = 0 . 3 GHz for L = 50 cm. The apA . Hirth, 1SL-Ber. 10/68. Dtsch.-Franz. Forschungsinst. St.-Louis, 1968.

718

8.

LIGHT SOURCES A N D RECORDING METHODS Pockels

dye c e l l

m i r r o r (100%)

/

cell

\

poldrizer

-

etolon reflector

20ns/cm

FIG.10. Monomode giant pulse ruby laser, pulse intensity versus time, spectral distribution.

pearence of only one circle line per order of interference thus indicates real single frequency operation. In measuring the width of the lines by means of a microdensitometer, one obtains a first rough information on the spectral width of the emitted light, thus yielding the coherence length AL = c AT of the wave trains. The output energies of such a system incorporating a 3-in, ruby rod is typically of the order of 10-20 mJ. This is sufficient for most types of optical investigation even when relatively low sensitive holographic plates have to be exposed. PULSES.As already pointed out, the emis8.1.3:3.4. MODE-LOCKED sion spectrum of all lasers consists of a large number of longitudinal and transverse modes. For simplicity, we assume in the following discussion fundamental transverse mode (TEMoo)operation. The number of axial modes is depending on the bandwidth of the laser transition and on the mirror spacing including the refractive indices which are determining the mode separation. Strong coupling of the phases of all simultaneously excited and oscillating modes can be achieved by active and by passive modulation techniques. Active modulation is mainly employed with continuously running gas lasers. This calls for an amplitude, phase or frequency modulator inside the cavity which is driven at a modulation frequency v M . Let us assume first the oscillation on a single mode n that has just reached the threshold of oscillation, characterized by its amplitude a , and initial phase &:

8.1. LIGHT

V,(/) =

719

SOURCES

sin(2.rrunt+ 4,,).

(8.1.13)

(I,

Performing an amplitude modulation a,(/) = uno cos(2.rrvMt + have V,(t) = =

U,

cos(2wMt

we

+ (bM) sin(2?runt+ 4,)

{sin[2.rr(un-

+ sinP.rr(u, +

+M)r

u,)t

VM)f

+ +n

-

+M1

+ 4 n + 4J1,

(8.1.14)

indicating the formation of an upper and a lower side-band frequency. If the modulation frequency corresponds to the frequency spacing of two successive modes vM = Au, then u,, - uM = u, - AU = u,,-~and the two sideband frequencies are identical with resonator eigenfrequencies which are then amplified in phase with the originally oscillating mode. This procedure is repeated until all the modes of the spectrum are oscillating in phase. The time behavior of the laser output can be described mathematically by the inverse Fourier transform corresponding to the characteristic frequency spectrum. This yields the experimentally observed train of mode locked pulses, the duration of which can be shown to be proportional to the inverse number of coupled modes N , whereas the peak intensities are proportional to W . In flash-lamp pumped dye or solid state lasers, this active modulation technique can no longer be used, because of the thermal drift of the optical cavity length causing frequency shifts and variations in frequency spacing of neighboring axial modes. This effect would necessitate automatic frequency pulling of the active modulator during the 1-2 ms pumping pulse. As found by De Maria,62this matching condition can nevertheless be fulfilled automatically by using nonlinear saturable absorbers similar to those applied to Q-switching. These absorbers are characterized by high absorption coefficients in narrow spectral bands for low light intensities, whereas, above a given threshold value, the dye levels are saturated so that the dye is highly transmitting. The relaxation time for the decaying transparency has to be smaller than the cavity round-trip time. Formally, the saturable absorbers can also be considered to cause modulation, the frequency of which is automatically identical with the inverse cavity round-trip time. LetokhoP3 and otherss4 have developed a theory which starts from initial small random intensity fluctuations due to A . J . DeMaria, W. H. Glenn, M. J . Brienza, and M. E. Mack, Proc. IEEE 57,2 (1969). S. Letokhov, Sov. Phys. -.IETP (Engl. Trunsl.) 28, 562 (1969). P. G. Kryukov, Yu. A. Matveev, S. A . Churilova, and 0. B. Shatberashvili. Sov. Phys. -JETP (Engl. Transl.) 35, 1062 (1972). "

gg

720

8.

n

Q

LIGHT SOURCES A N D RECORDING METHODS

madulator

(vbl =Au’

‘U

actively modulated c w loser passively modulated pulsed laser

saturable absorber

pulse train of a mode - Locked ruby Laser 2 0 ns/cm

5 ns/cm

FIG.1 1 . Active and passive mode-locking of cw and pulsed lasers.

spontaneous emission processes which are subsequently amplified traversing the active medium. The selection of a single pulse traveling back and forth in the cavity, originating from the highest fluctuation peak, is provided by the nonlinear transmission characteristics of the saturable absorbers which show higher absorption losses for the smaller intensity peaks. Figure 11 shows schematically the two methods of active and passive modulation as well as an example of the light output of a mode-locked ruby laser. This has been measured by means of a fast photodiode and an oscilloscope which, because of the limited bandwidth of about 1 GHz, cannot follow the actual rise and decay times of the ultra-short pulses. The pulsewidth has to be measured by rather sophisticated methods such as two-photon fluorescence techniques, nonlinear correlation techniques, or by rapid scanning image-converter streak cameras. Reference is made to the l i t e r a t ~ r efor ~ ~details which are out of the scope of the present contribution. 8.1.3.4. Superradiant Light Sources. Laser oscillation usually occurs if a medium is suitably pumped, inverted and if feedback is provided by an appropriate resonator. If stimulated emission dominates, the amplificaBJ

R. Dhdliker,

( 1970).

A. A. Griitter, and H. P. Weber, IEEE J . Quantum Electron. 6, 687

8.1.

LIGHT SOURCES

72 I

tion of a wave propagating in z-direction is given by the following equation: Z(z>

=

I(0)e-a'Z.

(8.1.15)

a is the gain factor, a = cr A N , where cr is the cross section for stimulated emission (correlated with the dipole matrix element of the transition) and AN is the population inversion between the two lasing levels involved. If az is sufficiently high, spontaneously emitted photons propagating in &z direction are amplified by stimulated emission processes thus forming superradiant light pulses without any resonator. In the strict sense, the superradiation therefore proves to be spatially and temporally largely incoherent. The spectral linewidth is approximately identical with that of a fluorescence line. This means that the light output is quasimonochromatic but not restricted to discrete resonator eigenfrequencies. In long lasers or in laser-amplifier chains this effect is disadvantageous because the inversion can be reduced considerably by superradiance. In a large number of cases, however, the superradiant mode of operation is preferentially used. Due to their high gain, most NJasers, for example, emitting in the near uv at A = 0.3371 pm are operated in the superradiant mode. For high-speed photographic applications, special types of superradiant sources have been investigated using solid state materials such as ZnS, ZnO, CdSe, ZnTe, GaAs, CdTe. These materials are showing strong fluorescence when they are irradiated with a high-energy electron beam. Short-duration electron pulses of some tens of nanoseconds with highcurrent densities can be generated by vacuum field-emission discharges by using high voltage Marx surge generators with voltages of some hundreds of kilovolts. The superradiant material is deposited on foils which are positioned near the exit window of the electron beam gun. The gain achieved by this method in these materials is so high that the excited wave provides a very intense light output after a single pass in the amplifying thin layer. The halfwidths of the emitted pulses are of the order of some nanoseconds. By using different materials, a large range of the visible spectrum can be covered.66 It should be pointed out that by removing the superradiant plate the same installation can be used as a pulsed source for electron beam or for x ray recording techniques.

8.1.3.5. Generation of Coherent Radiation Using Nonlinear Optical Methods. The nonlinear behavior of material at optical frequencies, as it can be described mathematically by field dependent dielectric constants BB J. L. Brewster, J. P. Barbour, F. M. Carbonnier, and F. J. Grundhauser, Proc. fnt. Congr. High-speed Phorogr., 9rh, Denver, 1970. p. 304. SMPTE, New York, 1970.

8.

722

LIGHT SOURCES A N D RECORDING METHODS

and magnetic permeabilities, is well known. The polarization, for example, can be expressed as a function of the electric field by a power series in the following form: y =

E+x‘”E.E++X‘:”E.E.E+ . . . (8.1.16) L- higher-order nonlinear susceptibilities Llowest-order nonlinear susceptibility linear susceptibility

x(l) *

L

Using the high field intensities of existing lasers, the nonlinear effects can be used for the generation of a large number of new lines of coherent radiation, see Fig. 12. 8.1.3.5.1. GENERATION OF HARMONICS. The nonlinear properties have first been strikingly demonstrated in 1961 by Franken and coworkersG7by the observation of the harmonic of a ruby laser beam. According to the notations of Bloembergen,B8the lowest order nonlinearity at an angular frequency w3 is given by P L ( w 3 )= x(w3 = w1 + wz) EIEz e‘(k1+k)z-i(W1+W2)I)

where

x is a third-rank tensor.

(8.1.17)

Second harmonics are obtained when

w1 = w2 = w and w3 = 2w. Since k3 = 2k, = 2kz, the propagation velocities at w and 20 are different due to dispersion ((ki/= 2.rr/hi). Thus it is

necessary to match these two phase velocities. This can be done if the nonlinear crystals are birefringent as in the case of ADP or KDP (NH4H2POI,KHzPO,). The fundamental and harmonics have then to be attributed to the ordinary and extraordinary ray so that for specific angular conditions with respect to the crystal axis the color dispersion is compensated for the anisotropy of the two phase velocities. As the nonlinear polarization proved to be proportional to the square of the laser electric field amplitude, the efficiency can be increased by increasing laser intensity. Taking higher order susceptibilities into account, higher order harmonics can be generated as well. Terhune was the first researcher to have observed third harmonic generation.sB 8.1.3.5.2. RAMANLASERS.The investigations of stimulated Raman processes gave rise to numerous applications including the generation of molecular or lattice vibrations, the measurement of the lifetimes of excited vibrational states and the production of intense coherent light at new frequencies. Early laser studies have shown that ruby lasers gain 67

P. A . Franken, A. E. Hill, C. W . Peters, and G. Weinreich. P h y s . Rev. L e f t . 7, 118

( 1961).

“ N . Bloembergen, Nonlinear optics. In “Quantum Optics and Electronics,” p. 411. Gordon & Breach, New York, 1965. 6e R. W. Terhune, P. D. Maker, and C. M. Savage, Phys. Rev. Lett. 8,404 (1962).

8.1.

LIGHT SOURCES

1 p q

723

external

e.9. KDP

Zw, operation

Roman octive material le.g.Ht)

pump source

Stokes or Anti-Stokes frequencies frequencies

frequencies cavity resonant far US, or for both q , o n d w,

(C) I

pump source

wL- qi+ q

i;,

.

r,

*

I s.y.

wL = loser * signal

WJ

-

w, idler

7

angular frequency

FIG.12. Generation of coherent radiation using nonlinear optical methods. (a) Harmonic generator. (b) Raman laser. (c) Parametric oscillation. (Vector quantities are indicated by arrows over letters in figure and by boldface letters in text.)

switched with nitrobenzene Kerr cells emitted additional light at 0.767 pm. This means that quanta of energy hwL are absorbed, one part of which is converted to quanta of energy nos. A large number of liquids, solids, and gases are showing typical frequency shifts from the exciting laser line which correspond to the vibrational frequencies of the molecules involved. This can be interpreted as a scattering process. Low laser intensity variations are linearly influencing the intensity of the new line. High laser intensities are capable of generating large numbers of scattered photons which are then amplified exponentially due to a transition from a spontaneous to a stimulated scattering process. By characterizing the molecular vibrational wave by the angular frequency wv , the following equation holds for the angular frequency w, of the new Stokes-shifted Raman line: wL = w s + w v . If both frequencies, i.e., the laser and the Stokes frequency are present, higher order Stokes lines, oL- 2 w v , wL - 3wv, and so on, can be obtained. As there will be a polarization term at the anti-Stokes frequency wAs = 2wL - w s , it is also possible to obtain blue shifted anti-Stokes

724

8.

LIGHT SOURCES A N D RECORDING METHODS

lines. The gain of the Stokes line is proportional to the Stokes susceptibility which can be expressed by the differential Raman scattering cross section. The above gain is proportional to the square of the electric field strength of the laser. The amplification of the scattered wave thus grows exponentially with the incident laser intensity. Providing a feedback by resonant mirrors, such a medium constitutes a Raman laser in which oscillation can start from noise. Suitable frequency selective mirror reflectivities can force oscillation on the first- or higher-order Stokes lines. The excitation is mostly obtained by giant pulses from solid state lasers. The application of tunable dye lasers in the visible thus provides tunability of the Raman laser emission to an extended range in the infrared.'O 8.1.3.5.3. OPTICALPARAMETRIC OSCILLATORS.As compared to normal lasers where amplification is obtained by population inversion, the gain in parametric amplifiers is produced by the interaction of three electromagnetic waves with a nonlinear medium which is characterized by its second-order nonlinear coefficient x'~'. In Raman processes, two electromagnetic fields and a molecular vibrational mode were interacting, whereas in parametric oscillator studies we are concerned with three purely electromagnetic waves, one high frequency pump wave (up)and one pair of lower frequency waves called the signal (us,)and the idler (wi). The three frequencies are related by the formula w, = wSi + wi which corresponds to the energy balance. Above a threshold value of the pump, the signal and idler waves experience a net gain. They can grow in such a way that their fields are comparable to that of the pump. The parametric gain is critically dependent on the amount of momentum mismatch Ak = k, - ksi - k i . In a medium without dispersion Ak would be zero. In practice, Ak may be large making the parametric gain relatively small. As in the case of harmonic generation this can best be compensated by using birefringent nonlinear crystals. Experiments have been performed using double resonance oscillators, where mirrors are used which reflect both for the signal and the idler wave whereas the mirrors are transparent to the pump radiation. Under optimum conditions, one half of the pump power goes into the signal and the idler, one quarter is transmitted and one quarter is reflected. Efficiency can be increased by using ring cavities with which the upper theoretical limit of 100 percent can be attained. Oscillation has also been obtained using single resonance oscillators with mirrors which only reflect for wsl or w i . A third possibility is given by internal parametric oscillators where the nonlinear crystal is incorporated in the cavity of the pump source. The materials used for parametric oscillators are the same as for second 'O

J . Kuhl and W.Schmidt, Appl. Phys. 3, 251 (1974).

8.2.

RECORDING METHODS

725

harmonic generation. They must have a lack of symmetry centers, a large value of the nonlinear second-order electro-optical coefficient, and a large transparency range. They should further be homogeneous, phase-matchable, and resistant to optical damage. Besides, ADP and KDP, LiNbOs, and B%NaNb,015 are frequently used. The first operation has been achieved by Gi~rdrnaine.~’Since then, pulsed and cw operation has been studied successfully under different conditions.‘* Ruby laser pulses and second or third harmonics of neodymium laser emission have been used as pump sources, cw operation was possible by using the Ar 11-5145-i% line or neodymium-doped cw YAG lasers. Tuning of the parametric oscillators can be achieved by varying the index of refraction of the crystal which can be done by temperature changes or by varying the angle between the three waves in the case of noncollinear interaction. The largest tuning range from 0.684-2.36 pm has. been obtained in a double resonance oscillator, pumped by a frequency-doubled neodymium laser, by using three crystals and a set of three mirrors. The tuning ranges obtained are thus considerably larger than those achieved with dye lasers.

8.2.Recording Methods 8.2.1. Introduction A large number of cameras has been developed in the past for the investigation of rapidly varying phenomena. These include single exposure as well as cinematographic techniques. Because of the use of lasers, the range of possibilities of conventional photography has been considerably extended. Holographic methods yield informations both on amplitudes and phases of the wavefronts. A survey of the most important possibilities is schematically shown in Fig. 13. For the sake of clearness, overlapping ranges are not indicated. The single exposure techniques are devided into two groups, one of which applies to short illuminating pulses whereas the other one uses high speed shutters. The cinematographic methods are classified following the mainly applied methods of image separation. It is obvious however, that the different techniques can be combined such as for example in the case of the operation of high speed shutters or periodical pulse trains with mechanical cameras.

’*J . A. Giordmaine and R. C. Miller, Phys. Rev. L e f t . 14, 973 (1965).

72 R. G. Smith, Optical parametric oscillators. In “Laser Handbook (F.T. Arecchi and E. 0. Schulz-Dubois, eds.), Vol. 1 , p. 837. North-Holland Publ., Amsterdam, 1972.

726

8.

LIGHT SOURCES A N D RECORDING METHODS

FIG 13 Schematic classification of recording methods and systems.

The aspects of mechanical cameras, with the exception of cineholo~ importante of graphy, have mainly been considered by D ~ b o v i k , ’the electron-optical high speed photographic systems by Courtney-Pratt ,74 whereas the applicability and vertisability of spark cinematography and electrooptical methods are treated comprehensively by Vollrath and Th~mer.’~ 8.2.2. High Speed Photographic and Cinematographic Methods.

8.2.2.1. Spectral Sensitivity and Resolution of Photographic M a t e rial. The main parameters characterizing photographic emulsions and their applicability for high speed recording are the sensitivity and the spatial resolution power. The sensitivity (ASA or DIN) gives a measure for the blackening as a function of the amount of illumination (exposure). Each photographic plate has its characteristic curve showing the optical density as a function of the exposure. The light levels involved should be adapted to work in the linear range of this curve. The spectral sensitivi73 A . S. Dubovik, “Photographic Recording of High-speed Processes.” Pergamon, Oxford, 1968. “ J . S . Courtney-Pratt, Phofogr. J . , Sect. B 92, 137 (1952). 75 K . Vollrath and G . Thomer, eds., “Kurzzeitphysik,” p. 76. Springer-Verlag. Berlin and New York, 1967.

8.2.

RECORDING METHODS

727

ties are mainly determined by sensitizations, that means by the addition of small amounts of dyes, so that various distributions can be obtained in different ranges in the uv, visible, and ir part of the spectrum. The material can thus be adapted to the special type of thermal or laser light source used for the investigations. The spatial resolution is limited by the graininess. Improved image quality necessitates fine grain materials in which, however, sensitivity is decreased. In most conventional procedures including laser photography, the spatial resolution is not limited by the photographic material (which is normally of the order of some hundreds of lines per millimeter), but by the optical system itself. In contrast, holographic recording requires higher resolution up to several thousands of lines per millimeter. For this purpose, special materials have been developed such as the Kodak 649 F or the Agfa 10E70 or 10E75. The last type is mostly used for pulsed ruby laser holographic techniques, whereby about 2800 lines/mm can be resolved, and the energy necessary for the exposure is of the order of 50 erg/cm2. 8.2.2.2. Single Exposure Techniques

8.2.2.2.1. APPLICATION OF SHORT DURATION LIGHTPULSES.Singleexposure techniques can be performed by using rather simple cameras and short duration light pulses that can be provided by either type of thermal or laser light source discussed in the previous sections. Pulses are ranging from several picoseconds up to several milliseconds. The pulse duration required is, thereby, only limited by the maximum admissible blur of the recorded image. The relatively inexpensive thermal light sources can be used for the investigation of many fluid dynamic problems. Lasers are more suitable in the field of strongly self-luminous effects such as flames, deflagrations, detonations, or plasmas. The single-exposure technique can be applied to the investigation of objects in the reflected light or, provided the objects are partially transparent, in the transmitted light. In the latter case, the known optical methods such as shadowgraphy , interferometry, or schlieren techniques yield valuable informations. Three-dimensional information can even be obtained by using coherent pulsed radiation sources for holographic techniques such as single-exposure holography or double-exposure holographic interferometry.76 8.2.2.2.2. H I G HSPEEDSHUTTERS. Most optical shutters are based on the linear or quadratic electro-optic or magneto-optic effect. The propagation of a light wave is then influenced by electrically o r magnetically in‘s J .

1971.

C . Vienot, P. Srnigielski, and H. Royer, “Holographie Optique.” Dunod, Paris,

728

8.

LIGHT SOURCES A N D RECORDING METHODS

duced birefringence which is acting on the phase velocities of different linearly or circularly polarized wave components. In image converter type shutters, photoelectrons are set free on photosensitive cathode materials due to the Hallwachs effect. Gating can be achieved by applying suitable high voltage pulses between the cathode and the fluorescence screen. 8.2.2.2.2.1. PolNrizution-Dependent High Speed Shutters. Propagation characteristics of light in nonisotropic media are properly described by the index-ellipsoid representation:” (8.2.1)

Suis the impermeability (inverse permeability fractive indey p by

E)

which is related to the re(8.2.2)

By choosing the Cartesian coordinates xI ,x2,and x3 such that they correspond to the main axis of the ellipsoid, the above equation is simplified to read (8.2.3) for biaxial materials and (8.2.4) for uni-axial materials, where po and pe are indices of the ordinary and extraordinary ray. For isotropic materials p holds for all directions. Distortions of the index ellipsoid can be induced electrically, optically, magnetically, and mechanically. The last case of electro-acoustic effects shall not be considered, however. The distortions can be taken into account by replacing Suby (Sij + ASij). Figure 14 shows the different types of shutters, based on the polarization-dependent velocities of propagation to be discussed in the following sections. Pockels Cell Shutters. In the linear electro-optic effect which is also called “Pockels effect,” the distortion A( l/p2)uis proportional to the electric field E . This dependence can be expressed by a third rank r tensor ” S . H. Wemple, Electro-optic materials. I n “Laser Handbook” (F. T. Arecchi and E. 0. Schulz-Dubois, eds.), Vol. 1, p. 977. North-Holland Publ., Amsterdam, 1972.

8.2.

729

RECORDING METHODS

&

mode-locked laser

(C)

~

carner

beam ident

ps -gated optical E - fieldstrength

(d)

FIG. 14. Schematic representation of polarization-dependent high speed shutters. (a) Pockets cell (longitudinal effect). (b) Kerr cell. (c) Optically gated Kerr cell. (d) Faraday shutter.

A

e;v+

(8.2.5)

In the same way, afu,k tensor can be used to relate A ( ~ / P ~to) the ~ polarization P k . In the reduced index notation, the following abbreviations are used for ij: 1 1 = 1 , 22 = 2, 33 = 3, 23 = 32 = 4, 13 = 31 = 5, 12 = 21 = 6. Depending on the symmetry conditions of the materials, only some nonvanishing matrix elements have to be retained. The field induced birefringence can then be calculated from the ellipsoid distortions AP =

&I

- PI.

The indices “parallel and perpendicular” refer to the polarization of the wave with respect to the electric field direction. The exact notations are depending on the special type of material used and the direction of propagation and polarization of the beams. For shutter applications, the material has to be placed between two polarizers orientated perpendicular to

730

8.

LIGHT SOURCES A N D RECORDING METHODS

one another. Both longitudinal and transverse electric field configurations can be applied. The phase retardation 6 of the two polarization components of the incident beam after the traversal of the medium of length 1 is written as (8.2.6) Let us consider briefly KDP crystals which are well known from laser Q-switching techniques. They are optically uniaxial, and are belonging to the tetragonal symmetry class. If E , for example, is orientated along the crystal axis x3, E = E 3 , the induced index distortion yields7* A

-

-

r12,3

E3 = rwE3,

so (8.2.7) and

po is the refractive index of the ordinary ray, r, the relevant electro-

optical coefficient. The above relations can be used to derive the halfwave voltage U,,,: (8.2.8) which is necessary for a 90-degree rotation of the plane of polarization. Besides the longitudinal effect, transverse effects can also be applied. Kerr Cell Shutters. The induced optical birefringence, in the case of the quadratic electro-optic effect (Kerr effect) proves to be proportional to Ez. The relations are thus more complex (8.2.9) A similar equation can be written relating A( l/pz)uto the polarizations P k and P l , thus defining the polarization optic coefficients Gi,,kl. For simplicity, the reduced notations are preferred again.

’’ R . T. Denton, Modulation techniques. I n “Laser Handbook” (F.T. Arecchi and E. 0. Schulz-Dubois, eds.), Vol. 1, p. 703. North-Holland Pub]., Amsterdam, 1972.

8.2.

73 I

RECORDING METHODS

For the construction of high speed shutters, liquids such as CS, or nitrobenzene are often used. The phase retardation for the two polarization components parallel and perpendicular to the applied electric field which is transverse to the direction of beam propagation, is then given by 6 = 2rBlE2. B is the Kerr constant which is dependent on wavelength and temperature. A halfwave voltage can be defined as in the case of the linear electro-optic effect by Uh,, = ~ / ( 2 B l where ) ~ / ~ a is the width of the Kerr cell ( E = U / a ) and U the applied ~ o l t a g e . ’ ~ As the electrically induced birefringence has extremely short relaxation times, those shutters can even be operated with hf electric fields up to the optical frequency ranges. The application of mode-locked lasers, therefore, yields the possibility of generating ps shutters. This can be achieved by focusing the laser radiation into a cell of CS, or nitrobenzene, respectively, which is placed between two crossed polarizers. By this method, Duguay et al.*O first visualized the spatial shape of ps light bundles. The output pulses of a mode-locked Nd-glass laser, thereby, passed through a nonlinear optical crystal thus generating the second harmonic. This part, the green pulse, was split using a wavelength-selective mirror which, after being optically delayed, traversed a cell of milky water placed in front of the camera Kerr cell shutter configuration. The Kerr cell was gated by the remaining part of the infrared pulse so that the extension of the green pulse can be photographed due to its own straylight. Faraday Shutters. In the magneto-optic effect, the plane of polarization of light is rotated by a certain amount when it is passed through magneto-optic materials such as different types of glasses with a magnetic field orientation parallel to the direction of propagation. The rnagnetization causes a change of the refractive indices (+)* = E k 6 of the two circularly polarized components of the light. We describe the two components by the following expressions:

(8.2.10) a plane polarized incident light beam will be rotated by an angle 4=(p+-p-)-

7T

A

.i=e.i,

4 (8.2.11)

78 W . Miiller, Elektrooptische Verschlusse. In “Kurzzeitphysik” (K.Vollrath and G. Thomer, eds.), p. 207. Springer-Verlag. Berlin and New York, 1967. aa M. A . Duguay, J. W. Hansen, and S. L. Shapiro, IEEE J . Quantum Electron. 6, 725 (1970).

732

8.

LIGHT SOURCES A N D RECORDING METHODS

where 8 is the rotation angle per centimeter length. In some para- or diamagnetic materials, large values of 8 can be observed. 0 is thereby proportional to the applied magnetic field H , where 8 = V . H . V is the Verdet constant which is mostly given in the literature in (deg/(cm Oe)). Because of the high currents which are to be switched to obtain the required magnetic fields, Faraday shutters are operating at lower speeds than do electro-optic shutters. A flint glass, for example, of 2 cm in length with a diameter of 1 cm necessitates magnetic fields of 45,000 Oe ( V = 0.001 degs/(Oe cm)) to obtain a halfwave retardation corresponding to a rotation of the plane of polarization of 900.79 8.2.2.2.2.2. Image-Converter-Type oj' Shutters. Image converter techniques advanced rapidly during the last few years providing electronically controlled shutters which allow to stop motion at precise times with exposures in the nano- or microsecond range. The operation principle involves a photosensitive cathode on which the image is formed by means of conventional optics.*' The emitted photoelectrons are accelerated in the evacuated chamber by using suitable electric fields. The anode utilizes fast response cathode-ray tube-phosphors. The images on this screen are formed by electric, magnetic, electromagnetic, or proximity focusing techniques and can be photographed directly by means of a second lens system. Shutter times are determined by the high voltage pulses applied to the anode or to an extraction mesh grid. The proximity focus biplanar tubes are especially useful because they are virtually distortion free (with the exception of the peripheral region around the edges), and because they allow for obtaining high spatial and temporal resolution. The strong electric fields are generated by a high voltage, applied to the closely spaced parallel electrodes. In commercially available systems, images can be switched on or off with exposure times of 5 ns. Due to the achievable high radiant gain across the image tubes (50- lo4),considerably lower light levels can be tolerated than with Kerr cell shutters. The main features of the long focus tubes including deflection electrodes with their ps capabilities will be discussed in the following sections concerned with cinematographic techniques. 8.2.2.3. Cinematographic Methods. In Fig. 13, a classification of cinematographic techniques has been chosen, following the different methods of high speed image separation. The optical information can, thereby, be obtained in the way, that all the individual elements contribute to the photographic recording or that only a smaller number of raster J . S . Courtney-Pratt, Research (London) 2, 287 (1949).

8.2.

RECORDING METHODS

733

points or lines are to be considered. In the extreme case only one single line will be extracted. The first method is utilized in most types of cameras, in the mechanically driven cameras with intermittant o r optically compensated continuous film transport, in electronic image-converter cameras and in multiple spark cameras using optical image separation. The second way, to split up the information into a large but finite number of raster-elements is utilized in image dissection cameras. The extraction of a single line finally leads to streak records, where the only part of the image of an object which is transmitted through a small aperture slit is temporally smeared by the relative motion of the slit image with respect to the film. This can be obtained mechanically by moving the film (drum cameras), or by mirror scanning o r slit scanning (e.g., rotating mirror cameras), or electronically in image-converter cameras by applying suitable deflection voltages inside the tubes. The streak techniques proved to be especially useful, if fast unidimensional motions of waves in fluids, luminous fronts in plasmas, flames, or detonations have t o be investigated. 8.2.2.3.1. MECHANICAL C A M E R A S . As already mentioned, an extensive description of the large variety of experimental techniques and apparatus has been given by D ~ b o v i k , 'so ~ that only the most important features of these cameras shall be pointed out. Techniques using intermittant film transport are restricted to relatively low repetition frequencies of less than 600 pps. Considerably higher image repetition rates are obtained with continuously moving film. Drum cameras for example, with externally mounted film can be operated up to rotation frequencies of 50-70 rev/s, whereby velocities up to 100 m/s are obtained. This value corresponds to the limit, given by the maximum admissible centrifugal forces that film materials are able to withstand. Higher velocities of about 200 m/s can be achieved, however, if the film is mounted internally. Drum cameras are often operated in the streak mode in the case of self-luminous phenomena or they are combined with periodically emitting light sources such as stroboscopes. The temporal resolution can be increased, if the relative motion of the image with respect to the continuously transported film is compensated, for example optically by means of an additional rotating cube of glass through which the image is transmitted. With rotating mirror cameras, even higher resolutions can be achieved.6Z Figure 15 shows schematically the operation principles of rotating mirror cameras both in the streak and in the framing mode. In the streak mode, an image of the object to be studied is formed in the plane of 82

E. B. Turner, SPIE J . 8, 157 (1970).

734

8.

LIGHT SOURCES A N D RECORDING METHODS

I

lens

lens -

'ob ects

e

FIG.15. Schematic of rotating mirror cameras. (a) Rotating mirror streak camera. (b) Rotating mirror framing camera with optical compensation.

the slit. By means of a relay lens, the final image is subsequently generated in the film plane, after being reflected on the rotating mirror or prism surface. Object movements parallel to the direction of the slit are thus transformed into a component of motion perpendicular to the axis of the slit image. From known scanning speed, the real speed of the object motion can be determined with high accuracy. Streak velocities of 10 mm/ps are currently obtainable with commercially available cameras providing a temporal resolution power of only a few nanoseconds. Transformation of such a camera to the framing mode necessitates the incorporation of some further components. This is indicated in the upper part of Fig. 15. Optical compensation of the mirror scanning is thereby achieved by using a series of aperture stops with an additional array of relay lenses. An intermediate image is formed near the surface of the rotating mirror by means of the field lens. Following the mirror rotation, the light is subsequently transmitted through successive apertures and relay lenses, thereby causing the framing action. The number of discrete images corresponds to the number of relay lenses. Framing rates of several lo6 frames per second are possible, whereby the exposure times of the individual frames are about the half interframe time. Commercially available cameras even allow for simultaneous recording in the streak and in the framing mode. 8.2.2.3.2. IMAGE DISSECTION C A M E R A S . An important technique applied for the construction of high speed cinematographic cameras which is quite different from those discussed in the previous sections uses the principle of image d i s ~ e c t i o n . The ~ ~ main feature is that the images are di-

8.2.

RECORDING METHODS

735

vided into a large number of small elements (typical values are about 620 dots/cm*). As compared to their own diameter, the interspaces between the individual dots on the recording film are relatively large. To obtain a complete separation of two subsequent images, each element has, therefore, to be displaced only by a small amount so that extremely high recording rates can be achieved.84 Different systems have been developed, applying, e.g., simple dissecting plates with a moving film o r by combining dissection plates with rotating mirror cameras. Higher performance of operation has been obtained using lenticular plates to dissect the image and aperture or mirror scanning for sequential recording. Thereby, an image of the object to be studied is formed in front of the lenticular plate which provides the dissection of the original image, as each one of the small lenslets sees only a portion of the whole image. The addition of a movable aperture, for example a rotating disk (Nipkow disk using a spiral row of aperture holes) near the objective lens, allows scanning of different raster elements proportional to the displacement of the aperture. The different pictures thus obtained can finally be restored after processing the film by uniformly illuminating the photographic plate in a holder with the same or a similar movable aperture and lenticular-plate array. Cameras of this type have been designed allowing 3000 pictures to be exposed with rates of lo6 p p ~ More . ~ ~ elaborate systems even include fiber optics or combine image dissection with deflecting image converter tubes. In USSR, cameras with maximum repetition rates up to lo9 pps have been realized.86 8.2.2.3.3. IMAGE CONVERTER A N D I N T E N S I F I E R S . Deflection image converters combined with intensifiers have proved to be one of the most useful tools in high speed cinematography with subnanosecond and even picosecond r e s ~ l u t i o n . ~ In ’ ~ ~present-day ~ cameras of this type, long focus tubes are applying electrostatic or magnetic focusing techniques, whereby the electrostatically focused tubes are mainly used. Diodes, triodes, and even tubes with larger numbers of electrodes have been developed. These tubes provide flexibility, high gain, and deflection capability. Mesh extraction grids which are located only a few millimeters be83 H . Bender, Die Rasterverfahren der Hochfrequenzphotographie. In “Kurzzeitphysik” ( K . Vollrath and G. Thomer, eds.), p. 301. Springer-Verlag, Berlin and New York, 1967 J . S. Courtney-Pratt, J . SMPTE 82, 167 (1973). M. P. Battaglia, SPIE 8, 175 (1970). A. S. Dubovik and N . M. Sitsinskaya, J . S M P T E 80,691 (1971). n7 D. J. Bradley, P r o r . Int. C o n g r . , High-Sprrd Photog., 11th. London, 1974, p. 2 3 . Chapman and Hall, London, 1975. 88 R. Hadland, Lecture presented at the Technical Seminar of the British Electro-Optics and Laser Equipment Exhibition, Tokyo, December, 1975.

8.

736

LIGHT SOURCES A N D RECORDING METHODS

hind the cathode are often incorporated to accelerate the emitted photoelectrons. As the high voltages applied are of the order of 5-20 kV, the initial energies become insignificant. Variations due to the different velocities of the electrons thus do not cause a serious spread in the arrival times on the anode, typical values of which are smaller than 2-4 ps. A schematic diagram of an electrostatically focused tube is shown in Fig. 16 where the electrons traverse the acceleration or gating grid, respectively, the focusing cone and further acceleration and deflection plates. These tubes can be operated both in the framing and in the streak mode. Special sweep circuits have been developed that can achieve final sweep rates on the photoanode of 75 mm/ns. A typical streak recordsg (Fig. 16) reveals the temporal evolution of a laser-supported detonation wave produced by the impact of a 10-15-5 COz laser pulse on an aluminium target surface. The sweep velocity, thereby, achieves a value of 0.25 mm/ns. To obtain higher streak rates or shorter exposure times with a bright enough image on the screen, the brightness has to be in-

photocathode

objective lens X

I

100

focusing cone

I

gdting

grid

-

200

300

-t

AL-target

acceleration electrodes

def l i c t i o n plates

photoanode

/

film plane

evolution o f lasersupported detonation waves

Ins]

measurements o f picosecond mode Locked laser pulses (scanning speed 63 ps/mm)

--I

1.2 ns

L-

FIG. 16. Long focus tube image converter camera. Application of the camera in the streak mode for the recording of fast luminous events. 8D M . Hugenschmidt and K . Voh'dlh, Proc. Int. Congr. High-Speed Photog. 12th, Toronto, 1976. p. 427. SPIE, Washington, 1977.

8.2.

RECORDING METHODS

737

creased by using further intensifier stages. This will be necessary because the beam current must be kept low to avoid image distortions due to charge repulsion. As an example for the application of an image converter camera including an intensifier, Fig. 16 contains a streak record of the temporal shape of the output-pulses of a mode-locked Rhodamine 6G dye laser. As the streak rate is known to be 63 mm/ns, the evaluation of the photodensitometer traces of such photographs allows for the exact determination of the real pulse durations The image intensification can be obtained by magnetically focused intensifiers with 3 or 4 stages (for example in the ,Imacon 600 system) as it was used to take the photographs shown in Fig. 16 or by microchannel-plate intensifiers. Conventional intensifiers are optically coupled by a lens system, the transfer efficiencies of which are only a few percent, so that the maximum gain of typically lo6 will be reduced to about 1 to 2 x lo4. The development of multichannel plate image intensifiers potentially yields the best solution to this problem.g1 These components allow for obtaining high gain of the order of lo6 by applying electric voltages of about 1000 V. The plates are, thereby, a few millimeters thick. In Livermore, a camera has been built that incorporates a wafer-type channel plate, which is proximity focused and does not require focus cones. Because of technical problems in manufacturing, focused-type channel plate intensifiers are often used. Such types are incorporated in the Imacon 675 image converter streak cameras where, due to the absence of optical coupling, the full gain of the channel plate intensifier is obtained at the film. As compared to the model 600, this camera is more compact, provides improved time resolution and has an increased total recording time from 0.9 to 1.5 ns. In addition, the signal-to-noise ratio is considerably higher. 8.2.2.3.4. MULTIPLE SPARKCAMERAS.A very simple, most effective and relatively inexpensive method for high-speed recording of phase objects in the field of fluid dynamics excluding mechanically driven components uses the optical separation of subsequent images. This was introduced by Cranz and Scharding2in 1929. The operation principle of a multiple spark camera is schematically represented in Fig. 17. The main components are first a series of n light sources, usually open sparks in air, a field lens which can also be replaced by a large aperture spherical mirror, and an equal number n of small objective lenses which are genA. Hirth, Dissertation, ISL-Ber. 26/74. Dtsch.-Franz. Forschungsinst. St.-Louis, 1974. J . Graf and R. Polaert, Acra Electron. 16, 11 (1973). O2 C. Cranz and H . Schardin, Z . Phys. 56, 147 (1929).

738

8.

LiCHT SOURCES A N D RECORDING METHODS

field lens

Laser-produced shock waves

FIG.17. Schematic of multiple-spark cameras. (a) Electrically triggered spark camera. (b) Optically delayed laser system.

erating n images of the object under investigation on the photographic plate. Due to the characteristics of the field lens, the light output of each source is imaged exactly on the aperture of the corresponding objective lens. Subsequent triggering of the individual sources thus allows for obtaining the different temporal phases of the object, geometrically separated on the photographic plate. Information-theoretical considerations and practical experimental aspects led to the development of different types of cameras with 8, 24, or 36 frames. Mostly used is the 24 multiple-spark camera as realized at the ISL.93 The repetition frequencies are, thereby, only limited by the duration of the light pulses. Low inductance spark circuits yielding exposure times of only a few tens of nanoseconds allowed for the realization of framing rates up to 10 MHz. Reference is made to the 1iteratu1-e'~for the large number of modifications including the application of prisms, fiber optics or even combinations with mechanically driven systems. Figure 83

A. Stenzel, ISL-Tech. Mitt. T 26/70. Dtsch.-Franz. Forschungsinst. St.-Louis, 1970.

8.2.

RECORDING METHODS

739

17 further shows a few shadowgraph-records (chosen from a series of 24 pictures) revealing the growth of a COz laser produced shock wave propagated from a plexiglass target surface into the surrounding air. The application of coherent radiation sources to cameras or systems of this type proved to be most vertisile for the investigation of transient fast self-luminous phenomena. The schematic arrangement of such a system which was used for the investigation of laser produced plasmas is also shown in Fig. 17. Starting from a single short duration laser pulse (a few nano- or picoseconds), a series of temporally delayed illuminating pulses is obtained by means of an optical delay line. This includes a row of mirrors, the reflectivities of which are chosen to yield equal intensities of the different reflected parts of the original beam. An interference-filter has to be inserted to suppress self-luminosity of the object. Figure 17 shows four frames of a laser induced gas breakdown in Thereby, a framing rate of 60 MHz has been applied. Higher rates can be achieved, however, up to the GHz-range by using mode-locked lasers. Valuable information can even be obtained in a much simpler way by directly photographing the object with a periodically pulsed source such as by a mode-locked train of ps pulses, with an open camera. This will be possible, if the object movements are so fast, that subsequent exposures do not overlapg5as in conventional low speed stroboscopic systems. 8.2.3. lnterferometric Methods

In the field of fluid dynamics, interferometric methods provide a large number of quantitative information on refractive-index changes or optical path differences introduced by phase objects, respectively. Thermal light sources can be applied as well as lasers. The detection of the interference patterns can be performed either by using photographic or photoelectric recording techniques. Photographic recordings yield the spatial distribution at a fixed time, photoelectrical registrations provide high temporal resolution capabilities along a given optical path. Both techniques are able to give the whole information, however, if optical scanning or combinations with cinematographic methods are used. Then, temporal variations and spatial distributions of refractive-index fields can be determined simultaneously. This is most important for the investigation of transient phenomena. Interference effects are always observed when two or even more light beams, the phases of which are strongly correlated are superimposed. In M. Hugenschmidt, K . Vollrath, and A . Hirth, Appl. Opt. 11, 339 (1972). K . Vollrath and M. Hugenschrnidt, Pro(,. Int. Congr. High-speed P h o t o g . , IZth, Toronto. 1976, p. 407. SPIE, Washington, 1977. 94

740

8.

LIGHT SOURCES AND RECORDING METHODS

the case of two interfering beams, for example, the intensity as a function of the phase difference 6 is described by a cos26 distribution. 6 is related to the refractive index p(r, t ) (which is both depending upon the spacecoordinate r and time t ) by the following equation 6 = (2.rr/h) J &, t ) dl. The integration has to be performed along the direction of the optical path. In the case of a large number of interfering rays, as in a FabryPerot interferometer, the fringes of maximum intensity are becoming considerably narrower than the cos2 6 profiles. This narrowing is depending on the finess F of the Fabry-Perot interferometer which is related with the mirror reflectivities R by the equation F = 4R/(1 - R)2.51 8.2.3.1. Classical lnterferometric Systems. Classical interferometric systems are designed for the use of nonmonochromatic or even white light sources. Thereby, two main groups of interferometers have to be considered: (1) The two-beam interferometer group, represented by the MachZehnder type (Mach-Zehnder, Michelson, etc.) and by the shear type (differential interferometer). (2) The Fabry-Perot types, on which belong to the group of the multiple beam interferometers.

The Mach-Zehnder types have in common that the rays intersecting the phase objects are spatially completely separated from their reference rays. In the shear type, as well as in the Fabry-Perot type, the interfering beams are largely overlapping. In particular, the differential interferometers using two Wollaston prisms have proved their versitility in gas dynamic research.e6 8.2.3.2. Laser Interferometry. Since lasers provide monochromatic and mainly coherent radiation, the adjustment of an interferometric system is greatly facilitated. By the use of small-band interference filters, investigations of self-luminous phenomena (such as flames or plasmas) can be carried out in a straightforward manner without loss of information. Figure 18 shows in the upper part two interferograms of laser-produced, rapidly expanding plasmas one of which was taken with the giant pulse of a ruby laser (7 = 20 ns, P = 5-10 MW), whereas the other picture reveals a spark illuminated exposure. The differences can be seen clearly. In the laser interferogram, the fringe contrast, even for high orders of interference is much greater than in the case of the spark interferogram. Furthermore, the fringe shift can be determined with high accuracy throughout the whole area of the object including the central G. Smeets, ISL-Tech. Mitt. T 21/70. Dtsch.-Franz. Forschungsinst. St.-Louis, 1970.

8.2.

74 1

RECORDING METHODS

spark interferogram

laser interferogram

evaluation of the fringe shift Aa/a along the axis A-B (At = 390 ns)

i

U

a

ne

t tCni-9

fP

+2

1.002

0 -2

-4

2.35ps

-iI

-6

6

i i

[mml

0.9 9 6

‘2.35 ps I

1

v

2 rmmi

1 [mml

FIG.18. Laser interferometry. Quantitative evaluation (electron density of a ruby laser produced Xe plasma).

part which is overexposed by the plasma luminosity in the spark photography. The lower part of the figure shows an example indicating the evaluation procedure. This interferogram was taken with a Wollaston prism interferometer. The laser produced plasmas are assumed to expand in a rotational symmetric way around the axis of the incident laser beam. The measured fringe shift Au, normalized to the distance a

742

8.

LIGHT SOURCES A N D RECORDING METHODS

between the undisturbed fringes V ( x ) = A a / a , is then related to the radial profile of the refractive index by an Abelian integral equation. If i different particle groups are concerned, the refractive index p is related to the physical parameters such as the local densities ni by the equation p - I = C 274X)nt.

(8.2.12)

The polarizabilities of the different groups at(X)are dispersive. Their relative influence has to be estimated for each experimental condition concerned. In the case of atoms or molecules in excited states, &$(A) shows pronounced resonances due to anomalous dispersion. By using dye lasers as interferometric light sources, laser frequencies can be tuned to resonance rendering the other terms of (8.2.12) negligible. Laser interferometric techniques are thus able to provide quantitatively partial densities of specially excited particles. In highly ionized plasmas as shown in Fig. 18, the free electron term considerably exceeds the other terms so that, starting from the measured fringe shift, the procedure allows for the calculation of the electron densities.g7 8.2.3.3. Two-Wavelength Interferometry. By applying light sources emitting simultaneously at two or more wavelengths, the above mentioned dispersive behavior of the refractive index can be used to yield more detailed information. However, it must be proved that the wavelength ranges chosen are not affected by anomalous dispersion. In this case, the refractive index is mainly influenced by the group of electrons n, and by the group of heavy particles (essentially neutral particles a,,). eZhZ n, ; m-%Oc2m,

(8.2.13)

e0 is the dielectric constant of vacuum. Frequency doubling of a dye or ruby laser output provides a simple method to generate at the same time two monochromatic short intense light pulses starting with a single laser. *~~ generation is Figure 19 shows a schematic a r r a t ~ g e m e n t . ~ *Harmonic achieved by means of an optically nonlinear crystal such as KDP or ADP. The separation of the two interferograms can be performed by using a wavelength selective mirror. Suitably chosen interference filters again suppress background illumination. The evaluation of fringe shifts proceeds in the same manner as already described yielding the two refractive index profiles from which the neutral particle and electron densities can be determined. A similar setup can be applied using continuous-wave

@' M. Hugenschrnidt, 2. Angew. Phys. 30, 350 (1971). 9*

ss

M.Hugenscbrnidt and K . Vollrath, Opt. Loser Techno/. 3,93 (1971). A. J . Alcock and S . A . Ramsden, Appl. Phyhys. L e f t . 8, 197 (1966).

8.2. ruby

laser

743

RECORDING METHODS

KDP

crystal interferometer

’

imaging system

L8

FIG. 19. Two-wavelength interferometry. Evaluation of fringe shifts V ( x ) .

lasers, for example two He-Ne lasers, one of which emits in the red (0.6328 pm), and the second in the infrared (e.g., A = 1.15 or 3.39 pm). Accurate time resolution is obtained by the use of high speed photoelectric detectors.loO 8.2.4. Holographic Methods

Wave front reconstruction of images has been discussed by a large number of physicists since about 1920. The first experimental results were obtained in 1949 by Gabor who suggested the name “holography.” This new optical procedure allowed the registration of amplitudes and phases of optical waves,lol which are schematically designed in Fig. 20 by the phase fronts C. The indices 0, Rf, and Rc refer to object, reference, and reconstruction. The method of Gabor can be applied to partially transparent objects. One part of the incident light wave is directly transmitted; the other part is scattered by the object. On the photographic plate these two parts are superimposed thus forming an interference pattern containing the whole information. The reconstruction of such an in-line hologram can be obloo lol

G . Smeets, ISL-Notiz N 608/75. Dtsch.-Franz. Forschungsinst. St.-Louis, 1975. D. Gabor, Electron. & Power 12, 230 (1966).

744

8.

LIGHT SOURCES A N D RECORDING METHODS

Irecordings 7

[-reconstructions-\

,...,',

ti

FIG. 20. Schematic of holographic recording and wavefront reconstruction. P = arbitrary object point, P' = normal image point, P" = conjugate image point. (a) Inline holography. [After G a b ~ r . ~ O ~(b) ] Off-axis holography. [After Leith and U p a t n i e k ~ . ' ~ ~(c) ] Holograms in reflected light. (d) Holograms in transmitted light.

tained by illuminating the developed plate with a parallel beam of monochromatic light. Diffraction effects are responsible for the appearance of the image which is, however, always disturbed in this simple setup by a conjugate image. This difficulty was overcome by Leith and Upatniekslo2 in 1962 by the technique of the off-axis holography which was able to be realized because of the availability of laser light sources. Since then, holography has grown into an expanding field of scientific research and technical application. 8.2.4.1. Basic Principles of Holography. Applying the off-axis technique in the lower part of Fig. 20, the experimental setup is shown both for the registration of objects in the reflected and in the transmitted light, a being the angle between the object beams and reference beams. As indicated, this can be done experimentally by appropriate optical elements such as prisms and mirrors. The wave front reconstruction is simply obtained by illuminating the developed plate as indicated by means of a reconstruction beam. Due to the off-axis condition, the real and virtual images are then geometrically completely separated. Mathematically, the exposure and reconstruction of a hologram can be described by the following simple set of equations. lo*

E. N . Leith and J . Upatnieks, J . Opt. SOC. A m . 52, 1123 (1962).

8.2.

745

RECORDING METHODS

8.2.4.1.1. EXPOSURE OF THE HOLOGRAM. The complex light amplitudes Vo scattered from an object are superimposed upon a reference wave described by the complex amplitude VRf. For simplicity, Vo is represented by a spherical wave emanating from an arbitrary object point P , whereas V,, shall be represented by a plane wave. The amplitudes incident on a point ( 6 , q) of the holographic plate are then V(t) = Vo(t) + VRXr), and the resulting intensity is I(?) = W O + VRf)(V,* + V&).

(8.2.14)

After an exposure time 7E, the optical density D of the plate in the considered point will be proportional to the energy E = I ( t ) T ~ Substituting . the above relations yields the expression E = { ( V O+ ( ~ lv~rl'+

vov&4-

VzVRf}TE.

(8.2.15)

The transmission T, the ratio of transmitted to incident intensity, is related to the optical density by the equation D = log 1/T.

(8.2.16)

In the linear range of the characteristic curve D versus log E , the transmission will be proportional to the energy, thus yielding T=

T - P ( E - 0.

(8.2.17)

Tand E are mean values of the transmission and energy, respectively. p yields the slope of the curve. Assuming that E is mainly determined by the intensity of the reference beam, E can be approximated by E = IV,f(27, so that the following equation holds: T=

T - /~TE{IVO~~ + VoV& + ViVRf}.

(8.2.18)

Thus T is dependent on a term proportional to V,, and on a second term proportional to the complex conjugate Vg , which both contain information on the amplitude and the phase relations of the object wave. 8.2.4.1.2. RECONSTRUCTION OF THE WAVE FRONT.The process of wavefront reconstruction can be described in a similar way by multiplying the transmission T with the complex amplitude of a reconstruction wave V, which (with respect to the wavelength or the angle of incidence) must not be identical with the original reference wave. For any point (6, q) one obtains Vm . T = VRc

*

T-

P ~ E V ~ ( l v 0 1+' VoV&

+ V,hV,f}.

(8.2.19)

The first term describes a mean attenuation. The second term indicates a further attenuation of the reconstruction wave by diffraction due to 1 VOl2. Both terms are thus concerned with the directly transmitted part of the

746

8.

LIGHT SOURCES A N D RECORDING METHODS

wave VRc. The amplitude and phase information concerning the object wave is contained in the third term directly and in the forth term with inverted polarity of the phases. The spherical wave approximation can easily be extended to describe more complicated objects by summing or integrating the contributions of all object points. The same mathematical formalism can be applied to calculate the geometrical location of the normal and conjugate image points and the magnification. The treatment of these questions, the discussion of orthoscopic or pseudoscopic images, the distinction between Fresnel, Fraunhofer, and Fourier holograms, amplitude and phase holograms, numerically computed holograms, Bragg-Lippmann holography, to mention only a few topics, is beyond the scope of this presentation. Reference is made to the l i t e r a t ~ r e . ~ ~ ~ - * ~ ~ 8.2.4.1.3. APPLICATION OF HOLOGRAPHIC TECHNIQUES. Holographic techniques are well suited for investigations in fluid dynamics.lo6 Simple experimental arrangements can be used if lasers are available, the coherence lengths of which are larger than the maximum optical path differences between the object and reference beam. Ruby lasers have mostly been used for studying transient phenomena (more recently dye lasers have been applied as well). As holograms store the amplitudes and phases, the objects can be reconstructed without loss of their threedimensional character. The images can be observed and evaluated in different planes. Furthermore, holograms can be evaluated following different optical procedures; see Fig. 21. The reconstruction of the wave field of an aerodynamic flow or of a plasma, for example, can be visualized by means of shadowgraph or schlieren techniques, depending on the absence or presence of an edge.lo7 By using a Wollaston prism, the same holographic plate can yield an interferogram which can easily be subject to quantitative evaluation. 8.2.4.2. Holographic Interferometry. As already mentioned, the holographic information can be measured by using classical interferometers. It is most important, however, that due to the storing capabilities of photographic plates, different wave fronts be registrated even in the event Io3 H. Kiemle and D. Ross, “Einfiihrung in die Technik der Holographie.” Akad. Verlagsges., Frankfurt, 1969. lo‘ J. C. Vienot, Holography. In “Laser Handbook” (F. T. Arecchi and E. 0. Schulz-Dubois, eds.), Vol. 2, p. 1487. North-Holland h b l . , Amsterdam, 1972. M. FranCon, “Holographie.” Masson, Paris, 1969. lO8 E. R. Robertson, “The Engineering Uses of Coherent Optics.” Cambridge Univ. Press, London and New York, 1976. lo’ A. Hirth, C . R . Hebd; Sronces Acod. S c i . , S r r . B 268, 961 (1969).

8.2.

RECORDING METHODS

747

(a)

reference beam

shadowgraph

schlicre n picture

interferogram FIG. 21. (a) Recording of phase objects. (b) Reconstructions of holograms following different optical procedures.

that they expose the hologram at two separate times.108 The two superimposed holograms then can be considered stored independently from one another in the same emulsion. Upon reconstruction, both waves are restituted giving rise to interference effects. In this way, small changes of the optical path length due to variations of the refractive index or due to macroscopic displacements of an object can be measured with high accuracy. Optical path changes of A are just cause the fringes to be shifted by the amount of the undisturbed fringe spacing.'08 A major advantage of the holographic interferometric technique is that accuracy is not affected by using inexpensive schlieren grade windows, mirrors, lenses, or prisms in the optical setup, because these distortions do not change in the time interval between the two exposures so that they are canceled. By varying the time delay between the two exposures, direct temporal variation of the optical path lengths can be obtained. Figure 22 shows some double exposure holographic interferograms of the

lo@

F. Albe, 1SL-Ber. R 114/76. Dtsch.-Franz. Forschungsinst. St.-Louis, 1976. P. Smigielski and A . Hirth, ISL-Ber. 11/71. Dtsch.-Franz. Forschungsinst. St.-Louis,

1971.

748

8.

L I G H T SOURCES A N D RECORDING M E T H O D S

TEA-CO1 laser

ruby laser

cell

cell

etalon

\ diffusor /

% / ; y ; s t

hologram

r uct ion

FIG.22. Investigations of the temporal variation and spatial distribution of refractive index fields by holographic interferometry.

electrical discharge of a pin-type TEA CO, laser, where the infrared laser mirrors have been replaced by simple glass plates. The object beam is transmitted end on through the plasma tube, whereas the reference beam is split off by means of a prism."' The light pulse of the monomode ruby laser is synchronized with respect to the TEA discharge by a Pockels cell inside the laser cavity which is driven by an adjustable delay time. The holographic plates are first exposed without discharge plasma. A few minutes later, the discharge is initiated to obtain the second exposure at the desired moment. The left row of Fig. 22 shows some recordings of the temporal development of the plasma. The lower row demonstrates the capability of spatial scanning the complex locally varying fields of refractive indices. These reconstructions reveal the fringe distributions at a fixed time. To obtain this spatial information, a diffusor sheet has to be placed near the entrance window of the CO, laser discharge tube. The holographic technique proves to be the only method with which such irregular spatial density profiles lacking symmetry can be analyzed. 'Io M. Hugenschmidt and K . Vollrath, ISL-Ber. 21/71. Dtsch-Franz. Forschungsinst. St.-Louis, 1971.

8.2.

RECORDING METHODS

749

Further double-exposure techniques can be applied in a modified form by using lasers emitting two successive pulses. These can be separated in time by some tens of microseconds to a few nanoseconds, so that changes of rapidly varying events which are introduced in short time intervals can be tested dynamically."' Periodically vibrating processes are investigated by real time holography. Thereby, the reconstruction of the hologram is arranged in such a way that the image of the object is superimposed upon the real object which is also illuminated by the reconstruction beam. Distortions due to mechanical stresses or vibrations thus give rise to interference effects which then can directly be seen or photographed. Material testing of periodically vibrating objects can also be carried out by a time averaging analysis, where the object is exposed for time intervals longer than the vibration period. In this case, high fringe contrast is only obtained for the nodes of the vibration, whereas in other parts of the vibrating surface the contrast is eliminated 8.2.4.3. Two-Wavelength Holography. The interpretation and evaluation of holographic interferograms proceed in the same way as in the case of classical interferometry. The refractive index again has to be related to the particle densities involved for objects so that the application of multiple wavelength coherent sources also will be able to provide more information. Experimentally, the pulse of the previously described monomode ruby laser transmitted through a KDP crystal which is properly aligned to hold for the phase-matching conditions is split up to provide the object beam and the reference beam, each of which contains both wavelengths. The sensitivity of photographic plates (Kodak 649 F) proves to be adapted to the whole wavelength range so that the double exposure technique can directly be applied. Thereby, the photographic plate stores four different holograms. Upon reconstructing the two interferograms, the red and the ultraviolet can be separated if certain geometrical conditions concerning the overall dimensions of the object and the angles between object beam and reference beam are met. This is shown schematically in Fig. 23. The image separation is due to the fact that different Bragg conditions are valid for the diffraction of the reconstruction which depend upon the wavelength. As expected, the magnification ratio of the two images corresponds to the wavelength ratio.113 A. Felske and A. Happe, in "The Engineering Uses of Coherent Optics" (E. R . Robertson, ed.), p. 595. Cambridge Univ. Press, London and New York, 1976. R . L. Powell and K . A . Stetson, .I. O p t . Soc. A m . 55, 1593 (1965). A . Hirth, C. R . A r a d . Sci., Srr. B 271, 28 (1970).

750

8.

I

rubv __ a .

LlGHT SOURCES A N D RECORDING METHODS

laser -

,reference beam

KDP

*I""'

'

'ogram

object beam

h=691.3 nm

h=347.1 nm

/--

imaging sys tem

FIG.23. Two-wavelength holographic interferometry arrangement.

As an example, Fig. 23 contains two interferograms of an electric spark discharge. 8.2.5. Infrared Imaging

Infrared recording techniques are becoming increasingly important especially in the field of plasmaphysics. The sensitivity in schlieren experiments, the amount of rotation of the direction of polarization in Faraday rotation measurements, and the polarizabilities of the free electrons in interferometric measurements, are revealed to be proportional to h2. The last mentioned fact shall be discussed more in detail. An infrared light source combined with an interferometric technique will thus provide a marked increase in ~ e n s i t i v i t y .As ~ ~ compared to the fundamental waves of a ruby laser, COz lasers emitting at 10.6 p m yield a gain of 250, as compared to their harmonics; this will even imply a gain factor of lo3. Pulsed TEA-COZ lasers represent simple and convenient infrared sources which are emitting short pulses with half-widths in the range of some tens to some hundreds of nanoseconds, and peak powers of the order of several megawatts. Shorter pulses in the nano- and subnanosecond range can be obtained as well by mode locking techniques which require, however, more elaborate laser systems. 8.2.5.1. Infrared Recording Systems. The registration of infrared radiation which can be performed electrically by means of a great number of thermal detectors or quantum detectors covering a large range of spectral sensitivities and detection bandwidths (from slow thermal systems to high speed pulse detection) will not be considered in the present section.

8.2.

75 1

RECORDING METHODS

If photographic recording techniques are required, the main problem will be the conversion of the infrared radiation towards the visible. No infrared photographic materials are available beyond 1.1 to 1.2 pm. Some of the methods used for the visualization are: the Czerny evaporograph, the detection by means of liquid crystals, the evaporation of thin sheets of material (metals or paraffin wax), the Lippmann plates, or the quenching of fluorescence. Some of these methods are indicated in Table IV. Their applicability is dependent upon parameters such as the threshold power or energy density and the spatial resolution limit. Some of these values are also indicated in Table IV.lI4 The highest resolution of 70 lines/mm has been obtained with thin sheets of various materials which are evaporated by the infrared radiat i ~ n , ”whereas ~ the lowest threshold power densities have been achieved with Czernys evaporograph. Liquid crystals are characterized by medium resolution power but relatively low threshold energy densities. They show good performance characteristics: the process of the temperature dependent wavelength selective reflectivity in the case of cholesterinic liquid crystals is reversible, they are easy to handle, and can be adapted to different temperature range^.^^^^^^' TABLE1V. Limiting Values for Different Infrared Recording Techniques” _

_

_

Threshold power

~

~

Energy density

Resolution

( J /ern*)

(lines/mrn)

density (W/crn‘) ~~

~

evaporography

10-e

I

10

liquid erystols

10-I

lo-z

20

metallic sheets

3.10-l

70

paraffin wax

3.10-l

70

10

4

0.06 - 1.8

10

Lipprnann plates quenchlng of fluorescence

“A

power

=

10-I up to 60

10.6 p m .

W. Waidelich, “Kurzzeitphysik,” Vortrag auf Friihjahrstagung, Kiel, 1972. DPGVerhandlungen, Phys. Verlag, Weinheim, 1972. A . Darr, G . Decker, and H . Rohr, Z . Phys. 248, 121 (1971). 118 J . Fergason, A p p l . Opt. 7, 1729 (1968). M . Hugenschmidt and K . Vollrath, C. R . Acud. S c i . , Ser. B 274, 1221 1972).

8.

752

LIGHT SOURCES A N D RECORDING METHODS

Further image converters have been built using different semiconducting materials such as SeCr which provide a temperature and wavelength dependent absorptivity. The optinicon developed by Ulmer*18 uses temperature induced changes of the refractive index of thin liquid film layers. This causes changes in the reflectivities for the additionally incident visible radiation (for example of a He-Ne laser) proportional to the infrared radiation distribution. 8.2.5.2. Applications to Flow and Plasma Diagnostics. An example of quantitative analysis of an infrared interferometric method is shown in Fig. 24. The Michelson interferometer uses two gold coated quartz mirrors and a Ge beam splitter with a 50 percent reflection coating on the front and an antireflection coating on the rear side. The light source con-

.># I

I

camera

.,--

,--0

**-

Michelson interferometer (b)

v(x)=

%

,i ri-4-

+0.2

-0.2-

-0.4-

1

/

-0.6-

1.000

o'~-

1 0.998

- 0.80 1 2[

m

m

I ~ 2 ~[mmj~

~

0~

1~ 2 h m 3

FIG.24. (a) Infrared interferometric recording of spark plasmas. (b) Quantitative evolution of the electron density distribution.

W. Ulrner, Infrared Phys. 11, 221 (1971).

8.2.

RECORDING METHODS

753

sists of a TEA-COz laser emitting pulses of about 3 MW (7 = 200 ns) on the P(20) line at 10.56 pm. The output mirror of the laser is given by the plane surface of a plane-convex Ge lens which together with a mirror in a confocal arrangement acts at the same time as a beam expander. The phase object to be studied (an electric spark discharge), is located in one arm of the interferometer. A further mirror serves to form an image on a suitable infrared converter. The sensitivity of the liquid crystal detector (a mixture of cholesterol-oleyl-carbonate and -nonanoate, 75 : 15) can be chosen in such a way that small temperature variations of a few tenths of degrees are spectrally changing the reflected part of a white light illuminating source (flash lamp) from the red to the blue. The color distribution thus directly indicates the ir radiation pattern, which can then be photographed by means of an ordinary camera and electronic flash equipment. The numerical evaluation and calculation of the refractive index and electron density is ~traightforward.'~The results are also given in Fig. 24. It should be pointed out that an increasing number of laser lines in the ir are becoming available by molecular gas lasers, optically pumped lasers, and dye-laser-pumped Raman lasers. In most cases their power is still relatively low. For the investigations of transient phenomena, more powerful systems would be preferable. It seems possible, however, that such systems can be designed in the near future, e.g., by the transversely excited pulsed HCN lasers which emit in the far ir at 337 pm.119

B. Adam, H. J. Schotzau, and F. K . Kneubiihl, f h y s . Left. A 45, 365 (1973)

lln