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A saturable absorber for the iodine laser
Volume 18, number 4
OPTICS COMMUNICATIONS
September 1976
A SATURABLE ABSORBER FOR THE IODINE LASER
E. FILL and K. HOHLA Max-Planck-Gesellschaft zur F~Jrderung der Wissenschaften e. V., Pro]ektgruppe fffr Laserforschung, D-8046 Garching, Fed. Rep. Germany
Received 12 May 1976
An absorber for iodine laser pulses has been developed using iodine atoms in the ground state as the absorbing species. The iodine atoms have been produced by dissociation of gaseous molecular iodine in a heated cell. The absorption is easily saturated at moderate energy densities.
1. Introduction
The use of a high-gain medium such as iodine in high power laser systems offers the advantage of a reduced number of amplifier stages and a high energy extraction efficiency [1 ]. On the other hand, the large difference between small signal and saturated gains resuits in reduced contrast between prepulses, amplified superfluorescence and the main pulse. In addition, the high gain gives rise to spurious oscillations of the whole amplifier chain. Both problems can be overcome by the use of a saturable absorber (s.a.) with high enough extinction ratio, i.e. the small signal transmission should be as small as possible whereas transmission of the main pulse should be close to 100%. A number of saturable absorbers exist for the CO 2 neodymium and dye lasers. However, at 1.315/a, the wavelength of the iodine laser, only one single dye (of undisclosed composition) with a saturable absorption has been reported [2]. The absorption of gaseous molecular absorbers starts at considerably longer wavelengths. In this paper we describe a s.a. which uses the lasing transition in absorption as the saturable transition. Thus, iodine atoms in the ground state 2P3/2 are the absorbing species. Three different methods have been investigated as a source for iodine atoms in the ground state: 1. Photolysis of C 3 F7I (as in a laser) and subsequent
quenching of the excited iodine atoms by means of 0 2 added to the iodide. 2. Photolysis of HI. 3. Thermal dissociation of I2-molecules in a hot cell [3]. With all three methods saturable absorption of iodine laser pulses could be demonstrated. The most reproducible results were obtained with the third of the above methods. This method additionally offers the advantages of a relatively simple technology and cw operation. The results reported in this paper are all obtained with the hot iodine dissociation cell.
2. Theory Starting from rate equations one obtains for the energy transmission of a s.a. depending on input energy density [4]
est E (e )l
T =eOln
1+
xp
-1
exp(-oNl)
/
,
(1)
where e 0 = input energy density, e s = h v / o b = saturation energy density,
o = cross-section for absorption at line centre, N = difference of populations/cm 3 of lower and upper level, 431
Volume 18, number 4
OPTICS COMMUNICATIONS
= length of the absorber, = 1 + glower/guppe r = degeneracy factor; for the transition involved b = 3, if complete relaxation of the hyperfine sublevels within the upper and lower level is assumed. This formula is valid if the relaxation of the upper level is slow compared to the pulswidth. Also, the spectrum of the pulse must be much narrower than the width of the absorbing transition, i.e. formula (1) applies at line centre. Two limiting cases are comprised in (1): a) Small signal behaviour: TO = exp ( - a N l )
if e 0 "~ e s.
(2)
ife 0>>e s.
(3)
b) Behaviour at complete saturation: hvNl be 0
Ts=l
Thus, for a small enough signal Beer's law is obeyed, whereas for a signal with high energy density the absorber simply substracts a number of photons corresponding to its population difference from the number of incoming photons. To derive a condition relating absorber and amplifier parameters in a laser amplifier system we consider a configuration consisting of an amplifier with gain g = OvNvl v followed by an absorber with absorption oL = OANAI A . Then the condition for low gain in the small signal region (relevant for noise and prepulses) is ovNvl v ~. OA N A I A .
(4)
For a strong signal (the main pulse) the energy extracted from the amplifier should be much larger than the energy taken out by the absorber. It follows that
September 1976
sary condition for parameters of a s.a. combined with an amplifier Ovbv/A v ~ OAbA[A A.
(6)
To fulfill this condition two modes of operation of a s.a. are conceivable: 1. The beam diameters in the amplifier and absorber are equal, but the absorption cross-section of the absorber is much larger than the cross-section for stimulated emission of the amplifier. 2. The cross-sections for stimulated interaction are equal, but the beam diameter in the absorber is reduced by passing the beam through a telescope. Mode 1 can be accomplished with an iodine saturable absorber by reducing the pressure in the absorption cell to narrow the pressure broadened line. In this case, however, the performance of the device as an optical isolator is deteriorated, because the line of the absorber does not cover the entire linewidth of the amplifier. This problem is eliminated in mode 2, but one has the inconvenience of a more complicated beam adjustment. In our experiments a combination of the two methods has been applied. The number density of absorbing atoms, i.e. the number of iodine atoms in the ground state per cm 3 can be calculated as follows: The partial pressures of molecular iodine Pl and atomic iodine PI in thermodynamic equihbnum are related by the equation p 2 / p l : = rp(T), where Kp(T) is the equilibrium constant for dissociation [5], which is strongly dependent on temperature. Relating pressures and number densities by the equation of state for an ideal gas one obtains for the number density N of dissociated iodine atoms U = N~
|/Kp (T)m
(7)
V 254RTV' NvlvAv/b v >~ N A IAA A/b A, Av Aa bv ba
= = = =
(5)
area of the beam in the amplifier, area of the beam in the absorber, degeneracy factor of the amplifier, degeneracy factor of the absorber.
Eq. (4) and inequality (5) together give as a neces432
N~ m R V
= = = =
Avogadro's number, mass of iodine contained in the cell, ideal gas constant, volume of the cell.
This equation is valid, if the fraction of dissociated molecules is small compared to the total number of
Volume 18, number 4
OPTICS COMMUNICATIONS
molecules, a condition fulffiled at the temperatures used in our experiments. Also, at these temperatures (< 1200 K) the number of iodine atoms thermally excited to the upper laser level is negligible. Therefore the number density of absorbing iodine atoms is equal to the population difference and is given by eq. (7). From eqs. (2) and (7) the absorption at line centre can be calculated using the well known formula for the cross-section for absorption at the centre of a lorentzian line cr = X2A/47r2 Av,
(8)
where A = first Einstein coefficient = 5.12 for the transition involved, and Av = pressure broadened linewidth (full width at half maximum). It turns out, however, that under our experimental conditions a straightforward application of (2), (7), and (8) to calculate small signal absorption for given cell parameters is not possible. First, the exact value of the pressure broadening coefficient of iodine molecules is not known, so that only an estimate of the small signal absorption at line centre can be given. Second and more important, the nanosecond pulses used in our measurements have a spectrum which is comparable to the linewidth of the absorber. Therefore not the absorption at line centre but a much smaller absorption is measured. An exact calculation of this experimental absorption is only possible by numerical calculations. However, an upper limit for the absorption of a short pulse by an absorber with a lorentzian lineshape can be derived analytically [6]. If the pulse is assumed to be Gaussian in time, then the transmission is T~> exp ( - 2 ~/r~AVA/AVp),
(9)
Av A = full width at half maximum of the lorentzian absorber line, AVp = full 1/e width of the gaussian pulse spectrum, ct --- absorption coefficient at line centre. In our experiments AVA/AVp ~ 1, which means that significant deviations from Beer's law should occur. This has been confirmed experimentally, as will be discussed later. Eq. (9) states that the pulse gets less and less absorbed as it travels through the absorber. This can be understood by realizing that the pulse is reshaped by the action of the absorber on its spectrum, making the absorber less and less effective. Numerical calculations including pulse transmission and pulse shaping are under way [7] and will be published elsewhere. It should be pointed out, that the equation for complete saturation [eq. (3)] remains valid even for nanosecond pulses and can therefore be used to determine the transmission of the absorber for a strong signal.
3. Experimental arrangement A sectional drawing of the iodine saturable absorber is shown in fig. 1. The dissociation vessel is a sealed quartz cell of 2 cm 0 and 42 cm length, terminated with Brewster windows. Heat is applied to the cell by about 2 meters of tantalum ribbon around the cylindrical side walls. 600 watts of electric power are sufficient to reach a temperature of 800°C. The quartz cell is contained in a cylindrical glass vessel which is evacuated to 10 -3 torr for thermal isolation. The inner wall of the glass vessel is lined with aluminum sheet, to avoid loss of heat by radiation. The windows of the outer cell are water cooled. The temperature of the quartz cell can be measured by means of a thermocouple. With this arrangement continuous operation of
heating ribbon
Laser
Beam
/">
September 1976
to pump
/ ,,T, ~1i'0~0000h00].0ei!oc00o0u p011 00~001i~~__~ / 10-3Torr
water cooled windows
Fig. 1. Sectional view of iodine dissociation cell. Overall length is 86 cm. 433
Volume 18, number 4
OPTICS COMMUNICATIONS
the dissociation cell at 900°C is possible. Small signal and saturation measurements were carried out with various amounts of iodine contained in the cell. For the small signal measurements a single switched out pulse from the oscillator of the iodine laser system Asterix II [8] was used as the probe signal. The beam was expanded by putting a distance of about 25 m between the laser and the cell, so that energy densities well below 1 m J / c m 2 were applied as an input signal. The pulses before and behind the cell were passed onto a vacuum diode (Valvo XA 1003) feeding a Tektronix 7704 oscilloscope. The risetime of the detection system was sufficiently long so that energy transmissions were measured. For the saturation measurements the full preamplifier section of Asterix II was used, which delivered a pulse of about 1 joule in slightly less than a nanosecond. Before entering the cell the beam was narrowed by means of a telescope. Energy densities of up to 15 joules/cm 2 could be obtained, limited only by optical damage of the windows. Two calorimeters, one before and one behind the cell, were used to measure the transmission of the pulse.
4. Results and discussion Small signal absorption versus temperature is plotted in fig° 2 for two different iodine contents of the cell. Each data point represents an average of 5 shots. The error bars result from a scatter in the measured absorption from shot to shot. Since the cell was run cw in these experiments with the temperature well
t7
1000
100
t
1 300
400
500
J
t 700
600
i 800
900
I00C
t °C
Fig. 2. Small-signal absorption of nanosecond pulses versus temperature of the cell. 434
September 1976
10 000
I
V////// ///
1000
~oo
I
10
100 As~ -
1000
P
Fig. 3. Double pass absorption versus single pass absorption. Full line represents exponential law (Beer's law), dashed line from a law of form exp (- k,f~). stabilized it is unlikely that the cell parameters changed between shots. A more probable explanation of the scatter is that the shape of the pulse generated by the oscillator was not entirely reproducible, which means that the spectrum of the input pulse to the cell varied from shot to shot. The invalidity of Beer's law for nanosecond pulse was demonstrated by comparing single and double pass absorption of the cell. It turned out, that double pass absorption was much lower than the value obtained by squaring the value of single pass absorption. In fig. 3 double pass absorption is plotted versus single pass absorption on a doubly logarithmic scale. In such a plot Beer's law gives a straight line with slope 2 (drawn full in fig. 3), whereas our measured points lie closer to a line with slope x/~-(drawn dashed). The dashed line is obtained when it is assumed that the equality holds in the law given by (9). The transmission of the iodine cell under saturating conditions is shown in figs. 4 and 5. In fig. 4 transmission is plotted versus energy density of the input pulse for 700 and 800°C, at 50 mg iodine content. Transmissions in the range of 90% are obtained at energy densities above 3 joules[cm 2. Theoretical curves for complete
Volume 18, n u m b e r 4
OPTICS COMMUNICATIONS
September 1976
100
T 5O
ix 700 °C x
0
o
1
2 .Ioules/cnn
800oc
t
I
3
i.
2
Pig. 4. Transmission o f the cell u n d e r saturation conditions. Iodine c o n t e n t is 50 mg. Drawn lines are theoretical from the approximation of complete saturation.
5. Conclusion
100
'°7Y"
/
T
/o / 0.0
*~ 5o
i
Joules/Cm 2
Fig. 5. Transmission of the cell under saturation conditions. • O Temperature is 800 C.
saturation, obtained from eq. (3) are also shown in fig. 4. As expected, the agreement with the experimental transmissions gets better the higher the energy density, i.e. the better the pulse saturates. The theoretical curve for complete saturation gives a lower limit for the experimental transmissions if only single photon absorption occurs. Within experimental error, no multiphoton absorption can be inferred from our data. Multiphoton absorption (as observed for example in the absorption of intense CO 2 laser pulses in SF6) would lower the transmission of the absorber at high energy densities.
We have observed strong absorption of iodine laser pulses in atomic iodine. The absorption is easily saturated with pulses of a few joules/cm 2. No signs of nonlinear effects, such as multiphoton absorption or selffocusing could be observed. The cell is well suited to improve the contrast of the main pulse to prepulses and noise from superfluorescence. To a certain extent it also acts as an optical isolator of amplifier stages. A further application of the absorber is the generation of iodine laser pulses of subnanosecond duration by the method of optical induction decay [9]. First experiments carried out in our laboratory have given encouraging results. A detailed description will be given elsewhere.
Acknowledgement This work was performed under the terms of the agreement on association between Max-Planck-Institut fiir Plasmaphysik and Euratom.
References [1] K. Hohla, Max-Planck-lnst. f/Jr Plasmaphysik, Rep. IV/33 (1971).
435
Volume 18, number 4
OPTICS COMMUNICATIONS
[2] M.G. Gal'pern, V.A. Gorbachev, V.A. Katulin, O.L. Lebedev, E.A. Luk'yanets, N.G. Mekhryakova, V.M. Mizin, V.Yu. Nosach, A.L. Petrov and G.G. Semenovskaya, Sov. J. Quant. Electron. 5 (1976) 1384. [3] V.A. Gaidash, G.A. KiriUov,S.B. Kormer, S.G. Lapin, V.I. Shemiakin and V.K. Shirigin, ZhETF Letters 20 (1974) 243. [4] P.V. Avizonis and R.L. Grotbeck, J. Appl. Phys. 37 (1966) 687.
436
September 1976
[5] D.R. Stull and G.C. Sinke, Thermodynamic Properties of the Elements, Advances in Chemistry Series, Amer. Chem. Soc. ed. (1956), p. 109. [6] G. Schappert, unpublished result. [7] J. Olsen, to be published. [8] K. Hohla, G. Brederlow, W. Fuss, K.L. Kompa, J. Raeder, R. Volk, S. Witkowski and K.J. Witte, J. Appl. Phys. 46 (1975) 808. [9] E. Yablonovitch, Phys. Rev. A10 (1974) 1888.
OPTICS COMMUNICATIONS
September 1976
A SATURABLE ABSORBER FOR THE IODINE LASER
E. FILL and K. HOHLA Max-Planck-Gesellschaft zur F~Jrderung der Wissenschaften e. V., Pro]ektgruppe fffr Laserforschung, D-8046 Garching, Fed. Rep. Germany
Received 12 May 1976
An absorber for iodine laser pulses has been developed using iodine atoms in the ground state as the absorbing species. The iodine atoms have been produced by dissociation of gaseous molecular iodine in a heated cell. The absorption is easily saturated at moderate energy densities.
1. Introduction
The use of a high-gain medium such as iodine in high power laser systems offers the advantage of a reduced number of amplifier stages and a high energy extraction efficiency [1 ]. On the other hand, the large difference between small signal and saturated gains resuits in reduced contrast between prepulses, amplified superfluorescence and the main pulse. In addition, the high gain gives rise to spurious oscillations of the whole amplifier chain. Both problems can be overcome by the use of a saturable absorber (s.a.) with high enough extinction ratio, i.e. the small signal transmission should be as small as possible whereas transmission of the main pulse should be close to 100%. A number of saturable absorbers exist for the CO 2 neodymium and dye lasers. However, at 1.315/a, the wavelength of the iodine laser, only one single dye (of undisclosed composition) with a saturable absorption has been reported [2]. The absorption of gaseous molecular absorbers starts at considerably longer wavelengths. In this paper we describe a s.a. which uses the lasing transition in absorption as the saturable transition. Thus, iodine atoms in the ground state 2P3/2 are the absorbing species. Three different methods have been investigated as a source for iodine atoms in the ground state: 1. Photolysis of C 3 F7I (as in a laser) and subsequent
quenching of the excited iodine atoms by means of 0 2 added to the iodide. 2. Photolysis of HI. 3. Thermal dissociation of I2-molecules in a hot cell [3]. With all three methods saturable absorption of iodine laser pulses could be demonstrated. The most reproducible results were obtained with the third of the above methods. This method additionally offers the advantages of a relatively simple technology and cw operation. The results reported in this paper are all obtained with the hot iodine dissociation cell.
2. Theory Starting from rate equations one obtains for the energy transmission of a s.a. depending on input energy density [4]
est E (e )l
T =eOln
1+
xp
-1
exp(-oNl)
/
,
(1)
where e 0 = input energy density, e s = h v / o b = saturation energy density,
o = cross-section for absorption at line centre, N = difference of populations/cm 3 of lower and upper level, 431
Volume 18, number 4
OPTICS COMMUNICATIONS
= length of the absorber, = 1 + glower/guppe r = degeneracy factor; for the transition involved b = 3, if complete relaxation of the hyperfine sublevels within the upper and lower level is assumed. This formula is valid if the relaxation of the upper level is slow compared to the pulswidth. Also, the spectrum of the pulse must be much narrower than the width of the absorbing transition, i.e. formula (1) applies at line centre. Two limiting cases are comprised in (1): a) Small signal behaviour: TO = exp ( - a N l )
if e 0 "~ e s.
(2)
ife 0>>e s.
(3)
b) Behaviour at complete saturation: hvNl be 0
Ts=l
Thus, for a small enough signal Beer's law is obeyed, whereas for a signal with high energy density the absorber simply substracts a number of photons corresponding to its population difference from the number of incoming photons. To derive a condition relating absorber and amplifier parameters in a laser amplifier system we consider a configuration consisting of an amplifier with gain g = OvNvl v followed by an absorber with absorption oL = OANAI A . Then the condition for low gain in the small signal region (relevant for noise and prepulses) is ovNvl v ~. OA N A I A .
(4)
For a strong signal (the main pulse) the energy extracted from the amplifier should be much larger than the energy taken out by the absorber. It follows that
September 1976
sary condition for parameters of a s.a. combined with an amplifier Ovbv/A v ~ OAbA[A A.
(6)
To fulfill this condition two modes of operation of a s.a. are conceivable: 1. The beam diameters in the amplifier and absorber are equal, but the absorption cross-section of the absorber is much larger than the cross-section for stimulated emission of the amplifier. 2. The cross-sections for stimulated interaction are equal, but the beam diameter in the absorber is reduced by passing the beam through a telescope. Mode 1 can be accomplished with an iodine saturable absorber by reducing the pressure in the absorption cell to narrow the pressure broadened line. In this case, however, the performance of the device as an optical isolator is deteriorated, because the line of the absorber does not cover the entire linewidth of the amplifier. This problem is eliminated in mode 2, but one has the inconvenience of a more complicated beam adjustment. In our experiments a combination of the two methods has been applied. The number density of absorbing atoms, i.e. the number of iodine atoms in the ground state per cm 3 can be calculated as follows: The partial pressures of molecular iodine Pl and atomic iodine PI in thermodynamic equihbnum are related by the equation p 2 / p l : = rp(T), where Kp(T) is the equilibrium constant for dissociation [5], which is strongly dependent on temperature. Relating pressures and number densities by the equation of state for an ideal gas one obtains for the number density N of dissociated iodine atoms U = N~
|/Kp (T)m
(7)
V 254RTV' NvlvAv/b v >~ N A IAA A/b A, Av Aa bv ba
= = = =
(5)
area of the beam in the amplifier, area of the beam in the absorber, degeneracy factor of the amplifier, degeneracy factor of the absorber.
Eq. (4) and inequality (5) together give as a neces432
N~ m R V
= = = =
Avogadro's number, mass of iodine contained in the cell, ideal gas constant, volume of the cell.
This equation is valid, if the fraction of dissociated molecules is small compared to the total number of
Volume 18, number 4
OPTICS COMMUNICATIONS
molecules, a condition fulffiled at the temperatures used in our experiments. Also, at these temperatures (< 1200 K) the number of iodine atoms thermally excited to the upper laser level is negligible. Therefore the number density of absorbing iodine atoms is equal to the population difference and is given by eq. (7). From eqs. (2) and (7) the absorption at line centre can be calculated using the well known formula for the cross-section for absorption at the centre of a lorentzian line cr = X2A/47r2 Av,
(8)
where A = first Einstein coefficient = 5.12 for the transition involved, and Av = pressure broadened linewidth (full width at half maximum). It turns out, however, that under our experimental conditions a straightforward application of (2), (7), and (8) to calculate small signal absorption for given cell parameters is not possible. First, the exact value of the pressure broadening coefficient of iodine molecules is not known, so that only an estimate of the small signal absorption at line centre can be given. Second and more important, the nanosecond pulses used in our measurements have a spectrum which is comparable to the linewidth of the absorber. Therefore not the absorption at line centre but a much smaller absorption is measured. An exact calculation of this experimental absorption is only possible by numerical calculations. However, an upper limit for the absorption of a short pulse by an absorber with a lorentzian lineshape can be derived analytically [6]. If the pulse is assumed to be Gaussian in time, then the transmission is T~> exp ( - 2 ~/r~AVA/AVp),
(9)
Av A = full width at half maximum of the lorentzian absorber line, AVp = full 1/e width of the gaussian pulse spectrum, ct --- absorption coefficient at line centre. In our experiments AVA/AVp ~ 1, which means that significant deviations from Beer's law should occur. This has been confirmed experimentally, as will be discussed later. Eq. (9) states that the pulse gets less and less absorbed as it travels through the absorber. This can be understood by realizing that the pulse is reshaped by the action of the absorber on its spectrum, making the absorber less and less effective. Numerical calculations including pulse transmission and pulse shaping are under way [7] and will be published elsewhere. It should be pointed out, that the equation for complete saturation [eq. (3)] remains valid even for nanosecond pulses and can therefore be used to determine the transmission of the absorber for a strong signal.
3. Experimental arrangement A sectional drawing of the iodine saturable absorber is shown in fig. 1. The dissociation vessel is a sealed quartz cell of 2 cm 0 and 42 cm length, terminated with Brewster windows. Heat is applied to the cell by about 2 meters of tantalum ribbon around the cylindrical side walls. 600 watts of electric power are sufficient to reach a temperature of 800°C. The quartz cell is contained in a cylindrical glass vessel which is evacuated to 10 -3 torr for thermal isolation. The inner wall of the glass vessel is lined with aluminum sheet, to avoid loss of heat by radiation. The windows of the outer cell are water cooled. The temperature of the quartz cell can be measured by means of a thermocouple. With this arrangement continuous operation of
heating ribbon
Laser
Beam
/">
September 1976
to pump
/ ,,T, ~1i'0~0000h00].0ei!oc00o0u p011 00~001i~~__~ / 10-3Torr
water cooled windows
Fig. 1. Sectional view of iodine dissociation cell. Overall length is 86 cm. 433
Volume 18, number 4
OPTICS COMMUNICATIONS
the dissociation cell at 900°C is possible. Small signal and saturation measurements were carried out with various amounts of iodine contained in the cell. For the small signal measurements a single switched out pulse from the oscillator of the iodine laser system Asterix II [8] was used as the probe signal. The beam was expanded by putting a distance of about 25 m between the laser and the cell, so that energy densities well below 1 m J / c m 2 were applied as an input signal. The pulses before and behind the cell were passed onto a vacuum diode (Valvo XA 1003) feeding a Tektronix 7704 oscilloscope. The risetime of the detection system was sufficiently long so that energy transmissions were measured. For the saturation measurements the full preamplifier section of Asterix II was used, which delivered a pulse of about 1 joule in slightly less than a nanosecond. Before entering the cell the beam was narrowed by means of a telescope. Energy densities of up to 15 joules/cm 2 could be obtained, limited only by optical damage of the windows. Two calorimeters, one before and one behind the cell, were used to measure the transmission of the pulse.
4. Results and discussion Small signal absorption versus temperature is plotted in fig° 2 for two different iodine contents of the cell. Each data point represents an average of 5 shots. The error bars result from a scatter in the measured absorption from shot to shot. Since the cell was run cw in these experiments with the temperature well
t7
1000
100
t
1 300
400
500
J
t 700
600
i 800
900
I00C
t °C
Fig. 2. Small-signal absorption of nanosecond pulses versus temperature of the cell. 434
September 1976
10 000
I
V////// ///
1000
~oo
I
10
100 As~ -
1000
P
Fig. 3. Double pass absorption versus single pass absorption. Full line represents exponential law (Beer's law), dashed line from a law of form exp (- k,f~). stabilized it is unlikely that the cell parameters changed between shots. A more probable explanation of the scatter is that the shape of the pulse generated by the oscillator was not entirely reproducible, which means that the spectrum of the input pulse to the cell varied from shot to shot. The invalidity of Beer's law for nanosecond pulse was demonstrated by comparing single and double pass absorption of the cell. It turned out, that double pass absorption was much lower than the value obtained by squaring the value of single pass absorption. In fig. 3 double pass absorption is plotted versus single pass absorption on a doubly logarithmic scale. In such a plot Beer's law gives a straight line with slope 2 (drawn full in fig. 3), whereas our measured points lie closer to a line with slope x/~-(drawn dashed). The dashed line is obtained when it is assumed that the equality holds in the law given by (9). The transmission of the iodine cell under saturating conditions is shown in figs. 4 and 5. In fig. 4 transmission is plotted versus energy density of the input pulse for 700 and 800°C, at 50 mg iodine content. Transmissions in the range of 90% are obtained at energy densities above 3 joules[cm 2. Theoretical curves for complete
Volume 18, n u m b e r 4
OPTICS COMMUNICATIONS
September 1976
100
T 5O
ix 700 °C x
0
o
1
2 .Ioules/cnn
800oc
t
I
3
i.
2
Pig. 4. Transmission o f the cell u n d e r saturation conditions. Iodine c o n t e n t is 50 mg. Drawn lines are theoretical from the approximation of complete saturation.
5. Conclusion
100
'°7Y"
/
T
/o / 0.0
*~ 5o
i
Joules/Cm 2
Fig. 5. Transmission of the cell under saturation conditions. • O Temperature is 800 C.
saturation, obtained from eq. (3) are also shown in fig. 4. As expected, the agreement with the experimental transmissions gets better the higher the energy density, i.e. the better the pulse saturates. The theoretical curve for complete saturation gives a lower limit for the experimental transmissions if only single photon absorption occurs. Within experimental error, no multiphoton absorption can be inferred from our data. Multiphoton absorption (as observed for example in the absorption of intense CO 2 laser pulses in SF6) would lower the transmission of the absorber at high energy densities.
We have observed strong absorption of iodine laser pulses in atomic iodine. The absorption is easily saturated with pulses of a few joules/cm 2. No signs of nonlinear effects, such as multiphoton absorption or selffocusing could be observed. The cell is well suited to improve the contrast of the main pulse to prepulses and noise from superfluorescence. To a certain extent it also acts as an optical isolator of amplifier stages. A further application of the absorber is the generation of iodine laser pulses of subnanosecond duration by the method of optical induction decay [9]. First experiments carried out in our laboratory have given encouraging results. A detailed description will be given elsewhere.
Acknowledgement This work was performed under the terms of the agreement on association between Max-Planck-Institut fiir Plasmaphysik and Euratom.
References [1] K. Hohla, Max-Planck-lnst. f/Jr Plasmaphysik, Rep. IV/33 (1971).
435
Volume 18, number 4
OPTICS COMMUNICATIONS
[2] M.G. Gal'pern, V.A. Gorbachev, V.A. Katulin, O.L. Lebedev, E.A. Luk'yanets, N.G. Mekhryakova, V.M. Mizin, V.Yu. Nosach, A.L. Petrov and G.G. Semenovskaya, Sov. J. Quant. Electron. 5 (1976) 1384. [3] V.A. Gaidash, G.A. KiriUov,S.B. Kormer, S.G. Lapin, V.I. Shemiakin and V.K. Shirigin, ZhETF Letters 20 (1974) 243. [4] P.V. Avizonis and R.L. Grotbeck, J. Appl. Phys. 37 (1966) 687.
436
September 1976
[5] D.R. Stull and G.C. Sinke, Thermodynamic Properties of the Elements, Advances in Chemistry Series, Amer. Chem. Soc. ed. (1956), p. 109. [6] G. Schappert, unpublished result. [7] J. Olsen, to be published. [8] K. Hohla, G. Brederlow, W. Fuss, K.L. Kompa, J. Raeder, R. Volk, S. Witkowski and K.J. Witte, J. Appl. Phys. 46 (1975) 808. [9] E. Yablonovitch, Phys. Rev. A10 (1974) 1888.