LASERS | Carbon Dioxide Laser

L LASERS Contents

Carbon Dioxide Laser Dye Lasers Edge Emitters Excimer Lasers Free Electron Lasers Metal Vapor Lasers Noble Gas Ion Lasers Optical Fiber Lasers Organic Semiconductors and Polymers Planar Waveguide Lasers Semiconductor Lasers Up-Conversion Lasers

Carbon Dioxide Laser C R Chatwin, University of Sussex, Brighton, UK q 2005, Elsevier Ltd. All Rights Reserved.

Introduction This article gives a brief history of the development of the laser and goes on to describe the characteristics of the carbon dioxide laser and the molecular dynamics that permit it to operate at comparatively high power and efficiency. It is these commercially attractive features and its low cost that has led to its adoption as one of most popular industrial power beams. This outline also describes the main types of carbon dioxide laser and briefly discusses their characteristics and uses.

Brief History In 1917 Albert Einstein developed the concept of stimulated emission which is the phenomenon used in lasers. In 1954 the MASER (Microwave Amplification by Stimulated Emission of Radiation) was the first device to use stimulated emission. In that year Townes and Schawlow suggested that stimulated emission could be used in the infrared and optical portions of the electromagnetic spectrum. The device was originally termed the optical maser, this term being dropped in favor of LASER, standing for: Light Amplification by Stimulated Emission of Radiation. Working against the wishes of his manager at Hughes Research Laboratories, the electrical engineer Ted Maiman created the first laser on the 15 May 1960. Maiman’s flash lamp pumped ruby laser produced pulsed red electromagnetic radiation at a wavelength of 694.3 nm. During the most active period of laser systems discovery Bell Labs made a very

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significant contribution. In 1960, Ali Javan, William Bennet and Donald Herriot produced the first Helium Neon laser, which was the first continuous wave (CW) laser operating at 1.15 mm. In 1961, Boyle and Nelson developed a continuously operating Ruby laser and in 1962, Kumar Patel, Faust, McFarlane and Bennet discovered five noble gas lasers and lasers using oxygen mixtures. In 1964, C.K.N. Patel created the high-power carbon dioxide laser operating at 10.6 mm. In 1964, J.F. Geusic and R.G. Smith produced the first Nd:Yag laser using neodymium doped yttrium aluminum garnet crystals and operating at 1.06 mm.

Characteristics Due to its operation between low lying vibrational energy states of the CO2 molecule, the CO2 laser has a high quantum efficiency, , 40%, which makes it extremely attractive as a high-power industrial materials processing laser (1 to 20 kW), where energy and running costs are a major consideration. Due to the requirement for cooling to retain the population inversion, the efficiency of electrical pumping and optical losses – commercial systems have an overall efficiency of approximately 10%. Whilst this may seem low, for lasers this is still a high efficiency. The CO2 laser is widely used in other fields, for example, surgical applications, remote sensing, and measurement. It emits infrared radiation with a wavelength that can range from 9 mm up to 11 mm. The laser

transition may occur on one of two transitions: (0001) ! (10 00), l ¼ 10:6 mm; (0001) ! (02 00), l ¼ 9:6 mm; see Figure 1. The 10.6 mm transition has the maximum probability of oscillation and gives the strongest output; hence, this is the usual wavelength of operation, although for specialist applications the laser can be forced to operate on the 9.6 mm line. Figure 1 illustrates an energy level diagram with four vibrational energy groupings that include all the significantly populated energy levels. The internal relaxation rates within these groups are considered to be infinitely fast when compared with the rate of energy transfer between these groups. In reality the internal relaxation rates are at least an order of magnitude greater than the rates between groups. Excitation of the upper laser level is usually provided by an electrical glow discharge. However, gas dynamic lasers have been built where expanding a hot gas through a supersonic nozzle creates the population inversion; this creates a nonequilibrium region in the downstream gas stream with a large population inversion, which produces a very highpower output beam (135 kW – Avco Everett Research Lab). For some time the gas dynamic laser was seriously considered for use in the space-based Strategic Defence Initiative (SDI-USA). The gas mixture used in a CO2 laser is usually a mixture of carbon dioxide, nitrogen, and helium. The proportions of these gases varies from one laser system to another, however, a typical mixture is 10%-CO2;

Figure 1 Six level model used for the theoretical description of CO2 laser action.

LASERS / Carbon Dioxide Laser 391

10%-N2; 80%-He. Helium plays a vital role in the operation of the CO2 laser in that it maintains the population inversion by depopulating the lower laser level by nonradiative collision processes. Helium is also important for stabilization of the gas discharge; furthermore it greatly improves the thermal conductivity of the gas mixture, which assists in the removal of waste heat via heat exchangers. Small quantities of other gases are often added to commercial systems in order to optimize particular performance characteristics or stabilize the gas discharge; for brevity we only concern ourselves here with this simple gas mixture.

Molecular Dynamics Direct Excitation and De-excitation

It is usual for excitation to be provided by an electrical glow discharge. The direct excitation of carbon dioxide (CO2) and nitrogen (N2) ground state molecules proceeds via inelastic collisions with fast electrons. The rates of kinetic energy transfer are a and g; respectively, and are given by eqns [1] and [2]:

a¼

FCO2 £ IP ðsec21 Þ E000 1 £ n0

½1

g¼

FN2 £ IP ðsec21 Þ Ev¼1 £ n4

½2

Figure 2 Electron energy distribution function.

Ee ; FCO2 ; and FN2 are obtained by solution of the Boltzmann transport equation (BTE); the average electron energy can be optimized to maximize the efficiency (FCO2 ; FN2 ) with which electrical energy is utilized to create a population inversion. Hence, the discharge conditions required to maximize efficiency can be predicted from the transport equation. Figure 2 shows one solution of the BTE for the electron energy distribution function.

where:

Resonant Energy Transfer

FCO2 ¼ Fraction of the input power (IP) coupled into the excitation of the energy level E000 1 ; n0 is the CO2 ground level population density; FN2 ¼ Fraction of the input power (IP) coupled into the excitation of the energy level Ev¼1 ; and n4 is the N2 ground level population density.

Resonant energy transfer between the CO2 (0001) and N2 ðv ¼ 2Þ energy levels (denoted 1 and 5 in Figure 1) proceeds via excited molecules colliding with ground state molecules. A large percentage of the excitation of the upper laser level takes place via collisions between excited N2 molecules and ground state CO2 molecules. The generally accepted rate of this energy transfer is given by eqns [5] and [6]:

The reverse process of the above occurs when molecules lose energy to the electrons and the electrons gain an equal amount of kinetic energy; the direct de-excitation rates are given by h and b; eqns [3] and [4], respectively: ! E000 1 ðsec21 Þ h ¼ a £ exp ½3 Ee !

b ¼ g £ exp

Ev¼1 ðsec21 Þ Ee

½4

where Ee is the average electron energy in the discharge.

K51 ¼ 19 000 PCO2 ðsec21 Þ

½5

K15 ¼ 19 000 PN2 ðsec21 Þ

½6

where PCO2 and PN2 are the respective gas partial pressures in Torr. Hence, CO2 molecules are excited into the upper laser level by both electron impact and impact with excited N2 molecules. The contribution from N2 molecules can be greater than 40% depending on the discharge conditions.

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Collision – Induced Vibrational Relaxation of the Upper and Lower Laser Levels

The important vibrational relaxation processes are illustrated by Figure 1 and can be evaluated from eqns [7– 10]; where the subscripts refer to the rate between energy levels 1 and 32; 21 and 31; 22 and 31; 32 and 0, respectively: K132 ¼ 367 PCO2 þ 110 PN2 þ 67 PHe ðsec21 Þ

½7

where T is the absolute temperature and n refers to the population density of the gas designated by the subscript. This expression takes account of the different constituent molecular velocity distributions and different collision cross-sections for CO2 ! CO2, N2 ! CO2 and He ! CO2 type collisions. Equation [13] also takes account of the significant line broadening effect of helium. Neglecting the unit change in rotational quantum number, the energy level degeneracies g1 and g2 may be dropped. n100 0 is partitioned such that n100 0 ¼ 0:1452n2 and eqn [12] can be re-cast as eqn [14]:

K2131 ¼ 6 £ 105 PCO2 ðsec21 Þ

½8

K2231 ¼ 5:15 £ 105 PCO2 ðsec21 Þ

½9

g ¼ s ðn1 2 0:1452n2 Þ cm21

½10

where n1 and n2 are the population densities of energy groups ‘1’ and ‘2’ respectively.

K320 ¼ 200 PCO2 þ215 PN2 þ3270 PHe ðsec

21

Þ

K132 , K2131 ; and K2231 are vibration/vibration transfer rates and K320 is a vibration/translation transfer rate. Note the important effect of helium on eqn [10]; helium plays a major role in depopulating the lower laser level, thus enhancing the population inversion. PHe is the partial pressure of helium in Torr. Radiative Relaxation

Spontaneous radiative decay is not a major relaxation process in the CO2 laser but it is responsible for starting laser action via spontaneous emission. The Einstein ‘A’ coefficient for the laser transition is given by eqn [11]: A ¼ ðtsp Þ21 ¼ 0:213 ðsec21 Þ

½11

Gain

The gain (g) is evaluated from the product of the absorption coefficient (s) and the population inversion, eqn [12]:   g g ¼ s n000 1 2 1 n100 0 cm21 ½12 g2 For most commercial laser systems the absorption coefficient is that for high-pressure collisionbroadening (P . 5.2 Torr) where the intensity distribution function describing the line shape is Lorentzian. The following expression describes the absorption coefficient, eqn [13]:

s¼ TnCO2

692:5 ! ðcm2 Þ nN2 nHe þ 1:4846 1 þ 1:603 nCO2 nCO2 ½13

½14

Stimulated Emission

Consider a laser oscillator with two plane mirrors, one placed at either end of the active gain medium, with one mirror partially transmitting (see Figure 4). Laser action is initiated by spontaneous emission that happens to produce radiation whose direction is normal to the end mirrors and falls within the resonant modes of the optical resonator. The rate of change of photon population density ðIp Þ within the laser cavity can be written as eqn [15]: Ip dIP ¼ Ip cg 2 dt T0

½15

where the first term on the right-hand side accounts for the effect of stimulated emission and the second term represents the number of photons that decay out of the laser cavity, T0 is the photon decay time, given by eqn [16], and is defined as the average time a photon remains inside the laser cavity before being lost either through the laser output window or due to dispersion; if dispersion is ignored, Ip =T0 ; is the laser output: 2L

T0 ¼ c loge

1 RB RF

!

½16

where L is the distance between the back and the front mirrors, which have reflectivities of RB and RF , respectively. The dominant laser emission occurs on a rotational – vibrational P branch transition Pð22Þ; that is ð J ¼ 21Þ ! ð J ¼ 22Þ line of the ð000 1Þ ! ð100 0Þ; l ¼ 10:6 mm transition, where J is the rotational quantum number. The rotational level relaxation rate is so rapid that equilibrium is maintained between rotational levels so that they

LASERS / Carbon Dioxide Laser 393

feed all their energy through the Pð22Þ transition. This model simply assumes constant intensity, basing laser performance on the performance of an average unit volume. By introducing the stimulated emission term into the molecular rate equations, which describe the rate of transfer of molecules between the various energy levels illustrated in Figure 1, a set of molecular rate eqns [17 –21] can be written that permit simulation of the performance of a carbon dioxide laser: dn1 ¼ an0 2 hn1 þ K51 n5 2 K15 n1 2 Ksp n1 dt     n1 2 K132 n1 2 n32 2 Ip cg n32 e     dn2 n ¼ Ksp n1 þ Ip cg 2 K2131 n21 2 21 n31 dt n     31 e n22 2 K2231 n22 2 n n31 e 31      dn3 n ¼ 2K2131 n21 2 21 n31 2 2K2231 n22 dt n     31 e    n22 n1 2 n31 þ K132 n1 2 n32 n31 e n32 e     n32 n 2 K320 n32 2 n0 e 0

½17

½18

½19

dn5 ¼ gn4 2 bn5 2 K51 n5 þ K15 n1 dt

½20

dIp Ip ¼ Ip cg 2 dt T0

½21

The terms in square brackets ensure that the system maintains thermodynamic equilibrium; subscript ‘e’ refers to the fact that the populations in the square brackets are the values for thermodynamic equilibrium. The set of five simultaneous differential equations can be solved using a Runge – Kutta method. They can provide valuable performance prediction data that is helpful in optimizing laser design, especially when operated in the pulsed mode. Figure 3a illustrates some simulation results for the transverse flow laser shown in Figure 9. The results illustrate the effect of altering the gas mixture and how this can be used to control the gain switched spike that would result in unwelcome work piece plasma generation if allowed to become too large. Figure 3b shows an experimental laser output pulse from the high pulse repetition frequency (prf – 5kHz) laser illustrated in Figure 9. This illustrates that even a

quite basic physical model can give a good prediction of laser output performance.

Optical Resonator Figure 4 shows a simple schematic of an optical resonator. This simple optical system consists of two mirrors (the full reflector is often a water cooled gold coated copper mirror), which are aligned to be orthogonal to the optical axis that runs centrally along the length of the active gain medium in which there is a population inversion. The output coupler is a partial reflector (usually dielectrically coated zinc selenide – ZnSe –that may be edge cooled) so that some of the electromagnetic radiation can escape as an output beam. The ZnSe output coupler has a natural reflectivity of about 17% at each air– solid interface. For high power lasers (2 kW) 17% is sufficient for laser operation; however, depending on the required laser performance the inside surface is often given a reflective coating. The reflectivity of the inside face depends on the balance between the gain (eqn [14]), the output power and the power stability requirements. The outside face of the partial reflector must be anti-reflection (AR) coated. Spontaneous emission occurs within the active gain medium and radiates randomly in all directions; a fraction of this spontaneous emission will be in the same direction as the optical axis, perpendicular to the end mirrors, and will also fall into a resonant mode of the optical resonator. Spontaneous emission photons interact with CO2 molecules in the excited upper laser level, excited state (0001), which stimulates these molecules to give up a quanta of vibrational energy as photons via the radiative transition ð000 1Þ ! ð100 0Þ; l ¼ 10:6mm: The radiation given up will have exactly the same phase and direction as the stimulating radiation and thus will be coherent with it. The reverse process of absorption also occurs, but so long as there is a population inversion there will be a net positive output. This process is called light amplification by stimulated emission of radiation (LASER). The mirrors continue to redirect the photons parallel to the optical axis and so long as the population inversion is not depleted, more and more photons are stimulated by stimulated emission which dominates the process and also dominates the spontaneous emission, which is important to initiate laser action. Light emitted by lasers contains several optical frequencies, which are a function of the different modes of the optical resonator; these are simply the standing wave patterns that can exist within the resonator structure. There are two types of

394 LASERS / Carbon Dioxide Laser

Figure 3 (a) Predicted output pulses for transverse flow CO2 laser for different gas mixtures, (b) Experimental output pulse from transverse flow CO2 laser.

Figure 4 Optical resonator.

resonator modes: longitudinal and transverse. Longitudinal modes differ from one another in their frequency of oscillation whereas transverse modes differ from one another in their oscillation frequency and field distribution in a plane orthogonal to the direction of propagation. Typically CO2 lasers have a large number of longitudinal

modes; in CO2 laser applications these are of less interest than the transverse modes, which determine the transverse beam intensity and the nature of the beam when focused. In cylindrical coordinates the transverse modes are labelled TEMpl ; where subscript ‘p’ is the number of radial nodes and ‘l’ is the number of angular nodes. The lowest order mode is the TEM00, which has a Gaussianlike intensity profile with its maximum on the beam axis. A light beam emitted from an optical resonator with a Gaussian profile is said to be operating in the ‘fundamental mode’ or the TEM00 mode. The decrease in irradiance ðIÞ with distance ‘r’ from the axis ðI0 Þ of the Gaussian beam is

LASERS / Carbon Dioxide Laser 395

described by eqn [22]: IðrÞ ¼ I0 expð2r2 =w2 Þ

½22

where w is the radial distance, where the power density is decreased to 1=e2 of its axial value. Ideally a commercial laser should be capable of operation in the fundamental mode as, with few exceptions, this results in the best performance in applications. Laser cutting benefits from operation in the fundamental mode; however, welding or heat treatment applications may benefit from operation with higher-order modes. Output beams are usually controlled to be linearly or circularly polarized, depending upon the requirements of the application. For materials processing applications the laser beam is usually focused via a water cooled ZnSe lens or, for very high power lasers, a parabolic gold coated mirror. Welding applications will generally use a long focal length lens and cutting applications will use a short focal length, which generates a higher irradiance at the work piece than that necessary for welding. The beam delivery optics are usually incorporated into a nozzle assembly that can deliver cooling water and assist gases for cutting and anti-oxidizing shroud gases for welding or surface engineering applications.

Laser Configuration CO2 lasers are available in many different configurations and tend to be classified on the basis of their physical form and the gas flow arrangement, both of which greatly affect the output power available and beam quality. The main categories are: sealed off lasers, waveguide lasers, slow axial flow, fast axial flow, diffusion cooled, transverse flow, transversely excited atmospheric lasers, and gas dynamic lasers. Sealed-Off and Waveguide Lasers

Depopulation of the lower laser level is via collision with the walls of the discharge tube, so the attainable output power scales with the length of the discharge column and not its diameter. Output powers are in the range 5 W to 250 W. Devices may be constructed from concentric glass tubes with the inner tube providing the discharge cavity and the outer tube acting to contain water-cooling of the inner discharge tube. The inner tube walls act as a heat sink for the discharge thermal energy (see Figure 5). The DC electrical discharge is provided between a cathode and anode situated at either end of the discharge tube. A catalyst must be provided to ensure regeneration of CO2 from CO. This may be accomplished by adding about 1% of H2O to the gas mixture, or alternatively,

Figure 5 Schematic of sealed-off CO2 laser, approximately 100 W per meter of gain length, gas cooled by diffusion to the wall.

recombination can be achieved via a hot (300 8C) Ni cathode, which acts as a catalyst. RF-excited all metal sealed-off tube systems can deliver lifetimes greater than 45 000 hours. Diffusion-cooled slab laser technology will also deliver reliable sealed operation for 20 000 hrs. Excitation of the laser medium occurs via RF excitation between two water-cooled electrodes. The water-cooled electrodes dissipate (diffusion cooled) the heat generated in the gas discharge. An unstable optical resonator provides the output coupling for such a device (see Figure 6). Output powers are in the range 5 W to 300 W and can be pulsed from 0 to 100 kHz. These lasers are widely used for marking, rapid prototyping, and cutting of nonmetals (paper, glass, plastics, ceramics) and metals. Waveguide CO 2 lasers use small bore tubes (2 –4 mm) made of BeO or SiO2 where the laser radiation is guided by the tube walls. Due to the small tube diameter, a gas total pressure of 100 to 200 Torr is necessary, hence the gain per unit length is high. This type of laser will deliver 30 W of output power from a relatively short (50 cm long) compact sealed-off design; such a system is useful for microsurgery and scientific applications. Excitation can be provided from a longitudinal DC discharge or from an RF source that is transverse to the optical axis; RF excitation avoids the requirement for an anode and cathode and results in a much lower electrode voltage. Slow Axial Flow Lasers

In slow flow lasers the gas mixture flows slowly through the laser cavity. This is done to remove the products of dissociation that will reduce laser efficiency or prevent it from operating at all, and the main contaminant is CO. The dissociated gases (mainly CO and O2) can be recombined using a catalyst pack and then reused via continuous recirculation. Heat is removed via diffusion through the walls of the tube containing the active gain medium. The tube is frequently made of Pyrex glass with a concentric outer tube to facilitate water-cooling of the

396 LASERS / Carbon Dioxide Laser

Figure 6 Schematic for sealed-off slab laser and diffusion cooled laser with RF excitation (courtesy of Rofin).

laser cavity (see Figure 7). Slow flow lasers operate in the power range 100 W to 1500 W, and tend to use a longitudinal DC electrical discharge which can be made to run continuously or pulsed if a thyratron switch is build into the power supply; alternatively, electrical power can be supplied via transverse RF excitation. The power scales with length, hence high power slow flow lasers have long cavities and require multiple cavity folds in order to reduce their physical size.

Fast Axial Flow Lasers

The fast axial flow laser, Figure 8, can provide output powers from 1 kW to 20 kW; it is this configuration that dominates the use of CO2 lasers for industrial applications. Industrial lasers are usually in the power range 2– 4 kW. The output power from these devices scales with mass flow, hence the gas mixture is recycled through the laser discharge region at sonic or supersonic velocities. Historically this was achieved using Rootes blowers to compress the gas upstream of the laser cavity. Rootes compressors are inherently inefficient and the more advanced laser systems utilize turbine compressors, which deliver greater efficiency and better laser stability. Rootes compressors can be a major source of vibration. With this arrangement heat exchangers are required to remove heat after the laser discharge region and also after the compressor stage, as the compression process heats up the laser gases. Catalyst packs are used to regenerate gases but some gas replacement is often required. These laser systems have short cavities and use folded stable resonator designs to achieve higher output powers with extremely high-quality beams that are particularly suitable for cutting applications. They also give

Figure 7 Slow flow CO2 laser, approximately 100 W per meter of gain length, gas cooled by diffusion to the wall.

excellent results when used for welding and surface treatments. Fast axial flow lasers can be excited by a longitudinal DC discharge or a transverse RF discharge. Both types of electrical excitation are common. For materials processing applications it is often important to be able to run a laser in continuous wave (CW) mode or as a high pulse repetition rate (prf) pulsed laser and to be able to switch between CW and pulsed in real time; for instance, laser cutting of accurate internal corners is difficult using CW operation but very easy using the pulsed mode of operation. Both methods of discharge excitation can provide this facility. Diffusion Cooled Laser

The diffusion-cooled slab laser is RF excited and gives an extremely compact design capable of delivering 4.5 kW pulsed from 8 Hz to 5 kHz prf or CW with good beam quality (see Figure 6). The optical resonator is formed by the front and rear mirrors and two parallel water cooled RF-electrodes. Diffusion cooling is provided by the RF-electrodes, removing the requirement for conventional gas recirculation via Rootes blowers or turbines.

LASERS / Carbon Dioxide Laser 397

Figure 8 Fast axial flow carbon dioxide laser.

This design of laser results in a device with an extremely small footprint that has low maintenance and running costs. Applications include: cutting, welding, and surface engineering.

designs. For this reason this type of laser is suitable for a wide range of welding and surface treatment applications. Transversely Excited Atmospheric (TEA) Pressure

Fast Transverse Flow Laser

In the fast transverse flow laser (Figure 9a) the gas flow, electrical discharge, and the output beam are at right angles to each other (Figure 9b). The transverse discharge can be high voltage DC, RF, or pulsed up to 8 kHz (Figure 9c). Very high output power per unit discharge length can be obtained with an optimal total pressure ðPÞ of ,100 Torr; systems are available delivering 10 kW of output power, CW or pulsed (see Figures 3a and b). The increase in total pressure requires a corresponding increase in the gas discharge electric field, E; as the ratio E=P must remain constant, since this determines the temperature of the discharge electrons, which have an optimum mean value (optimum energy distribution, Figure 2) for efficient excitation of the population inversion. With this high value of electric field, a longitudinaldischarge arrangement is impractical (500 kV for a 1 m discharge length); hence, the discharge is applied perpendicular to the optical axis. Fast transverse flow gas lasers provided the first multikilowatt outputs but tend to be expensive to maintain and operate. In order to obtain a reasonable beam quality, the output coupling is often obtained using a multipass unstable resonator. As the population inversion is available over a wide rectangular cross-section, this is a disadvantage of this arrangement and beam quality is not as good as that obtainable from fast axial flow

If the gas total pressure is increased above , 100 Torr it is difficult to sustain a stable glow discharge, because above this pressure instabilities degenerate into arcs within the discharge volume. This problem can be overcome by pulse excitation; using submicrosecond pulse duration, instabilities do not have sufficient time to develop; hence, the gas pressure can be increased above atmospheric pressure and the laser can be operated in a pulsed mode. In a mode locked format optical pulses shorter than 1 ns can be produced. This is called a TEA laser and with a transverse gas flow is capable of producing short high power pulses up to a few kHz repetition frequency. In order to prevent arc formation, TEA lasers usually employ ultraviolet or e-beam preionization of the gas discharge just prior to the main current pulse being applied via a thyratron switch. Output coupling is usually via an unstable resonator. TEA lasers are used for marking, remote sensing, range-finding, and scientific applications.

Conclusions It is 40 years since Patel operated the first high power CO2 laser. This led to the first generation of lasers which were quickly exploited for industrial laser materials processing, medical applications, defense,

398 LASERS / Carbon Dioxide Laser

Figure 9 (a) Transverse flow carbon dioxide laser gas recirculator, (b) Transverse flow carbon dioxide electrodes, (c) Transverse flow carbon dioxide gas discharge as seen from the output window.

and scientific research applications; however, the first generation of lasers were quite unreliable and temperamental. After many design iterations, the CO2 laser has now matured into a reliable, stable laser source available in many different geometries and power ranges. The low cost of ownership of the latest generation of CO2 laser makes them a very attractive commercial proposition for many industrial and scientific applications. Commercial

lasers incorporate many novel design features that are beyond the scope of this article and are often peculiar to the particular laser manufacturer. This includes gas additives and catalysts that may be required to stabilize the gas discharge of a particular laser design; it is this optimization of the laser design that has produced such reliable and controllable low-cost performance from the CO2 laser.

LASERS / Carbon Dioxide Laser 399

List of Units and Nomenclature The direct excitation of carbon dioxide (CO 2) ground state molecules (sec21) b The direct de-excitation of nitrogen (N2) (sec21) g The direct excitation of nitrogen (N2) ground state molecules (sec21) h The direct de-excitation of carbon dioxide (CO2) (sec21) l wavelength (m) s absorption coefficient (cm2) t Electrical current pulse length (ms) tsp Spontaneous emission life time of the upper laser level (sec) A ¼ ðtsp Þ21 The Einstein ‘A’ coefficient for the ¼ 0:213ðsec21 Þ laser transition c velocity of light (cm sec21) Cc Coupling capacitance (nF) CO2 Carbon dioxide e subscript ‘e’ refers to the fact that the populations in the square brackets are the values for thermodynamic equilibrium E Electric Field (V cm21) FCO2 Fraction of the input power (IP) coupled into the excitation of the energy level E000 1 FN2 Fraction of the input power (IP) coupled into the excitation of the energy level Ev¼1 g gain (cm21) g1 and g2 energy level degeneracy’s of levels 1 and 2 He Helium I beam irradiance (W cm22) I0 beam irradiance at the center of a Gaussian laser beam (W cm22) Ip Photon population density (photons cm23) Ip Input current (A) IP Electrical input power (W cm23) J the rotational quantum number K51 ; K15 Resonant energy transfer between the CO 2 (0001) and N2ðv ¼ 2Þ energy levels proceeds via excited molecules colliding with ground state molecules (sec21) K132 ; K2131 ; are vibration/vibration transfer K2231 rates (sec 21) between energy levels 1 and 32; 21 and 31; 22 and 31, respectively (see Figure 1)

K320

a

Ksp ; A L

n N2 P PCO2 ; PHe and PN2 Pin r RB RF t T T0 w

is a vibration/translation transfer rate (sec21) between energy levels 32 and 0 (see Figure 1) Spontaneous emission rate (sec21) The distance between the back and the front mirrors, which have reflectivity’s of RB and RF (cm) molecular population (molecules cm23) Nitrogen Pressure (Torr) The respective gas partial pressures (Torr) Electrical input power (kW) radius of laser beam (cm) Back mirror reflectivity Front mirror reflectivity time (sec) Temperature (deg K) the photon decay time (sec) the radial distance where the power density is decreased to 1=e2 of its axial value

See also Fiber and Guided Wave Optics: Overview. Lasers: Noble Gas Ion Lasers.

Further Reading Anderson JD (1976) Gasdynamic Lasers: An Introduction. New York: Academic Press. Chatwin CR, McDonald DW and Scott BF (1991) Design of a High P.R.F. Carbon Dioxide Laser for Processing High Damage Threshold Materials. Selected Papers on Laser Design, SPIE Milestone Series, pp. 425 – 433. Washington: SPIE Optical Engineering Press. Cool AC (1969) Power and gain characteristics of high speed flow lasers. Journal of Applied Physics 40(9): 3563. Crafer RC, Gibson AF, Kent MJ and Kimmit MF (1969) Time-dependent processes on CO2 laser amplifiers. British Journal of Applied Physics 2(2): 183. Gerry ET and Leonard AD (1966) Measurement of 10.6-m CO2 laser transition probability and optical broadening cross sections. Applied Physics Letters 8(9): 227. Gondhalekar A, Heckenberg NR and Holzhauer E (1975) The mechanism of single-frequency operation of the hybrid CO2 laser. IEEE Journal of Quantum Electronics QE-11(3): 103. Gordiets BF, Sobolev NN and Shelepin LA (1968) Kinetics of physical processes in CO2 lasers. Soviet Physics JETP 26(5): 1039. Herzberg G (1945) Molecular Spectra & Molecular Structure. Infra-red and Raman Spectra of Polyatomic Molecules, vol. 2. New York: Van Nostrand.

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Hoffman AL and Vlases GC (1972) A simplified model for predicting gain, saturation and pulse length for gas dynamic lasers. IEEE Journal of Quantum Electronics 8(2): 46. Johnson DC (1971) Excitation of an atmospheric pressure CO2 – N2 – He laser by capacitor discharges. IEEE Journal of Quantum Electronics Q.E.-7(5): 185. Koechner W (1988) Solid State Laser Engineering. Berlin: Springer Verlag. Kogelnik H and Li T (1966) Laser beams and resonators. Applied Optics 5: 1550– 1567. Levine AK and De Maria AJ (1971). Lasers, vol. 3. New York: Marcel Dekkar, Chapter 3. Moeller G and Ridgen JD (1965) Applied Physics Letters 7: 274. Moore CB, Wood RE, Bei-Lok Hu and Yardley JT (1967) Vibrational energy transfer in CO2 lasers. Journal of Chemical Physics 11: 4222. Patel CKN (1964) Physics Review Letters 12: 588.

Siegman A (1986) Lasers. Mill Valley, California: University Science. Smith K and Thomson RM (1978) Computer Modeling of Gas Lasers. New York and London: Plenum Press. Sobolev NN and Sokovikov VV (1967) CO2 lasers. Soviet Physics USPEKHI 10(2): 153. Svelto O (1998) Principles of Lasers, 4th edn. New York: Plenum. Tychinskii VP (1967) Powerful lasers. Soviet Physics USPEKHI 10(2): 131. Vlases GC and Money WM (1972) Numerical modelling of pulsed electric CO2 lasers. Journal of Applied Physics 43(4): 1840. Wagner WG, Haus HA and Gustafson KT (1968) High rate optical amplification. IEEE Journal of Quantum Electronics Q.E.-4: 287. Witteman W (1987) The CO2 Laser. Springer Series in Optical Sciences, vol. 53.

Dye Lasers F J Duarte, Eastman Kodak Company, New York, NY, USA A Costela, Consejo Superior de Investigaciones Cientificas, Madrid, Spain q 2005, Elsevier Ltd. All Rights Reserved.

Introduction Background

Dye lasers are the original tunable lasers. Discovered in the mid-1960s these tunable sources of coherent radiation span the electromagnetic spectrum from the near-ultraviolet to the near-infrared (Figure 1). Dye lasers spearheaded and sustained the revolution in

atomic and molecular spectroscopy and have found use in many and diverse fields from medical to military applications. In addition to their extraordinary spectral versatility, dye lasers have been shown to oscillate from the femtosecond pulse domain to the continuous wave (cw) regime. For microsecond pulse emission, energies of up to hundreds of joules per pulse have been demonstrated. Further, operation at high pulsed repetition frequencies (prfs), in the multikHz regime, has provided average powers at kW levels. This unrivaled operational versatility is summarized in Table 1. Dye lasers are excited by coherent optical energy from an excitation, or pump, laser or by optical energy from specially designed lamps called flashlamps. Recent advances in semiconductor laser

Figure 1 Approximate wavelength span from the various classes of laser dye molecules. Reproduced with permission from Duarte FJ (1995) Tunable Laser Handbook. New York: Academic Press.