High average power solid state lasers in the 2 - 3 kW-range with fiber transmission are now available and higher power systems are under development in the labs. A review is given on the latest efforts to increase output power and improve beam quality. Highlights are the lamp pumped systems: 3 kW-Nd-YAG, cw and pulsed, 1.7 kW-Q-switched Nd:YAG, 0.2 kW-Ti- Sapphire, 0.1 kW-fundamental-mode laser, using phase conjugation. Besides these conventional systems new pumping schemes are operating in the lab. Diode pumped lasers up to 1 kW were realized and lamp pumped systems up to 10 kW are within reach in the next years. A very exciting new development is the direct application of high power diode lasers in material processing. Diode output powers of about 50 W can be transmitted by 500 micrometers - fibers, 100 W will be available in the near future. Although many efforts were made to grow better crystals, the results are disappointing with concern to high power. Only one promising new system was proposed recently and is under investigation in the lab, the Ytterbium-YAG crystal.

For most of the past twenty years, the development of carbon dioxide laser sources for industrial material processing has seen a strong trend away from diffusion cooling in favour of forms of convective cooling using gas flow techniques. For many years the latter provided the only effective route to high power discharges, and hence to lasers of modest resonator lengths at output powers of a kilowatt and higher. However, recently there has been a revival of interest in the possible use of discharge technologies based on diffusion-cooling for the excitation of lasers for materials processing and other high power laser applications. This possibility has arisen because the potential has been recognised for significant practical improvements in laser performance characteristics, in overall device compactness and in capital and running costs. This renewed interest in diffusion-cooled devices is based on the realisation that, under certain conditions, it is possible to: i) operate transverse capacitative radiofrequency dischar. between metallic electrodes of large area at very high power densities, under conditions where high volumetric power optical extraction may be achieved, even though the gas is static and the cooling is by diffusion; ii) construct lasers where the laser output power scales as the area of the discharge rather than as the discharge length as for conventional carbon dioxide lasers (both diffusion-cooled and convection-cooled devices); iii) design resonators for gain media ofunconventional geometry, with which it is possible to extract laser beams of high optical quality. Although isolated reports ofprevious work exist 1,2,the origins ofthe evolution ofcurrent work on large area lasers lie in research on RF excited lasers in the early 1980s, which itself drew on earlier work on hollow waveguide resonator devices6. This basis, coupled with parallel research on parallel plate transverse RF discharges7' led to research on large electrode area carbon dioxide devices in both planar slab8 and annular9 conflgnrations. Subsequently, veiy compact devices at power levels to 240W were demonstrated using hybrid waveguide-conlocal unstable resonators10, and later this approach was scaled to power levels in excess of 1 kW'1'2 The past few years has also seen commercial exploitation of this technology13'14, as well as the extension ofannular large area RF devices towards 2 kW 15• Inthis paper, a brief review is presented of the principal factors involved in the design of high power area-scaled lasers, including an outline of some of the relevant radiofrequency discharge physics, and a discussion of several types of optical resonators which may be used in combination with large area gain media to produce high quality laser beams

Results of power scaling experiments on high power CO₂ lasers for industrial applications are given. Rf-excited systems with fast axial gas flow and fast gas flow in an annular discharge gap (coaxial flow) are compared. A resonator for coaxial lasers delivering a beam with high focusability is described.

A compact size high power Q-switched CO₂ laser has been developed using a combination of intense pulsed radio frequency discharge excitation and a high speed rotating Q-switch chopper. Performance characteristics have been investigated in detail as functions of input discharge energy, Q-switching speed, pulse repetition frequency, laser gas composition and oscillation wavelength. Typical output characteristics for free running oscillation wavelength (10P20) show -500 kW pulse peak power, 170 mJ pulse energy, 250 ns pulse duration (FWHM) and 680 W average power at 4 kHz repetition frequency.

Q-switched low pressure CO₂ lasers allow the production of maximum laser pulse power and both high average laser power at high pulse repetition rates. The laser beam power can be calculated by a six temperature model including rotational relaxation, intramode and intermode vibrational energy exchange. A Q-switched CO₂ laser oscillator amplifier systems consisting of a DC discharged, wavelength tunable and diffusion cooled oscillator and a microwave excited amplifier is described. By a mechanical Q-switching of the oscillator laser pulses with a pulse repetition rate of 4 - 200 kHz and TEM₀₀ beam quality can be produced. After amplification the peak power reaches 0.9 MW at a pulse repetition rate of 10 kHz and 0.2 MW at 100 kHz repetition rate. The pulse duration (FWHM) is about 0.2 microsecond(s) . The maximum average power of 7 kW in the Q-switched mode reached nearly the maximum cw laser power of 10 kW (with TEM₀₀ beam quality). The laser pulse duration can be shortened by the combination of cavity dumping and electro-optical Q-switching. This pulsed operation leads to laser pulses with 35 ns pulse duration and 1.9 MW maximum power at a pulse repetition rate of 10 kHz and an average laser power of 0.8 kW. The self-oscillation of the oscillator amplifier system can be suppressed by an optical shutter between oscillator and amplifier.

Application of radio frequency excitation to transverse flow CO laser has led to the successful realization of high output-high-power high-conversion-efficiency oscillation at room temperature. The operating performance of the laser system thus developed has been examined in such aspects as the effects brought by changes in the laser operating parameters on the discharge and laser output characteristics, and on the beam profile. In experiments performed to date, an output power of 992 W at a conversion efficiency of 15% has been achieved with a gas temperature of 286 K, using gas made up of He, N₂, CO and O₂, without Xe addition. Compared with operation at cryogenic temperature, room temperature operation has proved to ensure more stable and more uniform excitation over the entire discharge space, and to permit extraction a beam of distinctly higher quality, thus demonstrating the superiority of room temperature operation.

The first experimental results on a repetitively pulsed e-beam sustained discharge CO-laser operating at room temperature are presented. The active laser volume was 3.41. A laser gas mixture of CO:N₂:He equals 1:6:7 has been used in a pressure range between 300 mbar and 600 mbar. The pumping current pulse duration was 7 microsecond(s) . The laser resonator consisted of a concave rear copper mirror (R equals 9 m) and a plane ZnSe output mirror of 97% reflectivity. The laser operated in a burst mode, the burst length varying between 0.1 s and 10 s at pulse repetition rates between 1 Hz and 50 Hz. The maximum laser energy of a single pulse was 24 J, at 45 Hz pulse repetition rate a medium output power of about 1 kW has been obtained. Further improvements of the laser and its application potential are discussed.

In this paper we introduce some kinds of the high power industrial CO₂ laser which researched successfully and several special techniques used among them.

Paint stripping of aircraft with pulsed laser radiation has several advantages compared to traditional methods of depainting: selective removal of individual layers possible, suitable for sensitive surfaces, workpiece ready for immediate repainting, and considerable reduction of contaminated waste. For paint stripping of large aircraft pulsed lasers with average power of at least 2 kW are required. Amongst the various types of pulsed lasers technical and economical considerations clearly favor TEA CO₂ lasers for this application. The first commercially available TEA CO₂ laser with an average power in excess of 2 kW, especially designed for depainting, has been developed by Urenco. The key data of this laser are: pulse energy up to 9 J, repetition rate up to 330 Hz, and beam quality: `flat top'.

The RF-discharge region of a gasdynamically cooled carbon monoxide laser has been optimized. Thereby the laser power was increased from 2.5 kW to a value of 7 kW (stable resonator) with an efficiency of 14%. An unstable resonator has been applied to improve the beam quality resulting in a 4.7 kW laser beam with a total divergence of 2.5 mrad. The beam quality is strongly influenced by water vapor absorption in the beam delivery system. The design of the RF-excited laser will be described and experimental results will be presented.

In this contribution we present a high power cw CO laser operating at room temperature. A scheme of forced convective cooling by fast axial gas flow combined with microwave discharge excitation has been developed. A T-shaped laser tube made of Pyrex glass with 5.1 cm i.d. is located in a microwave resonator of 12.5 cm (W) X 5.6 cm (H) X 30 cm (L) in dimension. The microwave at 2.45 GHz of up to 6 kW power generated by a magnetron is coupled into the resonator using the R26 waveguide technique. The laser gas flows through the T-shaped laser tube and is circulated by a rootspump in a closed loop. Two water cooled heat exchangers cool away the waste heat and the compression heat. The optical cavity consists of a total reflecting gold coated copper mirror and an output coupling flat from CaF₂ with 10% transmission. A maximum output power of 440 watts (or 1.4 kW/m) has been obtained from this laser system with a maximum efficiency of 8%. The laser characteristics have been studied. It is found that both Xe and O₂ are important for the high power CO laser operation at room temperature. The laser is characterized by the modular construction of the discharging section, so that it can be easily extended to a multimodular system to increase the laser output power.

Mathematical model of subsonic discharge CO laser is presented. Boundary layer type equations are used. This approach permits to define the value of the thermal flow to the wall for lasers in axisymmetrical channel with cooling. In some cases the laser parameters are expedient to calculate on the base of one dimensional gasdynamic equations. With the use of the one dimensional gasdynamic equations experimental data of various installations are analyzed. The influence of temperature on laser characteristics is studied. Theoretical optimization of laser parameters is presented.

The performance of high power gas dynamic laser facility for application in technology of heavy industry branches using high energy consumption and large scale equipment is described. Design of supersonic diffuser which allowed to obtain high pressure recovery is presented. Preliminary results of gas dynamic laser test runs are provided.

High power high repetition rate TEA CO₂ laser has potential importance in material processing such as shock hardening, glazing, drilling, welding, and cutting for high damage threshold materials, as well as in chemical reaction and isotope separation. This paper describes a transverse-flow closed-cycle UV-preionized TEA CO₂ laser with peak pulse power of 20 MW, maximum average power of 1.5 KW at repetition rate of 300 HZ. The laser has compact constructure of gas flow circulation system using tangential fans. With addition of small amounts of H₂ and CO to the normal CO₂-N₂-He gas mixture, one filling sealed operating lifetime is up to millions of pulses. A novel spark gap switch has been developed for very high repetition rate laser discharge in the condition of high pulse power.

The properties and possible application fields of several comparatively new types of resonators are discussed. Among them are: (1) Unstable resonator with semi-transparent output mirror. This scheme provides an increase in axial luminous intensity. (2) Half-confocal stable resonator with diffraction output coupling. This resonator comprises a big concave and a little plane mirrors; its properties are similar to those of an unstable resonator with spatial filtration (SFUR) proposed by Gobbi et al, but the half-confocal resonator is simpler and more convenient. (3) Multipass unstable resonator of high stability with regard to misalignments. This resonator consists only of large curvature concave mirrors and has the axis position stability by two-three orders better than the conventional arrangements. Resonator comprises a Sagnac interferometer with splitting into two beams rotating in mutually opposite directions, which has low sensitivity with regard to azimuthal inhomogeneities. The possibility of designing a Sagnac interferometer made up only of non-transparent mirrors is discussed. (5) Resonator with high effective length based on an astigmatic telescope transforming the annular into rectangular beam cross section. This scheme is perhaps one of the best for lasers with annular cross section of the active medium.

High energy laser (HEL) beams propagating through the earth's atmosphere are influenced by a wide variety of effects which, in general, reduce the beam irradiance (or, power concentration) at the target. In this paper an overview of atmospheric effects modeling for HEL systems is presented. The most important atmospheric propagation effects for cw HEL systems include transmission losses, turbulence, and thermal blooming. The general characteristics of these effects are described including scaling parameters and models useful for estimating their effects on HEL beam propagation. The wavelength dependence for propagation of an uncompensated cw HEL beam over a sea level path is shown by comparing the peak target irradiance for seven laser wavelengths ranging from the 0.337 micrometers N₂ laser to the 10.59 micrometers CO₂ laser. The use of adaptive optics for compensation of turbulence and thermal blooming effects is discussed and its effectiveness compared for near- field/ground-to-space and distributed/horizontal path propagation effects. Adaptive optics compensation is shown to be much more effective and the system requirements less stressing for near-field rather than distributed propagation paths since the turbulence induced scintillation effects tend to be smaller and the compensation is less likely to increase the blooming effects for near-field paths.

The progress in efficiency and beam quality parameters of fast-flow CO₂-lasers depends upon application of new ideas and results of fundamental and applied researches of gas discharge and beam quality problems. Recent results of gas discharge researches conducted by NICTL are shortly presented, in particular, numerical and experimental modeling of large- area, full-scale discharge systems with 2D segmentation of electrodes. A new approach to the philosophy of discharge stability has been developed. Some fundamental results on discharge homogeneity and stability problems have been obtained. The problems of beam quality are being investigated in NICTL as well, in particular, inhomogeneities of active medium gain and refraction coefficients caused by above mentioned inhomogeneities of discharge plasma and by gas flow turbulence. A positive influence of the turbulent diffusion on gain homogenization has been shown. Some fundamental results on nonlinear interaction of the beam with active medium are presented, including the laser radiation induced development of large scale and small scale (turbulent) inhomogeneities. Several approaches to obtaining pulsed mode of generation of basically CW CO₂-lasers are being developed, such as Q-switching by a molecular gas cell, utilization of controllable mirrors and modulation of discharge input power.

The method is proposed for calculation of lasers with unstable resonators which is based on Chebyshev polynomial approximation of nonlinear gain in ray coordinates of opposite waves simultaneously. Method is applied to calculation of gas dynamic laser which is known as active media with strongly nonuniform intensity distribution in unstable resonator.

The three types of laser resonators are newly proposed for the low gain, large aperture lasers such as COIL. The calculation clearly show the usefulness of these resonators.

Shock waves and disturbed gas generated by an excitation discharge in an excimer-laser cavity have been visualized by the shadowgraph technique. The influences of HCl and Xe concentration on the generation of shock waves have been investigated in a He/Ne gas mixture. At the higher HCl concentration, the distribution of gas density in the discharged region, where the gas is rarefied due to the expansion, seems to be strongly jagged, and the strong shock waves propagate toward the upstream and the downstream directions along the flow axis. At the lower HCl concentration even with the high Xe concentration, on the other hand, the density distribution in the heated column is fairly smooth and the shock waves are very weak. We have also successfully demonstrated the double-pulse glow discharge of a pulse interval of 200 microsecond(s) using a high-speed He-Ar gas flow of approximately 200 m/s. At the pulse interval of 100 microsecond(s) , however, the second discharge, which seems to be arc, occurs through the heated column which is swept downstream but still contains the waste products of the first discharge.

The wavefront of the laser beam propagating through the atmosphere is distorted temporally and spatially due to the atmospheric turbulence, resulting in beam spread, intensity fluctuation, and beam wander on the target. The adaptive optical system can compensate the wavefront distortions of the laser beam by deforming the mirror surface to match the conjugate wavefront. Thus, by using an adaptive optical system, the laser beam could be continuously directed to a certain distant position with the near diffraction—limited spot size under the turbulence. The multidither Coherent Optical Adaptive Technique (COAT) is one of several existing adaptive optical techniques, whose primary merit is to be able to correct the wavefront remotely with the signals returned from the target and need no complicated wavefront sensors. In the 1970's, Pearson et al.1'2 first fabricated the multidither COAT system for a visible (argon ion) laser, and demonstrated the effectiveness of the COAT system in compensating its wavefront distorted by atmospheric turbulence. However, since the first demonstration, considerable few experiments of atmospheric compensation using the COAT system for CO2 laser have been reported only except for several papers describing the usage as an intracavity adaptive optics.36 We fabricated the COAT system to correct the distorted wavefront of the CO2 laser beam,7 and performed the in—house experiments to evaluate the system performance and optimize it. In turn, preliminary experiments for atmospheric compensation were conducted, in which our COAT system was integrated with a large Cassegrainian type telescope and tried to correct the distortions of a focused CO2 laser beam. In this paper we describe the characteristics of our COAT system compensating the beam wander and the intensity fluctuation due to the atmospheric turbulence.

We have investigated higher-order anti-Stokes Raman scattering with injection of Stokes or anti-Stokes pulses for the efficient generation of vacuum ultraviolet radiation. Spatial profiles of anti-Stokes pulses were modified and their energies were increased with injection of pulses with the wavelength of the first Stokes pulse to the pump pulse of the fourth harmonic of Nd:YAG laser. Enhancement factors become larger for the higher orders of anti-Stokes pulses. The shift of optimum pressure together with the change of profile from ring to Gaussian-like shape induced by injection explains that the enhancement effect is due to adjustment of phase matching condition for anti-Stokes generation.

Lasers that are equipped with stable optical resonators having large to very large Fresnel numbers and uniform active medium with respect to gain and refractive index, are running multimode, and the highest order transverse mode has the largest mode coefficient. By using this dominant mode feature and an incoherent superposition of the coherent modes, a simple approximate relation was inferred, that allows to estimate beam quality factor M² from the geometrical data of the cavity. A way to evaluate the global degree of coherence of dominant mode laser beams by using a Gauss-Schell model beam having the same beam quality factor M², is proposed.

Computer models have been developed for the beam mode analysis of CO laser. The models are improved from the authors' previous one which was deduced from asymptotic method. In order to analyze by the variety of PCs (personal computers) and workstations, two types of computer codes have been developed. The compact code is described in language C, which can be compiled by most of PCs. The simple code is described in Mathematica, of which high- level mathematical functions are useful. Both codes are carried out under the same conditions and compared.

An investigation of the spatial and temporal properties of the laser pulse in the far field of an pulsed electron-beam sustained CO₂ laser was made. The data obtained show subtle details of the intensity distribution, and different pulse shapes for different concentric apertures.

The results of experiments on four-wave interaction of electron-beam-controlled discharge long pulsed CO₂-lasers' (t-20 microsecond(s) ) and CO- lasers' (t approximately 200 microsecond(s) ) radiation inside their active medium are presented. Linearly polarized probe CO₂ (CO-) laser beam intersects strong electromagnetic waves counterpropagating through an inverted medium inside the laser resonator. The laser beam reflected from the active medium has been registered both in near-field zone and in far field one. The experiments on recovering of optical images in near-field zone and recovering of angular divergency of laser radiation in far-field zone demonstrated that reflected beam was phase-conjugated one. The reflectivity of phase-conjugated beam was up to 2% for CO₂-laser and up to 0.2% for CO- laser. The time-history of phase-conjugation reflectivity and comparison with theoretical results are discussed.

The results on investigation of UV ((lambda) equals 271 nm) Cu vapor laser sum frequency ((lambda) 1 equals 510 nm, (lambda) 2 equals 578 nm) generation in crystal DKDP are presented. For interaction type ooe the mean UV power about 0.2 W and conversion efficiency (eta) approximately equals 2.5 + 3% are obtained for laser mean power about 7 W. Beam divergence, yellow and green line pulse amplitudes and relative delay are of key importance for sum frequency generation. Optimization of standard 20 W mean power Cu vapor laser was necessary for efficiency frequency conversion. The possibility is shown to create 5 divided by 10 W source of UV radiation, using Cu vapor laser sum frequency generation in DKDP.

This is a review of tl Chemical Oxygen - Iodine Laser (COIL) technology, paying particular attention to historical perspectives in terms of highlighting the unique characteristics of COIL that have allowed it to develop into a high efficiency and a very forgiving system. Chemical pumping of lasing transitions has been around for a long time (HCI Lasers in 1965 and HF/DF Lasers in 1972), but most of the reaction energy ends up as heat in the lasing medium, generating the associated system engineering difficulties. COIL, on the other hand, has a very high efficiency in converting reaction energy to electronic excitation, and that fraction of the reaction energy which is released as heat does not end up in the laser cavity, but remains in the oxygen generator. This has tremendous engineering advantages for high efficiency systems.

A 5 KW supersonic Chemical Oxygen-Iodine Laser (COIL) has been constructed. A rotating mesh type singlet-delta oxygen generator (SOG) is used that appears to be simpler, lighter in weight and more efficient than the rotating disk SOG. An output power of 1 kw from this supersonic COIL has been achieved by using the rotating mesh SOG at a Cl₂ flow rate of 150 mmole/sec. Theoretical modeling of supersonic COIL based on a simplified premixed model is given.

Results of experimental and theoretical study of the liquid jet O₂(¹(Delta) ) generator in P.N. Lebedev Institute Samara Branch are presented. This study includes hydrodynamic and gasdynamic effects, heat and mass transfer, droplet generation and separation. The dependence of O₂(¹(Delta) ) and Cl₂ yields on geometrical and physical parameters of generator are presented. High O₂(¹(Delta) ) yield and operation of small scale subsonic chemical oxygen-iodine laser up to 100 torr of active gas pressure in jet liquid generator have been achieved. Counterflowing jet liquid O₂(¹(Delta) ) generator is a very perspective for high power supersonic oxygen-iodine laser without water vapor trap and droplet separator.

This is an overview of the development of Chemical Oxygen-Iodine Laser (COIL) technology in the United States. Key technical developments will be reviewed, beginning in 1960 and culminating in 1977 with the first COIL lasing demonstration at the Air Force Weapons Laboratory (now the Phillips Laboratory). The discussion will then turn to subsonic laser development, supersonic lasing demonstration and efficiency improvements, and finishing with a brief discussion of some spin off COIL technologies. Particular emphasis will be placed on how the O₂ (¹(Delta) ) generator and O₂-I₂ mixing nozzle technologies evolved.

Parametric studies of the gain and the power of a small scale supersonic chemical oxygen- iodine laser are presented. The laser is of 5 cm long active medium, and utilizes a simple sparger-type O₂(¹(Delta) ) chemical generator and a medium size pumping system. A grid nozzle is used for iodine injection and supersonic expansion. 45 W of CW laser emission at 1315 nm are obtained in the present experiments. The small size and the simple structure of the laser system and its stable operation for long periods make it a convenient tool for studying parameters important for high power supersonic iodine lasers and for comparison to model calculations. The gain and the lasing power are studied as a function of the molar flow rates of the various reagents, and conditions are found for optimal operation. Good agreement is found between the experimental results and calculations based on a simple 1D semi-empirical model, previously developed in our laboratory and modified in the present work. The model is used to predict optimal values for parameters affecting the laser performance that are difficult to examine in the present experimental system.

A supersonic flow chemical oxygen-iodine laser is developed utilizing a jet-type singlet oxygen generator and the output power of several hundred watts is successfully attained. In the present study, the reaction zone structure of the supersonic flow chemical oxygen-iodine laser is also simulated solving the 2D Navier-Stokes equations. The numerical results clearly show that the enhancement of mixing is indispensable to improve the laser performance.

Iodine dissociation has been measured in the supersonic cavity of a chemical oxygen-iodine laser during lasing under a wide variety of flow conditions. By varying flow conditions, measured dissociations from 0 to 100 percent were observed. A simple model of the initial step in the dissociation process was developed that adequately rationalizes the measurements.

A study was made which examines the potentials of subsonic and supersonic oxygen-iodine lasers for industrial applications. Comparisons with CO₂ and Nd:YAG lasers are given. After a short introduction into the theory and design of the Chemical Oxygen Iodine Laser (COIL), a model is presented which enables calculation of laser kinetics and gasdynamics. This model is subsequently used to perform an analysis of COIL systems and to calculate costs of investment and operation. In addition, potential advantages in material processing are surveyed. As a result it is found that operating costs are significantly higher than for the classical processing layers if they are compared on the basis of equal power. But, advantages in the production process may overcompensate these higher costs.

The Chemical Oxygen Iodine Laser (COIL) is an attractive candidate for easy power scaling at short wavelengths. High specific power output from supersonic operation leads to compact devices. The German Aerospace Research Establishment (DLR) has recently completed construction and assembly of a multikilowatt supersonic COIL and started experimental investigations at its Lampoldshausen rocket test site. The excited oxygen is produced by a rotating disk generator, provided by the Phillips Laboratory, Albuquerque, NM. Currently the laser is operated without a cold trap. After the injection of the iodine, the laser gas is expanded to an isotropical Mach number of 1.8 by a multi-element grid nozzle. At present, laser power is extracted with a single pass stable resonator located immediately after the nozzle exit plane. Pressure is recovered by a supersonic diffusor and a 3-stable pumping system.

The spatial distribution of gain has been investigated on the Research Assessment and Device Improvement Chemical Laser, a supersonic chemical oxygen-iodine laser (COIL). A frequency-stabilized, narrow linewidth diode laser system operating on the F equals 3 yields F equals 4 hyperfine levels of the (²P_1/2) to (²P_3/2) spin-orbit transition in atomic iodine was used as a small signal probe. A peak gain of 1.2%/cm was measured along the horizontal centerline of the single-slit, supersonic nozzle is about two times greater than measurements made on ReCOIL and compares favorably with measurements made on the Rotating Disk Generator (RotoCOIL) device. Gain distribution was investigated under three I₂ flow conditions. Scans across the supersonic expansion indicate a gradient in gain distribution due to higher gas temperatures along the walls and mixing phenomena.

The behavior of the ²P_1/2 yields ²P_3/2 lasing transition in an atomic iodine in the presence of a magnetic field has been studied. The small-signal gain of a chemical oxygen iodine laser in the presence of a magnetic field was measured by utilizing a semiconductor laser. Gain measurements are given for (pi) and (sigma) components for field strength between 0 - 540 Gauss.

The output laser power of a small-scale subsonic Chemical Oxygen-Iodine Laser has been improved from 5 to 34 watts with the same gas pumping system. To obtain a good agreement between the model and the experiments we have introduced the temperature effects on the reaction kinetics. We show also that the addition of a buffer gas such as SF₆ achieves a significant cooling of the active medium, which in turn allows a better laser power extraction.

The pulsed operation of COIL is now a subject of interest. The different approaches are used to obtain such a mode. The method of instant volumetric iodine generation is considered. The experimental results are presented. The advantage and disadvantages, problems, perspectives and possible applications are discussed.

The general scalability of jet generators may be limited due to the inevitable breakup of the thin liquid jets. The fluid mechanic theory of liquid jets is reviewed. An experiment was set up to simulate jets of basic hydrogen peroxide and measure their velocity and breakup lengths as a function of the orifice diameter and length, and of the viscosity of the liquid. The experimental setup and its future extensions are described. The jets were run with atmospheric pressure into a chamber at low pressure. The results are compared to predictions from the simple theory and extrapolated to driving pressures of 10 bar.

This paper brings experimental results on a repetitively pulsed Chemical Oxygen-Iodine Laser (COIL) using a gain modulation or switching by means of an externally applied magnetic field, based on the Zeeman effect. A peak-to-average laser power enhancement factor and a power conservation factor were evaluated from these experiments in dependence on a magnetic field intensity and a length of magnetic pulses, as well as their shape. A critical view on the pulsed method used is presented.

We obtained 12.4 W visible radiation by Intracavity second harmonic generation of continuous wave chemical oxygen-iodine laser. A chemical oxygen-iodine laser with a maximum fundamental wave output power of about 10 W in a concentric cavity composition was used. Experiments were performed for three types of optical cavities using a 1 cm LBO crystal.

A closed-loop flowing basic hydrogen peroxide (BHP) system with real-time cooling was constructed and coupled to a supersonic COIL, resulting in a 20-min. continuous run at an average power of 500 W. An overall BHP heat transfer coefficient of 150 BTU/(hr(DOT)ft²(DOT) degree(s)F) was measured.

The chemical oxygen-iodine laser (COIL) has been studied at the Phillips Laboratory since its invention in 1978. One of the most difficult challenges in COIL technology is to produce constant power for more than a few seconds; an essential feature for most applications. The key to developing a laser with these operational characteristics is management of the heat released during the production of singlet delta oxygen. Approximately 10 joules is deposited into the singlet delta oxygen generator (SOG) for every joule extracted as laser power. A recent test series demonstrated run times of 120 seconds at 9 kW by controlling the SOG reaction temperature with a flowing aqueous solution of cold hydroperoxide (BHP). This method of managing the energy released is quite effective but requires a large reservoir of cold BHP.

A small scale experiment has been set up to investigate the deactivation of O²(¹(Delta) ). The effect of different buffer gases and defoamer on the O₂(¹(Delta) )-yield has been studied. With a deconvolution calculation water vapor can be measured by mass-spectrometry. Cold traps and Raschig-Rings were used to diminish the water content in the gas flow. O₂(¹(Delta) )-deactivation measurements show, that non-metallic materials should be preferred for construction of chemical oxygen-iodine lasers. To compare different experiments the O₂(¹(Delta) )-deactivation may be described by simple equations. Comparison of these equations with experimental data leads to the conclusion, that the reaction order of the O₂(¹(Delta) )-deactivation varies from 1.825th to 1st order in dependence of time and O₂(¹(Delta) )-partial pressure.

A hollow electrode fast flow oxygen RF discharge was examined experimentally as an alternative source of molecular singlet delta oxygen. The relative yield of singlet delta oxygen was measured under the following experimental conditions: the RF frequency 13.66 MHz, the RF power up to 500 W, the oxygen output pressure 0.5 - 2.5 Torr, the oxygen flow rate 0.1 - 2.5 Nl/min, the hollow electrode inner diameter 1 - 4 mm, the hollow electrode length 1 - 4 mm, the gas composition--99.5% and 99.995% oxygen, respectively. The singlet delta oxygen yield was increasing with the RF power and decreasing with the pressure. The dependence of the yield on the flow rate and the geometrical dimensions was not monotonous and exhibited an optimum. In such cases, the singlet delta oxygen yield up to 16% was achieved.

High-explosive charges were used in the early 1980's at Los Alamos National Laboratory to pump high-energy atomic-iodine lasers. Laser outputs at the kilojoule level were measured in a series of experiments. Two techniques were used to convert the high-explosive (HE) energy release to optical radiation for the photolysis of the perfluoroalkyliodide fuel. One technique used strong shockwaves propagating through argon gas and driven by the detonation as an intense optical pump source. The second approach used exploding metal films driven by megampere-level current pulses from explosive-driven magnetic flux compression generators. The optical extraction system for both types of single-pulse lasers was a power oscillator configuration using a stable resonator. The purpose of these experiments was to evaluate the scaling potential of HE-driven lasers for a number of applications including inertial confinement fusion. The HE field experiments were supported by a number of laboratory laser experiments. Exploding wires were used to pump 100-J atomic-iodine lasers (and 20-J molecular iodine lasers). Atomic-iodine lasers were also pumped with exploding metal films. In support of this work, several types of optical pump sources were characterized. These included HE-driven shockwaves in a variety of rare gases, exploding metal wires and films, surface discharges, ablating-wall flashlamps, and xenon flashlamps. Equivalent blackbody temperatures as a function of various parameters were measured for each source using absolutely calibrated photodetectors equipped with optical bandpass filters.

The single beam Asterix IV high power iodine laser ((lambda) equals 1.32 micrometers ) provides at present a maximum output energy of 2.1 kJ at a pulse length of 5 ns and an output power of 4 TW at 0.3 ns pulses. This laser is designed to deliver pulses with lengths ranging from 0.2 to 5 ns with a maximum power of 5 TW and an energy of up to 2 kJ. Asterix IV has been developed on the 10 years experience with the 1 TW Asterix III laser and with the support of a 1D and a 3D pulse propagation code. Special emphasis has been put on achieving a high overall system efficiency and laser beam intensity profile as homogeneous as possible. In this paper the measures for optimizing the laser performance and the results obtained will be described.

Alpha is a megawatt-class hydrogen fluoride, continuous wave, space based chemical laser brassboard which demonstrates and validates technology for space-based applications. It consists of a cylindrical gain generator that exhausts radially outward through circumferential nozzles forming an annular lasing media and an annular ring resonator, which extracts the laser energy. Technical innovations first demonstrated on Alpha include: (1) use of extruded aluminum components, (2) diamond turned, annular optics made of molybdenum, (3) uncooled silicon mirrors, (4) light weight optical benches, and (5) active alignment. Alpha first lased in 1989, and has repeatably demonstrated megawatt-class power and excellent beam quality. Using Alpha, TRW has demonstrated the use of low weight uncooled mirrors in very high power lasers to reduce system jitter. They have performed flawlessly and beam jitter levels were significantly reduced.

Overtone performance of the cw HF chemical laser was optimized by the same flow rates that optimized fundamental performance. When the absorption/scattering losses of the mirrors were taken into account, an overtone efficiency of 70 - 90% was achieved. The overtone efficiency was a strong function of medium saturation. There was no significant change in overtone power and efficiency as the mode volume increased. However, there was an increase in the number of lasing lines and a shift to higher rotational lines. Overtone performance was as sensitive to cavity pressure as fundamental performance. Rigrod theory showed that a higher medium saturation yields a higher overall overtone efficiency, but does not necessarily yield a higher measurable power (power in the bucket). For low absorption/scattering loss overtone mirrors and a 5% penalty in outcoupled power, the intracavity flux and hence the mirror loading may be reduced by more than a factor of two when the gain length is long enough to well saturate the medium. For the UIUC overtone laser which has an extensive data base with well characterized mirrors for which the Rigrod parameters g₀ and I_sat were firmly established, the accuracy to which the reflectivities of high reflectivity overtone mirrors can be deduced using measured mirror transmissivities, measured outcoupled power and Rigrod theory is approximately +/- 0.07%.

High-power pulsed lasers in repetitive mode show an output energy decrease of the laser beam. In such lasers, the characteristics time depending on the laser production effect is weak in front to those of the flow. Consequently, the decrease of the output energy is mainly due to the residual pressure and density perturbations, remaining in the laser cavity after the strong electric energy deposition. For a better understanding of the flowfield, a numerical approach is carried out using flux corrected transport algorithms (FCT methods). In previous works, numerical studies of the unsteady 2D flow in both excimer and chemical laser cavity have been presented. The limitations of the 2D modelizations are clearly put in evidence in this paper. The numerical method is extended in order to take into account the optical direction, which has never been modelized and to better understand the 3D flow evolution in the chemical laser cavity. Shock waves travelling in the optical direction, generated by a side effect of the electrodes are clearly put in evidence. These waves have a long time effect on the flowfield and lead to a high residual perturbation level, which is directly responsible of the output laser beam decrease.

During the reaction H(Dz)+F2 similar quantities of the chemical interaction energy are spent on the excitation of vibrational and rotational levels of 1W and DF molecules. However the known energy characteristics of pulsed chemical DF laser are 2—3 times lower than those of the HF laser [1J. It is connected on opinion of various researchers with larger (as compared with hF) number of vibrational arid rotational levels of molecule I)F among which the energy of chemical reactions is distributed with smaller values of Einstein coefficients and rate constants of chain reaction. lt results in lower value of gain for majority of vibrational-rotationai transitions of DF molecule. Therefore for extraction of laser energy from active medium of DF laser an optical resonator with small losses is required.

The compact, repetitively pulsed HF chemical laser built 2 years ago has been modified to increase the output laser energy. The discharge volume is now 1.4 1 instead of 0.54 1 for the previous electric discharge. Due to high voltage limitation, the discharge gap was only slightly increased. The discharge width along the flow direction was increased from 4 to 8 cm. To achieve an easily usable beam, a novel optical configuration consisting of two passes with 2 folding mirrors has been used. It gives a 35 X 35 mm² square output beam. To date, a maximum output laser energy up to 12 J per pulse was obtained at the HF wavelength using C²H₆ as the hydrogen donor. At DF wavelength the energy per pulse was about 3 J using D₂. In repetitive operation, the repetition rate was limited to about 65 Hz due to the available flow rate of the gas loop. The average power obtained was 610 W.

A thermodynamic approach to the problem of maximum efficiency for a chemical laser is developed. Assuming the laser radiation entropy to be negligibly small in comparison with the entropy of the chemically reacting compounds, thermodynamic limitations for the laser efficiency are obtained. A number of kinds of chemical reactions, each one being a source of laser pumping, are explored. It is shown that the most significant restrictions appear for reactions of a recombination type reducing the number of particles in the laser mixture. Photorecombination lasers are investigated in detail. The maximum efficiency for those lasers is computed over a wide range of parameters of the initial mixture.

The main objective of this work was to determine the extent to which lasing on the overtone suppresses the gain on the fundamental transitions P₁(4-9) and P₂(4-9) as a function of media saturation on the overtone. This was accomplished by a comparison of the residual fundamental gain (RFG) data obtained at three different levels of media saturation with the corresponding zero power gain (ZPG) data. Comparison of the residual fundamental amplification ratio (RF-AR) data with the zero power amplification ratio (ZP-AR) data indicated that the gains of the low J lines P₁(4-6) and P₂(4-6) were suppressed more than the gains of the high J lines even through their upper or lower levels were not directly involved in overtone lasing. Analysis of the HF mole/mass ratios calculated by a rotational nonequilibrium computer model, ORNECL, showed that the fundamental gains are determined by three independent mechanisms when lasing occurs on the overtone. The first mechanism is the `direct lasing effect' that depopulates the v equals 2 states and populates the v equals 0 states that are directly involved in overtone lasing. The second mechanism is the `rotational relaxation effect' that reduces the rate at which the low J v equals 2 states are populated and increases the rate at which the low J v equals 0 states are populated. The third mechanism is the `collisional deactivation effect' that reduces the rates at which the HF(O,J) and the HF(1,J) states that are not directly involved in overtone lasing are populated by the various collisional deactivation processes that transfer molecules from the high J v equals 2 states (that are involved in overtone lasing) to these lower energy states. Further analysis of the HF(v,J) concentrations and the ZPG and RFG calculations indicated that rotational relaxation is the primary mechanism responsible for the suppression of the low J lines whose upper and lower levels are not involved in overtone lasing.

To state more precisely our knowledge of the vibrational energy transfer with Cl₂ molecules in active media of the gas flow and chemical lasers, the study of nonequilibrium vibrational relaxation of the thermally heated and partially dissociated molecular chlorine in the supersonic flow is carry out. An experimental technique, based on the sensitive spectroscopic diagnostics and numerical gasdynamic and kinetical modeling is used. The parameters of vibrational kinetics of the molecular chlorine is found. The effect of chemically active Cl atoms on the rate of V-T deactivation of Cl₂ molecules is determined.

We discuss the effect of laser induced continuum structure and the modification of quasi bound state of atomic systems as well as its possible applicability towards amplification of short wavelengths in schemes in which no inversion of population is established.

An X-Ray preionized LC inversion discharge XeCl laser has been modified to work in the X- Ray phototriggered mode. X-rays are produced by a secondary emission electron gun. Homogeneous discharge are obtained at low electric field between 1.1 and 1.5 kV/cm.bar with good laser results: 1.2 J of laser energy at 2.6% efficiency and 1.5 J/liter specific energy. Moderate repetition rate is possible without arcing or missing shots.

Excimer lasers are the most efficient and powerful sources for ultraviolet radiation. Our new laser cavity design NovaTube^TM is the result of many years of extensive material research. Several test runs with NovaTube^TM lasers at 193 nm (ArF) and 157 nm (F₂) demonstrated excellent gas lifetime data when compared to conventionally designed lasers. For the first time a 50 W KrF laser was successfully operated sealed-off for more than 1 billion pulses. By this outstanding performance the operating costs can be cut 10 times using the new laser tube technology.

A parametric study of long pulse XeCl laser is undertaken at IMFM in the frame of EUREKA EU 205 program, with the aim to optimize such lasers in terms of efficiency and laser average power at high PRF. These developments have important issues in applications. In particular there is a need for short wavelength, high energy lasers with long pulse duration, in order to overcome the limitations of present laser sources for transmission through optical fiber, namely fiber input surface degradation and two photons absorption which are optical power dependent. An X-ray preionized spiker-sustainer XeCl excimer laser operating at high repetition frequency (1000 Hz) and 220 W of average power with 2.4% efficiency is demonstrated with use of LUX Test-Bed. Pulse energy and pulse duration (160 ns) are virtually independent of pulse rate frequency. Comparison with CLC circuit XeCl laser excitation for the same operating conditions shows superior properties of spiker-sustainer excitation for high frequency operation due to higher efficiency and discharge homogeneity and stability. As an example with CLC circuit excitation laser average power saturate at 400 Hz without acoustic dampers and at 800 Hz with use of dampers while with spiker-sustainer circuit no saturation is observed up to 1200 Hz (limited by available electrical power) without use of acoustic dampers. Experimental results on laser behavior at high PRF are presented as well as a study of acoustics perturbation induced by the discharge inside LUX by means of fast pressure piezoelectric probes. Pressure probes signals show a reduction by a factor 2.5 of pressure waves amplitude in the immediate vicinity of laser cavity with spiker-sustainer circuit as compared to CLC circuit. This strong decrease is attributed mostly to the higher discharge stability and also to the higher efficiency obtained with spiker-sustainer circuit.

A method of finding the time dependent resistance and inductance of the discharges in the switch system and laser chamber in pulsed gas lasers is described in the present work. According to this method the current waveform is digitized and the first and second derivative is calculated through a computer. For a certain time instant, substituting the value of the current and its first and second derivative into the integrodifferential equations describing the performance of the circuit loops, we form relationships which connect the values of the resistance and inductance for this particular time instant. Combining relationships originated from very closed adjacent time instants, the values of the resistance and inductance can be found. Scanning the entire time region of the discharge, the time history of the resistances and inductances of the discharges are revealed. Their behavior shows for the resistances an abrupt drop while for the inductances a sharp peak, both during the formation phase. After that the above characteristic quantities fluctuate slowly around constant values.

The efficient pumping of N₂ and He/Kr/F₂ (45:3:1) laser mixture by longitudinal electric discharge in the form of powerful breakdown ionization wave at driving pulse voltage of -120 kV amplitude, 40 ns duration, 3 ns rise time has been demonstrated. The developed gives us a possibility to excite a nitrogen and an excimer laser mixture with specific stopping energies about 2,1 J/cm³ and 1,9 J/cm³ corresponding as well as to produce laser action in N₂ with output power up to 100 kW in pulse having a duration of 10 ns FWHM with excellent laser beam quality.

Although much efforts have been made to develop higher performance excimer lasers mainly in Europe and Japan supported by the community and the government, an interdisciplinary subject in the paper title has not ever been investigated in detail. To take care of this un- attacked research region lying between various research groups, we tried to measure the beam quality of a discharge-pumped KrF laser under a rather high repetition rate operation. As we are interested in the diagnostics of various high speed phenomena, we measured the beam quality with advanced high speed photography. The spatial and temporal beam patterns were observed under various experimental conditions, and they were analyzed to start our high repetition-rate experiments. The results showed that an unstable resonator could produce high quality laser beams with the lower beam divergence angle.

Laser-pumped molecular lasers can be the source of laser wavelengths that are otherwise unavailable. In these laser sources a gas cell containing selected molecules is optically pumped with an available laser to generate output at a wavelength longer than that of the pump. For example, the CO₂ laser has long been used to generate far-infrared coherent radiation from optically pumped gases exemplified by methanol and heavy water. Indeed, laser-pumped molecular lasers have generated thousands of laser lines from the vacuum ultraviolet to the millimeterwave region since their inception in the early seventies. While these lasers are useful for specified wavelengths, they also offer the advantage of very good beam quality. The pump laser is not required to have good beam quality in order to achieve this performance. Interest in laser-pumped molecular lasers is expected to increase in the near future because of the needs of various users. Examples may be found in the mid-infrared region where compact sources are largely unavailable. The use of improved solid-state lasers or diode lasers as pump sources for molecular lasers will result in useful sources for remote sensing, ladar, and other applications.

We describe the results of an analytical and experimental program that is investigating the feasibility of developing mid-IR lasers based upon laser pumped gas phase molecules. We present results for lasing on overtone pumped HF, HCl, and DF. In addition, describe several possible excitation sources including diode lasers and alexandrite lasers.

Population inversions in small gas-phase molecules are produced by optical pumping of vibrational overtone transitions. Efficient frequency down conversion of the pump radiation is obtained by lasing on the inverted transitions. The performance of the lasers is governed by kinetic relaxation and energy exchange processes. The effect of these processes on the ultimate scalability of this class of laser will be discussed.

A design, as well as verification measurements, are presented for an end pumped, 20 Watt output power, single frequency, Tm:YAG laser driver for pumping a HBr mid-IR laser. Efficient end pumping of the Tm:YAG is achieved by `close lens coupling' 15 Watt average power, room temperature, 785 nm diode bars to several Tm:YAG rods. The Tm:YAG laser is operated single frequency (injection seeded) in order to couple efficiently its output to the narrow absorption band of HBr. A 2 micron laser operating multi-line, but with a bandwidth less than 1 - 2 GHz, is also under consideration using a HBr laser with increased pump absorption characteristics obtained by increasing the pressure or by placing the HBr laser inside the 2 micron laser using intra-cavity 2 micron pumping.

A long pulse molecular F₂ laser ((lambda) equals 157 nm) with an optical pulse width of 160 ns and an output energy of 1.7 J (4.6 MW/cm²) pumped by an electron beam has been realized. The only restriction for the optical pulse width of the laser seems to be the duration of the excitation pulse. No signs of self terminating laser pulses due to bottle-necking in the lower laser level have been observed.

The high pressure atomic xenon laser is becoming the most promising light source in the wavelength region of a few microns. The merits are high efficiency (so far up to 8 percent), high output energies (15 J/liter at 9 bar), high continuous output power (more than 200 W/liter), no gas dissociation and thermal heating of the lower laser level. Compared with the well-known low pressure xenon laser the power performance is now roughly a factor thousand higher. The operation of the system, based on three-body-collisions, uses the metastable state of the xenon atom as the ground state so that in the recirculation of energy a high quantum efficiency is obtained. Furthermore the homogeneous line broadening caused by the high collision frequency has also a strong beneficial effect on the efficiency. However, the required intense homogeneous excitation of the gas medium at high density is from a technical point of view a great challenge. From our experimental and theoretical work we found that at optimum performance the input power must be 1 to 2.5 [KW cm^-3 atm^-2]. We describe our results obtained with e-beam sustained and x-ray preionized systems delivering pulsed energies in the range of joules per liter. Furthermore we describe our recent results on continuous RF excited wave guide systems of about 37 cm length with output powers in the range of watts.

The highly selective Na₃ + X (Cl,Br,I) reactions have been shown to create a continuous electronic population inversion based on the chemical pumping of Na₂. Optical gain through stimulated emission has been demonstrated in regions close to 527, 492, and 460 nm ((alpha) equals 8 X 10^-3 cm^-1 for an individual rotational level at -527 nm). A device has ben constructed with a focus to increasing amplifier gain length and amplifying medium concentration based on the controlled intersection of supersonically expanded sodium and halogen atom sheaths. The interaction forms an extended reaction-amplification zone centered on-axis in an optical cavity, thus facilitating the conversion of the observed amplifiers to chemical laser oscillators. Initial results with this upscaled device, where the sodium metal expanded in both pure and seeded supersonic expansion is intersected by a bromine atom flow, provide the first example of chemically enhanced Raman scattering. Unique Raman signals are induced by and correlate with emission from the Na D-line components formed in the chemical reaction zone primarily as a result of Na₂ + Br yields Na^* + NaBr reaction, cannot be readily generated by light scattering due to an external light source, and appear to be enhanced by the environment of the reaction zone itself. The Na D-line emitters interact with cooled sodium dimers in a resonance Raman scattering process, for which computer simulations suggest a scattering linewidth, (Gamma) -4 cm^-1. These results suggest an unusually fast resonance Raman scattering process which appears to be chemically enhanced. The results of initial double pass gain measurements suggest that a stimulated Raman process, similar to that associated with optically pumped alkali dimer lasers, has been observed.

Pulsed photodissociation of iodine monobromide at 532 nm provides a high yield of spin-orbit excited atomic bromine. Near resonant electronic-to-vibrational energy transfer from Br(²P_1/2) to NO(v equals 2) is rapid, k equals 2.4 X 10^-12 cm³/molecule-s, and selective, with a branching ratio of NO(v equals 2) of 0.89 +/- 0.21. An NO(v equals 2 yields 1) laser operating at 5.4 microns was demonstrated at NO pressures from 0.1 - 1.4 Torr. Temporal profiles were obtained as a function of IBr and No pressures and photolysis energy to analyze laser gain, threshold, and efficiency. The threshold photolysis pump energy was 25 mJ/pulse. Lasing pulses were delayed by 150 ns from photodissociation and persisted for 100 - 200 ns. Device efficiency is limited by NO V yields V relaxation, and the maximum observed NO laser energy was 0.01 mJ for 85 mJ photolysis energy. Comparison to similar Br(²P_1/2 yields ²P_3/2) and Br(²P_1/2)/CO₂(101 - yields 100) is provided.

Advantages of axial flow system of pure gas lasers are analyzed for application in Ar/Kr gas mixture lasers. It is possible to achieve constant partial atomic concentrations along the discharge under corresponding conditions. Under other conditions the discharge promotes gas separation that leads to growing output characteristics of the white laser. Up to 60% of pure gas lasers efficiency was achieved.

This paper describes an investigation on the behavior of laser induced vapor bubble in cryogenic liquid nitrogen. The bubble is produced by the pulse ruby laser beam focused in the special cryostat. The dynamics of the laser-induced bubble is visualized by high speed photography. The pressure pulse signals from the bubble motion in the liquid nitrogen are measured under equilibrium or nonequilibrium conditions. Furthermore a numerical study is also performed on the dynamics of single spherical bubble in cryogenic liquid under the conditions corresponding to our experiment. Among the results the rapid growth of bubble and its rebounds have been visualized even in near-equilibrium conditions. Calculated results are compared with the experimented data.

In the development of fast axial flow (FAF) high power CO₂ lasers the increase of massflow and the reduction of phase distortions are of main interest in the realization of high output power with high beam quality. Both topics strongly depend on the layout of the flow system for the convective cooling of the medium. Reducing the pressure losses in the closed loop gas circuit guarantees an enhanced mass flow in the system. The element at the outlet of the discharge tubes is the place of the largest pressure losses in FAF CO₂ lasers. Optimized geometries have been developed to increase the mass flow significantly. The inlet into the discharge tube is of major interest due to phase distortions of the active medium. Besides stationary, statistical phase distortions strongly influence the output power and beam quality. The influence of inlet geometries on the flow profile has been investigated. The discharge stability could be clearly improved by new developed geometries. Measurements of statistical phase distortions under different flow and discharge conditions have been obtained. The work on optimizing the geometries enabled the enhancement of power incoupling into the discharge with an simultaneous decrease in the statistical phase distortions. Experimental results are presented.

Industry has growing interest in lasers with high output power of high beam quality. Therefore, laser developers have to concentrate their efforts on the optimization of every component involved in the beam generation, e.g. the rf-power incoupling and the gas flow generation. At present CO₂ lasers of very high output power are developed by increasing the dimensions of the whole system. Inevitably, a more or less pronounced reduction of beam quality and efficiency will occur, if scaling laws are not considered. In order to reach the goal of a well designed system with high performance data the knowledge of homogeneity and stability of the discharge as a function of the various parameters involved is essential. The evolution of filaments for example increases with the rf-input power. This filamentation, however, is a complex mechanism that also depends on gas temperature, gas turbulence and contamination as well as on the excitation frequency of rf-discharges. In this work, diagnostic methods for determining the degree of filamentation are presented. The results achieved are used to determine the frequency-dependency and scaling laws of rf-excited CO₂ discharges. The range of parameters favorable for a stable discharge is generally not the same compared to the parameters yielding high values of small signal gain and saturation intensity. The optimal discharge geometry (length and diameter) depends on various quantities. The values for mass flow and of rf-input power are given by the gas circulating system and the rf- generator. The maximum rf-input power density depends on the limit of stable operation, which itself is a function of gas pressure and velocity and various other parameters. Taking into account the gain saturation of the medium, the optimal gas pressure itself depends on the input power. Measurement data of small signal gain and saturation intensity spatially resolved parallel to the gas flow direction as well as electro-optical efficiency will be given. Based on these measurements, scaling laws are derived and will be presented.

To clarify the characteristics of the generation and propagation of shock waves generated by pulse discharges in an excimer laser, numerical simulation using a TVD scheme and a grid- distortion splitting method is carried out. The calculations are conducted in conditions corresponding to our visualization experiments and high-frequency operations. The propagation and attenuation of the shock waves in the high-frequency operation are visualized from the numerical results.

By numerically solving the Kirchhoff-Fresnel integral equations, the effects of temporally varying phase distortions on the beam quality of a four-wave mixing MOPA system were investigated. Our simulation revealed that the performance of such a four-wave mixing MOPA system depends strongly on the percentage of the overall power impinging on the active area of the phase conjugate mirror. Additionally, the onset time of the laser-induced medium perturbation was calculated for a set of different gas mixtures and a range of output energies. The phase distortions produced by the transit of a shock wave orthogonal to the optical axis could be completely cancelled by the phase conjugate mirror, i.e., no effect was found when comparing the far field distributions at different times during passage of the shock wave through the mode volume.

A new free vortex aerodynamic window, developed for industrial high power CO₂-lasers, is presented. To meet the industrial requirements improvements with regard to a reduction of the total nozzle pressure were necessary. The lower pressure results in reduced turbulence and therefore in less influence on the laser beam. This was verified by interferometric measurements as well as by experiments on an industrial laser. Some results of a numerical simulation of the flow in an aerodynamic window aimed at a better understanding of the phenomena involved are presented.

This paper presents a numerical study of the flow structure and heat transfer in the discharge tube of a high power CO₂ laser. A compressible turbulent model, in connection with simplified assumptions, was developed. The resulting set of partial differential equations describing the flow was solved by the PHOENICS code. Several types of thermal boundary conditions were tested and numerical results were found in good agreement with the experimental data obtained in a previous study. The location of the reattachment point was found to be highly correlated with the values of the turbulent energy and the dissipation rate of turbulent energy in the entrance section. The size of the recirculation zone and the shock waves created near the exit section of the nozzle have no significant effects upon the structure flow downstream the reattachment point.

The trend of development in fast axial flow CO₂ lasers with transversal rf excitation leads to increasing laser power with high beam quality. Since the power density of the discharge is limited due to thermal instabilities scaling the output power enforces to increase the discharge dimensions and the mass flow for the convective cooling of the medium. Under the boundary conditions of large discharge volume and high mass flow the active medium affects the optical path of the laser beam by stationary as well as statistical phase distortions. Output power and beam quality are strongly impaired by phase distortions of the active medium. The description and diagnosis of the instationary, statistical phase distortions is an important field of activity in the development of high power CO₂ lasers and an important step in optimizing the beam source. In order to estimate the influence of gas flow state and discharge conditions, a measurement technique to evaluate the statistical phase distortions is presented. Parallel beams of coherent light being chopped high frequently pass the medium to be investigated. Propagating through the medium the beams suffer different refractions which can be measured as a relative motion of the beams to each other. The different refractions are caused by density and temperature inhomogeneities and fluctuations in the discharge. The relationship between measured signals and characteristical quantities of the statistical phase distortions is discussed. The measurement technique as well as experimental examples are presented.

In the laser cutting process the gas flow is of main importance for exhausting molten and evaporized material from the cut kerf. The present paper investigates the gas flow of conventionally used conic-cylindrical nozzles and newly designed supersonic nozzles in a cut kerf. A quantitative description of pressure gradients and velocity profiles of the flow field is given as a result of numerical simulation. The calculations are compared with experimental Schlieren photographs. For both nozzle types the dependence of the flow field on the variation of the nozzle adjustment and the nozzle stand-off distance is investigated. In case of the conic- cylindrical nozzle the non-linear pressure and velocity distribution inside the kerf indicates both the shock formation caused by expansion to ambient pressure and the detaching of the flow field. The important result for the supersonic nozzle is the independence of the flow field from the nozzle stand-off distance which leads to an unchanged cut quality. Therefore the supersonic nozzle is of special interest for 3D laser cutting and cutting of thick materials.

In general, the beam quality of industrial high-power CO₂ lasers does not reach the maximum theoretical value, but it is dependent on the operation parameters of the particular system. This is due to the distortion of the intracavity wavefront which results from the non- homogeneity of discharge and gas flow and from the power absorption of mirrors and windows. For this reason, investigations have to be done in order to identify and then to minimize or to compensate the distorting effects caused by basic `elements' of a fast axial flow rf-excited laser. Conventional linear electrodes, improved helical electrodes, with or without a discharge-stabilizing air gap between electrodes and quartz tube, have been compared with respect to the minimum achievable phase distortions. Bending mirrors, resonator mirrors and outcoupling windows absorb a part of the beam power and of the uv-luminescence of the discharge and, therefore, become deformed. The temperature of the quartz tubes changes with laser operation time leading to a transient thermally induced lens in the laser active medium. This effect in combination with the time constants of mirrors and windows determines the warm-up period of a laser system. The detrimental transient effects can be reduced either by optimization of the basic elements of the system or by inserting optical elements which can be used for active compensation. One example is the use of an intracavity adaptive mirror offering the possibility to alter the mirror surface curvature. An experimental high-power CO₂ laser has been used to demonstrate and to verify approaches to minimize phase distortions caused by the effects mentioned above.

Lasers are widely employed in laboratories and in certain industrial applications, notably for welding, cutting and surface treatment. This paper describes a new application, incineration, which appears warranted when the following features are required: high-temperature incineration (> 1500 degree(s)C) with close-tolerance temperature control in an oxidizing medium while ensuring containment of toxic waste. These criteria correspond to the application presented here. Following a brief theoretical introduction concerning the laser/surface interaction, the paper describes the incineration of graphite waste contaminated with alpha-emitting radionuclides. Process feasibility has been demonstrated on a nonradioactive prototype capable of incinerating 10 kg(DOT)h^-1 using a 7 kW CO₂ laser. An industrial facility with the same capacity, designed to operate within the constraints of an alpha-tight glove box environment, is now at the project stage. Other types of applications with similar requirements may be considered.

Two mid-IR gas lasers, the CO (5.3 micrometers ) and the HF (2.7 micrometers ) lasers afford higher absorptivities on metals, lower plasma absorption as compared to the CO₂ laser. On the other hand, the resonant cavity can be improved in order to deliver smaller spots sizes than the Nd:YAG laser. A whole array of these four high power IR lasers in the kW range are available at the ISL. Comparative studies in metal processing with these lasers have been undertaken. It has been shown that, due to higher absorptivities, the remelting is obtained with lower laser power.

This paper reports two topics in the material processing using TEA CO₂ lasers. We demonstrated selective ablation of hydrogenated amorphous silicon (a-Si:H) thin layer on a quartz substrate by the second harmonic (SH) radiation of TEA CO₂ laser generated by AgGaSe₂ nonlinear crystal. Si-H bonds contained in a-Si:H strongly absorb the 5 micrometers SH radiation and resulted in the selective ablation of the a-Si:H layer. The successful ablation processing of ethylenetetrafluoroethylene (ETFE) copolymer by the 9.6 micrometers fundamental wavelength TEA CO₂ laser is also reported. Only ETFE thin film adhered to an aluminum substrate can be ablated by the TEA CO₂ laser.

Simultaneous doping and deposition of Si onto stainless steel 304 by irradiation of KrF excimer laser beams in an ambient SiH₄ gas is demonstrated. The specific process is referred to as laser implant-deposition (LID). Dependence of the total quantities of supplied Si atoms (dose) on the experimental conditions is examined to analyze the LID mechanism. The Si depth profiles in the LID samples are simulated by a liquid phase diffusion model, which show good agreement with the experimental results.

A tough adhesivity of teflon and stainless steel using an epoxy resin-based bonding agent was performed. The chemical stability of teflon is attributed to the C-F bond composed. Thus, it is considered to pull out the fluorine atoms selectively from the area irradiated with an ArF excimer laser light and to substitute a functional group displaying excellent affinity with bonding agents to create a powerful adhesivity. The defluorination of the surface was performed with boron atoms which were photodissociated from orthoboric acid water solution. The hydroxyl groups, which have a good affinity with epoxy bonding agent, were substituted only at area exposed to the laser light. The modified teflon surface with the epoxy bonding agent, and shearing tensile strength were performed. The strength was 110 Kgf/cm2.

A new phase shifting technique based on interferometry has been developed which is especially suited for deep-UV microlithography. Using only a single layer chromium mask, with no additional phase shift elements, significant resolution and contrast enhancement over conventional transmission lithography can be achieved. Both computer simulations, as well as experiments using a CCD camera and UV photoresist confirm the capabilities of this new approach. Using a relatively simple experimental setup and an illumination wavelength of 355 nm, lines with feature sizes as fine as 0.3 micrometers were achieved.

The working quality in laser material processing is influenced by a number of parameters as to laser source, beam guiding system and material. Although the reliability of the lasers and the stability of the system components have been improved there are still some weak points. In the following a closed-loop control system is presented which will help to meet the requirements. The output power of a rf-excited CO₂ laser is fluctuating in various frequencies, which influences the working quality in laser material processing. In order to avoid these effects, a closed-loop control system for stabilization of the output power of the laser has been developed. This system consists of a beam diagnostic device positioned at the rear outcoupling window of the laser resonator, a signal transformer and a modified PID controller. With the combination of two pyroelectric detectors--one of them having a chopper wheel in order to detect the signal portion of the laser running in continuous wave operating mode--the actual output power of the laser can be measured very quickly. An 11% decrease of the output power in a time range of 30 seconds and fluctuations of +/- 4% with a frequency of about 300 Hz have been measured. The closed-loop control is tested with the laser running in a cw- mode. The amplitude of the rf-exciting signal is used as correcting variable. Referring to basic investigations on the dynamic behavior of the output power of a laser, a strategy for a PI- controller has been derived and the optimal parameters have been found experimentally. Using this closed-loop control, the fast fluctuations can be reduced to +/- 1%. The above- mentioned decrease of the output power is completely avoided.

CO₂-laser processing of plastics has been studied experimentally and theoretically. Welding of cylindrical parts made from polycarbonate and polypropylene, cutting of polymethyl-methacrylate plates, and drilling holes in polypropylene are presented as examples. A good coincidence between theoretical and experimental results in case of laser welding has been found. Some practical aspects of laser processing of plastics has been given.

Due to their fabrication process, the ceramic materials usually exhibit a granulary surface. That leads to a non negligible roughness and porosity, in so far as the intergranular interstices could be regarded as pores. On the other hand, the hardness of such materials makes the processing (polishing for instance) somewhat difficult. The purpose of this work is the surface modification of different ceramic samples using UV laser radiations in order to improve the tribological properties of the workpieces. Experiments have been performed on alumina (Al₂O₃), aluminum nitride (AIN) and silicon carbide (SiC) in order to determine the influence of different parameters on the roughness and porosity of their surface. Three laser wavelengths (193 nm, 248 nm and 308 nm), a range of energy density up to 8 J/cm², a pulse duration of 25 ns and different repetition rates (1 - 200 Hz) were the experimental parameters. Results concerning roughness were analyzed using a mechanical profilometer. The photographs of treated surfaces obtained by scanning electron microscopy have been numerized; the porosity of the material has been measured by image processing. For instance, in case of alumina, we obtain a 90 surf.% decrease of porosity after laser treatment (2 J/cm²). At such a fluence, the interstices (which typical width is about 1 micrometers ) are no more visible, resulting in a minimal porosity in spite of the appearance of numerous circular pores and microcracks. A good choice of laser parameters allows us to obtain a significant improvement of the surface: the granular structure and microcracks no longer appear. Absorptivity was determined at 248 nm using a thermocouple.

A 2D laser surface remelting problem is numerically simulated. The mathematical formulation of this multiphase problem is obtained using a continuum model, constructed from classical mixture theory. This formulation permits to construct a set of continuum conservation equations for pure or binary, solid-liquid phase change systems. The numerical resolution of this set of coupled partial differential equations is performed using a finite volume method associated with a PISO algorithm. The numerical results show the modifications caused by an increase of the free surface shear stress (represented by the Reynolds number R_e) upon the stability of the thermocapillary flow in the melting pool. The solutions exhibit a symmetry-- breaking flow transition, oscillatory behavior at higher values of R_e. The spectral analysis of temperature and velocity signals for particular points situated in the melted pool, show that these oscillations are at first mono-periodic then new frequencies appear generating a quasi- periodic behavior. These oscillations of the flow in the melted pool could induced the deformation of the free surface which could explain the formation of surface ripples observed during laser surface treatments (surface remelting, cladding) or laser welding.

Superconducting Y₁Ba₂Cu₃O₇ thin films were prepared by in-situ oxygen plasma-assisted XeCl excimer laser deposition. A multistep superfast CO₂ laser annealing was investigated as a method of modification of the Y₁Ba₂Cu₃O₇ thin films on Si. It was revealed that cw CO₂ laser heating of substrate surface may be also a method for reduction of particulate density. The buffer layers such as Ba(Sr)TiO₃ and YSZ(111) were successfully experimented for preparation of good superconducting thin films. It is shown that N₂ laser ablation may be a proper technique for patterning Y₁Ba₂Cu₃O₇ thin films. The films were characterized by EDAX, SEM, XRD and Raman spectroscopy.

Hard coatings, e.g. TiN or WC on high quality tools are regenerated several times, due to their high costs. Conventional decoating techniques are of chemical nature and problematically regarding the handling of the chemical residues. In addition to that the lifetime of recoated tools after chemical decoating of the damaged functional layers is drastically reduced compared to new tools. Excimer laser treatment using the so-called `Duplex-Technique' enables a damage-free removal of the hard coatings with much longer lifetime of recoated tools than those of chemically decoated. The handling of the waste material is extremely easy using a laser processing head with an integrated exhaust system, that was designed at ATZ- EVUS. The paper gives a detailed presentation of the developed Duplex-Technique, the influence of the laser parameters and the obtained surface properties. Results of internal stress measurements, roughness values, changes in chemical composition and the surface appearance are described. From the technological point of view the removal rates, the productivity and last not least the superior performance of excimer laser decoated and PVD recoated tools in a lifetime test are demonstrated, compared to newly coated and chemical decoated tools.

An unstable resonator (M equals 2) has been applied to reduce the beam divergence of a gasdynamically cooled supersonic CO laser operating at 105 K in a semiclosed gas cycle. A 4.7 kW laser beam with a total divergence of 2.5 mrad is obtained with an efficiency of 9.4%. The results of welding experiments are compared with those using a CO₂ laser. The weld depth obtained with the CO₂ laser are drastically reduced using Argon as assist gas; whereas the results with the CO laser are independent of the assist gas because of the lower plasma absorption coefficient for the shorter CO laser wavelength. The beam quality of the CO laser is strongly influenced by water vapor absorption in the beam delivery system.

This work investigates the possibility of ablating paint from aerospace material with a XeCl- laser. The main advantage of this type of laser is the low heat generation during the ablation process. This is important when stripping thermally sensitive materials such as polymer composites. The dependence of the ablation process on energy density, pulse frequency as well as other laser parameters are presented. The results show the influence of chemical and UV artificial aging processes on ablation depth. Further, the behavior of the time-averaged transmission of the laser beam through the plasma is described as a function of the energy density. The time-varying temperature in the substrate at the point of ablation was measured during the process. An abrupt change in the temperature variation indicates the end of point ablation. This measured temperature variation is compared with the calculated temperatures, which are derived from the 1D heat equations. Finally, first results of repaintability and ablation rates will be presented.

The temperature of the high temperature superconducting (HTSC) sample surface, T, irradiated by the c.w. IR CO-laser was measured by the pyrometric method. The YBa₂Cu₃O₇ single crystals, YBa₂Cu₃O_6.2, YBa₂Cu₃O_6.9, and the Nd_1.85Ce_0.15CuO₄ ceramic samples were studied. The experimental data were compared with the theoretical estimations. The verified values of the heat conductivity, k, were derived from this comparison. Using these k values, the T data for the case of the pulse laser irradiation obtained in our previous investigations of laser irradiation effects were refined. The results of the present work confirmed that effects of the IR laser irradiation seen in our previous works cannot be explained by pure thermal action as the degree of the IR laser thermal effects, i.e., the heating, is relatively small in that experiments, and the T values are lower than it is needed for appreciable changes in properties of the HTSC samples being under consideration.

The sub-picosecond response of the surface reflectivity of different polymers during ablation was measured using a pump-and-probe technique. Ablation was generated by a 0.5 ps duration KrF excimer laser. The time-dependent reflectivity of the surfaces was measured by a 496 nm, 0.5 ps long, low power laser. It showed an up to 4-fold increase from the unirradiated sample to a maximum of 94% within less than a picosecond and with a few picoseconds falltime. The spectrum of the ultraviolet light reflected from the spot is blue-shifted and exhibits more than three times the bandwidth of the incident radiation.

The present contribution focuses on fundamental investigations and possible methods for quality and process control mechanisms, especially for the removal of thin hard films and deformation layers from metallic substrates. Extended fundamental investigations including short time photography and plasma emission spectroscopy were carried out to characterize the plasma formation and propagation during excimer laser treatment. The influence of both the laser and process parameters (wavelength, energy density, number of pulses, ambient gas type and pressure) on the process and the plasma properties is determined. The investigated plasma emission spectra is strongly correlated to the surface modifications achieved. It will be outlined how these signals can be used for process control in excimer laser assisted processing.

A large field of applications as microelectronics, micromechanics and optics needs to overcome the deposition of various materials showing dielectric, superconducting, piezoelecinc properties under thin film form. Numerous methods were developed as Chemical Vapour Deposition, Sputtering, Thermal Evaporation. Since the early 1980's, a new process based on laser ablation is developed.1 The Pulsed Laser Deposition (P.L.D) method is based on the laser evaporation of a target and the subsequent deposition of the ablation plume on a substrate (see fig. 1). A wide variety of materials was successfully deposited in thin film form by this method. Recently, Nb5Te4 thin films were realized for the first time by using P.L.D process.3 The Nb5Te4 compound presents strong anisotropic properties and finds applications in microelectronics (1D conductors) and micromechanics (solid lubricant). These first results3 shown the key roles played by the laser fluence and the substrate temperature on the film composition and crystallisation. For example, increasing the laser fluence decreases the interreticular distance observed by X-ray diffraction and increases the Te/Nb ratio. This shows the importance of kinetic parameters of the laser-induced plume on the crystallisation process. In order to understand the influence of the laser parameters on the film formation, it is necessary to study the ablation process and the expansion of the ablation cloud.

The energy transfer to a metallic sample and the resulting thermal coupling depend strongly on the experimental conditions: incident fluence and intensity, pulse duration, pressure and nature of the surrounding gas. In order to optimize laser-material processing such as surface treatment (ablation, control of roughness, changes of the hardness, residual stresses or surface compounds) the expansion of the produced plasma in front of the surface must be studied. The purpose of this paper is to present an experimental study concerning the characterization of different plasma regimes using an interferometric method and a fast image converter camera. The samples (Ti alloy) have been irradiated from 5 J/cm² to 130 J/cm² using a 248 nm laser radiation. The experiments have been performed in various ambient gases (nitrogen, argon, air, and helium) under different ambient pressures (10^-3 to 10⁵ Pa). Time-evolved structures (spherical blast wave, LSC, LSD) have been shown depending on the experimental conditions.

The ablation process of materials by an excimer laser is in most cases of thermal nature. The high intensity of the laser radiation leads within a few nanoseconds to evaporation of the material which compresses the ambient gas. A strong shock wave and further discontinuities are formed. These gasdynamic processes were detected by schlieren photography and shadowgraphy. The investigations show that the propagation mechanism of the shock wave depends on the ambient gas pressure and the laser pulse energy. The measured distances are compared with the calculated values using the Sedov-Taylor blast wave theory. A very good agreement between theoretical values and experimental data is found. Further, an interpretation of the appearance of the other discontinuities and their behavior will be given. A special nozzle was constructed in order to form a 1D gas flow. The modeling of this gas flow leads to the density, pressure and velocity of the jet at any point of the gas flow. The presence of such an external gas jet parallel to the target surface shows a drastical effect on the gasdynamic processes as well as on the formation of debris.

Pulsed high power CO₂-laser irradiation can cause damage to optical materials. Some results obtained at ISL with a repetitively pulsed CO₂-laser with pulse energies up to 24 J are presented in this paper. In production facilities with CO₂-lasers, optics transmitting in the visible spectral range like glass or PMMA are used as protection windows against scattered light. These materials have small skin depths for electromagnetic waves at 10,6 micrometers , typically in the order of some micrometers , so the interaction takes place in thin surface layers. Under high power laser radiation the transparency of the optics is lowered. On the other hand infrared transmitting optics like KCl or ZnSe show a low intrinsic absorption for CO₂-laser radiation. Theoretical estimations matching with the experimental observations showed, however, that strong heating occurs, if a thin layer of inhomogeneities, typically some micrometers thick, is included in the surrounding material with slightly higher absorption than the surrounding lowless material. Under these assumptions the thermally induced stress inside the materials can explain the experimentally observed mechanical damage. Besides these thermal damage effects mechanical momenta are transferred by pulsed laser radiation to the optics. Experimental results as obtained by a ballistic pendulum are reported.

We present the reflection investigation results for the first stage of the Reflection and Conduction Wave (RCW) appearance (low energy density conditions). The phenomenon of the RCW represents a reversible process which arose after irradiation of the sample at room temperature by long pulse of IR laser.

The TEA-CO₂-laser (transversely excited atmospheric pressure) is a tool for the pulsed processing of materials with peak power densities up to 10¹⁰ W/cm² and a FWHM of 70 ns. The interaction between the laser beam, the surface of the work piece and the surrounding atmosphere as well as gas pressure and the formation of an induced plasma influences the response of the target. It was found that depending on the power density and the atmosphere the response can take two forms. (1) No target modification due to optical break through of the atmosphere and therefore shielding of the target (air pressure above 10 mbar, depending on the material). (2) Processing of materials (air pressure below 10 mbar, depending on the material) with melting of metallic surfaces (power density above 0.5 10⁹ W/cm²), hole formation (power density of 5 10⁹ W/cm²) and shock hardening (power density of 3.5 10¹⁰ W/cm²). All those phenomena are usually linked with the occurrence of laser supported combustion waves and laser supported detonation waves, respectively for which the mechanism is still not completely understood. The present paper shows how short time photography and spatial and temporal resolved spectroscopy can be used to better understand the various processes that occur during laser beam interaction. The spectra of titanium and aluminum are observed and correlated with the modification of the target. If the power density is high enough and the gas pressure above a material and gas composition specific threshold, the plasma radiation shows only spectral lines of the background atmosphere. If the gas pressure is below this threshold, a modification of the target surface (melting, evaporation and solid state transformation) with TEA-CO₂- laser pulses is possible and the material specific spectra is observed. In some cases spatial and temporal resolved spectroscopy of a plasma allows the calculation of electron temperatures by comparison of two spectral lines.

A TEA-CO₂-mini-Laser (pulse duration 1 microsecond(s) , 200 mJ/pulse) has been used to generate plasmas on the surface of opaque and transparent solids. The power density in the laser focal plane leads to decomposition and vaporization of material and finally to a fast plasma creation. The combined effects of laser induced plasmas and surface ablation generate both shock waves in the ambient atmosphere and high pressure transient acoustic waves in the solid. The processes of plasma ignition and shock wave propagation have been visualized by means of laserdiagnostic techniques such as interferometry or shadowgraphy. The technique used was a quasi-cinematographic sampling method. Therefore a probing laser (repetitively pulsed Nd:YLF-laser, q-switched, frequency-doubled) illuminated the plasma with an adjustable delay to the CO₂-pulse. This delay ranged from a few nanoseconds to tens of microseconds. The same method was simultaneously used to observe the shock wave inside the transparent target.

An experimental study of the damages induced by laser irradiation by different materials used as well as IR optics (germanium) or detector bulk materials (silicon) has been performed. The irradiation source is a repetitively pulsed Nd:YAG laser operating in fundamental mode ((lambda) equals 1.06 micrometers ) and single pulse selection. Instantaneous output power densities of 6 X 10³ to 5 X 10⁵ W/cm² and pulse durations of 1 to 20 ms have been achieved. Different types of damages have been observed, depending on laser power density and spot size: mechanical fractures along privileged directions, ripples formation and principally surface protuberance rise as a sharp tipped peak during the melting pool resolidification when the laser is turned off. Emphasis is placed on the study of this last effect. We measure the final height of the resolidification peak and correlate it with target material and irradiation parameters. A numerical model of laser-material interaction including the density variation between the different phases has been used to correlate the experimental results. Qualitative agreement has been demonstrated for the surface growth time history.