Laser micro processing has grown to a mature technology at many places in the electronics industry. When high beam quality lasers are combined with dedicated imaging and diffractive optics, excellent solution s can be offered to expand the field of applications. Standardization of laser specifications is necessary.

The beam quality of conventional flash lamp pumped laser rod systems is improved by using phase conjugating mirrors based on stimulated Brillouin scattering (SBS). Pulsed multi amplifier set-ups with SBS were investigated. Nd:YALO as active medium was used to avoid stress birefringence. In case of serial arrangements an optimized optical system between the individual rods is used to achieve high efficiency independent of thermal lensing. With a two amplifier system an average output power of 1 Watt up to 210 Watt in a near diffraction limited beam was realized. In case of parallel arrangements phase coupling of the individual beams was achieved using a common SBS-cell after the first amplifier pass. Instead of a Brillouin-cell a fiber phase-conjugator can be used to reduce the power threshold down to 6 kW. An average output power of 35 Watt from a pulsed MOPA-system with such a phase conjugator could be obtained. As applications high power second harmonic generation and applications in materials processing were realized.

The combination of our unique capabilities at LLNL in diode development, crystal growth, and system designs have allowed us to pursue a variety of research areas of interest to the commercial, medical, and defense industries. We have developed a flexible diode pumping technology which utilizes low cost silicon microchannel coolers to enable high average power diode operation and a shaped cylindrical microlens technology which allows the radiance conditioning of large two- dimensional laser diode arrays. The flexibility that this diode technology has brought to pump power generation in both average power and radiance have broadly expanded the number of ion-host combinations that can be efficiently excited and used in diode pumped solid state lasers. Some of our laser developments are briefly discussed.

Developments concerning high-power diode lasers, optical systems for beam shaping and superposition and complete systems for direct materials processing are discussed, along with examples of their industrial applications.

Narrow spectral width oscillation of an AlGaInP index-guided diode laser (638 nm) has been achieved with an external stimulated photorefractive backscattering phase-conjugator. The emission spectral width was narrowed down to 5.3 GHz (7.2 pm) due to the very narrow spectral bandwidth of the 2k gratings formed by the stimulated photorefractive backscattering process in a photorefractive barium titanate crystal. The spectral brightness increased approximately 50 times as high as a free-running diode laser. We have also suppressed the self-scanning of the laser frequency, and successfully achieved wavelength-stable oscillation in the phase-conjugator coupled-cavity diode laser.

A planar YAG waveguide with a 200 micrometer Nd doped core, produced by a diffusion bonding process, has been face pumped by an array of qcw laser diode bars, producing 9.3 W of average output power at 8% duty cycle, with 24% conversion efficiency. The multi-pass pumping using a slotted mirror reflective pump chamber has been modeled by a matrix method, accounting for both laser diode temperature tuning effects and the pumping efficiency found in the experiment.

The design and operation of a one kilohertz diode pumped all solid-state Nd:YAG master oscillator power amplifier system with a phase conjugate mirror is presented. The setup allows high power scaling without reduction in beam quality.

Design and performance details of a high energy diode pumped slab amplifier (HEDPSA) system are given. The output from a 200 millijoule commercial Gaussian coupled Nd:YAG laser is amplified to 1 Joule per pulse with good beam quality at a repetition rate of 20 Hz. The system comprises two zigzag geometry Nd:YAG slab amplifier stages pumped by 2D semiconductor laser diode arrays. Details of the amplifier optical design are given, highlighting the techniques used for compensation of thermal distortions in the laser slabs. Thermal lensing results show good agreement with thermal and optical models of the system. Good agrement is also shown with theoretical models of amplifier performance.

A novel multi-rod cw Nd:YAG laser with 4 kW output power through an optical fiber has been developed. We adopted a unique YAG rod arrangement which minimizes the distance between adjacent rods. By using such a resonator, the laser output power of 4.8 kW was extracted from the laser resonator at an oscillation efficiency of 3.9%. The fiber power transmission with a fused silica fiber of 1.0 mm diameter is possible for the delivered laser power of 4.4 kW with transmission efficiency of about 92%, which is the same as a theoretical reflection loss at end surfaces. Welding experiments using the developed 4 kW YAG laser with a fiber- connected multi-joint robots showed excellent performance of 10 mm penetration for stainless steel welding and of more than 4 mm penetration for aluminum alloys.

A new method for obtaining high-power laser beams with drastically improved beam quality for fiber coupling is presented. The central idea is to use variable reflectivity mirrors (VRM) as outcoupling mirrors for high-power solid- state lasers with stable resonators, operated near the maximum power. The reflectivity profile of the VRM, properly chosen, acts similar to a mode aperture. Without any further modifications of the laser, the beam parameter products were substantially reduced: the maximum values at intermediate pumping powers were reduced to the beam parameter product occurring at the highest input power. The laser power, however, was not reduced within the principal working interval. Different Nd:YAG laser systems with 350 W, 1.9 kW, and 3 kW maximum output power were equipped with VRM. The 1.9 kW system was coupled without any loss of power into a 400 micrometer fiber instead of the 600 micrometer fiber normally used. The influence of the VRM spot size on laser power and beam parameter product (BPP) was investigated. Furthermore, it was shown that better beam quality is achieved by adding further laser cavities inside the resonator instead of using them as external amplifiers.

Laser based on Nd:Cr:GSGG low-energy oscillator/multiple-pass amplifiers produced 1.7 J pulses in a M² approximately equals 2 divergence beam at 2.4% electrical efficiency. Thermal lensing and birefringence correction were major factors.

Passive Q-switching of high power cw pumped Nd:YAG laser by using phototropic Cr⁴⁺ ion centers in YAG was investigated in detail. Over 180 W average output was obtained at 15 kW input power with two cavities. The Q-switch efficiency is about 70%. The beam parameter product is 7.5 mm mrad and longitudinal mode selection was observed.

We report on the status of an experimental activity to scale Cr:LiSAF to high average power using a unique zig-zag optical cavity design.

Copper lasers are the highest efficiency high average power visible lasers (510.6 and 578.2 nm). The most efficient of them all is the copper hybrid laser, which has been demonstrated to produce 200 W average power with 1.9% efficiency. The prospects for increasing the average output power from single or double tube devices into the kilowatt regime are discussed. With practically no plasma skin effect, a long gain duration (approximately 100 ns), and rapid plasma relaxation between pulses, copper lasers with hydrogen halide additives (hybrid lasers) hold promise for scaling in tube bore, active length and PRF to kilowatt average output powers with efficiencies of approximately 2%, and approximately 600 W at 3.5% efficiency.

The results of a detailed study investigating the effects of HBr, Br donors and other halogens on the electrical and optical lasing characteristics in an elemental copper vapor laser is reported.

A CuBr laser with a central electrode is reported. Up to the 2 kW input power the laser produce maximum output power of 47 W (at efficiency 2.35%) and maximum efficiency 2.5% (at 42 W output power). Some features of the excitation circuit and laser operation are given.

Results of parametric studies on a high efficiency, 2.7 micrometers atomic bromine laser, produced by photolyzing IBr with 532 nm radiation, are presented. Concurrently, the results of a rate equation computer model show excellent agreement with experimental measurements.

A high pulse repetition rate helium-xenon laser is described. The laser produces output at four principal wavelengths and can be tuned to several others. The primary laser excitation mechanism is identified as recombination pumping in the afterglow. The importance of rapid quenching of the discharge current is demonstrated and an excitation circuit to achieve this is described.

A coaxial carbon-dioxide waveguide laser with rf excitation at 96.5 MHz and an output power exceeding 1.3 kW is described. This laser uses a new concept of electrode segmentation for discharge homogenization in addition to a new kind of optical resonator for the coaxial geometry called the toothed mirror resonator.

A 100 MHz rf discharge excited slab carbon-dioxide laser with the near diffraction limited output power of 1.07 kW and the efficiency of 12.6% was studied. The size, including the laser head and the rf generator, is 1/4 of the fast axial flow laser. The self-excited rf generator with a cavity resonator made the laser simple and compact. To determine the rf excitation frequency, the discharge characteristics were studied at both frequencies of 40 MHz and 100 MHz.

The suitability of a slab-shaped dc-excited discharge as active medium of a compact high power carbon-dioxide laser is investigated. Measurements of several plasma and laser parameters indicate the effectiveness of the adopted discharge stabilization technique.

A high power, two-dimensional, 16 by 4 element waveguide array carbon-dioxide laser is presented and properties such as beam quality and spatial power distribution are assessed, together with beam reformatting techniques to produce compact beams.

A marked improvement is observed in the performance of pulsed microwave excited slab carbon-dioxide laser when the gas is permitted to expand following a discharge pulse. The gas pressure changes following each discharge pulse indicate a heating followed by expansion and contraction stages. A sequence of over ten reciprocating cycles is observed after each pulse. Two modes of operation (open versus closed) were investigated. The gas motion in and out of the discharge space in the open structure, leads to a faster overall cooling than for fully confined gas. Pressure changes driving the reciprocating motion of the activated gas are monitored. Maximum average output power of 20 W with 10% efficiency and about 1 KW peak power were obtained from the open structure at gas pressure of 210 mmHg. The absence of the expansion cooling mechanism in the closed structure caused about 40% reduction in the output power and efficiency. The results of the present research suggest a new architecture for pulsed lasers that provides a gas cooling markedly faster than the conductive cooling of a stagnant gas in the same geometry.

A 1 kW laser capable of full depth modulation in excess of 200 kHz has been developed to satisfy a significant need within the printing plate engraving industry. The Howden Laser LE1500-OA design is based upon a 1.5 kW diffusion cooled carbon-dioxide laser converted into an oscillator-acousto- optic-modulator-amplifier configuration. A novel method is used to limit unwanted noise emissions from the high gain amplifier at the 'zero' setting. The design is being used successfully within systems manufactured by ZED Instruments Ltd to engrave flexographic printing plates at several production sites involved in packaging and label printing.

An electron-beam controlled, pulsed carbon-dioxide laser with an average output power of more than 15 kW is presented. Laser operation has been achieved for pulse energies up to 180 J at repetition rates of 100 Hz, pulse durations variable between 3 and 10 microseconds, and bursts of 1000 shots with an unprecedented reliability.

For delivery of high-power IR laser light, hollow-core fibers have advantages of ruggedness, environmental stability, and high-power threshold over the IR solid-core fibers, such as fluoride glasses, metal halides, and chalcogenides fibers. In this paper, we focus on the hollow-fiber beam delivery system for high power mid-IR lasers with more than 1-kW cw power or 1000 mJ pulse energy and review the current status of hollow fibers by showing the system designs and the tested results.

This paper gives an overview of the recent developments of non-rotationally symmetric optical elements (NOEs) for beam shaping of high power laser radiation at the Lehrstuhl fur Lasertechnik and the Fraunhofer-Institut fur Lasertechnik. Several applications of NOEs in the IR- and UV-wavelength region demonstrate the innovative potential of NOEs for laser materials processing.

A device capable of drastically improving the energy efficiency of present mask based laser materials processing systems is presented. Good accordance between experiments and simulations for a TEA-carbon-dioxide laser system designed for laser marking has been demonstrated. The energy efficiency may be improved with a factor of 2 - 4 for typical mask transmittances between 10 - 40%.

The results of the investigation of the beam quality in the high-power industrial carbon-dioxide laser generating in the 10 kW range are presented. As a result of this investigation, a scheme of the modified stable-unstable resonator is proposed, that uses only reflective mirrors. In this scheme, the plane of the unstable telescopic resonator is transverse to the flow in the discharge chamber. In the plane parallel to the electrodes this scheme represents a single-mode stable resonator. High efficiency of this resonator and high beam quality have been pointed out.

Continuously tunable output in the mid-infrared was generated via four-wave mixing in a parahydrogen Raman cell. Continuously and line tunable carbon-dioxide master oscillator power amplifier chains produced the 10 micron input for the Raman cell.

An influence of different parameters such as gas pressure, specific input energy, laser mixture content, intensity ratio for probe and co-propagating pumping wave, coherency and geometry, etc., upon characteristics of phase conjugation (PC) signal and PC reflectivity (PCR) at intracavity degenerate four-wave mixing (DFWM) of long pulse carbon-dioxide and carbon lasers radiation in their inverted medium has been studied.

The results of high efficient excimer (exciplex) lasers investigations are presented. Processes of exciplex molecules formation are described as well as the data on the highest radiation parameters of the noble-gas-halides lasers are given. Also conditions of high efficiencies production of the noble-gas-halides excimer (exciplex) lasers are analyzed.

The development of high energy KrF lasers to the high average power required by inertial confinement fusion and industrial applications is reviewed. Techniques which maintain high beam quality are emphasized.

One kW average power (13 J multiplied by 80 Hz or 10 J multiplied by 100 Hz) has been obtained in 1992 and up to 22 J per pulse and per module has been reached in 1995 in the frame of Eureka Programm EU205 by SOPRA. This work is the prolongation of these studies and gives to the flat panel industry a unique tool in the world for silicon annealing. Gathering three modules, we obtain 60 J per pulse at 308 nm. The first module is an oscillator; the two others work as amplifiers. We tested different kinds of cavity (stable and unstable); finally, we choose an unstable cavity (M equals 1.2) with special output mirror. The pulse duration is about 200 ns FWHM but can be tuned if necessary. The flat panel industry asks a repetition rate of 0.16 Hz, so there is no need for circulation of the gas mixture. Work on synchronization of the 3 modules is shown. Main applications of these lasers are shock hardening, soft x-rays generation, surface treatment by photo-ablation and 'single shot excimer laser annealing' (SSELA). For the first ones, the laser beam has to have a low divergence and for surface treatment, the beam must be very homogeneous (typically plus or minus 3%). That is the reason why we designed a homogenizer and we solved problems of self focusing and plasma creation. The 3 modules and the process in cleanroom are driven by a control command system for an industrial use.

For high repetition operation of excimer-lasers care has to be taken of the changing performance of the electrical circuit, gas dynamic effects and contamination of the gas mixture to avoid deterioration of the laser performance. The parameters that influence the stability of the discharge are discussed. With the proper settings 1 kW of average power can be reached.

An excimer laser in master oscillator power amplifier (MOPA) configuration (E equals 2.3 J, tau equals 25 - 50 ns, f equals 10 Hz) was used for shock treatments of metallic samples. The process was studied and optimized using physical investigations concerning the pressure development. The effect of the laser treatment on the microstructure of the material could be demonstrated by micrographs of irradiated samples and could be quantified by roughness and hardness measurements as well as by residual stress analysis.

A high-power industrial carbon-dioxide laser with fast axial gas-flow was set up with eight radio-frequency-excited discharge tubes and four turbo-blowers. With an unstable resonator the maximum output power is 25 kW. The gas flow and the electrodes for the rf-excitation were shaped carefully to achieve homogeneous gas discharges and thus an active medium that introduces only minor aberrations into the beam of the unstable resonator. The beam propagation behavior near focus was studied for a diagnostic optics with long focal length and for processing optics with shorter focal lengths, respectively. Various welding applications in mild steel and stainless steel were carried out.

A rectangular cross-section flow tube has been used in the fast axial gas flow configuration to investigate the excitation of the carbon-dioxide laser with narrow electrode gap rf discharges. Stable discharges at a power density of 80 W/cm^-3 have been obtained, and the influence of excitation frequency in the range 27 to 125 MHz investigated. No localized thermal instability has been detected, and discharge shrinkage at high frequency is the main frequency dependent effect found. A beam power of 132 W/cm^-2 per discharge section has been obtained. Current density measurements have confirmed that conditions similar to planar waveguide, diffusion cooled carbon-dioxide lasers are attained.

An investigation of the parameters influencing the small- signal gain and the beam quality of the laser emission from a stable TEM₀₀ cavity within a large aperture e-beam controlled carbon-dioxide laser discharge unit was performed. An almost linear dependency of the small-signal gain as a function of the energy loading was monitored. Small-signal gains of up to 2.4%/cm in the center of the discharge region were found for an energy loading of up to 140 J/(1 bar) at a wavelength of 10.59 micrometer. The beam quality evaluation was done by M² parameter measurements according to the ISO standard. The fluence distribution was recorded with a beam analyzing system of 100 micrometer spatial resolution. No increase of the M² parameter was found within the limits of measurement accuracy for a stable resonator configuration when the energy loading almost doubled from 61.7 J/(1 bar) to 112.5 J/(1 bar).

The operational characteristics of a 135 kW continuous wave carbon dioxide laser system are described. A brief description of the fast-flowing electrical discharge coaxial laser system is presented followed by a detailed discussion of the operational and output characteristics of the device. Diagnostics systems configured to measure electrical discharge voltage and current, mass flow, laser cavity pressure, laser output power, output spatial intensity distribution and output temporal stability are described. The data collected with these systems are summarized with subsequent analyses presented and compared with theory. The 135 kW carbon dioxide laser is located at the Laser Hardened Materials Evaluation Laboratory (LHMEL) at Wright-Patterson Air Force Base, Ohio, USA. The device was developed and is currently operated for the purpose of characterizing the thermal response of materials.

The properties of the laser active medium were investigated using a drift chamber, and the effect of gas additives on the laser power is evaluated. The transport coefficient drift velocity is measured in carbon-dioxide-laser gas mixtures. The dependence of the drift velocity on the applied electric field is very similar for the different gas compositions in the positive column of the discharge, and a semi-empirical formula is given. The influence of gas additives is investigated (1) a selective energy reduction due to elastic impacts with Ar, (2) striking voltage reduction by adding C₄H₁₀ to the laser gas.

We have characterized three compact carbon-dioxide laser modules in terms of pulsed and cw power, beam quality and temporal stability. The three modules differ in the electrodes dimensions, and have been equipped with resonators based on two different hybrid stable-unstable schemes. Advantages and drawbacks are shown.

In this paper, a model of the temperature field in a laser cavity, is proposed. This model includes the calculation of the electron concentration in a turbulent flow. First, a 1D study is validated with experimental data of Leys et al. Second, the model is applied to a 2D argon flow. This study shows that the knowledge of the temperature field allows us to characterize the correlation between the plasma oscillations and turbulence fluctuations of the neutral gas.

We present the marking technique based on computer-controlled electromechanical diaphragm array and focusing system, which allows codes to be laser-written on dot-array sequences basis.

Microwave discharge has been proposed for high power laser oscillator. However, there are few practical high power (over 1 kW) microwave excited carbon-dioxide laser oscillator since it is difficult to establish a homogeneous discharge region across and along a laser tube for coaxial fast gas flow laser. This paper proposes a practical high power microwave excited carbon-dioxide laser oscillator by (1) employing orthogonal crossing electric field for radial homogeneity of the discharge, (2) dividing the axial discharge region of the laser tube into several discharge sections for stable discharge, and (3) selecting the optimized gas flow to maximize the laser output laser power. Applying these technologies, the experimental apparatus achieved 270 W laser output at 1.4 kW input microwave power per the waveguide. The equipment is also able to scale up by connecting tubes, which consist of five waveguides per tube, and confirmed 1.8 kW for the same volumetric gas flow rate and the structure of a conventional 1.8 kW dc laser. The proposed methods may provide a practical high power microwave excited carbon-dioxide laser oscillator that exceeds the conventional dc discharge one.

In an e-beam controlled carbon-dioxide laser unit the spatial, temporal and spectral properties of the small-signal gain were investigated in a single pass geometry. A smooth spatial distribution was found with peak values of 2.9%/cm near the cathode at 112.5 J/(1 bar). No reduction in the gain value was monitored up to repetition rates of 60 Hz when circulating the laser gas with a gas flow velocity of 100 m/s. A HeNe laser probe beam, being collinear to the carbon-dioxide laser beam, was used to monitor shock waves or mechanical vibrations by measuring its deflection. This investigation showed, that the small-signal gain measurements were not interfered by any disturbances.

Two methods of exciting a trigger wire double discharge electrode system are investigated for the generation of high repetition frequency pulsed discharges in a sub-atmospheric, transversely excited carbon-dioxide laser. Specifically, a 10 microsecond pulse forming network excited, 2 liter, transverse discharge was initiated by a corona discharge generated from trigger wires placed above a planar cathode. The discharge was pumped at pulse repetition frequencies up to 10 kHz and input pulse energies of 10 J. Two electrical systems for exciting the auxiliary preionizing discharge are considered viz. direct capacitive coupling from the anode voltage and separate high voltage pulse excitation. The laser itself was designed to improve materials processing capabilities of carbon-dioxide lasers. This followed from a detailed theoretical analysis of the laser-material interaction process and it was concluded that pulse repetition frequencies of about 5 - 10 kHz were needed with optical pulse lengths of 6 - 8 microseconds and plateau powers of between 20 - 150 kW. A stable single pass resonator was installed and used to assess the performance of both discharge initiation circuits by measuring the laser output pulse energy with a photon drag monitor. For the separately excited preionizer, the delay time between the preionizing pulse and the main discharge pulse was optimized by maximizing the laser output pulse energy as a function of the delay time. The discharge was modelled using SPICE to investigate the effects of the preionizing electron density on the discharge voltage and current.

Single-shot performance of tandem and folded frequency doubling schemes has been investigated in AgGaSe₂. These give rise to enhancements of up to a factor of approximately 3 in conversion efficiency. Experimental results are compared to theoretical predictions.

The design of compact and efficient laser sources having high beam-quality and high beam-power is an objective of gas laser research. Therefore, it is necessary to realize highest input power densities while producing only minimized distortion of the optical quality of the laseractive fluid. A non-concentric helical configuration of electrodes in combination with a particular swirl flow allowing for homogeneous deposition of 28 kW of electrical power in a single tube element and operating at the choking limit, while producing only 350 nm of maximum phase distortion, is shown in theory and experiment.

A numerical model is presented to describe the influence of the diffraction on the temporal and spatial properties of a Q- switched carbon-dioxide laser. The model includes the interaction of the spatial distribution of the laser field with the spatial distribution of the gain medium, the diffraction of the laser field due to edge effects of the rotating chopper disc, as well as the unstable resonator configuration. An rf-excited 5 kW carbon-dioxide laser has been used to produce a Q-switched peak power of 600 kW at a pulse duration of 200 ns and an average output power up to 1.3 kW. Comparison of the numerical results with the experimental data are shown.

Recently carbon-dioxide laser excited by microwave discharge has been studied to achieve higher efficiency and lower cost laser system. In this study a stripline was used to guide microwave with TEM mode. This mode has almost homogeneous intensity distribution of electric field and almost homogeneous discharge could be obtained.

A new resonator design for a transverse flow high power carbon-dioxide laser based on a fast tunable Fabry-Perot modulator was developed. It allows a very effective modulation and pulse generation.

Spectroscopic measurements of N₂ molecular bands emitted by the discharge of a sealed off test chamber and a 2 kW carbon-dioxide slab laser deliver detailed rotational temperature and intensity profiles in the gap between the electrodes as a function of different discharge parameters. The results give basic information on the behavior of the discharge and the main plasma parameters for further investigations.

Two independent methods are applied to measure the impedances of an rf excited carbon-dioxide slab laser and of a discharge test chamber. The rf feedthroughs are chosen as reference ports for the measurements. One method using an adjusted matching network and a network analyzer is compared with another method measuring two signals representing current and voltage with a digital oscilloscope and processing them using a computer.

A new concept for a master oscillator power amplifier (MOPA) system that is based on phase conjugation by four-wave mixing is presented. The system has a rigorous rotational symmetry with respect to amplification and phase conjugation allowing for a compact design. Numerical resonator calculations on the basis of the Fresnel-Kirchhoff technique were used to support the layout.

It was shown theoretically and by experiment that a thermal defocusing gaseous lens formed in industrial fast-axial-flow carbon-dioxide lasers may affect the diffraction losses of the generator and the beam parameters.

A transverse radio frequency discharge excited, carbon-dioxide slab laser has been operated cw with a hybrid waveguide and unstable resonator to produce an output power of 54 W from the active volume of 280 by 40 by 2.5 mm³. An impedance matching network consists of coaxial cable lines and has not discrete L, C reactive elements.

Research and development of pulsed-repetitively slab rf- excited carbon-dioxide laser is presented. The experiments were carried out for two modes operation: meander mode (duty factor delta equals 50%) and pulse repetition mode at delta equals 12.5%. The range of 240 divided by 4000 Hz of pulse repetition frequency was used. Pulse power about 400 W for meander mode and up to 700 W for the second mode have been obtained. Input power, radiation pulse shape and output beam divergence were measured.

The gains were measured for the lines in the traditional band 00⁰1-10⁰0, 00⁰1-02⁰0, in the second band of the 00⁰2-10⁰1 sequence, and in the hot band 01¹1- 11¹0 emitted by the active medium of an electric-discharge carbon-dioxide module. The experimental results were used in a generalized theoretical model to find the translation and vibrational temperatures and the concentration of the active molecules. The Fermi resonance between the 10⁰0 and 02⁰0 did not ensure a Boltzmann distribution of their populations under the conditions in the electric-discharge module. The degree of dissociation of the carbon-dioxide molecules in the discharge did not exceed 22% when the maximum input power density was near 6 W/cm³.

Epitaxial rare-earth (Re: Nd, Yb) doped yttrium aluminum garnet (Re_x:Y_3-xAl₅O₁₂ or Re:YAG) films have been grown on various substrates by pulsed laser deposition for the purpose of fabricating diode-pumped waveguide lasers. The films were characterized by Rutherford backscattering, x- ray diffraction, and photo-luminescence measurements. These Nd:YAG films on (100) silicon substrate having a large lattice mismatch show oriented stoichiometric growth. On the other hand, the films of rare-earth doped YAG on garnet substrates show the epitaxial growth with the smooth surfaces. Their characterization of rare-earth doped YAG thin films on various substrates was comparable to that of the Nd:YAG bulk laser crystal.

High power lasers operating at high repetition rates at the kilohertz regime are attractive for a variety of applications. Such high repetition rates can be achieved by Q-switching a cw laser using either an acousto-optic, an electro-optic or a passive Q-switch. Since passive Q-switching needs no external electric circuit, it may be valuable for solar pumped lasers to be used at space. In recent years, Cr⁴⁺:YAG has been extensively used for passive Q-switching of flashlamp pumped Nd:YAG lasers. For high average power lasers, the excellent thermal characteristics of Cr⁴⁺:YAG give it an edge over other saturable absorbers. In the present paper we report on the first passive Q-switching of solar pumped high power Nd:YAG laser. This solar pumped Nd:YAG laser, which employed 3 and 4 mm diameter Nd:YAG rods, emitted up to 50 watts in the cw mode. The laser rod was side-pumped by solar irradiation using a three-stage concentrating system. The Cr⁴⁺:YAG device rear surface was coated by high reflection dielectric coating, serving as cavity rear mirror, while the output mirror had a reflectivity of 90%. An average output power of 9 watts was obtained from the passive Q-switched solar pumped laser at 15 - 40 kHz, twice previous results for a passive Q- switched Nd:YAG laser, results which have been obtained under continuous flashlamp pumping.

In this contribution experimental results in high-precision processing of various reflecting metals employed in microelectronics carried out with a 10 W copper bromide laser are presented.

Copper vapor lasers in a MOPA-chain (MOPA, master-oscillator- power-amplifier) configuration with low divergence can be used for the high precision machining of metals and ceramics. The fundamental interaction phenomena, ablation process and possible industrial applications are presented. The following paper relates the results and experiences in the operation of a copper vapor laser MOPA chain, consisting of an oscillator and up to three amplifiers, with the triggering points for these lasers exactly variable through a master-timing-system. In principle, a low-divergent laser beam is generated (511 and 578 nm wavelengths) via an off-axis unstable resonator scheme, with precise synchronization of the amplifiers producing average powers of over 140 W. Due to the excellent beam focusability, peak power densities of some 10¹⁰W/cm² are achievable in a 50 ns pulse duration, which provides almost material-independent precision machining at high velocities. Beginning from the principles of beam-target reciprocation, the removing and cutting of metallic as well as non-metallic materials with copper vapor lasers is described. Additionally, the potential of copper vapor lasers for industrial applications is illustrated through precision machining examples.

A small-bore (12.5 mm) hybrid laser delivers its highest average output power for a gas flow rate of around 1 liter/atm/hr^-1 and an HBr pressure of 2 - 4 torr in 34 torr Ne. It is estimated that the atomic bromine pressure at the wall is approximately mtorr. The number density of copper in atomic and molecular forms averaged throughout the tube is 4 multiplied by 10¹⁶ cm^-3. Significant numbers of Cu_xBr_x polymers and CuH must exist in the laser discharge to lock up the large pool of copper atoms in the discharge. The copper hybrid laser produced an average output power of 12.8 W, corresponding to a specific average output power of 348 mW/cm^-3, at a PRF of 35 kHz.

An experimental investigation was made of the parameters of lasing as a result of atomic transitions in xenon observed when Ar-Xe and He-Ar-Xe mixtures were excited by an electron beam in a laser chamber with an active volume of approximately 600 liters. Electron beam from six double acceleration diodes were injected axially symmetric into this chamber. This ensured a more efficient utilization of the pump energy and a uniform distribution of the laser radiation energy over a transverse exit aperture in the maximum specific input energies. An output radiation energy of the order of 100 J was generated with an efficiency of about 2% at lambda equals 1.73 micrometer and of the order of 50 J with an efficiency of about 1% at lambda equals 2.03 micrometer. An analysis was made of the experimental results and of criteria for the optimization of the pump parameters in order to improve the output characteristics of a wide-aperture laser operating on the basis of atomic transitions in xenon.

The simulation code for the analysis of CO laser has been adapted for the recent computer network environment. The workstations, personal computers (PCs), and Macintoshes (MACs) are linked to the network where the simulation is carried out by the fast back-end machine and the input/output interfaces are built in the front-end PCs and MACs. The typical example can be displayed in the Web homepage.

Spectrally resolved atmospheric absorption has been investigated to determine the problems of long path beam delivery of high power industrial CO laser light for material processing. A dry gas purged beam path is found to be essential if the natural CO laser spectrum is used. However, an unpurged beam delivery system may be satisfactory over typical distances when the spectrum is modified to avoid coincidence with strong water lines by the use of an intra- cavity absorption section.

The results are presented of computer modeling jointly with preliminary results of experimental investigations of sequentially Q-switched frequency selected CO laser. Calculations show the essential difference in pulse form and inversion restoration time between the models of single- and multi-quantum vibrational exchange kinetics.

A frequency selected (FS) Q-switched (QS) e-beam controlled- discharge (EBCD) CO-laser has been researched and developed. The laser generates short pulses (tau_0.1 approximately equals 1 - 10 microseconds) having different spectral content including single line one within 4.92 - 6.54 micrometer spectral range. An influence of spectral content of CO laser radiation 'locked' or 'blocked' inside a laser resonator on output energy and efficiency of lasing on selected wavelengths has been studied.

For operating conditions of the high power supersonic e-beam sustained CO laser a numerical investigation of the energy and spectral characteristics of the first overtone generation is conducted. It is shown that electro-optical efficiency of the first overtone generation can be more than 30%. The efficiency of a spectrum selection both on fundamental and first overtone transitions is evaluated. On the basis of numerical calculations the engineering evaluations for several variants of such a device are executed.

The results are presented of measurements of surface hardness and wear resistivity of rolling machine 2000 rolls which were under heat treatment by high power gas dynamic laser at Cherepovetz Metallurgy Work. Surface hardness of rolls increased from 70 HSh to 80 - 85 HSh. Wear resistivity also showed considerable increase as compared with untreated rolls.

New methods to obtain higher power from the laser with the unstable resonator are proposed. Calculations are performed for expanding radius of concave or convex mirror to reduce the diffraction loss power from the concave mirror and to convert the power spread out from it into the output power. The Fresnel-Kirchhoff formula is used for the iterative computer calculation in the passive resonator and for a carbon-dioxide laser. The calculations show that the increase of the output power can be obtained more than 20 percent with expanding concave mirror in the carbon-dioxide laser.

The mode coupling of hollow infrared optical fibers was investigated as a function of beam launching conditions, i.e. fiber transverse and longitudinal misalignment and tilting. Experimental results with a TEM₀₀ carbon-dioxide laser and hollow sapphire and dielectric coated metallic waveguides are compared with geometrical optics and mode coupling theories.

We present a method and results of modeling of the large-area carbon-dioxide lasers, including gain saturation and dephasing effects. Numerical calculations are based on iterative evaluating of Fresnel-Kirchoff integral using fast convolution algorithm.

Calculations of the waveguide mode coupling losses due to free space propagation between the waveguide endings and the resonator mirrors of a coaxial Cu-waveguide are presented. The results show that the mode coupling losses have a strong influence on the mode selection in a coaxial waveguide laser.

A temporal behavior of a phase conjugation reflectivity R at degenerate four-wave mixing (DFWM) in InSb for a pulsed carbon-dioxide laser radiation is theoretically investigated. It is shown, that R at a transient DFWM is determined by a response time of a nonlinear medium, which depends on intensity of a radiation, and can be some times higher than a reflectivity for non-transient condition.

The high frequency temporal structure of probe (laser) and phase conjugation (PC) signal under intracavity degenerate four-wave mixing (DFWM) of long pulse carbon-dioxide and carbon laser radiation inside their inverted medium has been studied experimentally with nanosecond resolution over the full pulse length.

For xenon at temperature t equals 19.3 degrees Celsius in a range of density 0.3 g/cm³ less than or equal to (rho) less than or equal to 1.0 g/cm³ SBS gain coefficient (g) and phonon lifetimes are measured. It is shown that at the densities higher than (rho) equals 0.5 g/cm³ SBS gain g levels out, reaching the value g approximately equals 100 cm/GW, and the phonon lifetimes decrease down to (tau) _ph equals 25 ns with approaching to a critical point of Xe.

Propagation of super Gaussian beams, which can be generated by graded-phase resonators, has been treated by approximating these beams with generalized flattened Gaussian beams. An optimum approximation is discussed by using characterization parameters and the fitting of the amplitude profiles. Beam propagation factor M² and Kurtosis parameter add further quantitative information which is appropriate for and confirms the approximation.

Slab carbon-dioxide laser is the most compact system now. The goal of this paper is to consider conditions for abnormal high losses of regenerative amplifier unstable resonator main mode. In this case we can realize phasing without necessity of frequency locking into injector-regenerative system. For these conditions the optimal regime of input and propagation of radiation along regenerative amplifier is considered.

A method of phase locking two carbon-dioxide lasers through diffraction coupling is presented, which is achieved by the parallel propagation of the two laser beams along a common so- called coupling path. This is done by folding the resonator twice by mirrors and extraction of the output radiation by a common output coupler. As this method is only utilizing reflective optics (except of the output coupler), it is also practicable as a technique for phase-locking lasers with powers in the multi-kW range. Diffraction leads to an overlap of the optical fields of the two lasers, establishing a phase- locked state, if several locking conditions are fulfilled. The amount of energy, diffracted from one resonator to the second cavity which is necessary to establish phase-locking, depends highly on the length detuning between the two resonators. Phase-locked operation is detected by measuring the beat signal between the two laser beams. Since the coupling between the lasers is small, a stable phase-locked operation can only be obtained, if the length of one resonator arm is actively controlled with a PC-based electronic stabilization loop, driving a piezoelectric translator for resonator length tuning.

A comparison is made between the beam quality parameter M² and a new times-diffraction-limit-factor (TDL) for two high peak power XeCl lasers. The laser systems, equipped with unstable resonators, emit beams with different geometrical characteristic and optical quality. The results show that the M²-factor is always larger than the TDL value and this discrepancy increases if the beam is almost diffraction- limited. A novel method to obtain the far-field profile of coherent superGaussian-like beams and a simple technique to estimate the beam angular stability of a pulsed laser are also presented.

A 500 W, corona photo triggered XeCl laser has been upgraded up to 1100 W by increasing its input energy. Discharge efficiency in excess of 4% has been demonstrated. The maximum average power was obtained at 440 Hz repetition rate.

A series of experiments were performed to investigate the fundamental characteristics of a KrF laser pumped by electric discharge. The mode patterns of the laser were observed by a high speed camera. A YBCuO target was irradiated by the same laser and the surface conditions were observed by a microscope. A model x-ray source for pre-ionization which used a wire-initiated plasma source was constructed.

An x-ray preionized LC-inversion discharge pumped XeCl laser has been converted for KrF operation. The x rays were produced by a long pulse (400 nsec FWHM) secondary emission electron gun. A KrF laser energy of 1.2 J is obtained, only slightly lower than those obtained in an XeCl regime at the same pumping conditions.

High energy extraction from UV pre-ionized molecular fluorine lasers is hampered by the short period of discharge stability and the spatial inhomogeneity of the discharge. So far stable discharge operation is reported for small discharge cross sections with an area less than 0.8 cm² and efficiencies in the order of 0.15%. To increase the energy extraction of the molecular fluorine laser the discharge cross section should be enlarged. Here we report on the successful operation of a molecular fluorine laser with a large discharge cross section of 1.5 multiplied by 2.4 cm² (electrode spacing times discharge width) which operates at a high intrinsic efficiency of 0.45%. Crucial for obtaining this result is the development of a short pulse, high intensity x-ray preionization source.

Efficient VUV excimer lamps with two types of discharge configurations, expanding jet discharge and silent discharge (dielectric-barrier discharge) in a variety of rare gases and their mixtures, are presented. In the jet discharges VUV output power was 9 mW with an efficiency of 10^-2% at 126 nm for argon excimers. Output powers of other excimers were 300 mW with 1.0% efficiency at 146 nm for krypton excimers and 500 mW with 1.6% efficiency at 176 nm for xenon excimers. Simultaneous emissions from hetero-nuclear rare gas excimers (ArKr*, 135 nm) as well as homo-nuclear rare gas excimers (Ar₂* and Kr₂*) were observed by using rare gas mixtures of argon and krypton. Output powers and efficiencies of the silent discharge excimer lamps were 500 mW and 1.6% for argon, 5 W and 13% for krypton, and 5 W and 20% for xenon excimers. In the silent discharge extremely broad band excimer emissions were observed at the center wavelengths of 145 nm for an argon/krypton mixture and of 163 nm for a krypton/xenon mixture. A PMMA plate was photo-chemically etched at the rates of 1 - 2 nm/min by the irradiation of the 172 nm radiation in air and argon gas atmospheres.

The uniformity and stability of discharge process in a high pulse repetition frequency (PRF) long pulse XeCl laser are investigated for three different copper electrode roughnesses versus PRF (f less than or equal to 500 Hz; 10 shots). The discharge quality evolution is experimentally analyzed from discharge photographs obtained with a CCD video camera and pressure perturbation measurements achieved with a piezoelectric pressure probe placed very close to the discharge volume.

Alkali rare gas triatomic ionic excimers have been observed for the first time. These molecules have been produced by electron beam excitation of rare gas alkali mixtures at high pressure and high temperature. Emission spectra centered at approximately 159 nm have been observed and attributed to Kr₂⁺Cs ions. Lifetimes and binding energies of the lowest excited states have been estimated. Simple kinetic considerations allowed us to propose two formation processes of these ions and to estimate their rate constant.

Experimental and theoretical results are presented of a XeF(C- A) blue green laser driven by 5 kJ energy. The laser was pumped by a ferrite induced discharge of 90 cm long. An output energy of 0.22 J has been obtained with a plane-parallel resonator. A code to simulate the laser has been developed. Numerical results for a wide range of conditions are compared with experiments performed by us and other authors. It is found that the intracavity refractive losses limit the laser operation for XeF₂ pressures above 3 Torr. The laser output depends strongly on the discharge-cavity length ratio. Possible ways to increase the laser power and efficiency are discussed.

A dielectric barrier discharge type Xe excimer lamp has been used for etching of some organic materials which can be widely used for opt-electric applications. Using polymethylmetacrylate (PMMA) and polyimidic (PI) resin as samples, they were irradiated at an intensity of 12 mW/cm². In both cases, etch rates could be varied by changing the species and pressures of gases in the chamber. The maximum etch rates for PMMA and PI were approximately 7 and 10 nm/min, respectively. The etched surface was found to be very smooth compared to the surfaces etched by the ArF excimer laser. Because the excimer lamp produces incoherent radiation, uniform irradiation over a large area without speckling or interference fringe has been obtained. Furthermore, because the peak power of the lamp is significantly lower than that of excimer lasers, effective quantum effects and photochemical effects without thermal effects have been observed. We applied this technique for etching single crystalline organic opt-electric materials such as 1-arginin phosphate monohydrate and 1-histidine tetrafluoroborate.

In this contribution, the condensation theory of I. M. Lifshitz and V. V. Slyozov is used to calculate the formation of metal clusters in the plasma/vapor regime above an aluminum target in a nitrogen atmosphere of 1 bar irradiated with 248 nm, 30 ns FWHM pulses. This theory takes into account formation of clusters by fluctuations in the supersaturated medium (plasma/vapor). It also accounts for both cluster growth due to atom-cluster-collisions (nucleation) and collisions between clusters (coalescence state). This condensation model is integrated in a gasdynamic code which calculates the flow field and the thermodynamic properties of the plasma/vapor. In addition, the density of electrons, clusters, and neutral particles are determined and, finally, with the appropriate polarizability the refraction index can be calculated. The theoretical data are discussed together with the results of interferometric measurements.

Results of experiments investigating the influence of the XeCl and KrCl laser beams on the generation of electron beams up to short threshold are reported. The cathode was a lead- lanthanum-zirconium-titanate PLZT ceramic having the emission side coated with 200 micrometer large strips made up gold film. Under the laser irradiation the emission current increased more than 40%.

The kinetic parameters of ejected species during excimer laser ablation of a NbTe₄ target are investigated by optical time-of-flight spectroscopy. Velocities of monoatomic neutrals, ions and molecules are calculated for different laser fluences.

A brief review of laser ablation mechanisms and applications is presented. Topics covered include polymer ablation, thin film deposition, tissue ablation, and chemical analysis.

The polymers ablation by a Q-switched carbon-dioxide laser is demonstrated and compared with that of the conventional TEA laser ablation. The 20 - 100 micrometer depth ablative etching of PET with well defined edge were successfully obtained when the strong absorbed 9R(20) line at 1078.57 cm^-1 was irradiated. It was found the ablation characteristics by a Q- switched laser was almost same as that by a TEA laser.

Using excimer lasers in DuPont's laboratories in Wilmington, we showed that a wide variety of nylon and polyester fabrics can be successfully irradiated to produce fabrics which combine the attractive visual and tactile aesthetics of cotton or wool fabrics with the durability, wrinkle resistance, and wash-and-wear performance of fabrics of 100 percent nylon or polyester. Extensive wear tests in such demanding end uses as bicycle pants proved that the desired aesthetic improvements were retained in use.

In recent years the copper laser has undergone extensive development and has emerged as a leading and unique laser for micromachining. The copper laser is a high average power (10 - 250 W), high pulse repetition rate (2 - 32 kHz), visible laser (511 nm and 578 nm) that produces high peak power (typically 200 kW), short pulses (30 ns) and very good beam quality (diffraction limited). This unique set of laser parameters results in exceptional micro-machining in a wide variety of materials. Typical examples of the capabilities of the copper laser include the drilling of small holes (10 - 200 micrometer diameter) in materials as diverse as steel, ceramic, diamond and polyimide with micron precision and low taper (less than 1 degree) cutting and profiling of diamond. Application of the copper laser covers the electronic, aerospace, automotive, nuclear, medical and precision engineering industries.

Diffractive optical systems have been developed to allow high efficiency material processing using excimer lasers. These systems allow beam homogenizing and shaping for surface treatment and parallel focusing for multiple hole drilling. Beam utilization factors of 80% have been achieved.

A high power pulse CO laser operating at room temperature has been developed using the modulated pulse rf discharge excitation technique. The fast pulse response and the good reproducibility of laser pulse were obtained for pulse frequency up to 1 kHz with a stable and homogeneous discharge. A maximum peak power of 920 W with a pulse width of 1.0 ms FWHM has been attained by the optimizing N₂ and CO concentration.

An industrial CO laser with output powers in excess of 3 kW and capable of operation at room temperature has been developed within a collaborative Eureka project. The design of this laser employs dc excitation and fast axial gas flow technology within a closed-circuit flow loop. The design and performance characteristics of this device are described.

An industrial high-power laser is a technical system to be characterized primarily by the efficiency. For a high-power laser system to become like an industrial one the efficiency must be more than 10%. As is well known a steam-engine has such an efficiency. In welding and in cutting thick materials to provide required power density in a spot for the device with long focus the value of output power of radiation must be no less than 100 kW at beam divergence 10^-3 rad. At the present time there is a problem in concurrent fulfillment of the requirements on an output power, the divergence, and the efficiency as well as the requirements on the stability of output parameters, total resource of operation, the safety of operation, and the use of standard components. A line of attack on this problem is proposed by the present author through the use of continuous formation of a CO laser mixture by combustion of a chemical fuel and the use of atmospheric air as a buffer gas (up to 80%), which is cooled in supersonic nozzles followed by excitation in a radio-frequency (rf) electric discharge without an electron gun. A small-scale model system of electrogasdynamic CO laser was used by the present author and his colleagues to demonstrate for the first time the laser radiation was possible in a system with combustion products and air. A technical proposal for a multipurpose self-contained industrial cw high-power CO laser system is proposed. This laser system is based on standard electrical machinery with a gas-turbine drive without ejecting toxic CO into the atmosphere.

The calculations of energy characteristics, operating regimes and dimensions of the discharge zone of the supersonic e-beam sustained CO laser are presented. Calculations have been carried out for the laser modules with a power up to 40 megawatts.

The carbon-dioxide gas dynamic laser driven by methane-air combustion in an unsteady flame propagation combustor was proposed for the purpose of industrial application. The effect of coexisting water with high concentration on the population balance between carbon-dioxide(001) and carbon-dioxide(100) levels of vibration-rotation modes was theoretically analyzed. The analysis showed that the population inversion between the two levels can take place with the methane-air combustion at stoichiometric ratio if the gas flow is accelerated to Mach number 5.0. The population inversion was confirmed by the experimental measurement of the small-signal gain carried out applying the unsteady flame propagation combustion. High emission power of about 2.5 kW with the path length of 0.3 m may be expected from the magnitude of the gain.

The first purpose of combustion-driven carbon-dioxide-gas- dynamic lasers (GDL) was military applications. Just these applications aimed to development of more and more powerful systems led to the appearance of more and more expensive lasers using very dangerous and poison fuels. It is very hard to satisfy simultaneously both military and economic requirements, therefore, this idea could not be realized widely and successfully. In this way the lasers had no chance to be suitable for ordinary users. New approaches at the old idea of combustion-drive carbon-dioxide-GDL allow us to open some ways to realize the lasers for education and research projects. The most inexpensive 10 - 15 kilowatt and 400 megawatt lasers are considered. Experimental results, technical and financial estimates are presented also.

The recent trend in microelectronics towards patterning critical feature sizes of 0.25 micrometer and below has motivated the development of microlithography at the deep ultra-violet (DUV) laser wavelengths of 248 and 193 nm. In recent years the performance, reliability, and cost of ownership of excimer light sources have improved. Some key technologies needed for excimer lasers in microlithography include materials issues, gas lifetime, higher repetition rates and improved pulse-to-pulse energy repeatability. An experimental demonstration of a new method to generate nearly nondiffracting Bessel beams using a Fabry-Perot interferometer is described. It is experimentally demonstrated that the DOF can be increased by a factor of 2 and simultaneously the transverse resolution improved by a factor of about 1.6, when using this technique to image contact holes.

In response to the requirement for higher wafer throughput and increased dosage accuracy in DUV lithography steppers and scanners, Cymer has developed a 1 kHz KrF laser optimized for this application. We shall describe its performance and design features.

A 193 nm excimer laser microstepper has been developed for deep UV photolithography research and development and system details are presented. The tool incorporates a times 10, 0.5 NA, 4 mm field diameter, high-resolution imaging lens of either all-refractive or catadioptric design. An all-fused silica refractive lens has been used in the results reported here to carry out exposures in polymethylmethacrylate (PMMA) and polyvinylphonel (MX-P8) photoresists. Well-resolved images of 0.2 micrometer dense lines and spaces have been produced in the PMMA and MX-P8 resists.

Diamond-like carbon (DLC) films were deposited using the laser-assisted physical vapor deposition (LPVD) method. This work is dedicated to an XeCl-excimer laser PVD procedure. The influences of the power density, working distance, and substrate temperature on the DLC-deposition are discussed. The propagation velocity of the laser ignited plasma was determined using short-time photography and time-of-flight (TOF) method.

Picosecond laser pulses were used for microstructuring metals, ceramics and diamond. Using 40 ps laser pulses line patterns with a minimum line width in the range of 7 micrometer were formed in diamond without destroying the crystallinity. In Si₃N₄-ceramics holes were drilled with diameters smaller than 6 micrometer. Ultrathin copper and gold films of 1 micrometer thickness were removed from fused silica and silicon (100) without influencing the substrates. The ablation process was investigated by high-speed photography and pump-&- probe-measurements with a time resolution of 40 picoseconds. The measurements allow a detailed characterization of the melting, vaporization and solidification processes. Shock waves in the ambient atmosphere were detected and the energy in the shock wave was determined.

The paper reviews recent developments in high power excimer laser technology driven by industrial requirements. Technological achievements as NovaTube^TM laser tube technology and HaloSafe^TM halogen generator technology are discussed. Experimental results are presented for various lasers at the most important excimer wavelengths 351 nm (XeF), 308 nm (XeCl), 248 nm (KrF), 193 nm (ArF) and 157 nm (F₂) which have been designed for application in micromachining, thin-film-transistor annealing, marking as well as lithography.

The DF performance of the compact, repetitively pulsed HF/DF chemical laser, described at the last GCL, has been improved by using C₆D₁₂ instead of D₂ as deuterium donor. The output energy previously achieved with D₂ was low, less than 3 J per pulse compared to the HF energy of 10 J obtained with C₂H₆ as hydrogen donor for the same experimental conditions. Unfortunately, it was not possible to employ C₂D₆ because of the excessive cost of this deuterated compound. Therefore we made a search for a not too expensive deuterated hydrocarbon and we found that C₆D₁₂ could be an interesting product. We first tested C₆H₁₂ as a hydrogen donor instead of C₂H₆. Very good results were obtained on HF emission since the drop off in output energy was only about 5%. Consequently, C₆D₁₂ was ordered and tested. Output pulse energy up to 8 J was achieved at a charging voltage of 43 kV and a total pressure of 120 torr. In repetitive operation, typical average power of 450 W was achieved at a repetition rate of 65 Hz.

The design and performance of a closed cycle high repetition rate HF laser is described. A short pulse, glow discharge is formed in a 10 SF6:1 H2 gas mixture at a total pressure of approximately 110 torr within a 15 by 0.5 by 0.5 cm³ volume. Transverse, recirculated gas flow adequate to enable repetitive operation up to 3 kHz is imposed by a centrifugal fan. The fan also forces the gas through a scrubber cell to eliminate ground state HF from the gas stream. An automated gas make-up system replenishes spent gas removed by the scrubber. Typical mean laser output powers up to 3 W can be maintained for extended periods of operation.

A diffraction grating was used in the Littrow and off-Littrow configurations to select the lines in a cw HF stable resonator. The beam is outcoupled through a partially reflective mirror. Single line Littrow, single line off- Littrow, and off-Littrow line pair performance were measured. For the first time, as far as we know, in the operation of chemical lasers, Littrow and off-Littrow lines lased simultaneously. Two line (one Littrow line, one off-Littrow line) and three line (one Littrow line, two off-Littrow lines) Littrow/off-Littrow line combinations were demonstrated. The grating reflectivity was a strong function of the polarization of the laser beam. Data indicated that the grating generated a beam whose polarization was perpendicular to the grating grooves.

The non-chain reaction HF laser energized by an x-ray photo- triggered discharge has been studied for Ne/SF₆/C₂H₆ and Ne/SF₆/H₂ gas mixtures. For an active volume of 312 cm³, a high specific output energy of 10 J/l has been reached with an electric efficiency of 4.7% for the mixture with ethane, whereas only 6 J/l is obtained with an efficiency of 3.1% for the mixture with hydrogen. It is shown that discharge instabilities are responsible for the laser emission disruption in the case of the H₂-mixture. However, addition of C₂H₆ to Ne/SF₆, in place of H₂, induces the discharge stabilization. The higher laser performance obtained with the C₂H₆-mixture, compared with the performance of the H₂-mixture, is a direct consequence of this stabilization effect.

The characteristics of 45 liters active volume chemical laser, which chain reaction was initiated by two contrary electron beams with 1 Hz repetition rate, have been investigated. Maximum single pulse energy was about 5 KJ, radiation divergence -- near 10^-4 rad.

The role of laser beam parameters and material properties in establishing a stable deep welding process is investigated, experimentally and theoretically. Particular emphasis is placed on the mechanisms involved in the energy coupling. It is shown that high quality welds are achieved by a proper choice of parameters and a sophisticated process conduction.

Average electron densities and temperatures were measured for both carbon and carbon-dioxide laser induced aluminum welding plasmas using spectroscopic techniques. The plasma temperature is smaller in the case of carbon laser welding, whereas the electron density is slightly higher. Therefore the absorption length (inverse bremsstrahlung) of the carbon laser radiation in the welding zone is a factor of 2 to 7 longer. This is the reason for the better weld seam quality obtained with the carbon laser.

A numerical beam propagation model has been developed to calculate the absorption and defocusing of a carbon-dioxide laser beam by the laser induced plasma plume during deep penetration welding. By directly solving the paraxial wave equation with a finite difference scheme, the model allows us to calculate the laser beam propagation through the plasma plume and to determine the 'effective' intensity distribution in the focal plane. In combination with a previously published model that takes into account Fresnel absorption at the keyhole walls and plasma absorption inside the keyhole, now a calculation of the welding depth as function of the process parameter becomes possible. In particular, the propagation model clarifies the role of plasma absorption above the workpiece. The calculations identify the defocusing of the laser beam by the plasma plume as the main mechanism that degrades the laser deep welding process. Absorption by the plasma is shown to be of little importance. Additionally, the propagation model elucidates the process stabilizing potential of shielding gas mixtures, especially of helium and argon mixed in a ratio of 3:1; a result also of phenomenological investigations that are used frequently in practice.

A continuous carbon-dioxide laser of 1.35 kW has been used to study the welding of 5 mm thick stainless steel for pressures ranging from 0.1 to 0.8 MPa in increments of 0.1 MPa. Experimental data, including penetration depths, weld widths, and in some cases weld pool profiles, has been obtained for each value of the pressure using different mixtures of argon and helium shielding gases. In a previous paper it has been reported that keyhole welding could not be carried out for pressures significantly in excess of atmospheric pressure using pure argon and nitrogen shielding gases, but that the process was possible at pressures up to 0.8 MPa using helium. In the present paper the critical pressure for keyhole welding is determined as a function of the mixed shielding gas composition. The laser material interaction is analyzed by solving the heat conduction equation with line and point heat sources representing the keyhole and plume respectively. The line source strength is itself calculated from consideration of the inverse bremsstrahlung and Fresnel absorption processes in the keyhole. It is concluded that successful laser welding in the hyperbaric range crucially hinges on good plume control through the effective delivery of an appropriate shielding gas mixture.

We describe a non-intrusive optical sensor for process monitoring of Nd:YAG laser welding, using light returned through the core of the power delivery optical fiber. This sensor is referred to as the core power monitor (core PM), and uses the delivery fiber to collect the broadband light generated in the process, which is then divided into spectral bands (designated as UV/visible and IR). These optical signals exhibit a characteristic oscillatory intensity modulation within the frequency range 2 - 5 kHz, which is believed to arise from a combination of keyhole, and weld pool oscillations. The frequency content may be related to the size and shape of the welding keyhole, and an alarm system for overlap weeding has been developed based on this principle. This can detect both misalignment of the focused laser spot off the seam, and any excessive gap between the plates.

The thermal radiation from the weld-pool is measured at two wavelengths through the laser delivery fiber between Nd:YAG laser pulses. The chromatic aberration of delivery optics has been optimized to detect the variation of weld bead width. The design of optics and the signal processing algorithm for optical monitoring is described. Furthermore, the applications of optical monitoring to the detection of power variation and focus shift are shown.

Gain saturation and diffractive loss data have been collected on the Phillips Laboratory's VertiCOIL laser. These data have been applied to the COIL simplified saturation model to estimate the optical extraction efficiency of VertiCOIL.

The supersonic chemical oxygen iodine laser (SCOIL) is full of promise as an industrial laser due to its high power and efficiency. The most important part of the SCOIL is a singlet oxygen generator (SOG) which produces the energy source of the laser system. Although there are several types in SOG, a jet type SOG (JSOG) is expected to be the most suitable for the SCOIL. We conducted tests of a slit nozzle JSOG that is a new type of JSOG and compared it with the generally used round nozzle JSOG.

For a rotating disk type generator the dependencies of utilization and yield on the generator operating mode are experimentally investigated. The fundamental effects of rotational speed, reduced gas volume, gas flow and liquid phase composition on the efficiency of singlet delta oxygen generation are discussed together with theoretically predicted generator performace from literature.

The photolytic pulsed iodine system PERUN has already been described. This contribution presents its improvements within the last few years according to an increasing demand of interaction experiments. As examples of its use a short description of some interaction experiments currently being performed with the system PERUN -- self-smoothing effect of double-pulse laser plasma, production of highly charged ions for particle accelerators, passage of laser light through a hole (plasma shutter) -- are given.

Second harmonic generation of chemical oxygen iodine laser was investigated with a Brewster cut LBO crystal. By utilizing a Brewster cut LBO crystal the loss in the resonator can be suppressed. Further, by reducing crystal absorption, the crystal can't be heated and go off phase match or even crack due to thermal stress. We could obtain 16.4 W of second harmonic power and keep out the crystal from being destroyed by the damage of thermal stress.

Experimental investigation of a jet singlet oxygen generator for a supersonic chemical oxygen-iodine laser was performed aimed to evaluation of the effects of BHP temperature and composition on the water content and other output generator parameters. Laser experiments on a small-scale system were realized to prove the obtained results.

A 1D kinetic model of COIL performance based on the concept of mixing length is described. The results of calculations are compared with the experimental data measured on the COIL device in the Institute of Physics, Prague.

The flow field of a supersonic flow chemical oxygen-iodine laser is simulated solving three-dimensional Navier-Stokes equations, and the dependence of the mixing/reacting zone structure and the resulting gain region on the effective velocity ratio of I₂ jet to the primary flow is studied. It is assumed that the flow is laminar and the water vapor condensation due to the supersonic cooling is ignored. A chemical kinetic model encompassing 21 chemical reactions and 10 chemical species is used to determine the chemical composition of gas mixture. The I₂He ratio and plenum pressure of the secondary flow are varied in order that the amount of iodine injected into the primary flow is kept constant in each effective velocity ratio. The present results demonstrate that a pair of contrarotating vortices generated behind the I₂ jet greatly enhances the mixing and the simultaneous chemical reaction of I₂ and O₂(¹$DELTA). It is shown that the optimum condition for the secondary I₂ jet momentum exists. The I₂ jet which causes the high gain penetrates into the primary flow moderately deeply and does not collide with the counter one.

The instability of the liquid jets in a jet singlet oxygen generator for oxygen-iodine lasers may limit the operational conditions and the scalability of such generators. Therefore the type of instability and the breakup behavior of jets is investigated experimentally for varying operational conditions. Jets of basic hydrogen peroxide are simulated by the use of glycerin solutions with different viscosities. Transition from laminar to turbulent jets, the associated breakup modes and the beginning of jet dissolution by surface spraying are observed. Jet lengths have been measured for various parameters, including gas counter-flow.

The present paper describes the experimental results for aerosol-jet singlet oxygen generator with a twisted flux to separate the aerosol effectively. It was demonstrated that at 50 torr pressure and 30 m/s velocity, there does not exist the aerosol in the flux at the reactor output, and singlet oxygen contribution constitutes 70%, not less. The numerical simulations results are given for supersonic COIL up to 50 torr pressure in the nozzle input with and without inertia diluent in the main flux.

A 400-keV electron beam has been used to initiate HF chemical laser pumped by the nonchain reaction. Parametric studies of the energy deposited and laser performance were performed to determine optimum pressure and composition of SF₆/H₂ or C₂H₆ mixture.

We describe results of experiments on optically pumped HF(0-2) and DF(0-3) transitions using a tunable and narrow-line Cr⁴⁺:Forsterite laser. Cascade lasing transitions on 2- 1 band of HF and on 3-2 and 2-1 bands of DF have been obtained. A computer modeling on DF has also been developed in conjunction with experiments to predict and compare the results obtained.

A sliding-discharge excited SF₆:propane-butane:N₂ laser emitting simultaneously in the UV and IR region is investigated experimentally. The shapes of the voltage, current and laser (UV and IR) pulses are obtained and shown in their sequence. The IR laser spectrum consisting of more than 20 separated lines is registered and their intensity redistribution is found out when the N₂ pressure increased.

The aim of the present investigation is to forecast a principle opportunity of a pulsed DF to HBr laser frequency conversion and to evaluate an efficiency of that process. Computer modeling of the pulsed DF-HBr optical resonance transfer laser (DF-HBr ORTL) is performed and output characteristics of the system are predicted. The ORTL gas mixture contained DF at 10 torr, HBr at 10 torr and He at 100 torr. It was considered as a multiline pumping. The pump radiation flux was varied from 0.02 to 2 J/cm². Depending on conditions the length of an ORTL gas media must be between 2 and 30 cm at using the longitudinal pump geometry. When DF- HBr gas mixture is pumped by a pulsed flash-initiated chemical DF laser, it is feasible to produce HBr lasing with a specific output energy over 100 mJ/cm³ and efficiency of 40% relative to absorbed radiation energy. When used for pumping, a pulsed electrical discharge chemical DF laser can provide HBr lasing with specific energy up to 20 mJ/cm³ and efficiency up to 25%. Therefore one will anticipate that a highly efficient laser system of the 4.2-5.2 micrometer spectral region can be developed.

Performance has been investigated of a broadband chemical laser utilizing chain and nonchain reactions and emitting simultaneously from excited HF* and DF* molecules. Simultaneous stimulated emission of radiation in the spectral ranges of 2.7 - 3.0 micrometer and 3.7 - 4.1 micrometer has been obtained. Theoretical dependencies are presented of laser energy characteristics on the ratio of initial H₂ and D₂ concentrations in a working mixture.

Ways of achieving extreme energy characteristics of pulsed DF/HF lasers were determined as results of theoretical and experimental (at energy up to 5 kJ) investigations of generator and amplifier. The close output energies of DF and HF lasers and higher efficiency of the cascade circuit relative to an equivalent generator were obtained.

Atmospheric aerosols can limit laser beam propagation and reduce energy transfer to work-pieces by creating shielding plasmas. The purpose of this paper is to get results for plasma ignition thresholds depending on incoming laser power density, kind and density of impurities in the interaction volume.