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.