Two prototypes of a compact high average power XeCl industrial laser have been built and tested. They use the fast photo triggering principle with a 400 ns voltage rise time and a fast corona preionizer. This voltage rise time is short enough to eliminate self-breakdown and sufficiently long to strongly reduce the electrical stresses on the main thyratron. The design includes in the same stainless steel cylindrical vessel the complete gas circulation system, the discharge and the complete high current and high voltage circuitry. The overall volume, including gas bottles, vacuum pump, blowers motors and high voltage supplies, is less than 4 m³. Due to the special geometry of the discharge current loop, a very uniform field is achieved between the electrodes and along the insulators surfaces while keeping the inductance very low. This allows a good beam quality and a very good efficiency even at high repetition rate. Energy output and efficiency have been studied as a function of charging voltage, repetition rate frequency and preionization timing. Five hundred Hz continuous operation has been successfully tested on the first prototype while the second one has ben operated above at 400 Hz in burst mode. An intrinsic efficiency greater than 4% and an overall efficiency greater than 3% (blowers excluded) have been demonstrated.

At the NCLR an industrial 1 kW XeCl excimer laser, delivering 1 J at 1 kHz, is under development. Particular emphasis is on reliable long-term operation with moderate operational and maintenance costs. The design challenges can be grouped into characteristic operational time durations. The initial step to scale a single shot laser to a kilohertz device concerns the attainment of a sufficient clearing ratio in the active gas. Furthermore, the optical quality of the laser gas must be maintained also at high frequencies, which requires a low degree of turbulence in the gas flow and minimization of shock waves generated by the discharge. Laser operation for several hours primarily demands a thermal management of all active components. In addition the gas quality needs to be maintained, which concerns the chemical composition, the amount of impurities and occurrence of dust in the gas. Finally, continued laser action for billions of shots puts tight requirements on the lifetimes of the gas, the electrodes, the mirrors and the excitation system. These issues are largely determined by proper choice of materials but to a major part also by the conceptual layout of the laser. Therefore, a choice of technology is required which is very different from the spark-preionized charge transfer excitation predominantly found in commercially available excimer lasers.

An industrial high power laser with a maximum output power above 30 kW is presented. Particular reference is made to the compactness and the beam properties of the laser, which are comparable to lasers with much lower power. In order to meet these requirements, we had to optimize the gas flow, and to fine tune the discharge properties. The laser system consists of 8 discharge tubes and 4 turbo blowers; it can be operated from cw to pulsed operation up to 100 kHz over a power range from 5 to 100%. With an unstable resonator and an aerodynamic window the laser has an output power of 25 kW and an unmatched beam quality better than K equals 0.3. This high focusability, the high peak intensity of the far field beam of the unstable resonator and the temporally stable beam guarantee an excellent welding performance. Detailed beam quality analysis of the unfocused and the focused beam is described.

Diffusion cooled carbon-dioxide laser sources allow nowadays to produce more than two kilowatt output power with weights, dimensions and costs reduced with respect to the traditional fast axial flow sources of the same power level. In particular, they can be easily integrated in traditional as well as in robot-laser workstations. These kinds of sources are characterized by two large area water cooled coaxial or planar electrodes with small spacing (few millimeters) in order to guarantee a good mixture cooling. Consequently the discharge geometry results 'optically hard:' a thin ring or a thin rectangle. In this communication we present theoretical and experimental results concerning the design and development of the optical resonator and of the external optical chain for a carbon-dioxide laser prototype. The work was done in the frame of a national project. The prototype is a diffusion cooled, rf excited slab source with 1 kW output power. The surface of each electrode is 110 by 700 mm and their spacing is 2 mm. For this geometry a hybrid unstable-guided resonator has been adopted. The main problem of this configuration is that the extracted beam is elliptical and astigmatic, and therefore needs to be manipulated before the working point where high optical quality is required for material processing. A particular computer code has been developed to calculate the resonator modes and the calculated profiles of the beam have been compared with the measured ones with and without external optical chain. The good agreement of the results confirms that the criteria adopted for the simulation are correct and that the code developed can be successfully employed in the design stage. This is particularly significant for this class of sources because an extensive experimental study of different optical combinations of the resonator mirrors can become expensive. In fact the mirrors have large dimensions (about 110 by 40 mm) and non standard curvature radii. Moreover they need an optimized high reflectance coating for the high degree of pollution due to the particular discharge conditions. For these reasons a good simulation 'tool' can be useful to save time and money.

A frequency selected (FS) Q-switched (QS) e-beam controlled- discharge (EBCD) CO-laser has been researched and developed. The laser generates short pulses (tauout approximately 1 - 10 microseconds) having different spectral contents including single line one with 4.95 - 6.50 micrometer spectral range. A special optical scheme has been chosen, which has enabled us to study an influence of spectral contents of CO laser radiation 'locked' or 'blocked' inside a laser resonator on output energy and efficiency of lasing on selected wavelengths. The influence of laser mixture contents, gas pressure and temperature, laser pulse length and spectral width upon the laser characteristics also has been studied. A use of frequency selection and Q-switching decreases the laser efficiency dramatically from approximately 30% for non-selected (NS) free-running (FR) mode of operation down to approximately 0.5% (tauout equals 5 microseconds) for optimal wavelength (lambda approximately 5.3 micrometer) and to 0.1% at the long wave edge of laser spectrum (lambda approximately 6 micrometer). Output energy and laser efficiency strongly depend on the number of short pulses in a train for a single electrical pumping pulse. For instance, eight short pulses lasing (tauout approximately 5 microseconds) has increased the output energy up to approximately 0.5 J (0.6 J/l Amagat) and efficiency up to 0.6%. For some wavelengths the output energy and laser efficiency of FS QS CO laser does not strongly depend on temperature within 100 - 150 K interval. Comparative analysis of NS FR, FS FR, NS QS and FS QS mode of operation of carbon monoxide laser has been done.

We present the results of a compact three-electrode XeCl laser operated in a power oscillator-power amplifier (POPA) configuration with transverse mode selection. The smaller of two laser heads is an oscillator, equipped with a generalized self-filtering unstable resonator (GSFUR). The oscillator output beam has an energy of 9 mJ in a 75 ns FWHM pulse and is only 1.1 times over its diffraction limit. The single-pass amplified beam has an output energy of 90 mJ in a pulse 85 ns long and, with a times diffraction limit (TDL) equal to 1.6, reaches a brightness of 3 (DOT) 10¹⁴ Wcm^-2ster^-1. Operating the laser at low repetition rate (1 Hz), the beam pointing stability (BPS) of both oscillator and amplifier output beams has been measured. The angular fluctuations of the focused beams are each within 1/6 of their own total beam divergence.

Shock hardening of metals (e.g. Ti, stainless steel) by pulsed lasers offers the possibility of large hardening depth (several millimeters) without serious damage to the surface of the workpiece. Previous investigations for shock hardening have mainly been performed with high power solid state lasers. The adaptation of commercial, high power gas discharge lasers to the shock hardening process could make this process relevant for industrial applications, as high repetition rates may be used. Two different laser systems have been investigated: a TEA carbon-dioxide laser and a XeCl laser. Both systems have pulse energies of some joule, a pulse length of several ten nanoseconds, and pulse repetition rates of up to 10 Hertz. The divergence of the beam was minimized to improve focusing properties. Systematic measurements of the laser induced pressure by means of piezo probes have been performed. An enhancement of the hardness of illuminated Ti(RT15) targets has been found and is reported.

Results of investigations on pulse lengthening of a TE- carbon-dioxide-laser for materials processing is are reported. The laser is used in two main modifications both working at an absolute pressure of lower than 300 mbar. With a folded resonator pulse durations up to 20 microseconds have been achieved. As there is no longer the initial peak but a descending saw-tooth like form, on can get mean powers up to 100 kW and maximum powers up to 300 kW. Using a so- called hybrid-resonator containing a cw-carbon-dioxide- laser-section, there are even better pulse shapes within about 10 - 15 microseconds, but containing lower pulse energies up to 1/2 joule. The influence of parameters e.g., absolute pressure of working gas or gas mixture on pulse form and duration and results of cutting and drilling of metals like Al or Cu are presented.

The equivalence relations for pumping systems of the active media in gas lasers with an electrodeless gas discharge are obtained. They permit us to present the results of the model calculations and experiments in unique invariant form and to employ them in the developing of the gas laser's workable pumping systems.

The non-chain reaction HF laser energized by an x-ray photo- triggered discharge has been studied for Ne/SF₆/C₂H₆ and Ne/SF₆H₂ gas mixtures. For an active volume of 312 cm³, a high specific output laser 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. The study of the discharge development shows that the total production of HF molecules weakly depends on the gas mixture type, so that the plasma kinetic is not responsible for the difference observed between the laser performances obtained with the two mixtures. Time and spatially resolved measurements of the plasma fluorescence shows that discharge instabilities occur in the Ne/SF₆ discharge, which are responsible for the HF laser emission disruption when hydrogen is added. But addition of ethane at about 3% of the total pressure induces the discharge stabilization, and allows the laser emission to occur in an homogeneous medium. As a result, this work clearly establishes the physical reason of the observed laser performance improvement when hydrogen is replaced by some hydrocarbons in discharge pumped HF laser.

One-dimensional unstable resonators with compact output coupling and small magnification factors typical for molecular gas lasers are considered. The properties of the output beams for slab carbon-dioxide lasers with waveguide- unstable cavities have been investigated. It is shown that the losses of similar resonators without side walls depend strongly on the mirrors misalignment, and radiation power fluctuates in a wide range. At the same time, the reflecting ceramics side wall, positioned along the edge of active medium near optical axis allows to stabilize low cavity losses as well as high output power. The change of spatial structure of beams with distance from laser head is studied. It is found that the variations of near-field patterns from uniform distribution caused by diffraction is little affected by the far field radiation divergence. The distances that are necessary for conversion of output beam shape to smoothing-out one-peak profile were determined depending on the uniformity degree of initial distribution. The obtained results can be used in the development of slab gas lasers, when high specific power and good quality of output beam are required.

Results of experimental investigations of output power and beam divergence for a slab waveguide rf-excited carbon- dioxide laser are presented. Laser output power 150 - 250 W and maximum efficiency 10 - 13% in a beam with near- diffraction-limit divergence has been achieved. In repetitive-rate mode, the pulsed power was more than twice as much as the average one in the cw mode. After beam shaping lens, the sum beam divergence of 2.5 mrad at the 0.8 total power level has been obtained.

Degenerate four-wave mixing (DFWM) and phase conjugation (PC) of pulsed carbon-dioxide laser radiation on transient gratings inside its own laser medium have been studied both experimentally and theoretically. The operational mode of the e-beam controlled discharge carbon-dioxide laser has been chosen in such a way (CO₂:N₂:He equals 1:2:4, p equals 0.28 atm, t_in equals 30 microseconds) that the laser pulse length (tau_out approximately 10 - 20 microseconds) is comparable with an effective relaxation time of the upper laser level (tau_rel approximately 15 - 30 microseconds). The time-history of the carbon-dioxide laser and pc signal pulses, and pc reflectivity have been thoroughly investigated for different cavity Q factors and specific electrical energy inputs. It has been shown, that the formation of the PC signal under the intracavity DFWM on gain (amplitude) and thermal (phase) gratings is characterized by a complicated time history that reflects the main relaxation processes taking place inside the inverted medium. A feature of the transient pc process inside the laser active medium is the dependence of the effective relaxation time of the upper laser level on the intensity of pumping waves. Pc reflectivity has been obtained by numerical calculations. A comparison of the theoretical and experimental data confirms the contribution of two different mechanisms of grating formation under DFWM inside the active medium of the carbon-dioxide laser.

The focusability of a long pulse XeCl excimer laser has been improved using confocal positive branch unstable resonators where the outcoupling is done through the convex mirror. For the outcoupler different reflectivity profiles are used. A near diffraction limited output beam is obtained from hard edge unstable resonators. A beam with only one central spot in the focus of a lens can be obtained with a resonator fitted with a Gaussian outcoupling mirror.

Induced beam scattering at small angles in the acoustic resonator has been experimentally discovered and theoretically explained. For a single-mode, excitation scattering angle as well as the acoustic vibration frequency are proportional to the emission intensity and the area of the longitudinal section of resonator. The higher beam divergence is the higher is the initial field and acoustic vibrations intensity in the resonator. For rather high longitudinal dimensions of the acoustic resonators a theory has been summarized in case of multi-mode excitation of acoustic waves during Mandelstam-Brillouin stimulated scattering (MBSS). If it happens an angular spectrum of scattered waves must be excited which is located near angle Bregg corresponding to the lowest space frequency of acoustic vibrations. In case of considerable energy pumping into the scattering light, Bregg peak of angular distribution will be close to the half-divergence angle of pumping or priming. During the experimental studies of wave- front (WF) structure (by Hartmann method) of the emissions formed by ORU (optical resonator unit) with various magnifications and lengths of 'cold' focusing, the hartmonogrammes gave triplet crosses over the vertical which is the result of the presence of several beams with different wave fronts in the Hartmann matrix plane (HM). It may be shown indeed that for the emission lambda equals 10.6 micrometer primary energy transition into scattered light in the vertical plane may be done for very small acoustic waves reflection ratios, r_o greater than 10^-2. When a camera with face walls having windows for the beams path, the magnification of beams divergence was also detected, which coincides with the scattering angle calculated from the criterion of multimode excitation. The effect was not detected without the face walls. Conditions are determined under which wave-front distortions of the beams are minimal.

To increase the beam quality of existing lasers we developed flash lamp pumped master oscillator power amplifier systems (MOPA) with phase conjugating SBS-mirrors. The used Nd:YALO as active material shows no remarkable stress birefringence resulting in a simple optical arrangement. With a single amplifier rod average output powers of 4 watt up to 140 watt were realized with a beam quality of 1.1 times the diffraction limit. Serial arrangements of amplifiers were investigated to increase the average output power. By using an optical system between the individual rods the pump power can be tuned over a wide range. The parameters of the used two lens system can be optimized to avoid damage of optical components and secure a high efficiency independent of thermal lensing. With a two amplifier system an average output power of 1 watt up to 210 watts in a near diffraction limited beam could be realized. The system works with a repetition rate of 100 Hz and emits per flash a train of 13 Q-switched pulses with an FWHM of about 90 ns. Calculations for a serial arrangement of six amplifiers were made and show the possibility to achieve an average output power of 1 kW.

In order to increase the laser power for material processing, especially for welding applications, different set-ups for beam addition are used. The most widely used method is to overlay the beams of two or more independent lasers directly onto the workpiece. Welding results of aluminum alloys with two lasers in the 2 kW class are shown. The disadvantage of the large welding optics by using this method could be reduced if the laser beams are coupled into one optical fiber. Results of experiments with three identical cw Nd:YAG lasers with 150 W average power and a beam parameter product of 17 mm*mrad each are shown. Three different schemes for coupling these laser beams into one optical fiber are presented. One is to overlay the laser beams at the core of the fiber. Another scheme is to couple the laser beams into fibers and image the endfaces of these fibers onto one fiber endface. The last coupling scheme is to bundle the fibers and place them endface to endface in front of a thicker fiber. Losses and the beam parameter product behind the final fiber are compared. Also an oscillator-fiber-amplifier system is introduced. This yields a very simple system to increase the laser power under keeping of the beam quality.

One of the present tasks in high-power solid-state laser development is to increase the beam quality. For 1.06 micrometer Nd:YAG lasers with unstable resonators, superGaussian outcoupling mirror can be used to obtain a high central intensity peak. Still, it contains less than half the power. Part of the rest should be focusable if bifocusing is avoided by birefringence compensation. If used with stable resonators, graded reflectivity mirrors were able to reduce the maximum beam parameter product from 16 to 10 mm*mrad. The maximum output power of 360 W remained unchanged. For 1.44 micrometer Nd:YAG lasers, which are used in laser medicine, apodized gratings have to be used instead of gradient mirrors. Preliminary investigations and first results are presented

In this paper we demonstrate up-conversion emission at 550 nm into an Er-doped Ti:LiNbO₃ waveguide pumped by a 980 nm laser diode. The two main processes of the up-conversion are the sequential two photon absorption (STPA) and the energy transfer (ET). In our case, the fluorescence lifetime of the ⁴S_3/2 energy level of the Er³⁺ ion is measured as 32 microseconds. The fluorescence spectrum shows two peaks around 550 nm and 558 nm which are strongly polarization dependent. This effect is attributed to the different crystal fields induced by the location of Er³⁺ ions in the different LiNbO₃ crystal lattices. The variations of the fluorescence intensity and the transmitted pump power are studied versus the length of Er-doped waveguide. The fluorescence intensity has a quadratic behavior versus the coupled pump power which demonstrate that the up-conversion mechanism is mainly due to a STPA process.

High average powers at 532 nm are of great interest for material processing applications because the absorption of metals and other materials increases with shorter wavelength. As the fundamental laser source we developed a Q-switched Nd:YAG oscillator-amplifier system with an average output power of 215 watts and a beam parameter product of less than 1 mm*mrad. The system operates with 100 Hz repetition rate and emits a burst of twenty Q-switch pulses during each flash-lamp pulse. The pulse duration of a single pulse increases from 100 to 200 nanoseconds with increasing pumping power. The high beam quality of the amplified beam is realized by a double-pass configuration of the amplifier with a phase conjugating SBS-cell. Additionally the thermally induced birefringence of the amplifiers is compensated for by a 90 degree-rotator between two identical amplifier rods. An output power of 164 W is frequency doubled in a KTP-crystal to 82 Watt average output power in the green which corresponds to 50% of doubling efficiency.

The synthesis methods of the polarizers and polarizing beam splitters are shown. These methods are based on using the unequal thickness multilayer coatings with symmetrical periods. These enable us to design the polarizer with nearly up to a 100% degree of polarization of the transmitted and reflected beams in the spectral bands of 100 - 200 nm and polarizing beam splitters with apertures up to 20 - 25 grade.

In this paper we present a new theory for nonlinear self- refraction of Gaussian laser beams in Kerr-like materials and its application to determine the third order nonlinear susceptibility in silica sono-gels doped with copper tetrasulfonated phthalocyanine. In the theory we assume that the incident Gaussian beam induces a phase shift that varies as a Gaussian function of the beam radius in the sample. The profile of the intensity in the sample is furthermore assumed to remain Gaussian. The beam propagation after the sample is determined by using the Huygens-Fresnel integral formula. By solving this complicated Huygens-Fresnel integral we obtain analytical expressions for the spatial beam profile in the near-field and in the far-field after the nonlinear sample.

Results of experimental study of thermo-optical characteristics of cw, lamp-pumped Nd:YAG lasers with selective reflectors are presented. Laser heads' reflectors with both high reflective, wide wavelength band standard metallic coating and selective interferometric coatings were used in experiments. Moreover, a new method of lamp-pumped laser head reflectors realization is proposed. This method's feature is making of the reflectors in form of thin polymer film with selective interferometric coating. This proposition is a way to fabricate simple and low-cost high- power technological laser head reflectors.

The uniformity and stability of discharge process in a high pulse repetition frequency (PRF) long pulse XeCl laser are investigated for three different electrode materials (aluminum, copper, brass) and different roughnesses of the copper electrode versus PRF (f less than or equal to 700 Hz) for burst durations of 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.