This PDF contains the front matter associated with SPIE Proceedings Volume 6346, including the Title Page, Copyright information, Table of Contents, list of Conference Committees, Welcome Address, and Introduction.

High Power Lasers have been used for years in corresponding applications. Constantly new areas and new processes have been demonstrated, developed and transferred to fruitful use in industry. With the advent of diode pumped solid state lasers in the multi-kW-power regime at beam qualities not far away from the diffraction limit, a new area of applicability has opened. In welding applications speeds could be increased and systems could be developed with higher efficiently leading also to new perspectives for increased productivity, e.g. in combined processing. Quality control is increasingly demanded by the applying industries, however applications still are rare. Higher resolution of coaxial process control systems in time and space combined with new strategies in signal processing could give rise to new applications. The general approach described in this paper emphasizes the fact, that laser applications can be developed more efficiently, more precisely and with higher quality, if the laser radiation is tailored properly to the corresponding application. In applying laser sources, the parameter ranges applicable are by far wider and more flexible compared to heat, mechanical or even electrical energy. The time frame ranges from several fs to continuous wave and this spans approximately 15 orders of magnitude. Spacewise, the foci range from several µm to cm and the resulting intensities suitable for materials processing span eight orders of magnitude from 10³ to 10¹¹ W/cm². In addition to space (power, intensity) and time (pulse) the wavelength can be chosen as a further parameter of optimization. As a consequence, the resulting new applications are vast and can be utilized in almost every market segment of our global economy (Fig. 1). In the past and only partly today, however, this flexibility of laser technology is not exploited in full in materials processing, basically because in the high power regime the lasers with tailored beam properties are not available and consequently many applications are not developed yet or are not operatable at optimized parameters. Because of their systematic flexibility in the sense layed out above, solid state lasers are the class to be considered in this context, diode pumped solid state lasers as well as high power diode lasers. In the past years the intrinsic beam quality of diode lasers in terms of divergence, smile and stability as well as the lifetime increased significantly. Single bars with beam parameter products around BPP=20 mm*mrad have been demonstrated. Lifetimes of 30.000h (90%) @100W cw operation are available /29/, /30/. Consequently the market increasingly accepts this type of lasers and new application domains like welding and metal deposition are investigated. Also the beam quality of high power diode lasers has been increased at many groups working in the field of coherent coupling or incoherent superposition [1]. Corresponding by the direct application of high power high quality diode lasers increasingly enters the markets. For all concepts the key demand is its applicability in terms of the specification of the process the laser is designed for. The characteristics of the laser with respect to power and energy on the one hand and quality in terms of space (spacial coherence, focussability), time (puls e duration and duty cycle) and wavelength have to match the demands of the specific application. Therefore, we face an intense trend for diversification of lasers in the named characteristics. There may be dominant large markets and niches but on the other hand there will not be a "best" laser or laser concept in the general sense.

The results of first inductive gas lasers creation in UV, VIS and IR ranges of spectrum are reported. A pulsed cylindrical inductive discharge was used to population inversion formation on the transitions of atoms and molecules. The excitation system producing an inductive discharge in gases under a pressure within a range from 0,1 to 1,0 atm has been developed. Lasing on the transitions of nitrogen molecules (337,1 nm), fluorine atoms (623-755 nm) and carbon dioxide molecules (10,6 um) was achieved. The results of the experimental study of spectral, temporal, and energy parameters of these inductive lasers are presented.

EUV lithography (EUVL) is the candidate for next generation lithography to be introduced by the semiconductor industry to HVM (high volume manufacturing) in 2013. The power of the EUVL light source has to be at least 115W at a wavelength of 13.5nm. A laser produced plasma (LPP) is the main candidate for this light source but a cost effective laser driver is the key requirement for the realization of this concept. We are currently developing a high power and high repetition rate CO₂ laser system to achieve 50 W intermediate focus EUV power with a Tin droplet target. A high conversion efficiency (CE) from laser energy to EUV in-band energy is the most important issue for the concept to be realized. A CE of more than 2 % has been obtained with a Tin target and a CO₂ laser which is also predicted by numerical simulation analysis. The high CE requires a short laser pulse of less than 15 ns. This paper describes the development of a CO₂ laser system that is based on RF-excited, axial flow CO₂ laser amplifiers. The system produces a short pulse length about 15 ns and a nominal average power of several kW at a repetition rate of 100 kHz, An output power of 2.6 kW has been achieved with a pulse length between 15~30 ns at 130 kHz repetition rate at small signal amplification. The phase distortion of the laser beam after amplification is negligible and the beam can be focused to about 150&mgr;m diameter in 1/e². The conceptual design of the CO₂ laser system for LPP EUVL light source, and the laser performance are reported. The further increase of the amplification efficiency is discussed to obtain 5 kW average output power.

First experiments on fundamental band CO lasing in sealed-off cryogenically cooled slab facility with RF discharge excitation were carried out. Repetitively pulsed and CW modes of RF discharge excitation were studied. Average output power achieved 12 W. Lasing efficiency exceeded 10%. The output laser spectrum was observed within wavelengths range 5.08-5.34 &mgr;m. Stable lasing was obtained for tens minutes.

In this paper we present results on the influence of acoustic waves on the output laser beam from high repetition rate TEA CO₂ lasers. We show that acoustic waves generated inside the cavity lead to deterioration in beam quality, decreased output energy, and an increase in pulse to pulse energy variation. We investigate the impact of gas mix on the acoustic behaviour, and present experimental results on laser performance across a range of gas mixtures. Solutions to the acoustic wave problem are presented together with experimental results. The influence of acoustic damping measures on laser gain are demonstrated showing a significant improvement in gain and output power at high repetition rates. The link between the pre-ionisation method employed and the acoustic wave impact on laser performance is discussed.

A diode-pumped rubidium laser with an optical slope efficiency of 69% has been constructed. This study utilized a narrow-line diode laser pump source for the experiments. The trade space study included optimization of various parameters such as lasing cell composition, temperature, and output coupler reflectivities. The results of the experimental study are given.

This work is devoted to the experimental and theoretical study of rf frequency influence on the discharge structure and the O₂(a¹&Dgr;_g) yield in a rf discharge in the tubes with HgO coating for removing atomic oxygen to avoid fast O₂(a¹&Dgr;_g) quenching. Two series of experiments were carried in two discharge sets up: 1 - tube of large diameter of 14 mm, electrode length of 30 cm; 2 - tube of small diameter of 7 mm, electrode length of 10 cm. In the first series, the increase of rf frequency from 13.56 MHz to 81 MHz resulted in the singlet oxygen (SO) yield Y >15% at such high oxygen pressure as 10 Torr. 2D self-consistent model was developed to simulate the rf discharges in gas flow in a wide rage of discharge parameters both with and without HgO coating. Results of the simulation agreed very well with the experimental data. It is shown that at the high rf frequency the discharge operates in a mode of the normal current density so that the energy absorbed by electrons from the rf field increases with the frequency. In the second series we scaled up the rf discharge on the pd parameter (p-pressure, d-tube diameter) to increase the oxygen pressure. The pd discharge scaling at the high rf frequency allowed to reach the O₂(a¹&Dgr;_g) yield Y > 15% at oxygen pressure up to ~17 Torr. The effect of NO admixture on the O₂(a¹&Dgr;_g) production has been studied in series 2 at rf frequency of 160 MHz. NO admixture (5÷20%) results in the noticeable increase in the O₂(a¹&Dgr;_g) yield in comparison with the discharge tube without HgO coating. It is shown that combination of O₂ + NO mixture with HgO coating of the discharge tube walls provides the most optimal O₂(a¹&Dgr;_g) production with the efficiency up to 7-10 % and allows to reach the threshold yield in 15 % at oxygen pressure even above ~20 Torr.

The "Chemical Oxygen-Iodine Laser" (COIL) has passed through a tremendous development from the first milliwatt to a multi-kilowatt power, and thus offered a great opportunity for investigations in the COIL technology based on a multidisciplinary science. This overview has been aimed at a demonstration of enormous endeavor of the international COIL community in the last years to bring this laser system soon to the end-users by a focusing on advanced concepts of the hardware design, improvement and scaling-up the existing facilities. The overall COIL technology is considered as a sequence of the coexistent technologies of main laser components with a mutual impact: a technology of the singlet oxygen generator, gasdynamic mixing and expansion nozzle, energy extraction and optical resonator, and the exhaust and pressure recovery system. Advanced concepts of the named technologies based on inherent supporting research disciplines - a computational modeling, kinetic studies and diagnostic techniques - are briefly described. A critical insight into the COIL performance via an energy flow, energy losses, power extraction, and a chemical efficiency of this laser system utilizes the established heuristic phenomenology. The paper ends with a reference to developed projects and suggested potential applications of the chemical oxygen-iodine laser technology.

The dissociation of I₂ molecules at the optical axis of a supersonic chemical oxygen-iodine laser (COIL) was studied experimentally as a function of I₂ flow rate. The measurements revealed that the number of consumed O₂(¹&Dgr;) molecules per dissociated I₂ molecule depends on the experimental conditions: it is 4.2 ± 0.4 for typical conditions and I₂ densities applied for the operation of the COIL, but increases at lower I₂ densities. In addition, a new method for dissociating I₂ prior to its mixing with O₂(¹&Dgr;) and thus reducing the loss of O₂(¹&Dgr;) is reported. The method is based on applying corona/glow electrical discharge in the transonic section of the secondary flow in the COIL supersonic nozzle. 1.7% of I₂ is dissociated by the discharge resulting in 70% power enhancement at rather high I₂/O₂ ratio, 1.6%, close to the optimal value (~ 2.5%) for operation of COILs with supersonic mixing.

A centrifugal bubbling SOG is a perspective source of oxygen at high pressure with high depletion of the BHP in the single burn dawn. The theoretical estimations show that at high centrifugal acceleration gas-liquid contact specific surface 30cm^-1, frequency of the surface renewal can less than 10^-3s and bubble rise velocity up to 500 cm/s be realized in the bubble SOG. The results of the measurements of O₂(¹&Dgr;) yield, chlorine utilization and water fraction at the exit of the centrifugal bubble SOG are presented. A high O₂(¹&Dgr;) yield and chlorine utilization higher than 90% have been obtained at chlorine gas loading up to 6 mmole/s per 1 cm² of the bubbler surface. The ejector COIL powered by centrifugal bubbling SOG demonstrated ~25% of chemical efficiency with specific power 6 kW per 1 litre/s of the BHP volumetric rate.

A spray type singlet oxygen generator (SOG) for chemical oxygen-iodine laser (COIL) was studied. Mathematical modeling has shown that a high O₂(¹&Dgr;) yield can be attained with BHP (basic hydrogen peroxide) spray in the Cl₂-He atmosphere. It was found experimentally that O₂(¹&Dgr;) was produced with a ≥50% yield at a total pressure up to 50 kPa (375 Torr). A rotating separator was developed that can segregate even very small droplets (≥0.5 &mgr;m) from O₂(¹&Dgr;) flow.

A uniform droplet singlet oxygen generator (SOG) has been developed and used to power chemical oxygen-iodine laser (COIL). The uniform droplets images have been caught by a camera. The chlorine utilization, O₂(¹&Dgr;) yield, H₂O concentration, and laser power have been measured. The maximum power density through the nozzle is 121.3 W/cm².

A critical parameter for understanding the performance of Chemical Oxygen Iodine Lasers is the yield of singlet oxygen produced by the generator. Off-Axis Integrated Cavity Output Spectroscopy (Off-Axis ICOS) has been utilized to measure the absolute density of both ground-state and singlet oxygen in the cavity of a COIL laser.

For a 10kW-class Chemical Oxygen Iodine Laser (COIL), negative and positive branch hybrid resonators are investigated in theory and experiment. Calculations from Fresnel-Kirchhoff theory predict a nearly diffraction limited beam quality in the unstable direction. Theory is verified by measurements with a 2D-camera system in the far field.

Nozzle banks for an ejector-chemical oxygen-iodine laser consisting of two-dimensional slit nozzles with a trip-jet mixing system were tested in the cold- and hot-flow operation regimes. Horizontal Pitot scan experiments demonstrated that the mixing ability of the trips is excellent. The Mach number of the mixed flow was approximately 3. The gain measurements were conducted, and the results of measurements revealed that the maximum gain was around 0.6%/cm. The gain cut-off length was more than 200mm. The lasing experiments were also conducted, and the power was about 3kW at the chemical efficiency of about 20%.

In the hybrid electric discharge Oxygen-Iodine laser (ElectricOIL), the desired O₂(a¹&Dgr;) is produced using a low-to-medium pressure electric discharge. The discharge production of atomic oxygen, ozone, and other excited species adds higher levels of complexity to the post-discharge kinetics which are not encountered in a classic purely chemical O₂(a¹&Dgr;) generation system. Experimental studies over the past six years using electric discharges have demonstrated O₂(a) yields greater than 20%, gain, and cw laser power. Several modeling studies have also been performed for ElectricOIL and similar systems. As the development of this type of iodine laser continues, the roles of oxygen atoms and NO/NO₂ are found to be very significant in both the discharge region and downstream of the discharge region. A series of O₂(¹&Dgr;) emission, I* emission, O-atom titrations, gain, and O₂(¹&Dgr;) yield, NO₂* emission, and laser power measurements have been taken to explore the complex phenomena that are being observed. As the overall system is better understood improvements are being made in laser power and efficiency.

Oxygen-iodine lasers that utilize electrical discharges to produce O₂(a¹Δ) are currently being developed. The discharge generators differ from those used in chemical oxygen-iodine lasers in that they produce significant amounts of atomic oxygen and traces of ozone. As a consequence of these differences, the chemical kinetics of the discharge laser are markedly different from those of a conventional chemical oxygen-iodine laser (COIL). The reactions of O with iodine include channels that are both beneficial and detrimental to the laser. The beneficial reactions result in the dissociation of I₂ while the detrimental processes cause direct and indirect removal of I(²P_1/2) (denoted I*, the upper level of the laser). We have examined kinetic processes relevant to the laser through studies of photo-initiated reactions in N₂O/CO₂/I₂ mixtures. The reactions have been monitored using absorption spectroscopy, laser induced fluorescence and time-resolved emission spectroscopy. It has been established that deactivation of I* by O atoms is a critical energy loss process. We have determined a rate constant of (1.2±0.1)×10^-11 cm³ s^-1 for this reaction. As part of this effort the branching fraction for the formation of O₂(a) from the reaction of O(¹D) with N₂O was determined to be 0.38. This result has implications for lasers based on photolysis of O₃/N₂O/I₂ mixtures and the formation of O₂(a) in the upper atmosphere.

Experiments with a flow cell apparatus imitating conditions of oxygen-iodine laser, equipped with a chemical jet singlet oxygen generator and an electric discharge iodine generator have been performed. I₂ and CH₃I in the flow of Ar were used as atomic iodine precursors. The distributions of the electronically excited species along the flow were examined detecting their optical emissions. A straightforward comparison of two methods of oxygen-iodine medium production - conventional, by means of I₂ dissociation in the singlet oxygen flow and with iodine atoms produced externally in the electric discharge - was performed. It was found that stored electron energy lifetime had been about 30% longer, when iodine was produced from CH₃I in the discharge, compared to the conventional I₂ dissociation in the singlet oxygen flow. It was observed that maximums of the I(²P_1/2) and I₂(B) concentrations had shifted to the nozzle plane, when I₂ in Ar carrier was subjected to the glow discharge, pointing to a nearly twofold increase in the I₂ dissociation rate. Contrary to the known results for low iodine and singlet oxygen concentrations, squared dependence of the amplitude of the I₂(B) luminescence maximum with I(²P_1/2) concentration was observed in the dissociation region for both methods of iodine production.

The NCl-I laser has been demonstrated using HN₃ as a fuel with a combustor to produce chlorine atoms. In this paper, we study the auto-decomposition of NCl₃. This process could result in a new type of chemical laser that would not need a combustor. We present the results of experiments designed to understand the basic physical processes in this system. The NCl-I laser operates using only gaseous species, eliminating the need for heterogeneous gas liquid reactions such as used in COIL chemical lasers. The lasing species is the same as in COIL, simplifying the scaling process since many optical, tracking and propagation problems have been demonstrated in the ABL program.

The paper presents the results of experimental study of the singlet oxygen (¹&Dgr;O₂) lifetime in the solution, saturated by fullerene and oxygen in CCl₄ and irradiated by optical pump with various level of power and with various spectral content. It was shown that in the case of absence of the UV component in this optical pump, i.e. in the case using of only visible spectral range, the lifetime of ¹&Dgr;O₂ reaches ~ 50 ms for optical pump energy density of ~ 0/5 J/cm². Pulse duration was ~ 0.5-1 ms. Such criterion makes it possible to develop the outlook of the realistic singlet oxygen generator with optical pumping of fullerenes for pulse-repetitive and CW regimes of FOIL operation.

We present the design elements and the preliminary experimental results of an Extreme-UltraViolet (EUV) exposure tool driven by the high-output-energy excimer laser-facility Hercules, which is aimed at printing a sub-100-nm-pattern on a photoresist in a single-shot irradiation.

The net small-signal gain was measured in a discharge-pumped KrCl laser (222 nm) operated at high gas pressures of around 3.3 bar. The pump power-density was varied between 200-650 kW cm^-3. The experiments were carried out on a three-electrode laser system. The net small-signal gain was measured in an oscillator-amplifier configuration and reached up to 1.4 % cm^-1. Values for the small-signal gain g₀ of 1.50 % cm^-1, the nonsaturable absorption &agr; of 0.07 % cm^-1 and the saturation intensity I_sat of 0.8 MW cm^-3 were calculated for a specific power-density of 320 kW cm^-3.

The net gain and spontaneous emission at 193 nm have been measured in x-ray pre-ionized discharges excited by a single pulse charge transfer scheme in Ar and F₂ containing mixtures with He and Ne as a buffer gas. With a pumping pulse of ~ 100 ns (FWHM) and a specific peak power deposition of ~ 1 MW cm^-3 bar^-1 in a gas mixture containing F₂ : Ar : He (0.1% : 5% : 94.9%) at 2 bar total pressure the spontaneous emission with a peak intensity of ~ 200 W srad^-1 and a pulse width (FWHM) of ~ 60 ns was measured. The net gain profile with a peak value ~ 20% cm^-1 and a pulse width FWHM ~ 60 ns has been obtained under the same pumping conditions.

During the recent years extensive development of laser tube and pulsed power module in compact excimer laser systems has been done. Aim of all development was the enlargement of lifetimes and reduction of cost of ownership. Therefore, new laser chamber designs have been developed, tested and optimized. Especially the discharge unit was new designed. Additionally, a new generation of pulsed power modules has been developed. By replacing the solid state switch element by a faster device the efficiency of the module was enlarged and the matching to the discharge was improved. Due to the reduction of electrode erosion the lifetime of the laser chamber is enlarged. These new components were integrated in a new experimental laser system and tested to verify the estimated prolongation of laser tube lifetime. In long-term tests of this new laser tube connected with the newest generation of the pulsed power module at 193 nm wavelength, 8 mJ constant energy and 1 kHz repetition rate have been done. A laser chamber lifetime of more than 5 billion pulses was achieved during the first long-term tests. Implementing further improvement the lifetime of the laser chamber could enhanced to more than 6 billion pulses under these test conditions. The lifetime of the pulsed power module exceeds this high lifetime, no degradation of components in the pulsed power module was found.

The influence of electrode materials and surface roughness on the discharge homogeneity of F₂ based excimer laser gas mixtures is investigated in a small x-ray preionised discharge chamber. The temporal and spatial evolution of the discharge is monitored by taking photographs of the discharge luminosity with an ICCD camera. It is found that for the same surface roughness, discharges with nickel coated aluminum electrodes are more homogeneous than the discharges using chromium or gold coated aluminum, massive copper, aluminum, brass and steel electrodes. Moreover, the surface roughness of copper electrodes does not have a large influence on the homogeneity of discharges in F₂ doped excimer laser gas discharges.

We observed significant amplification of the argon excimer laser emission at 126 nm initiated by femtosecond high-intensity laser-produced electrons. By introducing an optical feedback with a vacuum ultraviolet multilayer mirror, the small signal gain coefficient of 2.3 cm^-1 was evaluated at 126 nm.

We demonstrated a debris-free, efficient laser-produced plasma extreme ultraviolet (EUV) source by use of a regenerative liquid microjet target containing tin-dioxide (SnO₂) nano-particles. By using a low SnO₂ concentration (6%) solution and dual laser pulses for the plasma control, we observed the EUV conversion efficiency of 1.2% with undetectable debris.

We introduce basic knowledge about diode lasers and point out some of the most important recent innovations in this field. Three key fields of progress are treated, namely laser diode material/structure, mounting/cooling/packaging-technology and optics/collimation/multiplexing. The most remarkable achievement is the continuous wave generation of 509 W by a single diode laser bar and 25 W by a single laser diode.

With the new industrial high power fiber lasers we have already stepped into a new generation of laser applications. These lasers are smaller, better, more cost-effective, and offer a processing "on the fly." Of utmost importance is their excellent beam quality which enables us to reduce the size of the focussing head including the scanning mirrors. With the reduced mass of the mirrors we can reach scanning frequencies up to 1.5 kHz and in special configurations up to 4 kHz. Using such mirrors with this high beam quality we can shape the key hole geometry, and thus it is possible to decrease the keyhole spiking, which always occur in the case of deep penetration welding. We can generate very thin and deep welding seams, which we have only experienced with electron beam welding. The excellent beam quality of the fiber lasers offers us a lot of new applications from deep penetration welding to high speed welding. By using beam scanning we are able to easily change the beam and the seam geometry. Furthermore, it is possible to work with this kind of laser from a distance of some meters between focussing/scanning head and the work piece. This technique is called remote processing or processing "on the fly." The excellent beam quality also enables us to cut very precisely, and due to the small cutting widths with a very high speed. In this case the main problem is that the roughness of the cutting edge increases a little bit. One reason for this is that we cannot blow out the mold as easily as we can do it with higher cutting widths. There are also polarized fiber lasers on the market where we can use the Brewster effect for different applications. The presentation will cover some physical basics including different industrial applications.

An efficient microchip laser utilizing domestically fabricated ceramic Yb:YAG is presented. In continuous-wave (cw) and Q-switched operation, the laser maintains linear polarization with 22 dB extinction and oscillates in the fundamental TEM₀₀ mode. In cw mode, the ceramic laser has an output power of 2.25 W and a slope efficiency of 66%. When passively Q-switched at 11.4 kHz repetition rate using Cr:YAG, the 1.9 ns pulse has an average power of 0.72 W and a slope efficiency of 46%. To our knowledge, this is currently the highest reported power from a ceramic Yb:YAG laser. The laser performance of the 5-at.% ceramic is compared to a 10-at.% single crystal, and we discuss how the scattering loss and storage efficiency of the ceramic medium affect its laser characteristics.

We describe the analysis of the propagation properties of the beam extracted from a slab-shaped ceramic Nd:YAG laser. The YAG slab is face pumped by two horizontal arrays of quasi-CW diode bars delivering 1200 W in 150 &mgr;sec pulses up to a 15% duty cycle. The use of a hybrid stable-unstable resonator enables a maximum extraction of more than 215 W at 28% slope efficiency level, with a beam parameter product smaller than 3mm.mrad in both transverse directions. M² values have been characterized as a function of the diode pumping current. Furthermore, by an analysis of the M² trend as a function of the knife-edge clipping levels, we have been able to infer the modal composition of the beam in both the stable and unstable direction.

We developed a Nd:YAG rod-based MOPA to generate high power in a radially polarized beam. Two pump-chambers in the amplifier section produced 2.1kW @ M²=9.5, while three pump-chambers yielded 3.1kW @ M²=14. Efficiency in the last pump-chamber was 33%. Several techniques were utilized to enhance beam-quality: a radially polarized oscillator, special pump-chambers, external compensation of lower-order aberrations, and high-order aberration compensation by pairing pump-chambers.

Water vapour absorption wavelengths have been directly generated by diode pumped Nd:YGG crystals emitting at 935 nm and with Nd:GSAG crystals emitting at 942 nm in cw and pulsed operation. In addition the 1064 nm fundamental wavelength from Nd:YAG pump lasers with pulse lengths of 10 or 20 ns was shifted using Stimulated Raman Scattering (SRS) or Ti:Sapphire (TiSa) lasers. The potential of Nd:GSAG, Nd:YGG, SRS and TiSa laser systems is compared for future incorporation into a satellite based Lidar system. High output energies are possible by recent advances of fiber coupled diode sources allowing pulsed longitudinal pumping of Q-switched solid state lasers.

The picosecond CO₂ gas laser has proven a valuable tool in strong-field physics applications. We review the merits of this approach, taking as an example, the Brookhaven Accelerator Test Facility (ATF) that affords a platform for exploring novel methods of particle acceleration and radiation sources. To carry out this mission, the ATF is equipped with a picosecond terawatt CO₂ laser system, PITER-I. We describe the physical principles and architecture of this multi-stage laser system and its application in two high-energy physics projects. The first is the intense Thomson scattering of the CO₂ beam from 60 MeV electrons with production of one x-ray photon per electron that opened the possibility for a Compton gamma-source generating a polarized positron beam for the next generation of electron-positron colliders, such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). The second is our new study of a high-brightness multi-MeV ion- and proton-beam source energized by this picosecond CO₂ laser. High-energy, collimated particle beams originate from the rear surface of the laser-irradiated foils. The expected advantage from using a CO₂ laser for this application, rather than an ultra-fast solid state laser, is the 100-fold increase in the electron ponderomotive potential due to the tenfold longer wavelength of the CO₂ laser. This innovation promises to substantially enhance energy efficiency and particle yield, and will facilitate the advancement of laser-driven ion accelerators towards practical applications. Finally, we address possibilities for generating CO₂ laser pulses of petawatt peak power and a few-cycles duration.

The multi-stage hybrid laser system producing ultrashort pulses of radiation with peak power ~10¹⁴ - 10¹⁵ W being now under developing at the Lebedev Physical Institute of the Russian Academy of Sciences is discussed. The distinctive feature of the laser system is direct amplification of ultrashort pulses produced by a solid state laser system, first going through a prism stretcher with negative dispersion, in gas active medium without using a rather expensive and complicated grating compressor of laser pulses. Two hybrid schemes are being developed now based on the amplification of femtosecond pulses of the third harmonic of Ti:Sapphire laser at the wavelength 248 nm in the active medium of KrF laser amplifier, and on the amplification of the second harmonic of Ti:Sa laser at the wavelength 480 nm in the active medium of photochemical XeF(C-A)-laser excited by VUV radiation of an e-beam pumped Xe₂ lamp. The final stage of the laser system is supposed to be an e-beam pumped facility with a laser chamber of 60 cm in diameter and 200 cm long in the case of KrF laser, and with another laser chamber of 30-40 cm in diameter put into the former one in the case of XeF(C-A) laser. The parameters of such e-beam facility are close to those of previously developed at the Institute of High-Current Electronics: electron energy ~600 keV, specific input power ~ 300-500 kW/cm³, e-beam pulse duration ~ 100-200 ns. A possibility of using Kr₂F as an active medium with saturation energy 0.2 J/cm² for amplification of ultrashort laser pulses is also under consideration. There was theoretically demonstrated that the energy of a laser pulse at the exit of the final stage of the laser system could come up to ~ 17 J with pulse duration ~50 fs in the case of KrF laser, and ~75 J with pulse duration of 25 fs in the case of XeF laser. Two Ti:Sa laser systems producing 50 fs pulses with energy ~0.5 mJ at the wavelength 248 nm and ~5 mJ at the wavelength 480 nm have been already developed and are being now installed at the Lebedev Institute.

We describe a new method for Q-switching. It uses a polarizer together with a photo-elastic modulator, i.e. a transmissive optical element, which oscillates in a longitudinal eigenmode. This induces due to the photo-elastic effect a modulated artificial birefringence which modulates the polarization of passing light and hence the transmission through the polarizer. With a proper choice of eigenmodes and corresponding phases and amplitudes the times of high transmission are short enough to act as a Q-switch in a laser cavity. The voltage amplitude of the excitation is in the order of 10 V. Hence a simple and robust pulsed laser with constant pulse repetition frequency can be built.

Welding and joining of different metallic materials such as steel, aluminum and titanium represents an increasing challenge for welding specialists, as intermetallic brittle phases form in the molten bath and grow to a connected chain during solidification. The brittleness of these phases leads to cracking and reduces the quality of the weld. One solution to avoid the negative effects of extended intermetallic phases is to directly and actively modify the motion behavior of the melt pool, in order to have intermetallics be broken up by the strong convection and attain a homogeneous and fine distribution in the solidified pool. Recent developments in laser technology allow for time dependent modulation of the laser power, so that Marangoni convection can be influenced as well. The basic effects of pulse shaping are presented in this paper for different material combinations.

Amplification of blue-green (470-490 nm) femtosecond pulses in a gaseous active medium of a photolytically driven XeF(C-A) laser amplifier is studied experimentally. The active medium is optically pumped by the vacuum-ultraviolet radiation from two large-area multichannel surface discharges initiated along opposite planes of the amplifier cavity. The amplification factors exceeding 100 are obtained in a multipass optical scheme in the regime of homogeneous gain distribution over a beam cross-section of up to 3 cm in diameter. A small-signal gain of 0.002 cm^-1 is observed at 488 nm for the continuous radiation of Ar-ion laser and for the 150 fs seed pulses as well. The registered spectra insure a good conservation of the temporal characteristics of amplified ultrashort pulses.

In view of the demands made by the end users in the field of application of vehicle construction for an ever-higher product quality and improved performance, continuous innovations are considered to be absolutely decisive for being successful. This especially applies to the welding technology, and therefore the goal is to develop new, better and more powerful welding processes. In joining technology the high welding speed on the one and the good gap bridging ability on the other hand play a significant part. However, both features cannot be achieved by conventional laser welding processes. Therefore, a hybrid process is being. It is no doubt that the laser beam welding and the MIG welding have been established in the welding technology for very long, and that both processes allow a wide field of application in the joining technology. New possibilities and synergetic effects, however, are based on the combination of both processes.

During forming processes high deformations rates can lead to cracks and rupture very easily. Especially brittle materials like titanium or magnesium make difficulties in forming. Due to the dependence of the yield strength on temperature, forming at elevated temperatures eases processing of such materials. Since forming takes place only at localized areas of the work piece selective heating is suffcient and advantageous in most cases. Selective Laser heating offers a possibility to heat only the areas of the work piece where strongest deformations are required. For this purpose several laser sources have been tested like CO₂, Diode and Nd:YAG Lasers and their advantages and disadvantages in localized heating of the work pieces will be discussed. The work presented here summarizes research activities at the Institute for Forming and High Power Laser Technology, Vienna University of Technology, on laser assisted deep drawing, laser assisted bending, wire drawing and so on during the last decade. Recent developments like roll profiling, incremental forming processes and hydro forming are discussed briefly.

A closed loop control for the laser beam welding process requires the collection of process parameters relevant for the machining result by means of suitable sensors, the identification of the control path as well as the feedback of the acquired data via a control algorithm and suitable actors into the process. The present paper describes such a control with the sensors respectively actors used and shows first results of controlled laser beam welding.

Laser welding is applied in large-volume production since the late eighties and has revolutionized the possibilities of designing and engineering products. Nevertheless, problems appear during application because the operational conditions in industrial environments fluctuate and can influence the welding process negatively. Contaminations, like lubricants and organic solids, are an example of changing conditions in laser beam welding. If a lap joint is welded, these materials have to be removed from the sheets, otherwise pores and surface failures may appear due to keyhole instabilities induced by uncontrolled outgassing. One possibility for solving this problem is the use of two separate laser beams. For producing these two beams several systems are available for all different kind of lasers. A bifocal optic is such a solution for an Nd:YAG laser. By using this system, the laser beam is divided after collimation with a prism. Afterwards the two beams are focussed with a lens to the surface of the sheet and two single spots are produced. If the distance between the two spots is low, one common, elliptical keyhole is created. With this system two different welding strategies are possible. The spots can be oriented parallel or normal to the feed direction. For stabilizing the laser welding of contaminated steel sheets the parallel arrangement is better, because the amount of contamination is nearly the same as in single spot welding but the total volume of the keyhole is greater and so pressure variations due to uncontrolled evaporation of contaminations are lower. In order to prove this theory and to determine the exact effects some investigations were made at the Chair of Manufacturing Technology of the University of Erlangen-Nuremberg. A 4 kW Nd:YAG laser with a beam parameter product of 25 mm*mrad and a focal distance of 200 mm was used to weld two 1 mm DC04 steel sheets together with a lap joint. Between the sheets a deep drawing lubricant, Castrol FST 6, was used to simulate the contaminations. The sheets were welded with mono- and bifocal optic, whereas with bifocal the power distribution between the two beams was varied. After welding, the seams were qualified by analyzing surface defects and mechanical properties. The results of the investigations show that the adoption of a bifocal optic can increase the stability of the welding process. The distribution between the two single spots has an essential influence on the welding result. In order to get a higher penetration and failure free seams, a 30 % to 70 % distribution between the two spots is better. Furthermore the blade angle has another slight impact on the welding result. For monofocus and bifocus a towing angle between the sheets and the beam produces better welding results. Considering these results it can be resumed that the application of a bifocal optic is a possibility to increase the quality and the stability of lap joint welding but the parameter settings for good results can only be varied in a tight tolerance zone.

This paper reports on a new arrangement for CO₂ laser welding, allowing the fast variation of the polarization of radiation (DP). The experiments with steel St37 and Ck45 show a significant influence of a DP on the melt pool dynamic, especially a remarkable smoothing of the top bead at certain frequencies of DP.

The extremely good focusability of thin disk lasers provides many advantages regarding the process as well as system aspects. Using commercially achievable devices up to 6 kW, considerable improvements in welding performance are demonstrated. At very small spot diameters, it was found that, in addition to the spot diameter itself, the divergence angle of the focussed beam is deciding for the seam shape.

The paper presents a new method for on-line acquisition of weld-piece surface topography during pulsed laser welding. The method is based on illumination of the weld-piece surface with multi-line laser projector and imaging of the illuminated surface with a digital camera. The acquired image is processed by a personal computer to get the three-dimensional (3D) weld-piece surface topography. The set-up was applied to minimize weld-piece distortion during laser butt-welding of low-carbon thin steel sheets. Methodology based on Taguchi methods was developed enabling optimization of processing parameters. Root-mean-square (RMS) of surface distortion was used as the scalar parameter quantifying the amount of distortion. The results show that laser pulse duration and welding speed have a major influence on RMS distortion.

The presented data are the results of the development of the continuous electric-discharge CO₂ laser with the cross discharge and convective cooling of the active medium, with the power up to 14 kW. Extended disc fans are used for gas pumping, they are heat-exchangers at the same time. Preliminary results of gas flow-rate, heat-exchange and energy characteristics are reported. Some experimental results of laser cutting of thick sheets are given as well as the results of the numerical and experimental simulation of the laser cutting gas-dynamics.

The material in issue is a case-hardening steel, type 20MnCr5 (UNI 8550), that has been previously undergone to a softening annealing process conferring therefore to the material one equal Hardness Brinnel 200 to 220 HB. This steel is very difficult to weld as by laser technology as by other welding technologies by fusion. It has been experienced a power at first laser of 1500 W (for requirements begins them of understanding of the process and the relative to you parameters to set up then) and of 3000 W, like previewed. They have been used a covering gas and a laser beam focusing mirror. All the welded tests have been at first subordinates to taken care of a visual examination to eye and then to an Image Acquisition System, computerized and connected to both metallographic and stereo microscope. Then cut to you with metallographic apparatus, workings to the abrasive papers and metallographic cloths, in order to end with a chemical etching of type NITAL. The best ones turn out to you have been then always visualize in shape of macro and micro-graphs, acquired with the same Image Acquisition System of type NIKON - LUCIA 4.82 vers. by LIM, to storage and automatically measure the cross-section area (melted zone) and then to calculate the efficiency level, on each joint and bead on plate, expressed in Dau unit, to verify the laser welding efficiency, correlated to the laser working parameters. To a better characterization of the produced joints many micro-hardness tests and relate family hardness trends and profiles have been carried out. At the end from the comparison of the values of ETE % and MR % (Model of Swift- Hook & Gick) and of the values of WE in Dau (Model DA.LU.), lead on some beads of the 20MnCr5, has turned out an ulterior and substantial validity and goodness (as well as easiness of employment and calculation of WE) of this last Model and Unit of Measure. In fact the values of ETE % and WE (in Dau) are appeared substantially similar, respecting both all the substantial correlations between the job parameters, to laser.

Cracking in laser cladding caused by inhomogeneous thermal expansion and/or phase transformations restricts the feasible feed rates and the use of high-strength coating materials. In order to better understand the physical reasons of thermal stress and cracking and to reduce the restrictions mentioned, the formation of beads and the evolution of the temperature and stress fields in laser cladding are simulated with and without inductive pre- or post-weld heating. The results of a semi-analytical analysis of the cladding process are transferred to a finite element model which calculates the temperature field, the phase transformations, and the residual stress and strain. These results show that both the danger of cracking due to high residual stress and strain and the distortion of the workpiece by irreversible plastic and thermo-metallurgical strain may be reduced by pre- or post-weld heating using inductors which can be directly integrated into the cladding process.

The laser cladding process allows the generation of high quality coatings. Otherwise the circular cross section of the single tracks, which is mainly defined by surface tension and gravity, shows potential for optimization. For cladding of large areas a flat and broad geometry is favoured, slim walls allow enhancing the generation of 3-dimensional structures. The paper reports about a novel technology, which uses the skin effect due to the application of high frequency magnetic fields and the resulting surface pressure to adapt the track geometry towards the requirement of the application.

Although laser cleaning is being applied nowadays in different fields, mainly for artworks conservation, laser cleaning of metals, namely precious metals, is still far from providing widely acceptable results, producing instead "dull" surfaces. In this paper, Silver samples with diverse surface characteristics have been used in order to test laser cleaning, with different kinds of contamination such as copper particles, silver sulphide and industrial production oil. Also different kind of laser sources, from UV (Excimer, KrF) to mid-Infra Red (CO₂) have been applied with several pulse lengths from nano seconds to milliseconds. An overview of the results in terms of cleaning efficiency and cleaning success is presented and correlated with the optical properties of the contaminations and thermal properties of sample substrate. The evaluation of laser cleaning has been performed by different analysis techniques such as SEM and EDX and colorimetry.

Three distinct techniques were used for the cleaning of Fe alloys objects that present surface corrosion. These objects are presently kept in a museologic space. The techniques used to remove the corrosion were mechanical, chemical (EDTA solution) and LASER (pulsed CO₂ and Nd:YAG - Q-Switch), and their efficiency and selectivity compared. The chemical characterisation of the metallic surface and the evaluation of the cleaning degree of the Fe objects were done by transmission 57Fe Mossbauer spectroscopy. The results of the Mossbauer spectroscopy presented here, together with optical microscope observations of the original and cleaned objects suggest that: - The mechanical process of cleaning does not allow for the total removal of corrosion, reaching easily the substratum and so causing mechanical damage on the object. - The cleaning methods using EDTA in confined environment and high control of the experimental conditions, remove the corrosion products efficiently, without damaging the surface of the object. - The cleaning methods using LASER although not achieving the complete removal of corrosion, showed clear characteristics of selectivity in a given range of operating parameters.

Laser plasmas generated in result of interaction of intense laser pulses with matter are efficient sources of XUV radiation in the energy range from about 50 eV to 500 eV. The results of experiments on micro- and nanoprocessing of organic polymers using a laser plasma XUV source based on a laser-irradiated gas puff target are presented and discussed.

In this paper, we review the history of laser space propulsion from its earliest theoretical conceptions to modern practical applicatons. Applications begin with the "Lightcraft" flights of Myrabo and include practical thrusters for satellites now completing development as well as proposals for space debris removal and direct launch of payloads into orbit. We consider laser space propulsion in the most general sense, in which laser radiation is used to propel a vehicle in space. In this sense, the topic includes early proposals for pure photon propulsion, laser ablation propulsion, as well as propulsion using lasers to detonate a gas, expel a liquid, heat and expel a gas, or even to propagate power to a remote conventional electric thruster.

This paper discusses different principles of inducing propulsive power using lasers and examines the performance limits along with pros and cons with respect to different space propulsion applications: satellite launching, orbital transfer, space debris clearing, satellite propulsion, and space travels. It concludes that a use of electrical propulsion, in conjunction with laser power beaming, is the most feasible application with technological and economic advantages for commercial use within the next decades.

Nowadays, combustion engines and other combustion processes play an overwhelming and important role in everyday life. As a result, ignition of combustion processes is of great importance, too. Usually, ignition of a combustible material is defined in such a way that an ignition initiates a self-sustained reaction which propagates through the inflammable material even in the case that the ignition source has been removed. In most cases, a well defined ignition location and ignition time is of crucial importance. Spark plugs are well suited for such tasks but suffer from some disadvantages, like erosion of electrodes or restricted positioning possibilities. In some cases, ignition of combustible materials by means of high power laser pulses could be beneficial. High power lasers offer several different possibilities to ignite combustible materials, like thermal ignition, resonant ignition or optical breakdown ignition. Since thermal and resonant ignitions are not well suited on the requirements mentioned previously, only optical breakdown ignition will be discussed further. Optical breakdown of a gas within the focal spot of a high power laser allows a very distinct localization of the ignition spot in a combustible material. Since pulse duration is usually in the range of several nanoseconds, requirements on the ignition time are fulfilled easily, too. Laser peak intensities required for such an optical breakdown are in the range of 10¹¹ W/cm². The hot plasma which forms during this breakdown initiates the following self-propagating combustion process. It has been shown previously that laser ignition of direct injection engines improves the fuel consumption as well as the exhaust emissions of such engines significantly. The work presented here gives a brief overview on the basics of laser induced ignition. Flame propagation which follows a successful ignition event can be distinguished into two diffrent regimes. Combustion processes within an engine are usually quite slow - the reaction velocity is mainly determined by the heat conductivity of the combustible. Such deflagrations processes show propagation velocities well below the speed of sound. On the other hand, detonations show much higher propagation velocities. In contrast to deflagrations, detonations show propagation velocities higher than the speed of sound within the combustible. The shock front which propagates through a combustible in the case of a detonation is responsible for a considerable pressure gradient moving at supersonic velocity. Basics and possible examples of laser induced ignitions of deflagrations and detonations are given and pros and cons of laser ignition systems are discussed briefly.

The Airborne Laser (ABL) uses a megawatt-class laser with state-of-the-art atmospheric compensation to destroy ballistic missiles at long ranges. The system will provide both deterrence and defense against the use of such weapons during conflicts. This paper provides an overview of recent laser test results and the risk reduction approach being utilized to ensure program success.

The Airborne Laser (ABL) is an airborne, megawatt-class laser system with a state-of-the-art atmospheric compensation system to destroy enemy ballistic missiles at long ranges. This system will provide both deterrence and defense against the use of such weapons during conflicts. This paper provides an overview of the ABL weapon system including: the notional operational concept, the development approach and schedule, the overall aircraft configuration, the technologies being incorporated in the ABL, and the current program status.

Conditions have been revealed, both theoretically and experimentally, where an optical pulsating discharge generates continuous periodical shock waves. It has been shown that this mechanism of shock waves merges into a low-frequency quasi-stationary wave operating in different gases (matter vapor) for a wide range of laser spark energies. The background for multi-megawatt laser based "Impulsar" engine will be presented.

Merging modes of optical pulsating discharge generated shock wave conditions are determined for the different parameters of gas pressure, energy and repetition rate of laser pulses. Quasi-stationary wave formation characteristics due to moving as well as stationary optical pulsating discharge for the cases of single reflector and matrix of reflectors will be discussed. Advantages of quasi-stationary wave mode for "Impulsar" engine applications will be demonstrated.

Following its first developments in the 1970's, and profiting by the increasing availability of laser sources delivering intensities above 10⁹ W/cm² with pulse energies in the range of several Joules and pulse widths in the range of nanoseconds, laser shock processing (LSP) is being consolidating as an effective technology for the improvement of surface mechanical and corrosion resistance properties of metals and is being developed as a practical process amenable to production engineering. The main acknowledged advantage of the laser shock processing technique consists on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Although significant work from the experimental side has been contributed by several authors to explore the optimum conditions of application of the treatments and to assess their ultimate capability to provide enhanced mechanical behaviour to work-pieces of typical materials, only limited attempts have been developed, however, in the way of full comprehension and predictive assessment of the characteristic physical processes and material transformations with a specific consideration of real material properties. In the present paper, a review on the physical issues dominating the development of LSP processes from a high intensity laser-matter interaction point of view is presented along with the theoretical and computational methods developed by the authors for their predictive assessment, and practical results at laboratory scale on the application of the technique to different materials are shown along with corresponding results on the mechanical properties improvement induced by LSP treatments.

Laser beam welding performed with a CO₂ laser by applying a magnetic field perpendicular to the welding direction influences the weld pool dynamics, which changes the seam properties significantly. From these results it was concluded that an intensive current density must exist in the melt. In repeating those welding experiments with a Nd:YAG laser, however, no significant effects could be observed. To explain this discrepancy, detailed trials with both CO₂ and Nd:YAG lasers were carried out and led to an explanation which is presented in this paper. Looking at the welding process with radiation of 10.6 &mgr;m and 1.06 &mgr;m, the only significant difference is the presence of a laser-induced plasma plume above the workpiece in the case of the longer wavelength. Therefore, the investigations were concentrated on its possible role in establishing a current flow through the weld pool. This current was directly measured during the welding process (bead on plate): Two aluminum plates separated by an insulated gap of 0.85 mm were moved under the focused beam (3 kW; 5 m/min) and the signal was recorded as function of the gap's position. From these measurements were deduced values of current that amounted to approximately 0.3 A with CO₂ and more then one order of magnitude less with Nd:YAG lasers.

The cryogenic liquid with vapor bubbles is regarded as phase-changing and unsteady field with heat and mass transfer phenomena. The cryogenic liquid has a characteristic feature of the small latent heat, surface tension, and viscosity, as compared with those of normal temperature water. As the cryogenic laser processing technology is still under development and research, there have been few reports on laser-matter interactions, for example, on micro/nano particle production. This paper firstly deals with behavior of a cavitation bubble induced by a pulsed YAG laser in liquid nitrogen. The interaction of the bubble with the solid wall has been studied by flow visualization, and, furthermore, the laser-particle processing in liquid nitrogen has been studied. As our research on cryogenic laser-submicron particle processing is in the first stage of experiment, the paper concentrates mainly on the microscopic observation of the laser-processed holes on Al surface and small particles.

With the advent of reliable Yb:YAG disk-lasers and Yb-doped fibre lasers the industry is now ready for adopting these novel sources in their laser material processing systems. Not only superior beam quality in comparison to conventional YAG lasers but also the simplified handling via multi-kW-fibres open up new high performance industrial applications. Recent results underline the importance of disk- and fibre-lasers.

The current trends and recent developments of Free Electron Lasers in the two spectral regions laying at the boundaries of the electromagnetic spectrum are reviewed in this paper.

The thin-disk laser (TDL) is a promising solid state laser concept since it combines good power scalability with high efficiency and small thermal lensing effects. Therefore this concept is a good candidate for near diffraction limited multi-kilowatt operation. In addition, due to its low depolarization losses, the TDL can be operated polarized without sacrificing its efficiency. To exploit the full potential of the TDL concept, the small thermally induced wave front distortions of the disk have to be compensated. At the same time, the intensity distribution of the output beam can be formed according to the requirements of the application. The wave front can be either corrected by a static graded-phase mirror or by an adaptive mirror. For the TDL, a thermally driven adaptive mirror was developed. The polarization state of the radiation field is another important aspect which can be optimized for specific processes. For example, a radially polarized beam promises significantly increased process efficiency for cutting sheet metal. The polarization state oscillating inside the resonator can be controlled by means of a polarization selective resonant grating-waveguide mirror. We describe the successive steps of development towards optical elements optimized for the intra-cavity beam shaping of the TDL.

The paper presents results of singlet delta oxygen yield (SDO) measurements in a high-pressure, non-self-sustained discharge and small signal gain measurements on the 1315 nm iodine atom transition in the M=3 supersonic cavity downstream of the discharge. The results demonstrate operation of a stable pulser-sustainer discharge in O₂-He flows at pressures of up to 120 torr and discharge powers of up to 2.2 kW. The reduced electric field in the DC sustainer discharge ranges from 6 to 12 Td. SDO yield in the discharge is up to 5.0-5.7% at the discharge temperatures of 400-420 K. The results suggest that SDO yield exceeds the gain threshold yield at the M=3 cavity temperature by up to a factor of three, which is confirmed by gain measurements. The highest gain measured in the supersonic cavity is 0.01%/cm.

Radially-polarized beams can be strongly amplified without significant birefringence- induced aberrations. However, further improvement of the beam quality is desirable. In effort to transform the radially-polarized beam to a nearly-Gaussian beam, we consider effect of a spiral phase element (SPE) on the Laguerre-Gaussian (LG) (0,1)* beam with radial polarization, and compare this with the case when the input beam is a LG (0,1)* beam with spiral phase and uniform or undefined polarization. The LG (0,1)* beam with radial polarization, despite its identity in intensity profile to the beam with spiral phase, has distinctly different properties when interacting with the SPE. With the SPE and spatial filter, we transformed the radially-polarized (0,1)* mode with M²=2.8 to a nearly-Gaussian beam with M²=1.7. Measured transformation efficiency was 50%, and the beam brightness P/(M²)² was practically unchanged. The SPE affects polarization state of the radially-polarized beam, leading to appearance of spin angular momentum in the beam center at the far-field.

In this paper we implement technique of intracavity bimorph flexible mirror control that allows to manipulate laser beam parameters (increase power, decrease divergence) and to form a given intensity profile at any distance including a far-field formation. Intracavity mirror surface profile is controlled by number of voltages calculated by means of genetic algorithm combined with hill-climbing one. Then the traditional Fox-Li approach is applied. It is numerically shown the possibility of successful formation of super Gaussian beam shape in near field, ring-like beam in far-field, possibility of significant power increase of TEM₀₀ mode and far-field peak intensity enlargement.

In this paper the performance of a positive branch unstable resonator under different conditions has been investigated. Using variable reflective mirror (VRM) in this resonator, wave front phase and electric field are evaluated numerically. Based on the Fresnel-Kirchhoff integral, the phase and intensity across the output mirror were calculated. Gaussian Reflecting Mirror (GRM) as well as super-Gaussian has been considered for the resonator front mirror and the results are compared with those of ordinary hard edge mirror. Variety of geometrical conditions for resonator was considered in these simulations and the behavior of resonator under different Fresnel numbers N and magnification M were evaluated. In the calculations the effect of finite misalignment on the front mirror was also assessed and the beam quality was inspected for each case. The energy extraction was also estimated for each condition by calculating Finesse.

Fluid dynamic processes in COIL medium represents considerable difference as compared with any other types of cw CL flows because of high Damkerrer number, high thermal factor as well as low Reynolds number. Experimental diagnostics of the COIL AM flowfields by traditional gas-dynamic measuring techniques is restricted due to low static pressure and chemical reactions. Therefore numerical simulation represents the main instrument for investigation. Numerical and experimental results as well as corresponding techniques and verification procedures are presented in the paper related to flows in two main parts of COIL: supersonic diffuser (SD) and nozzle bank (NB).

A theoretical model for the chemically reacting flow within a chemical laser is employed to conduct 3-D simulations of the laser flowfield for operational conditions. Issues regarding flow unsteadiness are explored in light of their impact on the optical field to gas coupling.

The concept of 1D subsonic COIL model with a mixing length was generalized to include the influence of a variable magnetic field on the stimulated emission cross-section. Equations describing the chemical kinetics were solved taking into account together with the gas temperature also a simplified mixing model of oxygen and iodine molecules. With the external time variable magnetic field the model is no longer stationary. A transformation in the system moving with the mixture reduces partial differential equations to ordinary equations in time with initial conditions given either by the stationary flow at the moment when the magnetic field is switched on combined with the boundary conditions at the injector. Advantage of this procedure is a possibility to consider an arbitrary temporal dependence of the imposed magnetic field and to calculate directly the response of the laser output. The method was applied to model the experimental data measured with the subsonic version of the COIL device in the Institute of Physics, Prague, where the applied magnetic field had a saw-tooth dependence. We found that various values characterizing the laser performance, such as the power density distribution over the active zone cross-section, may have a fairly complicated structure given by combined effects of the delayed reaction to the magnetic switching and the flow velocity. This is necessarily translated in a time dependent spatial inhomogeneity of output beam intensity profile.

Flow mixing and chemical kinetics in the afterglow of an electro-discharge source of singlet oxygen O₂(a¹&Dgr;_g) are studied theoretically using both a simple analytical approach and an advanced three-dimensional (3D) simulation with the FLUENT code. Calculated results are compared with titration measurements of atomic oxygen flow in the discharge oxygen-iodine laser (DOIL) system in which the first positive gain and continuous-wave laser oscillation were demonstrated. It was shown that atomic oxygen atoms in rf post-discharge O₂/He mixtures effectively deplete O₂(a¹&Dgr;_g) molecules through quenching of the excited iodine atoms. Thus, the precise technique for O atom flow measurements and methods of removing O atoms are of importance for DOIL operation. One technique is through the ad-mixing of NO₂ to the post-discharge flow which allows control of the O atom flow. Theoretical predictions of NO₂* production (and equivalently the emission), using both an analytical solution of a simplified system of equations and thorough 3D modeling of the reactive mixing flows, are compared with experimental measurements of NO₂* emission. Analysis of 3D calculated distributions of species concentrations and flow velocities shows how titration measurements should be calibrated correctly and validates the use of a simple analytical approach for the interpretation of titration measurements. It is recommended that NO₂ titrations for the flow conditions near to those examined use the value obtained from fully extinguishing the NO₂* emission.

We have developed a 3D beam propagation numerical code and applied it to simulations of 7(19)-core of hexagonal structure fiber amplifier experimentally studied recently. It was shown that the mechanism of phase synchronization of co-propagating beams is associated with mode filtering by gain spatial structure. The tolerance limits on micro-core parameter random variations for phase-locking taking place are numerically found for 7-core fiber amplifier with different random core index sets. It is shown that the smaller core index step stabilizes the phase locking effect. An original optical mode solver was developed. Spectra of modes and of their gain are studied for 7(19)-core fibers. Gain self- and cross-saturation effects are examined, and possibilities of core structure tailoring in order to increase singlemode stability are described.

This paper describes the overview of the past decade's progress of network sensing supported by the use of single mode fiber optics in which glass fiber material itself exhibits sensing function, comparing with conventional FBG/BOTDR techniques, and newly developed hetero-core techniques. The hetero-core fiber optic sensor has been highlighted in terms of the fabrication process, the sensing mechanism and characteristics, and a road map toward commercialization to the variety of industrial applications.

High precision laser drilling is getting more and more interesting for industry. Main applications for such holes are vaporising and injection nozzles. To enhance quality, the energy deposition has to be accurately defined by reducing the pulse duration and thereby reducing the amount of disturbing melting layer. In addition, an appropriate processing technology, for example the helical drilling, yields holes in steel at 1 mm thickness and diameters about 100 &mgr;m with correct roundness and thin recast layers. However, the processing times are still not short enough for industrial use. Experiments have shown that the reduction of the atmospheric pressure down to 100 hPa enhances the achievable quality and efficiency, but the use of vacuum chambers in industrial processes is normally quite slow and thus expensive. The possibility of a very fast evacuation is given by the use of an aerodynamic window, which produces the pressure reduction by virtue of its fluid dynamic features. This element, based on a potential vortex, was developed and patented as out-coupling window for high power CO₂ lasers by IFSW^{1, 2, 3}. It has excellent tightness and transmission properties, and a beam deflection is not detectable. The working medium is compressed air, only. For the use as vacuum element for laser drilling, several geometrical modifications had to be realized. The prototype is small enough to be integrated in a micromachining station and has a low gas flow. During the laser pulse, which is focussed through the potential flow, a very high fluence is reached, but the measurements have not shown any beam deflection or focal shifting. The evacuation time is below 300 ms so that material treatment with changing ambient pressure is possible, too. Experimental results have proven the positive effect of the reduced ambient pressure on the drilling process for the regime of nano- and picosecond laser pulses. Plasma effects are reduced and, because of the less absorption, the drilling velocity is increased and widening effects are decreased. So the process is more efficient and precise. Furthermore, the necessary pulse energy for the drilling of a certain material thickness is reduced and so laser power can be saved.

Solid Al alloy samples were analyzed by laser-induced breakdown spectroscopy (LIBS) either by employing an optical glass fiber as light waveguide or by direct laser irradiation in air. The samples had an Al content of 87.56 % to 99.2 % (w/w), a Si content of 0.07 % to 11.12 %, a Fe content of 0.15 % to 0.89 % and a Mg content of 0.005 % to 2.38 %. For the LIBS analyses, we used an Nd:YAG laser delivering 5 to 7 ns pulses with pulse energies from 50 to 500 mJ per pulse at a repetition rate of 10 or 20 Hz. The experiments were performed either at a wavelength of 1064 nm or (frequency doubled) at 532 nm. We obtained qualitatively similar results in experiments with direct laser irradiation in air and experiments where cleaved fibers were used as a laser light guide. Damage of the fibers sets an upper limit for the laser energy in the latter case. Experiments employing tapered fibers showed that a lateral resolution in the order of 10 &mgr;m is feasible with the presented setup.

The concept of asymmetrical thermal blooming (ATB) effect of intense laser beam propagation through atmosphere is presented. The numerical simulation results for simplified cases are given. Some analyses of the ATB effect have been done.

Multiple phase screen (PS) method is typically used in numerical simulations of light propagation through a turbulent atmosphere and an adaptive optics (AO) system for phase compensation. One of key problems in it is generation of turbulent PS for describing the turbulent atmosphere. The covariance approach is a relatively new approach for generating PS. In this paper, the covariance approach is used to generate turbulent PS and a preliminary numerical simulation investigation on this approach and its generated PS is carried out. We propose to use three methods to evaluate the generated PS in a combining way. It is found that a comparison of the phase structure function of generated PS with the theoretical one is not enough and often inefficient. By contrast, a comparison of open loop and close loop laser propagation results by using generated PS can give a deeper insight. Open loop and close loop results by using PS generated by covariance approach are obtained and compared with those by using PS generated by spectral approach for the first time. It is shown that PS generated by covariance approach includes more abundant frequency components and these components have obvious influences on light propagation through a turbulent media.

Some peculiarities of the use of adaptive optical elements and the whole system to correct for the aberrations of high power single pulse lasers are discussed in this paper. The examples of the use of adaptive system to correct for the aberrations of some lasers are presented. As a corrector we used bimorph multi electrode deformable mirror while as a sensor - Shack- Hartmann wavefront sensor.

We present a novel algorithm of bimorph mirror grid determination. Its usability is demonstrated on practical examples of defining electrode grid to form a vortex beam. We briefly describe the developed finite element bimorph mirror model, designed to evaluate an arbitrary electrode response function.

Numerical and experimental study of processes of absorption of pumping radiation of HF-laser, a laser oscillation and a radiation amplification in a (CO₂+N₂O) active medium of high pressure is carried out. Measurements and numerical estimation of an absorption coefficient for 21 spectral lines of the HF-laser radiation, which are strongly absorbed by the CO₂ and N₂O molecules, are performed. Measurements of an integral absorption coefficient for total pumping spectrum for the laser media of various content are carried out. About 63 and 35 % of radiation of the non-chain and chain chemical HF-laser, respectively, are absorbed using a four-component laser mixture. For the mono-component active media the maximum specific energy of an output radiation in 10 microns region, efficiency of an energy conversion and a small signal gain (¹²CO₂ - 200 J/l, 18 %, 9 m^-1 ; ¹⁴N₂O - 100 J/l, 7 %, 6 m^-1) are obtained. For two-component (¹²CO₂+¹⁴N₂O)-active media these parameters are 120 J/l, 7%, 7 m^-1 at maximum laser energy 12 J. According to the numerical estimation, a coefficient of conversion of energy accumulated in lasing mode to energy of a pico-second pulse in 10.6 micron region for mixture of the CO₂ and N₂O molecules may achieve 30 %.

LES (Large Eddy Simulation) model is derived and applied to the transonic-type COIL nozzle flow. This type of model appears to be good enough to obtain fine spatial/temporal resolution of the most energy-containing 3D structures. All non-linear chemical effects are reflected much more detailed than in highly-symmetrical RANS models.

The dissociation of I₂ molecules at the optical axis of a supersonic chemical oxygen-iodine laser (COIL) was studied via detailed measurements and three dimensional computational fluid dynamics calculations. Comparing the measurements and the calculations enabled critical examination of previously proposed dissociation mechanisms and suggestion of a mechanism consistent with the experimental and theoretical results. The gain, I₂ dissociation fraction and temperature at the optical axis, calculated using Heidner's model (R.F. Heidner III et al., J. Phys. Chem. 87, 2348 (1983)), are much lower than those measured experimentally. Agreement with the experimental results was reached by using Heidner's model supplemented by Azyazov-Heaven's model (V.N. Azyazov and M.C. Heaven, AIAA J. 44, 1593 (2006)) where I₂(A') and vibrationally excited O₂(a¹&Dgr;) are significant dissociation intermediates.

The paper reports on experimental studies of the active medium gain in supersonic electric-discharge oxygen-iodine laser (DOIL) based on traveling mw discharge. The measurements have included: absolute concentration, yield, and energy efficiency of production of SO in pure oxygen and oxygen-helium mixes at an oxygen partial pressure 3 to 15 Torr. For the gas flow to get rid of atomic oxygen, both heterogeneous mercury oxide coatings of the tube walls and homogeneous additives to the work mix, such as nitrogen oxide, have been used. The active medium of DOIL was formed using a nozzle array of the type of ejector sized as 10*50 mm². The singlet oxygen-helium mix was supplied through three rows of sonic cylindrical nozzles, while the iodine-carrier gas mix - through two rows of supersonic conical nozzles with a half-opening angle of 10°(arc). The gas-phase iodine was produced in a quartz cell filled with iodine crystals. Room-temperature iodine vapors were picked up with a carrier gas (nitrogen or helium) and thus delivered into the nozzle array. The active medium was investigated by the high-resolution laser diode spectroscopy approach that used the laser type Vortex 6025 purchased from New Focus, Inc. The laser medium gain factor was determined by the intra-cavity approach having a sensitivity about 1*10^-6 cm^-1. The static temperature of the medium was determined from the measurements of gain half-width. The gain of the active medium of electric-discharge OIL has been investigated. The DOIL in use was operating on a mix composed as O₂:He=1:1 at a total pressure of 6 Torr and flowrate - about 1 mmol/s. With helium as an iodine carrier gas at a flowrate ~3 mmol/s, we have recorded a positive gain in the DOIL medium.

We report numerical calculations of scattering and backscattering efficiency factors of water droplets illuminated by a quasi-white light radiation (a supercontinuum radiation) that results from self-focusing of a high-power femtosecond laser pulse in the atmosphere. The behavior of these integral optical characteristics for a wide range of droplet sizes that covers the size spectra of fog and cloud particles is analyzed. The results are compared to the values of the scattering factors obtained by the Lorentz-Mie theory.

A model to describe the transverse field distribution of the output beam from porro prism resonators is proposed. The model allows the prediction of the output transverse field distribution by assuming that the main areas of loss are located at the apexes of the porro prisms. Experimental work on a particular system showed some interested correlations between the time domain behavior of the resonator and the transverse field output. These findings are presented and discussed.

By means of radio frequency(RF) capacitively coupled discharge, pure CF₃I and the mixture of CF₃I and various inert gases were used to produce iodine atom for a chemical oxygen-iodine laser. The dependences of atomic iodine concentration on gas pressure and gas flow rate were studied. In the case of RF frequency of 13.56MHz, output power of 500W, gas pressure of 3.5Torr, pure CF₃I without any carrier gas ,and CF₃I flow rate of 1.2SLM, atomic iodine concentration of 4.0×10¹⁴ cm^-3 was achieved. The concentration of atomic iodine was of the order of 0.4×10¹⁴cm^-3 at p=15Torr in the case of the mixture of CF₃I and some inert gases, which was lower by one order of magnitude than that of pure CF₃I. A kinetics modeling was carried out and demonstrated that CF₃ played dominant role in the loss mechanism of atomic iodine.

Considered is the method of isothermal calorimeter intended for the measurement of singlet oxygen (SO) concentrations in oxygen flow. The method is distinguished for its simplicity and does not require expensive equipment. However, by now no calculation methods were available that would grant the necessary properties of the calorimeter at designed conditions. In this our work such calculation method is developed for calorimeters. Compared are the concentrations of SO produced in electric discharge generators as measured by calorimetric and optical methods.

An advanced Chemical Oxygen-Iodine Laser (COIL) using the chemical generation of atomic iodine was studied. Atomic iodine is produced by the reaction of atomic chlorine with hydrogen iodide (HI) in two separated reactors tightly attached to the supersonic laser cavity. The iodine-contained mixture is injected to the flow of singlet oxygen by means of the supersonic orifices located 5 mm downstream the nozzle throat. The atomic iodine number density in the laser cavity up to 1.2 x 10¹⁵ cm^-3 and a small-signal gain up to 0.35 %/cm were achieved. An rather high quenching of singlet oxygen by HI caused that the attained laser power was low. The results of small signal gain and the laser power are compared with the previous system including the mixing of reactants upstream the nozzle throat.

Effects of electric and magnetic fields on the discharge characteristics of gas mixtures have been analyzed. A part of Boltzmann transport equation is modified to process the addition of magnetic field. The differential equation is transformed to difference equation for numerical processing. The effects are analyzed by the modified computer code.

Photoisomerization has been recently studied in the various area of industry. A field of molecular motor and an optical memory with nanotechnology, for example, has been especially made progress acutely and hold the promise of the application to various industries. In our laboratory, photo functional materials using photochromic molecules with ultraviolet (UV) and visible (visible) laser has been studied and developed. Photochromic molecules are isomerized to absorb specified wavelength light and its property of the molecular structure changes reversibly by light. For a efficient isomerization, the high energy density light is necessary in large number of photochromic molecules. On the other hand, the laser beam is enable to focus on the several hundred nanometer range, it is possible to isomerize at the only several decades piece unit in the photochromic molecule, because the average of molecule length of the photochromic molecule is several nanometer. In this experiment, it has been UV (wavelength 355nm, pulse duration 5ns) and visible (wavelength 532nm, pulse duration 8ns) laser with 50Hz pulse repetition frequency for measuring the ratio of isomerized molecules from fluctuating absorption wavelength by laser irradiation. As a result, it is considered that the probability in the excited molecule is a increases using laser to irradiate the molecule, the number of photon that collides per molecule have been increased on the increasing irradiation time. In conclusion, it is suggested that microdevices to control submicroscopically have been constructed by combination of photochromic molecule and UV-visible laser.

A new personal identification method has been investigated which promises to be a useful technique for protecting society against the recent increase in card counterfeiting crimes. Micro cracks that is created into transparent acrylic material using second harmonic Nd:YAG (wavelength: 532nm, pulse width: 8ns, pulse energy: 0.5mJ) are used for writing ID information. Identical person is identified by image matching of speckle pattern from created crack. In this study, identification is attempted for constructing the new personal identification method. Various ID patterns are created with changing shapes, the arrangement of spot. In result of crack observation by optical microscope, cracks are created around the spot and the shapes are different respectively. Speckle patterns are successfully identified by image template matching with normalized correlation coefficient. In a case of identification on template and target image obtained from same object, strong correlation was obtained. In this result, processed objects were identified by discerning speckle pattern. For these reason, the feasibility of new personal identification system using the laser processing and the speckle pattern is implied.

The numerical end experimental study of the energy and temporal characteristics of the output radiation of the CW medium-scale chemical HF/DF-laser with mechanical modulation of the optical resonator quality by the mirror rotation with frequency up to 1 kHz is carried out. In experiment, a peak power of the laser radiation pulses has exceeded the radiation power in CW mode of the HF-laser operation not less than in four times. The average power in repetitively pulsed mode is lower than one in CW mode, but it grows with increase of the modulation frequency. Time of the complete recovery of a small signal gain of the active medium, which is about 6 - 7 &mgr;s, is determined. Two numerical models are developed, which describe the dynamics of the laser oscillation in a mode of the resonator quality modulation for both HF- and DF-laser, some features of their operation are analyzed using these models.

Fabrication technique of fiber Bragg gratings (FBG) sensors, which is used the measurement of distortions of large scale architectural structures, requires a skillful technique, and consequently the costing becomes a problem. The fabrication technique without removing a polymer jacket can realize much easier fabrication and the cheaper cost. We have investigated the laser damage threshold of jackets in order to realize such a fabrication technique. The 3rd and the 4th harmonics of repetitive Nd:YAG laser pulse were used as light sources. An optical fiber coated untinged polymer jackets is used as a sample. The numbers of the shots against the same sample position was varied from 1 to 1200. As a result, damage thresholds at the shot number of 1200 were 6.9 J/cm² and 0.9 J/cm² for the wavelength of 355 nm and 266 nm, respectively. These values are corresponding to intensity of 1.3 MW/cm² and 0.3 MW/cm², respectively. It is considered that the fabrication of FBG, which need a typical intensity of ~kW/cm², without removing the jacket should be possible by controlling the laser intensity between this level and the threshold.

We studied about singlet-delta-oxygen generation by using RF discharge on condition of supersonic gas flow to oscillate Discharge Oxygen Iodine Laser (DOIL) efficiently. We examined the various characteristics of singlet-delta-oxygen generation with the closed-cycle supersonic-gas-flow RF discharge excited oxygen-iodine laser oscillation system. As a result, there were the facts that we found some tendency, and the yield of singlet-delta-oxygen generation of 18.6% was achieved with the optimal gas composition ratio of Ar:N₂:O₂ = 40:40:20 in this experiment.

A high power 1kW pulsed transversely excited atmospheric CO₂ laser that has been developed for the paint stripping of missiles was used to test paint stripping on several metallic and composite aircraft panels to determine the rate at which this laser could remove paint from aircraft.

We demonstrated enhancement of extreme ultraviolet (EUV) emission at 13.5 nm from a lithium plasma by use of dual laser pulses. A single laser pulse produced a lithium plasma condition for the EUV emission far beyond its optimum. Utilization of dual laser pulses, however, enhanced the EUV emission energy, and its maximum in-band EUV conversion efficiency (CE) in a measured solid angle was observed to be 2% at a delay time between 20 and 50 ns.

DC discharge characteristics of NF3/He have investigated experimentally at many kinds of experimental conditions, for example, different electrodes material, a few of distance between the two electrodes, flow rates of the gas NF₃ or He, a series of series-wound resistances and give the steady and optimum discharge parameters finally. Fluorine atom yield from the DC discharge of NF3/He have studied experimentally and the relationship of fluorine atom yield and the load power is shown for the first time.

Execution, transfer, storage and display of laser simulation have been improved. The technology of virtual machine has been introduced to integrate different hardware and software environments. The same binary code for laser simulation is executed in the integrated environments.

Time-resolved fluorescence of Ar/Kr/F₂ mixtures and transient absorption spectra were measured at Berdysh preamplifier module of GARPUN KrF laser facility pumped by 50 A/cm², 350 keV, 100 ns e-beam. The experiments pursued an aim to obtain additional data on KrF laser kinetics, in particular, on the formation of Kr₂F molecules and their influence on the absorption and extraction of KrF laser radiation in large-scale amplifiers. A possibility of femtosecond laser pulses amplification at broad-band Kr₂F (4²&Ggr;→1,2 ²&Ggr;) transition is discussed.

Femtosecond lasers which have an extremely high peak power are expected to use for micro-processing, such as drilling and cutting. We have analyzed the drilling process by femtosecond laser pulses on surface of various materials including silicon with image-intensified CCD camera with a high speed gate. A femtosecond laser pulses (single and double pulses) were focused on surface of samples using a lens with a focal length of 100 mm. As a result, it was found that the smaller the pulse energy, the faster the formation of the hole. By measuring the rise time in 12 kinds of materials, it was found that the rise time strongly correlates with the thermal conductivity in these materials. Besides, it was also found that double pulsed laser suppressed the heat influence compared to the single pulse laser. These knowledges are thought to be very important and useful for developing microfabrication using a femtosecond laser.

Boltzmann equation was solved for the oxygen gas with the mixture of He, Ar, Xe etc. The electron energy distribution, electron average energy, ionization and attachment coefficient were obtained. The results have shown that the electron energy distribution over frequency is almost identical to that for the DC excitation at the same value of E/N, only in the microwave frequency range the frequency began to influence the electron energy distribution. The condition to obtain self-sustained discharge and the the influence of gas mixture on the production of singlet delta oxygen was discussed in this paper.

Laser ultrasonics is currently the optimal method for non-destructive testing of composite materials in the aerospace industry. The process is based on a laser-generated ultrasound wave which propagates inside the composite. The response at the material surface is detected and converted into a defect map across the aircraft. The design and optimisation of a laser system for this application is reviewed in this paper, together with the basic science involved. This includes the optimisation of laser parameters, such as output couplers and gas mixture, and the impact these choices have on the laser chemistry. We present a theory for the catalytic recombination of the gas which shows excellent agreement with experiment. Finally, an operating laser system for this application, yielding a six-fold improvement in performance over conventional laser systems, is described.

Laser ablation of Al, Ni, Fe, and Cu was investigated for pulse durations of 0.1, 1 and 5 ps at fluences up to 100 J.cm^-2. The ablation depth per pulse was measured and the ablation thresholds were estimated. The increase of the pulse duration results in a decrease of the ablation rate. The presence of a molten phase is clearly expressed at fluences above 10 J.cm^-2. Molecular Dynamics simulation was used to model the laser ablation process in the case of Fe, Al and Ni.

We demonstrate an approach for stabilizing the transverse mode structure in cases where there is strong coupling between the longitudinal and the transverse modes. In this approach, an intracavity phase element that discriminates and selects a specific transverse mode is inserted into the laser resonator. We show that the discrimination can be so strong that the selection of the single transverse mode remains stable despite changes of the resonator length. We calculated the ratio of the small-signal gain and the gain threshold value for the fundamental and (1,0) Hermite-Gaussian modes, as function of a tiny change &Dgr;z of the resonator length, with and without the phase element. Without the phase element, the tiny change of the axial coordinate z of one of the mirrors of CO₂ laser leads to periodical change of different transverse modes. Introducing intracavity phase element preserves a single transverse mode, which is kept practically unchanged with the change of the axial coordinate z, except for a slight periodical change of the output power and the beam quality, due to the periodicity of the resonance conditions.

Cryosorption vacuum system for COIL is researched and developed. Adsorption chiller has been proposed and developed by improving the heat exchanger chiller. Compared with the heat exchanger chiller, the volume and LN₂ consumption of the new chiller were favourably reduced. In the present work, the new adsorption operation process, cryogenic pressure swing adsorption is adopted. Compared with thermal swing adsorption, regeneration time is shortened and LN₂ consumption is saved at least 80% in the new operation process. The cryosorption vacuum system was integrated to COIL and tested successfully. The weight of sorbent in adsorption bed is 22Kg, the total gas flowrate is about 0.5mol/s, the COIL's power maintains over 2kW, the total COIL's working time accounts to 100 seconds. It is concluded that the cryosorption vacuum system has the same pressure recovery capability as the large vacuum tank.

Short overview is done about the activity of ISTC in a direction concerned with improving powerful laser beam quality by means of nonlinear and linear adaptive optics methods. Completed projects #0591 and #1929 resulted in the development of a stimulated Brillouin scattering (SBS) phase conjugation mirror of superhigh fidelity employing the kinoform optical elements (rasters of small lenses) of new generation designed for pulsed or pulse-periodic lasers with nanosecond scale pulse duration. Project #2631 is devoted to development of an adaptive optical system for phase registration and correction of laser beams with wave front vortices. The principles of operation of conventional adaptive systems are based on the assumption that the phase is a smooth continuous function in space. Therefore the solution of the Project tasks will assume a new step in adaptive optics.