The objective of the proposed method is a better knowledge of the spatial energy distribution (SED) supplied by multimodal cavities. The principle of the method consists of observing the laser beam intensity at different locations of a section. After recording the beam with a CCD camera, the information is treated in view to recompose the sources SED. The knowledge of the latter and the fundamental equations allows calculation of the propagation characteristics. The results obtained were compared with those from Badawi's method, and a good agreement was found. The authors concluded the method was valid and decided to continue. The SED after transmission by a step index fiber, is recording and confirmation of the literature results are obtained.

For large-core optical fibers of a few meters length, which are typical of those used in beam delivery systems for high power Nd:YAG lasers, it is shown that the near-field profile of the output beam is a strong function of the launching conditions. The output profile depends on both the input spot size and its alignment relative to the fiber axis. A theoretical model has been developed for step index fiber which shows that the output profile depends on the distribution of guided power between meridional modes and groups of skew modes. It is also shown that the modal distribution is a function of the launching conditions. A relationship is hence derived between the launching conditions and the output profile. The predictions of the theoretical model are consistent with experiment.

The development of a new modular CO2-laser with integrated gas-flow system and rf excitation has been successfully completed with models of 600 W, 1 kW, and 1.5 kW going into production now. Furthermore, a prototype model of a scaled-up version of this laser has been developed. The output power is 3 kW with a highly uniform and stable distribution of the power over the 32-mm-(Phi) output aperture. The radially symmetric gain distribution is due to the excitation system which employs two transverse double-helical electrodes on the outside of each discharge tube. The 3-kW laser uses a total of 6 discharge sections (L equals 300 mm, (Phi) equals 35 mm), each of which has its own gas-cooling system and rf generator. This modular approach makes it possible to easily add more discharge units in order to increase the output-power capability. The rf generators are mounted in the direct vicinity of the discharge sections. This fact together with the very compact gas-flow system permits the separation of the laser-head from the auxiliary equipment, such as power-transformer, gas-handling, micro- processor-control. The weight of the laser-head is low enough to allow for the mounting on moving machine parts, e.g., the bridge of a gantry machine with a large cutting area.

Lasers quickly penetrated into Japanese industries in the mid-80s. The paper reviews the present situation of industrial lasers and their applications in Japanese industries for materials removal, joining, and some surface modification technologies as well as their economical evaluation compared with competitive technologies. Laser cutting of metallic and nonmetallic thin sheets is widely prevalent even in small scale industries as a flexible manufacturing tool. As for the laser welding is concerned, industrial applications are rather limited in mass production lines. This mainly comes from the fact that the present laser technologies have not employed the adaptive control because of the lack of sensors, monitoring, and control systems which can tolerate the high-precision and high-speed processing. In spite of this situation, laser welding is rapidly increasing in recent years in industries such as automotive, machinery, electric/electronic, steel, heavy industries, etc. Laser surface modification technologies have attracted significant interest from industrial people, but actual application is very limited today. However, the number of R&D papers is increasing year by year. The paper also reviews these new technology trends in Japan.

Different optical fibers behavior has been studied in the transfer of a high-power YAG laser beam. With the NEC 1.2 kW YAG laser the authors studied mainly the light loss of the fibers and the capability of handling and connection between lasers and optical fibers. Gradient-index and step-index fibers have been compared. Different diameters and different lengths were tested.

Control of laser radiation characteristics using deformable resonator mirrors is considered on the basis of operator equation, which leads to several deterministic algorithms. Algorithms for control of unstable resonator loss, output intensity, and phase distributions are derived within the developed theory. In this framework, heat lens compensation in the Fabry-Perot resonator of a solid-state laser and control of output beam divergency is considered. For solving this problem a single-channel active mirror was designed and investigated. Active control of 60 W multimode repetitive-rate YAG:Nd3+ laser resulted in radiation brightness increasing with a 2 - 5 ratio and in energy distribution and mode structure improvement. Experimentally measured efficiency of control is in good qualitative agreement with developed theory.

The laser energy impinging on a metal workpiece is partially absorbed and partially reflected by the material surface. This work is aimed at gaining a better insight into the energy balance of the process, and it can also provide the correct input for process modeling and the optimum choice of parameters for increasing welding efficiency. Measurements of the absorption coefficient were made using platinum-platinum rhodium thermocouples which monitored the temperature rise. The radiation backscattered by the workpiece or plasma plume was also recorded, and tests were performed to measure the total amount of material lost by evaporation during laser welding. All the tests were performed on austenitic stainless steel. The resulting absorption curves show different behavior at low or high speed and this can be explained only by taking into account the influence on the process of both the size and inclination of the keyhole. To conserve the keyhole, the interaction process must be rapidly interrupted so as to freeze the molten material and preserve the cavity in the form assumed during the process. A fast mechanical switch has been devised and tests seem to confirm the assumption made.

High-power CO2 cw lasers proved to be very efficient in surface treatment of metallic materials in order to improve wear or fatigue resistance of workpieces. In the case of surface treatments which involve a molten zone, it is necessary to improve existing knowledge on convection which can take place inside the metallic bath during laser-material interaction. As a matter of fact, metallurgical investigations showed that this convection influences the homogenization and the structure of the metal specimen. It has been pointed out by some researchers that these movements are generated by high-temperature gradients which produce surface tension variations within the liquid surface. This kind of flow is generally called Marangoni Convection. A method which allows the measurement of some particle velocities on the pool surface is described. A CCD camera was used to estimate surface temperatures and a high-speed video camera allowed the analysis of the velocity fields. The first experiments showed that the order of magnitude of these velocities is typically about a few tens of centimeters per second in the case od cast-iron surface treatment. The scanning speed was about one centimeter per second with a 10 kW/cm2 shaped beam originating at a 6 mods UTIL laser.

Real-time control during multikilowatt CO2 laser surface treatments is required to improve reproductibility, to simplify applications to complex geometry workpieces, and to deal with every kind of unsteady situation. A strategy involving automatics was chosen using linearization around some working points to solve nonlinear problems. Surface temperature was measured by a scanning pyrometer and a real-time temperature calculation allowed estimation of maximum temperatures at the chosen treatment depth. In a second step, maximum temperatures both at the surface and in the volume could be regulated simultaneously by actuating the laser power and the scanning velocity. Applications to laser self-quenching are detailed. The experimental setup and the complete closed-loop automatic system are described. The experimental results of the closed-loop system are displayed. Improvements of the regularity of the treatments are developed for complex geometry pieces. Another regulator was considered to ensure temperature cycles for 10 X 10 X 1 mm3 samples at a standstill in controlled atmosphere chamber during titanium nitriding.

It is well known that the metallic plasmas observed in laser welding are of great importance for the processes of absorption and energy transport to the target. The microscopic parameters of this plasma, which are the electronic temperature and plasma density, have been studied using spectroscopic measurements. Typical temperatures of 6000 - 7000 K and densities of about 1017 cm-3 were obtained. From these measurements, the transmission of the plasma plume and its refractivity could be estimated and compared to the corresponding experimental results. For the absorption measurements, an integrating sphere has been used. It was observed that the absorption coefficient is closely correlated to the spatial and temporal behavior of the plasma and the effect of refraction is of primary importance for laser processing when using gases other then helium at high laser intensity.

Nickel-base superalloys have important applications in industry (i.e., aeronautic and nuclear), so deformation mechanisms of these superalloys have been extensively studied. Most of the results are coming from typical experiments at low-strain rates of deformation. Laser shock hardening provides a high amount of deformation. The purpose of the present study is to compare a high-rate deformed WASPALOY to what is known about deformation mechanisms of this alloy and some other nickel-base superalloys. Oriented single crystals of a nickel-base superalloy, strongly hardened by (gamma) phase, were exposed along the [001] axis to a laser shock (1.06 micrometers , 60 J, 25 ns, confined plasma configuration) at power densities of 3 and 9.5 X 109 W/cm2. Then, thin foils taken at depths of 50 and 700 micrometers below the impacted surface of the specimens were observed by T.E.M. All following observations have been made in areas submitted to plastic deformation. At the surface, deformation bands with planar walls (small size approximately equals 350 nm +/- 100 nm) and pairs of a /2 [110] dislocation have been observed. At the depth of 700 micrometers , deformation bands disappear, but pairs of a /2 < 100 > dislocation remain. In both cases, superlattice stacking faults have been brought into evidence and the deformation is inhomogeneous.

The basic physical processes of melting with laser radiation are reported. Two- and three- dimensional simulations for the fluid and heat flow in a laser-melted pool are performed, using the nondimensional form of the governing equations. The influence of the dimensionless parameters on the flow field, the temperature distribution, and the melt pool geometry are considered. A finite-difference code with boundary-fitted coordinate system has been developed for the analysis of a two-dimensional laser remelting process, including the dynamic of the free-selfadjusting liquid-air surface. The results show that the surface deformation due to the pressure variation by the thermocapillary flow is small compared to melt depth, but it may be up to 30% of the melt depth if the volume expansion due to the solid-liquid phase transformation is taken into consideration. The numerical results are related to experimental ones from metallography and high-speed photography.

Experiments have been performed with identical welding parameters (such as power and welding rate) and focus distance varying from f: 20 mm to f: 120 mm. For example, using a small focus lens, the fact that the welding head is in the plume modifies the weld shape. This focus distance provides different problems than the other parameters (gas, welding energy, etc.) which do not have the same effects with respect to the classic focus distance.

As the authors welded with a small focus distance in a small area, the shielding gas performed the following new functions: heat removal, optics shielding, and liquid metal shielding. Therefore, the gas usually used in laser welding (He, N2, Ar, or a mixture) was compared, with and without a small percentage of oxygen. The benefit influence of a small percentage of oxygen has been demonstrated for the three aimed points, but also an important and beneficial effect on the weldshape has been show; i.e., with the same welding speed value and the same welding energy value the weld penetration can be increased from 0.5 mm up to 2 mm (only by optimization of the gas composition with a small amount of oxygen).

The first known 10.6 micrometers wavelength mirrors entirely coated with the help of the dual- ion-beam sputtering deposition technique are presented. This new coating technology proved suitable for making good quality mirrors which can withstand severe civil and military standards such as climatic and temperature tests. Moreover, these mirrors exposed locally to a very high power density ($OM 100 kW/cm2 in cw mode) do not show any microdamage even after a long period of use. These up-to-date mirrors initially developed for military applications meet the requirement of high power cw CO2 lasers used for industrial applications.

Using a cw 2 kW CO2 laser, thin silica coatings have been deposited on steeply inclined Incoloy 800H substrates traversed beneath the incident laser beam. The small angle of incidence (10 - 15 deg to the direction of beam propagation) of the beam on the substrate resulting from the large angle of incline gave reduced substrate heating and eliminated melting of the substrate surface but allowed melting of the silica powder injected into focus of the beam. The focus was positioned a known distance above the substrate and the molten powder fell onto the laser-heated substrate below. By making overlapping passes complete surface coverage was achieved over a large area, the coating thickness being 2 - 3 micrometers ; this overlay filled points of surface roughness to give the component a microscopically smooth outer appearance. The silica coating formed a good bond with the metallic substrate, adhesion appeared to be improved by having a slightly rough finish rather than a highly polished one. Resistance to sulphidation attack was assessed by placing coated samples into a furnace containing a mixture of gases as in a simulated coal-gasifier heat-exchanger environment at 450 and 750 degree(s)C. Sulphidation resistance was greatly improved compared with that of the uncoated alloy, and coating spallation and cracking did not occur during testing; the few sulphides observed were at small discontinuities in the coating which were eliminated by applying further silica overlayers and also by rastering the coating surface with the laser beam in the absence of additional powder to seal it.

The PHEBUS facility has operated since 1986 for the French laser matter interaction program at Limeil. The performances of this system, which is the third most powerful one in the world, are presented as well as some new developments which aim at smoothing the laser beam in order to improve the overall laser-target behavior.

High-energy gain can be attained, considering the direct-drive approach (laser light illuminates directly the fuel capsule), using laser systems delivering a few magejoules (>=5 MJ) of ultraviolet light. These goals are attained with the condition of a spatial uniformity of 1% and an energy and power balance among the beams of 1%, which are the perfect conditions assumed in simulations. To attain these uniformity requirements some techniques are being developed like random phase plates (RPP) ILE Univ., Osaka, Japan; induce spatial incoherence (ISI) Naval Research, U.S.A.; and smoothing by spectral dispersion (SSD) Univ. of Rochester, U.S.A. The difficulty of arriving at those critical levels is connected to the proposition of the indirect-drive approach, where the laser light is first converted to x-ray radiation producing a very uniform illumination on the fuel capsule. The conversion efficiency of laser light to radiation is a function of laser wavelength, intensity, and converter material. Conversion efficiencies of 80% have been obtained in the 3 W Nd: glass laser OMEGA at the Univ. of Rochester, and is congruent to 70% with 3 W Nd: glass laser NOVA at Lawrence Livermore National Laboratory, with intensities of 1013 - 1014 W cm-2. Numerical simulations together with the experience gained with some last experiments (NOVA, CENTURION/HALITE) have demonstrated the possibility of obtaining convergence ratios of is congruent to 30 and energy gains of is congruent to 100 with laser energies is congruent to 10 MJ in the indirect-drive option. With direct or indirect approaches, if the laser technology is able to arrive at a repetition rate of a few pulses per second, a real alternative to producing electric power in large plants >=1000 MWe can be envisioned. Indirect drive is less demanding in laser technology but more costly in the minimum drive energy needed for high gain.

Laser fusion has achieved the level today that the physical mechanisms for an economic production (at the costs of light water reactors) of energy has been confirmed, based on underground tests using 10 MJ laser pulses. This scheme is based on conversion of the laser radiation into x rays for smoothing, called hohlraum or indirect drive, and on central spark ignition. The authors present an improvement of this scheme by the following two advancements: (a) More efficient direct drive. The long misunderstood 10 psec pulsating laser- plasma interaction, which cannot be dominated by stimulated Raman nor Brillouin scattering as confirmed experimentally by P.R. Drake and C. Labaune et at., is now understood from a very detailed computation showing how the laser radiation causes partial standing waves in the corona and spatial density ripple, followed by reflection far out at very low density (as measured by Maddever and Luther-Davies), with subsequent hydrodynamic decay of the ripples, repeated penetration of the light through the corona, standing ware generation etc., within 10 to 20 psec. (b) Use of volume ignition of ICF pellets. The mechanisms of self heat by charged fusion reaction products permit a very transparent computation of optimized fusion gains where the strongly lowered initial temperatures and the strong heating of the pellets after volume ignition result in fusion gains beyond 1000 for the not too high compression of DT.

Experimental results obtained in different conditions of interaction are reported. Several instabilities have been investigated including filamentation, stimulated Brillouin and Raman scattering, two plasmon decay, harmonic generation, self-phase modulation, and hot electron acceleration. Threshold and saturation levels of some of those processes have been measured. Time-resolved spectroscopy gave detailed information on the physics involved. The effect of several beam-smoothing techniques has been tested. Particularly, random phasing and induced spatial incoherence, when properly matched with focusing optics, showed reduction or suppression of most of the above mentioned instabilities.

Laser-matter interactions enter a new area with the advent of ultraintense laser beams in the 1018 - 1019 W/cm2 range. Such experiments are now within reach since new short-pulse laser technology has recently made possible the production of laser sources with output power at multiterawatt levels with picosecond or subpicosecond pulses. These experiments should yield a significant increase in the knowledge of fundamental aspects of interactions at this relativistic regime. The two main experimental approaches are based on excimer lasers, especially KrF lasers at 248 nm and chirped pulse amplification (CPA) in broadband solid-state materials such as Nd-glass amplifiers, followed by temporal compression down to the one picosecond regime /1/. The latter approach at 1.06 micrometers was chosen because most of the new physical effects expected at about 1019 W/cm2 are governed by the parameter I(lambda) 2, where I is the focused intensity and (lambda) the laser wavelength.

In this paper, a short conceptual review (avoiding details en physico-mathematical formulation for the sake of brevity) of the fundamental energy transport mechanisms in the laser matter interaction at high intensities will be presented along with the associate calculational tools normally used for their simulation, and, as a sample of this kind of calculational SchemeS, a description will be provided on a particular calculaticnal rrdel able to simulate the plasma behaviour under a coupled schen relating the radiative and fluiddynamic rrdium evolution.

Solid targets have been irradiated by 80 fs laser pulses at 1017 W/cm2 laser irradiance. Spectra obtained in the 0.7 - 0.9 nm wavelength range show that x ray emission occurs at near-solid electron densities (Ne $OM 1023 cm-3). The analysis of the K(alpha) emission line points out the role of hot, fast electrons in the interaction physics. A time-resolved Schlieren imaging technique shows that the ultrashort laser pulses strikes a preplasma created by the laser-amplified spontaneous emission (ASE) with an electron density gradient scale length less than 1 micrometers.

The laser-produced plasma is an intense x ray source of high brightness competitive with electron synchrotron sources. The potential of this source for the purpose of opacity studies of cold and hot dense matter are illustrated with results from experimental work recently performed. Measurements of the absorption coefficient of cold and radiatively heated material (carbon and gold) in a temperature range up to about 150 eV are presented.

Analysis is made of the relative contributions of the main electronic emission processes (like thermoionic, multiphoton, and field effects), to the production of very high electron density by an ultrashort (fs range), visible or IR, laser pulse on a metallic photocathode. At such a duration, temporal variations of electrons and lattice temperatures have to be considered separately. They can be calculated with Anisimov's coupled equations, assuming from local thermal equilibrium of electron and photon gas respectively, but this condition is far from being satisfied. The authors establish a relationship between the extracted electronic charge, the incident laser fluence, and the laser pulse duration proving that ultrashort laser pulses are more efficient for the production of a very high photocurrent density with no surface damage.

The case of in-process sensing is one of the strengths of laser material processing both in the variety of signals from the process and range of techniques for beam guidance. If the beam could be stabilized at a given location automatically this would be equivalent to relocating the laser beam. Automatic laser beam position sensing is the first step toward this goal. Methods of beam sensing without beam blocking are reviewed and a new device described. This device is based on a beam splitter and a rotating slot.

The hydrodynamic and implosion efficiencies, defined respectively as the ratios of the thermal energy in the fuel and the kinetic energy in the shell to the absorbed laser energy, have been estimated in a series of experiments using = 0.26 im laser illumination of D-T filled glass microballons.

High-power electron-beam controlled discharge (EBCD) carbon monoxide lasers and pulsed master oscillator-laser amplifier systems have been experimentally investigated at the Gas Laser Laboratory of Lebedev Institute. Requirements were formulated for an effective amplification in active medium and transportation through an atmosphere of multiwavelength CO-laser pulses with different spectral and temporal parameters. A dependence was found of spectral properties of multiwavelength signal at the laser amplifier output on the radiation energy density at the input. The supersonic EBCD CO laser operating at transient conditions of excitation with peak power of approximately 105 W and the repetitively pulsed subsonic EBCD CO laser acting with pumping pulse repetition rate of up to 120 Hz were created. Laser mixtures with density 0.4 and 1.1 Amagat were used in the subsonic laser. It was shown that CO-laser emission characteristics did not deteriorate up to fo/f on the order of magnitude of 1.2 (N equals 0.4 Amagat) and fo/f on the order of magnitude of 3 (N equals 1.1 Amagat), where fo equals V/q is the limit repetition rate (V equals gas flow velocity, 1 equals electrode dimension along the gas flow). The average power in subsonic gas-flow CO laser with active volume 5.1 was 10 kW.

An investigation of the magnetic properties of iron-contained quartzite irradiated by CO2 laser (power density of 103 - 105 W/cm2) has been made. It has been found that the decrease of the matters specific magnetization by the increase of irradiation intensity takes place. This decrease is caused by structure transformations in crystalline lattice, leading to a decrease of iron ion content in ferromagnetic phase (magnetite) and to an increase of ion content in paramagnetic glass phase.

The free-electron laser (FEL) amplification mechanism is discussed in this paper, including the influence of the electron beam quality (energy speed, emittance, stability) on the FEL performance. Considerations regarding the choice of the electron accelerator are presented, and some characteristic properties of the laser radiation are discussed. An overview of FEL facilities for the infrared spectral region, which are being designed and built worldwide, is given.

The discovery of high-temperature superconductors (HTS) has opened new opportunities for applications of superconductors in optoelectronics. The HTS perovskites represent a new class of solid-state materials exhibiting many very interesting and potentially useful electronic, optical, and electro-optical properties. They also operate in the 30-80 K temperature range, where refrigeration is cheap and the parameters of semiconducting devices are optimal. A review of the substrate materials and deposition techniques suitable for fabrication of high- quality epitaxial HTS films for electronic and optoelectronic applications is given. Laser processing techniques of HTS films are presented, with a special emphasis on the laser-writing method, which enables definition of superconducting and nonsuperconducting regions in the same epitaxial HTS film. Two possible approaches for the development of a complete optoelectronic system with the elements based on the HTS films and operational at liquid- nitrogen temperatures are presented. The first approach consists of manufacturing the devices made of conventional electro-optic materials and containing HTS transmission lines and electrodes. Design and properties of ultrafast HTS interconnects are discussed, and a new concept of the Mach-Zehnder-type YBa2Cu3O7-y-on-LiNbO3optical modulator is introduced. The second, more futuristic, approach is to exploit contrasting properties of the oxygen-poor and oxygen-rich HTS phases to fabricate novel, monolithic devices. Recent experiments are discussed which reveal intriguing optical properties of HTS films and are most relevant for the development of all-HTS optoelectronic devices. Several practical devices, such as high-frequency modulators, ultrafast-pulse generators, and sensitive photodetectors, are presented.