The United States government and industry have a long standing interest in developing high power semiconductor lasers for a variety of applications. These include material processing, long range sensing, and long range communications. The wavelength depends on the application, and sometimes on eye safety considerations. Key development goals are high brightness and high efficiency. This paper will review some of the applications, key laser performance features desired, and some of the accomplishments in high power diode lasers from the High Power Semiconductor Laser Technology program.

We have developed a high-power laser system that is based on actively cooled GaAs diode laser stacks. Fast axis collimation and subsequent beam rearrangement generates a symmetric laser beam in respect to the beam parameter product along the two main axes. By polarization and wavelength coupling 100 diode laser elements can be coupled into one fiber at a beam parameter product of less than 200 mm*mrad in both directions and more than 2 kW cw output power at the workpiece. At a spot diameter of less than 1 mm the power density exceeds 250 kW/cm². First material processing experiments show that deep welding at working speeds that meet industrial requirements in steel can be observed. High-power diode lasers show that they become suitable for industrial work.

In the already classical separate confinement (SCH) quantum well (QW) semiconductor laser diode structures many of the desired performances are contradictory coupled through the structural parameters -- i.e. a structural parameter modification leading to the improvement of one or more laser performances will produce the deterioration of at least another performance. Based on an analysis of this contradictory coupling a novel transverse layer structure that alleviates the problem and enables improved laser diode performances is proposed. Both optical simulation and a fully self-consistent model are used in a design optimization methodology and simple evaluation and optimization criteria for the new transverse structure are derived. A number of the analyzed high-power edge-emitting Ga_zIn_1-zP/(Al_xGa_{1- x})_yIn_1-yP/GaAs quantum well laser structures were prepared using all-solid-source molecular beam epitaxy (SS-MBE) for layer growth and remarkable performances were obtained (continuous wave output powers of 3 W at 670 nm, 2 W at 650 nm, and 1 W at 630 nm; threshold current densities of 350 - 450 Angstrom/cm² for 670 nm, 500 - 540 A/cm² for 650 nm, and 600 - 680 A/cm² for 630 nm). Although only a few of the optimization features were implemented the good agreement between measurements and simulations for the prepared structures indicate that significant performance improvements -- predicted by the simulations -- are still possible.

We have developed a 'FUNRYU' water cooled heat sink to efficiently cool the LDs and a new assembling system to reliably fabricate LD arrays and evaluated them using a conventional AlGaAs LD bar. We have achieved a maximum CW output power of 115 W limited by the driver circuit and the maximum conversion efficiency of 51% furthermore, we also design the 'FUNRYU' heat sink to be stackable and obtained a power density over 550 W/cm².

Highly efficient heat exchangers with uniform temperature distribution based on microchannel and porous structure are developed. The heat exchange efficiency dependence on construction peculiarities of the devices and cooling liquids properties are shown. Basing on the comparison of both experimental results and the numerical simulation, the ways to obtain the device thermal resistance about 0,2 C/W are discussed. Practical neededs for fabrication both linear diode arrays with power more than 1 kW will be observed.

Changing the layer structure of the AlGaAs LD bars, the dependence of degradation on conversion efficiency and internal loss is examined in this paper. Using four kinds of LD bars mounted on the water-coolers, we measured output and aging characteristics. The internal loss estimated from the free carrier absorption has a close relationship with the degradation of the AlGaAs LD bars. On the other hand, in the region we examined, the conversion efficiency is not the dominant factor in determining the degradation. The free carrier absorption locally raises the lattice temperature and accelerates the propagation of defects in the lattice. On the other hand, the thermal power caused by the injection current uniformly raises the LD temperature and affects the local lattice in minimal manner. Therefore, the degradation dominantly depends on the internal loss.

High power diode lasers are mainly used for applications such as pumping of solid state lasers, direct material processing (i.e. welding, soldering, hardening, annealing) and printing. The outstanding characteristics of diode lasers are their compactness, their high efficiency and their reliability accompanied by a long lifetime. Since high power diode lasers are composed of an array of single emitters (multi-stripe or broad area) their lifetimes can widely differ from those of the corresponding single emitters. The lifetime of high power diode lasers depends on the driving current and the cooling temperature they are run with. Their degradation is caused by different degradation mechanisms which have not been definitively clarified up to now. Defects and degradation of InGa(Al)As/GaAs DQW diode laser bars mounted on copper micro channel heat sinks were investigated. The analytical techniques used for this investigation are optical microscopy, scanning electron microscopy, white light interference microscopy. The high power diode lasers were investigatively accompanied through the different phases of their setup process (i.e. mounting, characterization and burn-in). Afterwards a long-term lifetest was performed. The influence of a raised current and a raised cooling temperature on their degradation was investigated respectively. Changes in surface morphology and surface composition of the facets were detected as well as changes in the threshold current, slope efficiency and emission spectrum. Due to the degradation the threshold current increases and the slope efficiency decreases while the emission wavelengths are shifted to higher values showing a broadened spectral width. Formation of micro cracks and dislocations through the facets was also observed. The influence of these changes on performance and lifetime of the high power diode lasers will be discussed.

Facet heating is an important mechanism limiting the performance of high power laser diodes. The temperature increase at the laser facets contributes to the gradual degradation and can lead to Catastrophical Optical Damage (COD). We present a two-dimensional optoelectronic and thermal model for Fabry-Perot Quantum Well power lasers. The model is applied to 808 nm AlGaAs laser bars, with the aim of identifying the main volume and facet heating sources. The results of the simulation show that the heating caused by free-carrier absorption is the main source that could be minimized by a proper layer design. We present a new physical mechanism leading to COD: optical absorption at the facets produces carrier accumulation that can trigger the thermal runaway process.

An analysis of intensity filamentation in a broad area semiconductor laser having an optical cavity with an angled grating has been performed, using both an analytical six-wave mixing theory and beam propagation method (BPM) simulations. With the grating at the Bragg diffraction angle, the analytical theory shows that the use of the grating gives rise to lateral optical anisotropy, which suppresses filamentation of the laser radiation. For a given semiconductor laser design and operating condition, the predictions of the analytical theory are compared with those from a beam propagation method simulation. 12

A robust, modular and comprehensive simulation model, built on a first-principles microscopic physics basis, includes the fully time-dependent and spatially resolved internal optical, carrier and temperature fields within an arbitrary geometry edge-emitting high-power semiconductor laser device. The simulator is designed to run interactively on a multi- processor shared memory graphical supercomputer by utilizing a highly efficient algorithm running in parallel over multiple CPUs. The experimentally validated semiconductor optical response is computed using a microscopic approach that includes the relevant bandstructure of the Quantum Well and confining barrier regions together with a fully quantum mechanical many-body calculation that takes all occupied bands into account. The latter quantity is introduced into the simulator via a multidimensional look-up table that captures the local dependence of the gain and refractive index of the structure over a broad range of frequencies and carrier densities. The simulator is designed in a modular form so as to be able to include differing device geometries (broad area, flared, multiple contacts, arrays, ..), filters (DBR or DFB grating sections), index/gain-guiding, temperature and current profiles and so on. Results will be presented for both broad area and MOPA devices.

The overview of the phase-locking problem for the powerful laser diode arrays is presented. Comparison of the external Talbot cavity configuration with the other ones is held. Attention is paid to both investigation and employment of the laser diode arrays consisting of wide aperture lasers. Such arrays, placed in the external cavity, allow easier solution of the scaling up problem that results in the output lobes with both significantly increased power and low divergence. Our experiments confirm feasibility of both QCW and CW phase- locking of the LDA in the external quarter-Talbot (L_c equals Z_T/4) cavity at the output power of more than 10 W with the lobes divergence of (delta) (Psi) approximately equals 0.5 mrad. Employment of the efficient porous heatexchanger along with the system that cancels both induced phase distortions arising due to heat release and existing optical imperfections has allowed a breakthrough in powerful arrays phase-locking.

Current commercially available diode lasers with output powers above a few watts lack beam quality, i.e. they have only limited possibility of small foci in combination with long Rayleigh lengths. Recent advances in coherent coupling of such lasers open view to a new generation of high power, high beam quality, low cost lasers suitable for a wide range of technical applications such as microshaping or cutting. Therefore, we performed experiments to couple the 25 diode lasers of a bar with specially coated low-reflection front facets. Mutual coherence can be improved in external resonators as opposed to the internal resonator absent in our case. Additional elements like mode stops can improve beam quality. Here we present results on the coupling of gain- guided broad-area diode lasers in external resonators, both of single emitters and bars of 25 emitters. In the single emitter case we achieved output powers up to 0.8 W at a beam quality of M² equals 16 or 0.4 W with M² equals 3.5 along slow axis. For the bars we achieved 10 W with M² equals 304.

We present a new concept to scale up the power of fiber lasers into the kW range and report on the first laser based on this concept: a 'fiber embedded tube laser.' An optical to optical slope efficiency of 28% and a laser output power exceeding 10 W were easily achieved.

In this paper a very compact device of a very efficient high- power fiber-laser based on a specially designed Ytterbium (Yb)-doped double-clad D-Shape silica fiber with an output power of more than 8 watts will be described. The development, preparation and characterization of rare-earth doped double- clad optical fibers based on silica for the application in high-power fiber-lasers were done at the Optics Division of the Institute for Physical High-Technology e.V. Jena (IPHT) in the last few years. The doping with Ytterbium as the laseractive component was one of the most important questions which were investigated. Many samples had to be made for the optimum concentration of Yb, the optimum codoping and geometry, respectively. As a result of these experiments we realized a double-clad fiber with D-shape geometry of the pump-core. This D-shape resulted from ray-tracing calculations performed at the Lasercentre Hannover e.V. (LZH) as an optimal solution for the high conversion efficiency of the pump-power. Considering these numerical results, such fibers were realized at IPHT by drawing a sidepolished preform by preserving the cross section of the preform. After some basic investigations on the optimum wavelength for the pump-light and the resonator length, respectively, a compact device with a launched pump- power of approx. 13 W and an output-power of more than 8 watts was developed and realized. The maximum output-power was only limited by the pump-power available at our laboratory at the chosen wavelength. Main advantages of the setup are the short length of the laseractive fiber (less than 10 m), the high efficiency (greater than 60%) and the compactness of the device. Additionally, the diameter of the pump-core was reduced to 125 micrometer. These kinds of fibers were tested with a specially designed pump-source of the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena. The first result with a double-clad Yb-doped fiber of only 4 meter length was a fiber-laser with an output-power of about 4 Watts. Last but not least, some basic experiments with Fiber-Bragg-Gratings (FBG) as one of the resonator mirrors were done. A fiber-laser with a FBG for the laser-wavelength had an output-power of about 700 mW with a FWHM of about 0.1 nm.

The 'Advanced Photon Processing and Measurement Technology' project was started in Aug. 1997 as part of the Industrial Science and Technology Frontier Program of the Agency of Industrial Science and Technology (AIST), MITI in Japan. In the project, 13 private companies and 1 university, which are the member of RIPE, and 4 national research institutes under MITI are developing new technologies using high-quality photon beams, by challenging 6 key themes in the 3 technology fields, 'Photon generation technology,' 'Photon-applied processing technology,' and 'Photon-applied measurement technology.' In the 'Photon generation technology,' we are developing 'High- power all-solid-state laser technology,' and 'Tightly-focusing all-solid-state laser technology.' The objective of the former theme is to develop LD-pumped all-solid-state laser devices of high power (greater than or equal to 10 kW), high efficiency (greater than or equal to 20%), and compact size (laser head less than or equal to 0.05m³). Recently, we obtained 3.3 kW output power from both rod-type and slab-type Nd:YAG laser oscillators. The objective of the latter theme is to develop compact all-solid-state laser devices of high power (greater than or equal to 1 kW), high efficiency (greater than or equal to 20%), for focusing the beam on a very small area of 50 micrometer in diameter of the processing object. In this theme we are developing two types of lasers, 'a disk or cylindrical shaped fiber laser pumped by LD from surroundings' and 'a high-brightness and high-rep-rate UV all-solid-state laser with CLBO crystal.' Recently, we obtained 10 W output power from the fiber laser and 20 W UV output power using CLBO crystal.

We have developed a high power LD pumped Nd:YAG laser using one zig-zag slab crystal, and obtained the average output power of 3.3 kW with the optical efficiency of more than 35% under the cooling condition of 12 degrees Celsius. Our method is to mount the LD stacks on both the sides of the 6 mm X 25 mm X 206 mm slab crystal in which the LD stacks mounted on one side of the slab pump the upper part of it while the ones on the other side pump the lower part. The LD pump light transmitted through the crystal is reflected by Au mirror and is introduced back into the slab. These LD stacks are arranged so that the thermally induced birefringence can be eliminated by maintaining uniform pumping in the width direction of the slab (non-zig-zag direction). The pumping distribution in the width direction was made constant by ray tracing simulation. The small signal gain distribution was measured constant in this direction, which indicates that the pumping distribution in the width direction has become uniform. The total heat generated in the slab was calculated to be less than 2.3 kW and the temperature distribution and thermal stress distribution were also simulated. According to this calculation, the maximum thermal stress of 40 MPa occurs in the surface of the slab, which is one-fourth of the fracture limit of YAG crystal.

As a first step of a driver development for the inertial fusion energy, we are developing a diode-pumped zig-zag Nd:glass slab laser amplifier system which can generate an output of 10 J per pulse at 1053 nm in 10 Hz operation. The water-cooled zig-zag Nd:glass slab is pumped from both sides by 803-nm AlGaAs laser-diode (LD) module; each LD module has an emitting area of 420 mm X 10 mm and two LD modules generated in total 200 kW peak power with 2.5 kW/cm² peak intensity at 10 Hz repetition rate. We have obtained in a preliminary experiment a 8.5 J output energy at 0.5 Hz with beam quality of 2 times diffraction limited far-field pattern.

We have demonstrated a highly efficient quasi-cw Nd:YAG laser with a novel side-pumping configuration using micro-lens free stacked-diode-bars. In this configuration, the fast-axis of the diode bars is arranged in parallel to the rod axis which means highly efficient P-polarization pumping of the Nd:YAG rod. The pumping beams are coupled into a cylindrical diffusive reflector by using wedge lenses (1-dimensional lens duct). The pump radiation in the slow axis direction is focused by the cylindrical surface of the wedge lens, and the radiation in the fast axis direction is transferred by the total internal reflection. Six micro-lens free diode bars are arranged around the Nd:YAG rod. The transfer efficiency of the wedge lens was 89%. Laser power of 270 W was obtained at the beam quality of 20 mm mrad at the electric efficiency of 18.4%. The experimental results are well explained by calculations and we consider further enhancement of the efficiency is possible by optimizing the diffusive cavity design.

In recent years, the high power Yb:YAG lasers have been actively investigated due to the advantage of the high quantum efficiency of 91% which reduces the thermal loading in the Yb:YAG crystal. So far, the Yb:YAG laser with the output power higher than several hundreds watts has been developed using the crystal configurations of rod and thin disk. We have developed the Yb:YAG laser by employing the rectangular slab crystal in order to examine the possibility of realizing the high power slab Yb:YAG laser. The dimension of the Yb:YAG crystal used is 1 mm X 5 mm X 10 mm and its configuration is a rectangular parallelepiped, and the density of Yb is 1.1 atom%. The LD (Laser Diode) pump light focused with plano-convex lens is introduced through the 1 mm X 10 mm plane of this slab which is AR-coated at 940 nm while the opposite 1 mm X 10 mm plane is HR-coated at the same wavelength. The Yb:YAG laser cavity axis is in the direction perpendicular to the 1 mm X 5 mm planes which are AR-coated at 1030 nm. The two 5 mm X 10 mm planes are cooled by being contacted with the copper heat sinks which are cooled by the water at the temperature of 18 degrees Celsius. The CW output of 35 W was obtained when the power of LD pump light was 496 W. The optical efficiency was 7.1% with the optical slop efficiency of 12.2%.

We are developing rod-type all solid-state laser with average power more than 10 kW as a tool for high speed and highly precise material processing such as cutting and welding. We developed a rod-type all solid-state laser system with average power of 3 kW level and succeeded in attaining an average power of 3.3 kW with multi head configuration with combined mode of CW and QCW operation. We also obtained electrical- optical efficiency of 19% in CW operation and that of 17% in QCW operation in the region of average power of 0.8 kW to 1.0 kW.

High average power (grater than 100 W) with high brightness operation of diode-pumped rod type Nd:YAG laser is investigated. The key technologies to compensate thermal distortions are described and the high brightness operations of normal and Q-switched modes are demonstrated.

Simultaneous multiple wavelength cw laser operations were achieved in two types of composite laser rods composed of Nd³⁺:YAG and Nd³⁺:YLF crystals, which were laser diode (LD) pumped within a single virtual-point-source cavity. Up to 30 W total output from wavelength of both 1064 nm and 1047 nm was obtained under 150 W LD input power, among which about 25% was from 1047 nm wavelength. Different bonding methods were compared which shows that the use of an optical adhesive is effective and presents no deterioration at low and middle power level. Simultaneous multiple wavelength operation at 1064-nm and 1053 nm was also studied.

A laser-diode end-pumped continuous wave hybrid Nd:S-VAP and Nd:YVO₄ laser was demonstrated. The two crystals were combined to construct a hybrid-crystal which has a broader effective absorption bandwidth. The hybrid laser can operate efficiently without the need to control the temperature of pump diode. In a combination with the c axes of Nd:S-VAP and Nd:YVO₄ perpendicular to each other, the polarization states of the laser output can be selected by changing the pumping wavelength of laser diode. Thermal compensated hybrid- crystal concept has been proposed and discussed based on the combination of laser crystals with negative and positive thermal coefficient of the refractive indices.

A scaleable diode end-pumping technology for high-average- power slab and rod lasers has been under development for the past several years at Lawrence Livermore National Laboratory (LLNL). This technology has particular application to high average power Yb:YAG lasers that utilize a rod configured gain element. Previously, this rod configured approach has achieved average output powers in a single 5 cm long by 2 mm diameter Yb:YAG rod of 430 W cw and 280 W q-switched. High beam quality (M² equals 2.4) q-switched operation has also been demonstrated at over 180 W of average output power. More recently, using a dual rod configuration consisting of two, 5 cm long by 2 mm diameter laser rods with birefringence compensation, we have achieved 1080 W of cw output with an M² value of 13.5 at an optical-to-optical conversion efficiency of 27.5%². With the same dual rod laser operated in a q-switched mode, we have also demonstrated 532 W of average power with an M² less than 2.5 at 17% optical-to-optical conversion efficiency. These q-switched results were obtained at a 10 kHz repetition rate and resulted in 77 nsec pulse durations. These improved levels of operational performance have been achieved as a result of technology advancements made in several areas that will be covered in this manuscript. These enhancements to our architecture include: (1) Hollow lens ducts that enable the use of advanced cavity architectures permitting birefringence compensation and the ability to run in large aperture-filling near-diffraction-limited modes. (2) Compound laser rods with flanged-nonabsorbing-endcaps fabricated by diffusion bonding. (3) Techniques for suppressing amplified spontaneous emission (ASE) and parasitics in the polished barrel rods.

Investigations of the factors that limit average power scaling of elemental copper vapor lasers (CVLs) have demonstrated that decay of the electron density in the interpulse period is critical in restricting pulse repetition rate and laser aperture scaling. We have recently developed the 'kinetic enhancement' (or KE) technique to overcome these limitations, whereby optimal plasma conditions are engineered using low concentrations of HCl/H₂ additive gases in the Ne buffer. Dissociative electron attachment of HCl and subsequent mutual neutralization of Cl^- and Cu⁺ promote rapid plasma relaxation and fast recovery of Cu densities, permitting operation at elevated Cu densities and pulse rates for given apertures. Using this approach, we have demonstrated increases in output power and efficiency of a factor of 2 or higher over conventional CVLs of the same size. For a 38 mm- bore KE-CVL, output powers up to 150 W have been achieved at 22 kHz, corresponding to record specific powers (80 mW/cm³) for such a 'small/medium-scale' device. In addition, kinetic enhancement significantly extends the gain duration and restores gain on-axis, even for high pulse rates, thereby promoting substantial increases (5 - 10x) in high- beam-quality power levels when operating with unstable resonators. This has enabled us to achieve much higher powers in second-harmonic generation from the visible copper laser output to the ultraviolet (e.g. 5 W at 255 nm from a small- scale KE-CVL). Our approach to developing KE-CVLs including computer modeling and experimental studies will be reviewed, and most recent results in pulse rate scaling and scaling of high-beam-quality power using oscillator-amplifier configurations, will be presented.

Tm and Ho doped solid-state lasers operate at 2-micron wavelength have many applications in medical, remote sensing, and military technologies. Using flash-lamp pumping, we demonstrated high-power Cr-Tm:YAG and Cr-Tm-Ho:YAG lasers at room temperature. The output energy in free-running operation exceeded more than 4 J. When an acousto-optic Q-switch was used, we obtained Q-switched single transverse-mode lasers at 2 micrometer wavelength. The maximum pulse energy of Cr-Tm:YAG and Cr-Tm-Ho:YAG lasers reached 0.7 J and 0.5 J, respectively, and the corresponding pulse widths were 140 ns and 165 ns.

A diode pumped Nd:YAG rod master-oscillator power-amplifier system that delivers 220 W average power in a near top-hat beam distribution has been developed. The repetition rate is 2.5 kHz (25% duty cycle) and the pulse width is approximately 50 ns. With a two-stage KTP crystal 131 W green average power was obtained at frequency-conversion efficiency as high as 65.2%. The system was continuously operated 100 hours starting with an initial green power of 106 W. As the experiments finished the green power was 97.4 W. The decrease in green power, which was mainly attributed to the gray tracing effect in KTP, was characterized by a slope of 0.07-%/hour. The amplifier heads incorporate new concepts for both the pump cavity and the pump source to cavity transport line. Thereby the results of which were an efficient absorption of the pumped light in the media and homogeneous pumped-beam distribution under various pump-power levels, Nd:YAG active media of different radii and concentration and shifts of the diode wavelength resulted.

IR FEL Research Center was newly established in April 1999 in the Noda Campus of the Science University of Tokyo (SUT). A mid-infrared free electron laser (MIR FEL) machine is now at the final stage of its construction and is expected to start its commissioning from the fall of 1999. In the second stage, the construction of a far-infrared free electron laser (FIR FEL) will begin from the fall of 2000. A project of constructing of an IR FEL facility is under the collaboration between the Science University of Tokyo and the Kawasaki Heavy Industries Ltd. The installed MIR FEL is based on the electron LINAC having maximum energy of 40 MeV as the injector of electron beam, tunable over the wavelength range from 5 to 16 micrometer, and the overall average power being 1 W at the repetition rate of 10 Hz. The above collaboration program, called 'FEL-SUT Project,' is of two folds; the one is the development of the technologies concerned with strong pico- second pulses IR FEL suitable for a wide application of IR FEL and the other is the exploration of new applications of IR FEL. It is also aimed to look for the possibility of industrial and medical use of IR FEL.

Free Electron Laser Research Institute (FELI) had been established by the Key Technology Center project in Japan. Now it is operated under the collaboration of several organization after the end of the project. The FEL with wide tunable wave range from 0.27 to more than 50 micrometer installed in FELI has being used for applications of various fields. Beside the application of FEL as a users facility at FELI, an effort to extend the wavelength toward shorter region has being performed. SASE FEL with micro-wiggler and seed X-ray is proposed and preliminary investigation is under way. The basic development of micro-wiggler is almost finished. For seed X- ray, we will use Laser-produced Plasma X-ray. The 3D computer simulation results shows the feasibility of SASE in the range of up to 0.01 micrometer with FELI accelerator. This article describe the status and investigation of FELI update.

The performance of laser pulses in the sub-picosecond range for materials processing is substantially enhanced over similar fluences delivered in longer pulses. Recent advances in the development of solid state lasers have progressed significantly toward the higher average powers potentially useful for many applications. Nonetheless, prospects remain distant for multi-kilowatt sub-picosecond solid state systems such as would be required for industrial scale surface processing of metals and polymers. We present operation results from the world's first kilowatt scale ultra-fast materials processing laser. A Free Electron Laser (FEL) called the IR Demo is operational as a User Facility at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, USA. In its initial operation at high average power it is capable of wavelengths in the 2 to 6 micron range and can produce approximately 0.7 ps pulses in a continuous train at approximately 75 MHz. This pulse length has been shown to be nearly optimal for deposition of energy in materials at the surface. Upgrades in the near future will extend operation beyond 10 kW CW average power in the near IR and kilowatt levels of power at wavelengths from 0.3 to 60 microns. This paper will cover the design and performance of this groundbreaking laser and operational aspects of the User Facility.

The JAERI superconducting rf linac based FEL has successfully been lased to produce a 0.3 kW FEL light and 100 kW or larger electron beam output in quasi continuous wave operation in 1999. The 1 kW class output as our present program goal will be achieved to improve the FEL lasing mode, optical out coupling method, electron gun, and electron beam optics in the JAERI FEL. As our next 5 year program goal is the 100 kW class FEL light and a few tens MW class electron beam output in average, quasi continuous wave operation of the light and electron beam will be planned in the JAERI superconducting rf linac based FEL facility. Conceptual and engineering design options needed for such a very high power operation and shorter wavelength light sources will be discussed to improve and to upgrade the existing facility.

Amplification in free-electron lasers exploiting media with periodically modulated refractive indices is studied in the regime of a large modulation. The conditions for realization of the large-modulation regime in a layered plasma medium and in a periodic dielectric superlattice-like medium are established. The maximized gain, the corresponding saturation field and efficiency, as well as the optimal electron energy and propagation direction are determined and compared to each other for different types of modulated media. It is shown that the large-modulation regime makes it possible to extend significantly the operation frequency domain of the FEL employing a low-relativistic electron beam. Relationship with the Cherenkov and stimulated resonance-transition-radiation FELs is discussed.

The use of lasers as the driver for inertial confinement fusion and weapons physics experiments is based on their ability to produce high-energy short pulses in a beam with low divergence. Indeed, the focusability of high quality laser beams far exceeds alternate technologies and is a major factor in the rationale for building high power lasers for such applications. The National Ignition Facility (NIF) is a large, 192-beam, high-power laser facility under construction at the Lawrence Livermore National Laboratory for fusion and weapons physics experiments. Its uncorrected minimum focal spot size is limited by laser system aberrations. The NIF includes a Wavefront Control System to correct these aberrations to yield a focal spot small enough for its applications. Sources of aberrations to be corrected include prompt pump-induced distortions in the laser amplifiers, previous-shot thermal distortions, beam off-axis effects, and gravity, mounting, and coating-induced optic distortions. Aberrations from gas density variations and optic-manufacturing figure errors are also partially corrected. This paper provides an overview of the NIF Wavefront Control System and describes the target spot size performance improvement it affords. It describes provisions made to accommodate the NIF's high fluence (laser beam and flashlamp), large wavefront correction range, wavefront temporal bandwidth, temperature and humidity variations, cleanliness requirements, and exception handling requirements (e.g. wavefront out-of-limits conditions).

Using light (photons) as a means to measure various kinds of physical quantities in their normal environment has the advantage of real-time and precise measurement without interference from other objects. In the semiconductor process, for example, the measurement limit of particle size is down to 0.1 micrometer and the measurement of elements that the particles are composed of is impossible. Presently, there is only one measuring instrument that can measure the elements and diameter of particles at the same time by putting particles in microwave induced He plasma. However, the minimum size with which particles of hydrocarbon material can be measured is limited to several micrometer by this method. This method can not be applied to the measurement of sub-micron particles, which is important to the semiconductor process and for functional particles. The Final goal of the research is to develop devices and systematization technology needed for the newly proposed particle measurement technology using laser induced breakdown (LIB). In particle Measurement using LIB, indispensable high peak, power pulse laser (HPPL) for breakdown generation, ultraviolet fiber for passing fluorescence light and a high speed photo detection device having sub-micro second time resolution are needed. Furthermore, it is necessary to develop systematization technology and obtain final system evaluation when using these device technologies. In this paper, we mainly report on the experimental results of High Peak Power Laser (HPPL) for LIB and its basic consideration in the fiscal year 1999.

The results of principal upgrade of high-pressure, X-ray preionized TE-CO₂ laser towards obtaining 10 atm volume self-sustained discharge in CO₂:N₂:He mixtures with reasonably high (up to 25%) percentage of molecular gases are reported. The estimated energy of radiation from such a laser is greater than or equal to 15 J. It corresponds to peak power of regeneratively amplified 2 ps 10 micrometer pulse formed in master oscillator of laser system greater than or equal to 1.5 TW.

For several years NCLR is working on a 1 kW, 1 kHz XeCl laser. Improvement of the beam quality at high power levels enabled us to do large-scale application experiments and industrial applications become feasible. The base for the good beam quality is a homogeneous discharge. It starts with a smooth gas flow from a classical flow loop. The combination of X-ray pre-ionization and the sophisticated spiker-sustainer circuit guarantees a stable discharge with a long optical pulse (250 ns). Due to the gentle discharge only weak shock waves are formed that are damped within 800 microseconds. An unstable resonator gives a nearly diffraction limited beam. We are now finding a market for this laser. Hole drilling is one of the most promising applications. We can drill holes of 10 to 100 micrometer diameter at a very fast production rate of up to 1000 holes per second. The holes can be drilled in many different materials: metals like aluminum, titanium, steel and nickel alloys, but also plastics, ceramics, glass and composites. The good results encouraged us to design a commercial version of the laser.

Energetic nanosecond UV sources could be advantageously used in laser material processing, biomedicine and to create laser- produced plasmas emitting soft X-ray radiation. SOPRA, in collaboration with IRPHE, is then developing an oscillator- regenerative amplifier XeCl laser system of short duration (1 - 3 ns), high energy and moderate divergence. Insertion in the amplification loop of the seed pulse and final extraction of the amplified laser pulse are realized by controlling the evolution of its polarization state by means of a HT driven Pockels cell and a half-wave plate. The experimental results are discussed and compared to numerical ones issued from a code describing the amplification of the seed pulse in the active medium. Finally, it is shown that the maximum output peak power is fairly low, P_L approximately 1.4 MW (E_L approximately 4.8 mJ, (tau) _FWHM approximately equals 3.4 ns), due to important energetic loss as the highly divergent amplified beam is truncated by low-diameter aperture.

We present the results of beam quality measurements of an XeCl ((lambda) equals 0.308 micrometer) laser, equipped with a generalized self-filtering unstable resonator (GSFUR), while operating in the burst mode at repetition rates of up to 50 Hz. In particular, we have measured the behavior of the laser- energy distribution (both in the near- and far-field) and of the beam-angular-stability (BAS) vs. the repetition rate. The time-evolution of the divergence within the single laser pulse was also measured. The GSFUR is able to achieve a nearly diffraction-limited divergence since the beginning of the laser pulse, and to maintain the values of the times- diffraction-limit number, of the M² parameter and of the BAS independent of the repetition rate. The BAS was measured by using two different techniques, and the results suggest that the reliability of the standards commonly used may depend upon the experimental set-up.

The optical quality of NCLR's 1 kW, 1 kHz XeCl excimer laser has been investigated. Nearly diffraction limited beams can be obtained in short burst mode up to 1 kHz. Long burst mode operation is currently limited by the quality of the optics. The beam pointing variation is reduced to half the divergence angle and is found to be independent of the repetition rate of the laser system.

For the light source of the gravitational wave detector, the injection-locked Nd:YAG laser has been developed in which high power with high frequency stability is required. The frequency of the injection-locked laser is stabilized to the high- finesse Fabry-Perot optical resonator to suppress frequency noise down to 2 X 10^-2 Hz/(root)Hz at 1 kHz. To improve the long term frequency-stability, the frequency of the injection-locked laser is locked to both the high-finesse Fabry-Perot cavity and to the hyperfine components of the rovibrational transition of ¹²⁷I₂ simultaneously by the offset-locking technique. The frequency drift of the high- finesse Fabry-Perot cavity is estimated which is measured from the frequency difference between Fabry-Perot resonate frequency and iodine transition frequency.

Thermal lens effects on highly pumped Yb doped phosphate glass was measured by a Shack-Hartmann wavefront sensor for the development of compact chirped pulse amplification systems. High energy pump pulses of 1 - 2 Joules were produced by a flashlamp pumped Ti-sapphire laser. The pumping intensity on the Yb:glass surface exceeded 800 kW/cm². The pulse energy of 330 mJ from Yb:glass was obtained with 53% slope efficiency with 0.5 Hz reputation. The absorbed pump energy generated the thermal lens effects inside the Yb:glass. The wavefront distortion completely disappeared after 300 ms of pump pulse. Neither heat accumulation nor pumping damage was observed on the Yb:glass.

During the last decade, Nd:YVO₄ has been developed as a promising substitutes for Nd:YAG in diode-pumped lasers due to its high absorption and emission cross-sections. However, the applications of YVO₄ are limited due to its poor physical- mechanical properties and growth difficulty etc. Now, in this present paper, we have developed the high-doped Nd:YAG(SUPER- Nd:YAG) crystals. It shows high absorption cross-section and has many advantages over Nd:YVO₄: (1) Due to the cubic symmetry and high quality, Nd:YAG is easy to operate with TEM₀₀ mode. (2) Nd:YAG can be Q-switched with Cr⁴⁺:YAG directly (sandwich). (3) Nd:YAG can produce blue laser with the frequency-doubling of 946 nm. (4) Nd:YAG can be operated in a very high power laser up to kW level.

A review of problems concerning up to date CO laser physics and engineering is presented. Kinetic processes, including multiquantum processes of vibrational-vibrational exchange, optical pumping, explosive absorption, amplification of multiline radiation, its optical quality and phase conjugation are analyzed. Modes of operation (overtone lasing, Q- switching, etc.), laser geometry, methods of pumping and cooling, scalability of CO lasers and delivery of their radiation are discussed. A special attention is paid to the quite new results on first-overtone ((Delta) V equals 2) CO laser jointly obtained at the Lebedev Physics Institute (Russia), TRINITI (Russia) and Air Force Research Lab (USA).

Development of Chemical Oxygen-Iodine Laser (COIL) in Tokai University is described. From FY1996, we have conducted a three-year research project sponsored by NEDO (New Energy and industrial technology Development Organization), and it was finished in March 1999. As a result, high-efficiency operation (23.4%) of COIL with nitrogen as a buffer gas was demonstrated. Reduction of the vacuum pump size by the high- pressure subsonic mode operation with turbo blower was demonstrated. Specific energy reached to 3.5 J/liter. Output power stabilization/modulation technique by the external magnetic field was developed. Twisted Aerosol Singlet Oxygen Generator (TA-SOG) was tested and its performance was compared to liquid-jet SOG. TA-SOG was operated at the internal gas velocity of 85 m/s. Novel unstable resonator was developed with the aid of newly developed FFT code. We are now conducting a one-year project whose goal is a development of a 1 kW-class system capable of one-hour stable operation. Finally, three operation modes of future industrial COIL are proposed.

The objectives of this study are to achieve high-power, efficient operation of a room-temperature CO laser and to collect data for designing the CO laser system for nuclear reactor decommissioning. The influence of the H₂O concentration in the laser gas on the output performance was investigated, and it was found that the H₂O concentration should be kept as low as possible (less than 260 ppm) to obtain stable, high-power outputs. To improve output performance, the rf frequency was increased from 13.56 MHz to 27.12 MHz. The output power for the 27.12 MHz excitation was increased by 10 to 20% compared with that for the 13.56 MHz excitation. The laser output was scaled by extending the discharge tube inner diameter from 19 mm to 30 mm. By optimizing the air gap length and the curvature radius of the outer metallic electrode, the operating gas conditions, and the reflectivity of the output coupler, a maximum output of 830 W was obtained at a laser efficiency of 12.2% with adding neither Kr nor Xe. The addition of Kr was more effective for increasing the output than the addition of Xe. A maximum output of 910 W was obtained at a laser efficiency of 14.8% with Kr addition, and a maximum output of 810 W was obtained at a laser efficiency of 16.2% with Xe addition.

We have developed Gd_xY_1-xCa₄O(BO₃)₃ (GdYCOB) crystal in order to control birefringence. As a result, GdYCOB is noncritically phase matchable for third- harmonic generation of a 1064 nm light by type-I mixing (1064 + 532 yields 355 nm). However, during high-power operation, degradation of output power and distortion of beam pattern occurred due to photo-induced damages and thermal dephasing. In this paper, we report on nonlinear optical properties and improved the performance of GdYCOB by suppression of photo- induced damages and thermal dephasing.

Effect of ion beam etching on surface damage resistance was investigated in CsLiB₆O₁₀(CLBO) crystal. In high-power UV operation, an as-polished CLBO surface was damaged due to absorption of the polishing compound embedded inside the crystal surface. In the as-polished surface of CLBO, polishing compound ZrO₂ (absorption edge is about 300 nm) was detected to a depth of 60 nm. We have removed polishing compound with ion beam etching without degrading the surface quality. The effects of polishing compound removal on surface damage were characterized for the surface laser-induced damage threshold (LIDT) at 355 nm (pulse width 0.85 ns) as a function of etching depth and surface lifetime for the generation of fourth-harmonic of ND:YAG laser (266 nm, 20 ns, 4 kHz). We found an improvement of the surface damage resistance. LIDT of etched surface increased up to 15 J/cm² as compared with that of the as-polished surface of 11 J/cm². Etched CLBO surface also exhibits an improvement lifetime 4 times longer than that of as-polished surface.

The improved Sellmeier's equations and thermo-optic dispersion formula that reproduce well our experimental results for harmonic generation of CO₂ laser harmonics at 3.5303 - 5.2955 micrometers are presented. These formulas are believed to be highly useful for predicting the temperature-tuned 90 degree(s) phase-matched OPO in the mid-infrared.

Tunable single-line first-overtone (FO) CO lasing on wavelengths from 2.7 up to 4.2 micrometer corresponding to overtone vibrational transitions from 13 yields 11 up to 38 yields 36 on 413 ro-vibrational lines was experimentally obtained. A parametric study of energetic and spectral characteristics of the single-line FO CO laser was carried out. Energy distribution over ro-vibrational lines was measured. The maximum specific output energy (SOE) came up to approximately 3 J/l Amagat, with single-line output efficiency being up to 0.6%. For the first time, a multi-quantum theoretical model was used to describe the tunable single-line FO CO laser. This multi-quantum approach demonstrated better agreement between theoretical calculations and observed experimental data for laser output as a function of vibrational quantum numbers.

Output power enhancement of Chemical Oxygen-Iodine Laser (COIL) by pre-dissociation of molecular iodine using a microwave discharge was demonstrated. Two types of approach, transonic operation with grid nozzle and supersonic mixing with ramp nozzle array were tested. In the transonic operation case, a gas velocity at the laser cavity was estimated to be 167 m/s, and I₂ dissociation rate was found to be 50%. As a result, 9% of output power enhancement with the microwave pre-dissociation was obtained. In the supersonic injection case, we have initially obtained very poor output power. When we changed the gas flow rates, cavity flow returned to subsonic and we have obtained 284 W output power with 16% of chemical efficiency. No output power enhancement with microwave adoption was observed for ramp nozzle array. It was due to the insufficient dissociation of iodine due to the high stagnation pressure of secondary flow.

A hybrid oxygen-iodine laser (HOIL) using discharge singlet oxygen generator (DSOG) was studied experimentally. We used a microwave discharge as a plasma source. The microwave frequency was 2450 MHz and the input power for discharging was up to 30 W. Oxygen excitation tests were carried out using the DSOG by changing conditions of input power, oxygen pressure and flow rate, gas-mixing ratio, and so on. Spectrum analyzer was used as a diagnostic device for measuring the singlet oxygen of its wavelength 1.27 micrometer. The best results of excitation efficiency was 21% on condition that oxygen output pressure was 1.4 - 1.9 torr and oxygen flow rate was 30 sccm.

Copper vapor lasers (CVL) are high power, pulsed, visible lasers capable of diffraction limited beam quality and kilohertz pulse repetition frequencies. Kinetic enhancement of elemental CVLs has been reported to significantly increase the output power and pulse repetition frequency range of a given laser architecture. This paper reports the performance of medium scale CVLs when kinetically enhanced using small partial pressures of hydrogen halides to the Ne:H₂ buffer gas. Kinetic enhancement increased the output power by factors of up to 2.5. Using a 1 X 0.025 m discharge tube, an output power of 60 W was achieved at 15 - 20 kHz and over 50 W at frequencies in excess of 30 kHz. Using a high magnification unstable cavity, over 35 W was achieved at close to the diffraction limit. Sealed-off operation of the laser with excellent power stability was also demonstrated. The combination of high power, visible, diffraction-limited beam with short pulse widths, high pulse repetition frequency from a compact industrial package is of great interest for a number of micromachining and advanced manufacturing applications. In addition it is an excellent source for generating high power, narrow linewidth UV (255 nm) by frequency doubling.

We describe an all solid-state, high power, deep-UV (DUV) source based on sum-frequency mixing (SFM) of two single- frequency laser outputs. The system consists of a CW diode- pumped, Q-switched Nd:YLF laser operating at 1047 nm, a Ti:sapphire laser at 785-nm, and cascading SFM stages. Both laser sources are configured with an injection-seeded oscillator followed by amplifier to produce high power, single-frequency, TEM₀₀ outputs. The third harmonic of Nd:YLF MOPA is mixed with the output from Ti-sapphire MOPA to generate the first UV, which is used for the second mixing with the residual fundamental output to generate the DUV radiation. CLBO crystal is employed for each SFM process. The system produced UV pulses at 241.6 nm with 3.4 W, and also DUV at 196.3 nm with 1.5 W of average powers at a 5-kHz pulse- repetition rate. The linewidth of the DUV output was measured to be less than 0.05 pm.

High-power all-solid-state UV lasers are used for precise material processing applications in industrial fields with the potential advantage in the maintenance cost compared with other UV lasers. In this paper we report on the evaluation of nonlinear crystals by high-power intracavity second harmonic generation, the evaluation of nonlinear crystals for UV generation by the transmission loss and the surface damage threshold measurements, and high-power UV beam generation up to 20 W with the repetition rate of 10 kHz.

Intracavity frequency doubling with (beta) -BaB₂O₄ (BBO) crystal in a passively mode-locked unstable confocal resonator Nd:YAP (Nd:YALO₃) pulsed laser is described. Without any amplification and pulse selection, a powerful quasi-single psec pulse at 0.54 micrometer with an energy greater than 20 mJ and power density 5 GW/cm² was obtained by totally passive mode locking for the first time. Energy conversion efficiency 81% from 1.08 to 0.54 micrometer was achieved. The technique of intracavity second harmonic generation (ISHG) leads to stable output energy of SHG with an energy stability of 95.96%. This has a higher stability than that of fundamental wave with an energy stability of 92.78%.

In 7.7 divided by 39.6 cm¹ range investigated for the first time refraction and absorption spectra of AgGaSe₂ and AgGa_{1- x}In_xSe₂ crystals allowed to compute a phase matching characteristic and efficiency values of difference frequency generation with total laser intensity from 60 MW/cm² up to 15 GW/cm². The maximum efficiency values were close to 1*10^-3. Powerful half-cycle pulse generation is proposed with the investigated crystals.