Dr. Eugen Saenger, The Father of Laser Propulsion, 1905 - 1964.

I want to talk today about the enthusiasm for technological progress today and compare it with the situation 30 or 40 years ago. Then most of us looked forward eagerly to advances in technology and to the expansions of humanity's horizons these advances would make possible. We looked forward not only to the advances we could foresee but especially to the surprises which we had come to expect would be the greatest advances. We were full of hope that our children would live better lives.

This account of the beginning of the program on laser fusion at Livermore in 1962, and its subsequent development during the decade ending in 1972, was originally prepared as a contribution to the January 1991 symposium 'Achievements in Physics' honoring Professor Keith Brueckner upon his retirement from the University of San Diego at La Jolla. It is not a review of the international effort in laser fusion research. It is a personal recollection of work at Livermore from my vantage point as its scientific leader, and of events elsewhere that I thought significant. This period was one of rapid growth in which the technology of high-power short-pulse lasers needed to drive the implosion of thermonuclear fuel to the temperature and density needed for ignition was developed, and in which the physics of the interaction of intense light with plasmas was explored both theoretically and experimentally.

The progress of implosion physics research and relevant technologies enable us to examine technical and economical feasibility, and to plan the realistic strategy to the commercial power plant. The most important key issue for IFE is the driver technology. The development of the laser fusion driver is opening new industrial technologies based on the photon processes and new fields of high energy physics.

Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 10¹³ W/cm² with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the laser generated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.

The photophysical mechanism of laser ablation related to the change in activation energy for desorption (or destruction and further elimination) of electronically excited molecules is discussed. Despite the obvious physical reasons, up to now there is no experimental confirmation that this mechanism plays an important role with nanosecond laser ablation where duration of the laser pulse is longer than the corresponding relaxation time (typically, tens of picoseconds). On the other hand there was no special study of this mechanism for Ultra Short Laser Pulses (USLP) with pulse duration significantly shorter than the electronic energy relaxation time. Here we present the theoretical investigation of the photophysical ablation features with USLP and compare predictions which follow for the photophysical and pure thermal models.

Time-resolved measurement of the stress transients generated by pulsed infrared irradiation and ablation of tissue has demonstrated that these processes are governed primarily by photothermal processes. For ablation of porcine dermis at 2.79 micrometer (Q-sw Er:YSGG) and 10.6 micrometer (CO₂), the onset of material removal has been shown to be delayed with respect to irradiation and the magnitude of the generated stress transients are consistent with a model for explosive material removal. Upon consideration of the threshold radiant exposure for ablation, it appears that the mechanism and dynamics of these processes are controlled by explosive boiling as the tissue water is likely to be significantly superheated. To examine this issue further, we employed time- resolved optical interferometry to measure the surface displacement generated by Q-sw Er:YSGG laser irradiation of pure water for radiant exposures below the ablation threshold. This was done to directly measure the dynamic thermal expansion and interphase mass transfer generated by pulsed laser heating. These results are compared to a model which computes the dynamic thermal field within a semi-infinite pool of water undergoing pulsed irradiation while subject to a surface heat flux condition given by kinetic theory. We find that the measured mass fluxes exceed that predicted by simple kinetic theory arguments. The implications of the experimental and model results to pulsed laser ablation of tissue are discussed.

The development of new and more efficient schemes for improving the energy conversion from the optical radiation field into thermal, mechanical and or ionizational energy of target materials is a main concern in numerous laser effect studies, both in fundamental research and industrial applications. The present paper deals with recent investigations, carried out at ISL, by using a novel type of repetitively pulsed CO₂-laser. Infrared pulse energies up to 150 J and repetition rates up to one hundred pulses per second provide powers up to 15 kW. Peak powers (close to 100 MW) are depending on pulse lengths, which can be set from about 2 to 10 microseconds. Experiments have been performed to study repetitively pulsed large area, 'out-of-band'-plasma supported processes, induced on optically transparent dielectric materials. Further so-called 'in-band'-studies of semiconductor materials were carried out, by using a frequency doubled Nd-YAG-laser and a dye-laser. Due to the small spot approach, experiments could be performed with rather small pulse energies of 500 to 600 mJ to yield fluences up to several tens of J/cm². Examples, both concerning the high energy, (out-of-band) and the lower energy (in-band) measurements will be discussed for various types of targets. Mechanisms involved include quasi-explosive thermal ablation and absorption-wave supported impulsive loading due to thermo- mechanical processes.

This paper investigates the laser ablation of materials at high intensities. It is known that when drilling organic materials at low and moderate fluences (0.1 - 50 J/cm²) the etch depth per pulse increases exponentially with fluence, essentially following the Beer Law absorption characteristic. For carbon fiber composite the ablation rate reaches a level of about 2 micrometer per pulse for high fluences. However when approaching very high intensities (e.g. greater than 10¹⁰ W/cm²) a sharp increase in ablation rate to greater than 30 micrometer per pulse has been observed and used for drilling experiments. Extensive studies of this regime have subsequently been carried out to characterize it. This has included the effect of different focusing lenses and the effect material thickness. For high quality beams the hole quality is good with no heat affected zone and no significant mechanical damage. This new regime may make the large scale excimer laser drilling or cutting of carbon reinforced fiber an economically feasible application due to the increased drilling and cutting rates. Also this method only requires a simple optical system for high beam utilization factors. These issues will be discussed.

Ultrashort-pulse lasers are at an increasing rate being used for laser-induced surface modification of insulators, including ablation. Ti:sapphire chirped-pulse amplifier systems, with fundamental wavelengths in the near infrared, can produce efficient ablation and other desirable surface modifications with little collateral damage because the laser energy is deposited on a time scale much shorter than thermal diffusion times. Little is known, however, about how ultrashort pulses interact with insulators at wavelengths in the vibrational infrared. This paper describes surface modifications achieved by picosecond laser irradiation in the 2 - 10 micrometer range. The laser source was a tunable, free- electron laser (FEL) with 1-ps micropulses spaced 350 ps apart in a macropulse lasting up to 4 microseconds, with an average power of up to 3 W. This unusual pulse structure makes possible novel tests of the dependence on fluence and intensity, as well as the effects of resonant vibrational excitation. As model materials systems, we studied calcium carbonate, its isoelectronic cousin sodium nitrate, and fused silica. Particularly intriguing are surface modifications achieved by tuning the laser into vibrational resonances of the target materials, or by tailoring the energy content of the pulse. The mechanisms underlying these effects, and their implications for materials-modification strategies, are discussed.

A performance update of DLR's supersonic chemical oxygen iodine laser shows output powers of typically 10 kW CW and a chemical efficiency close to 20%. It is suggested to use a quasi-CW magnetically Q-switched mode of operation with considerably enhanced peak-to-CW power as a new ground based laser candidate for near earth space debris clearing. A preliminary design demonstrates the attractive features of such a laser system as compared to previous approaches. In particular, the chemical oxygen iodine laser promises to be rather compact and to substantially reduce the total debris clearing time.

We review the progress in the development of ultrashort- wavelength lasers based on discharge excitation. The observation of large soft x-ray amplification in Ne-like Ar at 46.9 nm in a capillary discharge created plasma [Phys. Rev. Lett. 77, 1476, (1996)] have opened a new path for the development of compact and practical soft x-ray lasers. These results have shattered the earlier perception that discharge- created plasmas are insufficiently stable and uniform for soft x-ray lasing. We report results of a detailed characterization of the capillary discharge Ne-like Ar laser, that include near-field and far-field images of the laser output and the measurement of its spatial coherence. The recent demonstration of lasing in Ne-like S at 60.8 nm in material ablated from a solid target by a discharge is also discussed.

The overview of the phase-locking technique is presented. Analysis of the parameters that influence on the phase-locking is given. Admissible range of the deviation of the linear laser diode array, external cavity parameters is estimated. 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 that opens an avenue to high power output radiation in a phase-locked mode. Special consideration is given to the cavity supermodes selection. Our experiments both confirm actual feasibility of 1D and 2D phase-locking of LDA's with specified parameters in the external Talbot (L_c equals Z_T/4) cavity and illustrate theoretical predictions of the system stability and selectivity, so that: (1) phase-locking in 1D configuration allowed diffractional limited lobes width (delta) (Psi) equals 0.5 mrad; (2) tilting of the output mirror provided 'in-phase' supermode selection; (3) phase-locking in 2D configuration of the two LDA's of N equals 8 lasers, separated by 1600 mkm resulted in diffractional lobes full width at half maximum (delta) (Psi) equals 0.5 mrad in slow axis and (delta) (Psi) equals 0.25 mrad in fast axis.

The review is devoted to the condition of researches on lasers pumped by nuclear reactors. In the review the brief description of NPL experiments is given, the parameters of perspective laser media and short explanation of processes taking place in gas NPL are also given. Possible schemes of NPL's, LAEL's and concepts of RL's are under consideration. The problems and perspectives of NPL's and RL's are touched upon.

The use of pulsed lasers for microprocessing material in several manufacturing industries is presented. Deep-uv photolithography, via hole and ink jet printer nozzle microdrilling, solar panel thin film scribing, texturing of hard disks, annealing amorphous silicon in flat panel displays, fiber Bragg grating production and micro-electro- mechanical system (MEMS) fabrication applications are discussed.

Laser-induced pressure generation and surface modifications induced by laser-shock processing were investigated with the use of different laser conditions including a large range of pulse durations and wavelengths. With the use of several lasers sources, original results could be pointed out concerning the precise influence of laser parameters on the shock generation in water-confined plasma regime. The beneficial influence of short pulse durations (1 to 10 ns) and the deleterious effect of short wavelengths (below 0.532 micrometer) were demonstrated with the use of a VISAR Doppler velocimetry system. Lastly, beneficial effects of LSP were shown on many surface applications such as fatigue and fatigue contact resistance, pitting corrosion and stress corrosion cracking behaviors.

We describe the principle of the polychromatic laser guide star (LGS) to recover the tilt information in imaging through the atmosphere. Observations using the AVLIS laser at the Lawrence Livermore National Lab are discussed in terms of returned flux in the ultraviolet. The major items of the program ELP-OA, starting now in France, are briefly reviewed, as well as the organization of the LGS R&D in Europe. Finally the conclusion outlines the possible improvements of the polychromatic LGS to allow us to reasonably implement it at large astronomical telescopes.

Diffractive optical systems have been developed to allow high efficiency material processing using excimer lasers. These systems allow beam homogenizing and shaping for surface treatments or parallel focusing for multiple hole drilling and production of microrelief. Beam utilization factors of 80% have been achieved.

The 193 nm argon fluoride (ArF) excimer laser can effectively be used to change the radius of curvature of the cornea and thus alter the refractive state of the eye. This change allows myopic (nearsighted) patients to see well with less dependence on glasses or contact lenses. The two major techniques of laser refractive surgery currently in effect in the United States are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). This paper will discuss these refractive cornea surgical techniques.

In most of pulsed laser material processing of which drilling, surface treatment, laser cleaning or shock hardening, the incident energy is distributed among several complex phenomena such as reflection, absorption, radiation or hydrodynamic depending on wavelength, polarization, pulse duration, energy end power density, spot size, nature of the material and last but not least the nature and the pressure of the surrounding gas. Experiments were conducted in order to characterize all these phenomena involved in different applications and to contribute in a better understanding of the laser material processing. Experimental data has been obtained from several diagnostic techniques such as integrating sphere, thermocouple probes, ballistic pendulum, pressure gauges, electromagnetic field detectors, spectrographs and high-speed camera over a wide range of energy and power density and gas pressure.

We have characterized laser-induced shock wave in water. Pressure measurements have been carried out using piezoelectric sensors and optical methods (interferometer, high-speed visualization). These experiments give us information on the pressure amplitude, the rise time, the pressure pulse duration, the energy, the shock wave velocity, and the pressure decay. These researches find applications in various fields such as: laser cleaning of sensitive surface, pressure probes calibration, medical treatments (intraocular surgery, lithotripsy).

A spatially modulated copper target surface (groove diameter (lambda) _s approximately equals 70 plus or minus 2 micrometer and groove depth d_S equals 20 plus or minus 2 micrometer shows an enhanced X-ray fluence and wider angle of emission as compared to optically polished ((lambda) /2), planar copper target when irradiated with laser energy density E_L approximately equals 600 J/cm². Experiments were performed using a 40 ps (FWHM), 1.06 micrometer laser. An array of 5 X-ray detectors at various angle to the laser axis measured the angular distribution of X-ray fluence. Higher X- ray flux and wider angle of emission are attributed to invoking of resonance absorption at the curvature, shape of curvature of the spatially modulated surface groove respectively. In this article we address to the issues of large angle emission of X-rays, their magnitude and scaling laws for both types of targets. Since X-ray emission time depends on laser pulse duration, present experimental results are discussed explicitly on the laser energy density dependance (J/cm²).

Multishot laser damage thresholds of Au-, Ni-, Cr-, and Mo- films on fused silica have been measured with 200 fs/400 nm pulses for a sequence of film thicknesses. We found a pronounced dependence of threshold fluences on film thickness in the ranges below 50 nm for Ni and Cr, below 70 nm for Mo, and extending up to 700 nm for Au. These variations reflect the depth of hot electron diffusion which is governed by the strength of the electron-phonon coupling. The onset of damage is defined as melting for Au-, Ni- and Mo-films. For Cr-films, we could identify the brittle-to ductile phase transition as well as melting. The change of damage thresholds can be described by the two-temperature model, provided the temperature dependence of optical and thermal properties of the material is properly accounted for. By fitting model predictions to the extrapolated single-shot damage thresholds, we find the following values of electron-phonon coupling constants (in units of 10¹⁶ W/Km³): g (Au) equals 2 plus or minus 20%, g (Ni equals 36 plus or minus 40%, g (Cr) equals 42 plus or minus 40%, g (Mo) equals 13 plus or minus 40%.

Irradiation of metals and dielectrics with subpicosecond laser pulses reveals a variety of different microscopic processes compared to longer pulses. We choose a transport theoretical description of electrons and phonons, which allows us to investigate the contribution of particular collision processes on macroscopic material response, even for highly nonequilibrium subsystems where a hydrodynamic description fails. The absorption of laser energy is described by photon- absorption of free electrons with assistance of phonon collisions. We considered relaxation of the electron gas by electron-electron and electron-phonon collisions. In the case of a dielectric, terms for multiphoton ionization and impact ionization are included. A resulting system of coupled time- dependent Boltzmann equations is solved numerically. For SiO₂ as an example of dielectrics we calculate changes in occupation numbers of electrons and phonons, respectively, determining the sequence of excitation and relaxation of the subsystems under ultrashort laser pulse irradiation. It is shown that the essential process of free-electron generation is multiphoton ionization. Additionally, we investigate polarization effects for the case of irradiation of metals. Using parameters for gold we find anisotropic absorption of laser energy, which depends on polarization of the electric field and material parameters as well as on photon energy of irradiating laser light.

We have investigated the material processing of oxides and fluorides using ultrashort laser pulses and have demonstrated a strong improvement when compared to results using longer pulse widths in the nanosecond range. High laser fluences (well above the damage threshold) at 800 nm and 248 nm are used to generate channels with high aspect ratios. Careful beam alignment can eliminate any remaining stress-induced damage outside the channel. At intermediate fluences just above the front surface processing threshold we observe a low ablation rate. In this 'gentle etch' phase it is possible to generate well-defined, smooth pockets and periodic patterns or ripples. The ripples appear when the laser pulse width is shorter than the lifetime of the electrons excited into the conduction band. In the low fluence regime (below the surface damage threshold) the self-focusing of laser pulses in the ps and sub-ps range can be utilized to obtain microstructures inside and on the rear side of the transparent materials.

For the use of laser ablation from solid materials, laser intensities I in the range of few orders of magnitudes above or below 10¹⁰ Watts/cm² are of interest. Though these are of a modest magnitude it is known from the very beginning of laser plasma interaction studies that the ablation process mostly is not following the simple gasdynamic (thermokinetic) heating and hydrodynamic plasma mechanisms but that most complicated phenomena appear which are in the focus of research since their first observations in 1962 by Linlor and Honig. A number of basically new nonlinear mechanisms were discovered including the nonlinear (ponderomotive) force, emission of different groups of plasma where the fast ions show a separation by energy linearly increasing on their ion charge number Z, ponderomotive and relativistic self focusing, generation of higher harmonics in the backscattered radiation, double layer and surface tension effects, parametric instabilities and a stochastic pulsation of the interaction within some ten picoseconds. This all seems to have a threshold of about 1 MW laser power at least if beams of up to 100 wavelength diameter are being used. For the applications of the ablation mechanisms it may well be that further anomalous mechanisms may be discovered with the very wide beams. In order to simulate the awareness of these complexities, this paper is presenting a new and updated synopsis on the anomalous nonlinear mechanisms whose knowledge seems to be settled at present.

In spite of soliton was discovered firstly by J. S. Russell as an unusual wave on the water (1834), its rediscovery was made by Zabusky and Kruskal (1965) in connection with the problem of heat transfer in lattice of solids, stated by E. Fermi et. al, which poses the question about nonvalidity of the Fourier law (1822). M. Toda (1979) formulated that in nonlinear lattices the energy is mainly transported by solitons. In 1992 we found experimentally the Wave of Reflection and Conduction (WRC), that can present solitonic mechanism of heat transfer in solids. It has a threshold approximately 10 kW/cm² and some solitary wave features: (1) the WRC amplitude is all the time of one sign, (2) its velocity U_WRC is nearly constant; (3) it reflects from sample surfaces without changing the velocity. WRC-reversible photoinduced phenomenon which appears and could be important in many laser-surface interaction studies. As example, it could be important for the investigation of processes in the materials after pulse laser irradiation, especially in the case of pulse-periodic irradiation. WRC is excited by IR pulse laser and consist from the whole series of more than 20 solitary kind waves, which velocity drop twice starting from speed of sound to less than cm/s. WRC shows an universal character: it can translate the pulse of heat energy through the samples of any of 4 main crystal types and of some amorphous solids (glasses) in the manner different from usual diffusion. WRC characteristics were measured from variation: (1) of the optical constants (by reflection on the surface or absorption in the bulk); (2) of the temperature (by thermovision camera) on the sample surfaces; (3) of the pressure (by pressure pick up); (4) of the conduction on the surface and in the bulk (in the case of conducting samples). Results of investigations are agreeable in spite of experiments were made in 4 laboratories (Russia, France, Japan) by different schemes. A new V.I. Emel'janov's theory of defect recombination wave describes solitons with U_WRC approximately cm/sec in solids, preconditioned by the action of powerful short laser pulse and could be considered as a possible physical mechanism of WRC.

Investigations of material transfer comparatively has been carried out using micro- to nano second laser-assisted removal of ceramics (Al₂O₃, ZrO₂) onto different substrates (steel, Si) for various laser parameters (wavelength, pulse energy, repetition rate, fluence) and processing variables (processing gas properties, target- substrate-distance). The transfer of the material was examined by high-speed photography, beam deflection, and optical spectroscopy. The experimental results, which are related to theoretical ones, are discussed in view of applications.

Pure iron samples were irradiated with short single laser pulses (6 ns) under atmospheric conditions. For the irradiation a Q-switched Nd:YAG laser (wavelength 1064 nm) was used. The second and third harmonics (532 nm and 355 nm) were used for investigations at different wavelengths. The composition and expansion of the laser induced plasma were investigated using Optical Emission Spectroscopy (OES) and a High Speed Framing Camera (HSFC), respectively. The composition and morphology of the target surfaces were investigated by means of Auger Electron Spectroscopy (AES) and Scanning Electron Microscopy (SEM). AES measurements indicate that the near surface region of all laser spots is enriched with nitrogen (about 15 at-%) and covered by a thin oxidized layer. This surface composition was found to be similar for all wavelengths. It was established that the density (10¹⁹. . . 10²³ cm^-3), the composition (Fe⁺, N⁺ and O⁺), the temperature (30,000. . . 35,000 K) and the expansion of the laser induced plasma are similar for all wavelengths as well. Because of the high density the plasma absorbs the laser irradiation completely and the wavelength becomes less important for the laser target interaction. We assume that the cooling process leads to a formation of compounds of iron and nitrogen which are deposited on the surface.

The flat panel display market is presently experiencing an annual growth rate of over 20%, and shows no signs of slowing down. Polycrystalline-silicon (poly-Si) thin film transistor (TFT) devices offer both improved quality and decreased cost over amorphous silicon displays, and will undoubtedly capture an ever increasing share of this market. However, producing poly-Si TFTs in high volume has presented a number of practical challenges. One of these challenges is transforming an amorphous silicon layer into poly-Si. For several years, researchers have investigated using excimer lasers for this purpose. In excimer laser annealing, a high-power excimer laser beam is scanned across the surface of a substrate coated with amorphous silicon. The intense UV light causes the silicon to melt, leading to subsequent recrystallization in polycrystalline form. This process is very non-linear, and places tight tolerances on laser beam parameters such as energy stability and beam uniformity. Furthermore, for this technique to be successfully integrated into a high volume production line, the laser must exhibit extremely high reliability, minimum downtime and low cost of ownership. In the past few years, excimer laser manufacturers have made significant technical improvements, leading to products that can successfully meet the rigorous demands of industrial TFT production. As a result, major display manufacturers in Japan, Korea and Europe have now adopted this technology for mass production.

This paper presents a study of laser ablation of Al₂O₃-TiC ceramics. The dependence of the ablation rate and of the surface topography and constitution after ablation as a function of laser fluence and number of pulses was investigated. At low laser fluences, the surface develops a globular topography which is responsible for the decrease of the ablation rate and the increase of roughness. At higher laser fluences, the removal rate and the surface topography slightly depend on the number of pulses and on the laser fluence. The roughness also increases with the number of pulses but smoother surfaces can be achieved.

For laser intensities in the range of 10⁸ - 10⁹ W/cm², and pulse lengths of order 10 microseconds or longer, we have modified the inertial confinement fusion code Lasnex to simulate gaseous and some dense material aspects of the laser-matter interaction. The unique aspect of our treatment consists of an ablation model which defines a dense material-vapor interface and then calculates the mass flow across this interface. The model treats the dense material as a rigid two-dimensional mass and heat reservoir suppressing all hydrodynamic motion in the dense material. The computer simulations and additional post-processors provide predictions for measurements including impulse given to the target, pressures at the target interface, electron temperatures and densities in the vapor-plasma plume region, and emission of radiation from the target. We will present an analysis of some relatively well diagnosed experiments which have been useful in developing our modeling. The simulations match experimentally obtained target impulses, pressures at the target surface inside the laser spot, and radiation emission from the target to within about 20%. Hence our simulational technique appears to form a useful basis for further investigation of laser-surface interaction in this intensity, pulse-width range.

Laser processing of hard materials, in particular, diamond and ceramics, demands an optimal laser system for a particular application: cutting, drilling, patterning, etc. A comparative experimental study of laser drilling of CVD diamond and 3D microstructuring of ceramics has been performed varying the main laser radiation parameters -- wavelength, pulsewidth and pulse energy. Ablation of CVD diamond films by nanosecond (5 - 9 ns) and picosecond (150 - 300 ps) pulses at three different wavelengths of 1078, 539 and 270 nm in the range of laser fluence 1 divided by 10³ J/cm² was investigated. Ablation rates at different stages of laser drilling -- for shallow craters, deep holes with aspect ratio of 3 divided by 5 and ultradeep channels (aspect ratio up to 30 divided by 40) -- were measured and compared. The effect of waveguide propagation of laser radiation in deep channel was revealed. The role of laser-plasma interaction as well as the modification of a surface layer are discussed. The results of 3D laser microstructuring of ceramics -- AlN, SiC, Al₂O₃, Si₃N₄ -- are demonstrated. The rectangular in plane structures (pockets) with typical size of 200 X 200 X 100 micrometer³ with steep walls and flat bottom were produced. The influence of radiation parameters on the quality of the tested structures, mainly, on the bottom roughness and walls steepness has been analyzed.

During the past few years high power lasers which incorporate intracavity optical waveguiding have been demonstrated in a number of different geometric formats. These include rectangular planar waveguide structures, two-dimensional multi-element waveguide array lasers and annular waveguide devices, all of which depend crucially on the operational flexibility of the transverse radiofrequency excitation technique. Here, we review the fundamental issues which underlie the attractions of the use of waveguiding structures in the design and construction of ultracompact, diffusion- cooled lasers which are efficient and operate at high average power levels. In particular, we review the properties of large area discharge planar waveguide CO₂/CO lasers, where multi-kilowatt cw power levels have been demonstrated with excellent beam quality and efficiency. It is shown that similar concepts may also be applied to solid state lasers. In addition, the use of the multi-element array concept for high power scaling will be examined, and the operating characteristics of an ultra-compact 64 (4 X 16) element array laser operating at 2 kW cw output power will be described.

In this paper, we summarize the electron pumping and molecular transfer kinetics of pulsed CO lasers, as to their influence, as well as the influence of gas pressure and temperature, on pulsed laser emission line structure, pulse duration and energy output per unit gain volume. Correspondence with existing experimental laser data is shown and compared to these factors. Implications for pulsed laser applications are explored.

High brightness, high average power, diode pumped Nd:YAG solid state lasers (DPSSL) are being developed by TRW as part of the Precision Laser Machining Technology Reinvestment Program. The use of diode pump arrays in place of flashlamps, and zig-zag slab geometry, allow lasers to be scaled to power beyond the current generation of lamp-pumped rod lasers while providing excellent beam quality. The efficiency is 3 - 4 times better using diode arrays in place of flashlamps resulting in less waste heat in the laser medium and reduced optical aberrations. The corresponding beam quality provides more than an order of magnitude increase in the average intensity available at the workpiece, thus enabling new machining capabilities.

Forty-five joule per pulse XeCl laser with good homogeneity (plus or minus 1%) and repeatability (plus or minus 1%) allow to increase the quality and decrease the cost of annealing of amorphous Silicon compared to Solid Phase Crystallization and Scanning Excimer Laser Annealing.

In this paper we describe the concept, and the basic scaling relationships of solid state heat capacity lasers. Intermediate between single shot and average power systems, the heat capacity concept scales solid state lasers to MW levels of burst power.

Design and performance details of a 5-kW cw arc-lamp-pumped Nd:YAG laser are given. Two types of 5-kW Nd:YAG lasers have been developed, which have enough laser power and beam intensity necessary to realize deep penetration and high speed welding of metals. Experiments of welding and thick plate cutting were performed in order to investigate the ability of 5-kW Nd:YAG lasers. Maximum penetration of 15 mm is achieved with stainless steel and that of 6 mm with aluminum alloys. Moreover high speed welding up to 8 m/min is realized.

A novel non-waveguide, non-free-space CO₂ laser resonator cavity, referred to as the split-wave hybrid (SWH) resonator, is described. Traditional resonator mirrors combined with two specially designed light reflecting electrode walls, which enclose the active medium, form the SWH resonator cavity. Light reflecting walls in the split-wave resonator act as wave-front-splitting mirrors in an interferometer, similar to a Fresnel double mirror or Lloyd mirror interferometer. Wave- front of the intra-cavity laser beam is significantly tilted with respect to the resonator walls, which facilitates lowest order mode selection in this resonator. Additionally, electrode wall surfaces contain discontinuities, which further enhances non-waveguide mode discrimination in the SWH resonator.

The short wavelength of excimer lasers offers advantages for industrial applications such as cutting, drilling surface treatment and annealing. At NCLR a 1 kW (1 J, 1 kHz) XeCl- laser is under development. This laser has several advantages above commercial excimer lasers that enables it to go to the high average power of 1 kW combined with an excellent beam quality.

The O₂-I₂ transfer laser is gaining interest as a high-power laser for a variety of applications. There remains an interest in alternate pumping techniques for some situations, however. Here we review earlier work on the potential use of nuclear pumping, either for direct pumping of O₂ or for pumping of an excimer flashlamp in a photolytic iodine laser system.

Ordinary Free-Electron Lasers (FELs) can be found in successful operation in the spectral range from millimeters to ultraviolet wavelengths. However the operation of the common FELs in the extreme ultraviolet and X-ray wavelength regimes faces certain adverse effects. Some of the main obstacles in the way of the realization of X-ray FEL are electron momentum spread and angular divergence. Another point to keep in mind is that ordinary FELs work on the principle of 'momentum population inversion.' By this one means that electrons with momenta larger than the resonant value contribute to the gain whereas electrons with momenta smaller than the resonant value contribute to the loss. Thus to ensure a net gain we need more electrons with momenta lying in the upper momentum domain than in the lower one i.e. a 'momentum population inversion.' Keeping these points in mind Scully and co-workers have proposed using the ideas of Lasing Without Inversion (LWI) to achieve the successful operation of short-wavelength (extreme UV and X-ray) FELs. The purpose of this work is to, as a first step, critically analyze the theoretical and the practical feasibility of the proposals by Scully and co-workers. In particular we take a look at the following issues: Can the LWI FEL's operate efficiently even with a strongly inhomogeneous, broad electron momentum distribution? How practically feasible is the two-section Cherenkov Transition Radiation (TR) FEL? Does this Cherenkov TR FEL allow a complete absorption cancellation and LWI operation even in the case of a very broad electron momentum distribution compared to the homogeneous width? Is the gain of LWI FEL greater than the gain of the usual FEL by a factor of 100 (i.e. by two orders of magnitude)? How realistic is the proposed 'classical selective interference?' Moreover is the 'classical selective interference' free of the angular spread limitation? How valid is the small-signal gain calculation? The saturation would invalidate the small-signal analysis: when precisely does the saturation set in? The saturation is expected to affect the phase coherence required for the interference: what is the form of dependence of phase coherence on the saturation?

The results of the experimental study of UV and IR lasers pumped by various methods are presented. The accelerators with radially converging or planar e-beams pumping gas mixtures at pressures up to 3 atm and self-sustained discharges were used. The highest laser radiation energies in the UV up to 2 kJ have been obtained at (lambda) equals 308 nm. Output of 100 J at (lambda) equals 1.73 micrometer in Ar-Xe mixture and 50 J at (lambda) equals 2.03 micrometer in He-Ar-Xe mixture was obtained from the e-beam laser with active volume of 600 l. Output energies of 110 J at (lambda) equals 308 nm and 90 J at (lambda) approximately 250 nm, respectively, were achieved in compact high-power e-beam laser with chamber of 20 cm in diameter and 30 l active volume. This e-beam geometry was shown to be very promising for excitation of non-chain HF- laser media. HF-laser efficiency with respect to deposited energy as high as approximately 10% and energy up to 200 J at (lambda) approximately 2.8 micrometer were demonstrated. Amplification of XeCl-laser beam from master oscillators and amplifiers under conditions of strong amplified spontaneous emission is considered. Investigations of CO₂ laser excited by e-beam controlled discharge and e-beam ignited discharge were performed. The highest laser output in the IR of 3 kJ have been obtained at (lambda) equals 10.6 micrometer.

Phase conjugation process at intracavity degenerate four wave mixing of long pulse CO₂ laser radiation in its active medium has been studied experimentally. Parametric dependencies of energetic and temporal characteristics of phase conjugation signal are analyzed. Phase conjugation reflectivity comes up to 7.5 - 10%. A periodical dependency of the reflectivity on optical delay with a period of approximately 10^-1 and 10^-3 cm has been observed.

The construction of CO₂ laser system generating a train of subnanosecond laser pulses with total train energy up to 5 J is reported. A record level of laser energy was obtained due to utilization of unique 5 X 5 cm² aperture, 6 atm X- ray preionized CO₂ amplifier. The estimations of individual pulses durations in the train are given. The prospective of upgrading of present configuration of laser system towards shortening of laser pulses duration and increasing the pressure of working gas mixture of the amplifier and its efficiency are discussed.

In Pulsed Laser Deposition the laser parameters pulse duration, repetition rate, and average output power are important parameters for control of the resulting film properties. In order to obtain high deposition and coverage rates the use of powerful laser radiation sources is required. A new approach is the use of Q-switch CO₂ lasers, making accessible the range of kW average output powers for PLD. A common industrial fast axial gas flow rf-excited CO₂-laser is equipped with a mechanical Q-switch assembly. The characteristic properties of this laser radiation source are studied by measurements of the average output power and of the spatial and temporal intensity distribution of the laser radiation. Results show, that the laser operates in multimode or in TEM₁₀ mode depending on the use of apertures in the resonator. Variation of the electrical rf-excitation (in synchronization with the Q-switch) influences the pulse duration and shape, repetition rate, and average output power. Based on the experimental findings model calculations are carried out, in order to predict the pulse energy and average output power of the laser radiation source, when operating with different Q-switch parameters, e.g. higher repetition rates.

In a series of spectacular experiments conducted at the High Energy Laser Systems Test Facility (HELSTF), White Sands Missile Range (WSMR), NM, using 13- to 15-cm diameter, 40- to 60-g vehicles designed to fly on the 10 kW PLVTS pulsed carbon dioxide laser (1 kJ pulses for 30 microsecond duration at 10 Hz), Prof. Leik Myrabo of Rensselaer Polytechnic Institute (RPI) and Dr. Franklin Mead of the Air Force Research Laboratory's (AFRL) Propulsion Directorate, have been successfully flying laser propelled Lightcraft under a joint Air Force/NASA flight demonstration program. The axisymmetric Lightcraft vehicles are propelled by airbreathing, pulsed- detonation engines with an infinite fuel specific impulse. Impulse coupling coefficients have been measured with ballistic pendulums as well as a piezoelectric load cell and fall in the range of 100 to 200 N/MW. Horizontal wire-guided flights up to 400 ft, using a unique laser beam pointing and tracking guidance system, have demonstrated up to 2.0 G's acceleration measured by a photo-optic array. Spin-stabilized free-flights with active tracking/beam control have been accomplished to altitudes of 15.25 meters. This paper will summarize the progress made to date on the Lightcraft Technology Demonstration flight test program, since the first 12 - 14 July 1996, experiments at HELSTF.

Space debris at low Earth orbits (LEO) in the size range of 1 to 10 cm in diameter poses a severe threat on the International Space Station and other valuable space assets. High-power laser radiation may be the most feasible means to mitigate this problem. Under the irradiation of a high-power laser beam part of the debris material is ablated and provides an impulse to the debris fragment. Proper direction of the impulse vector allows either to deflect the object trajectory to miss the station (defense option) or to reduce the orbital energy of the debris and force it on a trajectory through the upper atmosphere. There the debris burns up instantaneously or after a few revolutions (cleaning option for LEO). A space based deployment of the laser is favored for several reasons: The lack of laser transmission through the atmosphere reduces the total the total system substantially, laser range and detection requirements are inferior and the laser can be used against an immediate threat. Peculiarities of the geometrical situation in the orbital plane are described. Based on a 100 kW average power laser, aluminum as a typical material, and some other assumptions, the capability and limitations with respect to the debris velocity and mass are calculated for both options of the laser utilization.

ORION is a practical proposal for removing the 150,000 pieces of manmade space debris in the 1- to 10-cm size range now orbiting the Earth below 1500 km altitude which threaten large space systems in low Earth orbit. It is based on using the thrust produced by pulsed laser ablation of a thin layer on the debris surface to drop its perigee sufficiently for reentry and burnup. Applied when the object is rising between about 45 and 15-degree zenith angle, the necessary (Delta) v is of order 100 m/s. A laser of 30 kW average power at 10-ns pulsewidth and a 6-m mirror with adaptive optics can clear near-Earth space of these debris in 2 years of operation. Technical challenges faced by such a system include: heavy demands on detection, tracking and adaptive optics arising from the tiny optical cross section of the smallest debris and the required pointing accuracy and steering rate, stimulated Raman conversion and nonlinear refraction of the laser beam in the atmosphere, uncertainty of momentum coupling coefficients (C_m) for some materials, and high-average-power laser development. It is crucial that the system we propose be developed under international aegis, to insure that its installation does not increase international tensions. It should be viewed as a single-pay lifetime insurance policy for the World's space assets whose premium is less than 1% of the protected asset value, an excellent rate for such contracts.

Orbital debris in low-Earth orbit ranging in size from 1 to 10 cm in diameter can be detected but not tracked reliably enough to be easily avoided by spacecraft. In addition, shielding protection is extremely difficult and costly to accomplish for sizes above 1 - 2 cm. Debris in this size regime traveling at mean velocities on the order of 20000 miles per hour may cause catastrophic damage. Using adaptive optics technologies, a ground-based pulsed laser of sufficient power ablating the debris particle's surface to produce small momentum changes may, in several hundred pulses, lower a target debris particle's perigee sufficiently for atmospheric capture. A single laser facility could remove all of the 1 - 10 cm debris below 1500 km in altitude in approximately three years. A technology demonstration of ground based laser removal is proposed which would pave the way for the implementation of such a debris removal system. The cost of the proposed demonstration is comparable with the estimated annual cost of spacecraft operations in the present orbital debris environment.

This paper analyzes laser induced impulse and associated scaling laws appropriate to the intensity range of interest for laser space debris clearing (10⁶ - 10¹⁰ W/cm²). A simple radiative fluence model is used augmented by empirical values from some recent experiments.

Using KrF laser ablation the fabrication of Yb:YAG, and Cr:YAG planar waveguides as well as nonlinear Co:BaTiO₃ waveguide has been carried out. The waveguide structure was fabricated by deposition of a rare-earth-doped YAG layer on an undoped substrate. The optical waveguide was characterized by optical absorption spectra, photoluminescence spectral intensity, optical propagation loss, and valence state by XPS. Co:BaTiO₃ nonlinear optical, thin films have been fabricated with good quality using a new hybrid process of two-target pulsed laser deposition and in-situ pulsed laser annealing. Epitaxial Co:BaTiO₃ films with Co/Ti concentrations of up to 5% were achieved by this method. Particles on the growing film were almost eliminated by in-situ laser annealing. Cobalt dopant concentrations were easily adjusted and controlled.

Our measurements of laser coating removal rate for 14- micrometer-thick black organic paint coatings on a refractory substrate showed the rather astonishing result that total exposure time (Delta) t required to ablate the coating (for these particular thin organic coatings, in air) is described by (Delta) t equals C X I^-3/2 sec where x is the coating thickness, I the incident pulse intensity (W/cm²) and the constant C equals 1.4 X 10⁷ W^3/2-s/cm⁴. This was true for wavelength 350 nm less than (lambda) less than 10.6 micrometer and incident intensity 81 W/cm² less than I less than 424 MW/cm², covering a 7-order-of- magnitude. Such a simple result is phenomenal because of the range of different physical processes known to be involved (simple boiling, thermal conduction and differential thermal expansion for the long pulses, but the combined effects of shock spallation, large excursions within the solid equation of state, and acceleration, compression, shocking, thermal conduction, radiation, phase explosion and even some ionization for the shortest pulses). If extensible to other coatings and substrates, it is also very valuable for the design of expensive laser coating removal facilities, which are now receiving strong interest for very large-scale decontamination and aircraft repainting. Our first-order parametric model fits the experimental results to within better than a factor-of-3 in laser intensity throughout the above range.

Industrial-cleaning-rate laser systems have been built and tested for removing various types of coatings, such as rad- contaminated coatings, non-rad but hazmat-contaminated coatings (e.g., Pb-based paint), and non-hazardous coatings from various types of substrates such as concrete, metals, and composite materials.

The composite beam from a 64-element carbon dioxide array laser, which produces an average power output of 2 kW, has been used in a series of surface processing experiments. In one set of experiments related to nuclear power station decommissioning, the array beam has been used to process the surface of a range of different density concrete materials. Scabbling trials have achieved concrete removal rates of approximately 1200 cm³hr^-1kW^-1 and average scabble depths of approximately 5 mm with incident power densities as low as 80 Wcm^-2.