Pumped with two 970nm, InGaAs strained-layer diode arrays, a side-pumped Er,Yb:phospha te glass laser has been const ructed. The long pulse slope eff iciency was 14%, and threshold occurred wit h 60mJ input. The maximum output pulse energy was 20.5mJ, wit h output wavelengt hs at 1545 ± 12nm. When Q-switched, the laser prod uced 0.3mJ, 46ns FWHM pulses at an output wa velengt h of 1533 ± 1nm and wit h a repetition rate of 7Hz.

Intracavity pumping of Yb,Er:phosph ate glass with a diode-pumped Nd:Y AG laser allows efficient, tunable operation of the th ree-level Er laser in the 1530-1610 nm range. A multi­ longitudinal mode, 1535 nm output of more than 80 mW typically results with a 1 W, 808 nm pum p diode. Tunable, single-freq uency operation (20-80 mW) has been observed between 1533 and 1570 nm using a bi refringent tuner. The laser has also been acousto-optically Q­ switched to give 17 µJ pulses at 700 Hz. Further power scaling of the device will be limited by the relatively low thermal conductivi ty of the phosphate glasses.

A FFT based method is used to calculate the development of electromagnetic (EM) laser field inside a rotating mirror Q-switch laser. The method is applied to study the influence of pumping on the laser pulse temporal evolution and on the near and far field fluence distributions in an "eye-safe" Er: glass laser. Iterative computation of the lowest order modes for the misaligned plano--plano empty resonator shows that for small tilt angles resonator losses depend linearly on the tilt angle. Using linear Q-switching function an analytical expression based on rate equations can be developed which relates the maximum attainable single pulse output energy to the laser parameters. Theoretical results are compared with the characteristics of an "eye-safe" Er: glass laser.

An eyesafe source at 1.61 J.UD with 2.1 % wallplug efficiency, is demonstrated using a Nd:YAG pumped KTP optical parametric oscillator with total peak-power conversion efficiency of 70% and an energy conversion efficiency of 47 % to 1.61 J.UD or 30% to l.54J.UD.

Eyesafe lasers capable of operating at moderate pulse repetitive rates have direct application for optical atmospheric data, wire avoidance, air defense, and a variety of other military and commercial systems. Such applications demand a significant increase in performance over the uncooled single-shot erbium:glass laser widely used in rangefinder systems. Varo Inc. is meeting this need with recently developed repetition-rated Er:glass laser systems. Separate solutions have been designed for the different thermal loading conditions imposed by various pulse rate/duty cycle combinations. For moderate thermal loads (up to 1 Hz for limited duty cycles), an uncooled resonator is usually adequate. For high loads (1 to 20 Hz at extended duty cycles), a flowing cooling system has been developed. For extreme conditions (continuous operation above 20 Hz), research is focused on reducing the heat load by using high-efficiency laser diode pumping.

Recent interest in lasers in the 2 micron wavelength region has lead to crystal growth improvements and flashlamp-pumped operation at room temperatures. Holmium near 2.1 µm, thulium between 1.93 and 2.07 µm, and cobalt magnesium fluoride tunable from 1.75 to 2.5µm are finding uses in medical and dental applications. The improved eyesafety for these wavelengths also makes them candidates as laser rangefinders, LIDAR sources, and for remote sensing. A comparison of performance for these three laser materials with respect to these applications is included.

Design and performance characteristics of InGaAs/InP avalanche photodiodes which have been optimi7.ed for use in eye­ safe rangefinding applications are desaibed. The devices have diameters ranging from 85 to 350 µm. The use of a ball lens mounted on a bade-entry version of the 200 µm diameter APD Oextends its useful diameter up to 500 µm for most applications. For a 350 µm diameter APD, operating at 23°C, and biased to achieve a responsivity of 10 A/W at 1540 nm, typical dark current and noise are, respectively, 150 nA and 1.5 pA/lHz. In addition to room temperature performance characteristics, reduced noise and dark current at lower temperatures are reported for the various device types mounted on single stage thermoelectric coolers. In general, operation of the devices cooled also makes possible the use of higher gains - frequently as high as 2.5 - while still maintaining acceptable levels of noise and dark current. In particular, a typical 85 µm. diameter APD, operating at O"C, has noise less than 0.8 pA/./Hz when biased to operate at a 1540 nm responsivity of 20 A/W.

Abstract not available.

Abstract not available.

Phase conjugation offers a practical, realistic approach for scaling solid-state lasers to high energies and high peak powers with a minimum increase in complexity. The present approach involves coherently combining the outputs of multiple parallel amplifiers in a single phase-conjugate oscillator-amplifier configuration. The use of phase conjugation can eliminate phase distortions that would otherwise result from individual amplifiers having optical lengths that differ from one another by many optical wavelengths. The laser output energy can be scaled well beyond the limits imposed by traditional volume constraints of crystalline media. Hence, instead of selecting a laser medium based solely on the available sizes, a laser system designer can base the medium selection on tradeoffs among many other important material parameters. Since an increase in output energy also requires an increase in the energy incident on the PCM, the energy scalability of PCMs based on SBS has been actively investigated over the past decade. These investigations have shown that, while practical single-cell PCMs offer somewhat limited energy scaling potential, a series combination of two Brillouin cells can accommodate energies of several tens of joules. We summarize our effort in developing such a dual-cell PCM that has achieved excellent performance at energies approaching 5 J and is scalable to even higher energies.

An engineering approach for the development of laser hardware that takes advantage of the special properties of phase conjugation is reported. The principal advantages of conjugate solid-state lasers are alignment stability and passive compensation of thermal distortion in a double pass laser amplifier chain. Attention is given to the application of phase conjugation techniques to the design, fabrication, and field testing of an Nd:YAG second harmonic 0.5 Joule, 30 Hertz flashlamp pumped laser in a master oscillator power amplifier configuration. The double pass amplifier featured a phase conjugate mirror and a second harmonic crystal within the amplifier chain. The basic Nd:YAG laser produced a doubling efficiency of 75 percent from 1064 to 532 nm. This output was subsequently used as the source, and a conversion efficiency of 80 percent into six wavelengths in the visible spectrum was obtained using stimulated rotational Raman scattering.

A description of a frequency doubled, double pulsed Nd:YAG laser that is to be used to pump an injection locked Ti:Sapphire power oscillator is presented. These two lasers make up the transmitter portion of the Lidar Atmospheric Sensing Experiment (LAWSE) instrument. LASE is a Lidar/DIAL experiment that is to measure water vapor in the troposphere. By utilizing the twin concept, both pulses can be produced with a single laser system, thereby minimizing cost, size, and weight. Alignment problems associated with having two separate lasers each produce one of the twin pulses are also alleviated. The LASE transmitter consists of a doubled pulsed Nd:YAG laser that will pump a Ti:Sapphire power oscillator that will be injection-locked by a diode laser. The wavelength of the Ti:Sapphire output will be tunable from 813 to 818 nm. A performance summary of the pump laser is given. The data verify that the pump laser can meet the performance requirements to pump the Ti:Sapphire power oscillator.

2.3 micron laser operation at room temperature, flash lamp pumped Tm;Cr:YAG and Tm;Cr:YALO₃ has been initially reported in 1975 by Caird et al. We could not find any records of continuation of work in this direction, probably because of the low efficiency reported in YAG and lack of high quality YAP crystals. Cascade operation at 2 and 2.3 micron in diode pumped Tm:YLF has been also reported by Kintz et al. In our paper we present theoretical evaluation and experimental results from our work. We extracted more than 150 mJ at 2.32 micron at room temperature in Tm;Cr:YAG-flash lamp pumped at 30 Hz and 15 mJ at 60 Hz.

Lamp pumping of Ti:Sapphire has some advantages over laser pumping and represents some interest due to possible applications. The paper will present laser behavior of Ti:Sapphire under very long lamp pulse pumping. Pulse lamp duration (FWHM) was more than 100 times greater than the lifetime of Ti³⁺. Output energy with no tuning element was achieved greater than 1.5 J with 0.12% electrical-to-optical efficiency. Dimensions of the rod used was 7 mm in diameter and 148 mm in length. The doping level of Ti³⁺ was 0.09% Ti₂O₃ in the rod. Tuning characteristics with different tuning elements are also presented. Further development to obtain CW lamp pumping operation will be discussed.

Many transient phenomenon with self-luminescence need to be recorded by high-speed photography. In accordance with this special requirement, we have developed the dynamic state adjusting Q-sequential pulse laser in the trajectory research. Photographing with this laser-source, the flying vehicle motion sequential film in different positions can be easily obtained. Changing the light-path a little, two- and three-dimensional outdoor large field-of- view pulse-wave photography can be conducted. According to this sequential film, the two- and three-dimensional signals can be processed conveniently and quantitatively. This paper generalizes the outdoor laser YDS system and laser-moire high-speed photography system, with discussion of the operating principle of this system, placing the emphasis on the analysis of two-stage amplifying sequential-pulse ruby laser dynamic state Q-adjusting technology. The laser parameters are as follows: the laser output interval is even, is a range of 8 - 100 microsecond(s) . It also can be continuously adjusted, the total output energy is 9.8 J, and the maximum repeating frequency is 200 KHz. In the end, it introduced the intermittent pulse laser.

Operation of the Q-switching of a Nd:YAG laser at 1.06 micrometers and for the tunable color- center laser over 1.1 - 1.26 micrometers has been obtained by using a LiF:F₂ color center crystal both as the Q-switch for the Nd:YAG laser and the active medium for the color-center laser. The interactive affection of the two lasers has been analyzed and calculated with the rate equation. We found the pulse duration of the YAG laser to be compressed, while the pulse duration of the color-center laser widened and its power improved significantly in this condition.

A rotating pipe geometry laser is one of the schemes for obtaining high average-power output. The theory on thermal effects of the laser is proposed and the corresponding experimental results are given. An output power of 200 W is obtained from this laser at 20 Hz with laser efficiency of 2.2% and slope efficiency of 3.5%. Finally, the parameters of the laser of KW are discussed.

We have developed and tested stackable microchannel cooled laser bar diode pump packages suitable for direct face pumping of slab lasers at high duty factor. A stack of 41 diode packages gives a pump array of 13.5 cm² and produces a peak power of 3750 watts and an average power of 1000 watts, for an average irradiance of 75 watts/cm². A high average power, total internal reflection face pumped Nd:YAG laser using 80 diode packages has been constructed. Average power of 275 watts is obtained at 2.5 kHz pulse repetition rates and 100 microsecond current pulse widths.

Visible laser diodes are used to pump a co-doped Cr,Nd:Gd₃Sc₂Ga₃O₁₂ (Cr,Nd:GSGG) laser rod, demonstrating efficient operation at 1.06 (mu) . Using a 10 cm ROC HR output coupler, the absorbed power required to reach threshold was 938 (mu) W. The round trip losses in the 5 mm long rod were measured to be 4 X 10^-3. The best slope efficiency was 42.1%, obtained with a 97% R output coupler. The photon slope efficiency was 66.8%.

The study reviews the research and development of a prototype laser used to study one possible method of short-pulse production and amplification, in particular, a pulsed Nd:YAG ring laser pumped by laser diode arrays and injected seeded by a 100-ps source. The diode array pumped, regenerative amplifier consists of only five optical elements, two mirrors, one thin film polarizer, one Nd:YAG crystal, and one pockels cell. The pockels cell performed both as a Q-switch and a cavity dumper for amplified pulse ejection through the thin film polarizer. The total optical efficiency was low principally due to the low gain provided by the 2-bar pumped laser head. After comparison with a computer model, a real seed threshold of about 10 exp -15 J was achieved because only about 0.1 percent of the injected energy mode-matched with the ring.

Some of the tradeoffs involved in selecting a laser source for space-based laser ranging are outlined, and some of the recent developments in the laser field most relevant to space-based lasers for ranging and altimetry are surveyed. Laser pulse width and laser design are discussed. It is argued that, while doubled/tripled ND-host lasers are currently the best choice for laser ranging in two colors, they have the shortcoming that the atmospheric transmission at 355 nm is significantly poorer than it is at longer wavelengths which still have sufficient dispersion for two-color laser ranging. The life requirement appears to demand that laser diode pumping be used for space applications.

Tunable continuous-wave and pulsed laser output was obtained from a Tm-sensitized Ho:YLiF4 crystal at subambient temperatures when longitudinally pumped with a diode laser array. A conversion efficiency of 42 percent and slope efficiency of approximately 60 percent relative to the absorbed pumped power have been achieved at a crystal temperature of 275 K. The emission spectrum was etalon tunable over a range of 16/cm centered at 2067 nm with fine tuning capability of the transition frequency with crystal temperature at measured rate of -0.03/cm/K. Output energies of 0.22 mJ per pulse and 22 ns pulse duration were recorded at Q-switch frequencies that correspond to an effective upper laser level lifetime of 6 ms, and a pulse energy extraction efficiency of 64 percent.

We have constructed a laser utilizing five 10 W cw diode bars to transverse-pump a Tm:YAG rod in a multiple-pass pump cavity. At an estimated rod temperature of 35 - 40 degree(s)C, 6.0 W of multi-mode and 3.8 W of single-mode output were obtained at 2.02 micrometers in a non- optimized resonator with 50 W incident pump power.

Results of a study to determine the optimum laser rod diameter for maximum output energy in a solid state neodymium laser transversely pumped with multiple laser diode arrays are reported here. Experiments were performed with 1.0 mm, 1.5 mm and 2.0 mm rod radii of both neodymium doped Y3Al5O12 (Nd:YAG) and La2Be2O5 (Nd:BeL) pumped with laser diode arrays having a maximum combined energy of 10.5 mJ. Equations were derived which predict the optimum rod radius and corresponding output mirror reflectivity for a given laser material and total pump energy. Predictions of the equations agreed well with the experiments for each of the laser materials which possessed significantly different laser properties from one another.

Particulate platinum (Pt) is well-known to cause damage in phosphate-based laser glass. Due to the high solubility of Pt in phosphate-based glasses, melters have been able to produce Pt- particulate free glass by converting the particulate to ionic Pt, which remains in the glass but does not cause damage. Unfortunately, the absorption of the ionic Pt that is primarily in the blue extends to approximately 600 nm in the visible. This absorption competes with the Nd⁺³ ions for pump photons. In relatively heavily Nd-doped (> 2 wt-%) elements such as disks and slabs there are sufficient Nd ions that this absorption does not cause a problem. The case for large rods is different: large clear-aperture rods require light Nd dopings in order to avoid radial gain variations. The ionic Pt can therefore seriously degrade performance both by decreasing gain and increasing heating of the host glass. This paper describes a series of calculations made that estimate the magnitude of this effect. Current production levels of ionic Pt are predicted to reduce the gain of a (phi) 90-mm, 0.55- wt-% doped rod by 10 - 15%. Results are presented for rods of diameters from 40 mm to 90 mm and for Nd dopings less than 1.0 wt-%.

The purchase of large (15- and 20-cm clear aperture) Brewster-angle laser disks involves the specification of a large number of often conflicting parameters, all of which bear on performance and cost. Furthermore, the laser requirements often approach the state-of-the-art in glass-melting technology and the parameter measurement. This paper enumerates the relevant parameters, the trade-offs made in their selection, and the test procedures and instrumentation required to ensure compliance with the specification.

A series of phosphate glass was prepared and the effect of glass composition on the thermal expansion coefficient was investigated. A promising phosphate laser glass which has a very low thermal expansion coefficient of 68 X 10^-7 degree(s)C^-1 has been developed, and the thermal, mechanical, spectral, and laser properties of this laser glass are reported.

The thermal conduction theory based on uniform heating in the slab laser has been presented before, but results from the theory are very different from the results of experiments. In this paper, a non-uniform heating model based on the absorption law of the medium is proposed. According to the model, the temperature distribution in the slab laser under CW and pulse pumping is solved. Thermal stress and fracture limitation of the slab are given. The new model is in good accordance with the experimental results.

The YAG:Nd single crystal boules with free-of-scattering centers, weighing 3000 g and 120 mm in diameter have been grown by the temperature gradient technique. The furnace enlarged on the excellent (phi) 80 mm YAG:Nd crystals to achieve such a large-size crystal.

Titanium-doped Al₂O₃(Tl³⁺:Al₂O₃) single crystals weighing 3 Kg and 120 mm in diameter have been grown by the temperature gradient technique. The crystals used had excellent optical homogeneity, less scattering particles, and low dislocation density.

A comparative evaluation of a stable resonator, self-filtering unstable resonator, and an unstable resonator with a graded reflectivity mirror (GRM) as the output coupler is presented. The GRM resonator produced a slope efficiency of 1.4 percent, best beam quality (1.4 times diffraction limit), and highest conversion efficiency (50 percent). Other accomplishments include determination of the importance of measuring thermal focusing when designing unstable resonators, development of techniques in measuring thermal focusing and birefringence in laser materials, and development of a high brightness laser system for efficient second harmonic conversion.

The University of Rochester's Laboratory for Laser Energetics has designed, built, and characterized a modern, 20-cm clear aperture, Nd:glass Brewster-disk amplifier. This device will be used as the final amplifier in the upgrade of the existing OMEGA laser system. This article describes results of the energy characterization of that amplifier. Specifically, spatially resolved measurements of small-signal gain and whole-beam measurements of large-signal gain are described. The amplifier met its small-signal design goal of 3.0 at only 70% of the design capacitor-bank energy with adequate spatial uniformity. A storage efficiency of 1.7% was achieved. Large-signal-gain measurements, both with and without 5-angstroms-FM bandwidth on the extracting beam, display no measurable change in gain because of the impressed bandwidth.

Il is shown numerically that in Talbot cavity being formed by a thin nonlinear amplifying layer and a pair of a flat imperfect mirrors the small phase distortions put a fundamental limitation on the size of aperture with in-phase lasing. This effect results in spreading of the far field distribution and it is not suppressed by plane wave injection.

Funding for the OMEGA laser Upgrade project has been approved and the design of a 30-kJ, 351-nm, 60-beam laser system has begun. This system will provide a unique capability to validate high-performance, direct-drive laser-fusion targets. We highlight various design features of a system that will attain 1% - 2% irradiation uniformity with versatile pulse shaping, thus providing a wide range of irradiation options for direct-drive experiments. The Nd:glass laser system consists of 60 beamlines using 20-cm disk power-amplifiers, frequency- tripling, phase conversion with diffractive optics, and smoothing by spectral dispersion (SSD). The initial pulse shape will be achieved by co-axial propagation of two beams having different pulse widths. These 'main' and 'foot' pulses will occupy different portions of the laser aperture. In addition, electro-optic technology will be used for shaping and truncation of these pulses.

A pulse-shaping system has been implemented on the OMEGA laser system. The system consists of a cw-mode-locked laser that seeds two regenerative amplifiers. In one regenerative amplifier the pulse width is stretched in time with an intracavity etalon. The other regenerative amplifier is used to trigger a photoconductivity switched Pockels cell, which shapes the stretched pulse.

Although a number of papers exist on the optimization of large-aperture disk amplifiers for use in fusion-laser systems, there is a relative paucity of information on the detailed mechanical design of these devices. In particular, there is little information on their reliability, maintainability, and producibility (RMP). This paper describes two recent designs developed and built by the University of Rochester's Laboratory for Laser Energetics. These 15- 20-cm clear-aperture Nd:glass amplifiers use longitudinal and transverse lamps, respectively, which are water cooled. We describe the design rationale and potential pitfalls. In particular, the disk sizing for a prescribed clear aperture is described. The various laser disk mounting techniques are described and the advantages and disadvantages are compared. Consequences for disk distortion, cladding expansion, cladding survivability, disk stability, and cleanliness are enumerated. The driving requirements for the amplifier frame, which is similar for both devices, are describe. The consequences for cleanliness, stability, and ease of assembly are also described. Finally, the modules containing the flash-lamp arrays are described. These presented the particularly interesting challenge to deliver high voltage and current, cooling water, not degrade pumping efficiency, to be readily serviceable in the event of a lamp failure, and to be cost effective. A working compromise among all of these conflicting requirements was achieved. The module designs, materials used, and the actual performance are described.

Plasmas can sustain many normal modes of oscillation (waves), including both electromagnetic and electrostatic modes. These waves can interact by a wide variety of linear and nonlinear mechanisms, including mode coupling, mixing, and instabilities. Furthermore, such mechanisms compete, so that a given wave might be absorbed, might mode convert, or might decay by one of several instabilities, depending upon the specific circumstances in which it is produced. Moreover, such waves are important in many applications, including for example laser fusion, x-ray lasers, plasma accelerators, and ionospheric heating. Laser-produced plasmas can provide an effective medium for the studies of such waves and the related mechanisms. New opportunities will be made possible by the advent of comparatively inexpensive nanosecond, kilojoule lasers. One can now contemplate affordable experiments, not limited by programmatic constraints, that could study such the basic physics of the waves in such plasmas with unprecedented precision and in unprecedented detail.

The net inverse bremsstrahlung (NIB) acceleration of high-energy electrons traveling in isotropic turbulent plasma waves and a laser light in the ultra-high intensity regime where the e times potential amplitude of the laser wave is much greater than the electron rest energy is inves­ tigated by means of quantum kinetics based on the Dirac equation. The NIB force is independent of the spread of the electron beam energy, and increases with the electron beam energy. However, the NIB force scales as the inverse of the laser intensity.

Two kinds of tunable coherent photons for positronium formation and annihilation can be generated by coalescing focused, high-intensity e and e+ beams which are accelerated in the same direction. Positronium formation can be enhanced by laser irradiation or by installing an undulator magnetic field in cases where the density of e and e+ plasma is not high. In the case of very high density, slender rod-type geometry plasma, positronium can be formed from the e and e+ beams cooled by synchrotron resonance maser process and by super­ radiant, super-fluorescence, or amplified spontaneous emission processes, and a coherent laser can be generated .

The picosecond soh X-ray source based on femtosecond laser-driven plasma and spherical multilayred mirrors is developed. Results on VUV laser plasma spectrum. investigation and second harmonic generation are reported.

We are exploring low Z Ni-like ions for use in a small-scale EUV laser experiment which is under development at MIT. In this paper, we discuss the basic motivation for our approach, the spectroscopy of the laser line positions, and basic physics models of the laser kinetics of the upper and lower laser states.

Laboratory x-ray lasers are currently being studied by researchers at LLNL using the Nova glass laser as a pump source. Laser action has been demonstrated at wavelengths as short as 35.6 angstroms while x-ray amplifier saturation has been observed with longer wavelength schemes. The most successful schemes to data have been collisionally pumped x-ray lasers which use the thermal electron distribution within a laser produced plasma to excite electrons from closed shells in neon- and nickel-like ions to higher energy metastable levels. The multitude of x-ray laser wavelengths produced by isoelectronic scaling of these laser schemes results in a quasi-tunable bright source of x-rays that presently spans the range 35 - 300 angstroms, (40 to 350 eV). Currently attempts are being made to quantify and improve the longitudinal and transverse coherence of collisionally pumped x-ray lasers in order to increase their usefulness for specific applications.

An overview of the X-ray Laser project at Princeton University will be given. Emphasis will be on improvements being made to the small scale soft x-ray laser (SXL). New target designs to enhance cooling and others to reduce losses due to beam refraction have been introduced though results are stilt preliminary. Proof of principle experiments for the application of the SXL to both transmission and reflection microscopy have been performed. To generate shorter wavelength x-ray lasers on a reasonable scale-size, a high power, 300 fsec pulse duration, ultraviolet KrF laser system (the PSP-laser) has been developed. Of the several theoretical schemes which exist, the two-laser approach and a newly proposed recombination approach will be described. The new approach proposes to scale the existing 18.2 nm recombination x-ray laser to shorter wavelength (<4 nm) by making use of the short-pulse pump laser and rapid cooling associated with the adiabatic expansion of µsphere targets.

Amplified spontaneous emissions of Li-and Be-like Al ions and Li-like Si ions were observed in recombining plasmas produced by a low-power driving laser. These were achieved by heating plasma with a train of laser pulses of short interval, and sharply focusing a pumping laser to a line 40 micrometers wide onto a slab target for rapid cooling of plasmas through adiabatic expansion. Mechanism of plasma heating and the optimum shape of the laser pulse for pumping soft X-ray lasers were also investigated.

The physical mechanisms associated with ablation of matter by laser irradiation are quite different in different regions of parameter space. The important parameters are the laser wavelength; the laser flux versus time, position, and angle of incidence at the target; and the target properties as well as the properties of the laser-transport medium adjacent to the irradiated target surface. Important target properties include surface contour, laser reflectivity and absorption depth, thermal diffusively, vaporization energy, Gruneisen coefficient, spall strength, ionization energies and plasma opacity versus temperature and density. As the flux increases, the process becomes less dependent on most of these target properties. Depending on the values of these various parameters, at relatively low fluxes targets can be vaporized and these vapors can be transparent to the laser beam. If a transparent liquid or solid transport medium exists in front of the vaporized target material, then a complicated contained- vaporization process takes place and the work done on the target by the vapors can be several orders of magnitude larger than with a gas or vacuum transport medium; the degree of work enhancement can depend strongly on the vapor condensability and condensed matter thermal conductivity. For short-pulselength irradiations of semi-transparent targets with a low- acoustic-impedance-laser-transport medium adjacent to the target, ablation needs to be a vacuum in order for the beam to be able to propagate to the target. For targets in a vacuum exposed to fluxes of this order (and considerably higher) and for long pulselengths, most of the laser energy will be absorbed (before reaching the critical surface) by inverse bremsstrahlung in material blown off from the target; at higher fluxes, the beam will be stopped at the critical surface producing localized absorption along with much higher energy densities and non-thermal equilibrium behavior. When the combination of pulselength, beam diameter, flux and target material are such that the blowoff becomes opaque to the laser and also the blowoff can traverse many beam diameters during the pulselength, then a complicated radiation-hydrodynamic process is involved with strong feedback between blowoff hydrodynamic expansion, laser absorption, radiation transport, and target ablation by plasma reradiation. In this paper the various ablation processes and potential applications are reviewed from the threshold for ablation up to fluxes of about 10¹³ W/cm², with emphasis on three particular processes; namely, front-surface spallation, two-dimensional blowoff, and contained vaporization.

The supersaturated solid solutions of Cr and Ni were formed in (alpha) -Al during laser treatment. Chromium solid solution was formed as a result of doublepass laser treatment of pure aluminum electroplated with chromium. It is reported that Cr distribution along the depth of the melted zone is not uniformal. The concentration of chromium is maximum at the alloy's surface, reducing to zero as depth increases. This data is related to microhardness tests. Ni solid solution was formed as a result of doublepass laser treatment of Al-5% Ni alloy. The distribution of nickel in solid solution is uniformal, but SEM study of the microstructure revealed that solid solution is only formed close to the melted pool bottom.

Laser treatment of aluminum alloys usually consists of surface remelting and alloying. Treatment including dissolution process is considered to be practically impossible because of the very short beam/alloy interaction time. It is demonstrated herein that eutectic dissolution in Al-Cu alloys is possible using double or multipass laser treatment. The structure of the heat affected zone (HAZ) was found to consist of fine spheroidized (alpha) + (theta) eutectic surrounded by (alpha) -Al solid solution. The copper concentration in (alpha) -Al profile within the HAZ exhibits maximum. The effect of alloy composition and the number of laser passes on HAZ microstructure, copper concentration profile within the zone, and microhardness was established.

The concept and demonstration of a 2.1-micron holmium laser directly pumped with a 2.0-micron laser are described. Experiments and calculations which explore the potential of this low quantum defect laser system are presented. The new technique exhibits higher gains, reduced upconversion losses, reduced temperature sensitivity, and lower thermal loading.

Analytical closed-form expressions for the pumping efficiency, passive loss, and the effective emission cross section are derived for threshold lasing in homogeneously broadened four-level laser media. These expressions are discussed with respect to existing discrepancies in the literature. The analytical results are applied to threshold-lasing measurements in flashlamp-pumped Cr:Nd:GSAG and Cr:Nd:YSGG as compared to Cr:Nd:GSGG.

Single crystals of Nd(3+):SrGdGa3O7 (SGGM) have been grown and characterized as candidate crystals for improved diode pumped lasers. The width of the 809 nm absorption in this disordered oxide crystal is 8 nm FWHM. The fluorescence lifetime of the 4F3/2 - 4I11/2 transition at 1.06 micron is 245 microsec. Dopant levels of 4 percent Nd have been achieved while maintaining high optical quality material.