Unique materials properties requirements for solid state high average power (HAP) lasers dictate a materials development research program. A review of the desirable laser, optical and thermo-mechanical properties for HAP lasers precedes an assessment of the development status for crystalline and glass hosts optimized for HAP lasers.

A variety of different axis cuts of Nd:BEL were investigated in both rod and straight through slab form. Using two cw diode pumped Nd:BEL lasers operating at 1.070 and 1.079 μm, the small signal gain was measured. This measurement was done for X, Y, Z, and athermal rods for both polarizations. The same procedure was done for Nd:YAG and Nd:Cr:GSGG for comparison tests. Using spectroscopic data for all materials, the stored energy was calculated as well as the pumping efficiency. Efficient, high quality beam, operation of the laser also requires prior knowledge of the thermal distortions induced by the flashlamp pumping. This information was obtained by operating the pump chamber in one arm of a Mach-Zehnder interferometer at flashlamp powers up to 600 watts at 20 Hz. This procedure can show a large degree of non-isotropic lensing and wedging. In order to help deconvolve the pump induced wedge terms, the rods were also rotated 90°. These measurements were made for both polarizations showing strong polarization dependent thermal distortion for Nd:BEL. Moire' deflectometry was also used to characterize the lensing. These results will be compared to the pumped interferometry. Knowledge of the thermal distortion profiles can lead to the design of straight through slabs with low thermal lensing terms. Long Pulse and Q-switched laser tests were conducted for the different axis rods and compared to Nd:YAG and Nd:Cr:GSGG in the identical optical arrangement. These results confirm the higher storage of Nd:BEL compared to Nd:YAG.

A slab geometry Nd:YAG laser with a zigzag optical path is described. The dimensions of the Nd:YAG slab are 5.6 x 18.4 x 153.9 mm, and Nei' ion concentration is 1.1 at.%. Two krypton flashlamps, one located on each side of the YAG slab, are used for pumping. The conditions for normal pulsed operation were as follows: the repetition rate was from 5 to 27 pps, and the pulse durations were 4 and 9.9 ms. With the above conditions, a maximum average output power of 500 W was obtained with an efficiency of 2 %, the slope efficiency being 2.4 %. The beam divergence was estimated to be 10x25 mrad. The stability of the laser output power was about ±1.5 %. Another oscillator that includes intra-cavity cylindrical lenses, was also designed. Using this resonator configuration reduced the beam divergence to about 7.6 x8.2 mrad. The preliminary laser processing experiment was attemped using this laser oscillator.

The potential of the gas cooled disk architecture to operate at kilowatt to megawatt levels of average power is explored. The key issues investigated are flow and heat transfer in the cooling channel, the power required to perform the cooling and its implications for overall system efficiency, flow conditioning in the drift region, losses due to turbulent scattering, and beam quality due to optical distortions. In all cases an understanding of the issues leads to ways to mitigate or eliminate deleterious effects. Our conclusion is that the gas cooled disk geometry is an architecture which can lead to devices which are capable of average power levels reaching from a few kilowatts up to hundreds of kilowatts per beam line without stressing existing engineering or technology.

Based on simple considerations for the mechanisms limiting average power operation of solid state lasers, we deduce some scaling laws for the gas cooled disk amplifier architecture. We discuss geometrical issues, gain and efficiency considerations, and output power scaling. The paper concludes with an outline for a high repetition rate GSGG system and a very large average power module suitable for a fusion driver.

A novel class of rare earth doped solid state lasers is described. The Ground State Depleted (GSD) laser is pumped by an intense (>tens kW/cm²) narrowband (<few nm) laser source and is characterized by: (1) an unusually low laser ion doping density (5-10 x 1018 ions/cc), (2) an unusually large fractional excited population inversion density (4-8 x 10¹⁸ ions/cc or >75%), (3) a gain element that is optically thick at the pump wavelength, (4) a gain element that has a substantially uniform gain distribution due to a bleaching of the pump transition at the pump intensity utilized. These features enable efficient room temperature operation of rare earth ion laser transitions terminating on the ground manifold. The relationships between laser parameters (cross sections, saturation fluences and fluxes, bleaching wave velocities, etc.) are given and laser performance scaling relationships are presented and discussed.

Ground state depleted (GSD) lasers have been described in an earlier paper as room temperature, four-level, high energy density lasers with uniform gain when pumped either transversely or longitudinally. Spectroscopic measurements have been performed at Livermore in various Nd doped glass and crystalline materials to extract parameters (stark resolved emission spectra, branching ratios, fluorescence lifetimes, and stimulated emission cross sections) important to the design of a GSD laser. This study has allowed us to identify several systems amenable to experimental demonstration. The results of a demonstration in one of these systems consisting of a sample of Nd doped Y₂Si0₅ which is pumped by a flash lamp pumped dye laser will be presented. Measurements are made to document the degree to which the ground state of the laser has been depleted. An important problem present in GSD lasers, illustrated by the results of our demonstration, is holding off the gain of the Nd ⁴F_3/2 - ⁴I_11/2 transition which typically has an emission cross section 5 to 10 times larger than the desired ⁴F_3/2 - ⁴I_11/2 transition cross section. Several possible techniques for holding off the ⁴F_3/2 - ⁴I _11/2 transition are discussed. These techniques include segmented laser designs, and the co-doping of laser samples with elements having a large absorption cross section in the 1.06 micron region while being relatively free of absorption in the 0.92 micron region of the spectrum. A system based on Y₂SiO₅ and co-doped with Nd and Sm will be presented as an example of the latter type GSD design along with available experimental results.

Highly efficient diode pumped solid state lasers at room temperature operating near the peak of the water absorption near 3 μm have many potential medical applications.' An excellent candidate for this type of laser is the 2.8 μm ⁴I_11/2 → ⁴I_13/2 laser Er:LiYF₄ (YLF). Laser diode pumping of an 8% Er:YLF sample producing continuous-wave laser emission at 2.8 μm was previously reported with a 0.7% slope efficiency and an absorbed power threshold of 60 mW.² In our current work we demonstrate a greatly reduced threshold for CW laser emission, 20 mW, and more than an order of magnitude increase in slope efficiency, 10%, in a 30% Er:YLF sample.

We report the polarization control of optically-pumped Nd:YAG via external fields, temperature and stress-induced anisotropy of lasing gains. The degree of polarization at various pumping wavelenths (0.58-0.81 microns) is measured.

A Q-switched diode pumped Nd:YAG zig-zag slab laser has been designed, packaged, and qualified for use as a laser source in a space borne laser radar (LADAR) experiment. This paper discusses the design of the diode pumped slab head, resonator cavity, rugged optics, electronics and thermal system. Also discussed are functional and qualification test results. Testing included operation over wide temperature ranges and while vented in a vacuum, as well as endurance through severe launch vibrations. High reliability, comparable cost, and reduced size, weight, and power consumption are advantages of diode pumped lasers. In addition, low operating voltages reduce EMI. This laser demonstrated the viability of diode pumped slab lasers for tactical and space applications

An intracavity-doubled, Q-switched, Nd:YAG laser side pumped by a pulsed, 3-bar, laser-diode array, produced a second harmonic output of 1.75 mJ/pulse at an optical conversion efficiency of 11.5%. We demonstrated the use of this laser to pump a pulsed dye laser via an optical fiber.

Diode pumped solid state lasers have been attracting significant interest in recent years due to advances in high power semiconductor diode lasers. They offer considerable advantages over flashlamp pumped lasers such as compact size, high efficiency, lower heat dissipation and solid-state reliability. In this paper, we report on the results of a Nd:YAG laser, transverse pumped by diode laser arrays. We have measured an output power of 1.14 Watts at 1.06 microns with a laser diode power consumption of 40 Watts. This represents the highest reported electrical efficiency (2.85%) for a transverse pumped, CW, TEM00 laser. The diode arrays were selected and tuned to emit at wavelengths close to the peak neodymium absorption line at 0.808 microns with Peltier coolers. Two diode laser bars side pumped a 20 mm long, 1.5 mm diameter Nd:YAG laser rod. The optical cavity is 13.8 cm long consisting of a high reflectivity mirror and a 95% reflectivity output mirror. The output beam divergence was measured to be near diffraction limited at 1.4 milliradians, and the beam diameter was 1 mm.

In this paper, we report the first systematical analyses of the room temperature noncritical phase match-ing (NPM) curves for biaxial crystals of KTP and KNb0₃. The advantages and potential applications using these NPM conditions are explored, with an emphasis on tunable lasers in diode-pumped systems.

High peak power rare gas halide lasers for applications in Inertial Confinement Fusion are being developed at laboratories across the world. The United States Department of Energy sponsors a program conducted at the Los Alamos National Laboratory and the Naval Research Laboratory. The Los Alamos laser development program is composed of three major elements; the Aurora Laser Facility is a 1 terawatt KrF laser designed as an integrated performance demonstration of a target qualified excimer laser system; an advanced design effort evaluates concepts that offer the improved performance and lower cost that will be essential for the construction of future lasers in the 0.5 to 10 MJ class; and a laser technology program that addresses both performance and cost issues that will be important in advanced laser system designs.

A new, visible chemical laser system analogous toexcimer lasers has been investigated. The present system utilizes chemically produced 0₄ complex showing a relatively broad spectrum centered at 703 nm. Laser gain as high as 2.8% has been observed in the visible spectral range. Laser oscillation has also been observed at several lines. We believe that this is the first demonstration of a chemical laser oscillation in the visible range. Experimental results are presented and discussed.

A laser system that produces terawatt-level pulses of 308-nm light with near-diffraction-limited beam quality is under development. Pulses of 175-fs duration are generated at 616 nm in a synchronously-pumped mode-locked dye oscillator. These pulses are amplified in a three-stage dye amplifier longitudinally pumped by the frequency-doubled output of a regenerative Nd:YAG amplifier. Output of the dye amplifier is frequency-doubled in a BBO crystal and amplified in a chain of XeC1 excimer stages. The optical design maintains high beam quality throughout the system.

Free electron laser (FEL) amplifiers driven by linear induction accelerators have considerable potential for scaling to high average powers. The high electron beam current produces large single pass gain and extraction efficiency, resulting in high peak power. The pulse repetition frequency scaling is limited primarily by accelerator and pulsed power technology. Two FEL experiments have been performed by the Beam Research Program at the Lawrence Livermore National Laboratory (LLNL): The ELF experiment used the 3.5-MeV beam from the Experimental Test Accelerator (ETA) and operated at a wavelength of 8.6 mm. This device achieved an overall single-pass gain of 45 dB, an output power of 1.5 GW, and an extraction efficiency of 35%. The microwave beam was confined in a waveguide in the 4-m-long wiggler. The PALADIN experiment uses the 45-MeV beam from the Advanced Test Accelerator and operates at a wavelength of 10.6 IA. Using a 15-m long wiggler a single pass gain of 27 dB was produced. Gain guiding was observed to confine the amplified beam within a beam tube that had a Fresnel number less than 1. The results of these expriments have been successfully modeled using a three dimensional particle simulation code. The Program also has ongoing efforts to develop wiggler, pulsed power and induction linac technology. A focus of much of this work is the ETA-II accelerator, which incorporates magnetic pulse compression drivers. One application of ETA-II will be to drive a 1 mm wavelength FEL. The microwave output will be used for a plasma heating experiment.

New short-pulse laser technology has made possible the production of extremely bright (>10¹⁸ W/cm²-sr) laser sources. The use of these new techniques on large scale Nd:glass based laser systems would make it possible to produce 1000 TW (Petawatt) pulses. Such pulses would yield focused intensities exceeding 10²¹ W/cm₂ corresponding to an electric field in excess of 100 e/a₀² and an energy density equivalent to that of a 10 keV blackbody. The first step towards the realization of such a source is the development of a compact 10 TW laser. The design of a 10 TW table-top laser and some potential applications are described.

Over the past year, the Nova laser at the Lawrence Livermore National Laboratory has been undergoing major refurbishment. Concurrently, an extensive research program has been undertaken to characterize and understand, in detail, the characteristics of Nova and the factors which limit its performance. As a result of these combined efforts, Nova now exceeds its original performance goals. Among the topics discussed are the substitution of platinum free laser glass in the power amplifier section of the Nova beamlines; increased frequency conversion efficiency, and large optic damage resulting from transverse stimulated Brillouin scattering. When we have implemented the necessary changes on all 10 Nova beamlines, we will soon be able to routinely deliver 120 kilojoules of energy at 1.053 μm, and 75 kilojoules at 0.351 μm.

HIgh Repetition and high EXcitation Solid state laser (HIREXS) of Zigzag optical pass slab geometry has been designed and developed for an application to the soft X-ray generation such as X-ray lithography. Design and performances of a zigzag optical path laser with a 230x65x5mm Nd:glass slab are described. The single pulse output energy of 98.9J was obtained at slope efficiency of 2.9% at 1.05μm wavelength. The laser beam divergences measured in the directions of width and thickness of the laser glass were about lmrad and 2mrad, respectively, without the transverse mode control. The maximum laser stored energy of 26J with the stored efficiency of 0.65% at 4kJ pumping was obtained. Q-switch operation of this laser resulted in 20J output at a pulse with of 28ns which results in the laser extraction efficency of 77%. The focused intensity was achieved 7x10¹²W/cm². A water-cooled laser slab of silicate glass LSG-91H of the same dimensions generates an output power of 45W at a repetition rate of 3Hz.

The University of Rochester's Laboratory for Laser Energetics is designing an upgrade to its 24-beam OMEGA laser system. OMEGA is a frequency tripled, all-rod system capable of producing 2 kJ at 0.8 ns on target. Important direct-drive-target-ignition physics could be investigated with an upgraded system capable of producing a shaped pulse consisting of a long (5 ns), low-intensity, "foot," smoothly transitioning into a short (0.5 ns), intense, compression pulse. The total pulse energy is 30 Id, which, from target-irradiation uniformity considerations, must be distributed over 60 beams.

A large signal theory is presented for frequency-mixing in diode-pumped Q-switched systems. Pulse lengthening, mode-mode coupling and the validity of small-signal theory are studied by numerical solutions of a set of coupled equations. Figures of merit of some nonlinear crystals are compared.