Self-focusing effects in large, high power laser amplifiers become manifest as small-scale beam instabilities and as large-scale phase aberrations. Spatial filtering has been shown to control instabilities; spatial filters constitute appropriate lens pair elements for image relaying as well. In this paper, image relaying is presented as a technique for preserving the transverse intensity profile of a high power beam as it propagates long distances through nonlinear elements. As a consequence, amplifier apertures can be filled more effectively, leading to a doubling of fixed-aperture system performance. A rationale for optimal selection of spatial filter bandpass is also presented. This selection, as might be expected, depends upon details of the beam's spatial structure as it enters any filter. A geometrical optics approach is used throughout; nevertheless, derived results remain valid when diffraction is included.

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The exchange of energy between nuclear spin system and lattice has been theoretically and experimentally studied for circumstances in which the nuclear Zeeman energy levels are not necessarily equally spaced. Starting from the master rate equations for the nuclear energy level populations, expressions are found for the population difference of an adjacent pair of energy levels as a function of time. For nuclear spin /, this population difference in general returns to thermal equilibrium with the lattice as a sum of (21) exponential terms. Under certain conditions, exact solutions of the rate equations may be obtained. As an example, detailed exact solutions are found for an artificial physical situation, in which the nuclear spins (/-5/2) are presumed to interact, independently of each other, with a rapidly fluctuating paramagnetic ion (the lattice). From the solutions to this model system, some conclusions are drawn which are consistent with more sophisticated statistical arguments. First, in the limiting case of equally spaced energy levels, these solutions reduce to a single exponential term; a unique spin-lattice relaxation time T\ may then be defined. Second, it is found that even for unequally spaced levels, any pair of level populations recovers to thermal equilibrium asymptotically as an exponential with this same time constant T\.The methods illustrated in the foregoing example are extended to include the effects of nuclear dipole-dipole interactions. Approximate solutions to the rate equations are found, for 7=5/2, in terms of a slight extension of previous descriptions of nuclear spin-lattice relaxation in dilute paramagnetic solids formulated by Bloembergen, de Gennes, and Khutsishvili. These solutions are applied to the particular example of the Al spins in A1 2 0 3 : 0.035% Cr 3 +, in order to predict the results of experimental measurements of transient nuclear magnetization made during this research. For the limiting case of equally spaced energy levels, our solution predicts that the Al spins should relax exponentially, with estimated time constant 7\«0.6 sec at 80°K, for an external field of 9 kG. Experimentally, we observe the Al spin relaxation proceed asymptotically as an exponential with (ri)asym P «0.78 sec at 80°K. The slight discrepancy is accounted for by introducing, in a qualitative manner, the effect of second-order quadrupole splitting of the nuclear Zeeman levels. Further measurements of the transient magnetization associated with an adjacent pair of nuclear energy levels are performed when the energy levels are far from equal spacing; the results of all measurements convincingly demonstrate the validity of the normal modes description of nuclear spin-lattice relaxation employed here. All experimental observations agree quantitatively with the estimated spin temperature time constant Ti -0.6 sec. A slight anisotropy in T\ as a function of crystal orientation in the field Ho is reported. It is believed that this anisotropy reflects anisotropy of the spin diffusion process in the nonc...

The evolution of solid-state laser systems over the past decade, both through technological advances and through increased understanding of the interplay between nonlinw effects and linear diffraction, is reviewed. The role of numerical methods to simulate the several physical processes (diffraction, self-focusing, gain saturation) involved in coherent beam propagation through large laser systems is discussed. A comprehensive simulation code for modeling all of the pertinent physical phenomena observed in laser operations (growth of small-scale modulation, spatial filtering, imaging, gain saturation, and beam-induced damage) is described in detail. The realism and accuracy of results obtained with this numerical code stem from an unambiguous identification of the sources of spatial noise, and from the use of spatial filters in modern lasers to limit the transverse beam modulation scale within the practical computational range of a two-dimensional numerical analysis. Several comparisons between code results and solid-state laser output performance data are presented. Finally, the design and performance estimation of the large Nova laser system presently under construction at the Lawrence Livermore National Laboratory (LLNL) are given. M

The Argus Nd:glass laser system, presently operating as an experimental facility for laser fusion experiments, is described. The laser consists of a master oscillator and two identical amplifier chains, each of 20-cm output aperture. Argus is presently capable of delivering more than 4 TW of power in short (<100-psec) pulses, or more than 2 kJ of energy in 1-nsec pulses, to 100-microm targets. Short pulse performance enhancement obtained by increased aperture filling and implementation of image relaying with high power vacuum spatial filters is described. Experimentally recorded near-field and far-field data for several power levels are presented and discussed in terms of the limiting effects of nonlinear beam instabilities upon focal spot intensity.