Efficient frequency doubling and tripling are critical to the successful operation of inertial confinement fusion laser systems such as the National Ignition Facility currently being constructed at the Lawrence Livermore National Laboratory and the Omega laser at the Laboratory for Laser Energetics. High-frequency conversion efficiency is strongly dependent on attainment of the phase-matching condition. In an ideal converter crystal, one can obtain the phase-matching condition throughout by angle tuning or temperature tuning of the crystal as a whole. In real crystals, imperfections in the crystal structure prohibit the attainment of phase matching at all locations in the crystal. We have modeled frequency doubling and tripling with a quantitative measure of this departure from phase matching in real crystals. This measure is obtained from interferometry of KDP and KD*P crystals at two orthogonal light polarizations.

Assuming the observed scaling of laser damage threshold fluence with the (almost) square root of pulse duration is due to thermal conduction, we develop a formalism for directly comparing pulses of different shapes and durations. We find, for example, that a top hat pulse leads to 15% higher temperature (presumably 15% lower damage threshold) than a Gaussian pulse of the same fluence. We also find that the damage threshold of the expected NIF type pulse should be estimated from a Gaussian pulse with the same peak intensity. We find that the deviation of the scaling of damage threshold from square root of pulse duration has contributions from both the small but finite size of laser energy absorbers and from the temperature dependence of thermal properties.

Experiments were performed on the 100-J class Optical Sciences Laser (OSL) at LLNL to characterize the saturation fluence and small-signal gain of a solid-stale Nd:glass amplifier utilizing LG-750 and LG-770, an amplifier glass developed for the National Ignition Facility (NIF). These high quality measurements of gain saturation at NIF level fluences, ie., 10-15 J/cm(2), provide essential parameters for the amplifier performance codes used to design NIF and future high power laser systems. The small-signal gain, saturation fluence and square-pulse distortion were measured as a function of input Fluence and pulse length in platinum-free LG-750 and LG-770. The input fluence, output fluence, small-signal gain and passive losses were measured to allow calculation of the saturation fluence. Least squares fits of the output vs. input fluence data using a Frantz-Nodvik[1] model was used to obtain an average saturation fluence for each data set. Overall, gain saturation in LG-750 and LG-770 is corn,Parable at long pulse lengths. For shorter pulse lengths, <5 ns, LG-770 exhibits a stronger pulse length dependence than LG-750, possibly due to a longer terminal level Lifetime. LG-770 also has a higher cross-section, which is reflected by its slightly higher extraction efficiency.

A standard third harmonic frequency converter consists of a single doubler crystal and a subsequent mixer crystal. This converter configuration has optimum performance for monochromatic temporally flat pulses. Significant degradation of conversion efficiency occurs when the input pulses have large intensity variations or applied bandwidth. Calculations have shown that dynamic range can be increased and bandwidth sensitivity reduced by adding a second doubler crystal and second mixer crystal, respectively.

The characteristics of the 3 omega focal spot are determined by the properties of the 1 omega beam and the frequency tripling process. The size of the 3 omega focal spot depends on the spectrum of spatial noise in the 1 omega beam. A perturbation theory for ripple transfer in frequency tripling is used to predict the characteristics of the 3 omega focalspot. The theory predicts that 1 omega phase noise grows 9X in power in the tripling process. This can cause a significant reduction in 3 omega energy delivered to the target. Results from Beamlet 3 omega focal spot characterization experiments are also presented.

In this paper we present experimental measurements and theoretical modeling of third harmonic (3(omega) ) conversion efficiency with optical bandwidth. Third harmonic conversion efficiency drops precipitously as the input bandwidth significantly exceeds the phase matching limitations of the conversion crystals. For Type I/Type II frequency tripling, conversion efficiency begins to decrease for bandwidths greater than approximately 60 GHz. However, conversion efficiency corresponding to monochromatic phase-matched beams can be recovered provided that the instantaneous propagation vectors are phase matched at all times. This is achieved by imposing angular spectral dispersion (ASD) on the input beam via a diffraction grating, with a dispersion such that the phase mismatch for each frequency is zero. Experiments were performed on the Optical Sciences Laser (OSL), a 1 - 100 J class laser at LLNL. These experiments used a 200 GHz bandwidth source produced by a multipassed electro-optic phase modulator. The spectrum produced was composed of discrete frequency components spaced at 3 GHz intervals. Angular dispersion was incorporated by the addition of a 1200 gr/mm diffraction grating oriented at the Littrow angle, and capable of rotation about the beam direction. Experiments were performed with a pulse length of 1-ns and a 1(omega) input intensity of approximately 4 GW/cm2 for near optimal dispersion for phase matching, 5.2 (mu) rad/GHz, with 0.1, 60, and 155 GHz bandwidth, as well as for partial dispersion compensation, 1.66 (mu) rad/GHz, with 155 GHz and 0.1 GHz bandwidth. The direction of dispersion was varied incrementally 360 degrees about the beam diameter. The addition of the grating to the beamline reduced the narrowband conversion efficiency by approximately 10%. Sufficient dispersion to allow nearly full phase-matching of all frequency components along the sensitive axis of the tripler allowed recovery of the narrow band conversion efficiency with bandwidth. However, even partial dispersion compensation was shown to significantly increase broadband 3(omega) conversion efficiency.

A novel four-color beam smoothing scheme with a capability similar to that planned for the proposed National Ignition Facility has been deployed on the Nova laser, and has been successfully used for laser fusion experiments. Wavefront aberrations in high power laser systems produce nonuniformities in the energy distribution of the focal spot that can significantly degrade the coupling of energy into a fusion target, driving various plasma instabilities. The introduction of temporal and spatial incoherence over the face of the beam using techniques such as smoothing by spectral dispersion (SSD) can reduce these variations in the focal irradiance when averaged over a finite time interval. One of the limitations of beam smoothing techniques used to date with solid state laser systems has been the inability to efficiently frequency convert broadband pulses to the third harmonic (351 nm). To obtain high conversion efficiency, we developed a multiple frequency source that is spatially separated into four quadrants, each containing a different central frequency. Each quadrant is independently converted to the third harmonic in a four-segment Type I/Type II KDP crystal array with independent phase-matching for efficient frequency conversion. Up to 2.3 kJ of third harmonic light is generated in a 1 ns pulse, corresponding to up to 65% intrinsic conversion efficiency. SSD is implemented by adding limited frequency modulated bandwidth to each frequency component. This improves smoothing without significant impact on the frequency conversion process. The measured far field irradiance shows 25% rms intensity variation with four colors alone, and is calculated to reach this level within 3 ps. Smoothing by spectral dispersion is implemented during the spatial separation of the FM modulated beams to provide additional smoothing, reaching a 16% rms intensity variation level. Following activation the four-color system was successfully used to probe NIF-like plasmas, producing less than 1% SBS backscatter at greater than 2 multiplied by 1015 W/cm2. This paper discusses the detailed implementation and performance of the segmented four-color system on the Nova laser system.

The requirements for laser uniformity are discussed in terms of the l-mode spectrum. It is shown that the choice of smoothing methods can significantly alter this spectrum and that this choice should be made in the context of the target physics. Although two dimensional smoothing by spectral dispersion yields a high quality near field beam profile, it results in poor smoothing for low spatial frequency. The partially coherent light method (fiber smoothing) leads to superior smoothing at low spatial frequencies, but has very poor near field beam quality. As a result, it may be desirable to use partially coherent light during the drive pulse foot (at low intensity and when minimizing the laser imprint is critical) and smoothing by spectral dispersion during the main pulse.

Conditions which seed the self focussing of high-power broadband laser beams are determined by examining growth rates for plane-wave perturbations on a strong pump field as a function of frequency and angle. Measurements verifying predictions of growth based on the linearized stability analysis of Bespalov and Talanov extended to broadband fields are reported.

The National Ignition Facility (NIF) is a proposed 1.8 MJ laser facility for carrying out experiments in inertial confinement fusion, currently designed for indirect drive experiments. The direct drive approach is being pursued at the 30 kJ Omega facility at the University of Rochester. In this paper we discuss the modifications to the NIF laser that would be required for both indirect and direct drive experiments. A primary concern is the additional cost of adding direct drive capability to the facility.

We report on the design and fabrication of continuous contour (kinoform) phase plates for homogenizing the focal plane irradiance of high-power, inertial confinement fusion laser systems. These kinoform phase plates are designed using an iterative algorithm. They offer the flexibility of controlling the overall shape of the far-field irradiance profile and the ability to concentrate the energy within a central region of the focal profile. These properties make kinoforms superior to the conventional, binary random phase plates for many applications. Potential methods for fabrication of such kinoform phase plates are discussed.

Large bandwidth in lasers for inertial confinement fusion is potentially capable of controlling laser-plasma instabilities. We have demonstrated the efficient generation of large bandwidth, DELTAnu/nu=2%, in a frequency-doubled Nd:glass laser at 527 nm, using stimulated rotational Raman scattering in atmospheric pressure nitrogen gas. Its use in a fusion laser would involve the use of a multilens array to produce multiple foci where the broadband conversion takes place. This technique is also effective for frequency tripled or quadrupled lasers at 351 or 263 nm.

We report the elastic constants and thermoelastic coefficients of silver gallium selenide and deuterated l-arginine phosphate. We also report their fracture strength as measured by indentation tests. These data are used to calculate their thermal fracture resistance, a parameter which is important in high-average-power laser systems. We compare their thermal fracture resistance to other nonlinear crystals.

We describe in detail the design and performance of a multikilowatt Pockels cell for use in high average power laser systems. The Pockels cell is a gas-cooled, transversely excited electro-optic switch based on KD*P as the electro-optic material. Matched pairs of crystals with different crystallographic orientations are used to make the switch performance insensitive to the operating point and ambient temperature. Excellent switching performance and low-wave-front distortion have been achieved at average power levels in excess of 1 kW.

Two figures of merit, the threshold power (Pth) and the limiting volume (Vmin) can be used to compare the relative efficiency and economy of new harmonic generating crystals. The properties of barium metaborate and L-Arginine phosphate are used to illustrate the effect of nonlinearity, birefringence, and damage threshold on these figures of merit.

We report measurements of all the material constants necessary to fully characterize barium borate as a nonlinear optical material. All data was taken on crystals supplied by Professor Chuangtien Chen, Fuzhou, People’s Republic of China. We have determined the crystal structure, the optical absorption, the refractive indices from the UV to the near IR, the thermo-optic coefficients, the nonlinear optical or coefficients, the resistance to laser damage, the elastic constants, the thermal expansion, thermal conductivity and dielectric constants, and the fracture toughness. This data is used to evaluate barium borate for a variety of applications. We find that, in general, barium borate has a low acceptance angle, and that despite its higher optical nonlinearity, it is therefore not significantly more efficient than other commonly available materials, except in the UV below 250 nm. On the other hand, it has a high damage threshold, it is physically robust, it has good UV and IR transparency, and it has excellent average power capability. It permits deep UV generation, and has great potential for generating tunable visible and IR light as an optical parametric amplifier.

It is well-known that under uniform heating, a thin plate develops a quadratic temperature profile with the thermal gradient normal to the plate surface (Figure 1). The central part of the plate is warmer than the surfaces, and also undergoes thermal expansion. The thermal expansion normal to the surface produces no stress, but the thermal expansion parallel to the plate surface puts the surface in tension and the central part in compression. A simple but useful model of the plate treats it as a set of three isothermal plates arranged in the order cold-warm-cold (Figure 2). The warm part is AT warmer than the cold surfaces, and left to itself it would expand by aAT. Being constrained by the surface, it is in compression. If the warm part had infinite modulus, the resulting strain in the surface would be aAT. Because it relaxes, the actual surface strain is reduced by 1/3. By Hooke's law the stress is proportional to the strain, so that the surface stress is simply 2/3.EaAT, where E is Young's modulus.

The optimization of nonlinear optical devices such as frequency doublers is discussed. A nonlinear material will convert efficiently if the laser brightness (P/Q2, where P is the peak power, and Q is the number of times diffraction-limited the pulse is) exceeds a material parameter called the threshold power. The conversion efficiency is shown to be independent of the beam aperture. Thus surmounting optical damage requires only that the beam aperture be large enough. Data on the threshold powers of common nonlinear materials is presented.