This paper describes theoretical and experimental investigations of induced spatial incoherence (ISI), a technique for achieving the smooth and controllable target beam profiles required for direct‐drive laser fusion In conventional ISI, a broadband laser beam (coherence time tc=1/Δν≪tpulse) is sliced into an array of mutually incoherent beamlets by echelon structures that impose successive time delay increments Δt>tc A focusing lens then overlaps those beamlets onto the target, which is usually located at the far field Here, we evaluate the ideal target beam profiles for practical ISI focusing configurations, and examine the perturbing effects of transient interference, laser aberration, and plasma filamentation Analytic and numerical calculations show that nonuniformities due to interference among the beamlets are smoothed by both thermal diffusion and temporal averaging Under laser‐plasma conditions of interest to inertial confinement fusion (ICF), average ablation pressure nonuniformities ∼1% sho

Theory of induced spatial incoherence

We have demonstrated that single point diamond turning can be used to generate high optical quality finished surfaces on a new organic nonlinear crystal, L-arginine phosphate (LAP). The proper choice of cutting conditions can produce surfaces with <5-A rms local roughness. Local softening or melting near the cutting tool tip may play a key role in the machining process by ensuring that material is removed by ductile cutting rather than brittle fracture. At the same time, the low melting temperature of LAP makes lubrication and cooling especially important to prevent extensive melting and tool fouling. In spite of the presence of a weak cleavage plane in LAP, the surface quality is relatively insensitive to crystallographic orientation. Tool wear is apparently negligible, so that surface flatness is governed by the stability of the diamond turning machine. These results suggest that it may be possible to fabricate large aperture LAP harmonic converters for use in inertial confinement fusion lasers.

Diamond turning of L-arginine phosphate, a new organic nonlinear crystal.

To provide high-energy, high-power beams at short wavelengths for inertial-confinement fusion experiments, we routinely convert the 1.05-microm output of the Nova, Nd:phosphate-glass, laser system to its second- or third-harmonic wavelength. We describe the design and performance of the 3 x 3 arrays of potassium dihydrogen phosphate crystal plates used for type-II-type-II phase-matched harmonic conversion of the Nova 0.74-m diameter beams. We also describe an alternate type-I-type-II phasematching configuration that improves third-harmonic conversion efficiency. These arrays provide conversion of a Nova beam of up to 75% to the second harmonic and of up to 70% to the third harmonic.

Harmonic conversion of large-aperture 1.05-microm laser beams for inertial-confinement fusion research.

A laser medium is particularly useful in high average power solid state lasers The laser medium includes a chormium dopant and preferably neodymium ions as codopant, and is primarily a gadolinium scandium gallium garnet, or an analog thereof Divalent cations inhibit spiral morphology as large boules from which the laser medium is derived are grown, and a source of ions convertible between a trivalent state and a tetravalent state at a low ionization energy are in the laser medium to reduce an absorption coefficient at about one micron wavelength otherwise caused by the divalent cations These divalent cations and convertible ions are dispersed in the laser medium Preferred convertible ions are provided from titanium or cerium sources

Synthetic laser medium

Supergaussian rings: focusing properties

Experiments and analysis indicate that graded random phase modulation can be used to apodize a laser beam. In the case of an obscuration or a hard edge it can prevent the formation of Fresnel-diffraction ripples. For example, here the interaction of a 1-μm-wavelength laser beam with a central obscuration of half-width a = 100 μm is studied theoretically. It is found that if the exit surface of a window, placed immediately downstream of the obstacle, is randomly modulated with a Gaussian amplitude transverse correlation length l = 50 μm and a mean-square amplitude that decreases exponentially from a peak height of ~1 μm2 away from the center of the obscuration with transverse scale length L = 500 μm, then the Fresnel-diffraction ripples normally produced by the obscuration are eliminated. The scaling of these results is also discussed. The calculations are in general agreement with experimental results.

Laser-beam apodization with a graded random phase window.

The technology of frequency converting intense laser light was demonstrated during the early 1960s1. In the initial experiments, the deep red wavelength of a ruby laser produced ultraviolet light, a shorter wavelength harmonic of the fundamental laser radiation, as it was passed through a quartz crystal. Shortly afterwards the Nd:YAG infrared laser was used to produce visible or ultraviolet light as it passed through a crystal of potassium dihydrogen phosphate (KDP).2.3 Today frequency conversion is a well characterized and widely used method of producing intense, coherent light at wavelengths unavailable from the source laser medium.

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Harmonic Generation At High Peak Power

https://www.osapublishing.org/viewmedia.cfm?uri=CLEO-1986-WQ1&seq=0

Progress in high-average-power solid-state laser development at Lawrence Livermore National Laboratory

Laser-beam apodization with a graded random phase window: erratum.

The Nova laser is equipped with ten large-aperture (74-cm) potassium dihydrogen phosphate (KDP) crystal arrays capable of converting the fundamental wavelength (1.054 μm) to the visible (0.527-μm) or ultraviolet (0.351-μm) light. Each array assembly consists of a 3 × 3 deep-web aluminum eggcrate, which supports two crystal arrays configured in the quadrature 2ω/cascade 3ω architecture.1

M. A. Summers

Papers

Laser-beam apodization with a graded random phase window.

Harmonic Generation At High Peak Power

Progress in high-average-power solid-state laser development at Lawrence Livermore National Laboratory

Laser-beam apodization with a graded random phase window: erratum.

Design and performance of the Nova frequency conversion system