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The effect of instantaneous cubic nonlinearity on second-harmonic generation of femtosecond laser pulses is investigated. The possibility of improving generation efficiency and shortening pulse duration is analyzed. Energy conversion efficiency of 60% was obtained in an experiment in a 1 mm thickness KDP crystal at the peak intensity of 0.6 TW/[cm.sup.2] and the duration of 60 fs. OCIS codes: 190.0190, 190.2620, 190.5940.

We discuss the theoretical and experimental investigation of acousto-optic dispersive tunable filters, based on quasi-collinear geometry of light-sound interaction in a tellurium dioxide single crystal. The geometry uses the effect of strong acoustic anisotropy in the paratellurite as well as peculiarities of acoustic wave reflections at the free boundary of the crystal. A mathematical concept for determination of optical, electrical, and constructional parameters of the filters is developed. Different experimental acousto-optic filters intended for femtosecond pulse shaping are designed and tested. Preliminary experiments are performed in a subpetawatt optical parametric chirped pulse amplification laser system. The experimental data conform completely with the predicted data. OCIS codes: 230.1040, 140.7090, 190.4970.

Two Nd:YLF lasers with a pulse duration of about 2 ns have been synchronized. The duration and synchronization of two laser pulses are provided by two pairs of Pockels cells with synchronized voltage pulses. One of the Pockels cells in the first pair ensures Q switching, and the other cuts out a 2 ns pulse from the giant 20 ns pulse. In the second pair, one of the cells, driven by a two-step voltage pulse, forms a giant pulse synchronized with the pulse of the first laser, and the other cell cuts a short pulse out of it. The proposed scheme allows a rather simple and reliable synchronization of two Q-switched lasers with a jitter of 150ps. OCIS codes: 140.3510, 140.3580.

We present a novel, to the best of our knowledge, method for measuring the intensity profile of far-IR beams. The method is based on the measurements of nonstationary variation in optical thickness of a fused-silica plate heated by the studied radiation. The optical thickness is observed by means of a reflecting interferometer. Purpose-made experimental setup allows one to measure beams with an aperture of up to 60 mm with a spatial resolution of 1 mm. The accessibility of the utilized technologies and the possibility to easily increase the aperture are the major advantages of this approach. The probable area of application for the method is measurements of beams produced by powerful industrial far-IR lasers. OCIS codes: 040.3060, 140.6810, 120.3180.

With any form of phase distortions there is the need to qualitatively characterize beam quality. Three different qualitative criteria are most commonly used for this purpose, each of them describing the beam with one ratio: the overlapping integral, the Strehl ratio, and the [M.sup.2] parameter. We have analyzed the interrelation of the above-mentioned criteria in the three most common types of beam quality degradation: thermal lens, electronic self-focusing, and spherical aberration. Approximate analytical expressions for all three criteria and three types of beam distortion are derived for Gaussian and super-Gaussian intensity profiles. The efficiency of characterizing those beams by various criteria is discussed. OCIS codes: 220.1000, 140.6810.

An optical system that comprises two devices for remote measurements, a broadband optical interferometer and a scanning Hartmann sensor, is described. The results of simultaneous measurements with both devices and the results of numerical modeling of sample surface heating are presented. OCIS codes: 120.5050, 120.3180, 120.6810.

We have developed a model that describes thermally induced birefringence in polycrystalline ceramics that are exposed to a magnetic field. Conditions under which traditional compensation techniques (for glass and single crystals) can be effective for ceramics have been found. It is shown that a ceramic is almost equivalent to a [111]-oriented crystal if the ratio of the rod length to the grain size is ~300 or more. In particular, residual depolarization (after the compensation techniques are applied) is inversely proportional to this ratio, which is an important consequence of the random nature of thermally induced birefringence in ceramics. OCIS codes: 190.4870, 160.3820, 230.3240.

It is shown that the use of slabs instead of rods makes it possible to fabricate Faraday isolators and Faraday mirrors operating at a multikilowatt power. Analytical dependences of thermally induced depolarization in Faraday devices on radiation power and on slab aspect ratio have been obtained. OCIS codes: 140.6810, 230.2240.

A decrease in the depolarization ratio by more than one order of magnitude by use of a recently proposed Faraday mirror (FM) in comparison with the traditional FM has been demonstrated experimentally. At a high average laser power, the possibility of an increase in the accuracy of compensation of depolarization in the active element by means of a [lambda]/4 plate with both the traditional and the novel FM is shown. OCIS codes: 260.1440, 140.3580, 230.3810.

We present a comprehensive and systematic investigation of the fundamental physical ]imitations of Faraday isolation performance at high average powers that are due to thermally induced birefringence. First, the operation of various Faraday isolator designs by use of arbitrary orientation of cubic magneto-optic crystals is studied theoretically. It is shown that, for different Faraday isolator designs, different crystal orientations can optimize the isolation ratio. Second, thermo-optic and photoelastic constants for terbium gallium garnet crystals grown by different manufacturers were measured. Measurements of self-induced depolarization are made for various orientations of crystallographic axes. The measurements are in good agreement with our theoretical predictions. Based on our results, it is possible to select a crystal orientation that optimizes isolation performance at high average powers, resulting in a 5-dB enhancement over nonoptimized orientations. OCIS codes: 230.3810, 260.1440, 140.6810.