Table of contents

The experimental data on the initiation of a number of nuclear processes in a plasma produced by an ultrashort laser pulse of intensity 10¹⁶—10²¹ W cm^-2 are summarised. These processes include excitation of nuclear levels, fusion and fission reactions, production of unstable isotopes, generation of ultrashort pulses of elementary particles, etc. The trends and outlook for studies aimed at increasing the intensity, energy, and average power of ultrashort laser pulses are analysed.

The operation of an electric-discharge repetitively pulsed DF laser with an 8 — 10-mm discharge gap and a working mixture circulating along a closed gas-dynamic channel is experimentally studied. The main laser parameters and the experiments are described, and the experimental results are analysed. The reasons for the decrease in the output pulse amplitude and lasing suppression at high pulse repetition rates are discussed. The acoustic waves arising due to the repetitively pulsed initiation of the mixture were shown to be the principal factor reducing the average output power at high pulse repetition rates and leading to the output instability and even to lasing suppression. The methods for the suppression of the perturbations, resulting in an increase in the output power are described. An average output power of 25 — 30 W was achieved for a pulse repetition rate of up to 1200 Hz and a technical efficiency of ~2%.

Spontaneous emission and optical gain are studied in MOCVD-grown epitaxial layers of GaAs co-doped with erbium and oxygen. Optimal growth conditions are used to obtain the high erbium concentrations up to 8 × 10¹⁸ cm^-3. The measurements by the Shaklee — Leheny method revealed the optical gain ~45 cm^-1 at rather low pump intensity ~0.1 kW cm^-2 77 K.

The use of an optimal CO₂ — N₂ — H₂ gas mixture with a high concentration of hydrogen (30% — 40%) was shown to allow the formation of high-power TEA CO₂-laser pulses with a base duration of ~ 200 ns whose energy and peak power significantly exceed the parameters of pulses obtained in binary CO₂ — H₂ mixtures. Considering all the parameters, the helium-free 56% CO₂ — 14% N₂ — 30% H₂ gas mixture at a pressure of 0.7 atm is optimal for the generation of short high-power pulses in the TEA CO₂ laser. A small addition of nitrogen ([CO₂]/[N₂] ~ 10 — 12) to the binary CO₂ — H₂ mixture not only substantially increases the pulse energy and peak power (by a factor of 3 — 3.5) but also shortens their duration at half-maximum with retention of the total (base) duration.

The spatiotemporal smoothing of the intensity distribution of a focused laser beam was investigated on the Iskra-4 laser setup using a dynamic plasma phase plate produced by vaporisation of a special target placed at the beginning of the caustic of the focused beam. It is shown that the transmittance of the plasma target in the 10¹³— 2 × 10¹⁴ W cm^-2 intensity range is no less than 70% — 80%. The introduction of the dynamic plasma phase plate lowered the relative fluctuations of the spatial intensity distribution from 100% to ~10%. The characteristic time of the intensity distribution variation in the focal plane is 0.4 ps.

The steady-state two-wave interaction on a transmission photorefractive grating produced in a cubic gyrotropic crystal of the symmetry group 23 in an external dc electric field is studied. The analytic solution of the equation for coupled waves is obtained for an arbitrary orientation of the crystal and arbitrary polarisation of incident light beams in the fixed pump approximation. It is shown that a nonunidirectional contribution to the energy exchange between the waves can exist, which is caused by the local component of the photorefractive grating. In a particular case of light waves with a wavelength of 633 nm propagating in the (001) plane of a Bi₁₂TiO₂₀ crystal and of the orientation of the grating vector along the [110] axis, this contribution greatly exceeds a usual unidirectional contribution. The dependence of the two-wave gain on the modulation coefficient of the interference pattern on the input face of the crystal is studied by numerically integrating a total system of coupled-wave equations.

The theory of intracavity quasi-phase-matched self-frequency conversion in an active nonlinear periodically poled structure is developed. The processes of quasi-phase-matched self-frequency doubling, self-halving and mixing using the pump wave in a periodically poled Nd:Mg:LiNbO₃ crystal are studied. The dependences of the efficiency of nonlinear optical conversion in these processes on the reflection coefficient of the output mirror and on linear losses in the medium are investigated.

Nonstationary SRS equations are solved analytically for short samples in the approximation of a prescribed exciting field, and numerically for a longer medium taking into account the pumping depletion. The dependence of the energy of Stokes and anti-Stokes waves on their spatial mismatching is studied. The optimal angle of anti-Stokes generation is determined as a function of the length of the system, pump pulse energy, and the rate of transverse relaxation.

The propagation of powerful light beams in nonlinear media with the Kerr nonlinearity is considered. The effect of local inhomogeneities on self-focusing of the beams is studied.

The absorption and luminescence spectra of silica fibres doped with chromium and various dopants are studied. Fibres doped with aluminium and gallium exhibit a broad (300 nm) luminescence band of Cr⁴⁺ ions at 1100 nm at room temperature. The quantum yield of luminescence is estimated to be 10^-4-10^-5.

Long-period gratings are fabricated in an optical fibre for the first time by second-harmonic radiation from a femtosecond Ti:sapphire laser without the use of an amplifier. The photosensitivity of different fibres exposed to femtosecond pulses is studied. The estimates of the photosensitivity and the thermal properties of long-period gratings showed that the mechanism of the change in the refractive index induced by the 400-nm second-harmonic radiation differs from the corresponding mechanisms upon exposure to UV radiation from excimer lasers or irradiation by amplified 800-nm femtosecond pulses.

An electrostriction model is proposed for the photorefractive effect observed during the writing of Bragg gratings in germanosilicate fibres. Electrostriction is caused by a spatial charge grating formed upon the exposure to UV radiation. According to our estimate, the contribution of electrostriction to the photorefractive effect under real writing conditions is comparable with the contribution from colour centres and exceeds the contribution from the electrooptical effect by more than an order of magnitude. The electrostriction model explains the production of the IIA type Bragg grating in fibres with a high content of germanium in the core, as well as a number of effects that could not be explained earlier.

The existence of the Lax representation for a model of soliton management under certain conditions is shown, which proves a complete integrability of the model. The exact analytic solutions are obtained for the problem of the optimal control of parameters of Schrodinger solitons in nonconservative systems with the group velocity dispersion, nonlinear refractive index, and gain (absorption coefficient) varying over the length. The examples demonstrating the non-trivial amplification dynamics of optical solitons, which are important from practical point of view, are considered. The exact analytic solutions are obtained for problems of the optimal amplification of solitons in optical fibres with monotonically decreasing dispersion and of Raman pumping of solitons in fibreoptic communication systems.

It is shown that optical solitons in nonlinear fibre-optic communication systems and soliton lasers can be represented as nonlinear Bloch waves in periodic structures. The Bloch theorem is proved for solitons of the nonlinear Schrodinger equation in systems with the dispersion, the nonlinearity, and the gain (absorption coefficient) periodically changing over the length. The dynamics of formation and interaction, as well as stability of the coupled states of nonlinear Bloch waves are investigated. It is shown that soliton Bloch waves exist only under certain self-matching conditions for the basic parameters of the system and reveal a structural instability with respect to the mismatch between the periods of spatial modulation of the dispersion, nonlinearity or gain.

A novel tomographic method that can be applied in strongly scattering optical media is proposed. 1-MHz focused ultrasound is used to tag the scattering photons in the biological tissue; it carries a 10-KHz sinusoidal wave to act as a detection wave through amplitude-modulation (AM). The scattering photons that come from the focused zone carry the modulated information. Their optoelectronic signal is demodulated by real-time FFT. By detecting and discriminating ultrasound-modulated information carried by scattered photons, the optical tomographic images of the media simulating biological tissue and of a buried object are reconstructed by the AM spectral intensity. This ultrasound-tagged optical tomography can be applied to tissue structures with different optical parameters. For the first time, by using this method, we obtained the tomographic image of a 5 mm-wide soft rubber cube buried in a biological tissue-simulating media with a detecting depth of 30 mm.

The relaxation times τ of the bleached states of polymethine dyes absorbing light in the 750 — 850-nm are determined by the direct pump — probe method. The effect of the dye structure and the solvent type on the relaxation time is discussed. The role of different intra- and intermolecular interactions in the relaxation of excited electronic states of the dyes is analysed. Polymethine dyes are found (with τ=11 — 75 ps) that are promising for passive mode locking in Cr³⁺:LiCaAlF₆, Cr³⁺:KZnF₃, and Cr³⁺:LiSrAlF₆ crystal lasers.

It is shown that the diffraction reproduction of a periodically modulated wave field takes place when light propagates through Gaussian optical systems. Generally, such a reproduction is accompanied by image scaling. Equations are derived that relate the reproducton distance and scaling factor to the ABCD matrix elements of the optical system. The Talbot effect in a convergent (divergent) wave is considered.