Table of contents

Methods used to generate high-energy ultrashort laser pulses and their applications in various branches of science are reviewed. Special attention is given to the use of ultrastrong laser fields in the initiation of 'fast ignition' thermonuclear reactions.

Theoretical and experimental investigations were made of two-pass and four-pass amplifiers. The losses induced between the passes were taken into account. The two-level model approximation was used to derive expressions for the output energy and for the energy after any pass in the three cases of a complete time overlap of the pulses, of nonoverlapping pulses, and of combinations of these two cases.

The dependence of the rate of photodecomposition of the dye rhodamine 6G in polymer active elements on the average pump intensity was studied. The service life of such active elements pumped transversely with low-intensity (less than 1MW cm^-2) pump pulses was also investigated. When the average intensity was reduced below a certain value, the dye photodecomposition rate decreased significantly and the service life of the active elements rose strongly.

Semiconductor lasers with a new planar waveguide structure were developed and investigated experimentally. The structure made it possible to reduce the width (by a factor of 1.7) of the angular distribution in a plane perpendicular to the p — n junction and still retain a high value of the factor representing the optical confinement of waves in the waveguide. A single-lobe angular distribution in the same plane was achieved by a new method based on the difference between the optical confinement and optical loss factors for the fundamental and high-order transverse modes.

A linear two-frequency cw Nd³⁺ : YAG laser, pumped by a laser diode, emitted the λ = 1.06 mm wavelength. Stable generation of two linear orthogonally polarised axial modes was achieved for mode spacings ranging from 5 MHz to ∼ 9 GHz.

A pulsed dye laser, pumped by radiation supplied to the active zone by a multimode optical fibre, was proposed and built. Stable emission of a single longitudinal mode was achieved. The average output power was 270 mW when the pump power was 4 W.

An investigation was made of the operation of a gain-switched chromium-doped forsterite crystal laser pumped by radiation from a quasi-cw Nd : YAG laser. Optimisation of the cavity parameters made it possible to attain a 55 μJ threshold energy of the pump pulses and a slope efficiency of 10%. Estimates were obtained of the excited-state absorption cross section at the pump frequency and of the influence of this cross section on the slope efficiency.

A method is proposed for the optimisation of the cavities of solid-state lasers with self-mode locking by self-focusing. The feasibility of using cavities with two, three, and four mirrors for this purpose is considered.

A wide range of film materials, suitable for the fabrication of interference optics for the middle-IR range, was investigated. The selected film pair (YF₃—ZnS) had minimal internal porosity, low mechanical stresses, and good adhesion to substrates made of quartz, fluorite, barium fluoride, and leucosapphire. Experiments were used to determine the optimal film deposition procedures so as to minimise the residual microporosity, mechanical stresses, and losses resulting from diffuse scattering and absorption. Highly reflecting and output mirrors were made for the middle-IR range, as well as mirrors for an optical parametric oscillator (λ=3.5–12 mm) and antireflection coatings for a nonlinear GaSe crystal. The optical strength of such coatings was 25–30 J cm^-2. The interference optics developed in this way was used in an optical parametric oscillator with a GaSe crystal emitting at 4.2 mm when pumped with λ = 2.12 μm radiation.

A theoretical analysis is made of the influence of the spontaneous scattering noise on parametric excitation of stimulated Brillouin scattering (SBS) in a system of intersecting laser pump and signal wave beams. The line broadening of parametric oscillation is calculated. A study is made of the limit imposed on the sensitivity by excitation of a non-phase-conjugated SBS wave and also of the effects of the wave mismatch.

Experimental and theoretical investigations are reported of the spectral characteristics of photoinduced in-fibre refractive-index gratings for coupling the fibre core and cladding modes. A theoretical model is developed for predicting the transmission spectra of such gratings and the structure of the coupled cladding modes is considered.

It is shown that in stimulated Brillouin scattering of picosecond laser pulses the proportion of the energy scattered in a strongly inhomogeneous short dense-plasma layer reaches a fraction of a percent at intensities below 2 ×10¹⁵ W cm^-2 in the absence of a prepulse or less than 2 × 10¹⁴ W cm^-2 if a prepulse is present. This can be used to detect a prepulse and to obtain an experimental estimate of the contrast. In the case of longer (∼10 ps, ∼2 × 10¹⁵ W cm^-2) pulses the proportion of the energy scattered by the stimulated Brillouin process may reach several percent in the absence of a prepulse.

A refractive-index grating was formed for the first time in a germanosilicate fibre by cw Ar⁺ laser radiation in the near-UV range (333–364 nm). The refractive index change induced in a fibre with 10 mol.% GeO₂ in the core was 1.9 × 10^-4 when the radiation power density was 1.7 × 10⁵ W cm^-2. The gratings formed in this way were as therma lly stable as those formed by KrF laser (248 nm) radiation.

The problem of two-dimensional or quasi-three-dimensional flow of a multicomponent mixture in small-scale nozzles of a supersonic cw HF laser is solved numerically. A narrow-channel model taking account of chemical reactions in the interior and on the surfaces of nozzles is used. A comparison is made with a boundary-layer model and a model based on the two-dimensional Navier — Stokes equations. A good agreement between these models is obtained.

An analysis is made of the feasibility of using strong plasma outflow from focus-type constrictions of an axial-discharge plasma column. Selective ion recombination as a result of charge transfer with ions carrying a smaller charge in the main plasma column or with neutral atoms in the residual gas in the discharge chamber can create a population inversion for transitions in the soft x-ray and VUV parts of the spectrum. General problems encountered in the investigated system are discussed and quantitative estimates are obtained of the population inversion of levels participating in the 4 → 3 transitions in Li-like oxygen ions formed, with the effective charge Z=6, in pinch discharges with pulsed gas admission and a current of several hundreds of kiloamperes.

A harmonic analysis was made of the spatial scales of regular optical inhomogeneities in high-power laser amplifiers with a continuously flowing active medium. The wavefronts of the probe radiation reconstructed by interferometry made it possible to determine the distribution of the Strehl number over the spatial scales of inhomogeneities in the active medium of a cw chemical laser. The greatest contribution to the overall level of optical distortions in this laser came from a large-scale inhomogeneity, whereas small-scale periodic perturbations of the phase had practically no influence on the divergence of the output radiation, because their amplitude did not exceed the permissible limit (1 rad).

A variational method is used to study propagation of chirped optical pulses in media with periodic amplification. A nonlinear resonance and chaos in oscillations of the pulse duration are considered. A stochastic instability of evolution of the pulse duration and also pulse decay under the influence of periodic amplification are predicted. A calculation is reported of the stochastic pulse-decay length and a comparison is made with the results of a numerical simulation. The applications of the results to long-haul optical communication lines with amplifiers and to optical fibres with a periodically modulated core diameter are considered.

The thermal blooming regimes are identified and analysed for a vertical laser beam under conditions of free convection in an absorbing medium. The characteristic times, velocities, scales of temperature changes, and scaling parameters are determined for all the regimes. The equations of conservation of the medium are obtained and solved numerically in the approximation of paraxial wave optics under strong and moderate convection conditions. A comparison is made with the published experimental results. The reason for the appearance of turbulence at low Reynolds numbers is considered on the basis of these experimental results.

An analysis is reported of the use of the Schwarzschild objective in soft x-ray and VUV (2–20 nm) microscopy. Estimates are given of the influence of the objective alignment and of the precision of fabrication of spherical mirrors on the resolution of the objective (point spread function, line spread function, and modulation transfer function).

The geometric-optics approximation is used to derive an equation describing evolution of ultrashort solitons taking account of the temporal dispersion and of the Kerr nonlinearity. This evolution equation is of the Korteweg — de Vries type in which the role of the unknown function is played by the Poynting vector amplitude.