Table of contents

The possibility of efficient and symmetric compression of a target with a low-density structured absorber by a few laser beams is considered. An equation of state is proposed for a porous medium, which takes into account the special features of the absorption of high-power nanosecond laser pulses. The open version of this target is shown to allow the use of ordinary Gaussian beams, requiring no special profiling of the absorber surface. The conditions are defined under which such targets can be compressed efficiently by only two laser beams (or beam clusters). Simulations show that for a 2.1-MJ laser pulse, a seven-fold gain for the target under study is achieved.

The spectral radiative losses are investigated in a plasma under conditions typical of a plasma produced upon irradiation of solid targets by high-intensity (up to 10¹⁷ W cm^-2 ultrashort (10^-13-10^-9 s) laser pulses. The comparison of the calculated X-ray spectra with the experimental data for aluminum and carbon plasmas shows their satisfactory agreement. These studies made it possible to test the methods in use and to conclude that it is necessary to introduce supplements into the collision — radiation model for calculating the optical characteristics of a nonequilibrium plasma of complex chemical composition.

The results of studies on the improvement of the laser beam quality initiated by N.G. Basov and performed at the Laboratory of Quantum Radiophyisics (FIAN) are briefly discussed.

The use of electron beams for pumping dense gases made it possible to obtain lasing on atomic and molecular transitions in different spectral ranges and to develop high-power pulsed lasers. N.G. Basov and coworkers made a substantial contribution to the formation and advancement of this field. A brief review of the research on efficient elevated-pressure active media and high-power pulsed lasers utilising plasmas produced both by an electron beam and an electron-beam-controlled discharge is presented. These are excimer and exciplex lasers, lasers utilising atomic transitions in xenon and neon, an Ar —N₂ mixture laser, a molecular nitrogen ion laser, and a high-pressure CO₂ laser. Data obtained in the investigation of the radiation of rare-gas halide complexes are given.

Investigations of high-pressure lasers on Ar — Xe mixtures pumped by electron beams and electroionisation discharges are reviewed. A new method of pumping laser transitions by an electroionisation discharge is proposed. The method uses the excited states of Xe atoms as the ground laser level. An electroionisation laser on the IR transitions of a Xe atom with a volume of the active region of 10 L and a pulse duration of 5 μs is developed on this basis. Its specific (up to 8 J L^-1) and absolute (up to 80 J) energy outputs exceed those of electric-discharge noble-gas lasers by more than one and two orders of magnitude, respectively. It is shown that the efficiency of conversion of the electric energy deposited in the discharge into laser radiation can be as high as 5%. The laser beam divergence obtained at output energies close to the maximum one is ~3 × 10^-5 rad. It is shown in principle that a repetitively pulsed Xe laser emitting at 1.73 μm with a specific energy output of 0.5 — 1.0 kW L^-1 and an efficiency of 2.0% — 3.2% can be created.

A consistent theory of stochastic resonance is presented taking into account the effective dynamic relaxation time. It is shown in the small signal amplitude approximation that the output signal-to-noise ratio of a bistable system can exceed this ratio at its input only due the filtering properties of the system.

Temperature-dependent transient processes initiated by a direct photogeneration of singlet oxygen in suspensions of human erythrocytes and solutions of serum albumin are studied. The processes appear as anomalous jumps in the temperature dependences of the deformability coefficient of erythrocytes and the refractive index of the extracellular medium and protein solution. In the temperature regions of anomalous jumps, cells and proteins transfer to a metastable state of a lower activity, but they can be isothermally photoreactivated. Simultaneously, a reversible rearrangement of the aqueous phase occurs near the cell and protein surfaces, accompanied by the formation of an extended corona (hydrogel). The transient processes are interpreted as phase transitions in the membrane of erythrocytes and conformation transitions in proteins. The interaction between erythrocytes and albumin via hydrogel is discovered (hydro-conformational interaction). A qualitative physical model of the early stages of the light-oxygen effect is proposed, in which collective magnetic interactions between the electron spins of oxygen molecules and the nuclear magnetic moments of protons in H₂O molecules play a dominant role.

Self-mode-locking was observed in a Nd³⁺:YAG laser with a negative feedback without introducing any nonlinear elements into the laser cavity. The laser generates during pumping 300 — 500-ps single pulses on an axial period.

Passively Q-switched and cw operation regimes of a diode-pumped Yb:KYW microchip laser have been investigated. The maximum slope efficiency for cw operation of 23% relative to incident pump power has been achieved. Self-frequency Raman conversion for microchip cavity configuration has been realised.

The influence of self-action effects on the efficiency of SHG of phase-modulated laser pulses in a nonstationary regime is analysed using a generalised method of strong interaction of nonlinear waves. An analytic solution for the SHG efficiency is obtained taking into account the depletion of pump radiation, the influences of the linear phase mismatch, mismatch of group velocities, and higher linearities. It is shown that the effects of self-action, mismatch of group velocities, and linear phase mismatch can compensate for each other, resulting in an increase in the efficiency of SHG of phase-modulated pulses.

Electronic processes stimulated by a train of nanosecond laser pulses in a weakly absorbing semiconductor are studied theoretically. It is shown that these processes cause the transverse Dember effect — the appearance of a radial field and an electric potential difference between the illuminated and unlit parts of the semiconductor. The transverse Dember potential difference in silicon is comparable with a typical surface potential and, hence, plays an important role in surface nonlinear-optical processes.

The possibility of using perforated output mirrors in stable resonators of cw CO₂ lasers is discussed. The main factors responsible for the intracavity losses, including those connected with the diffraction of beams at the edges of holes in the mirrors are considered. The results of the experimental investigation of a 100-kW combustion product gas-dynamic CO₂ laser with stable and unstable resonators and output mirrors of various types are compared with the results of numerical simulation. The theoretical and experimental studies performed over a wide range of parameters explain some peculiarities of the beam formation process in resonators with perforated mirrors. Practical recommendations are offered for choosing the parameters of such mirrors.

A waveguide HCN laser with an active medium of length 0.5 m is described. The laser resonator contains a half-wave phase mirror and two plane-parallel one-dimensional wire gratings crossed at a right angle. The active medium was excited by an alternating current (50 Hz) and high-frequency radiation (40.7 MHz). The dependence of the output laser power on the resonator length is studied for different azimuthal positions of the phase mirror with respect to the grating. Lasing was observed at any azimuth, from zero to 360°.

Based on the time-correlation approach, a mathematical model is developed for analysis of the influence of anisotropy defects in an optical path on the correlation characteristics of low-coherence radiation. Several interference schemes of practical importance for optical coherence tomography (OCT) are analysed. The experimental data are presented which demonstrate the influence of anisotropy defects in an optical path on an output interference signal and the quality of two-dimensional images (tomograms) of biological objects obtained by the OCT method.