Table of contents

A review of the theory of the natural waves (eigen-modes) in a dielectric sphere is presented. A special attention is paid to the eigenmodes with large radial and azimuthal indices, the so-called whispering-gallery modes. The experimental results of the spectroscopic study of modes in a dielectric sphere are reported. The fields of applications of whispering-gallery modes are discussed.

A 10-fs Ti:sapphire laser with a standard 1-cm-long rod is built. The laser uses a broadband (low-dispersion) dispersion compensator and an original ring resonator.

The gain dynamics in the active medium of an overtone CO laser operating on the transitions between high vibrational levels is studied experimentally and theoretically. The gain dynamics is measured on the vibration — rotation transitions of the CO molecule [the P(12) spectral line in the vibrational bands from 20 → 18 to 36 → 34] in CO — He and CO — N₂ laser mixtures. The maximum small-signal gain was 0.43 m^-1. The theoretical model of the CO laser was significantly improved. It was shown that multiquantum and asymmetric VV-exchange processes should be included in the analysis of the population dynamics of high vibrational levels of the CO molecule.

It is shown that, according to the quantum theory of light, the spatial period of an interference pattern formed by light incident on a medium and reflected from it is determined both by the wavelength of light and the number of coherent photons in a scattered mode. The scattered signal is assumed arbitrarily weak.

The self-diffraction of light appearing upon optical poling of bulk glass samples is studied. During poling, a refractive-index grating is accumulated in the medium, on which one of the beams or both beams diffract efficiently. A theoretical expression is obtained for the amplitude of diffracted radiation. The results of the experimental study of this phenomenon in oxide glasses are in agreement with the theory, which explains the formation of a spatially periodic electric field in the medium by the coherent photogalvanic effect. The self-diffraction of light can be efficiently used for studying the physical properties of purely optical poling of media.

A method of increasing the pumping efficiency of active optical fibres doped with Er³⁺ ions by decreasing the diameter of the inner cladding is analysed. An active optical fibre is described by using the electrodynamic model of a three-layer dielectric waveguide. The dependence of the average pump power flux through the core on the ratio of the radii of the core and the inner cladding is obtained. It is shown that this dependence can also be interpreted as the dependence of the damping constant averaged over all the waves in the core and the cladding.

Idealised three-layer model optical fibres consisting of an optically dense core with the refractive index n₁ surrounded by a 'holey' intermediate cladding having a much lower optical density and the refractive index n₂ and by a rather thick outer cladding with the refractive index n₃ are investigated. The idealised model assumes that the intermediate cladding is homogeneous. It is also assumed that n₁ > n₃ > n₂. It is shown that in such fibres with large differences n₁ — n₂ and n₃ — n₂, a single-mode regime can be realised in fact, when the nearest higher modes H₀₁, E₀₁, HE₂₁ have additional radiative losses of several hundred dB km^-1, whereas the main operating mode HE₁₁ has no such losses in the single-mode regime. It is important that the zero dispersion can be obtained in these fibres in the spectral region below 1.3 μm.

A method for fast radiation wavelength tuning is proposed for a single-frequency fibre laser. A diffraction grating written on a waveguide electrooptical modulator serves as the mirror of the external cavity and as the selector. The voltage supplied to the grating electrodes shifts the reflection band of the grating and hence tunes the emitted radiation range. The radiation frequency tuning within the grating reflection band is carried out by using an intracavity waveguide electrooptical cell. The spectral characteristics of laser radiation are calculated. It is shown that the tuning range of such a laser achieves 0.2 nm for a control voltage up to 5 V, the switching time being shorter than 0.7 ns.

Nonlinear light scattering appearing upon air breakdown induced by high-power ultrashort pulses (110 — 5200 fs) from a Ti:Al₂O₃ laser is studied. As a result of forward scattering, the beam profile is severely deformed, which is accompanied by spectral conversion of the incident radiation to a series of shorter-wavelength peaks extending into the visible spectral range. Measurements are made of the thresholds and the scattered radiation energy, which amounts to 75% of the incident energy. The effect of scattering on the material ablation in air is investigated. The obtained data offer an explanation for the experimentally observed paradoxical morphology of the channels ablated by high-power femtosecond pulses.

The formation of fractals during laser evaporation of various media (metals and dielectrics) by 10-ms Nd laser pulses is studied experimentally as a function of the laser radiation power density and external pressure. It is established that fractals are formed during the action of a laser pulse in the bulk of a plasma plume. It is found that finely dispersed phase forms under a certain critical pressure a bound shell (macrofractal) in peripheral layers of the plume. The presence of the shell confining the plasma expansion leads to a threshold variation of the characteristics of the optical discharge.

Transmission of radiation from a free-running 2.94-μm Er³⁺ : YAG laser by strongly absorbing liquids is studied. It is shown that transmission of radiation is caused by the development of a channel in the liquid, which is formed by a laser pulse itself. The transmission in water, ethanol, and glycerol is studied as a function of the laser pulse energy and the liquid thickness.

The possibility of improving the electric discharge stability, increasing the efficiency, and expanding the dynamic operating range of an industrial self-sustained discharge CO₂ laser using additional ionisation by high-voltage nanosecond pulses is studied.

An electron-beam-pumped laser on Xe atomic transitions is experimentally investigated at various pump durations and powers within wide ranges of pressures and working mixtures including additions of molecular gases. It is shown that the maximum specific lasing powers are achieved at high specific pump powers (above 200 kW cm^-3 atm^-1) and durations of the beam current pulse of tens of nanoseconds in high-pressure Ar — Xe mixtures with molecular gas additions (N₂ and CO₂). A specific output radiation power of ~ 4 kW cm^-3 is obtained. For a pump pulse durations from hundreds of nanoseconds to 1 μs, the highest lasing energies are reached without molecular additions at a comparatively low beam-current density (the specific pump power is ~ 10 kW cm^-3 atm^-1). However, in setups with specific pump powers above 40 kW cm^-3 atm^-1 and a working-mixture pressure limited by the strength of the laser chamber, molecular additions result in an increase in the radiation energy and efficiency. In wide-aperture facilities with high pump powers, molecular additions improve the distribution of the radiation power density over the laser-beam cross section.

The dependence of the Raman-laser output power in a standing-wave mode on the pump frequency located near the resonance with ionic levels is studied. The emission spectra in two Λ-schemes of Ar II with different lifetimes of the final levels are compared. In the scheme with a long-lived final state, a sharp peak with an amplitude exceeding the amplitude of a broad contour by a factor of 1.5 — 2 is observed at the exact resonance, whereas no spectral features are observed near the resonance in the scheme with a short-lived final level. The effect is analysed within the framework of a model taking into account the Coulomb scattering of ions. It is shown that the peak forms due to a difference in the broadening of the Bennett structures in the population distribution of the operating levels over velocities. In the second Λ-scheme, the contribution of the final level is small in amplitude, and a sharp peak in the spectrum disappears in accordance with the predictions of the model.

The emission of a submillimeter laser pumped by a high-power (up to ~ 1 kW) CO₂ laser is studied. It is found that lasing of CH₂F₂ molecules saturates at the pump power ~350 W. A delay of submillimeter lasing depending on the gas pressure in the active medium and the pump intensity is observed. The possibility of a rapid tuning (in about 1 ms) of the submillimeter laser emission by scanning the emission from the CO₂ laser is demonstrated experimentally.

New physical objects, ispalators based on free soap films, exhibit persistent flows of the soap solution in open and closed volumes in air with additions of gases of the C₈F₁₈ type (p = 20 Torr) at temperature drops on the films of the order of tenths and hundredths of kelvin. The flows move continuously at a velocity of 5 — 20 cm s^-1. It is found that the parts of an inclined ispalator film show anomalous behaviour upon heating: their weight increases and they move downward over the film, whereas the unheated parts of the film move upward. Continuous radial vortex flows accompanied by the formation and washing of the regions of a thin black film are observed on circular films in closed volumes upon their uniform external cooling by evaporating water for 5 — 10 hours. The rapid flows make film ispalators the efficient heat carriers, which operate at small temperature drops (tenths and hundredths of kelvin) and surpass copper in the amount of thermal energy being transferred. The outlook for the further study and applications of film ispalators for detecting thermal fields and laser radiation is discussed.