Table of contents

A model of self-lateral-mode-locking in an injection laser with a parabolic inhomogeneity of the dielectric constant of the active medium is proposed. It is shown that an appropriate choice of the pump current profile provides a stable scan of the radiation pattern. The role of the antiwaveguide parameter in the violation of the scan stability is discussed in detail. The spectrum of lateral modes is shown to split with increasing this parameter. Subgroups of satellite modes appear inside each mode, which efficiently interact with each other, resulting in periodic violation and recovery of the scan periodicity.

The propagation of light beams in a nonlinear cubic medium with the refractive index periodically modulated along one transverse coordinate is considered. The profiles of soliton beams are found and their stability is studied. It is shown that the refractive-index modulation causes the collapse suppression and soliton stabilisation almost within the entire region of their existence.

A comparative analysis of the fluctuation structure of laser radiation over urban near-the-ground paths is performed for different turbulent states. The data obtained in experiments allow changes in the statistical characteristics of laser beams, which are induced by aperture effects, to be compared to those caused by changes in the turbulent state of the propagation medium. The influence of intermittency of small-scale turbulence on the radiation characteristics is considered. It is found that the intermittency has a weak effect on the rms shift of the beams' 'centers of gravity' but affects substantially the spatial and temporal intensity fluctuations.

The general type of boundary conditions in heat exchange problems, including heat exchange in laser systems, is discussed. The appearance of convection in a plane layer of a nonequilibrium gas with the volume energy release and its temperature dependence is briefly considered. Convection in a cylindrical system and in a system of coaxial cylinders with a cooled central part is considered in detail. Such systems simulate real laser devices. It is shown that the maximum temperature in the cylindrical system decreases due to convection, whereas the maximum temperature in the system of coaxial cylinders increases, i.e., the analysis of heat removal in a laser system reveals a very important role of convection.

The S₀(0) and S₀(1) rotational transitions in the H₂ molecules are studied by the method of transient CARS spectroscopy at temperatures 296 and 80 K in a broad range of gas densities — from the density at which Doppler dephasing occurs to the density corresponding to collision dephasing. The slowest decay of the responses of the scattered probe pulse at the molar gas density ∼0.2 amagat corresponds to the Dicke line narrowing. It is shown that the dephasing of the S₀(1) transition in the natural H₂ mixture leads to a substantial discrepancy between the experimental results and the model of statistically independent perturbations of the rotational and translational motion of molecules at densities within a few tenths of amagat. This discrepancy becomes quite significant at liquid nitrogen temperature. At the same time, the dephasing of the S₀(0) transition better corresponds to this model, as well as the dephasing of the S₀(1) transition in the buffer He gas. The deviation from the model of statistically independent perturbations is described by introducing the correlation parameter. It is assumed that the features of the dephasing of the S₀(1) transition observed in the natural H₂ mixture are mainly caused by resonance dephasing collisions.

Model signals of an optical coherence Doppler tomograph (OCDT) are obtained by the Monte Carlo method from a flow of a light-scattering suspension of lipid vesicles (intralipid) at concentrations from 0.7% to 1.5% with an a priori specified parabolic velocity profile. The velocity profile parameters reconstructed from the OCDT signal and scattering orders of the photons contributing to the signal are studied as functions of the suspension concentration. It is shown that the maximum of the reconstructed velocity profile at high concentrations shifts with respect to the symmetry axis of the flow and its value decreases due to a greater contribution from multiply scattered photons.

An adaptive system for the compensation of static and thermally induced wavefront aberrations of the amplification path of the 'Luch' laser facility is described. This system provided the reduction of the amplitude A of wavefront aberrations of high-power radiation and the standard deviation σ by a factor of ∼3: from A = 9.6 μm, σ = 2.4 μm to A = 3.2 μm, σ = 0.6 μm, which decreased the radiation divergence by half.

The adaptive phase correction of distortions of spatially limited laser beams and a plane wave transmitted through a turbulent atmosphere layer is considered. The requirements to the size of the mirror element and the bandwidth of the correction system are determined, which proved to be the same as in the case of weak intensity fluctuations.

The method is proposed for tuning CO₂ laser to a required oscillation line without a spectral instrument based on the coincidence of transition frequencies belonging to different vibrational—rotational bands of the CO₂ molecule. This coincidence leads to the anomaly in the gain distribution over rotational sublevels, thereby affecting the laser output parameters. The method was successfully applied to a completely automated low-pressure, longitudinal-discharge cw CO₂ laser and a pulsed TEA CO₂ laser.

The behaviour of the amplitude of a saturated absorption resonance observed in laser radiation propagated through an active interferometer with nonlinear amplifying and absorbing media is studied experimentally for the first time. Upon variation of the gain in the amplifying medium and preserving a constant saturating field in the interferometer, a nonlinear increase in the amplitude of the saturated absorption resonance was observed. In this case, an increase in the detected signal exceeded an increase in the noise, i.e., the signal-to-noise ratio also increased.

An analytic theory of harmonic generation in the plasma produced from the gas of hydrogen-like atoms in excited states is considered for relatively intense radiation. The consideration of l-degeneracy of the electrons in these excited states allowed deriving the dependence of generation efficiency on the principal quantum number. In the context of the Bethe model of gas ionisation, we revealed the threshold nonlinear dependence of the maximum generation efficiency on the degree of circular polarisation of the pump field for its given intensity. Analytic calculations were performed for the fifth and seventh harmonics. The results of these calculations allowed generalising to the case of excited atoms the previously obtained results for the third harmonic in the plasma arising from hydrogen-like atoms in the ground state.

The interaction of two counterpropagating light waves on reflection holographic gratings is studied in cubic photorefractive crystals with the natural optical activity. The anisotropy of contributions from intramode (without a change in the refractive index of natural circularly polarised waves) and intermode processes to the interaction of the waves in a bismuth titanate crystal is considered. It is shown that the polarisation dependences of the effective gain and the polarisation vectors of the interacting light beams measured at a wavelength of 633 nm in a (100) cut Bi₁₂TiO₂₀:Fe, Cu crystal of thickness 2.6 mm well agree with numerical calculations with the use of the two-beam coupling constant equal to 6.8 cm^-1.

Wavelength-division-multiplexing fibreoptic communication links with optical 2R regenerators based on a saturable absorber are mathematically simulated. The results of optimisation of specific configurations of symmetric lines are presented, and it is shown that the transmission distance in systems with the periodic optical regeneration of signals considerably exceeds that in systems without optical regenerators.

The mode and ray-optics techniques based on the representation of the input field as a spectrum of propagating modes or ray tubes are proposed for calculating the propagation of radiation in wide circular hollow waveguides excited by linearly polarised Gaussian beams. Optimal excitation conditions are determined theoretically and experimentally, and the degree of polarisation of the beams transmitted through the waveguides is evaluated.

Small-angle scattering of laser radiation in purified (twice-distilled) water is studied experimentally. The scattering indicatrix shows that such water contains scattering micron impurities. The parameters of the size distribution of these impurities are estimated from the experimental data. The results obtained in the paper confirm the earlier proposed hypothesis about the presence of stable microbubbles of gas, bubstons, dissolved in pure liquids.

The formation of a laser photoacoustic response in an inhomogeneous medium is considered taking into account the temperature dependence of the coefficient of thermal expansion. It is shown that in the one-dimensional or three-dimensional cases in the presence of individual absorbing centres (spherical particles), the shape of the photoacoustic response is the wavelet transform of the spatial distribution of heat sources. The parameters (level) of the wavelet expansion are determined by the characteristics of the laser pulse. The possibility of multiscale wavelet analysis of the medium structure is demonstrated by the example of a numerical model. The possibility of direct visualisation of individual cells in biological tissues is discussed.

The peculiarities of holographic reverse-shearing interferometry, which make it possible to obtain two simultaneous real-time interferograms of the phase object free from aberrations of the optical system are studied. The behaviour of fringes in the interferograms is identical to their behaviour in the conventional two-beam interferometry with a standard reference wave. The peculiarities of fringe alignment in the interferograms are considered. The real-time interferograms of a glass test plate with various fringe alignments are obtained, confirming the practical prospects of this technique.

A new algorithm is proposed for optimising the thickness of layers in multilayer aperiodic mirrors. An important feature of the algorithm is the separation of the explicit dependence of the reflection coefficient on the thickness of one layer. This reduces the amount of calculations required for the mirror optimisation by a factor of N (N is the number of layers of the mirror). The algorithm allows one to perform optimisation of different types: the obtaining of the maximum reflection coefficient at one frequency, the maximum of the integrated reflection coefficient within a specified frequency interval, the constant reflection coefficient in the working frequency or angular interval, etc. Examples of practical applications of the algorithm demonstrating its efficiency and versatility are presented.

The effect of polarisation of a Gaussian beam on the radiation absorption during laser cutting of metals is investigated. A generalised formula is proposed for calculating the absorption coefficient, which describes the polarisation of three types (linear, elliptical, and circular), taking into account the fact that the beam may interact with a metal surface of an arbitrary shape. A comparison with the existing analogues (in the cases of linear and circular radiation polarisation) confirmed the advantage of employing the formula for the spatial description of the shape of the surface produced, which is highly important for processing (cutting, welding, drilling) of thick materials. The effect of laser radiation characteristics on the surface shape and cut depth in cutting stainless steel sheets is investigated numerically. It is shown for the first time that the cutting of materials by the TEM₀₀ beam is most efficient when the beam has elliptical polarisation directed along the direction of beam displacement and characterised by a specific axial ratio.