Table of contents

A self-consistent mathematical model of a He—Sr⁺ recombination laser is developed and used to study the establishment of a repetitively pulsed regime in the active medium. The spatiotemporal characteristics are analysed in the stationary regime under different excitation conditions, the contraction of a repetitively pulsed discharge in helium is investigated and the discharge decontraction upon addition of strontium vapour, which is fundamentally important for metal vapour lasers is studied. Numerical simulations are shown to be a convenient tool for investigating the kinetics of the active medium of a He—Sr⁺ laser, which enables predicting the optimal excitation conditions and calculating the limiting lasing characteristics.

The mechanisms limiting the improvement of the energy characteristics of He—Sr⁺ recombination lasers with increasing pressure, active volume, and pulse repetition rate are analysed. It is found that the existence of optimal pressure is related to the cooling rate limitation for electrons at high pressures arising from the heating action of the trailing edge of a current pulse and that the average power saturation with increasing discharge tube diameter and pulse repetition rate occurs due to the formation of radial nonuniformity as well as due to the growth of the electron temperature in the afterglow. Calculations involving a mathematical model showed that the attainable maximum of the linear power is equal to ≈6.2 W m^-1 for self-heating active elements of BeO ceramics and to ≈7.8 W m^-1 with its surface blackened, to ≈7.7 W m^-1 for an independent introduction of the vapour and to ≈9.4 W m^-1 in combination with blackening, to ≈17 W m^-1 for intense forced cooling of the active elements of cylindrical geometry and to ≈29 W m^-1 with active elements of rectangular cross section for a wall dimension ratio of 1:3. We demonstrate the feasibility of raising the output power by ≈21% in the excitation of the active medium by pulse bursts with a short interpulse separation.

A model for calculating the parameters of a laser diode with an external fibre cavity containing a fibre Bragg grating (FBG) is presented. It is shown that by using this model, it is possible to obtain single-mode lasing by neglecting the spectral burning of carriers. The regions of the laser-diode current and temperature and the FBG temperature in which the laser can be tuned to the D₂ line of cesium are determined experimentally.

A three-stage power amplifier is developed for an IR terawatt femtosecond laser system generating 80-fs, 1240-nm pulses with energy of up to 90 mJ, a pulse repetition rate 10 Hz, and the intensity contrast above 10⁶. The efficiencies of different schemes of multipass amplifiers are compared. It is shown that an optical scheme with plane mirrors is optimal for achieving the terawatt power level.

The possibility of obtaining efficient emission at purely rotational transitions of HF molecules in a pulsed chemical hydrogen fluoride laser is studied theoretically. The operation of a H₂—F₂ laser with a gas pressure of 1.1 atm emitting at the v, j → v, j - 1 (v = 1 - 6, j = 10 - 14) transitions is simulated taking into account resonance VR processes. The total specific laser energy release calculated over all the vibrational levels is 5.5 J L^-1 on purely rotational transitions at λ≈17 μm (j = 14), 3.5 J L^-1 at λ≈18.5 μm (j = 13), and 2.5 J L^-1 at λ≈20 μm (j = 12).

BaY₂F₈ crystals are studied as promising active media for UV and VUV lasers. The up-conversion pumping of rare-earth activators is proposed to solve problems related to the solarisation of the medium and the selection of pump sources. The technology of growing oriented BaY₂F₈ single crystals is developed and the influence of the crystal orientation on the growth rate and quality of single crystals is determined.

Based on the two-dimensional heat conduction equation solved under boundary conditions of the third and fourth kind, a new approach is proposed for determining the gas temperature in the cross section of a copper bromide laser. Temperature profiles in the cases of the natural and forced convection are obtained taking into account the basic laser parameters such as the laser design, the input electric power, etc. In particular, computer simulations for the case of forced convection are performed in order to increase the input electric power at a fixed maximum discharge temperature.

The evolution of the orbital angular momentum (OAM) of a Laguerre—Gaussian beam interacting with turbulent inhomogeneities of the atmosphere is studied theoretically. The integral representations are obtained for the OAM in terms of the distributions of the random intensity and random field of the permittivity of the medium, and also for OAM statistical characteristics in terms of corresponding correlation functions. It is found that the average OAM value is preserved during the propagation of the laser beam in a random medium. The dependence of the dispersion of OAM fluctuations on the atmospheric turbulence and beam parameters is calculated. It is shown that the dependence of the OAM dispersion on the initial angular momentum of the laser beam disappears in the case of very strong turbulence.

The dynamics of the envelope of a pulsed light beam with the transverse field profile described by the Bessel function is studied theoretically and experimentally. The beam is formed by an axicon from a pulse with the Gaussian spatiotemporal envelope. The appearance of a satellite pulse behind the axicon is predicted. The possibility of generating pulsed Bessel light beams with the required diameter and intensity of the central maximum in optical schemes containing an axicon and a lens is shown.

A device consisting of a pin-hole line and an apodizing aperture, which provides the fill factor of the laser amplifier aperture equal to 0.8, is proposed and studied experimentally. The use of this device allowed the generation of 300-J, 1-ns pulses due to the efficient energy extraction from a six-stage neodymium phosphate glass amplifier with the final stage aperture equal to 10 cm.

A new method for the wavefront reconstruction is described which is based on measurements of the beam intensity distributions in two cross sections and of wavefront tilts by a Hartmann sensor in one of these cross sections and makes it possible to find the field phase distribution taking into account possible phase jumps. The iteration algorithm for finding this distribution is developed and its mathematical description based on the calculation of the field in the Fresnel approximation is presented. The algorithm is studied numerically. Relations are obtained to estimate the optimal parameters of a measurement system, in particular, the distance between the cross sections and the number of subapertures of the Hartmann sensor depending on the mode composition of laser radiation.

The cross modulation method of transformation of the spatial coherence of low-power pulsed laser radiation in a nonlinear medium is proposed. The method is realised experimentally in a multimode optical fibre. The estimates of the degree of spatial coherence of radiation subjected to the phase cross modulation demonstrated the high efficiency of this radiation decorrelation mechanism.

The generation of nanosecond and subnanosecond pulses in a XeCl laser is studied. The short radiation pulses are generated in a resonator with a SBS mirror. By focusing laser radiation inside and on the surface of a nonlinear medium, it is possible to generate pulses of duration 3 ns and 150 ps, respectively. The laser beams obtained in this way contain more than 70% of energy within the diffraction angle and have the signal-to-noise ration exceeding 10⁴.

Highly efficient lasing at difference frequencies is obtained in a nematic liquid crystal excited by several visible lines from a cw argon laser with a total power of 0.08—1.5 W. The maximum conversion efficiency was ≈1% and the quadratic susceptibility was ≈2×10^-6 m V^-1. The field of application of the approximate mechanism of quadratic nonlinearity and frequency conversion considered in the paper requires specification. The nonlinear conversion of radiation in a nematic crystal has specific features compared to lasing at difference frequencies in solid crystals.

The phase-conjugation dynamics in a self-pumped phase-conjugate mirror (SP PCM) based on a BaTiO₃ photorefractive crystal (PRC) with a linear resonator is simulated numerically. It is shown that a phased array of dynamic holograms is formed in such a SP PCM under optimal conditions, and the interference of the waves scattered by this array provides the very high stationary phase-conjugation efficiency (the nonlinear reflection coefficient and overlap integral exceeding 80% and 95%, respectively). It is found that the development of instabilities is caused both by the violation of phase relations in this array and the formation of additional dynamic holograms in the PRC in the geometry typical of a SP PCM with a semi-linear resonator and by the self-bending effect typical of PRCs.

The role of the group velocity dispersion in the spectral compression of subpicosecond laser pulses is analysed based on numerical and experimental studies. It is shown that the group velocity dispersion in an optical fibre can substantially change the physical pattern of the spectral compression process.

A scenario of the experiment on the observation of the specific narrowing of nuclear radiative gamma lines in a Bose—Einstein condensate (BEC) due to the quantum coherence of atoms is proposed which could be used to confirm the appearance of the so-called megaatom state in the BEC. The expected narrowing of the gamma lines can occur due to the motion suppression of individual atoms and partial elimination of the inhomogeneous (Doppler) broadening. Quantitative estimates are performed, in particular, for the case of Mössbauer spectroscopy.

Spectral characteristics of laser systems used for precision two-photon spectroscopy of the 1s—2s transition in the hydrogen atom are analysed. The spectral properties of radiation from a 972-nm external-cavity semiconductor laser stabilised by external monolithic cavities of different configurations are considered in detail. The classical cavity scheme with a horizontal axis and a new scheme with a vertical axis, which is weakly sensitive to vibrations, are considered. The emission spectrum of a stabilised laser exhibits a narrow central peak and a pedestal caused by the residual phase noise. The semiconductor laser stabilised with respect to the vertical-axis cavity emits the line of width less than 0.45 Hz, which contains ≈99% of the total output power. The transformation of the emission spectrum upon frequency doubling in nonlinear crystals is investigated. The laser system developed in the study provides the narrowing of the recorded 1s—2s transition line in the hydrogen atom down to 2 kHz at 121 nm.

The energy characteristics of radiation of halides of inert gases excited by a volume discharge without additional preionisation are studied. The pressures of working mixtures and relations between the inert gas and halogen optimal for obtaining the maximum pulsed power and radiation efficiency are determined. The peak UV radiation power density achieved 5 kW cm^-2 and the radiation efficiency was ≈5.5%. The pulse FWHM was 30—40 ns.

An algorithm for the wind profile recovery from spatiotemporal spectra of a laser beam reflected in a turbulent atmosphere is presented. The cases of a spherical wave incident on a diffuse reflector of finite size and a spatially limited beam reflected from an infinite random surface are considered.