Table of contents

A new type of a femtosecond self-mode-locked erbium fibre laser is proposed and fabricated. The original hybrid design of the laser cavity taking advantage of ring and linear cavity lasers allows continuous tuning of the pulse repetition rate in a broad range (more that 30 kHz) and provides a high reliability of the self-mode-locking regime.

Lasing in a Cr²⁺:CdS crystal is demonstrated for the first time. The output power of a Cr²⁺:CdS laser pumped by a pulsed Tm:YAP laser at 1.94 μm achieved 4 mJ and the slope efficiency with respect to the absorbed pump energy was 39%. The continuous tuning was obtained from 2.2 to 3.3 μm in the laser with a dispersion prism resonator.

Repetitively pulsed generation (200 μs, 40 Hz) was obtained in a neodymium phosphate glass laser pumped by a 870-nm diode array. The maximum slope lasing efficiency with respect to the optical pump energy equal to 13% is restricted by the factor (≈0.23) of active-medium filling by the mode field. By adjusting the laser cavity, the single-transverse mode regime, in particular, the generation of the TEM₀₀ mode is obtained in the entire range of pump energies.

By focusing radiation from a F₂⁺:LiF colour centre laser (emitting in the range from 0.89 to 0.95 μm) on plates of a Yb:YAG crystal with the 20% concentration of Yb and of ytterbium glass with the 10% concentration of Yb, we observed nanosecond radiation pulses of Yb³⁺ ions in the spectral region from 1.00 to 1.06 μm with the spectral width up to 20 nm in Yb:YAG and up to 50 nm in the ytterbium glass. Lasing appeared in the active medium in the region of excitation of the SBS of pump radiation with a diameter less than 200 μm. The angular divergence of the broadband laser radiation (10^-3—10^-4 rad) was one—two orders of magnitude smaller than the diffraction limit. The mechanism of generation of short broadband high-directional laser pulses in the spatial structure of thin layers with inversion produced in the region of propagation of an intense acoustic wave in the medium is discussed. The interpretation of experimental data on the angular divergence of radiation is based on a new concept of the spatial distribution of the electromagnetic field of a photon not in the form of a travelling wave but with the field structures located in fixed positions along the propagation direction. The features of the temporal picture and narrowband lasing spectra of Yb:YAG and ytterbium glass in a resonator upon relaxation of the SBS excitation region in the active medium are considered. The possibility of diagnostics of medium parameters using the shift of spectra of the lasing at the resonator modes in different sites of the SBS excitation region is discussed.

A laser on the self-contained helium 2¹P₁⁰ — 2¹S₀ transition pumped by a pulsed electron beam generated in an open discharge is simulated and experimentally studied. Lasing without decreasing the pulse energy in a tube of diameter 31 mm was observed up to the pump pulse repetition rate of 10 kHz determined by the parameters of a power supply. Dynamics of the electric field in an accelerating gap and populations of the working levels of helium are calculated. The radiation power calculated in accordance with its radial distribution measured in the saturated amplification regime is 7.85 times higher than that in the lasing regime. The practical laser efficiency of 0.056% is achieved for the quantum efficiency of 0.7%.

The emission power of zinc selenide plates placed in the air gap between electrodes is determined as a function of the amplitude of applied high-voltage nanosecond pulses and the gap width. The laser emission spectra of samples excited by an electron beam and an electric field are presented. The influence of the electrode shape on the emission pattern is studied. An increase in a laser pulse duration compared to the duration of an exciting high-voltage pulse was observed. It is assumed that this is explained by recombination emission processes proceeding in a dense electron—hole plasma.

The method for measuring the rate constants of the VV' exchange between CO and O₂ molecules is developed. The method is based on a comparison of the measured and numerically calculated small-signal-gain (SSG) dynamics in the active medium of a pulsed electron-beam-controlled cryogenic CO laser. The SSG dynamics was measured in CO—He—O₂ mixtures with the fixed CO:He = 1:4 ratio by varying the oxygen content from 0 to 4%. The SSG dynamics was measured by probing the active medium of the pulsed CO laser by a beam from a tunable cw CO laser. The mathematical model of the CO laser takes into account the influence of oxygen on energy exchange processes. The parameters of the analytic approximation for constants of the near-resonance VV' exchange CO(v)+O₂(u=0) → CO(v-1)+O₂(u=1) are determined by the developed method for quantum vibrational numbers v = 18—24. In particular, the rate constant for v = 20 is (3.1±0.5)×10^-12 cm³ s^-1 at the gas temperature of 150 K. The extrapolation of the obtained analytic expression for rate constants in gases at room temperature gives the reasonable agreement with the measurements performed earlier for v = 12 and 13.

We studied mechanical, optical, and lasing parameters of neodymium-doped yttrium oxide ceramics synthesised by using a new technology involving the laser synthesis of nanopowders and their magnetic pulsed compaction. The fracture toughness of ceramics to cracks and its microhardness were measured to be K_IC = 0.9—1.4 MPa m^1/2 and H_ν = 11.8 GPa, respectively. Ceramic samples sintered in the temperature range from 1550 to 2050°C have the porosity (1—150)×10^-4% and the optical loss coefficient α_1.07 = 0.03—2.1 cm^-1 at a wavelength of 1.07 μm. It is shown that such porosity does not affect the optical loss coefficient of light. Lasing at ≈1.079 μm with a slope efficiency of 15% was obtained in a 1.1-mm-thick sample pumped by laser diodes.

The effect of the phase difference of the coupling coefficients on relaxation frequencies in the emission spectrum of a solid-state ring laser operating in the self-modulation regime of the first kind is studied theoretically. A strong dependence of one of the frequencies of relaxation oscillations on the phase difference of coupling coefficients is found. The stability of the self-modulation regime is studied analytically.

The phase conjugation (PC) of a focused Gaussian laser beam with a partial spatial coherence of the wave front is studied numerically and theoretically upon SBS within the framework of a three-dimensional nonstationary SBS model, which takes into account transient processes and SBS saturation. The dependences of the PC coefficient h on the laser radiation power are obtained for different excesses of the angular divergence over the diffraction limit ξ. It is found that for the given reflectance of laser radiation from the SBS mirror, the PC coefficient monotonically decreases with increasing the divergence. For example, under the near threshold SBS conditions, h decreases from 70% to 30%, when ξ increases from 1 to 10. For the given divergence, the PC coefficient increases with increasing the reflectance and approaches the ideal one (h > 90%) upon deep SBS saturation, when the reflectance exceeds 90%—95%.

The possibility of efficient generation of difference-frequency radiation in the far- and mid-IR ranges in a two-chip laser based on gallium arsenide grown on a germanium substrate is considered. It is shown that a laser with a waveguide of width 100 μm emitting 1 W in the near-IR range can generate ≈40 μW at the difference frequency in the region 5—50 THz at room temperature.

The propagation of intensity-modulated laser radiation in a barium—sodium niobate crystal is studied in an external electric field. The possibility of controlling a nonlinear local response of the crystal is demonstrated. It is shown experimentally that the conditions of formation of a one-dimensional spatial soliton can be changed by varying the nonlinear response of the crystal.

The formation and collisions of breathers excited by laser radiation at the inhomogeneously broadened J = 0 → J = 1 quantum transition are studied by numerical simulations in the slowly varying envelope approximation. Conditions are obtained under which laser pulses with the initial shape quite simply realised in experiments can be transformed into elliptically polarised breathers in the medium, each of the components of their field being a breather described by the theory of self-induced transparency at a nondegenerate quantum transition. It is shown that the collision of such breathers is not elastic in the general case and leads to the appearance of more general types of resonance breather-like pulses. Taking into account relaxation processes, the possibility of the formation of a breather at the 6p²³P₀ → 6p7s ³P₁ transition in the ²⁰⁸Pb isotope is investigated. It is found that relaxation in some case not only causes the pulse decay but also changes the eccentricity of its polarisation ellipse.

The development of a longitudinal—transverse instability in a plasma with the anisotropic velocity distribution of electrons produced by the tunnelling ionisation of a target by a short laser pulse is studied. The dependences of the growth rate of this longitudinal—transverse instability on the wave number and the propagation angle of perturbations are investigated. It is shown that the increasing longitudinal electrostatic field in a rather broad angular region is comparable with the exciting magnetic field. It is pointed out that the longitudinal—transverse instability of the anisotropic plasma can lead to the additional absorption of laser radiation by the nonthermal fluctuations of the electron density.

Simple analytic expressions are derived to approximate the bit error rate for data transmission through fibreoptic communication lines. The propagation of optical pulses is directly numerically simulated. Analytic estimates are in good agreement with numerical calculations.

A new method for analysing the transmission and scattering of the guided TE mode in an inclined reflector located in an optical waveguide is proposed and studied. The reflection of an inhomogeneous optical beam from the inclined reflector is described semi-analytically for the first time by using the theory of coupled waves and taking into account the interrelation and transformation of energy between all the waves of the discrete and continuous spectra of the optical 2D-waveguide (even and odd guided, radiation, and evanescent waves). The results of calculations of the propagation of light through the inclined reflector in the form of a thin (10—500 nm) homogeneous strip obtained by our method and by the finite difference time domain (FDTD) method are in excellent quantitative agreement. The calculation rate of our method considerably (by one—two orders of magnitude) exceeds that of the FDTD method and our method has a better accuracy.

The lifetimes of rubidium atoms in a dark magneto-optical trap are measured at different populations of the 'bright' and 'dark' hyperfine states of captured atoms. It is found that the lifetime of atoms in the trap decreases if they spend more time in the bright state. A simple explanation of this effect is proposed which is based on the increase in the transport cross section for collisions of thermal rubidium atoms surrounding the trap with cold rubidium atoms upon their electronic excitation.

The problem of the wind velocity profile reconstruction from random intensity fluctuations of a laser beam diffusely scattered by a screen and received by a telescope is considered. Expressions for a spatiotemporal correlation function and spectrum of weak intensity fluctuations of the reflected beam are presented. An algorithm is proposed for the reconstruction of the profile and direction of the wind velocity from the intensity fluctuation spectra of reflected radiation received by the telescope. The efficiency of the algorithm is confirmed by the results of numerical experiments.