Table of contents

The great men do not die. They pass to eternity. Alexander Mikhailovich Prokhorov, who would be 90 on 11 July 2006, passed to eternity in January 2002.

A silica Bragg fibre with optical losses lower than 10 dB km^-1 is fabricated for the first time. The Bragg fibre manufactured by the MCVD method is intended for operation at a wavelength of 1.06 μm and has the mode-spot diameter 18.5 μm (the mode-spot area is 270 μm²). The fibre is considerably less sensitive to bending than step-index fibres and microstructure fibres with the same mode-spot size. The possibility of fabricating a Bragg fibre with the record mode-spot area (530 μm² at the operating wavelength of 855 nm) for all-silica fibres is demonstrated.

Single-mode cw lasing was obtained for the first time in Yb³⁺ : GGG crystals at room temperature at 1.030 and 1.037 μm upon pumping Yb³⁺ ions by a 0.925-μm neodymium-doped fibre laser. Lasing was observed at the transitions between the lower Stark sublevel of the ²F_5/2 level and the upper Stark sublevels of the ²F_7/2 ground state of Yb³⁺ ions. The study of lasing characteristic showed that lasing occurs according to the four-level scheme. The lasing threshold with respect to the absorbed power was 85 mW and the slope lasing efficiency was 20%. Upon 1.7-W pumping, the output power of 310 mW was achieved with the output resonator mirror with the transmission coefficient of 1%. The effective cross section of the lasing transition was 0.9×10^-20 cm².

CW lasing is obtained in Er³⁺(5%) : CaF₂ and Er³⁺(5%) : SrF₂ crystals near 2.75 μm with 0.4 and 2 W of output powers, respectively, upon transverse diode laser pumping into the upper ⁴I_11/2 laser level of erbium ions at 980 nm. Continuous tuning of the laser wavelength between 2720 and 2760 nm is realised in the Er³⁺ : SrF₂ crystal.

The spectral and kinetic properties of yttrium—aluminium garnet crystals co-doped with Er³⁺ and Yb³⁺ are studied in the range from room temperature to 800 °C. It is found that the lifetime of the ⁴I_11/2 level of the erbium ion decreases approximately by a factor of twenty with increasing temperature and the efficiency of nonradiative energy transfer from Yb³⁺ to Er³⁺ increases considerably. Analytic expressions describing the spectral and temperature dependences of the absorption and emission cross sections for the transition between the ⁴I_13/2 and ⁴I_15/2 levels of the erbium ion are proposed.

The spectral and lasing parameters of Yb³⁺ ions in zirconium dioxide crystals with a stabilised cubic structure (fianites) are studied. It is found that the absorption and luminescence spectra of Yb³⁺ in these crystals are characterised by a considerable inhomogeneous broadening. The nature of this broadening is investigated and the model of optical centres consistent with the properties of the inhomogeneous spectra of Yb³⁺ in the cubic zirconium dioxide crystal is proposed. It is shown that fianites can be used in solid-state lasers. Lasing in Yb³⁺ ions in diode-pumped fianite crystals was obtained and studied.

An amplifier of picosecond pulses with an output power up to 10¹⁰ W and an energy up to 30 mJ at 1180 nm is developed on the basis of F₂^- : LiF colour-center crystals. A 3—5-ps, 0.03-mJ probe pulse at 1.18μm was obtained upon intracavity SRS conversion in a passively mode-locked Nd³⁺ : KGd(WO₄)₂ laser. The F₂^- : LiF crystals were pumped by 1053-nm nanosecond pulses from a Nd : YLF laser, amplified in a GLS-22 phosphate glass to an energy of 5 J. The probe SRS pulses were amplified in a four-crystal two-cascade amplifier based on F₂^- : LiF crystals with the total length of the active medium of 360 mm, by using counterpropagating pump beams. The dependences of the output radiation energy on the pump and input signal energies are measured.

The spectra of coherent librations of H₂O molecules, coinciding in frequencies with the rotational spectrum of gaseous H₂O, are observed for the first time in aqueous solutions of α-chymotrypsin protein and hydrogen peroxide and in deionised Milli-Q water by the method of four-photon laser scattering in the frequency range 0—100 cm^-1. The resonance contribution of molecular librations to the four-photon scattering signal considerably increases in aqueous solutions compared to water. The resonances related to the lines of the ortho- and para-modifications of the natural isotope of the H₂O molecule in the gas phase are recorded in the four-photon scattering spectra. It is found that the protein molecule in aqueous solution selectively interacts with the para-H₂O, which makes it possible to interpret some features of the interaction of microwave radiation with biological objects and to develop a new class of laser biotechnologies.

The efficient technology is developed for growing high-quality doped single-crystal fibres by the mini-pedestal method with laser heating. The technology can be used to fabricate high-quality doped single-crystal fibres with a controllable radial gradient of the refractive index. The refractive-index profile in single-crystal fibres is studied experimentally. The efficient operation of a Nd³⁺ : YAG single-crystal fibre laser in the waveguide regime was demonstrated upon longitudinal pumping by a laser diode.

The efficient technology is developed for growing Cr- and Ca-doped gadolinium—gallium garnet single-crystal films of thickness up to 100 μm on large substrates (76 mm) by the method of liquid-phase epitaxy from a solution—melt. The dependences of the absorption spectra of films and optical losses at 1 μm on the growth conditions and the solution—melt composition are studied. It is shown that Cr⁴⁺ ions are formed in the films, and these films can be used as passive Q switches in lasers.

A brief review of recent theoretical and experimental studies of multipulse laser drilling keyholes in polymers is presented. The stationary keyhole profile is obtained after irradiation by a quite great number of laser pulses due to self-organisation processes and is a new more stable state of the surface irradiated by intense light. This concept together with ample experimental data obtained for various polymers form the basis of the analytical model of multipulse drilling holes with a superhigh depth — diameter ratio (300—600) (the so-called aspect ratio) by using UV excimer KrF laser radiation. The model reveals the main factors controlling the parameters of drilling holes by nanosecond UV pulses and, in particular, determines the conditions for drilling holes with virtually parallel side walls and very high aspect ratios.

We report our experimental investigations of the formation and development dynamics of laser plasma produced in gas microvolumes (microplasma) upon multiple ionisation by tightly focused (to a spot 2—3 μm in diameter) high-intensity (up to ≈10¹⁷ W cm^-2) femtosecond pulses of a Ti:sapphire laser (τ_p ⋍ 130 fs, λ = 800 nm). Precision interferometric measurements (with a spatial resolution of ≈1.5 μm) were made of the spatiotemporal distribution of the refractive index and electron density in the microplasmas of the air and helium immediately during the action of the exciting femtosecond laser pulse and at the initial stage of free plasma expansion. The microplasma formation was shown to occur as a result of almost complete (up to bare nuclei) ionisation of the initial gas. For the first time the spectral continuum and the dynamics of spectral line formation in the UV and visible spectral ranges were investigated with a picosecond time resolution for the femtosecond laser-produced microplasmas of the air, N₂, Ar, and He at normal conditions. For the first time the generation of the second (even) laser radiation harmonic was recorded in a femtosecond subcritical-density plasma of gases.

A self-sustained volume discharge in SF₆ mixtures with C₂H₆, He, and Ne preliminarily irradiated by CO₂ laser pulses was investigated. The radiation energy density absorbed by SF₆ in the discharge ignition region amounted to 6.5 J atm^-1 cm^-3. The discharge structure and the current distribution in the discharge gap were found to change radically with increasing the time delay between the laser and discharge pulses. In particular, brightly glowing narrow channels are formed at the boundary of the irradiation region. The observed effect is shown to arise from the development of a shock-wave process due to a temperature jump at the boundary between the irradiated and unirradiated gas. The velocities of shock wave propagation and the main thermodynamic gas parameters in the perturbation region were calculated. A comparison was made between the calculated and measured velocities of the shock waves.

The propagation of light in restricted homogeneous systems of tunnel-coupled waveguides is studied. The Fourier analysis of modes at the output of the waveguide system depending on the end-excitation angle of waveguides revealed the region of excitation angles corresponding to excitation of Bragg modes. The expressions for the Bloch vector for a great number of separate waveguides are presented which describe the reflection of light in a periodic layered medium. It is shown that the Bragg diffraction of light waves in film lasers can considerably change their radiation parameters.

The radiation characteristics of a planar CO₂ laser excited by a diffusion-cooled rf discharge at a frequency of 40 MHz are studied. A single-mode cw lasing power of ≈50 W is achieved with an efficiency of ≈10% for a nearly diffraction-limited radiation divergence of 4—7 mrad. The spatial structure, output power, stability and laser radiation quality are studied as functions of longitudinal and angular alignments of the resonator mirror for two types of hybrid unstable-waveguide resonators of the laser. It is shown that for the resonator corresponding to the negative branch of the stability diagram, a misalignment of 0.02 rad of the mirrors leads to a 50% decrease in the output laser power, while its value for the positive branch resonator is about 100 times smaller. It is found that for the resonator corresponding to the negative branch, the sensitivity to the violation of confocal arrangement of the mirrors upon an increase in the resonator length is an order of magnitude higher. The dependence of the density of input rf power on the working gas pressure is studied experimentally in the interval 50—110 Torr. Power density values of 1—4 W cm^-2 are obtained for normal discharge current density. These values are important for optimisation and scaling of the lasing characteristics of high-power planar CO₂ lasers.

Stimulated Compton scattering of counterpropagating laser beams in a moving plasma is studied theoretically. It is shown that, by using Compton scattering, picosecond or femtosecond laser pulses can be amplified under certain conditions by two-three orders of magnitude.

A new approach to the development of a laser jet engine (LJE) is considered which is based on the resonance merging of shock waves generated by an optical pulsed discharge (OPD). The OPD can be produced by using high-power 150—250-ns, 20—200-J laser pulses with a pulse repetition rate of 50—100 kHz. The OPD is formed with the help of a reflector array. This makes it possible to increase the efficiency of utilising laser radiation by a few times, to avoid strong shock loads in the LJE, to eliminate the thermal action of a laser plasma on a reflector, and to reduce the screening of laser radiation by the plasma. The possible thrust of the LJE based on this mechanism is estimated.

Experimental data of Lang and Leiderer [New J. Phys. 8, 14 (2006)] on the movement of a thin liquid film due to its explosive boiling on a substrate heated by a nanosecond laser pulse are analysed. It is shown that the maximum value of the pressure pulse produced in this case without any singularities may be much lower than the estimate obtained by these authors and is in good agreement with the assumption that the explosion is initiated in the vicinity of the spinodal.

A high-resolution spectroscopic method is developed for recording Raman spectra of molecular transitions in transient objects during a laser pulse with a resolution of ≈0.1 cm^-1. The method is based on CARS spectroscopy using a Fabry—Perot interferometer for spectral analysis of the CARS signal and detecting a circular interferometric pattern on a two-dimensional multichannel photodetector. It is shown that the use of the Dual-Broad-Band—CARS configuration to obtain the CARS process provides the efficient averaging of the spectral—amplitude noise of the CARS signal generated by a laser pulse and, in combination with the angular integration of the two-dimensional interference pattern, considerably improves the quality of interferograms. The method was tested upon diagnostics of the transient oxygen—hydrogen flame where information on the shapes of spectral lines of the Q-branch of hydrogen molecules required for measuring temperature was simultaneously obtained and used.