Table of contents

A high space — time resolution is achieved in detection of radiation from a Ti:sapphire laser by the methods of femtosecond photoelectronics. A time resolution of 2 × 10^-13 s is obtained at wavelength of 800 nm in a linear scan regime for a streak speed of 5 × 10¹⁰ cm s^-1, an input power density of (1 — 5) × 10³ W cm^-2, and a linear dynamic detection range no more than 10.

It is shown that the imposition of external magnetic field on a gas of ionised particles may give rise to inversionless radiation amplification by the ions having certain velocities, which is due to their Larmor rotation. In this case, the majority of ions may reside in the ground state. The integrated absorption coefficient remains positive due to an increase in the radiation absorption by the other half of ions.

The accelerated ambipolar particle diffusion is simulated in the afterglow of discharges in He — Cu and Ne —Cu mixtures. The calculations showed that a heating field E ≈ 0.3 — 1.3 V cm^-1 imposed on the afterglow discharge at a buffer gas pressure of 8 — 10 Torr and a tube diameter of 4 — 10 mm reduced the electron density by a factor of 10² — 10⁴ for ~100 μs. The effect was shown to be most pronounced in helium. According to the calculations, the application of accelerated ambipolar diffusion during the interpulse period will increase the pulse repetition rate of copper vapour lasers with low-diameter tubes (4 — 6 mm) and low buffer-gas pressures (6 — 8 Torr) by 20% — 50%. The production of excessive prepulse density of the metastable states of copper can be prevented by interrupting the plasma heating 3 — 6 μs before the onset of the next excitation pulse.

The possibility of observation of the hidden nuclear inversion below the lasing threshold from a change in the optical spectra of atoms containing the nuclei is considered.

The temperature regime of a laser cathode-ray tube are studied upon the electron-beam raster scanning. Three main factors determining the temperature of the active region of a laser are distinguished: the adiabatic heating by the electron beam during excitation of an individual pixel, heating caused by adjacent lines, and heating averaged during previous scans, which is determined by cooling of the laser screen. The numerical estimates of the temperature rise caused by these factors are presented. The influence of the temperature of the active region on the lasing power is discussed.

The characteristics of single-frequency lasers are investigated experimentally and theoretically. It is shown that the model of spectral hole burning with a varying interband relaxation time adequately describes the spectral and modulation characteristics of the laser (taking into account the transport of carriers). The time of carrier capture in a quantum well is 4 ps and the time of their escape is 80 ps.

Focusing femtosecond radiation with a refracting axicon lens is stimulated within the framework of the second approximation of the dispersion theory, using the Kirchhoff — Fresnel method. It is shown that, within the angles considered (up to 0.2 rad), an increase in the pulse duration due to its dispersive spreading inside the axicon has the major effect on the field distribution under study, while the angular dispersion is rather small.

A solution to the key problem of femtosecond metrology — elimination of the frequency offset related to the intracavity dispersion of a femtosecond laser — is proposed. The proposed method involves stabilisation of the intermode interval between equidistant spectral components in a frequency comb produced by a mode-locked femtosecond laser by phase-locking the frequency difference between a pair of discrete spectral components in this comb to the frequency of a reference laser. An introduction of a nonlinear-optical crystal for frequency doubling into the scheme for frequency-comb stabilisation allows the frequency offset related to the intracavity dispersion of the femtosecond laser to be eliminated, thus suggesting the way for absolute stabilisation of frequency combs generated by femtosecond mode-locked lasers. Radiation of a reference laser with such an approach plays the role of an anchor in the femtosecond clockwork.

An expression is obtained for approximate calculation of optical coupling coefficients for lasers in a 1D array phase-locked with a focal spatial filter. The phase-locking region for three lasers is found. The phase-locking efficiency is shown to be substantially dependent on the filter transparency. The results of calculations agree with the experimental data.

Computer simulation and experimental investigations of the dynamics are carried out for a solid state laser with an intracavity nonlinear reflector. The nonlinearity appears upon the internal reflection of radiation from the interface between a transparent dielectric and an absorbing liquid due to a change in the refractive index of the latter upon its heating by a refracted laser wave. Our calculations reveal the dynamics of the reflection coefficient and the power of the laser radiation taking into account the variation of temperature and pressure in the boundary layer of the liquid. The dependences of the lasing parameters on the parameters of the nonlinear reflector and pumping power are studied theoretically and experimentally. It is shown that a Q switch based on the thermal nonlinearity of reflection provides the generation of giant laser pulses whose duration varies from a few hundred nanoseconds to a few nanoseconds. Such a Q switch can be fabricated for any spectral region because it is based on a linear absorber rather than on a saturable absorber. Another advantage of this Q switch is the absence of residual absorption, which is a characteristic feature of all phototropic Q switches.

The nonreciprocity of the characteristics of counterpropagating waves in a ring resonator formed by totally reflecting prisms is analysed. The polarisation-inhomogeneous properties of a prism resonator and their effect on the nonreciprocity of the frequencies of counterpropagating waves are studied.

The acceleration of a plasma jet by an optical pulsating discharge is studied. The optical discharge generated periodic shock waves or shock waves combined in periodic trains. For a pulse repetition rate f > 50 kHz, the line at frequency f dominated in the spectrum of periodic waves. The spectrum of trains following with the frequency F₀ ≪ f contained an ultrasonic component and a low-frequency component. The trains produces a strong acoustic effect. The average power of the waves amounted to 160 W, and the conversion efficiency of laser radiation to shock waves was ~10% — 25%.

A mechanical damage of transparent solids caused by local laser-produced heating is considered. Applicability of thermoelasticity equations to the analysis of the mechanical stage of laser damage is substantiated for both short and ultrashort pulses. Damage criteria have been obtained for the crack-formation mechanism. On the basis of these criteria, the conditions for the transition from the crack-formation damage to defect generation and ablation have been clarified. The analysis can be applied to both intrinsic (impact and multiphoton ionization) and extrinsic (initiated by absorbing inclusions) absorption mechanisms of laser radiation.

Ultraviolet radiation is generated in two lithium tetraborate (Li₂B₄O₇ or LB4) crystals by Type I walk-off compensated sum-frequency mixing (SFM) of the commonly available Nd:YAG laser radiation and radiation from a dye laser pumped by the second harmonic of the same YAG laser. In the walk-off compensated configuration, an enhancement in the conversion efficiency by a factor of 3.8 relative to the single pass generation is realised.

A numerical model is proposed for the interaction of ionising and UV laser radiation with CaF₂ crystals. It is shown that the appearance of short-lived electronic excitations in the crystal has a strong effect on the nonlinear absorption of laser radiation for pulse durations exceeding 30 ps. The consequences of this absorption for the crystal structure are analysed.

The effect of background light on the sensitivity of a laser receiver with an iodine active quantum filter (λ=1.315 μm) was theoretically and experimentally investigated. Upon the reception of a 40-ns pulsed signal against the background of 2.5-fold attenuated radiation of a pulsed light source with a brightness temperature of 4 × 10⁴ K, the sensitivity of this receiver for a signal-to-noise ratio of three and a diffraction-limited acceptance angle was experimentally shown to be equal to 20 photons, which exceeds the quantum limit by about a factor of two. This is consistent with the results of theoretical treatment and suggests that upon the detection of optical signals against the background of the solar disk, the sensitivity of this receiver should decrease by only 12%. This receiver was compared with a receiver employing a photomultiplier of the visible range. Upon the reception of optical signals with the same parameters against the background of the solar disk and an interference filter with a transmission band width of 5 nm, the sensitivity of a receiver equipped with an FEU-115 photomultiplier was shown to be equal to about 1400 photons for a signal-to-noise ratio of three.

An electrodynamic problem of laser radiation scattering in an integrated-optical waveguide containing small statistical irregularities (interface roughness and irregularities of the refractive indices of the waveguide-forming media) is considered. The possibility of using the waveguide scattering of laser radiation for extracting the information on the statistical properties of irregularities from noisy data of the scattering diagram in a far-field zone is shown. An algorithm for reconstructing the autocorrelation function of irregularities for the correlation interval changing within a wide range is described. The possibility of restoring a given Gaussian autocorrelation function that describes statistical irregularities of the waveguide substrate surface for a correlation interval changing between 10 nm and 10 μm and a high-level additive white real noise is shown by computer simulation.

The impedance of a modal multichannel wavefront corrector is studied theoretically and experimentally for the first time. The theoretical model takes into account the dependence of liquid crystal parameters on the control voltage frequency. These dependences were measured separately. It is found that the equivalent capacitance and resistance of the corrector in the frequency region up to 1 kHz are determined by the ionic conductivity of the liquid crystal, whereas at higher frequencies the frequency dependence of the local voltage comes into play. The theoretical results well agree with the experimental data.

The tangential thermal diffusivity D_|| and thermal conductivity k_|| of diamond plates grown from the gaseous phase by chemical vapour deposition (CVD diamond) are determined by the transient grating technique in the temperature range 25 — 200 °C. Samples of insulating and boron-doped polycrystalline diamond of thickness about 0.3 mm and diameter 63 mm were synthesised in a microwave discharge in mixtures of methane and hydrogen. In view of the intense light scattering by the samples, a photosensitive grating recording technique was developed and used for measurements. It was found that the value k_|| amounts to 18 — 20 W cm^-1 K^-1 at room temperature, approaching the thermal conductivity of the highest purity single crystals of diamond. A comparison of the value of k_|| with the normal thermal conductivity k determined by the flash method reveals a thermal conductivity anisotropy of about 10% — 20% associated with the texture of the diamond film, the normal component of thermal conductivity being larger than the tangential component. Boron-doped diamond displays a dependence of the transient grating kinetics on the excitation wavelength. The obtained results indicate that CVD diamond is a promising material for preparing efficient heat sinks, especially of large size, used in microelectronic devices and laser engineering.