Table of contents

Continuous emission was obtained from a phase-locked (in an external cavity) linear array of laser diodes with a power in excess of 10 W. This was done by using an efficient heat sink based on a porous structure with a thermal resistance better than 0.3 °C W^-1. For a power of 1.5 W, a divergence of the lobes was ~0.1 mrad in the plane of the p—n junction, corresponding to the diffraction limit at the radiation wavelength λ=0.94 μm for an aperture of 1 cm. The promising trends in the development of technologies based on laser-diode arrays are considered.

A theoretical model of superradiant pulse generation in semiconductor laser structures is developed. It is shown that a high optical gain of the medium can overcome phase relaxation and results in a built-up superradiant state (macroscopic dipole) in an assembly of electron—hole pairs on a time scale much longer than the characteristic polarisation relaxation time T₂. A criterion of the superradiance generation is the condition αc_mT₂>1, where α is the gain coefficient and c_m is the speed of light in the medium. The theoretical model describes both qualitatively and quantitatively the author's own experimental results.

The line width and the emission frequency of an atom located near the vertex of an ideally conducting cone or inside a conical cavity in an ideal conductor are analysed. It is shown that the influence of the vertex diminishes with decrease in the vertex angle. On the other hand, the line width and the emission frequency of an atom located in a conical cavity may both increase and decrease greatly, depending on the position of the atom and on the vertex angle of the cavity. The results obtained may prove useful in the development of both monatomic microlasers and spectrally selective near-field nanoscopes.

A description is given of a high-power excimer laser system. The first results are reported of the amplification of a beam with a divergence close to the diffraction limit. Beyond the preamplifier, the 5 cm × 6 cm laser beam contains 50% of the radiation energy in the diffraction core. At the output from the system an irradiation brightness of 2 × 10¹⁴ W cm^-2 sr^-1 was obtained. A numerical model of the amplification of the radiation, taking into account the influence of the amplified spontaneous emission, was devised. It demonstrated a satisfactory agreement between the calculated and experimental data.

The energy characteristics and the spatial distributions of the radiation generated in the free-running lasing regime were investigated for a configuration (proposed previously by the authors) with compensation of the optical inhomogeneities in the active media of powerful solid-state lasers. Slab active elements made of Nd³⁺:KGd(WO₄)₂ and Nd³⁺:NaLa(MoO₄)₂ crystals were used. A uniform transverse intensity distribution in an output beam (37 mm × 8 mm in cross section) and a nearly limiting angular divergence were achieved for a pulse energy in excess of 60 J, a repetition rate up to 2 Hz, and a slope efficiency of 2.5%.

The spectrotemporal and energy characteristics of the radiation of a pulsed chemical H₂—F₂ laser were calculated on the basis of a multilevel model taking into account the nonequilibrium population of the rotational levels, and were compared with experimental data. The theoretical model took into account the stimulated emission of radiation as a result of the ν, j−1 → ν−1, j transitions (ν = 1–6 and j=4–12) in the HF molecule. The rotational relaxation rate constants of the HF molecule were varied in the calculations. Comparison of the calculated and experimental spectral characteristics of the laser radiation indicates that the rate of the rotational transitions involving high (j=10–12) rotational sublevels of HF is insufficient, leading to an appreciable reduction in the energy of the output radiation for lines with such values of j, compared with lines characterised by j=7, 8.

An algorithm for the correction of the atomic lines in the spectrum of a laser plasma was developed for determination of the depth profile of the elemental composition of layered materials in an ablation crater. The application of this algorithm made it possible to eliminate the profile smoothing effect as a consequence of a change in the line intensity as the crater becomes deeper. The correction of the profile is illustrated by an example of measurement of the thickness of a diffusion layer of zinc-plated sheet steel before and after its heating at 452°C.

An analytical solution is obtained to the problem of propagation of a 2-D nonlinear heat conduction wave from a cylindrical energy source, which acts in a planar layer of a material surrounded by a medium with different mass density and degree of ionisation. A theoretical justification is given of several interesting phenomena of 2-D thermal wave propagation through an inhomogeneous medium. These phenomena are related to the difference between the thermal wave velocities in the media with different thermal diffusivities. When the mass density in a layer experiencing the action of an energy source exceeds the density of the surrounding medium, the thermal wave front is shown to glide along the layer boundaries with a spatial velocity exceeding the velocity of the wave inside the layer. Moreover, there is a possibility of 'themal flow' of a layer across the boundaries between the layer and the surrounding medium in front of a thermal wave propagating inside the layer. The problems of heat transfer in multilayer targets for laser thermonuclear fusion are considered as an application.

It is proposed that narrow optical resonances be employed as reference points in the stabilisation of the frequency components of a laser continuously emitting ultrashort pulses. Such a laser has a broad spectrum of stable frequency components and a high pulse power, which permits a sharp increase in the frequency-conversion efficiency. A system for the stabilisation of a Ti:sapphire laser generating ultrashort pulses is examined. This system is based on a two-photon absorption resonance as a result of the 1S—2S transition in the hydrogen atom.

A method for the approximate analytical calculation of the parameters of laser output radiation is considered. The aperture truncation of the optical cavity mirrors is taken into account within the framework of the Gaussian formalism. The influence of the aperture constraints is described with the aid of equivalent Gaussian stops. The distances from the focusing lens to the beam waist, measured experimentally and calculated by the proposed analytical method, are compared. Analytical expressions are proposed for the Gaussian beam parameters, approximating the beam from a laser with aperture-truncated cavity end mirrors.

The characteristic features of the self-reflection of a powerful electromagnetic wave in a system of coherent excitons and biexcitons in semiconductors were investigated as one of the manifestations of the nonlinear optical skin effect. It was found that a monotonically decreasing standing wave with an exponentially falling spatial tail is formed in the surface region of a semiconductor. Under the influence of the field of a powerful pulse, an optically homogeneous medium is converted into one with distributed feedback. The appearance of spatially separated narrow peaks of the refractive index, extinction coefficient, and reflection coefficient is predicted.

For a photoionised gas in the mode of suppression of the ionisation barrier, it is shown how the photoelectron distribution function in the field of high-intensity radiation is determined by the distribution of intra-atomic electrons prior to their ionisation. The partial nonlinear conductivities of the current of the harmonic generation source are determined. It is established how the harmonic intensities scale up with the harmonic order number and the intensity of the pump radiation field.

Self-formation of one or several soliton-like pulses from a nonsoliton powerful laser pulse during its propagation through a two-component resonant amplifying—absorbing medium with an energy dissipation channel is studied numerically. Both components of the medium are simulated by two-level particles. Homogeneous mixtures of these components and media with consecutive amplifying and absorbing layers are considered. A qualitative analysis of the results is based on a simplified model of the process of soliton-like pulse propagation.

The relationships governing nonlinear transient transmission and reflection of ultrashort resonant laser radiation pulses with rectangular and Gaussian profiles by a thin semiconductor film were investigated taking into account the exciton—photon interaction and the concentration enhancement of the dipole moment of the exciton transition. It is shown that the transmission of a Gaussian pulse is characterised by the formation of two subpulses with identical areas, which are separated in time and have different profiles.

The optical properties of a new promising nonlinear mixed AgGa_xIn_1-xSe₂ crystal were investigated. Its absorption coefficients and refractive indices were determined at wavelengths 0.8 — 19 μm. For the first time the coefficients of the Sellmeier dispersion equations were determined for different indium contents. The corresponding phase-matching diagrams for second-harmonic generation and an optical parametric oscillator pumped by a Er³⁺:YAG laser were calculated and plotted. An experiment on the generation of the second harmonic of a TEA CO₂ laser and a Q-switched laser was carried out and a frequency doubling efficiency of 10.9% was obtained. AgGaSe₂ and ZnGeP₂ crystals were also investigated for comparison.

It is shown theoretically that, when a laser beam is focused by an axicon (a conical lens) in a Kerr-type nonlinear medium with nondissipative saturation of a square-root nonlinearity, an extended homogeneous optical channel may form and its length may be comparable with the length of the axicon caustic.

The nucleation and random movement of a nonlinear moving focus in a turbulent atmosphere during the propagation of a high-power subpicosecond laser pulse was investigated for a medium with fluctuations of the refractive index. It was shown by statistical sampling methods that under turbulent conditions the length corresponding to the establishment of a nonlinear focus in which the intensity reaches the ionisation threshold is on average reduced compared with the similar length in a regular medium. The focus, moving along a random path, outlines in air a 'curved' filament, the random displacement of which may reach several millimetres in the cross-sectional plane.

Problems of the precision measurements of spectral line parameters by diode-laser spectroscopic methods were examined. The instrumental function of a diode laser spectrometer was investigated in detail. The knowledge of the latter makes it possible to solve correctly the problem of the 'optical zero' and to reconstruct the true line profile with an error less than 1%. The instrumental function of a diode laser spectrometer was determined for the example of a laser diode emitting radiation at the wavelength λ =1.53 μm. The function represents the convolution of two Lorentzian functions with the characteristic half-widths Δν_l and Δg, which vary not only from mode to mode but also within the limits of a single radiation mode.

A highly sensitive detection system with time-resolved photon counting was developed for modulation optical tomography (diode laser with a power of 15–30 mW, modulation frequency 100 MHz) of extended scattering objects. Experiments with model objects (linear dimensions up to 140 mm, absorption coefficients of 0.005–0.015, scattering coefficients of 1.4 mm^-1) showed that the propagation of optical radiation in such media is described by a relative path-lengthening coefficient which varied in the range 1.2–4.2, depending on the absorption coefficient and on the size of the object. A rapid projection algorithm for the solution of the inverse problem was used in the reconstruction of images of strongly absorbing inclusions known as 'phantoms' (cylinders 10–23 mm in diameter). An algorithm was developed for the solution of the direct problem, which made it possible to reduce the solution time to 5–10 min.

An investigation was made of the interaction of pulsed Nd:YAG laser radiation with mixed Zn_xCd_1-xTe (x=0.47) crystals. These crystals were grown by the directional solidification method. Various physical phenomena occurring during the interaction of the laser radiation with the crystal surface were observed. Melted regions (craters) appeared as a result of the interaction. Surface microtopography and microsegregation of the elements were investigated with a scanning electron microscope and an x-ray microanalyser. The experimentally determined crater diameters were less than 1 mm, whereas our calculation (based on the one-photon absorption model) predicted the maximum temperature in the irradiated zone to be less than the melting point of Zn_xCd_1-xTe. The two-photon absorption must therefore be taken into account. There were no changes in the chemical composition in the irradiated zone. Soliton-like waves originated on the melted and solidified surface of a crater. A ring-like distribution of granules was attributed to the diffraction in the optical system.

Consideration is given to the capabilities of gamma-ray spectroscopy of the nuclei of free neutral atoms cooled employing modern laser light-pressure techniques. This spectroscopy is comparable with the Mossbauer spectroscopy in respect of the expected resolving power.