Table of contents

The spectral characteristics of the longitudinally diode-pumped Tm : YLF laser radiation are experimentally studied. The laser wavelength was tuned within a spectral range of 1860 - 1940 nm with a linewidth not exceeding 1 nm. The effect of water vapour on the spectral characteristics is studied.

Lasing in Nd:NaLa(MoO₄)₂ crystals is obtained without stabilisation of the diode pump wavelength. A dependence of the cw laser power (at a wavelength of 1059 nm) on the pump diode temperature is found within a range of 10—458C. It is shown that the variations in the diode temperature within this region change the lasing efficiency no more than by 30%. In the passive Q-switching regime, the experiments were performed under both pulsed and cw pumping. Upon pulsed pumping, the laser energy was 16 μJ at the output pulse duration of 11 ns. The laser wavelength was 1059 nm, as well as in the case of cw operation. Upon cw pumping with a power of 1.5 W, laser pulses were obtained with an energy of 15 μJ.

An analytic model is proposed to calculate the buffer-gas temperature in the discharge-tube cross section of the copper bromide vapour laser. The model is the generalisation of the previous models developed by the authors. Assuming that the volume electric power is arbitrary distributed over the tube radius, the general solution of the quasi-stationary heat conduction equation with the boundary conditions of the first and second kinds is presented. Application of the model is considered by the example of a copper bromide vapour laser emitting at 510.6 and 578.2 nm at different specific radial distributions of the volume power. The obtained results are compared with the temperature profiles known to date. Application of this model to molecular lasers is also discussed.

The development of detachment instability in active media of electric-discharge non-chain HF(DF) lasers due to the electron-impact detachment of electrons from negative ions is considered. This instability is initiated in large volumes of SF₆-based gas mixtures, spatially separated from electrodes and heated by a pulsed CO₂ laser. The self-organisation of self-sustained volume discharge upon laser heating, which results in the formation of quasi-periodic plasma structures within the discharge gap, is experimentally investigated. The evolution of these structures, depending on the gas temperature and specific deposition of electric energy, is analysed. The possible relationship between the self-organisation and detachment instability is discussed. A mechanism of development of single plasma channels in the working media of HF(DF) lasers, based on electron-impact destruction of negative ions is proposed.

We demonstrate the feasibility of low-threshold (10^-4 to 10^-2 W cm^-2) anti-Stokes conversion of photons in the range 1.72 — 2.00 eV to luminescence at 2.14 — 2.38eV at temperatures from 77 to 300 K in Zn_0.6Cd_0.4S crystals with nanoscale complexes of dye molecules and silver subnanoclusters adsorbed on their surface. The spectral characteristics of the centres involved in anti-Stokes frequency conversion indicate that this process in the crystals studied is due to sequential two-photon interband optical transitions. The absorption of a photon by a dye molecule and electron excitation transfer to an adsorbed silver subnanocluster produce a free hole in the valence band. Subsequent photoionisation of the subnanocluster produces a free electron. The last step of the anti-Stokes frequency conversion process is radiative recombination of the electron with a hole localised at an emission centre.

It is shown that when an s-polarised wave is mixed with a wave of the same frequency, which is incident along the normal to the metal surface, three frequency-doubled waves are generated. Two of them are p-polarised and one is s-polarised. The effect of the electron and lattice temperature evolution on the SHG efficiency is described quantitatively. An anomalously strong change in the generation efficiency of an s-polarised harmonic is found during temperature variations, which leads to a noticeable change in the electron collision frequency.

Linearised equations for the amplitudes of harmonic perturbations of strong waves in quadratic and cubic nonlinear media are obtained within the model of plane monochromatic waves. The dependences of the gains of noise components of the first and second harmonic waves upon frequency doubling on the B integral are found. The maximally admissible noise level in the fundamental radiation beam is calculated by the example of a 0.5-mm-long KDP crystal at the peak intensity of 4.5 TW cm^-2.

The thermophysical parameters (linear thermal expansion coefficients, thermal conductivities, and heat capacity) of the lithium triborate (LBO) crystal are measured and compared with previously published data.

We report the preparation of a high-purity optical-quality four-component glass of composition 68TeO₂ — 22WO₃ — 8La₂O₃ — 2Bi₂O₃, containing (2.7±0.5)×10^-5 mol % OH groups. Its refractive index has been determined in the range 0.9 — 5.45 μm using interference refractometry. The data are used to assess the dispersion and guidance characteristics of microstructured optical fibres potentially attractive for supercontinuum generation in the range 1 — 5 μm

The polarisation beats accompanying decay of LP-mode groups in multimode optical fibres with a step refractive index profile are considered. The theoretical dependences of the degree of linear polarisation on the optical fibre length and light exit angle are determined based on the waveguide properties of these groups. The results obtained are compared with the existing theoretical concepts. It is shown that the light emerging from a multimode optical fibre near its axis can retain linear polarisation in fibres up to several meters long. Some experimental results are reported, which confirm the basic theoretical conclusions.

This work examines the effect of gold nanoshells and titania nanoparticles on the imaging contrast of structural components of skin in optical coherence tomography (OCT). Experimental data are compared to Monte Carlo (MC) simulation results. In experiments with pig skin in vivo, the epidermis — dermis contrast is improved from 0.78 ± 0.03 to 0.92 ± 0.04 by gold nanoshells applied to the skin surface and from 0.78 ± 0.03 to 0.86 ± 0.04 by titania nanoparticles. The contrast of glands is enhanced by titania from 0.68 ± 0.12 to 0.84 ± 0.07. The highest contrast is reached 120 — 150 min after applying gold nanoshells and 160 — 200 min after applying titania. According to the MC simulation results, the contrast of inclusions increases from zero to 0.85 and 0.65, respectively.

Spectrally resolved fluorescence diffuse tomography (SFDT) is demonstrated to be an effective approach to the reconstruction of the fluorescent agent distribution in biological tissues. Analysis of the measured optical power spectrum enables more accurate fluorophore localisation in solving the inverse tomography problem, because the dispersion of the optical parameters of the imaging subject leads to characteristic changes in the shape of its fluorescence spectrum. The SFDT system built by us includes a common optical fibre output of light sources and a single fibre input of the detector in an on-axis configuration, and an electromechanical scanner. The system enables studies of laboratory animals having tumours labelled with various fluorescent agents (fluorescent proteins, quantum dots and others). The first in vivo experiments have demonstrated the feasibility of tumour detection and localisation in laboratory animals by SFDT.

Based on the generalised source method, a new method for calculating the light diffraction on one-dimensional dielectric diffraction gratings with an arbitrary profile is proposed. The method is an alternative to the widely used Fourier modal method because it possesses the same possibilities in defining different grating profiles; however, it has a significant advantage in calculating complex-shaped gratings because it requires many fewer mathematical operations. The applicability of the method is shown by comparing the calculation results by this method with the results obtained by the conventional methods of light diffraction calculation on the gratings.

The charge exchange of multiply charged fluorine and lithium ions with Ne atoms in a gas jet was recorded from the line spectra in the 125 — 350 Å range, which arise from the radiative decay of the excited states of Li II — III and F III — VIII ions populated in the charge exchange. In the F III — VI ion spectra obtained from the plasma — gas interaction region we observed transitions from levels with an open (partly filled or completely open) 2s-shell, which may be populated in sequential events of single-electron charge exchange as well as in double- and many-electron charge exchange. Partial cross sections were calculated for the single-electron charge exchange of F VIII ions with Ne atoms. By way of Hartree — Fock energy level calculations it was possible to reveal resonances between the ground states of k-fold (k = 1 — 4) ionised donor atoms and the states of k-fold excited multiply charged fluorine ion resulting from the k-fold charge exchange of F VIII ions. These resonances may be responsible for the relatively large cross sections of double- and many-electron charge exchange

We discuss the possibility of laser Doppler visualisation and measurement of the field of three-dimensional velocity vectors by suppressing the multiparticle scattering influence on the measurement results, when using one CCD camera. The coordinate measuring basis is formed due to switching of the directions and the frequency of spatially combined laser sheets, the frequency being synchronised with the CCD-camera operation. The field of the velocity vectors without the contribution from the multiparticle scattering is produced from the linear combinations of normalised laser sheet images detected with a CCD camera in a frequency-demodulated scattered light. The method can find applications not only in laser diagnostics of gas and condensed media but also in the Doppler spectroscopy of light fields scattered by multiparticle dynamic structures.

An operator model of a mirror as a reflector with a reflectance variable over the aperture is developed on the basis of a Fabry—Perot interferometer. The main characteristics of the interferometer (mirror) are studied in the geometric, diffraction, and modal approximations. The differences in the calculation results obtained using the conventional geometric approximation and the diffraction approach proposed in this study are discussed. It is shown that, when the effective Fresnel numbers are comparable to unity, it is necessary to use the diffraction approximation. The possibility of using an interferometer-based mirror as a selector of transverse laser modes is demonstrated.

Energy, time, and spectral characteristics of emission of the second positive system of N₂ molecules in gaseous nitrogen, Ar — N₂ mixture, and air are investigated. An FPG-10 generator with voltage pulse FWHM of 200 and 400 ps and matched-load amplitudes of 14 and 6 kV, respectively, is used to excite gases. It is shown that excitation can be performed in two regimes using this generator. In the first regime a diffuse discharge is formed at atmospheric pressure, which opens ways to design miniature nanosecond UV lamps. A diffuse discharge is formed due to the generation of runaway electrons, with the aid of electrodes having a small radius of curvature and voltage pulses with a sharp leading edge. In the second regime an elevated average radiation power is obtained under excitation by a barrier discharge. However, the operating pressure is lower in this case, and the sizes of the emitting region and the UV pulse width significantly increase.