Table of contents

The basic methods of laser spectroscopy that are used for standoff detection and identification of explosive traces in the form of particles on the surfaces of objects tested under real or close-toreal conditions are briefly considered. The advantages and drawbacks of all methods are discussed and their characteristics are compared. Particular attention has been given to the prospects of development and practical implementation of the technologies discussed and justification of their most preferred applications.

The first passively mode-locked holmium fibre laser has been demonstrated, with a semiconductor saturable absorber mirror (SESAM) as a mode locker. Semiconductor disk lasers have been used for the first time to pump holmium fibre lasers. We obtained 830-fs pulses at a repetition rate of 34 MHz with an average output power of 6.6 mW.

Traditional (in the AlGaAs/GaAs system) and phosphorus-compensated (in the AlGaAs/AlGaPAs/GaAs system) laser heterostructures emitting at a wavelength of 850 nm are grown by MOVPE and studied. Laser diode bars are fabricated and their output characteristics are studied. The method used to grow heterolayers allowed us to control (minimise) mechanical stresses in the AlGaPAs/GaAs laser heterostructure, which made it possible to keep its curvature at the level of the initial curvature of the substrate. It is shown that the use of a compensated AlGaPAs/GaAs heterostructure improves the linear distribution of emitting elements in the near field of laser diode arrays and allows the power — current characteristic to retain its slope at high pump currents owing to a uniform contact of all emitting elements with the heat sink. The radius of curvature of the grown compensated heterostructures turns out to be smaller than that of traditional heterostructures.

We have studied the pump parameters of a CuBr laser (pulse repetition rate of 14 kHz) with an independent heating of the active medium and an additional excitation source, providing an operational control of the radiation parameters in a given time interval. We have fabricated an experimental setup that makes it possible to change the average output power by applying a control pulse which is ahead of the main pulse by 0.12 — 20 μs. It is shown that to ensure the effective control of the laser power, the voltage amplitude of the additional excitation pulse should be within 12 % — 28 % of the main pulse amplitude. In this case, the proposed control regime allows a 2.5-fold decrease in the time delay between the control and main excitation pulses due to a higher energy of the additional pulse compared to self-heating regime of the laser operation.

A two-dimensional numerical model of the active medium of a fast-flow electron-beam-controlled CO laser, taking the latest achievements of state-to-state vibrational kinetics into account, is developed. The model includes detailed kinetics and spatial description of non-self-sustained discharge in the gas flow. Using this model the basic characteristics of two schemes of a gas-flow CO laser with subsonic and supersonic flow of the gas mixture are compared.

A compact diode-pumped eye-safe Nd : YVO₄ laser with an acousto-optic Q-switch and an intracavity BaWO₄ Raman converter is developed. The laser power at a wavelength of 1536 nm with a pulse repetition rate of 20 kHz is 0.6 W, the diode-to-Stokes slope conversion efficiency reaches 44 %. Laser pulses with an energy of 35 μJ and a duration of 10 ns are achieved at a repetition rate of 15 kHz.

For the first time it has been shown that the pulse repetition rate of the lasers on the self-terminating transitions of metal atoms may be as high as ∼1 MHz. The highest pulse repetition rate equal to ∼830 kHz was realised on self-terminating IR transitions in Sr I atoms (λ = 6.456 μm and ∼3 μm) and in Sr II ions (λ ∼ 1 μm) in a strontium vapour laser operating in a self-heating pulse periodic regime. The energy yield of a Sr laser was found to be proportional to the energy input into the active medium in a wide range of excitation pulse repetition frequencies; in this case, the average total specific output laser power is equal to 30 — 40 mW cm^-3.

Radiation from a semiconductor target excited by an electron beam in a gas-filled diode was investigated at different gas (air) pressures. Subnanosecond high-voltage pulses (up to 200 kV) were applied to the pointed cathode of the diode. The targets in the form of a 15 — 20-μm-thick single-crystal CdS film with reflecting coatings forming an optical cavity and 0.7 — 1-mm-thick ZnSe plates were used. As the air pressure increased from 0.1 to 5 Torr, a decrease in the amplitude and duration of laser radiation pulses from the targets was observed. Lasing (λ = 520 nm) of CdS targets terminated at pressures greater than 2.2 Torr. The laser pulse duration varied from 125 to 20 ps. The study of the dynamics of radiation from ZnSe targets (λ = 460 nm) at atmospheric pressure showed that when the gap between the target and the electrodes was 0.2 — 1 mm thick, an intense near-surface glow was observed that consisted of a few pulses with the duration from 20 to 100 ps, caused by runaway electrons. Investigations showed that the runaway electrons may play an essential role in the excitation of semiconductors by subnanosecond high-voltage pulses.

We have studied the nonlinear absorption of nanosecond 532-nm laser pulses by zinc phthalocyanine (PcZn), lead phthalocyanine (PcPb) and zinc porphyrin (PrZn) incorporated into a nanoporous-glass/polymer composite and determined the basic nonlinear absorption characteristics of these compounds in the composite host. The composite is shown to be suitable for designing nonlinear optical elements activated with organic compounds. The correlation between the characteristics of the three compounds in the composite host and liquid solvents is analysed.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to determine the parameters of gaseous combustion products of hydrogen and hydrocarbon fuels with oxygen at high temperatures and pressures. The methodical aspects of CARS thermometry, which are related to the optimal choice of molecules (diagnostic references) and specific features of their spectra, dependent on temperature and pressure, are analysed. Burning is modelled under the conditions similar to those of real spacecraft propulsion systems using a specially designed laboratory combustion chamber, operating in the pulse-periodic regime at high temperatures (to 3500 K) and pressures (to 20 MPa) of combustion products.

We set forth the data of experimental investigation of X-ray spectral absorption coefficients in the 1.1 — 1.6 keV photon energy range for Al and Ge specimens bulk heated by soft X-ray radiation. Two experimental techniques are described: with the use of one facility channel and the heating of specimens by the X-ray radiation from a plane burnthrough target, as well as with the use of four channels and the heating by the radiation from two cylindrical targets with internal input of laser radiation. The X-ray radiation absorption coefficients were studied by way of transmission absorption spectroscopy using backlighting X-ray radiation from a point source. The results of investigation of X-ray spectral absorption coefficients on the 1s — 2p transitions in Al atoms and the 2p — 3d transitions in Ge atoms are presented.

We consider acceleration of ions through the interaction of a laser pulse with a sharp leading edge with nanofilms. At sufficiently large amplitude of the pulse, all the electrons can be expelled from the film, which provides an effective regime of ion acceleration. Limiting the maximum energy of ions can result from the longitudinal reverse motion of electrons to the initial position and by the transverse motion of electrons along the nanofilm surface, which causes the ion charge compensation. The characteristic parameters of the dynamics of ions and electrons in the system are analytically evaluated, which agree well with the results of twodimensional numerical simulations by the particle-in-cell method. Optimisation of the acceleration process by using the analytical estimates makes it possible to select the optimal parameters of the laser pulse and nanofilm. Numerical simulation of ion acceleration at these parameters shows that the maximum energy of ions can be substantially increased.

The results of experiments on initiating the ignition of a CH₄ — O₂ — SF₆ triple gas mixture in a closed volume by the radiation of a high-power CO₂ laser are presented. It is shown that spatially nonuniform (in the direction of the laser beam) gas heating by the laser radiation leads to formation of a fast combustion wave, propagating along the chamber axis and giving rise to 'instantaneous' ignition. At the threshold value 16.5 J of the laser radiation energy the fast combustion wave is transformed into a detonation wave, which causes an explosion and destruction of the reaction chamber

We have demonstrated the possibility of measuring the femtosecond laser frequency comb shift by the position of a Fabry — Perot interferometer's transmission bands with a statistical error of 10^-2 and a systematic shift of 10^-1.

This paper compares the statistical properties of the backscatter intensity in a single-mode optical fibre for semiconductor lasers with a high and a low degree of coherence. We demonstrate that, when short probe pulses are used, shorter than the coherence times of the lasers, the statistical properties of the backscatter intensity obtained with the two lasers are identical, and the intensity distribution over an ensemble of independent fibre sections has an exponential form for both. With increasing probe pulse duration, the backscatter intensity distribution obtained with the shorter coherence time laser approaches a Gaussian one, whereas the distribution function obtained with the high-coherence laser remains nearly exponential. Reflectograms of a coherent reflectometer that relies on the detection of backscattered radiation have the highest contrast for an exponential backscatter intensity distribution over an ensemble of independent fibre sections. The reflectometer then probably has the highest sensitivity to external influences. This leads us to conclude that, when short probe pulses are used, which ensure high spatial resolution (10 m and better), one can use a laser with a short coherence time, equal to the pulse duration.

We discuss the results of experimental studies of competition of absorption and scattering of laser radiation propagating in dispersive media with resonant absorption. As media under study, use is made of a suspension of polystyrene particles in solutions of rhodamine 6G in ethylene glycol probed by laser light with a wavelength of 532 nm. It is found that an increase in the dye concentration leads to an increase in optical transmittance of suspensions and an increase in speckle modulation of the forward-scattered radiation. We interpret these features as a manifestation of Christiansen effect in disperse systems with resonance absorption.

Optical fields are studied in a planar channel inside a metal layer taking into account a complex dielectric function of the metal. An influence of an exit aperture on the incident field propagating along the channel is considered; a mathematical approach is developed for analysing the effect. Attention is paid to the fact that the incident field should not be assumed a priori known if the wave transformation at the exit is neglected. For a chosen dimension of the channel and parameters of a metal, the coefficients for the lowest mode transformation into higher modes are found. The spatial field structure at the exit aperture and on the exit surface of a metal is studied. Existence of domains possessing an anomalously high intensity is revealed.