Table of contents

The creation of a caesium vapour laser with closed-cycle circulation of the laser-active medium is first reported. The power of the laser radiation amounted to ∼1 kW with the 'light-to-light' conversion efficiency of ∼48 %. Quasi-two-dimensional computational model of the laser operation that provides adequate description of experimental results is considered. Calculated and experimental dependences of the laser radiation power on the temperature of the cuvette walls, laser medium pressure and pump power are presented.

Lasing on a self-terminating transition 2 ¹P^o₁ — 2 ¹S₀ (λ = 2.058 μm) in helium atom is studied for a single- and double-pulse operation regimes under electron beam pumping in pure helium and its mixtures with H₂, N₂, O₂, CO₂, H₂O, NH₃, and N₂O. In pure helium, the maximal pulse duration is ∼50 ns, which agrees with the calculated value. Recovery of lasing in the second pulse is observed at a time delay between the pulses of longer than 1.25 μs. In adding CO₂, N₂O, NH₃, and H₂O, the relaxation rate for population of the metastable state He(2 ¹S₀) increases, which makes the delay, needed for recovering lasing, shorter up to pulse merging in the case of H₂O. At the exciting pulse base-level duration of 1.2 μs, in mixtures of helium with NH₃ and H₂O, laser pulses with a duration of ∼0.8 μs are observed, which testifies that collisional quasi-cw lasing occurs. Mechanisms of collisional lasing are discussed.

A new method for obtaining Q-switching simultaneously with mode-locking using one travelling-wave acousto-optic modulator in a frequency-doubled Nd : YAG laser cavity is described. Further shortening of output laser pulses (from 40 to 3.25 ps) is achieved by forming a Kerr lens in the frequency-doubling crystal. At an average power of ∼ 2 W and a Q-switching rate of 2 kHz, the peak power of the stably operating reached ∼ 50 MW.

A saturated nonresonant two-photon absorption (TPA) theory of quasi-monochromatic radiation by an atom is developed in a model of a three-level cascade quantum system. It is shown that doubled-frequency radiation, arising from a multipole interaction of radiation with the dipole-forbidden transition, experiences spatial oscillations at arbitrary intensities of the pump. The influence of the total phase of the waves with the initial and doubled frequencies on TPA is considered under conditions of ring frequency mixing in a medium.

We present new specific analytic solutions of a system of nonlinear Schrödinger equations, corresponding to elliptically polarised cnoidal waves in an isotropic gyrotropic medium with spatial dispersion of cubic nonlinearity and second-order frequency dispersion under the conditions of formation of the waveguides of the same type for each of the circularly polarised components of the light field.

This paper presents an experimental study of frequency doubling of a tunable ytterbium-doped fibre laser in KTP crystals phase-matched in the XY and YZ planes. In the XY plane, we obtained continuous tuning in the range 528 — 540 nm through intracavity frequency doubling. The second-harmonic power reached 450 mW for 18 W of multimode diode pump power, which was five times higher in comparison with single-pass frequency doubling. In a single-pass configuration in the YZ plane, we obtained a wide tuning range (527 — 551 nm) in the green spectral region and a second-harmonic power of ∼10 mW. The tuning range was only limited by the mechanical performance of the fibre Bragg grating and can potentially be extended to the entire lasing range of the ytterbium-doped fibre laser.

The interaction of two coherent femtosecond laser pulses, propagating at a small angle with respect to each other in a sapphire crystal in the filamentation regime, has been investigated numerically and experimentally. Distributions of the fluence and free-electron density in the laser-plasma channels formed in the crystal are obtained. Additional filaments are found to form outside the plane of initial pulse propagation.

Some mechanisms of the formation of an extended (∼1 km) conducting channel in atmosphere using the laser engine technology (Impul'sar program) are considered. The electric conductivity of a channel formed by electric explosion of a 90-μm copper wire up to ∼1.6 m long (in electric breakdown) at a voltage not higher than 10 kV is studied. The requirements to the products of laser engine exhaust to atmosphere for producing a conducting channel, containing dispersion and condensation aerosols of hot particles of copper oxides (CuO, Cu₂O) and copper with an oxidised surface (from 50 nm to 300 μm in diameter), are formulated. Possible mechanisms of electric breakdown at the field strength E ≥ 52 V cm^-1 in the discharge gap are discussed. The electric conductivity of the channel formed by the laser plasma upon focusing the solid-state laser radiation on targets made of different materials has also been studied.

A method is proposed for simulating optical object images formed by oblique or grazing-incidence coherent beams. The theoretical approach relies on the solution of a parabolic equation, which generalises the Fresnel integral. Our numerical results are given for experimental conditions close to those realised when use is made of modern soft X-ray lasers. The newly developed method may also be employed to simulate X-ray imaging systems developed around synchrotron and free-electron laser beams.

We review the works related to the development of aperiodic multilayer structures — optical elements for the soft X-ray range. The potentialities of aperiodic multilayer mirrors as regards reflection of soft X-ray radiation in a broad wavelength range, first and foremost at normal radiation incidence, as well as the capabilities of broadband polariser mirrors are investigated. The results of multiparametric optimisation and experimental results for Mo/Si aperiodic mirrors (λ ≥ 12.5 nm) as well as calculations for several promising material pairs (Pd/Y, Ag/Y, etc.) for λ ≤ 12.5 nm are outlined. The effect of transition layers on the reflectivity is considered, in particular by taking into account the smooth variation of the permittivity near interfacial boundaries. The use of broadband mirrors in laser-plasma spectroscopic experiments is discussed.

The radiation of discharge plasma was investigated in the nanosecond breakdown of short interelectrode gaps in a nonuniform electric field. Voltage pulses with an incident wave amplitude of ∼10 kV, a FWHM duration of ∼1 ns, and a rise time of ∼200 ps were used. X-ray radiation from the discharge gap was recorded for an interelectrode gap of 0.5 mm, which confirms the generation of runaway electrons in the discharge formation. In the pulse-periodic air breakdown of a 0.5-mm wide gap at atmospheric pressure, the lines of electrode materials and the continuum were shown to make the main contribution to the plasma emission; the highest radiation intensity fell on the 200 — 300 nm region, which accounted for ∼40% of the total emission energy.

The efficiency of the electrical power transfer to the gas mixture of a XeCl dielectric barrier discharge (DBD) exciplex lamp is analysed. An equivalent circuit model of the DBD is considered. It is shown that the excilamp power can be controlled by applying current to the lamp. This highly desired property is ensured by means of a specific power supply topology, whose concepts and design are discussed. The experimental prototype of a current-mode converter operating in the pulsed regime at pulse repetition rate of 50 kHz is presented and its capability to control the amount of energy transferred during each current pulse is demonstrated. The capability of this power supply to maintain specific operating conditions for the DBD lamp, with a very stable behaviour (even at a very low current, in the regime of a single discharge channel), is illustrated. The experimental results of a combined use of this converter and a XeCl excilamp are presented. The influence of the supply parameters on the 308-nm XeCl excilamp is analysed. The shape of the UV pulse of the lamp is experimentally shown to be similar to that of the current, which actually flows into the gas mixture. The UV radiation power is demonstrated to be tightly correlated to the current injected into the gas and controlled by the available degrees of freedom offered by the power supply. The measured UV output characteristics and performance of the system are discussed. Time resolved UV imaging of a XeCl DBD excilamp is used to analyse the mechanisms involved in the production of exciplexes at various power supply regimes. It is shown that a pulsed voltage source leads to formation of short high intensity UV peaks, while current pulses lead to formation of sustained discharge filaments. Based on the results of modelling of the above-mentioned operation conditions, the two power supply regimes are compared and analysed from the point of view of the UV power and radiative control.

We have studied the properties of silica-core photonic bandgap fibres with a small diameter to pitch ratio (no greater than 0.4) of high-index rods and small refractive-index difference (no greater than 0.03) in the cladding. Theoretical analysis and experimental data demonstrate that the number of core modes depends only on the number of pure-silica elements that form the core and is independent of cladding parameters. If there is one such element, only one dispersion curve of a core mode falls in the fundamental bandgap and the fibre is single-mode. The fibres having seven such elements are multimode.

It is shown that by varying the external electric field with different polarity from 0 to 10⁶ V m^-1 in the course of laser processing with the mean radiation flux density ~10⁶ W cm^-2 the change in the evolution features of the plasma torch at the surface of some metals (Cu, Al, Sn, Pb) at early stages is quantitative rather than qualitative. At the same time the characteristic size of the target material droplets, carried out from the irradiated zone, becomes essentially (by several times) smaller as the amplitude of the external electric field strength grows, independently of its polarity.

Fragmentation of nanoparticles in a liquid under the action of pulsed laser heating is studied theoretically and experimentally. Fragmentation is simulated by solving the kinetic equation for the nanoparticles size distribution function, taking into account the temperature dependence of the thermophysical parameters of the medium. It is shown that fragmentation occurs after separation of smaller fragments from a molten nanoparticle. The simulation results are in good agreement with experimental data obtained in the fragmentation of gold nanoparticles irradiated in water by a copper vapour laser with a peak radiation intensity of about 10⁶ W cm^-2.

We discuss the physical feasibility of association of particles noninteracting with each other, which arises in accordance with the uncertainty relation under the 'corporate' spatial confinement of the particle ensemble as a whole. Investigation is conducted by the example of an ensemble of ultracold neutrons placed in a common potential well of infinite depth. We present quantitative estimates and indicate the expected properties of the arising crystal-like spatially periodic structures.

It is shown that when coherent population trapping (CPT) resonance is excited by a narrow laser beam, the presence of elastic collisions with the cell wall significantly affects the line shape of the CPT-resonance. We have constructed a theoretical model, which is based on averaging over the random Ramsey sequences of the atom dwell time in the beam and dark zones and takes into account the probability of elastic bounce of an atom from the wall.