Table of contents

A new scheme providing self-compression of femtosecond laser radiation upon filamentation of a collimated beam in argon at a pressure of ∼0.8 atm is experimentally realised. Pulses with the initial duration of 55 fs were compressed down to 8 fs, the peak power of the compressed pulse exceeding 20 GW.

CW passive mode locking is achieved in a bismuth-doped fibre laser using a semiconductor saturable absorber mirror optimised for operation in the range 1100–1200 nm. The pump source is a cw ytterbium fibre laser (1075 nm, maximum output power of 2.7 W), and the pulse parameters can be tuned by varying the intracavity group velocity dispersion using a diffraction grating pair. Stable laser pulses are obtained with a duration down to τ_p ≈ 1.1 ps.

Pulse synchronisation with an accuracy of no worse than ±5 ns is demonstrated in passively Q-switched neodymium phosphate glass and Nd:YAG lasers. Two operating regimes are realised: the 'sub-threshold' regime (when the slave Nd:YAG laser does not generate a giant pulse if its passive Q switch is not irradiated by the master Nd:glass laser) and the 'above-threshold' regime (when the pulse irradiating the passive Q switch of the slave laser advances its generation).

A new ytterbium laser glass based on an alumoborophosphate composition is developed. It is shown that the chemical and thermal stabilities of this glass are record-high for phosphate glasses and that its spectral and luminescent characteristics compare well with popular laser glasses. A mould of laser-quality glass doped with ytterbium with a concentration of 5×10²⁰ cm^-3 is synthesised. Active laser elements 5×5×2 mm in size are prepared from this glass for longitudinal diode pumping. These elements were used to fabricate a laser, whose output power in the cw regime reached 783 mW and maximum slope efficiency was 28.9%. Pulses with a duration of ∼150 fs and a peak power of about 5 kW are obtained in the passive mode-locking regime.

The depolarisation dynamics in a neodymium glass rod laser amplifier is experimentally studied. A method for determining the temperature distribution and the thermally induced phase distortions from the known transverse depolarisation degree distribution is proposed. The experimental results obtained by this method perfectly coincide with the theoretical results even at a strong radial inhomogeneity of the heat release.

Lasing at 337.1 and 357.7 nm is obtained upon excitation of nitrogen molecules by a toroidal pulsed inductive discharge. A system for formation of a pulsed inductive discharge for exciting gas lasers is described. The spontaneous emission spectra of the nitrogen inductive discharge plasma and the emission spectrum of the nitrogen laser are recorded and interpreted. The dependences of the energy and duration of laser pulses on the resonator Q factor and pump level are studied. The output energy of the inductive nitrogen laser emitting 15±1-ns pulses achieves 4.5 mJ. The generation of high-power 300-kW pulses is obtained for the first time at a low (∼1 Torr) pressure of pure nitrogen. The spatial distribution of the laser radiation intensity in the discharge tube cross section is investigated. The cross section of the radiation beam of the inductive nitrogen laser had the shape of a ring with the external diameter of 34 mm and width ∼4 mm, its divergence being ∼0.8 mrad. The average output power of the laser achieved 120 mW at a pulse repetition rate of 30 Hz.

We report a single-frequency 1-W fibre laser source emitting at 1093 nm, composed of a distributed-feedback ytterbium fibre laser and fibre-optic amplifier. The laser frequency was stabilised by side-locking to a transmission peak of a Fabry — Perot interferometer, and the residual frequency noise spectrum of the laser was measured. Our results indicate that the laser linewidth can be narrowed down below 1 kHz.

We report an experimental study of the performance parameters of a gas discharge pumped thallium vapour laser operating on the 7s²S_1/2 - 6p²P_3/2^o self-terminating transition at 535 nm. The switch we used, a TPI3-10k/25 cold-cathode thyratron, ensures a rise time of the load voltage pulse less than 15 ns. The laser output power is shown to increase in proportion to the energy stored in the discharge capacitor (up to the maximum voltage of the thyratron). The optimal pump pulse repetition rate for He(Ne)—Tl mixtures is ∼1.75 kHz. Even small hydrogen additions reduce the lasing energy and average output power. The addition of bismuth vapour increases the optimal pulse repetition rate (up to 3 kHz) and average output power. The factors responsible for the lower lasing efficiency in comparison with the copper vapour laser are analysed.

The nonstationary double resonance is numerically simulated in the Λ-scheme of degenerate energy levels with the quantum number J of the total angular momentum equal to 0, 2, and 1. The analysis is performed in the slowly varying envelope approximation taking into account the inhomogeneous broadening of quantum transition lines. In the case of a high-power input low-frequency pulse of long duration with a flat top switched on before the application of a comparatively weak and short input high-frequency pulse and switched off after the end of the latter, a specific pulsed structure, the so-called double adiabaton, can appear. It differs from an adiabaton known from the theory of electromagnetically induced transparency by the decomposition of the high-frequency pulse into two pulses with oppositely directed elliptic polarisations.

Two methods for the experimental study of the nonlinear interaction of the fundamental beam and noise component of high-power laser radiation under conditions of small-scale self-focusing in the nondestructive regime are proposed and realised. In the case of the nonlinear phase shift B = 1, the theoretically predicted circular structure of the spatial spectra of the field and laser beam intensity (pulse energy 20 J and peak power 20 GW) propagated through a glass sample was experimentally observed.

The dynamics of the transient light absorption induced in K8 optical glass by filamented femtosecond laser pulses have been studied using time-resolved transmitted-light microscopy at wavelengths from 450 to 700 nm. The transient absorption measured as a function of probe beam wavelength is compared to that predicted by the Drude plasma model. We conclude that, just 450 fs after a pump pulse, the transient absorption is dominated by transient electronic states, presumably, self-trapped excitons, with an excitation energy of 2.6 — 2.7 eV. These states are filled with free-carriers from a long-lived plasma, which acts as a 'carrier reservoir'. The relaxation of transient absorption has two components. The slow component, with τ₁ ∼ 17-17.5 ps, is governed by the plasma thermalisation time, whereas the second, with τ₁ ≫ 300 ps, is determined by the plasma lifetime.

The hyper-Raman scattering (HRS) of light in water is detected reliably by the active spectroscopy method of coherent light scattering, in particular, by the method of four-photon parametric light scattering in a medium in which HRS is a 'signal' wave in the parametric process involving simultaneously two high-power laser photons and IR photons of an 'idler' wave. Hyper-Raman scattering by libration vibrations of water molecules, which virtually cannot be detected by conventional methods of Raman scattering, was observed.

The optical characteristics of two Russian-made CaF₂ ceramic samples are compared to those of single-crystal CaF₂. The results indicate that the ceramic possesses high optical quality and a small absorption coefficient (∼10^-3 cm^-1 at a wavelength of 1.07 μm) and is suitable as a material for optical components. Experimental evidence is presented for spatial modulation of the thermally induced depolarisation in the ceramic.

The propagation of light through subwave photonic barriers formed by dielectric nanofilms with the refractive indices changing across the films according to the specified law n(z) is considered. Generalised Fresnel formulae depending on the gradient and profile curvature of the refractive index and describing reflection and transmission of such inhomogeneous films are found. For the specified material and thickness, the optical properties of such nanofilms can change from total transmission to total reflection by producing a technologically controlled profile n(z). The obtained results are based on exact analytic solutions of Maxwell's equations for new multiparametric models of inhomogeneous dielectric media. The possibility of producing new subwave dispersion elements, whose action is based on the dependence of the reflection and transmission spectra of gradient photonic barriers on their local dispersion determined by the shape and geometrical parameters of the profile n(z), is shown. The schemes are considered for producing such spectra in the visible and IR regions with the help of periodic nanostructures containing subwave photonic barriers with the normal and anomalous nonlocal dispersion.

An inhomogeneous ring optical resonator of a special type is considered, which contains two identical dielectric plates of finite thickness separated by an arbitrary distance. The refraction coefficient of these plates is significantly higher than that of the medium filling the rest of the resonator. This system can be treated as a ring resonator formed by two linear resonators coupled through the plates confining them. The classical spectral problem for such a resonator is solved in the plane-wave approximation. It is shown that in the case of a comparatively low reflectance from the plates, it is possible to obtain analytically a physically acceptable description of the spectrum of eigenfrequencies and modes. The method for solving the spectral problem is proposed in which the analytic approach is combined with the numerical experiment. It is shown that the resonator spectrum is simple and is formed by a sequence of doublets. The modes corresponding to these doublets are real and orthogonal. Conditions are found under which the splitting of eigenfrequencies in doublets disappears.

The influence of the amplitude—phase polarisation anisotropy of the interference mirrors on the polarisation characteristics of a ring laser (ellipticity, frequency shifts, and losses) is analysed. The combination of the mirror parameters, at which the maximum sensitivity of the polarisation characteristics of radiation to the nonplanar deformation of the axial contour is observed, is determined. It is shown that there exists a range of optimal phase anisotropies of the mirrors.

The method of laser Doppler visualisation and measurement of the velocity field in gas and liquid flows by suppressing the influence of multiparticle scattering is discussed. The cross section of the flow under study is illuminated by a laser beam transformed by an anamorphic optical system into a laser sheet. The effect of multiparticle scattering is eliminated by obtaining differential combinations of frequency-demodulated images of the laser sheet in different regions of the angular spectrum of scattered light.

The development of high-power discharge sources emitting in the 13.5±0.135-nm spectral band is of current interest because they are promising for applications in industrial EUV (extreme ultraviolet) lithography for manufacturing integrated circuits according to technological precision standards of 22 nm and smaller. The parameters of EUV sources based on a laser-induced discharge in tin vapours between rotating disc electrodes are investigated. The properties of the discharge initiation by laser radiation at different wavelengths are established and the laser pulse parameters providing the maximum energy characteristics of the EUV source are determined. The EUV source developed in the study emits an average power of 276 W in the 13.5±0.135-nm spectral band on conversion to the solid angle 2π sr in the stationary regime at a pulse repetition rate of 3000 Hz.

A system for recording a dynamic holographic grating in an optically addressed liquid-crystal spatial light modulator is studied. The system provides the asymmetrisation of the grating profile by using a TV-locked optical feedback loop (nonlinear or adaptive interferometer).

An optical system for laser radiation focusing, which consists of parabolic and elliptic mirrors, is considered. It is shown by the method of elementary reflections that the maximum concentration of laser radiation on the target can be achieved at a certain position of these mirrors.

Coherent summation of fibre laser beams, which can be scaled to a relatively large number of elements, is simulated by using the stochastic parallel gradient descent (SPGD) algorithm. The applicability of this algorithm for coherent summation is analysed and its optimisaton parameters and bandwidth limitations are studied.

The channeling of microwave radiation is demonstrated experimentally in a double line in which a plasma filament produced in air by intense femtosecond laser pulses serves as one of the conductors. It is shown that during the propagation of microwave radiation in this line, ultrashort pulses are formed, their duration monotonically decreasing with increasing the propagation length (down to the value comparable with the microwave field period). These effects can be used for diagnostics of plasma in a filament.