Table of contents

A method for spatial regularisation of chaotically located filaments, which appear in a high-power femtosecond laser pulse, is proposed, numerically substantiated, and experimentally tested. This method is based on the introduction of regular light-field perturbations into the femtosecond-pulse cross section.

A review of the development and investigation of moderate-power (10^-1—10² W) cw fibre lasers is presented. The properties of optical fibres doped with rare-earth ions and methods for fabricating double-clad fibres are considered. The methods for fabrication of fibre Bragg gratings used as selective reflectors are discussed and the grating properties are analysed. The main pump schemes for double-clad fibre lasers are described. The properties of fibre lasers doped with neodymium, ytterbium, erbium, thulium, and holmium ions are also considered. The principles of fabrication of Raman converters of laser radiation based on optical fibres of different compositions are discussed and the main results of their studies are presented. It is concluded that fibre lasers described in the review can produce moderate-power radiation at any wavelength in the spectral range from 0.9 to 2 μm.

The effect of the pump parameters on the efficiency of operation of a KrF gas-discharge excimer laser on a He—Kr—F₂ mixture is studied. A theoretical model of the excitation system and the kinetic processes in the plasma of this laser is developed. A pump system based on an LC inverter with a spark gap as a high-voltage switch, automatic UV preionisation, and a low-inductance discharge circuit is created. To increase the efficiency and the output energy of the KrF laser based on a He—Kr—F₂ mixture, it is proposed to enhance the pump intensity to 4 MW cm^-3 by increasing the inductance between the LC inverter and the discharge circuit to 100 nH. An output energy of 1 J at an efficiency of 2% is achieved for the first time for the KrF laser operating on this mixture.

The energy balance is considered in a two-impurity, four-level self-cooling laser. It is shown that a self-cooling solid-state laser based on a conventional four-level scheme cannot be built by using traditional active media. Self-cooling can be achieved with the help of new materials or different energy level diagrams.

The parameters of 494–555-nm Cd(Zn)Se/ZnMgSSe green lasers with differently designed active regions pumped by a 8–30-keV electron beam are studied. The minimum threshold current density (0.6–0.8 A cm^-2 at room temperature) is obtained for a structure with the active region consisting of a ZnSe quantum well with a CdSe fractional monolayer insertion.

The lasing characteristics of a Fe:ZnSe single crystal grown from the vapour phase by the free-growth technique using the chemical transport in hydrogen are studied. A Fe²⁺:ZnSe laser cooled to liquid nitrogen temperature and pumped by 2.9364-μm radiation from an Er:YAG laser produces the 130-mJ output energy with a slope efficiency of 40% in terms of the absorbed energy, which corresponds to a quantum efficiency of 55%. The lasing spectrum in a dispersive resonator can be continuously tuned between 3.77 and 4.40 μm.

Emission of single quantum-well superluminescent diodes of two types in spectral ranges between 810 and 870 nm and between 860 and 970 nm is combined with the help of an original broadband fibre Y-coupler. Such a light source produces more than 5 mW of the cw output optical power and has a record small coherence length of about 5 μm. Its relative intensity noise does not exceed −135 dB Hz^-1 in the 100-kHz—10-MHz frequency range. The use of this source in an optical coherence tomography provides the recording of tomograms of biological objects in vivo with a spatial resolution of 2–3 μm.

The metalorganic vapour phase epitaxy was used for growing GaInP/AlGaInP periodic structures with 25 quantum wells. The active elements based on these structures were prepared for a laser longitudinally pumped by a scanning electron beam. The structure is intended for resonance periodic gain in the case when the quantum wells are at the antinodes of the resonator mode corresponding to the peak of the gain line. The effect of the structural inhomogeneities over the thickness (up to 5%) on the lasing parameters is studied, as well as the temperature detuning from the resonant gain conditions. It is shown that the period of the structure must differ from the optimal value by no more than 0.7% for attaining a 10% uniformity in the lasing threshold along the active element.

The appearance of hysteresis in the dependence of the efficiency of transient SRS on the energy of picosecond laser excitation is studied. The hysteresis appears due to the frequency-phase modulation of laser radiation which changes the SRS gain with changing the degree of modulation.

Third-harmonic generation is obtained for the 1245-nm laser radiation in monolayers of J-aggregates of a cyanine dye in a diallydimethylammonium chloride polymer matrix. The third-order susceptibility of J-aggregates is two orders of magnitude higher than that of fused silica. The wavelengths of fundamental and third-harmonic radiation lie in the transparency region of the material.

A simple scheme is proposed for efficient frequency doubling in a Nd:YAG laser transversely pumped by diode lasers. The average output TEM₀₀-mode power of the 1064-nm Q-switched laser at the Q-switching rate exceeding 20 kHz is 15 W. The radiation power at the second harmonic frequency (λ=532 nm) is found to be 12 and 8.3 W for KTP and LBO crystals for Q-switching rates f≥20 kHz and 10 kHz, respectively. The maximum second-harmonic conversion efficiency is ∼80%.

A method is developed for calculating SHG in linearly homogeneous periodically poled nonlinear crystals (PPNC) by specifying a spatially inhomogeneous periodic distribution of the quadratic-nonlinearity parameter in the form of a 'small-scale' elliptic sine, whose half-period forms one domain with the characteristic 'microplateau' of the nonlinearity parameter and interdomain walls. It is found that, because the domain length should be equal to the coherence length when the quasi-phase-matching condition is fulfilled, and if the coherence length is calculated in the fixed-field approximation, the dependence of the harmonic amplitude on the longitudinal coordinate has the form of a 'large-scale' elliptic sine with a broad 'macroplateau' corresponding to a certain (in the case of quasi-phase matching, to virtually 100%) transformation; however, the mismatch in a domain is never completely compensated by the reciprocal lattice vector. In this case, the phase trajectories inside one domain have the form of a sequence: an unstable focus, a limit cycle ('macroplateau'), a stable focus. This picture repeats in the next domain. It is shown that the width of the SHG phase-matching curve in a PPNC in the regime of strong energy exchange, taking secondary maxima into account, can be considerably (by several times) larger than the width calculated in the fixed-field approximation.

The conditions under which an optical pulsed discharge stably generates periodic shock waves are determined theoretically and experimentally. It is shown that the mechanism of merging shock waves into a low-frequency quasi-stationary wave is operative in various gases (and vapours) in a wide range of laser spark energies. The application of such a wave for increasing the coupling factor in a laser engine is considered.

The dependence of a recoil momentum on the radius of a target irradiated by a single-pulse Nd³⁺:YAG laser (λ=1.064 μm, τ=20 ns, E≤300 mJ) in the air is studied. The recoil momentum decreases three-fold with increasing the relative target radius from 0.3 to 5 and tends to saturation for r>3. The calculation of the recoil momentum on the basis of the Euler and Navier—Stokes equations gave understated values for r>1, which lowered to negative values. The reasons for the qualitative discrepancy between the experimental and calculated data is discussed.

The production of nanoparticles upon ablation of copper and brass by pulsed radiation from Nd:YAG and copper lasers in water, ethanol, and acetone is studied. The nanoparticles were investigated by the methods of X-ray diffractometry, optical spectroscopy, and transmission electron microscopy. The produced copper and brass nanoparticles were shown to exhibit a plasmon resonance lying in the visible spectral range near 580 and 510 nm. The brass nanoparticles produced by ablation in ethanol have a shell approximately 10-nm thick for an average dimension of 20–30 nm. A chemical modification of ethanol was observed, which manifested itself in the appearance of intense UV absorption bands. Upon laser irradiation of brass nanoparticles in a liquid their absorption spectrum gradually transformed into the spectrum of copper nanoparticles.

It is shown theoretically and experimentally that the transformation of modes by an astigmatic π/2 converter is invariant with respect to the input-beam displacement and tilt. The possibility is considered of using this property for manipulating microobjects and simultaneous generation of Laguerre—Gaussian modes of different orders with the help of the same astigmatic π/2 converter.

Several optical schemes performing the complex-order fractional Fourier transform are considered. It is shown that these schemes, containing only Gaussian apertures or their combination with lenses, have eigenbeams represented by Hermite—Gaussian modes with transverse indices m, n≤1 and Laguerre—Gaussian modes with p=0 and l=1. The wave front of the eigenbeams is, as a rule, spherical.

Two-level schemes for data communication are proposed in which the upper level consists of phase-matched chaotic lasers synchronously perturbing the generation dynamics of a transmitter and receiver at the lower level and forming the communication link. For four schemes (with radiation injection from the controlling level to the controlled level, with mutual radiation injection between the two levels, and with a random synchronous perturbation of the pump-period modulation of lasers at the controlled level in each of the above schemes), the signal recovery errors are studied numerically depending on the coupling coefficients between the levels. It is shown that the recovery errors are minimal only for controlled receivers and the code functions reconstructed on them are close to the specified function.

The problem of designing a two-wave single-mode-fibre 3-dB coupler is considered. The method is proposed for adjusting the parameters of the coupler for simultaneous use of radiation at 0.83 and 1.3 μm in one fibre interferometer. The dependence of the coupling efficiency on the wavelength is studied experimentally for different angle mismatches of fibres in the coupler. The results of experiments and numerical calculations are presented.

The possibility of development of a Faraday isolator consisting of several thin magnetooptical discs was investigated. An analytical and numerical analysis was performed taking into account the photoelastic effect, the temperature dependence of the Verdet constant, and the Fresnel reflections between the discs. The depolarisation ratio caused by thermal effects was shown to be much weaker in the disc geometry than in the rod geometry. The results obtained suggest that it is possible to make a Faraday isolator for laser radiation with a multikilowatt average output power.

It is shown that, when the angular resolution of a receiving optical system is insufficient, the angular dimensions of a located object can be estimated and its shape can be reconstructed by estimating the parameters of the fourth-order correlation function (CF) of scattered coherent radiation. The reliability of the estimates of CF counts obtained by the method of a discrete spatial convolution of the intensity-field counts, the possibility of estimating the CF profile counts by the method of one-dimensional convolution of intensity counts, and the applicability of the method for reconstructing the object shape are confirmed experimentally.

A new source of the bichromatic laser field with a stable mean value of the difference frequency is used to study coherent population trapping (CPT). A high stability of the difference phase of the fields of the radiation source is achieved by phase locking the radiation frequency of two semiconductor lasers to longitudinal modes of a femtosecond laser. The first experimental results on the spectroscopy of CPT resonances in rubidium are presented. The possibility of using the new method for spectroscopy of CPT resonances in rare-earth atom vapours is discussed.