Table of contents

The interaction between self-modulation and relaxation oscillations in a solid-state ring laser operating in a self-modulation regime of the first kind is investigated theoretically. It is shown that the regions of parametric resonances between the self-modulation and relaxation oscillations have a strong influence on the stability of the former oscillations. The limits of the parametric instability bands and growth increments of the relaxation oscillations are found.

Single-mode self-pulsations repeated at rates in excess of 28 GHz were generated for the first time by high-current injection in a 1.5 μm multi-quantum-well diode laser with phase-shifted distributed feedback and a triple contact. The build-up of the pulsations was observed. The effect was attributed to dispersive self-Q-switching in distributed feedback lasers.

An rf (27.5 MHz) discharge was used for the first time to pump a cw H₂O laser emitting at λ = 118.6 μm. This made it possible to reduce the noise by more than two orders of magnitude compared with dc lasers. Optimisation of the active medium, by adding various gases (H₂, He, Xe), and of the type and geometry of the laser cavity, was carried out. The unsaturated gain, the saturation intensity of the active medium, and the internal cavity losses were measured. The maximum output power of a laser was 48 mW when the cavity was 2.5 m long. The noise/signal ratio was less than 10^-4 in the frequency range 0.01— 40 kHz.

An investigation was made of a high-pressure quasi-cw laser operating on the basis of the 5d—6p transitions in the Xe atom. The duration of the output radiation pulses was up to 7 ms. Competition between the lasing lines at λ = 2.63 and 2.8 μm was observed in a Kr—Xe mixture. The maximum output power was 150 W (corresponding to the efficiency of ~0.2%) and it was obtained for the 2.63 μm line in a mixture of the Kr : Xe = 50 : 1 composition when the pressure was 0.35 bar. The lasing threshold was reached when the thermal neutron flux was about 3×10¹² cm^-2 s^-1. Lasing of pure Xe at pressures of 0.13–0.6 bar was observed at the 3.51 μm wavelength (in this case the maximum output radiation power was about 90 W).

The conceptual design of a nuclear-pumped cw laser is put forward. Alternation of laser cells with plane uranium layers and heat exchangers (radiators) in a shared gas loop can reduce the gas velocity down to ~10 m s^-1. The results are reported of an investigation of optical inhomogeneities which appear in He and Ar due to excitation of the active medium in a prototype flow laser. It is shown that, in a section perpendicular to the plane of the uranium layers, a pumping inhomogeneity creates a positive parabolic gas lens and in a section parallel to these layers an optical gas wedge is formed. A vortex zone is observed in the gas flow at the exits from heat exchangers. Simulation experiments demonstrate that this effect increases tens of times the thermal diffusivity of the gas and results in considerable refractive losses of radiation in the effective heat-exchange region. Methods of compensating for optical inhomogeneities and for reducing the influence of vortices are proposed.

An investigation was made of the polarisation characteristics of the nonlinear absorption of λ = 1.055 μm radiation in a V³⁺ : YAG crystal at the saturation stage. The duration of a pulse propagating in the crystal was considerably less than the lifetime of an excited state of the V³⁺ impurity centres. A comparison of the experimental results with those of a theoretical calculation showed that the best description was provided by a phenomenological model in which resonant absorption in a crystal is attributed to three linear dipoles oriented along the main crystallographic axes of YAG.

An investigation was made of the formation of negative ions in HCl(BCl₃)—Xe—He gaseous mixtures used as the active media in XeCl lasers. The rate constants of the formation of negative ions in mixtures with boron chloride were 2.5 times higher than the rate constants for mixtures with hydrogen chloride, amounting to (5.8–2.0)×10^-10 cm³ s^-1 when the parameter E/N was varied from 2×10^-17 to 4×10^-16 V cm². A hypothesis was put forward that there are additional processes of negative ion formation in mixtures with BCl₃.

The refractive indices of new Nd³⁺ : GdVO₄, Tm³⁺ : GdVO₄, and Er³⁺ : GdVO₄ laser crystals were determined with an error of ±5×10^-5 in the range 400–1100 nm. The birefringence of these crystals was on the average 0.24, and the dispersion was described by the formula 1/Δn² = a+ b/λ². The thermal conductivity of the Nd : GdVO₄ crystal (atomic neodymium concentration 1.3%) was determined in the temperature range 77-300 K. At 300 K the conductivity along the <110> direction was 11.7 W m^-1 K^-1, which was higher than the conductivity of Nd : YAG crystals.

A study is reported of the dynamic characteristics of the superradiance emitted by three-level media. It is shown that the intensity and duration of bichromatic superradiance pulses can be controlled by varying the amplitude reflection coefficients at the boundary of a medium.

An experimental demonstration was made of the effectiveness of an electric discharge, in the form of a high-velocity ionisation wave, in pumping a coaxial nitrogen laser operating at pressures of 5–120 Torr. The specific input energy delivered by this nitrogen-exciting wave was up to 2.1 J cm^-3 in a time of the order of 10 ns. Lasing action was observed at the wavelength 337.1 nm under superradiance conditions. The peak power of an output radiation pulse reached 100 kW and the half-maximum duration was 10 ns. The range of the nitrogen pressures with maximal energy deposition in the gas coincided with the range of the minimal amplitude attenuation of the ionisation wave moving along the discharge gap. The input energy, the velocity and attenuation of the high-speed ionisation wave, and the laser radiation intensity were determined as a function of the nitrogen pressure.

An active automatic electronic control system, based on a beat signal, was used in phase locking of two Ar⁺ lasers with an error not exceeding 0.1 rad. An intracavity electro-optical modulator and a phase detector with a dynamic range ±2π rad ensured reliable operation of the laser system in the beat signal frequency range 1–15 MHz.

An experimental investigation was made of the dependence of the efficiency of transversely pumped ethanol-solution dye lasers on the orientation of the pump radiation polarisation relative to the optic axis of the laser. The results showed that the output radiation energy of such dye lasers had its maximum value for the pump polarisation oriented along the optic axis, but was independent of the orientation of the output radiation polarisation. A comparison was made of the relative efficiencies of conversion of the pump radiation in a dye laser amplifier for various mutual orientations of the pump and output radiation polarisations. The efficiency of such an amplifier obtained for the collinear and orthogonal (relative to the laser optic axis) polarisations was 1.3–2 times higher than the efficiency obtained for combinations of mutually orthogonal polarisations.

The efficiency of diffraction optics, in the form of transmission gratings and zone plates, in the soft x-ray range is considered. Analytic expressions are obtained for the diffraction efficiency. These expressions contain all the parameters of transmission gratings and zone plates: the optical constants of the materials, thickness, focal length, magnification, aperture angles, etc.

Sum-frequency generation by reflection of light from the surface of a nonabsorbing isotropic and gyrotropic medium is investigated for plane elliptically polarised pump waves. The boundary conditions for an electromagnetic field used in the calculations take account of the spatial dispersion of the optical response and of the surface inhomogeneity of the media in contact. The dependences of the electric field of the reflected sum-frequency wave on the angles of incidence, frequencies, and states of polarisation of the pump waves are determined. The results also include the dependences of this field on the Cartesian components of the tensors which represent the quadratic (in respect of the field) parts of the surface density of a bound-charge current and of the polarisation of the bulk of the medium with its spatial dispersion taken into account.

A theoretical investigation is reported of the propagation of light waves in the course of frequency doubling as a result of a fourth-order dipole nonlinearity of an isotropic noncentrosymmetric weakly nonlinear and weakly absorbing medium characterised by a slight spatial dispersion in the case of noncollinear interaction between the waves. Calculations are carried out by expansion into normal waves, taking account of the self-interaction of a strong wave and of its influence on a weak one. The equations describing propagation of the second-harmonic wave in the noncollinear interaction configuration are derived by the method of slowly varying amplitudes. It is shown that an interference field is formed as a result of the interaction between the strong and weak waves. Consequently, the amplitude of the second-harmonic wave acquires a complex spatial distribution along transverse coordinates and this distribution depends on the wave mismatch. An analysis of the angular spectrum of the second-harmonic wave is given.

A physical interpretation of the formation of squeezed light at the doubled frequency is proposed. A quantum theory of second-harmonic generation in a ring cavity, with a resonance at the second harmonic, is developed. The efficiency of squeezing quantum fluctuations of the quadrature components is considered as a function of the mirror reflection coefficients and of the efficiency of conversion to the second harmonic in one cavity trip.

A theory of second-harmonic generation by reflection from the surface of a centrosymmetric semiconductor is proposed. This theory takes account of the absorption of the pump and of the second harmonic in the semiconductor, and also of modulation of the nonlinear optical properties of the surface layer of the semiconductor by an electrostatic field (nonlinear electroreflection). An exponential approximation of the distribution of the modulating electrostatic field in the semiconductor is used to deal with electroreflection. Expressions are obtained for the dependences of the second-harmonic intensity and of the nonlinear electroreflection coefficient on the surface potential of the semiconductor. It is shown that the nature of these dependences is governed by the relationship between the characteristic depths of penetration of the pump, second harmonic, and modulating field in the nonlinear medium. A method for determination of the flat-band potential is suggested.

An investigation was made of the spectral characteristics of an SRS-active mode in a barium nitrate crystal by the method of two-photon Raman amplification. At room temperature the width of this vibrational mode was 0.6 cm^-1. The experimental temperature dependence of the linewidth was analysed within the framework of a theory of multiphonon relaxation.

The feasibility of constructing an all-optical dynamic memory based on stimulated Raman scattering (SRS) in a fibre waveguide is discussed. A possible use of an SRS inverter for regeneration of optical signals is considered. Theoretical and experimental investigations are reported of the functional properties of an SRS inverter. The construction and investigation of a prototype all-optical fibre memory with an SRS inverter are described. The repetition frequency of the optical pulses along a fibre loop is 100 MHz and the loop capacity is 500 bits. Pulse circulation is reported over a period of 10 min at an average pump power of 2 W. The response time and the energy requirements of devices with an SRS inverter, and ways of improving their characteristics are considered.

An investigation is reported of the special features of the temporal and spectral characteristics of radiation reflected from and transmitted by a plane-parallel plate when this radiation is in the form of a train of plane electromagnetic waves which is limited in space. Attention is drawn to the possibility of conversion of trains of electromagnetic waves into a sequence of shorter trains separated by distances equal to their length. Possible applications of such trains are considered.

Long delays of stimulated emission from homojunction lead chalcogenide lasers are shown to be due to the appearance of a thermoelastic waveguide because of inhomogeneous heat release, low thermal diffusivity, and high strain sensitivity.

Two-dimensional numerical calculations were made in a study of heat-conduction and hydrodynamic equalisation of the asymmetry of heating and compression of 'laser greenhouse' targets. It is shown that small-scale perturbations become equalised by the high thermal conductivity and by transverse shock waves in the absorber component of a target. It is proposed to use working targets with a surface relief for compensating the strong heating asymmetry associated with coupling of laser radiation through apertures in an outer chamber.

An analysis is made of the influence of the scattering-particle size on the spatial and temporal structure of the optical signal in a laser anemometer with a probe interference field. Estimates are obtained of the power spectrum and of the average signal frequency for a Poisson flow of finite scattering particles.