Table of contents

An investigation was made of an optical parametric oscillator (OPO) in which a Ba₂NaNb₅O₁₅ crystal was pumped synchronously and the repetition frequency of picosecond pulses was 140 MHz. The OPO was pumped by the second harmonic of the radiation from a cw YAG:Nd³⁺ laser with active (acoustooptic) mode locking. The average power of the OPO radiation reached 56 mW near the degenerate regime; the energy efficiency was 12% and the duration of the output pulses was 37 ps.

A study was made of the feasibility of reducing the width of the emission spectrum of a single-frequency injection laser of the ILPN-202 type (λ = 1.3 μm) by establishing an external optical feedback. The temperature of the laser was stabilized near 18°C to within 5 × 10 ⁻³°C and the current was kept constant to within 10 μA. The emission spectrum was recorded using a Fabry–Perot scanning interferometer. The experimental results were interpreted on the basis of a theory allowing for the amplitude–phase coupling. Criteria of stable single-frequency emission in the presence of an external optical feedback were established. The following quantities were determined: the amplitude–phase coupling constant α = 4 ± 0.5; the width of the emission line of an isolated injection laser Δν₀ = 60 MHz obtained when the injection current exceeded the threshold by the factor J/J_th = 2; the coherence length ofthe radiation of an isolated laser (236 cm) in the case when J/J_th = 1.3. An estimate was obtained of the width of a laser line in the presence of an external optical feedback and the value obtained was Δν = 600 kHz. The spectrum of an optical signal frequency modulated at 30 MHz was determined when the depth of modulation of the pump current was less than 1%. The slope ofthe modulation characteristic of the injection laser in the presence of an optical feedback was 200 MHz/mA.

A reduction in the divergence of radiation emitted by InGaAsP heterolasers with increasing temperature was observed experimentally for the first time. A model of a waveguide with varying parameters was used in calculations which can explain the reduction in the width of the angular distribution and the increase in the threshold current with temperature. The results support the waveguide mechanism of the anomalous rise in the threshold current of long-wavelength InGaAsP heterolasers with temperature.

Lasing was achieved in barium vapor near an absorption line at the λ = 553.5 nm wavelength corresponding to the 6¹S₀–6¹P₁ resonance transition. Barium vapor produced in an arc discharge was placed in the resonator of a dye laser having a stimulated emission spectrum of ~ 5 pm half-width. In the presence of additional "weak" broad-band radiation, the lasing spectrum consisted of two components symmetric relative to the center of the absorption line. This lasing was due to stimulated Raman scattering by a population grating and was most efficient in regions where the difference between the frequencies of the strong and weak fields coincided with the Rabi frequency. A quantitative comparison was made between theory and experiment on amplification without population inversion in a two-level system. Holographic interference spectroscopy was used to study experimentally the behavior of the susceptibility (the absorption coefficient and the refractive index) under conditions of resonant interaction. Good agreement was obtained between the behavior of the dispersion of the refractive index and the theoretical dependence. The highest gains were k_v ≈ 10 cm ⁻¹ when the barium atomic concentration was N ≈ 5 × 10¹⁶ cm ⁻³.

Experimental and theoretical investigations were made of the characteristics of the radiation emitted by a two-frequency helium–neon laser when mutually orthogonal transverse magnetic fields were applied to its active medium. It was found that the dichroism and birefringence of the active medium were minimized when the magnetic field strengths were equal. As a result, a stable two-frequency lasing regime was realized in which the waves were of similar intensities and had quite a low beat frequency which was stable in the presence of changes in the resonator length, in the pump intensity, and in the magnetic field strengths. An analysis was made of the energy and frequency characteristics of the laser as a function of the amplitude and phase anisotropies of the resonator, magnetoplasma effects, and the isotopic composition and pressure of the active mixture.

The contrast, against the background of Q switching of radiation from a neodymium laser with a phase-conjugation mirror, was determined in an oscillator–amplifier system. The mirror ensured contrast enhancement by a factor of 10²–10³, which was in agreement with calculations.

An experimental investigation was made of the operation of a KrF electron-beam laser operating under conditions of high specific powers and excitation energies. An optimal Ar–Kr–F₂ gas mixture operating at a pressure of 5 atm ensured a specific output energy of 60 J/liter and an efficiency of ≈ 6%. The electrodynamic effects of the interaction of the electron beam with the plasma influenced the laser efficiency.

Life tests were carried out on stripe and wide-contact GaInAsP/InP heterolasers emitting in the spectral range 1.06–1.1 μm at room temperature. Stripe diodes emitting spontaneous radiation were tested at heat-sink temperatures 20, 40, 60, and 70 °C. The activation energy of the degradation process was estimated to be 0.65 eV. Stripe heterolasers were tested by cw operation for 420 h at 50 °C, equivalent to 4.6 × 10³ h at room temperature. Pulsed heterolasers were tested at 80 °C for 400–600 h. The equivalent lifetime at room temperature was (3.2–4.8) × 10⁴ h. The predicted lifetimes were (5–9.4) × 10⁴ and 3 × 10⁴ h for pulsed and cw heterolasers, respectively.

The harpoon mechanism of formation of XeCl* excimer molecules was identified experimentally and various characteristics were studied when Cl₂–Xe–He mixtures were excited by an electric discharge in the chamber of a standard ÉLI excimer laser. Time dependences of the spontaneous emission of XeCl* molecules and of the gain and absorption coefficients were investigated as functions of the Cl₂ and Xe pressures. The investigation showed that, firstly, the harpoon mechanism of formation of XeCl* molecules predominated over the ion recombination mechanism (at least at the leading edge of the discharge pulse) and its role increased with decreasing input energy. Secondly, there was no additional absorption compared with conventional HCl-based mixtures, except for the absorption due to the presence of Cl₂.

An analysis is made of methods for improving the quality of radiation propagating in laser systems using "soft" diaphragms (apertures) and spatial filters. These involve solving the problem of diffraction of a plane electromagnetic wave by a circular diaphragm for arbitrary distances from the diaphragm, taking into account large diffraction angles. By solving the appropriate diffraction problems, the diaphragm parameters can be optimized. It is found that the amplitude distribution of the field can be efficiently controlled by varying the diaphragm radius of a spatial filter. It is suggested that absorbing diaphragms may be used to suppress depolarization of the radiation due to diffraction.

An XeCl excimer laser excited by nanosecond microwave pulses of up to 500 MW was assembled. The optimal mixture in terms of lasing efficiency was found to be Ne:Xe:CCl₄ = 300:60:1 at a total pressure of 1.5 atm. The coefficient of energy coupling of the incident microwave power into the discharge plasma was ~ 70%. The maximum efficiency was 5.3% when the resonator length was 70 cm and the specific microwave pump power was ~ 1 MW/cm³. It was found that, in terms of the power deposited in the medium, the lasing efficiency was no lower than that under electron beam pumping and the total efficiencies in these cases could be comparable.

It is shown that an inverted medium may be cooled as a result of amplification of light both in the case of propagation of a pulse of duration less than the VT relaxation time in a gas at rest and also in the case of continuous emission by a moving gas. The main mechanisms responsible for the cooling of a molecular gas in the case of resonant amplification of light are analyzed.

The interaction between coherent subpicosecond ultraviolet radiation and an XeCl molecule was modeled numerically. It was established that a two-level model of the spectrum of a molecule having stable upper and rapidly decaying lower levels can be applied to study the amplification of 0.1–0.7 ps pulses. A numerical analysis was made of the amplification of 0.2–0.5 ps pulses and the influence of coherence effects was assessed. The conditions of validity of the Frantz–Nodvik equation to describe the energy characteristics of the amplified pulses were analyzed.

An investigation was made of the luminescence and lasing spectra of a calcium niobium gallium garnet crystal activated with Cr³⁺ and Nd³⁺ ions. An effective nonradiative energy transfer from the chromium to the neodymium ions was discovered. The probability of an elementary Cr→Nd interaction event was determined. A detailed study was made of the Stark structure of the ground (⁴I_9/2) and metastable (⁴F_3/2) levels of Nd³⁺. Selective laser excitation was used to identify four main optical Nd³⁺ centers for which the splitting schemes of the ⁴I_9/2 and ⁴F_3/2 levels were constructed. Lasing was observed at 300 K as a result of the ⁴F_3/2→⁴I_11/2 transition in crystals activated with Cr³⁺ and Nd³⁺.

An investigation was made of the kinetics of changes in the absorption spectra of sapphire crystals containing radiation color centers at various annealing temperatures. Studies were made of the excitation and luminescence spectra, the quantum efficiency, and also the feasibility of stimulated emission from color centers in the 1.0-μm range. It was established experimentally that the conversion efficiency of the radiation depended on the integrated neutron flux and the temperature of the subsequent heat treatment. It was found that, in a laser utilizing sapphire containing color centers, the emission wavelength could be tuned continuously between 0.75 and 1.1 μm by using a single pump source. The conditions for photostability of the color centers were determined.

An investigation was made of the spectra of oxotungstates Sr_2.95(Nd, Nb)_0.025WO₆ and MeLa₂WO₇:Nd (Me = Sr, Ba) prepared in the form of powders as a result of solid-phase synthesis. Powders of BaLa₂WO₇ exhibited stimulated emission of neodymium in the region of 1.07 μm at liquid nitrogen and helium temperatures, as demonstrated by a strong narrowing of the luminescence spectrum and a threshold. A characteristic "pedestal cutoff" dependent on the pump power was observed in the excitation spectra of the luminescence. A study of the luminescence and luminescence-excitation spectra of europium oxotungstates was carried out in order to obtain more detailed information on the structure of the luminescence centers.

Analytic and numerical investigations are reported of the self-interaction of a randomly modulated light pulse subject to quadratic and cubic dispersion. An analysis is made of the influence of the third-order dispersion on the dynamics of transformation of such characteristics of a pulse as the time envelope of the intensity, duration, shift of the energy center, frequency spectrum, coherence time, dispersion of intensity fluctuations, and mode composition of the radiation. A comparison is made of the results obtained by analytic and numerical methods.

Equations are proposed for the nonlinear interaction between optical pulses and dye solutions allowing for the contribution of the stimulated emission band to the nonlinear dispersion of the solution. A numerical solution of the equations showed that, for pulses having a carrier frequency near the absorption band of the solution, phase modulation of the pulses transmitted by the solution took the form of a deviation of the instantaneous frequency toward the red, and the "chirp" was positive.

Numerical modeling is used in an analysis of the characteristics of the self-interaction of high-power laser radiation propagating in the atmosphere in the presence of fluctuations of the wind velocity. An analysis is made of the cases of fluctuations of the velocity of the medium parallel and perpendicular to the direction of the average flux. The random sampling (Monte Carlo) method is used in estimating the average intensity and the variance of the intensity fluctuations in the focal plane of a radiation source. It is shown that fluctuations of the wind velocity weaken considerably nonlinear refraction and thermal spreading of focused beams.

An investigation was made of the infrared reflection spectra and of the frustrated total internal reflection spectra of single-crystal and polycrystalline samples of BeO, which is a promising material for quantum-electronic devices operating in the middle infrared range. It was found that "resonance" dips in the infrared reflection spectra of these materials observed in the frequency range of CO₂ laser radiation are due to excitation of surface polaritons. An analysis was made of the possibility of using surface periodic structures in BeO, formed as a result of interference of the incident laser radiation and surface polaritons, as frequency-selective components of resonators in CO₂ waveguide lasers.

An investigation was made of cascade generation of the fifth harmonic of radiation from a wide-aperture neodymium laser in KDP and ADP crystals. It was found that the main factor limiting the efficiency of the process was two-photon absorption of the radiation. The coefficients of two-photon absorption of the radiation at the frequency of the fourth harmonic of a neodymium laser were determined for KDP (0.5 cm/GW) and ADP (0.35 cm/GW). The efficiency of conversion of the second harmonic radiation energy of a neodymium laser to the fourth harmonic was 92%, the highest achieved so far. The efficiency of conversion to the fifth harmonic was 19%, close to the maximum attainable for KDP crystals.

Calculations are reported of the threshold and energy characteristics and of the kinetics of evolution of frequency-degenerate parametric oscillation in photorefractive crystals with a nonlinearity governed by spatially fluctuating photogalvanic currents.

A theoretical analysis is made of the compensation of a positive nonlinear refractive index of insulators containing impurities by saturation of the refractive index in the anomalous dispersion region. The exact solutions are obtained for self-similar amplification of a laser pulse under conditions of homogeneous broadening of a resonant transition. It is shown that profiling of the time envelope of a steady-state bleaching wave of a saturable absorber together with detuning of the pulse frequency upward from resonance makes it possible to increase the threshold of small-scale self-focusing.

An experimental investigation was made of thermal self-interaction of high-power laser beams of circular and elliptic cross section under conditions of induced convection in the case of vertical (downward) propagation of radiation in a cell containing a silicone liquid. The velocity of the induced convective flow was measured and its dependence on the power of the heating radiation was determined. In the case of axisymmetric beams it was found that the angular divergence increased as a result of thermal self-interaction by an amount which was an order of magnitude less than that predicted by a theory postulating a thick lens in a medium at rest. Changes in the shape of an elliptic beam, which became more symmetric, were observed. The characteristic times of the transient processes were measured. These processes were found to be determined mainly by convective heat transfer.

Concurrent resonance stimulated Raman scattering (RSTRS) of radiation from a CO₂ laser was observed in NH₃ for the Gs(5, 0) → 1ν₂s(5, 0) transition. Four new lasing lines were observed in the wavelength range 67 μm. The tuning range in the far infrared was 4.5 cm⁻¹. The power of infrared radiation reached 2 MW for a quantum efficiency of the process 36%. Depletion of the pump radiation, time dependence of the process, and forward–backward asymmetry of RSTRS were observed. A study was made of the difference between the RSTRS characteristics for one and three passes through a multipass cell. The stimulated Raman scattering thresholds were different for linear and circular pump polarizations.

An investigation was made of the spatial characteristics of the pump and "Stokes" beams under conditions of resonant stimulated Raman scattering by NH₃ in a multipass cell. In spite of the large difference between the wavelengths of the pump and Stokes radiation, some similarity was observed between the output dimensions of their beams. It was found that, in a wide range of frequency tuning, resonant self-interaction influenced strongly the propagation of CO₂ laser radiation and Stokes radiation in a multipass cell.

Numerical and experimental investigations led to the construction of a source of powerful stable high-contrast picosecond pulses in a YAG laser with double active modulation and of a high-efficiency fiber-optic compressor employing a nonlinear crystal.

An experimental investigation was made of a redistribution of the intensity in the transverse cross section of a surface optical wave focused by a spherical geodesic lens in a Ti-diffused waveguide formed in a Y-cut LiNbO₃ crystal. A wave theory of geodesic waveguide lenses was used to calculate the optical field in the focal and several other transverse cross sections for a spherical lens with parameters found experimentally. The results of these calculations were in good agreement with the experimental data.

The distribution of the probability of photocounts was determined for amplitude-modulated radiation transmitted by a multimode fiber waveguide. A dependence of the variance of the number of photocounts on the average power of the radiation coupled into the fiber waveguide was observed. This effect was attributed to the generation of sound at the frequency of modulation of the optical radiation due to electrostriction and thermal mechanisms. A formula describing the dependence of the variance of the number of photocounts on the average power was derived. A study was also made of the dependences of this characteristic on the frequency and depth of modulation of the radiation.

Experimental and theoretical investigations were made of acoustic processes occurring during laser breakdown of crystalline argon. It was found that the main mechanism of generation of elastic vibrations was the attenuation of "strong" (those with a gasdynamic stage) shock waves. The spectrum of the generated vibrations was described satisfactorily by a model of an ellipsoidal loaded region elongated along the beam axis and emitting waves into an elastic medium. The proximity of an uneven wall of the crystallization chamber to the optical damage zone had a considerable influence on the characteristics of the process.

A classical one-dimensional model of ultraviolet dissociation of molecules of iodides suitable for lasing is developed allowing for the anharmonicity of the vibrations, "bending" of the term representing an electronically excited state, change in the electron matrix element, and possible deviations of the vibrational distributions of the dissociating molecules from equilibrium. A detailed analysis is made of the ultraviolet dissociation spectra of the CF₃I molecules for which all the characteristic parameters are deduced. This model is in quantitative agreement with the experimental results obtained in a wide range of wavelengths (λ = 275–450 nm). The feasibility of ultraviolet absorption probing of vibrational distributions is demonstrated by considering the example of the spectra of the CF₃I molecules interacting with the radiation field of a CO₂ laser.

An investigation was made of the self-radiation from plasma sites which appeared when intense CO₂ laser radiation of microsecond duration propagated in the real atmosphere.

A study is made of the problem of finding the optical characteristics and level populations of H-like ions in an expanding planar plasma layer. An analysis is carried out of the results of recent experiments demonstrating enhanced emission and amplification due to transitions of multiply charged ions. It is shown that the experimentally observed anomalies are not in agreement with the present ideas concerning the conditions for the appearance of inversion and amplification due to the 3–2 transition.

A scanning electron microscope was used with a pulse-periodic CO₂ laser to discover the laws governing the correlation of the modified microrelief of metal surfaces, subjected to the action of multiple laser pulses, with the emission of charged particles and the luminescence of the irradiated zone. It was established that the influence of sorption and laser-induced desorption on the emission signals may be manifested differently depending on the regime of current generation in the "target–vacuum chamber" circuit.

A study of generation of a magnetic field in a plasma, formed by radiation of moderate power on the surface of an insulating target, is reported. It is assumed that the source of an emf is the thermoelectric effect in an inhomogeneous plasma. The structure of magnetic fields and the distribution of gasdynamic parameters in a jet are found by numerical solution of a familiar system of equations considered in a two-dimensional cylindrical geometry. The patterns of the resultant systems of currents and of the associated magnetic field are given. The problems of diagnostics of plasma jets are considered.

An analysis is made of the influence of a longitudinal electrostrictive force on the formation and growth of a laser spark. It is shown that both under steady-state conditions and during slow combustion an allowance for the longitudinal electrostrictive force alters the parameters of a weakly ionized plasma and, in particular, it ensures a better agreement between the experimentally determined and theoretically calculated velocities of propagation of a plasma filament under slow combustion conditions.