Table of contents

The process of recrystallization of the surface of GaAs during pulsed laser annealing was investigated using generation (by reflection from the surface) of the second harmonic of mode-locked radiation from another laser. The recrystallization time did not exceed 30–40 nsec. The results were accounted for by the melting theory of laser annealing.

A review is given of recent progress in the development and applications of spatial light modulators (controlled transparencies) in the form of photosensitive multilayer structures containing a liquid crystal. A considerable improvement is reported in the most important parameters of the process of modulation of light such as the sensitivity (now 10⁻¹¹ J/cm²), resolution (now 300 mm⁻¹), and speed of response (hundreds and thousands of frames per second). There has been an expansion in the functional capabilities: the modulators can now handle infrared radiation, identify boundaries of objects and regions with time-dependent illumination in converted images, modulate light at one or several stable transmission levels, etc. Some new applications are described, particularly those in processing of television images and in real-time interferometry.

An investigation is made of the feasibility of determining three temperatures of the active medium of CO₂ lasers (the gas translational temperature and the vibrational temperatures of the lower and upper active levels) by an analysis of the results of measurements of the small-signal gain for vibrational–rotational transitions in the regular 00⁰1–10⁰0 and 00⁰1–02⁰0 bands using the nonlinear least-squares method. It is shown that these data are inadequate for unambiguous determination of the vibrational temperatures.

The results are given of measurements of the electron density in the interval between dissociation and excitation pulses in CuBr and CuI lasers. The efficiencies of the different electron density relaxation mechanisms are analyzed. A comparison is made with similar results for a self-heating copper vapor laser.

An analysis is made of the use of solar radiation in space to pump a cw gas-flow iodine laser emitting at 1.3 μ. It is shown that the high probability of dissociation of I₂ molecules by solar radiation at an iodide vapor pressure of 0.25–0.5 kPa when the degree of concentration of solar radiation is 100–200 removes the constraint on the maximum duration of irradiation of the laser iodide associated with quenching of I(²P_1/2) by I₂ molecules. In this case, the maximum duration of irradiation is governed by heating and pyrolysis. Numerical calculations and analytical estimates are used to analyze three variants of a solar-pumped iodine laser: using pure CF₃I iodide purified from I₂, using a CF₃I–(CF₃N)₂ mixture not purified from I₂, and using (CF₃)₃ CI not purified from I₂. Problems of circulation, purification, cooling, and irradiation of the active gas are examined.

An investigation was made of the main characteristics of pyroelectric detectors consisting of a thin oriented polycrystalline organic film, designed for response to pulses of 10–100 nsec duration and of ≥ 10⁻⁶ J energy in the wavelength range 0.266–16 μ. Prospects for the future use of such detectors are considered.

A study is made of the generation of a giant second harmonic and of the giant Raman scattering of light on a surface separating a solid from an electrolyte. The intensity of the giant second harmonic is shown to depend on the electrolyte composition, nature of the solvent, and presence of additives in the form of organic compounds. An electric field in a double layer may alter the nonlinear optical susceptibilities of the components of the double layer. Generation of a giant second harmonic may provide a convenient method for telemonitoring of the surfaces of metals and semiconductors (for example, for the determination of the zero charge potential) in technological processes. The gain of the second harmonic generation process is (3–5)×10⁴, which is of the same order as the gain of the giant Raman scattering, (5–10)×10⁴, which depends reversibly on the electrode potential φ, suggesting a common mechanism of these phenomena: an increase in the local field near projections of the surface of a metal.

A detailed experimental investigation was made for the first time of the time-resolved spectral and energy characteristics of a pulsed electron-beam-controlled CO laser, as a function of the water vapor content in an intracavity cell. It was found that using such a cell made it possible to considerably increase the number of rotational lines in the emission spectrum and thereby to extend the tuning range of the emission frequency of a pulsed electron-beam-controlled CO laser. The introduction of water vapor into the resonator made it possible to match the emission spectrum of such a laser to the atmospheric transparency windows in the range 5–5.5 μ. The possibility was investigated of utilizing a column of atmospheric air as a selective absorption cell.

An investigation is made of the enrichment of the initial state with atoms or ions of rare radioactive isotopes by using cw laser radiation. Estimates of the enrichment of metastable levels are obtained for ¹⁴C and ²⁶Al atoms and ⁴¹Ca⁺ ions. The radioactive isotope ¹⁴C is used as an example to discuss the combination of enrichment of the initial state with multistage selective photoionization of atoms. It is shown that in this case, it is realistic to expect a selectivity higher than 10²⁰.

A method was developed for the stabilization of the emission frequency of a tunable laser by an extracavity interferometer. In contrast to the Stark stabilization method suitable only for some CO₂ and CO laser lines, the proposed method is universal and it makes possible stabilized frequency tuning of any transition in the emission spectrum of a laser. An analysis is made of the frequency stability achieved for different interferometer configurations. The requirements are formulated in respect of selection of the interferometer parameters and automatic frequency tuning systems. It is shown that the method of a two-section interferometer makes it possible to widen the band of stabilized tuning of the emission frequency compared with the methods currently available. A report is given of an experimental demonstration of stabilized tuning of two emission lines of a CO₂ laser.

A method is proposed for calculating, in the geometric-optics approximation, the energy characteristics of a ring variant of a chemical supersonic DF–CO₂ laser with an unstable telescopic resonator formed by mirrors of toroidal shape. Typical distributions of the gain in the active medium are obtained in the absence and presence of lasing, and the intensity of the radiation in the lasing zone is calculated. The chemical efficiency is found as a function of the parameters of the optical system.

Apparatus based on a YAG laser with a picosecond time resolution was developed for investigating fast processes in semiconductors. Experimental determination of the time dependence of the reflection coefficient made it possible to identify the mechanisms responsible for changes in the carrier density in silicon and in gallium arsenide. In particular, studies were made of the processes of diffusion and linear recombination in single-crystal, polycrystalline, and amorphous samples of silicon and of gallium arsenide.

A numerical simulation of a slow adaptive optical system is reported. It is suggested that the wavefront of a beam can be controlled by a phase corrector with three degrees of freedom: the angle of inclination of the beam and two focusing radii along mutually perpendicular directions. Various algorithms for the control of the corrector coordinates are considered and a study is made of the effectiveness of suppression of thermal defocusing in a moving nonlinear medium.

An investigation was made of the generation of light in II-VI semiconductors along a streamer channel. The establishment of a high gain amounting to (4–6)×10³cm⁻¹ inside a channel resulted in lasing in resonators with mirrors formed by optical inhomogeneities of a crystal. The radiation emitted in the direction of the streamer emerged from the lasing region and propagated through the bulk of the crystal, whereas in the opposite direction it traveled along an optical waveguide formed by the streamer channel. The angular distribution of the radiation in the forward direction was influenced by the Franz–Keldysh effect at the head of the streamer and by distortions of the light wave in secondary streamer channels. A considerable increase in the radiation energy in the forward direction observed at streamer velocities of at least 4×10⁹ cm/sec was attributed to the attainment by the streamer head of a velocity higher than the group velocity of light at the wavelength of the emitted radiation.

An experimental investigation was made of a five-pass telescopic amplifier with a phototropic switch in its resonator. The use of GLS-21 phosphate glass made it possible to increase the energy of single-pulse radiation to 90 J and the energy efficiency was 0.5%. The angular divergence of the radiation measured at 0.7 of the maximum energy level was 0.18 mrad and it was limited by optical inhomogeneities of the active element in its "cold" state. An analysis was made of possible configurations of a telescopic amplifier with a switch in its resonator. A simple model was developed for calculating the energy characteristics of such amplifiers on the basis of the available experimental results.

An analysis is made of the propagation, in a nonlinear medium, of wave beams with small-scale amplitude and phase modulation without assuming a low beam intensity. It is found that at high intensities there is a type of angular divergence that ensures accurate compensation for self-focusing. In this case, unlike low-intensity beams, the required angular divergence is characterized by the maximum intensity not in the direction of the beam axis but at a certain angle with respect to the axis.

A study was made of the influence of the pulse repetition frequency on the characteristics of lasing and of the electrical excitation pulses in a pulsed copper vapor laser. The work was performed at a constant voltage on the discharge capacitor while varying the pulse repetition frequency from 6 to 20 kHz in a 20-mm diameter discharge tube having a 70-cm long heated section. An analysis of these experimental and published data was used to discuss possible mechanisms limiting the pulse repetition frequency in these lasers.

An investigation was made of the behavior of the recoil pressure p(t) under quasisteady conditions of metal evaporation (Pb, Bi) induced by submillisecond neodymium laser pulses. The experimentally determined dependence p on the radiation intensity I did not agree with the predictions of the thermal model in which the ratio p/I depended weakly on I . This discrepancy could be explained allowing for the radial displacement of the melt from the irradiation zone.

A description is given of a lidar system for determination of the distance to the Moon. This system has a resolution of 1 nsec and it consists of a laser transmitter, a photodetector, a pulse processing subsystem, as well as timing and guiding subsystems. The system has been in operation since 1978. The parameters of the subsystems and the errors in the determination of time intervals are discussed.

A theoretical analysis is made of a resonant interaction of two-level atoms with the field of three strong spatially separated standing waves. It is shown that this interaction should give rise to a resonance of width equal to the reciprocal of the transit time between the fields and this resonance should be observed for all rational values of the ratio of the distances by which the waves are separated in space. Calculations are made of the corrections to the flux of particles excited in an atomic beam and to the power absorbed by a gas because of the interaction of atoms with the three fields. It is found that because of recoil a resonance of this kind splits into an infinite series of components separated by intervals of the order of the recoil frequency. The results are given of a numerical calculation of the strongest components of this resonance. The shift and deformation of lines due to quadratic Doppler effect are considered. The results of the calculations are compared with the available experimental data.

Resonant absorption of radiation due to the ²D_3/2→²P⁰_1/2 and ²D_5/2→²P⁰_3/2 transitions in the copper atom was used to measure the lifetimes τ of the ²D_3/2 and ²D_5/2 states, which are the lower active levels of a copper vapor laser. It is found that at temperatures and pressures of the active medium typical of a copper vapor laser the lifetime is τ ∼10⁻⁶ sec and decreases as the power deposited in the discharge increases. Possible mechanisms for decay of the states under study are discussed.

A calculation is reported of the temperature of an active element in an illumination enclosure containing a cw YAG:Nd³⁺ laser. It is shown that, depending on the absorption coefficient δ of the active element at the 1.06 μ wavelength, quenching of stimulated emission may result in an increase (in the case of low values of δ) or a reduction in the temperature of the active element. A calculation, carried out allowing for the self-heating of the active element by stimulated radiation and absorption of part of the spontaneous radiation, demonstrates that the heat balance in the active element inside a laser illumination enclosure is attained when the quantum efficiency of the luminescence originating from the ⁴F_3/2 level is close to unity. The calculations are supported by experimental data.

A study was made of the lasing characteristics of new active media in the form of microporous glass containing a dye. The efficiency of conversion of the pump radiation, and the spatial and frequency spectra of the output radiation were determined for lasers with dispersive and nondispersive resonators and with active elements in the form of an ethanol solution of rhodamine 6G or microporous glass containing either rhodamine 6G or its ethanol solution. It was found that the use of active elements made of microporous glass and containing rhodamine 6G improved considerably the spatial characteristics of the output radiation compared with those obtained using liquid active media.

A theoretical investigation was made of the dependences of the shape of emission pulses from a CO laser on the rates of elementary processes occurring in its active medium. It was found that the lasing enhancement following the end of a discharge, observed experimentally under certain excitation conditions, is determined by the ratio of the pumping and vibrational-vibrational (VV) exchange rates. Good agreement was obtained with experiment. It was concluded that the rates normally employed in the calculations for the VV exchange between CO molecules represent a fourfold or fivefold overestimate.

Theoretical estimates of narrowing of the emission line of an injection laser with an external resonator are obtained for different mechanisms that determine the line width. If the width of a line emitted by a laser diode without an external feedback ("initial" width) is governed by phase fluctuations because of the spontaneous noise, then the width of a line emitted by an injection laser with an external resonator is a quadratic function of the line width of the resonator itself. However, if the initial width is determined by fluctuations of the refractive index of the active medium, then the same dependence is linear. It is shown that external resonator configurations used so far have not been optimal. Ways are suggested for introducing high-Q interferometers into the external part of a composite resonator of an injection laser, which should make it possible to reduce the width of the emission line of a laser with an external resonator to ∼1–10³ Hz without altering the dimensions. An analysis is made of the influence of a wide-band electronic circuit for automatic phase control (based on an external high-Q interferometer) on the emission spectrum of an injection laser. It is shown that the "natural" width of an emission line may be reduced by between four and six orders of magnitude if the proposed method is used.

A study is made of the influence of resonant multiphoton ionization and of the saturation effect on the efficiency of a parametric interaction in gaseous media. Two cases are analyzed separately: generation of short-wavelength radiation when the frequency of the output wave corresponds to a transition from the ground state to a continuum and conversion of infrared radiation. The optimal conversion conditions are identified.

An analysis is made of continuous generation of the second harmonic as a result of thermal self-interaction in beams with an elliptic cross section. It is shown that cylindrical focusing of the fundamental-frequency beam in the direction of noncritical phase matching makes it possible to compensate partly thermal self-interaction only when the beam power is much higher than the critical value. The efficiency of conversion to the second harmonic can also be increased by some misalignment of the wave vectors on the beam axis. In the case of generation in the direction of critical phase matching the efficiency of such optimization methods depends on the ratio of the temperature gradients of the refractive index at both (fundamental and second harmonic) frequencies. Dependences of the harmonic power on these parameters are investigated for different birefringence angles and recommendations are put forward on optimization of the process in a wide range of crystals.

A description is given of a method for determining the fluctuating parameters of a scattered wave (amplitude, phase, and polarization) from the correlation functions of the photocurrent generated in a double-beam interferometer. It is shown that antisymmetric (relative to the carrier frequency) components of the mutual spectra can be separated and small frequency shifts can be detected. Asymmetric spectra with a shift of ∼10 mHz are reported for a nematic liquid crystal in the dynamic scattering regime.

An experimental investigation was made of the dependence of the sensitivity of thermal detectors on the power of λ =10.6 μ radiation. The detectors were in the form of large-mesh bolometric grids made of a platinum microwire, and the tests were carried out at radiation power densities up to 190 W/cm².

Output radiation pulses of ≲ 1 μsec duration (at half-height) and with a peak power up to 5 MW were generated by switching the Q factor of the resonator of a cooled electron-beam-controlled CO laser. The dependences of the lasing characteristics on the composition of the gas mixture, excitation energy density, rate of Q switching, and Q switching delay were determined. The time needed for the formation of a plateau of the distribution function of the populations of the vibrational levels of the CO molecule in the course of vibrational–vibrational (VV) relaxation found experimentally was considerably greater than the time predicted theoretically.

A promising method is suggested for regeneration of the active mixture of high-power technological electron-beam-controlled CO₂ lasers by adsorption on silica gel of nitrogen oxide impurities formed during an electron-beam-controlled discharge. It is reported that the use of silica gel makes it possible to stabilize the energy characteristics of lasers of this kind more effectively than by catalytic regeneration and this can be achieved at the expense of considerably lower (by more than one order of magnitude) energy losses.

An experimental investigation was made of the influence of the concentration of a lasing dye in an active medium with an optimal optical density on the efficiency of longitudinally pumped lasers. An increase in the dye concentration above a certain value reduced the lasing efficiency. An analysis was made of possible reasons for this behavior: it was found that the results could be explained by the convergence of the lasing thresholds in the longitudinal and transverse (relative to the pump beam) directions with the losses due to light scattering in the nonlinearly absorbing medium (these losses originated from thermal transient self-defocusing and also from stimulated thermal scattering).

An investigation was made of the influence of the pressure of a buffer gas (N₂) on three-stage photoionization of molecular iodine. A tunable dye laser and a nitrogen laser were used to provide the photoionizing radiation. Broadening of the spectral photoionization lines on increase in the nitrogen pressure was observed. The nature of the dependence of the number of ions formed by one pulse on the nitrogen pressure was influenced strongly by the dye laser wavelength. The experimental dependences were used to calculate the rate constants of the relaxation processes occurring in iodine molecules in electronically excited states.

It is shown that monolithic structures with two coupled waveguides can ensure effective utilization of the absorbing surface of a large-area photodetector. The linear dimensions of this surface in the direction of propagation of the radiation depend only on the parameters of the waveguides themselves and can be varied within wide limits. The distribution of the photocurrent in the same direction is sinusoidal.

It was found experimentally that the dynamic reflection range was ∼10⁷ for a four-wave interaction in SF₆ and BCl₃ gases absorbing resonantly at 10.6 μ. In an optically dense nonlinear medium the power efficiency of reflection of a signal wave was independent of its intensity right up to the value comparable with the intensity of the first reference wave. On the side of low signal intensities the dynamic range was limited by the scattering of the reference waves on the windows of a gas cell.

The feasibility of using mirrors with a variable reflection coefficient for wavefront reversal was demonstrated at the wavelength of 10.6 μ. The reversed wave was generated by reflection from a mirror formed as a result of profiled evaporation of a selenium film on a copper substrate. The coefficient of reflection into the reversed wave reached ∼45% (of the total energy). The results of theoretical calculations were found to be in satisfactory agreement with the experimental data.

A comparison of the experimental yields of x-ray lines of multiply charged silicon ions with the results of numerical calculations carried out using the Zarya program was used to find the coefficient representing the limit set on the electron thermal conductivity of a laser plasma. The experiments were carried out at laser radiation power densities I = (1–2)×10¹⁴ W/cm² (λ = 1.06 μ) reaching the surface of an SiO₂ shell target in the case of spherical irradiation geometry. The best agreement between the calculations and the experiments was obtained for a weak limitation on the electron thermal conductivity.

A theoretical analysis is made of the amplification of the Stokes component of stimulated Brillouin scattering in a single-mode fiber waveguide in the presence of external transient perturbations of the waveguide.

Inversion of the populations of the n = 5,4, and 3 levels of hydrogen-like F ix ions in a recombining laser plasma was studied. The calculated values of the gain were 10⁻²–10⁻¹ cm⁻¹ and these agreed with estimates based on measurements of the absolute intensities of the spectral lines representing spontaneous emission. Geometry of expansion of a plasma jet was studied.

A brief review is given of the more important papers presented at the Fourth International Conference on Lasers and Their Applications. The Conference topics were as follows: lasers (development of new and improvement of old types); laser thermonuclear fusion; nonlinear optics; laser spectroscopy; optoelectronics; applications of lasers in chemistry, biology, and medicine; fundamental investigations; other applications.