Table of contents

The assumption of subthreshold noise excitation of modes with higher transverse indices is used in theoretical estimates of the degree of spatial coherence of the radiation generated in a laser emitting modes with just one (lowest) transverse index. It is shown that a comparison between the theory and experiment can be made only for specific geometry and operating conditions.

An investigation was made of the cathodoluminescence of epitaxial films and bulk crystals of InAs with carrier densities in the range n = 5×10¹⁵–10¹⁸ cm^–3 and p = 6×10¹⁶–1.5×10¹⁸ cm^–3. Stimulated emission was obtained when epitaxial n-type InAs films with an electron density in the range (2–6) ×10¹⁶ cm^–3 were excited by a 50 keV electron beam at temperatures of 85-200°K. The dependences of the output power on the pumping rate and temperature were determined and the evolution of the emission spectra was studied. The maximum stimulated radiation power was 22 W. Failure to achieve stimulated emission in the range n> 6×10¹⁶ cm^–3 was attributed to the enhancement of the role of impact recombination with increasing electron density.

It is shown that a nonlinear interaction of laser radiation and an additional slow electromagnetic wave with a homogeneous electron stream may give rise to a plane wave with the sum frequency and a high degree of coherence. This wave travels along a direction different from the direction of propagation of the primary waves. The intensity of the new wave is proportional to the product of the intensities of the primary waves and to the square of the electron density. It is shown that a wave of this kind can be used to measure the electron density in a plasma.

It is shown that an oscillatory circuit with a current breaker can be used to increase severalfold the pump power supplied to a laser utilizing self-terminating transitions. A description is given of the apparatus and results are reported on stimulated emission from nitrogen at γ = 337.1 nm.

A change in the resonance frequencies of the Fabry-Perot resonator of a GaAs laser diode with increasing pump current was detected and investigated. The value of ∂∊/∂N (∊ is the permittivity and N is the density of free electrons) was 2.6×10 cm⁻²⁰ cm³ when the current density was 15 kA/cm², temperature was 300°K, and wavelength was about 901 nm.

Combination of the functions of a transmission line and a distributed-gain quantum amplifier in a single light guide makes it possible to construct an attenuation-free optical communication line (with equal output and input signals). A calculation based on the example of transmission of a Stokes signal in a parallel pumping field in a glass fiber demonstrates the possibility of practical realization of a system of this kind.

An investigation was made of the texture and selective scattering of light by polymeric films with a liquid-crystal filler. A comparison was made with the selective scattering spectra of a plane texture of the same composition. Differences between the curves obtained for these two cases were discovered and analyzed.

Semiconductor solid solutions are used widely in lasers because variation of their chemical composition makes it possible to cover completely the spectral range 0.32–32 μ. Recently quaternary compositions such as AlGaPAs, GaInPAs, GaAlAsSb, and others have been investigated as laser materials. Interest in these compositions has arisen because pairs of semiconductor materials with a specified difference between the energy band structures but identical lattice periods ("isoperiodic pairs") are needed in heterostructures. A preliminary calculation of some important parameters of multicomponent compositions can be made using interpolation formulas. The use of multicomponent solid solutions has made it possible to improve the threshold characteristics of heterojunction injection lasers and, for example, to obtain stimulated emission of wavelengths 1.06 and 1.78 μ at room temperature.

It is argued that there is a need for an All-Union standard which would include scientifically based maximum permissible levels of irradiation of individual organs and of human body as a whole. The standard should also provide a unified classification of all the laser equipment from the standpoint of the operational safety of the service personnel and describe the methods and means for laser radiation dosimetry. Some recommendations are made on the classification of the exposure criteria and methods for measuring the energy (power) density of laser radiation.

An investigation was made of the probability of error in an atmospheric laser communication line. A duplex communication system with a helium-neon laser transmitted discrete information (in combination with pulse-code modulation equipment or with a computer) under conditions of orthogonal polarization modulation. The probability of an error in the system depended strongly on the degree of turbulence of the atmosphere. A comparison was made of the influence of intensity fluctuations and spatial variations of the laser beam on the error probability. This probability was found to be governed not only by the intensity fluctuations but also by the fluctuations of the laser beam cross section at the point of reception; the influence of beam wandering could be eliminated by automatic beam alignment.

The geometric approximation is used in a calculation of the energy characteristics of a telescopic amplifier containing a medium characterized by a homogeneously broadened lasing transition line. The calculations are carried out for a wide range of values of the input signal intensity, resonator parameters, and number of passes through the active medium.

An investigation is made of the spatial filtering of spontaneous radiation by the active medium in a traveling-wave optical amplifier. It is shown that in the high-gain case this radiation is described satisfactorily by a spherical wave whose effective source is located in the front of the amplifier. The influence of refraction is estimated as a function of the wave frequency.

The geometric-optics approximation is used to analyze theoretically the formation of the angular spectrum of the radiation emitted by an iodine laser, relationship between optical inhomogeneities and internal losses, and the influence of the stray scattering noise on the mode composition of the radiation. The threshold conditions are found allowing for optical inhomogeneities in the active medium. It is shown that the presence of such inhomogeneities simply bends the phase front of the radiation without altering its brightness.

A theoretical analysis is made of the two-mode emission from a gas laser in the vicinity of the symmetric position of the modes relative to the center of the gain profile when the intensities of the modes in the symmetric position cannot be regarded as low. The example of a homogeneously broadened spectral line is used in an investigation of the range of stable two-mode emission as a function of the relative excitation of the medium.

A method was developed for investigating photolysis processes and laser characteristics of gaseous active media consisting of highly aggressive and unstable substances. An experimental investigation was made of four compounds with the As-I bond but the main attention was concentrated on the laser characteristics of (CF₃)₂AsI. It was found that the energy output of the (CF₃)₂AsI laser was limited by the heating of the working gas in the course of photolysis.

Theoretical formulas are derived for the Stark shift of molecular transition frequencies in a resonant optical field. The formulas are obtained for one-photon vibrational-rotational and electronic transitions and for twophoton vibrational-rotational transitions. The influence of the hyperfine splitting on the Stark frequency shift is considered. Numerical estimates are obtained for the frequency shift of transitions in CO, HCl, CH₄, and I₂ molecules in a resonant optical field.

An investigation was made of the contribution of radiative and nonradiative transitions to the deactivation of the upper laser levels ⁴S_3/2, ⁴I_11/2 and ⁴I_13/2 of the Er³⁺ ion in mixed yttrium-erbium aluminum garnets. The optimal activator concentrations, ensuring minimum concentration quenching of the luminescence, were determined. A comparison of the Einstein coefficients, deduced from the corresponding integrated absorption coefficients, with the experimentally determined room-temperature lifetimes of the levels ⁴I_11/2 and ⁴I_13/2 established that the quantum efficiency of the ⁴I_13/2 level was unity (to within the experimental error) and that of the ⁴I_11/2 level did not exceed 0.017. However, the possibility of using high activator concentrations made the laser utilizing the ⁴I_11/2–⁴I_13/2 transition sufficiently efficient.

An analysis is made of the coupling of radiation out of a smoothly tapered optical waveguide. It is shown experimentally that energy is extracted because of scattering of the guided modes in the tapered region when the thickness exceeds a critical value and because of emission of guided modes when the thickness is below this value. An investigation is reported of the far-field pattern of the radiation as a function of the taper angle of the waveguide. The experimental results are compared with calculations.

The results are given of a numerical analysis of the vibrational relaxation of CO-Ar mixtures in expanding supersonic streams. An investigation is made of the influence of the geometry, temperature, and pressure in the nozzle throat on the gain of the expanding medium. It is shown that at a distance of 4 cm from the throat the gain may exceed 5×10^–2 cm^–1 for one-quantum vibrational-rotational (VR) transitions in carbon monoxide when the degree of expansion in the nozzle is 100. A study is made of the influence of vibrational-translational (VT) and vibrational-vibrational (W) relaxation processes on the distribution over the vibrational states in the carbon monoxide molecule. The conditions are found under which complete inversion can be achieved in some VR transitions. It is shown that a gasdynamic laser may be based on two-quantum transitions in the CO molecule resulting in emission of wavelengths from 2.3 to 2.7 μ. An investigation is also made of the influence of the carbon monoxide content in CO-Ar mixtures on the gain and the results are compared with the experimental data.

A theoretical analysis is made of the influence of perturbation of the surface of a liquid on the acoustic field excited in this liquid by Q-switched laser radiation. The angular distribution of the average acoustic pressure is determined. The acoustic pressure variance is calculated for Gaussian statistics of slopes and arbitrary statistics of displacements of the liquid surface. A qualitative analysis of the results obtained is given.

A description is given of a method for determining changes in the concentrations of stable molecules in flash photolysis of active media. In this method flashlamps are switched off rapidly but in a controlled manner by a running arc and the media are then subjected to infrared spectrochemical analysis. The results are given of the application of a method to photochemical changes in CF₃I and (CF₃)₃CI vapors. The time dependences obtained in this way give information on the characteristic sequence of chemical processes which can be used to separate the long-term influence of light on an absorbing gas into three stages: laser, pyrolysis, and quasiequilibrium. The data on the pyrolysis stage can be used to estimate the velocity of pyrolytic bleaching waves, which are propagated in the investigated vapors.

The rate equations approximation is used in a calculation of the contrast of an inverted Lamb dip of a laser with an internal absorption cell. An approximate solution is obtained for the dependence of the contrast on the pressure in the absorption cell. Theoretical results are compared with the experimental data obtained with the aid of an He-Ne laser at a wavelength of 3.39 μ. It is shown that some information on the quantitative composition of a mixture of gases can be obtained by spectroscopy inside a Doppler line.

An analysis is made of the polarizations, spatial field distribution, and frequency spectrum of longitudinal (axial) modes of a cavity ring resonator with an arbitrary configuration of the axial contour. Use is made of the integral equation method based on the Huygens-Fresnel principle. It is shown that in the longitudinal mode case it is possible to decouple the equations governing the polarization and spatial field distribution. The two-dimensional integral equation for the field distribution has inseparable variables. Its solution is found on a reference plane which intersects normally the axial contour at an arbitrarily selected point. It is found that the equal-amplitude curves on the reference plane are ellipses and—in contrast to the planar resonator case—the directions of the principal axes of these ellipses need not coincide with the directions of the principal curvatures of the wave fronts. The important case of a four-mirror with a symmetric axial contour is discussed in detail.

An analysis is made of the determination of the transverse distribution of intensity in a laser beam when this distribution departs strongly from the Gaussian form. It is shown that in some cases a one-dimensional distribution can be approximated by a Charlier-type curve. The results are given of experimental investigations of a laser with a spherical confocal resonator; it is shown that the experimental curves can be approximated to within 10% by a Charlier distribution.

An investigation was made of the mechanism of the pressure exerted on plane and hemispherical targets by CO₂ laser pulses in air. The optimal laser pulse duration corresponding to the highest values of the specific pressure impluse was determined. The experimental results were interpreted on the basis of a theory of point explosions allowing for the atmospheric counterpressure.

A brief review is given of laser cutting units designed for various purposes. A detailed description is given of an automated system for programed laser cutting of materials, developed at the All-Union Scientific-Research Institute of Light and Textile Machine Construction. A gas CO₂ laser, cutting machine, and digital program unit are all discussed. A technical specification of the system is given. Some results are reported of studies aimed to develop the technology of laser cutting of a wide range of nonmetallic materials. Possible applications of the system are indicated.

The results are reported of an experimental investigation of the dependences of the output power of submillimeter lasers, pumped optically by CO₂ laser radiation, on the pump power, vapor pressure of the active substance, resonator length, diameter of the coupling aperture, etc. The investigation was carried out under steady-state conditions using strong emission lines of the HCOOH, CH₃I, C₂H₂F₂, and CH₃OH molecules. Recommendations were formulated for some design parameters of submillimeter lasers.

A model is proposed for the calculation of power and energy characteristics of pulse CO₂ lasers. Dependence of the output energy of the input energy, and on the composition and pressure of the active medium and obtained and discussed for the case of excitation by a self-sustaining discharge. The best agreement between the calculated and experimental results is obtained at low values of the specific energy input.

A theoretical analysis is made of the control of the shape of laser pulses leaving a system comprising a master oscillator and an amplifier. A pulse with a time-controlled spectral composition is applied to the input of the amplifying system. Scanning of the spectral line of the input signal in the gain band of the system during a pulse can be used to vary the output power within a wide range. Examples of a master oscillator and of a unit for shaping of input pulses with a variable spectrum are considered. An expression is obtained for the necessary rate of variation of the input signal wavelength. The results are given of a numerical calculation of spectral-temporal characteristics of an input pulse needed to obtain an output pulse of required form in an amplifier utilizing neodymium-doped phosphate glass.

An investigation was made of the damage threshold, partial melting, and bleaching of radiation-colored K-8 glass as a function of γ-ray dose. The threshold temperatures were estimated and it was found that the residual absorption coefficient of glass was constant right up to power densities at which bulk damage was observed. Two types of radiation defects were found in K-8 glass and these defects were responsible for the absorption at γ = 1.06 μ.

An efficiency of ~3% was obtained in generation of the second harmonic of nonmonochromatic neodymium laser radiation in an LiIO₃ crystal. The laser was operated under free-oscillation conditions and the dispersion of phase matching was compensated by a diffraction grating. An investigation was made of the angular and spectral characteristics of the harmonic radiation. It was found that compensation of the phase-matching dispersion widened the harmonic spectrum from 0.2–0.3 nm to half the width of the laser spectrum.

An experimental investigation was made of ultrashort pulses generated in yttrium aluminum garnet (YAG) and amplified in YAG and neodymium glass. When the power density was less than 3-5 GW/cm², the main mechanism which limited the amplified power was large-scale self-focusing of the amplified radiation in the active media, which altered greatly the laser beam divergence. Conditions were found under which this self-focusing was avoided completely. The investigated self-focusing produced, around the main beam, a scattered-radiation halo of divergence much greater than the beam divergence. The proportion of the laser beam energy lost to the halo increased with the amplification, so that beginning from a certain stage further amplification simply resulted in a reduction in the brightness. In this situation the enrgy density in the amplified beam could still be below the damage threshold of the active medium.

The principles are given of the design of optical systems to be used in spherically symmetric laser irradiation of small targets. Calculations are made of the illumination of the surface of a spherical target irradiated with six and nine Gaussian beams. An analysis is made of the influence of the positions and parameters of laser beams on the characteristics of irradiation. A discussion is given of methods for further improvement of the uniformity of target illumination.