Table of contents

Distortions due to a self-focusing nonlinearity of the medium are calculated for a beam with a strong "gray" angular divergence, i.e., with fine-structure inhomogeneities. The calculation is made by the method of perturbation of the mode solution. Ways of avoiding self-focusing in high-power pulse amplification are considered.

A description is given of a new experimental method for suppressing parasitic oscillations in high-power CO₂ lasers. A cell with SF₆ and a Fabry–Perot interferometer with a base exactly equal to 84.73 μ are used in this method. The SF₆ cell acts as a saturable absorber at the 10.6 μ wavelength, whereas the Fabry–Perot interferometer suppresses parasitic oscillation at the wavelengths of 9.2, 9.6, and 10.2 μ. The advantages of the method are pointed out.

A phenomenological analysis of the nonlinear susceptibility tensor of an isotropic medium is used to show the possibility of existence of nonreciprocal effects in the course of propagation of arbitrarily polarized opposite waves of the same intensity and frequency. A system for producing amplitude and phase nonreciprocal effects is discussed. The dependence of the effects on the polarizations (ellipticities and azimuths) and on the coefficients of nonlinear interaction between the opposite waves is analyzed for small amplitude and phase changes.

A solution is given to the problem of finding the fields in a resonator formed by two wavefront-reversing mirrors. It is shown that the resonator fields take the form of a Gaussian-Hermitian beam whose axis is generally inclined to the resonator axis.

The process of coupling of the frequencies of ultrashort laser pulses is considered allowing for the frequency dispersion within the width of the exciting spectrum (in the second approximation of the theory of dispersion). The energy of the second harmonic is analyzed in detail. The duration of the exciting pulses for which the energy of the second harmonic reaches its maximum value is found on the assumption of group phase matching of the interacting waves.

An investigation was made of the characteristics of 200–250 nm radiation emitted from the zone of shock interaction between plasma streams produced in a quartz tube by two magnetoplasma compressors. When the pulse duration was several microseconds, the brightness temperature of the emitted radiation was 27 000 °C in the region of 205 nm.

A calonmetric system was used to determine the spectral dependences of the bulk and surface absorption coefficients of highly transparent solids, which were in the 10⁻⁶–10⁻⁴ cm⁻¹range at wavelengths of 5.3–6.2 μ. The example of thallium chloride was used to show that a tunable CO laser could be used as a light source. The absorption spectra of optical ceramic and single-crystal samples of TlCl were found to have impurity bands at about 1620 and 1730 cm⁻¹: these bands were respectively due to the deformation vibrations of water and valence vibrations of the C=O bond.

coefficients of highly transparent solids, which were in the 10⁻⁴–10⁻⁶ cm⁻¹ range at wavelengths of 9.2–10.9 μ. A tunable CO₂ laser was used as the light source. The absorption spectra of cesium iodide crystals were determined.

A theoretical analysis is made of the process of parametric up-conversion of the frequency of an infrared signal involving two-photon resonance of a pump field whose spectrum is considerably wider than the widths of the atomic transition lines. It is shown that the maximum steady-state efficiency of conversion in a wideband field is only slightly different from the maximum efficiency of conversion in a regular and monochromatic field. A dynamic improvement in the conversion efficiency is possible under transient interaction conditions.

An analysis is made of the feasibility of constructing new stimulated emission sources based on superconducting Josephson elements or on the basis of planar structures made of normal metals (metal–oxide–metal diodes). It is shown that in the case of superconducting oscillators the continuously tunable range is 10⁹–10¹² Hz, whereas in the case of planar metal–oxide–metal diodes it extends up to 10¹⁵ Hz.

Measurements were made of the bulk damage threshold I_d of yttrium aluminum garnet (YAG) crystals, either free of deliberately introduced impurities or activated with Er³⁺ ions in different concentrations (up to 80 wt.%), and also of LiNbO₃ crystals. This threshold was determined using giant pulses from a YAG:Er³⁺ laser emitting at λ = 2.94μ; the energy per pulse was 70 mj and the pulse duration was 120nsec. In the case of YAG crystals it was found that Id varied considerably from sample to sample in the range I_d = (1.5–18) × 10¹⁰ W/cm², which was not associated with the presence of Er3+ but was clearly due to the presence of some accidental impurities. A considerable increase in I_d was observed for YAG:Er³⁺ samples characterized by low damage thresholds when these samples were annealed in air. In the case of LiNbO₃, a typical value of the damage threshold was I_d ≈ 10¹¹ W/cm². The results obtained indicated a sufficiently high optical strength of YAG:Er³⁺ and LiNbO₃ crystals, used respectively as the active and modulating elements in YAG:Er³⁺ lasers.

A method is proposed for observing nonlinear resonances due to higher-order saturation effects. The experimental results indicate narrowing of these nonlinear resonances compared with a homogeneous spectral line width.

A comparative study was made of stimulated emission from chemical H₂–F₂, D₂–F₂, and D₂–F₂–CO₂ lasers initiated by an electron beam. Under identical experimental conditions the output energy of the D₂–F₂–CO₂ laser was practically equal to the output energy of the H₂–F₂ laser, but the duration of the output pulses of the D₂–F₂–CO₂ laser was 2.5 times greater. A mixture of the F₂:D₂:O₂:CO₂:He = 150:60:12:150:375 Torr composition yielded a specific energy of 60 J/liter for a chemical efficiency of 3.5% and an efficiency of 330% when measured relative to the pump energy deposited in the active region. The He content of the mixture had little effect on the stimulated emission parameters of the D₂–F₂–CO₂ laser.

Communication channel crosstalk was investigated in a Y-shaped frequency-division data channel multiplexer of a fiber-optic communication line.

It is shown that the resonance nonlinear susceptibility of four-photon parametric processes in gaseous media can be increased by several orders of magnitude. The increase in the susceptibility is due to suppression of the Doppler broadening of two-photon transitions in strong pump fields when the frequencies of the fields interacting with these transitions can be very different. Under these conditions there is an additional opportunity for increasing the susceptibility by reducing the frequency detuning relative to intermediate levels participating in two-photon transitions.

The emission frequency of a submillimeter CH₃OH laser (λ = 70.5 μ) was frequency–phase locked to a ¹³C¹⁶O₂ laser [λ = 10.07 μ, P(28)] and the ¹³C¹⁶O₂ laser was locked to a ¹²C¹⁶O₂ laser [λ = 10.2 μ, R(30)], which made it possible to transfer the frequency characteristics of a highly stable He–Ne/CH₄ laser to the submillimeter range.

An analysis was made of the tunnel excitation of diffused waveguides with the aim of optimization of the coupling of H and E waves. The optimal profile of the gap between a prism and a waveguide was calculated for the case of "weak" (Δn≪n) waveguides. A study was made of the dependence of the tunnel emission coefficient on the refractive index of the prism. A method was developed for determining the shape of the gap between the prism and the waveguide. A high efficiency of tunnel excitation of the H waves (90%) and E waves (65%) was achieved experimentally.

A comparitive analysis is made of the principal characteristics of narrow-band and conventional semiconductor light amplifiers. It is shown that quasi-distributed filtering of the spontaneous radiation ensures a high gain and a low level of the spontaneous noise at the amplifier output.

Stimulated emission was observed from solutions of derivatives of aza-1,2,4-triazine excited with nitrogen laser (λ =337.1 nm) nanosecond pulses. The active substances were synthesized by an experimental laboratory technology. Preliminary luminescence spectra as well as energy, time, and other characteristics of the radiation were obtained for the compound designated as SKh-10, typical of several investigated dyes. The experimental methods used to observe stimulated emission are described.

The relative spectral power of an XeF laser depends on the rate of stimulated emission at a fixed pressure, i.e., the spectral power can be controlled by varying the pump parameters.

A review is given of investigations of tunable lasers utilizing color centers in ionic crystals. An analysis is made of the microstructure, spectral, luminescence, and kinetic properties of various complex electron color centers in alkali-halide crystals and alkaline-earth fluoride crystals used or capable of being used as active media in near infrared tunable lasers. Possible causes of optical and thermal instability are discussed for centers having diiferent structures and methods of improving the stability are considered. The parameters of existing color center lasers are presented and the prospects for their development and application are discussed.

Kinetic equations are used to analyze the emission of two modes with different losses allowing for spatial inhomogeneity of the fields of these modes in a common active medium. It is found that spatial inhomogeneity results in saturation of the depth of the dip in the stimulated emission spectrum. Some characteristics of the temporal evolution of the stimulated emission are clarified by computer calculations. An analysis is made of the sensitivity of the method (depth of the dip in the stimulated emission spectrum) and of the time for establishment of steady-state laser action as a function of the pumping rate and degree of spatial inhomogeneity of the mode fields in the active medium.

A description is given of a method of calculating optical resonators based on solving a parabolic equation. By applying this method to unstable resonators with lens-like media, it is possible to describe the mode structure with misaligned mirrors (in addition to finding the known properties). Relationships between the spatial characteristics of the laser beam and the misalignment parameters of the mirrors and inhomogeneity of the medium are determined.

It is shown that the lasing ability of benzoxazoles depends on the structure of different types of their electronic energy levels. Compounds with T_ππ*,S*_ππ*,T_nπ*,and S*_nπ* lower levels emit radiation. Stimulated emission was obtained from solutions of 2-phenylbenzoxazole derivatives in the shortest wavelength range of 319–360 nm and from solutions of 4-(benzoxazole-2-yl)–4'-phenylstilbene derivatives in the 426–446 nm range. A nonselective resonator pumped by λ_p=308 nm radiation from a pulse-periodic excimer laser was used. The laser action was achieved in two bands corresponding to the vibrational structure in the gain spectrum.

A study was made of the characteristics of the laser radiation emitted by a low-pressure pulse-periodic closed-cycle CO₂ laser having an average output power of 1 kW. It was found experimentally that the duration of the laser pulses could be increased by displacing the resonator axis downstream relative to the discharge axis, with subsonic flow of the gas mixture. A laser pulse duration of 300 μ sec was obtained for a current-pulse duration of 1.5 μ sec.

Theoretical and experimental investigations-were made of the process of energy transfer between two coherent beams during recording of a holographic grating in a cadmium sulfide crystal oriented in different directions with respect to the lines of the grating. It was shown that two mechanisms are responsible for the energy transfer: an electrooptic and a free-carrier mechanism. The type of carriers in the samples under study was determined from the direction of energy exchange.

Experimental investigations were made of the output power of a cw electron-beam-controlled CO₂ laser as a function of the composition and velocity of the gas mixture, and of the resonator position. The optimal ratios of the gas mixture components and the best resonator position were found. A qualitative theoretical explanation is given of the results obtained.

An analysis is made of the propagation of paraxial Gaussian beams in optical waveguides arbitrarily curved in one plane and having a parabolic refractive index profile. The propagation function is derived for this case and by analyzing this function, it is shown that the center of the beam moves along a specific trajectory. Moreover, the Gaussian intensity distribution with a periodically varying parameter characterizing the beam width is conserved along the trajectory in the transverse cross section of the beam.

An experimental investigation was made of the influence of various types of multilayer GaAs–AIAs heterostructure isotypically doped with Zn or Te on the threshold current of a laser pumped transversely by an electron beam in the energy range 10–40 keV. The best results were obtained for a three-layer p -type structure in which the active layer was situated between two wide-gap layers and the structure was pumped through one of these layers. The lowest threshold current density for an electron beam energy of 40 keV was 0.08 A/cm² at T=85 °K and 1.3 A/cm² at 300 °K.

A theoretical study is made of kinematic mode locking in a laser with a homogeneously broadened luminescence line, which occurs during translational motion of one of the mirrors along the resonator axis. The problem is reduced to a system of algebraic equations, the solutions of which determine the quasisteady parameters of a near-Gaussian weakly asymmetric pulse traveling in the resonator.

An analysis is made of the problem of determining the ion temperature profile from transit-time data and neutron energy spectra under conditions of an experiment involving laser irradiation of D–D and D–T targets. The conditions required to construct an approximate solution are formulated. Calculations made for model examples show that the ion temperature profile and its maximum can be determined. Real data are processed assuming a constant target density. An approximate expression is given for the neutron energy spectrum using a model which allows for directional motion of the reacting nuclei.

A theoretical analysis is made of the effects which take place in optically pumped lasers whose active medium is subjected to a dc electric Geld. The possibilities are discussed of tuning the frequency and modulating the intensity of the radiation emitted by a laser. The gain characteristics of an active medium in an electric field are calculated for a CH₃F laser operating on a wavelength of 496 μ. It is shown that the frequency tuning limits of the output radiation of such a laser can reach 3 GHz, if the active medium allows electric fields of up to 30 kV/cm to be applied.

The results are given of an experimental investigation into the parameters of laser absorption waves, propagating under the action of radiation of wavelength 10.6 μ at power densities of q = 2–10 MW/cm². The data obtained on the dynamics of their development and on the temperature, optical properties, and pressures at the target and in the plasma are compared with calculations of the parameters of laser absorption waves using the theories of optical detonation and subsonic radiation waves. These comparisons lead to the conclusion that a laser absorption wave propagating under the action of radiation of wavelength λ – 10.6μ is a subsonic radiation wave. Quantitative discrepancies between the experimental and theoretical data are explained as due to the existence of a mechanism for increasing the capture rate ω of a mass of gas by the radiation wave. Possible mechanisms for increasing ω include nonequilibrium heating of the electrons and the inducing of turbulent gas flow at the subsonic radiation wave front.

An analysis is made of the selection of the CO₂ laser emission lines by a diffraction structure consisting of a thin metal coating. A method is proposed for design calculations of a resonator containing such a structure. It is shown that the ratio of the selection sharpness to the losses experienced by the selected line can be varied within a fairly wide range by the choice of the refractive indices of the media surrounding the structure and of the relative proportion of the area occupied by the metal. It is shown that if the metal coating is characterized by a low (0.1–1%) transmission, the losses introduced in the resonator by such a reflecting interferometer are smaller than the losses in a good-quality gold mirror; moreover, the minimal thermal effects are also retained. The results are reported of an experimental study of a low-pressure CO₂ laser with diffraction structures in the form of silver and aluminum gratings. The experimental results are in agreement with the theory. It is shown that vibrational-rotational laser transitions can be selected without additional losses compared with a nonselective resonator. A method for experimental determination of the selector parameters is described.

It is shown that continuous generation of cw single-mode radiation with a power of up to 0.3 W and line width of less than 400 MHz may be achieved from a liquid-nitrogen-cooled homojunction injection laser. A loss mechanism for single-mode operation is discussed and calculations of the maximum single-mode power are presented. These do not contradict the experimental results.

A study is made of the propagation of a light wave having a complex transverse structure in an amplifier under saturation conditions. An equation is obtained for the signal-to-noise ratio at the amplifier exit.

Measurements are reported of the populations of the copper atom ²D_5/2 levels and of the frequency characteristics of a copper laser utilizing self-terminating transitions, in tubes of diameter 20, 27, and 40 mm. It was shown that the optimal repetition frequency was determined not by the relaxation rate of the metastable level but by the plasma recombination rate. In a pure Cu–Ne mixture the recombination took place as a result of ambipolar diffusion, so that a lowering of the optimal repetition frequency from 1316 to 4–5 kHz was observed on increasing the diameter of the coaxial tubes from 2 to 4 cm. The addition of hydrogen to the Cu–Ne mixture led to an increase in the efficiency of the three-body volume recombination mechanism and to a substantial raising of the optimal repetition frequency.

An analysis is made of the possibility of correction of random angular displacements of the energy center of gravity of an optical beam (traveling in a turbulent atmosphere) by tracking the random angular position of the center of gravity of the image of a reference beam. The reference beam may be provided by an auxiliary source or it may represent part of the reflected original beam. It is shown that variance of the residual random angular displacements of the center of gravity of a beam can be reduced, under certain conditions, to not more than 25% of the same variance in the absence of correction. In particular, it is found that the proposed correction method is sufficiently effective for apertures smaller than the external scale of the turbulence. The time constant of the loop for the compensation of random displacements should be less than the ratio of the initial beam size to the average wind velocity.

An analysis is made of some of the methods for reducing disperison in three-layer optical fiber waveguides with a step-like distribution of the refractive index. In particular, a calculation is made of the fiber parameters which ensure that the four lowest modes have the same group velocities. The results are given of a calculation of the material and waveguide dispersions in one-mode two- and three-layer waveguides. It is shown that the material and waveguide dispersions can be mutually compensated in a wide range of wavelengths (λ = 0.6 – 2.5 μ) and normalized waveguide frequencies (V = 1–6).

An experimental study was made of the dependences of the optical damage threshold of polymethylmethacrylate on the duration of exposure to laser radiation for two different sizes of focal spot. A theoretical interpretation of the experimental results is proposed, based on a model of the evolution of thermal instability at absorbing inclusions, which is generalized to the case when cumulative effects are substantial. This model can also explain experimental data obtained by other authors on cumulative effects.

A description is given of a spectrometer for coherent anti-Stokes Raman scattering (CARS) with an instrumental resolution of ~ 10⁻³ cm⁻¹ and an automated data acquisition and processing system. The results are given of CARS investigations of the rotational structure of the vibrational state ν₁(a₁) of the spherical-top molecules CH₄ and CD₄.

An experimental method is described for determining the stimulated emission cross section σ of a gas laser. The method was used to measure σ for the 00°1–10°0 vibrational transition of the CO₂ molecule in a compact waveguide laser having a beryllia ceramic discharge tube. The determination of σ was based on an experimental measurement of the luminescence drop at 4.3 μ wavelength, and of the laser output power at 10.6 μ wavelength. The value of σ₀ measured at zero photon flux density in the waveguide resonator, for a gas mixture Xe–CO₂–N₂–He at a pressure of 6.65 kPa, was 1.84×10⁻¹⁸ cm². The effective value of the upper active-level lifetime of the CO₂ molecule was also determined.

The first successful experiments on selective interaction with a specific type of base in DNA and RNA are reported. It is shown that when dilute aqueous solutions of bases are irradiated by high-power ultrashort ultraviolet laser pulses, two-stage excitation of high electronic levels takes place, resulting in the formation of irreversible photoproducts. The quantum yield of photoproducts depends on many parameters and may be made essentially different for bases of different types. By varying the experimental conditions, selective interaction with a specific type of base was achieved both by irradiating mixtures of bases and by irradiating more complex components of nucleic acids.

A radiative collisional model which allows for all the levels of the 1_s²2_s²2_p⁵3_s, 2_p⁵3_p, and 2_p⁵3_d configurations is used to calculate the populations of the first excited states of neon-like ions. An earlier conclusion that an inversion exists between levels of the lp 2_p⁵3_p and 2_p⁵3_s configurations in a stationary plasma is confirmed. It is shown that an inversion with a fairly high gain is due to not one but several types of transitions covering a wide wavelengths range.