Table of contents

An analysis is made of the kinetics of hysteretic changes in the transverse fxeld structure in nonlinear interferometers resulting from the time dependence of the intensity of the incident radiation. It is shown that time-dependent changes in the beam profile can occur in an interferometer even when the incident beam is constant.

Measurements were made of the specific mass removal on boring holes in metals with an electron-beam-controlled CO laser operating in the free-lasing regime. It was found that such a laser was in no way inferior to a pulsed

CO₂

laser as regards its efficiency for boring holes in copper and steel, and was considerably superior in the case of aluminum.

Pulsed lasing was obtained for the first time from Li₂ molecules by optical pumping with radiation from a pulsed copper vapor laser (578.2 nm, pulse repetition frequency 5 kHz). The laser transitions, with wavelengths in the range 867–907 nm, belong to the electronic

A¹Σ_u⁺–X¹Σ_g⁺

system. With a pump power of 190 mW, an average output power of 8 mW was achieved with an efficiency for the conversion of the optical pumping energy of 7%. A number of Li₂ laser emission lines were observed in the superradiant regime.

A chromatographic mass-spectroscopic analysis was made of the spent mixture in an iodine laser and amplifier pumped by an open high-current discharge. It was found that the chemical processes differed for the driven regime (amplifier) and the free-lasing regime (laser). This difference should result in the production of a larger quantity of

I₂

in the driven regime which may give rise to differently designed regeneration systems for these two regimes.

An analysis is made of the influence of the addition of various rare gases to the initial gas mixture on the yield (or rate) of chemical reactions taking place in this mixture. Conditions are determined when the change in the yield will be positive for thermal reactions and negative for laser-chemical reactions (i.e. for reactions which take place by the excitation of molecules which absorb laser radiation beyond the thermal level).

The first report is given of the attainment of high-contrast resonances in a linear two-mode

³He–²²Ne

laser {λ = 0.63 μ) with an internal

¹²⁹I₂

absorption cell. The maximum contrast ratio of these resonances was 25% and their width was the same as that of similar resonances in the single-mode regime within the measurement error.

A calculation is made of the condition necessary for lasing of a dye in a smectic matrix. It is shown that a distributed-feedback mechanism ensured by mechanical modulation of the surface of a liquid-crystal layer can give information on the critical parameters of an orientational phase transition.

An experimental investigation was made of a system for continuous pumping of a YAG:Nd laser using radiation from a high-frequency electrodeless discharge. Estimates were made of the efficiency and life of various designs of pumping system. It was found that pumping systems using high-frequency electrodeless flashlamps with powers of up to 2.5 kW can be successfully used in lasers with an output power of 20–40 W and a life of ≳ 1500 h.

A pulse-periodic (with a pulse repetition frequency of 7.6 kHz) coherent source of infrared radiation tunable between 3.8 and 6 μ, having a line width of 0.6 cm⁻¹ and an average power of 10–100 μW was developed. Infrared radiation was obtained by difference frequency generation involving mixing, in an

LiIO₃

crystal, radiation from a copper vapor laser and radiation from a rhodamine 6G dye laser pumped by this copper vapor laser.

The results are presented of an investigation of generation of the second harmonic of neodymium laser radiation in powders of 43 new organic compounds. Possible methods of synthesizing organic materials with nonlinear optical properties are discussed.

A comparison was made of the dispersion properties of a system of series-connected fiber waveguides all with similar properties. It was found that if the excitation conditions of each fiber were standardized by a mode mixer, the amplitude-frequency characteristic of such a system could be approximated quite accurately by the product of the characteristics of the individual fibers.

An investigation was made of the profile of the hyperfine component of the cesium line observed using a cw semiconductor laser. It was found that the profiles of the absorption and luminescence signals, their widths, amplitudes, and the positions of the maximum depend strongly on the relaxation time of the active medium, laser radiation intensity, and rate and direction of passage of the frequency through resonance.

It was established experimentally that linearly polarized light transmitted by a polymer graded-index light-focusing rod retains its polarization. The interference pattern of the chromatic polarization indicated that the transmission of linearly polarized light by this rod manifested properties similar to those observed after passage through a biaxial crystal.

An investigation was made of plasma fluctuations in He–Cd, He–Zn, He–Xe, and He–Ne lasers. It was found that Penning excitation gives rise to fluctuations in the discharge, associated with sum and difference combinations of striations, the latter causing appreciable deterioration in the noise characteristics of the radiation. An analysis is made of methods of decreasing the output radiation noise of lasers utilizing mixtures of helium and readily ionized elements by suppressing or synchronizing the striations. It is shown that if the striations are synchronized, the electron temperature increases, which increases the output power of ionic lasers and reduces that of lasers utilizing atomic transitions in readily ionized elements.

A theoretical analysis is made of stimulated scattering in opposite pump beams. A condition for suppression of stimulated scattering by a weak opposite pump wave is determined. The threshold conditions for parametric generation of opposite Stokes and anti-Stokes components are found.

The optical system of the Del'fin 1 facility is described and results are presented of preliminary experiments on the laser heating of spherical targets of ∼ 300 μ diameter. The laser radiation energy was ∼ 700 J (at the target), the divergence was

∼ 3.5×10⁻⁴

rad, the contrast ratio was better than 10⁶, and the duration was ∼ 2 nsec. It was found that plasma temperatures of 800–1000 eV were achieved experimentally.

An experimental investigation was made of the maximum modulation frequency of the polarization of radiation from a cw YAG:Nd³⁺ laser. It was found that intracavity modulation of the polarization using an electrooptic phase modulator can be observed up to frequencies of 100–150 MHz. In this case, the control signal is 5–15 times smaller than that for external modulation methods.

A method is proposed for the experimental investigation of the integrated characteristics of an optical beam undergoing self-interaction in the propagation medium. The method is based on analyzing the spectrum of intensity fluctuations of the scattered radiation. An experimental investigation is reported of transient thermal self-interaction in a liquid. The method can be used to estimate the operating efficiency of adaptive radiation-focusing systems and in many other applications.

Simple general relationships are obtained for the calculation of noncoUinear interactions of light beams in nonlinear single crystals. These relationships are used to determine the optimal (from the point of view of the effective nonlinear susceptibility) directions of propagation of optical waves in urea single crystals used for frequency doubling of neodymium laser radiation and addition of the resultant second harmonic to the fundamental-frequency radiation. It is shown that for all types of interaction

(e^ωe^ω → o^2ω

;

e^ωe^2ω → o^3ω

;

e^ωo^2ω → o^2ω → o^2ω

;

e^ωo^2ω → o^3ω

;

e^ωo^2ω → o^3ω

) there is a wide range of directions for which the relative effective nonlinear susceptibility exceeds 0.9 of its maximum value, which is 1.5–2 times greater than in the collinear cases

A theoretical analysis is made of fluctuations of the radiation emitted from a single-mode traveling-wave laser with homogeneous broadening of the transition line in the direction opposite to the strong field. Allowance is made for the distributed nature of the laser parameters. An expression is derived for the spectral density of the field fluctuations. A comparison is made with the experiments.

An analysis is made of the propagation of radiation in a single-mode optical fiber which does not conserve the wave polarization state. It is shown that nonlinear properties of the fiber may give rise to nonreciprocal phase shifts between the opposite waves in a ring fiber interferometer. General relationships are derived for a fiber with accidental birefringence and the nonreciprocity is estimated.

An experimental study was made of the influence of the specific pump power on the operating efficiency of an electron-beam-controlled atmospheric pressure

CO₂

laser for fixed near-optimal values of the remaining parameters influencing the laser output energy. It was found that increasing the specific pump power from 2 to 7

kW/cm³

led to a 40% rise in the output energy obtained from a

CO₂:N₂:He = 1:6:3

gas mixture.

An investigation was made of the angular characteristics of the output radiation of a neodymium-glass laser system with wavefront reversal by a three-wave parametric interaction. A telescopic amplifier with a gain of

∼ 10³

was used in the system. The proportion of the reversed radiation was 30% when the pump radiation divergence was close to the diffraction limit. The half-width of the angular distribution of the radiation energy at the output of the system was 10⁻⁴ rad. The reasons for failure to achieve complete reversal were analyzed.

Parallel spectral multiplexing of LED radiation by a diffraction grating accompanied by spectral filtering was proposed and implemented. This made it possible to reduce the spectral intervals between the channels to 20 nm and the crosstalk level to less than – 20 dB, which had been achieved earlier only for semiconductor laser radiation. Spectral multiplexing of radiation of five fiber waveguides was achieved in a transparency "window" near the wavelength of 0.8 μ. A single fiber with a quartz glass core and a polymer cladding transmitted five analog television signals over a distance of 4 km.

Investigations were made of the dependences of the output energy, efficiency, and duration of lasing on the specific energy (maximum value

1 J/cm³)

supplied to a gas. Lasing was observed as a result of four transitions in the

2⁺

system of nitrogen, for which the threshold values of the specific input energy were determined. An increase in the specific input energy increased the optimal concentration of nitrogen in the mixture. In the case of sufficiently intense pumping the duration of lasing exceeded the lifetime of the upper active level, indicating the restrictions imposed by self-termination of the transitions were lifted. The maximum efficiency in a low-Q resonator was 0.6%, the output radiation energy was 0.3 J, and the specific output energy was 5 J/liter.

An experimental investigation was made of multiphoton dissociation of

(CF₃)₃CBr

molecules in the field of

CO₂

laser pulses with the aim of obtaining high concentrations of

(CF₃)₃C

radicals. Multiphoton dissociation of

(CF₃)₃CBr

could result in successive detachment of the bromine atom and of the

CF₃

group. Selection of the excitation conditions ensured a high yield of the

(CF₃)₃C

radicals and a low yield of the

CF₃

radicals.

An investigation was made of the multiphoton dissociation of

CF₃Br

and

CF₃Cl

molecules at high gas temperatures created by cw

CO₂

laser radiation. An increase in the initial temperature made it possible to increase the multiphoton dissociation yield and to ensure efficient multiphoton dissociation even of those molecules whose linear absorption bands were detuned slightly from the frequency of the exciting laser radiation.

A description is given of an intracavity laser spectroscopy method utilizing a dye laser pumped by a pulse argon laser. The pulse duration reached 200 μ sec and could be increased further to ∼ 5 msec, which ensured a high sensitivity of the method.

An optical demultiplexer with a fused quartz plate 300 μ thick was developed. Energy from fiber waveguides with a core diameter < 300 μ and an aperture 0.4 was coupled into the demultiplexer without energy losses. The device was capable of demultiplexing five optical carriers in the 0.7–0.9 μ range for a linewidth < 3 nm at spectral intervals of 50 nm. The crosstalk level was less than

– 20 dB

in respect of the optical power, and the losses in each channel were 3 dB. Promising results were obtained when three-dimensional optical devices were replaced with planar integrated elements based on plate waveguides.

An increase in the output power and efficiency of an He–Ne laser was observed when a microwave field was applied to a dc discharge. The dependence of the output power on the microwave power was determined.

Measurements were made of the energy and time characteristics of a chemical HF laser initiated by radiation from an excimer XeCl laser (λ = 308 nm). Various compositions of the mixture in the chemical laser were employed. When the mixture composition was

F₂:H₂:O₂:He = 10:3.25:1:36

the efficiency (measured relative to the energy deposited in the active region of the HF laser) was ∼ 550% when the initial concentration of the F atoms was

∼ 5×10¹⁵

cm⁻³.

The degree of initiation and the efficiency were deduced from the experimental measurements of the energy of the excimer laser beam absorbed in the active region of the chemical laser.

It is shown that in the case of a small-diameter plasma column formed in a gas by a laser pulse, the plasma expansion in the lateral direction reduces the density and the absorption in the central part of the column. Absorption waves are generated as a result of this effect and as a result of emission of radiation by the plasma itself; these waves travel not only antiparallel (opposite) to the laser beam but also parallel to the beam. Numerical solutions of a two-dimensional transient radiation–gasdynamic problem yielded the parameters and made it possible to study the evolution of a plasma under these conditions.

Fiber waveguides were made of

As₂Se₃–As₂S₃

glasses and their parameters were investigated. The total losses in the emission region of CO lasers were 8–10 dB/m, whereas in the emission region of

CO₂

lasers they were 30 dB/m. The power densities corresponding to the damage threshold were 2 and 1

kW/cm²,

respectively.

A review is given of the papers presented at the now traditional (eighth in the series) Vavilov Conference on Nonlinear Optics, which took place in Akademgorodok near Novosibirsk on June 22–25, 1981. The scientific program covered the following topics: new methods and the phenomena in nonlinear laser spectroscopy and nonlinear optics, control of the motion of various microparticles (atoms, molecules, ions) by laser fields, effects of laser radiation on complex molecules (including biomolecules), and laser methods in fundamental physical experiments (in particular, in the search for parity nonconservation in atomic processes). Over 200 scientists from 12 countries participated in the Conference.

A theoretical and experimental investigation is made of the feasibility of wavefront reversal by four-wave interaction in a medium exhibiting Raman nonlinearity. Theoretical dependences of the reflection coefficient of the four-wave mixer on the gain increment, phase detuning, and pump asymmetry parameter are obtained for the case of plane waves. In experiments using a ruby laser and quasiplane waves the maximum reflection coefficient of the nonlinear mirror was 0.5%, which agreed with the theoretical estimate. An experimental investigation was made of the wavefront reversal quality, and possible methods of improving this are discussed.

An experimental investigation was made of the interaction between a pulsed high-current relativistic electron beam (having an energy of

∼ 1 MeV,

a current of

∼ 10 kA,

and a pulse duration of

∼ 60 nsec)

with neutral gases in the pressure range

1–750 Torr.

It was found that in gas mixtures used in chemical lasers the proportion of the beam energy deposited in a gas at a pressure of

750 Torr

in an interaction chamber 100-cm long may reach 40%

(100 J).

The experimental results were in good agreement with the single-particle theory of interaction between fast relativistic electrons and a gas.

An analysis is made of the acoustooptic interaction in a layer structure comprising an optical diffused waveguide and a dielectric film having a refractive index higher than the maximum refractive index in the waveguide. It is shown that the deposition of a dielectric film increases the acoustooptic interaction coupling coefficient and shifts the maximum of its frequency dependence toward higher surface acoustic wave frequencies.

The following fluorinated iodides were tested as iodine-laser active media:

C₄F₉I,

C₅F₁₁I,

CF₃OCF₂CF₂I,

and

C₃F₇OCF₂CF₂I.

A stimulated emission output energy of 80–85 mJ/cm³ was obtained from perfluorobutyl iodide (without an inert dilutent). This was comparable with the output achieved from n- and iso-perfluoropropyl iodides which are the most efficient active substances for iodine lasers. The lasing quenching mechanism was analyzed theoretically for a

C₄F₉I

laser.

Paraxial equations for the intensity and eikonal of a light wave, and also a kinetic equation for the difference between the level populations are used to analyze deformation of the space–time structure of a laser pulse occurring in a medium exhibiting two-photon absorption under noncoherent interaction conditions. The analysis is made using time and space compression functions, and the results of a numerical solution of two-dimensional equations for the propagation of radiation. A determination is made of the principal laws governing the changes in the time and space distributions of the intensity of radiation under quasisteady and transient interaction conditions, and also in the case of a small signal. It is shown that two-photon absorption results in a significant inhomogeneity of the space–time structure of a pulse. The results are given of experimental investigations of changes in the time, space, and energy characteristics of a neodymium laser pulse undergoing two-photon absorption in gallium arsenide.

It was established that acoustic oscillations are excited in the active media of pulsed gas discharge lasers having high specific pulse input energies. It was also found that in the pulse-periodic regime the excitation of acoustic oscillations leads to a redistribution of the density of the active medium in the discharge tube, and this results in a resonant dependence of the average laser output power on the repetition frequency of the excitation pulses.

An analysis is made of a steady-state theory of a striated plasma column of a long laser spark. Modulation solutions of the steady-state problem are found and their stability is investigated. An approximate analysis and the results of numerical modeling, are used to formulate general conditions for the existence of metastable optical striations. In general, states with spatial temperature oscillations having the largest possible amplitude are found to be the longest-lived. A numerical solution of the problem is obtained for a laser spark in air. If dust particles are present in the air, each of these may become a striation nucleus; in this case, the striation distribution will be random.

A theoretical and experimental investigation is reported of the behavior of optical radiation near single sharp bends in quartz multimode fiber-optical waveguides. The theory uses conformal mapping of complex-variable functions to transform a curved waveguide into a straight one. Allowance is made for spatial distortions of the electromagnetic fields at the bend. The experimental method uses the experimental observation that almost all

(∼ 80%)

of the optical radiation which has escaped from the core into the cladding in the bend area, is returned to the core when the waveguide becomes straight again. The technique involves measuring the dependence of the optical radiation intensity at the exit from the bent waveguide on the variable refractive index of the external medium in which the bend is immersed. By using this technique, it is possible to determine experimentally the interval of the propagation constants of the cladding modes excited at the bend, the intensity of the residual radiation which does not leave the waveguide core for the given bend radius, and the critical bend radius for which all the core radiation escapes into the cladding in the bend area. The theory satisfactorily explains the results.

It was found experimentally that when various organic liquids are exposed to optical fluxes of fixed duration, there is always an emitted energy density threshold above which intensive carbon black formation occurs in the liquid. This threshold is generally much lower than the optical breakdown threshold of the liquids. The effect is attributed to cracking of the organic compounds near foreign mechanical inclusions always present in real liquids. It is found that the threshold of carbon black formation decreases when catalysts are added and increases when cracking inhibitors are added to the liquids under study.

An experimental investigation was made of radiative recombination of chlorine atoms in order to study the feasibility of developing a gasdynamic recombination laser utilizing electronic phototransitions. Nonequilibrium atomic concentrations were obtained using a method of thermal dissociation of molecular chlorine in a shock tube followed by rapid cooling in a supersonic nozzle. Small-scale nozzles were used for rapid cooling. Recombination luminescence at high

(> 10¹⁷ cm⁻³)

nonequilibrium atomic concentrations was observed for the first time experimentally. The downstream distribution of the radiation intensity in the supersonic nozzles was studied. It was found that the radiation was in the near infrared in the

800–1100 nm

range. A shift of the spectra toward the red and narrowing with cooling was observed. A theoretical estimate was made of the rate of vibrational relaxation of chlorine molecules under the experimental conditions. It was shown that light amplification should occur for phototransitions to high vibrational levels of the ground state. From measurements of the absolute radiation intensity in various spectral ranges, it was possible to estimate the gains feasible under experimental conditions:

(5×10⁻⁴)–10⁻² cm⁻¹.

The Wentzel–Kramers–Brillouin approximation is used to calculate the intensity of transitions to highly excited stretching vibrations of molecular groups. The problem was solved using the three-dimensional single-particle potential approximation. The limits of validity of the local mode model are identified. Calculations are made of the wavelengths and intensities of transitions accompanied by excitation of overtones of stretching vibrations of hydrogen in the benzene ring. The results of the calculations are compared with an experiment to measure the transmission of a polystyrene fiber-optic line.

A comparative analysis is made of the optical and energy characteristics of lasers utilizing binary mixtures containing hydrogen halides in which nonresonant vibrational–vibrational

(VV' )

exchange is used to pump vibrations of the emitting molecules. Specific values are obtained for the gain and specific radiation energy achieved in these systems.

A reliable mode-locking device which can be used to generate short

CO₂

laser pulses has been developed and investigated. The device uses p-type germanium filters cooled to 78°K, having a low threshold (10–15

MW/cm³)

and a short bleaching time

(≲ 0.1 nsec).

As a whole, the system can withstand more than

10⁵

lasing cycles without appreciable damage to the antireflection coatings and laser elements.

Expressions are derived for the Stokes parameters of radiation propagating in a paramagnetic crystal having cubic symmetry. The polarization changes as a result of the crystal anisotropy induced by a magnetostatic field and optical radiation. The conditions for validity of the quasisteady-state approximation are formulated.

The bleaching of a polydisperse aqueous aerosol by cw 10.6 μ wavelength laser radiation and diffusive evaporation of the water droplets were investigated using a numerical solution to a set of equations. The spatial and temporal dependences and the parameters characterizing the bleaching process were found. The parameters of the bleaching process were analyzed as a function of the radiation energy flux density, initial temperature of the droplets, evaporation coefficient and optical constants of water, and initial aerosol dropletsize distribution function. The efficiency of the bleaching process was investigated and the results compared with experiment.

An analysis is made of the thermoelastic action of high-power pulse-periodic laser radiation on the surface of a solid. The analysis is made using relationships, similar to Duhamel integrals, between the characteristics of the thermally stressed state on exposure to cw and arbitrarily time-varying laser radiation. Calculations are made of the dependences of the maximum laser radiation intensities causing impermissible elastic distortions of the surface, plastic flow of the material resulting in residual stresses, fatigue failure and melting of the surface layer.

An analysis is made of switching waves, transverse structure, and field profile hysteresis in nonlinear bistable interferometers. It is shown that a switching wave of zero velocity exists in the hysteresis region and this wave governs the steady-state profile of a beam in an interferometer. The case of oblique incidence of light on an interferometer is considered.