Table of contents

A review is given of theoretical and experimental investigations of optical resonators excited by an external light beam and filled with a nonlinear medium. An analysis is made of various types of nonlinearities (Kerr, Raman, Brillouin, and quadratic nonlinearities, saturation in a medium of two-level particles) and equations which describe the dynamics of optical oscillations in a resonator, are derived. Descriptions are given of the main properties such as limitation of the transmitted beam intensity, bistability and hysteresis, intensity pulsations, and amplification of amplitude-frequency deviations in the optical beam. Transient processes are also discussed. In the description of the experimental results, not only the resonator parameters and the observed properties are indicated but attention is also paid to identification of the type of nonlinearity. An analysis is also made of hybrid systems in which the nonlinearity is simulated (and deliberately increased) by radioelectronics.

Nonlinear distortions of light beams traveling under thermal self-interaction conditions can be compensated by an algorithm for the control of the initial beam profile. In the linear case the proposed algorithm for the optimization of the process of focusing of a light beam by the conditional gradient method is identical with the familiar phase conjugation algorithm. This analogy is used to investigate a number of problems relating to the efficiency of phase-conjugate adaptive optical systems.

Spatially resolved spectra and photographs of a carbon jet in the radiation of the far vacuum ultraviolet lines were obtained for laser radiation focused onto a planar target using different geometries. The fine structure of the H_α transition of a CVI ion was observed. The profile of the electron density N_e was deduced from the Stark broadening of Balmer series transitions. The expansion velocity was measured and estimates were made of the electron temperature, plasma ionization state (the ratio CVII/VI), rate of evaporation of the target material, and acceleration at an early stage of the expansion. The gradient of N_e in the vicinity of the critical point was estimated. The feasibility of determining N_e from the Stark broadening of Balmer series transitions in hydrogen-like ions over a broad density range is discussed.

A theoretical and experimental study was made of the dispersive properties of two-layer waveguides with a rectangular core, which were compared with the corresponding properties of circular waveguides. The method of effective permittivities was used to calculate the dispersion curves, number of propagating modes, and their group delays for rectangular step and graded-index guides. The vapor reaction method was employed in the fabrication of a rectangular multimode waveguide with the following parameters: pulse dispersion ~4 nsec/km, losses ~7 dB/km, numerical aperture ∼0.17.

An analysis is made of the influence of amplitude distortions on the intensity distribution in the image of a diffuse object obtained in a randomly inhomogeneous medium assuming that the logarithm of the amplitude distortions is normally distributed. It is shown that the distortions in this distribution depend strongly on the parameter η = ρ_α²σ²/S (ρ_α and σ are the correlation radius and variance of the logarithm of the amplitude distortions; S is the area of the receiver aperture) and also on the resolution of the receiving equipment. This conclusion is confirmed experimentally.

An investigation was made of XeF and XeCl lasers excited with a discharge stabilized by a shortduration electron beam. It was found that excimer molecules are formed in the XeF laser as a consequence of ion-ion recombination and of interaction between excited Ar and Xe atoms and the halogen-bearing molecules. In the case of the XeCl laser, the highest efficiency was achieved by electron beam excitation. On applying an electric field, the discharge contributed to the emisson only when there was substantial electron multiplication.

An investigation was made of the distribution of dopants, OH impurities, and the refractive index n in preforms and glass fiber-optical waveguides (GFOW's) fabricated by the chemical vapor deposition method. It was found that diffusive redistribution of Ge and 6 dopants occurs during the drawing process. This was manifested by equalization of the dopant concentration and blurring of the boundaries of the waveguide structure over the effective length ~0.08α (α is the core radius). This effect should be taken into account in the development of GFOW's for wide-band fiber-optical communication lines. In addition to x-ray microanalysis, the absorption method was used for the first time to investigate the radial distribution of the GeO₂ concentration in preforms. This method was used to measure the GeO₂ distribution in an xGeO₂+(1–x)SiO₂ binary system at λ = 0.269 μ [the extinction coefficient was (1.02±0/05)cm^–1/mole %]. With an appropriate choice of spatial resolution, this method could be used to simulate diffusive redistribution of the dopant in GFOW's. The refraction method was used to investigate the dispersion of n in a step-profile preform. Considerable inhomogeneity of n₂ was observed in the quartz cladding (~5 × 10^–4) and Δn = n₁–n₂ was ~1 × 10^–4 (mole % GeO₂)^–1 lower than the value of Δn for homogeneous materials of the same composition. These values exceeded the calculated effects of mechanical stresses.

The results are presented of investigations of new cerment discharge gaps, having operating voltages of 9–7 and 20–16 kV and filled with various gas mixtures (argon and nitrogen, pure nitrogen). The dependences are given of the delays in firing the gaps, as a function of the voltage on the electrodes and of the energy of the laser pulses used for triggering (λ = 1.06 μ). The minimum triggering energy of the type 1 discharge gaps was 2 μJ, when using a radiation pulse having τ_rad = 70 nsec. The rise time of the voltage pulse produced was 0.5 nsec. The discharge gaps were used to control electrooptic switches in laser systems. A study was made of the radiation produced during the breakdown of the discharge gaps. The radiation pulse had a flat top and a rise time of ≤ 1.5 nsec.

A prototype four-channel heterodyne spectrometer was developed, having a threshold sensitivity of 2 × 10^–19 W/Hz in the λ = 9.586 μ region. The transmission through the full thickness of the earth's atmosphere was investigated along inclined paths with a spectral resolution of 0.002 cm^–1.

The necessary, estimates were obtained and it was demonstrated experimentally that the Bragg diffraction of light by ultrasonic waves in air may be used to measure the intensity distribution of laser radiation. A transducer utilizing an X-cut quartz crystal was developed to implement this technique. Measurements were made of the angular distribution of the ultrasonic field and its divergence was determined as 4×10^–3 rad at 0.9 MHz. In order to confirm this possibility, measurements were made of the laser radiation distribution at λ = 0.6328 μ in the incident and Bragg-diffracted radiation.

A method of controlling the spatial parameters of a radiation field using a "soft" aperture inserted inside the laser resonator is described. An analysis is also made of spatial radiation distributions for a planospherical resonator with "soft" or "hard" apertures. The results of the calculations show that specified spatial field distributions can be obtained in an arbitrary plane perpendicular to the direction of propagation of the radiation.

An on-line diagnostic system was used to investigate the kinetics of N₂F₄→2 NF₂ dissociation in a highpower pulsed CO₂ laser field. It was found that, at N₂F₄ pressures of 3–15 Torr and incident energy densities of 0.2–3 J/cm², collisions did not have a significant influence on the excitation and dissociation process and its rate was governed by the rate of radiative activation of the molecules. The intramolecular relaxation time of N₂F₄(τ_ννi < 15) nsec was estimated from the experimental results. The experiment was interpreted satisfactorily assuming that the behavior of N₂F₄ molecules in the field is characterized only by the dependence of the stimulated transition cross section on the molecular energy σ(E) and by the dissociation energy which corresponds to the breaking energy of the weakest N–N bond. The experimental data were used to construct the time dependence of the molecular energy distribution function and to determine the character of the dependence σ(E): σE^–k(1<k< 2).

Gaseous UF₆ was dissociated with radiation from a CF₄ laser. A considerable increase in the rate of dissociation of the UF₆ occurred under the combined action of CF₄ and CO₂ lasers. It was found that the CO₂ laser radiation acted most efficiently when its frequency was close to that of the ν₂+ν₃ overtone of the UF₆ gas. Visible luminescence, explained by emission from the UF₆ dissociation products, was observed in two-frequency dissociation of the UF₆. A mass-spectrometric investigation of the residual UF₆ gas failed to show any isotopic selectivity of the dissociation.

The dynamics of changes in the chemical composition of the gas mixture in a pulsed high-pressure gasdischarge CO₂ laser with photopreionization, a high current-pulse repetition rate, and transverse circulation of the gas around a closed loop was studied and the setting up of an equilibrium chemical composition was investigated. The use of a high repetition frequency for the current pulses facilitated the rapid establishment of an equilibrium chemical composition in the gas mixture (in about 10–15 min), in a relatively large volume (60 liters), without the mixture being strongly contaminated by desorbed gases. It was established that after this interval of time, following the initiation of a discharge in the gas, a dynamic equilibrium was set up between the dissociation of the CO₂ molecules and the recombination of the dissociation products. It was shown that a steady-state degree of dissociation of the CO₂ gas was a function of the electric-discharge parameters and of the initial composition of the gas mixture, and was independent of the pressure. Under typical laser discharge conditions, the steady-state degree of dissociation of the CO₂ molecules was 20–30%. It was found that hydrogen could be used as a catalyst for oxidizing carbon monoxide. On the basis of these investigations, the use of reduced pressures of the gas mixture is recommended for practically important operating regimes of pulsed CO₂ lasers which employ gas mixtures in chemical equilibrium.

An investigation was made of the feasibility of achieving a high specific output energy and a high electrical efficiency in an H₂–F₂ chemical laser excited by a relativistic electron beam. Using an H₂:F₂:O₂:He = 6:15:1.2:50 mixture at a pressure of 1.3 atm, a specific output energy of 100 J/liter was achieved with an efficiency of ∼ 900% with respect to the energy deposited in the active volume of the laser and a chemical efficiency of ∼ 4%. The degree of initiation and the efficiency were estimated using the experimentally measured electron beam energy absorbed by the active volume of the laser. A high efficiency of utilization of the electron beam energy, ∼ 200%, was obtained. It was found that the H(F)+O₂+M→H(F)O₂+M three-body recombination process was very undesirable in active mixtures at pressures of ∼ 1 atm. The rate constants of this reaction for M = He, Xe, and SF₆ were determined experimentally.

An analysis is made of the evolution of a light pulse in a two-level resonant medium as a function of the degree of preliminary excitation by another pulse. It is shown that, in addition to self-induced transparancy observed under certain conditions associated with the degree of excitation of the atomic system, resonance scattering is also found due to the existence of macroscopic polarization of the medium. This scattering is not observed when the medium is in a noncoherent state. The transverse reversible relaxation time T*₂ can be determined from the variation of the scattering intensity.

An experimental investigation was made of laser action due to the B³Π_Ou⁺–X¹Σ_g⁺ electronic transition in molecular iodine in the 0.67–1.24 μ range pumped by a cw rhodamine 6G dye laser. The total number of lasing lines in this range was estimted to be ~10⁵. The threshold pump power was 10–30 mW for the 1.1–1.2 μ range and 30–100 mW outside this range. An investigation was made of the lasing line profile.

An experimental investigation was made of the effect of laser radiation on the extraction process at the interface between two liquids in reactions involving uranium extraction from an aqueous phase by neutral organic extractants, organic acids, and organic bases. It was found that on exposure of the interface between the two phases to resonance laser radiation, a shift of the extraction equilibrium toward increasing extraction from the aqueous to the organic solution was observed only in extraction processes involving neutral organic compounds when the interaction in the complex compound was of thp coordination type. An atmospheric-pressure pulsed CO₂ laser was used in the experiments.

The results are given of an investigation into the possibility of increasing the efficiency of sealed watercooled CO lasers by reducing the overall losses of the optical resonator. The experimentally obtained efficiency of a laser oscillator was 18–20% at an output power of about 32 W. A study was made of the influence of the discharge-tube wall temperature on the efficiency and on the power emitted per unit length. On varying the temperature of the cooling liquid from 40 to 1°C the laser efficiency rose from 8.5 to 18%, and the power per unit length from 13 to 31 W/m. The maximum laser output power was 38.5 W. The possibility of a further increase in the efficiency of sealed CO lasers is briefly discussed.

It is shown that during the buildup of a convective parametric decay instability a high-intensity internal field can be generated throughout the spatial region in which excitation and propagation of electron Langmuir waves is possible. Under the conditions of a laser plasma, this region accounts for an appreciable proportion (~30%) of the plasma corona. A turbulence region of this size has the result that when the effective collision frequency exceeds the classical value, anomalous heating of the laser plasma corona may be found. An explanation of various experimental data on second harmonic generation and reemission is given.

A method for measuring the nonequilibrium carrier lifetime τ (of the order of 10^–9–10^–11 sec) was developed and tested on photoconductors: it is based on measurement of the phase shift of the photoresponse by an optical delay line. Measurements were made of the lifetime in Ge:Zn^II samples and the values of τ were found to be in the range 7×10^–11–7×10^–10 sec. The results obtained were compared with the lifetimes deduced from the amplitude-frequency characteristics of the photoresponse and with the results of a numerical calculation. The errors of the new method were analyzed.

Resonance selective excitation of the metastable state of neodymium was achieved in silicate glass. A high degree of selection in inhomogeneously broadened emission spectra was obtained. The positions of all five Stark components of the ground state were established and their dependence on the value of the exciting radiation quantum within an inhomogeneously broadened spectral profile was determined. An investigation was made of the evolution of resonance radiation spectra as a function of the temperature and also of the wavelength, and spectral width of the exciting radiation. It was concluded that in order to improve the energy characteristics of neodymium-glass laser systems, it is advisable to broaden the spectrum of the pulse being amplified to ~120 cm^–1.

An investigation was made of the lasing threshold, total output energy, duration of emission and its time structure, duration of a single spike, and transverse distribution of the intensity in the beam emitted from a neodymium-glass laser with an ultrasonic Q switch. These properties were determined as a function of the control voltage applied to the switch. The results obtained were interpreted on the basis of a theory of the diffraction of light by a standing ultrasonic wave.

An investigation was made of the possibility of using a transverse microwave discharge in compact (active length 3–4 cm) efficient He–Ne lasers pumped by a transistorized microwave oscillator. An experimental study was made of the energy characteristics of such lasers. It was found that an increase in the frequency of the pump field increased the output power of the laser and reduced the required pump power, and also increased the degree of localization of the discharge inside the resonator.

The geometric-optics approximation is used in the derivation of expressions for the square of fluctuations of the intensity level and for the structure function of fluctuations of the phase of a beam of probe radiation traveling in a bleached disperse medium. Fluctuations of the parameters of the medium appear as a result of mixing of inhomogeneous temperature and water-content fields by a random-velocity wind. A comparison is made of the calculated fluctuations of the intensity with those measured in an artificial fog chamber.

Stimulated emission was observed when ZnTe, ZnSe, and CdS single crystals were exposed to two-photon excitation by a train of ruby laser picosecond pulses. The lasing wavelengths for ZnTe, ZnSe, and CdS crystals were 530, 450, and 495 nm, respectively. The angular divergence was 19–21° and the laser action occurred at peak exciting pulse intensities of around 1 GW/cm².

An investigation is made of some features of the heating of metals in air by cw laser radiation obliquely incident on the target surfact. It is shown that, for a fixed power, there is an optimal angle of incidence of the radiation polarized in the plane of incidence for which the target is heated to a given temperature in the minimum time. At the optimal angle of incidence, an appreciable improvement in energy transfer may be achieved in the laser heating of various metals. This is confirmed by experiments on the angular dependence of the inflammation time of tungsten.

It is shown that the zero frequency of the spatial spectrum of an image can be filtered off by transient pumping of the intensity of this frequency to the reference wave and simultaneous amplification of the high-frequency components in the course of recording of a dynamic hologram in a nonlinear medium with local response. Experimental results are reported showing that the contrast of amplitude transparencies can be improved by recording holograms in iron-activated lithium niobate crystals.

An investigation was made of the spectral characteristics of the stimulated emission from cadmium sulfide platelet lasers (at T = 4.2, 77, and 360°K) in the case when one or several partial resonators were emitting. It was found that simultaneous stimulated emission from nearby partial resonators established an optical coupling as a result of which each resonator generated the same set of frequencies, the widths of the emission lines increased, and the spectrum was enriched with new mode maxima.

The deformation of the 1.06 μ Nd³⁺ luminescence band profile of glasses of different compositions (silicate and phosphate) was recorded under free-oscillation conditions. It was established that the type of distortion and its temperature dependence were similar to those found on exposure to a giant pulse. A significant lack of correlation between the Stark components which comprise the 1.06 μ Nd³⁺ band was confirmed.

A physical model is used to explain deviation of the frequencies of longitudinal semiconductor laser modes observed in the case of amplitude self-modulation. The essence of the effect is that amplitude fluctuations are accompanied by synchronous fluctuations of the excess electron density and the latter influence the optical length of the resonator and longitudinal mode frequencies. It is shown that fluctuations of the electron density by 10–20% can give rise to a deviation comparable with the intermode separation in the radiation emitted from AlGaAs heterolasers at room temperature.

An investigation was made of the power characteristics of a neodymium-glass laser near the damage threshold of the active element. Calculations of these characteristics were made for GLS1 and GLS4 glasses assuming different pump pulse repetition frequencies (1–100 Hz), dimensions and other parameters of the active element and pump radiation. By optimizing the laser resonator, output powers close to the calculated values were obtained experimentally at frequencies of 20–100 Hz.

Model cosmological problems include studies of creation of real particles (photons) from vacuum during motion of mirrors in a space-time region free of all nongravitational fields. It is shown that this process is equivalent to parametric generation or amplification of light under zero initial conditions, i.e., it is equivalent to parametric amplification of zero-point vibrations of vacuum on a moving mirror acting against Casimir radiation pressure forces.

An investigation was made of the x-ray emission spectrum of a magnesium plasma heated by ruby laser pulses of 10–20 psec duration and 10¹⁴ W/cm² power density on the target surface. The relative intensities of the spectral lines were used to determine the electron temperature (200 eV), density [(1–1.15) × 10²⁰ cm^–3], and ionization state ([Li]/[He] ≈ 1) of the luminous part of the plasma. The relative intensities of the resonance lines of the H- and He-like ions yielded an estimate of the electron temperature in the vicinity of the critical point (T_cr550–700 keV).

Direct measurements were made of the absolute quantum efficiency of the luminescence from the ⁴F_3/2 upper active level of neodymium in high-concentration Li–La_1–x–yNd_xCr_y phosphate glass excited at various absorption bands of neodymium and chromium. The quantum efficiency of the chromium luminescence was also determined by the integrating sphere method. The results showed good agreement with measurements of the excitation spectra and decay kinetics of the upper active level of neodymium. Efficient nonradiative energy transfer was observed from the chromium ions to neodymium levels situated above the metastable state. For x=0.3 and y>0.03, nonradiative energy transfer was also observed from the metastable state of neodymium to the chromium ions. It was found that for x=0.3 and y=0.03, when the Nd→Cr reverse transfer was still negligible, approximately 50% of the excited chromium ions transferred their energy to the metastable state of the neodymium ions at room temperature although the luminescence quantum efficiency of chromium was a few percent.

An investigation was made of the influence of Fe and Nd impurities (for different stoichiometric coefficients R of the mixture) on the 90° phase-matching temperature T_m and the width of the phasematching temperature range ΔT of lithium niobate crystals. It was established that the dependence of T_m on R and the type of impurity observed for doped crystals was quite different from that of pure crystals. It was found that the temperature T_m for lithium niobate crystals can be controlled by adding suitable impurities.

A new variant of active Raman spectroscopy is proposed. Instead of two-frequency pumping, radiation in resonance with the scattering transition is used and a static electric field is applied simultaneously. This variant may have an advantage over the conventional method in the case of a large Stokes shift which is found, for example, in the Raman scattering due to electronic transitions. An analysis is made of the dynamics of the interacting waves and the optimal length of nonlinear medium to which a static electric field should be applied is determined.

An investigation was made of the matching between various types of semiconductor sources and thin-film planar and stripe waveguides by the tunneling of the optical energy via a prism coupling device and via a system comprising an optical fiber and a planar waveguide. The maximum coupling efficiency was found to be ∼ 60%.

A description is given of the optical part of an interference microscope designed for the investigation of local phase inhomogeneities of transparent media. A frequency shift of one of the interfering beams makes it possible to increase the precision of the measurements and to automate them. The results are given of investigations of the refractive-index profiles of glass-fiber waveguides.

Generation, in a KDP crystal, of the sum frequencies of YAG:Nd³⁺ laser harmonics and the Stokes components of stimulated Raman scattering produced picosecond pulses of 218–316 nm wavelengths and 0.1–3 mJ energy.

An investigation was made of the use of a two-pass telescopic neodymium-glass amplifier in combination with a Brillouin mirror in high-gain wavefront-reversal systems. Small-signal gain of 107 was achieved for a single pulse of SO nsec duration and no self-focusing filaments were observed in the output stage when the energy was up to 100 J.

In the temperature range 250–375°C, measurements were made of the quantum efficiencies of fluorescence and one-photon photodissociation of rarefied molecular vapors of paraterphenyl (p–TP) and 2,5-diphenyl-1,3,4,-oxadiazole (PPO) excited by λ=266 nm radiation and of 1,4-bis-[2-(5-phenyloxazolyl)]-benzene (POPOP) excited by λ=355 nm and λ=266 nm radiation. The quantum efficiency of photodissociation under the experimental conditions was only a small fraction of the total quantum efficiency of nonradiative deactivation of the fluorescence state of these molecules. The low fluorescence quantum efficiencies of p–TP and PPO (0.07–0.02) in the operating temperature range could be one of the reasons for the absence of laser action in vapors of these compounds.

Measurements were made of the gain of stimulated Raman scattering by rotational levels of gaseous hydrogen in the case when the directions of rotation of the polarizations of the pump (neodymium laser) radiation and of the Stokes signal coincided or were opposite. The dependence of the gain on the hydrogen pressure was found. It was found that a ∼50% efficiency of conversion into just the first Stokes component could be acheived without any special measures to suppress the second component.

An investigation was made of the operation of xenon flashlamps emitting 2–4 μ sec pulses. The efficiency of emission of these flashlamps was determined in the spectral range 260–390 nm for various values of the energy in a storage capacitor system and various xenon pressures.

A review is made of the principal results reported at the Twelfth European Conference on Interaction of Laser Radiation with Matter and Laser Fusion, which was held on December 11–15 1978 in Moscow. The current status and future studies of various physical problems connected with laser fusion are discussed.