Table of contents

An experimental and theoretical investigation was made of the threshold conditions for the damage to single crystals of KCl caused by focused radiation from a 5-kW cw electron-beam-controlled CO₂ laser. The aim was to establish design criteria for exit windows in industrial CO and CO₂ lasers having a high average output power. The damage was found to start on the front surface of a crystal due to the high intensity of shear stresses. A study was made of the influence of the nature of surface treatments of crystals on their optical strength.

A local approximation is used to study the dynamics of the surface relief of a solid subjected to high-power laser radiation in the case when the rate of damage is proportional to the absorbed energy density. An equation for the dynamics of the surface relief is derived allowing for the dependence of the absorption coefficient on the angle of incidence. It is shown that in the case of a plane-parallel beam the problem of determination of the surface relief at different moments in time reduces to the problem of quadratures. A study is made of the dependence of the depth of a cut on the velocity of a sample, power of the incident radiation, and its polarization. The dynamics of damage to the walls of a waveguide under the action of its normal modes is considered.

The quantum effects and recording noise are allowed for in an analysis of statistical optimization of the algorithms for the processing of images distorted by the atmospheric turbulence. The Poisson statistics of the recorded images and the Gaussian statistics of atmospheric distortions are assumed in the derivation of the equations for the maximum likelihood of a spatial spectrum of one short-exposure image. An analytic solution of the equations of maximum likelihood for low and high values of the signal/noise ratio is utilized in the synthesis of the optimal processing algorithms and the physical meaning of these algorithms is discussed.

The intracavity laser spectroscopy method was used to determine the two-photon absorption of potassium vapor. The two-photon transitions from the ground state 4²S_1/2 to the states 6²S_1/2 and 4²D_3/2,5/2 were detected from absorption in the output spectrum of a wide-band dye laser emitting in the range 760–770 nm; the absorption occurred at a frequency corresponding to the difference between the energies of the upper state and the energy of a ruby laser photon. The ratio ofthe absorption cross section to the square ofthe field was σ₂/E² = 1.5 × 10⁻³³,1.3 × 10⁻³⁵,and 2.8 × 10⁻³⁶ m⁴/V², in satisfactory agreement with calculations carried out on the basis of perturbation theory.

An analysis is made of nonlinear optical phenomena associated with a change in the orientation of the director of a liquid crystal induced by an optical field and resulting in optical bistability. The usual configurations with a nonlinear Fabry-Perot resonator and several other configurations are considered. Bistable cells with cholesterics are proposed: in these cells the nonlinearity is associated with the absorption of the incident and reflected light and is related—via the temperature of the cell—to the cholesteric helix period. Devices are considered in which switching from one state to another takes place as a result of a change in radiation parameters (such as the frequency or polarization) other than the power.

A theoretical model of synchronous pumping developed earlier is used in a numerical calculation of transient emission from dye lasers and of the parameters of steady-state emission. A detailed study is made of the kinetics of establishment of the synchronous regime after instantaneous application of a continuous train of pump pulses. The calculated results are shown to agree well with experimental data.

A study is made of a change in the degree of nonequilibrium of an evaporation process occurring under different regimes of interaction between laser radiation and the surface of an absorbing medium. A numerical solution of the heat conduction equation for a condensed medium is used together with the equations of gasdynamics for a vapor subject to an allowance for additional relationships on the irradiated surface governing the kinetics of a nonequilibrium phase transition. The results obtained indicate the existence of a nontrivial region of transient evaporation characterized by a constant Mach number M = 1 between limits that depend strongly on the laser interaction regime.

A study was made of the operation of a dynamic distributed feedback (DFB) laser utilizing dye-activated polyurethane and pumped by a train of ultrashort pulses from a YAG:Nd³⁺ laser. An interference system with division of the pump pulse duration was used to produce the dynamic DFB. Direct measurements were made of the duration of the generated pulses using an Agat-SF streak camera and it was found that the duration of the output pulses was considerably shorter (up to four times) than that of the pump pulses. Tuning in the 560-600 nm range with an ultrashort pulse duration of ~8 psec, a divergence of ~1 mrad, and a lasing efficiency of ~ 3 % were achieved. The evolution of the ultrashort pump and lasing pulse trains indicated that a thermal phase grating had no influence on the operation of this laser.

A study was made of the use of middle-infrared pulsed laser diodes in investigations of the width and profile of absorption lines of molecular gases. Broadening of the absorption lines of CF₂Cl₂ and SF₆ located in the region of 930 cm^-1 was studied by correlation processing of the laser diode spectra. The coefficient representing the broadening of the CF₂Cl₂ lines in air depended on the buffer gas pressure and it increased from 5.7 to 12.9 MHz/Torr when this pressure was increased from 0 to 100 Torr. The broadening coefficient of the ³²SF₆ lines by helium was independent of the He pressure in the range 5-100 Torr and amounted to 6 MHz/Torr. The self-broadening coefficients of NH₃ and CF₂Cl₂ were found to be 28.5 and 31.2 MHz/Torr.

A theoretical analysis is made of some features of the evolution, in fiber-optic waveguides (FOWs), of spontaneous Brillouin, Raman, and Rayleigh light scattering excited by single optical pulses and developing in the forward and backward directions (relative to the radiation incident at the entry end of a FOW). It is shown that regardless of the widely varying duration of the incident radiation pulses the intensity of the spontaneous forward scattering passes through a maximum at a point in the FOW whose coordinates are governed by the waveguide losses. A variant of the familiar backscattering method is proposed. It is shown experimentally and theoretically that when studying stimulated Raman scattering, the sections of decaying intensity observed at the beginning of the backscattering oscillograms are the result of the growth of spontaneous Raman backscattering.

A study was made of the influence of the distribution of the pump intensity along the optic axis on the output characteristics of a CO₂ laser consisting of two active modules placed in a common telescopic resonator. When one of the modules was pumped, the efficiency of conversion of the excitation energy into radiation (at specific input energies below 0.5 J-cm~3-atm⁻¹) was higher if the module located near a concave mirror was pumped. When both modules were used simultaneously, the laser efficiency was higher when more energy was deposited in the module located near the exit mirror than in the module next to the concave mirror. The results obtained were compared with calculations carried out as a simplified mathematical model.

An investigation was made of the output parameters of a neodymium laser with a positive electrooptic feedback (PEF). Comparison was made of the kinetics of the laser radiation field in three cases: conventional Q switching, PEF regime, Q switching by a crystal of LiF(F₂⁻). It was found that the kinetics of the laser radiation field in the PEF regime was close to the kinetics obtained using the LiF(F₂⁻) switch. In contrast to a laser with a passive filter characterized by a random change of lasing filaments, the structure of the radiation field in the PEF regime remained practically constant during the generation of a giant pulse. The dependence of the operation of the PEF laser on the control voltages could be utilized in computer control of the laser.

It is shown that an increase in the volume of a copper vapor laser above a certain value must lead to a rise in the optimal buffer gas pressure. There is a corresponding change in the electron cooling mechanism (during the interpulse period) from diffusive to elastic cooling, and recombination processes involving three-body collisions in the plasma become important, setting the upper limit to the pressure and specific output energy of the laser. The results are given of a series of experiments and numerical calculations for a medium excited under these conditions by a transverse discharge.

A study was made of the influence of laser light on radiation color centers in glass fiber waveguides of the SiO₂–SiO₂:B₂O₃ type. The induced-absorption spectra were determined in the wavelength range 300–1900 nm for fibers subjected to γ-ray doses from 300 to 10⁴ rad at irradiation temperatures 77 and 300 K. Eight types of radiation color centers were identified and the induced-absorption spectra of the waveguides were almost entirely due to these color centers. The centers differed in respect of the stability when subjected to laser light and temperature variations. Infrared absorption appeared when the induced-absorption band at 340 nm was suppressed by λ = 337 nm laser light and then restored at room temperature.

An experimental investigation was made of the collective influence of a set of aerosol particles in a medium on the initiation of a low-threshold optical discharge in an Nd laser beam. It was established that the threshold intensities for discharge excitation did not exceed 10 MW/cm² for a laser pulse duration of ~1 msec. It was observed that initiation of an optical discharge depended on the concentration of aerosol particles in the medium. Investigations were made of the characteristics of the excited laser plasma and a model of the phenomenon was proposed.

An investigation was made of the generation of the second and fourth harmonics of laser radiation in magnesium vapor as a result of the forbidden transition 3s²¹S₀–4S¹S₀. The efficiency of conversion to the second harmonic was ~ 10⁻² % and the corresponding efficiency for the fourth harmonic was ~10⁻⁸ %.

It is suggested that far vacuum ultraviolet radiation can be coupled out of a laser resonator by diffraction on a sharp ("knife") edge of a mirror covering part of a beam. It is reported that a model experiment carried out on a neodymium laser shows that a considerable fraction of the energy can be coupled out in this way.

A report is given of the first application of an optoacoustic method with an optical parametric oscillator in recording the absorption spectra of atmospheric pollutants in the spectral range 2.2–3.6 μ (propane and ethane molecules were used to test the method). Estimates were obtained of the sensitivity and linearity of the measuring system. Recommendations were developed on improving the system for the identification of separate hydrocarbons in complex mixtures.

Optical anisotropy induced in silicon by picosecond light pulses was observed and investigated. The anisotropy relaxation time was demonstrated to be less than 2 psec.

Emission at wavelengths of 1.0615 and 1.074 μ from a self-mode-locked YAG:Nd³⁺ laser was observed at thresholds which were only 1.5 and 3.5 times higher than the threshold for the emission of radiation with λ = 1.064 μ. Efficient conversion (characterized by a quantum efficiency of ~ 15%) of the third harmonics of the radiations with λ = 1.064 and 1.0615 μ to the visible (590 nm) and ultraviolet (250 nm) ranges was achieved in atomic barium vapor. Electron stimulated Raman scattering due to a strongly forbidden transition in a system of Ba atoms was observed for the first time.

A method was developed for the reversal of the wavefront of laser radiation suitable for a wide range of wavelengths. It is based on parametric mixing of an input signal with a reference wave and conversion of its frequency to a different (intermediate) frequency range where the reversal takes place. During the return passage through the parametric mixer the reversed intermediate-frequency signal forms, together with the reversed reference wave, radiation at the frequency of the input signal but with the reversed wavefront. This reversal method was tested experimentally for radiation with the wavelength λ = 0.53 μ.

A determination was made of the energy, spectrum, and duration of emission of radiation generated as a result of vibrational-rotational transitions in HF and HCl molecules in a chemical laser utilizing ClF–H₂–He mixtures and subjected to short-pulse initiation of the pump reactions by a transverse electric discharge preceded by photopreionization. When the total energy of the output radiation pulses due to both HF and HCl molecules was 0.16 J, the efficiency of the laser measured relative to the energy deposited in the active medium was 60%, the specific output energy was 4 J/liter, and the power per pulse was 0.4 MW.

It was demonstrated experimentally that propagation of light pulses in active neodymium-doped glass fibers reduced the duration of the amplified pulses almost twofold in fibers 1-m long. A study of the amplification of a series of ultrashort laser pulses carried out using a glass-fiber quantum amplifier showed that the main cause of reduction in the duration of nanosecond light pulses was recombination of short-lived color centers because of the absorption of the leading edge. The shortening of light pulses was studied at different rates of excitation of active fiber waveguides.