Table of contents

The spectral structure of stimulated Raman scattering (STRS) by optical phonons and polaritons of the A(z) symmetry in a crystal of LiIO₃ in a resonator was investigated experimentally as a function of the intensity of pump radiation traveling at an angle to the STRS resonator. The STRS spectra were fully identified. The special characteristics of these spectra were attributed to an aperture effect which, in particular, reduced the gain of the polariton scattering because of the emission of a polariton wave from a pump beam even when the angle between this beam and the STRS resonator axis was small.

A solution is obtained to the problem of a resonant interaction of a gas of atoms with the field of three strong standing-wave pulses separated in time. A calculation is reported of a correction to the power absorbed by the gas, which oscillates near the center of an atomic line as a function of the field frequency with a characteristic scale of the order of the reciprocal of the time delay between the pulses. It is shown that a signal then appears for all rational values of the ratio of the delays between the pulses and it is related to the transfer—along the time axis—of higher spatial harmonics of the polarization of a medium and of the level populations.

Expressions are obtained for the cross sections of the stimulated bremsstrahlung effect as a result of scattering of electrons in the radiation field of a laser emitting an arbitrary number of modes. It is shown that the experimental results on the scattering of electrons by neutral atoms and molecules can be explained qualitatively if simultaneous allowance is made for multimode and pulsed nature of the radiation.

An experimental investigation was made of the wavefront reversal parameters and of the reflection coefficient of a stimulated Brillouin scattering mirror made of CCl₄; the measurements were carried out in a wide range of pump energies using pulses of 0.5–10 nsec duration. A reduction in the pulse duration reduced the dynamic range (i.e., the range of the pump energies) in which wavefront reversal was observed. Specially shaped pulses with different values of the contrast were used for the reversal of radiation with a high peak power.

A prototype was developed of a multichannel duplex fiber-optic communication line with spectral multiplexing in the region of 1.3 μ. The line length was 5 km and it provided simultaneous communication by two optical carriers in one direction and two in the opposite direction; the spectral interval between the carriers traveling in the same direction was 17 nm. At each end of the line there was a diffraction grating which was used for multiplexing and demultiplexing; it ensured formation of narrow-band optical carriers in the multiplexing stage. The terminal devices were capable of multiplexing of ten channels traveling simultaneously in the same direction.

Electromagnetic oscillations of 180 kHz frequency tunable in a range of 30 kHz were generated in a metal–barrier–metal–barrier–metal (MBMBM) structure characterized by a negative differential resistance. Such structures are potential tunable sources of electromagnetic radiation ranging from the radio to the optical frequencies.

An experimental investigation was made of the transition from steady-state to pulsed ionization–thermal breakdown of gases near the surface of a target irradiated by a laser. It was found that when the laser beam power was varied, the target temperature at the moment of formation of a plasma near its surface fell to a minimum at some moment from the beginning of irradiation but before breakdown. A study was made of the dependence of this time of onset of steady-state breakdown on the size of the irradiated spot and on the thermophysical properties of the gas.

An experimental investigation was made of stimulated Raman scattering in CO₂ filling a multipass optical cell. An increase in the number of the Stokes and anti-Stokes components of stimulated Raman scattering was observed when the CO₂ pressure was reduced to 0.3–4 atm. Measurements were made of the wavelengths and of the spectral width of the components of stimulated Raman scattering in CO₂ observed for the first time. The dependences of the energies of the Stokes and anti-Stokes components of the scattering in CO₂ on the pump energy (λ ∼1.06 μ) were determined using a multipass cell. A reduction in the pressure of CO₂ to 0.5–1 atm resulted in generation of just the axial anti-Stokes components of the scattering.

An analysis is made of the effect of the diffusion of excited particles in an active medium on the characteristics of the saturation of the gain and output power in the case of gas lasers. It is shown that the diffusion increases the effective zone of the interaction between the active medium and the radiation field. To a high accuracy, the influence of the diffusion on gain saturation can be allowed for by introducing an effective saturation parameter which is governed by the characteristics of the medium and by the geometry of the system. In a more detailed consideration an allowance has to be made for the distortion of the saturation characteristics, which is strongest in the case when the diffusion length is comparable with the transverse size of a light beam. The values of the saturation parameter calculated from the relationships derived in the present paper are in agreement with the published experimental results.

An experimental investigation was made of the optical properties of a plasma formed by irradiation of an Al₂O₃ target in vacuum with high-power neodymium-glass laser radiation. It was found that at some distance from the target surface the levels of hydrogen-like oxygen ions with the principal quantum numbers n = 2, 3, and 4 were inversely populated. The gain at the λ ∼29 nm wavelength (4→3 transition) was ∼0.1 cm⁻¹.

A study was made of the excitation of rhodamine 6G in ethanol by electric breakdown of the solution. The excitation mechanisms were identified and the spectral composition of the luminescence was determined. It was established that the excitation was electronic and its efficiency was 1%.

A study was made of the spatial distribution of the intensity of radiation scattered by a random phase screen and focused by a lens. Two maxima were observed experimentally in the distribution of the intensity along the direction of propagation of a beam: these maxima were located near the focal plane. The experimental results were interpreted with the aid of a model of a beam subject to random phase modulation. It was found that one of the maxima in the intensity distribution corresponded to the "noise" component of the scattered radiation, and the other to the coherent component.

It was found experimentally that photoionization of a gas by ultraviolet radiation emitted by a multichannel guided surface discharge generated electrons in a density sufficient for initiation of a high-current nonself-sustained discharge. The use of a surface-discharge plasma as an extended cathode gave rise to a current of 3.6 A/cm² density in a nonself-sustained discharge in nitrogen. This volume discharge method should find application in high-power CO₂ lasers.

A report is given of the results of detailed calculations of the specific output energy of cascade lasing due to 00⁰1–02⁰0–01¹0 transitions in CO₂ molecules pumped by a pulsed gas discharge. The results are given as a function of the initial temperature, pressure, composition of the gas mixture, rate of Q switching of the resonator, and degree of excitation of the CO₂(v₃) molecules. Under practical conditions the specific output energy at the wavelength of 16.2 μ may amount to 0.2–0.5 J/liter.

The optoacoustic method was used to investigate the change in the collisional profile of the 694.38 nm absorption line of H₂O subjected to strong fields of laser radiations with linear and circular polarizations. The measurements were carried out with a resolution of 0.015 cm⁻¹ using laser radiations of intensities up to 50 MW/cm². The study of nonlinear absorption was made for H₂O–air, H₂O–N₂, H₂O–Ar, and H₂O–Xe mixtures at total pressures in excess of 180 Torr using radiations of several intensities.

A YAG:Nd³⁺ laser was passively Q-switched with the aid of an LiF crystal which contained color centers and was characterized by an initial transmission that varied across the crystal. Such switching reduced the angular divergence of the neodymium laser radiation to 2' without altering the energy characteristics. A method was developed for determining the initial transmission profile of a passive Q switch. An increase in the efficiency of the recovery of the color centers in an LiF crystal containing F₂ centers was observed under the simultaneous action of the second and third harmonics of the neodymium laser radiation.

Some features are discussed of the pumping of a CO₂ laser having a waveguide resonator and using a capacitative rf discharge. The gain was measured for several carbon dioxide vibrational–rotational transitions in a discharge chamber with a slowly circulated active medium and the dependence of the gain on the total pressure was determined. Continuous lasing was achieved at active mixture pressures up to 37 kPa.

Stimulated emission of visible radiation was obtained from multicomponent semiconductor lasers pumped by an electron beam. The lasers operated at room temperature in a pulse-periodic regime. A cadmium sulfide laser emitted radiation pulses of 100 kW power.

The active element made of concentrated neodymium phosphate glass and placed in a short resonator emitted pulses of 2.5 nsec duration. The element was pumped by pulses of 30 nsec duration and 0.53 μ wavelength. The experimental results are analyzed.

An experimental study was made of lasing in dyes pumped by radiation from a cw YAG:Nd³⁺ laser with intracavity second harmonic generation in an LiIO₃ crystal. The parameters of the tunable dye laser radiation were not inferior to those obtained by pumping dyes with high-power argon laser radiation.

The optimal excitation conditions and energy characteristics are calculated for a pulse-periodic electron-beam-controlled CO₂ laser with a cooled active mixture. A comparison is made with the characteristics of electron-beam-controlled lasers operating at room temperature. It is shown that cryogenic cooling of the active mixture enables the average output power of pulse-periodic industrial electron-beam-controlled CO₂ lasers to be increased by a factor of 2–3.

Measurements were made of the absolute quantum efficiency of the luminescence of Cr³⁺ ions due to transitions from the ⁴T₂ state in gadolinium gallium (GdGaG) and gadolinium scandium gallium (GdScGaG) garnet crystals. The quantum efficiency of the luminescence of the Cr³⁺ ions in the ⁴T₂ state in GdGaG and GdScGaG crystals was close to unity at least up to chromium ion concentrations of 3×10²⁰ cm⁻³.

Optimization of the technology of preparation of active elements made of LiF crystals containing color centers ensured high values of the parameters of wide-band generation of light by the color centers in a nonselective resonator pumped with YAG:Nd³⁺ laser radiation (λ = 1.064 μ). The efficiency measured relative to the incident pump energy was 25%, the efficiency of energy absorption was 32%, and the differential efficiency was 50%. The lasing band had a maximum at λ =1.17 μ and the half-width was Δλ >1000 cm⁻¹. Stimulated emission of radiation by the centers was observed when the pump laser was operated under free-running conditions (smooth pulses) and the efficiency measured relative to the incident pump energy was then 0.5%. An efficient LiF: laser with a spectrally narrow (<0.05 nm) and continuously tunable (within the 1.09–1.23 μ range) emission line was constructed. The efficiency at the maximum of the tuning curve (λ = 1.17 μ) was 10% measured relative to the incident energy and 12% measured relative to the absorbed energy; the differential efficiency was 23%. The operating temperature of the active elements of the laser was the room value and the repetition frequency of the output pulses was up to 100 Hz.

A study was made of the efficiency of conversion of target-heating laser radiation into x rays emitted by a plasma in a wide range of laser radiation power densities (5×10¹¹–2×10¹⁴ W/cm²) and atomic numbers of the target material (4–92). Estimates were obtained of the exposure dose needed for x-ray photolithography of resists when such a laser-plasma source of soft x rays was used.

An experimental investigation was made of spatial, temporal, and power characteristics of a streamer cadmium sulfide semiconductor laser. The shape and amplitude of the exciting pulses had a considerable influence on the formation of a streamer and on the velocity of its head. When the pulse amplitude was increased from 4 to 90 kV, the velocity of the streamer head increased from 2×10⁷ to 3×10⁹ cm/sec. The output power of the laser increased on increase in this velocity in accordance with a near-parabolic law. A study was also made of the influence of the voltage drop in the streamer channel on the spatial and temporal characteristics of the laser.

An investigation was made of the efficiency of a laser utilizing a concentrated Li–Nd–La phosphate glass as the active medium and an LiF crystal with centers as a passive Q switch. The efficiency was about 0.5% in the pump energy range 5.6–13 J and the distribution of the output radiation corresponded to the TEM₀₀, mode. The main factor which limited the output energy of the laser pulses was the partial filling of the aperture of the glass active element. The experimental results were compared with calculations. Relationships were obtained for estimating (with an accuracy sufficient for practical purposes) the values of the parameters of the output pulses obtained under various experimental conditions. Estimates were obtained of the influence of the residual and induced absorption, and of the relaxation time of the passive switch on the lasing efficiency.

A simple method is proposed in which an allowance is made for the effects of the translational and rotational nonequilibrium of laser molecules (induced by the radiation field) on the specific energy characteristics of a cw chemical HF laser in the case of inhomogeneous broadening of spectral lines. The energy characteristics of the laser are calculated in the approximation of a quasihomogeneous flame front model and this is done for various rates of the translational and rotational relation of the HF molecules. An estimate is obtained of the influence of the effects of a translational and rotational nonequilibrium on the energetics of a diffusion-type HF laser operating under real conditions.

An investigation was made into the characteristics of a combined-action (pulse-periodic and cw) high-power CO₂ discharge laser, and of its construction. In forming the discharge the gas was preionized by high voltage electrodeless pulses.

A high power fast-flow closed-cycle industrial laser is described. The distribution of the unsaturated gain K₀ in the active medium was measured using the P20 line of the 00⁰1–10⁰0 CO₂ transition provided by a frequency-stabilized tunable CO₂ laser. A study was made of the dependences of K₀ on the discharge chamber input energy and on the composition and pressure of the CO₂–N₂–He active mixture. On the basis of the data obtained, a method was developed for optimizing the resonator parameters and excitation conditions in fast-flow electric discharge CO₂ lasers. A laser output power of 7.5 kW was obtained for an input energy of 60 kW.

The approximate method of smooth perturbations is used to study the induced fluctuations of the intensity of probe radiation propagating in a bleachable aerodisperse medium. Parts of the interaction zone where the intensity fluctuations are small and the attenuation of the probe radiation is weak are identified.

A theoretical investigation is made of the process of wavefront reversal in a medium whose nonlinear properties are governed by heat evolution. The conditions ensuring a high nonlinear reflection coefficient are found allowing for heat conduction. An estimate is obtained of the efficiency of reversal in media with a thermal nonlinearity.

An improved method was developed for determining the probability W₂₁ of relaxation of the population of the ⁴I_11/2 level of neodymium ions. The relationship between W₂₁ degree of dumping, and parameters of the dumping pulses was obtained by solving the rate equations for the populations subject to an allowance for the Stark structure of the energy levels ⁴F_3/2 and ⁴I_11/2. At T=300°K the probability values were W₂₁=10⁸ sec⁻¹ for Y₃Al₅O₁₂:Nd³⁺ and W₂₁=1.5×10⁸ sec⁻¹ for La₂O₂S:Nd³⁺. In the case of a BaGd_2-xNd_x(MoO₄)₄ crystal at 77°K the probability was W₂₁=2.5×10⁸ sec⁻¹. The influence of Wn on the efficiency of utilization of the stored energy was found to depend on the energy gap between the lower Stark sublevel of the ⁴I_11/2 level and the final sublevel in the stimulated transitions.

The results are presented of an experimental investigation into the divergence of radiation from a quasi-cw CO₂ laser having an unstable resonator. It was found that the onset of inhomogeneity in the active medium, caused by instability of the self-sustained gas discharge, led to a substantial increase in the divergence of the laser radiation before its power fell. This effect must be taken into account when optimizing laser operating regimes. Laser radiation entering the resonator after reflection from an irradiated target can greatly influence the power, stability, and divergence of the radiation, leading to disruption of the normal intensity distribution in the focal plane. This effect is observed even when the target is rotated through large angles relative to the direction of the incident radiation.

Theoretical and experimental investigations were made of the selective properties of a linear resonator with an asymmetrically located dispersive component in the case when the dimensions of the end reflectors are limited by stops. When the equivalent Fresnel number is less than unity, the resonator band depends strongly on the dimensions of the stops, position of the dispersive component in the resonator, telescopic coefficient, and mirror curvature. A good agreement between the theory and experiment shows that the proposed calculation method can be used in the optimization of the parameters of small-aperture dispersive resonators frequently employed in dye lasers.

Explicit expressions are obtained for the probabilities of transitions in a three-level system subjected to two-frequency laser radiation. The radiation or transition frequencies vary smoothly with time, passing through a resonance. The results obtained generalize the Landau–Zener solution to the case of three crossing levels, and they can be used to describe a number of physical phenomena.

A theoretical and experimental study was made of the special features of the method of inhomogeneous mirrors used to form gas-laser beams with a constant transverse intensity distribution. Quantitative characteristics were obtained and these indicated a high efficiency of the method. The main factors influencing this efficiency and the range of application of the method were identified.

The characteristics of absorption saturation in resonant polarization-inhomogeneous media of passive laser switches (made of LiF crystals with color centers and of a solution of 1052 dye) were investigated theoretically and experimentally. The induced dichroism was determined: in the case of the dye switch it agreed with the theoretical predictions, whereas in the case of the lithium fluoride switch it was slightly less than the calculated value because of depolarization of the bleaching wave in the crystal as a result of residual birefringence. It was concluded that allowance for the induced dichroism was essential in studies and interpretation of the polarization characteristics of lasers with passive Q switching.

A new method was developed for determining the mechanism of anti-Stokes filling of luminescent levels of an activator in a rare-earth crystal. An investigation was made of the dependences of the absolute populations of the ⁴I_13/2, ⁴I_11/2, ⁴F_9/2, ⁴S_3/2, H_9/2, ⁴G_11/2, and ²P_3/2 levels of Er³⁺ in a BaYb₂F₈ crystal on the energy absorbed within the Yb³⁺ sensitizer band. The mechanisms of anti-Stokes filling of the ⁴I_13/2, ⁴F_9/2, ⁴S_3/2, and ²H_9/2 levels of Er³⁺ were determined under conditions of weak (1 mJ/cm³) and strong (∼10 J/cm³) pumping.

A theoretical analysis is made of the response of a system of small liquid particles to resonance exciting pulses, as manifested in the Raman scattering spectrum. It is shown that coherent response signals may be useful in spectroscopy of small liquid particles.

A theoretical study is made of the parametric interaction of optical waves in nonlinear layer media. The mismatch between the group velocities of the interacting waves may be varied in such media and the phase-matching condition can be satisfied by the quasimomentum of periodic modulation of the nonlinear coefficient. The tuning curves of the group-velocity matching are given for an LiNbO₃ crystal. Analytic solutions of nonlinear equations are obtained by a secondary simplification method and these solutions are compared with the results of numerical calculations.

Detailed experimental investigations of the kinetics of the vibrational and translational temperatures of the active medium of a TEA CO₂ laser are reported for the first time. A comparison of the theoretical and experimental results confirms the validity of the generally adopted temperature model for times ≳1 μsec/atm after the start of the discharge. A considerable difference between the experimental and theoretical results in the range <1 μsec/atm shows that in this range a dynamic equilibrium is still absent between the lower levels of the symmetric and bending modes. A reduction discovered in the rate of transfer of vibrational energy from N₂ to CO₂ for excitation in an electric discharge is explained by the excitation of the higher vibrational levels of N₂, a low VV relaxation rate in nitrogen, and anharmonicity of the N₂ molecule.

The amplification of ultrashort pulses in a N₂-laser-pumped amplifier was studied both theoretically and experimentally. A good agreement was found between the theoretical and experimental results over six orders of magnitude of input energies if the excited-state absorption of the dye was taken into account. The output energy density of the last amplifier stage was 16.0 mJ/cm², which ensured an energy extraction efficiency as high as 7.7%. The calculated pulse duration increase was 20%. The amplified spontaneous emission background was found to be 1%. The stabilization of the output pulse energy by saturation of the amplification is discussed.

A numerical investigation is made of the modulation of radiation in a gas whose resonance frequency is varied in time by the application of an external electric field. A good agreement between the calculated and experimental results is reported. An analysis is made also of total internal reflection from a plasma whose degree of ionization increases with time.

Attention is drawn to the possibility of generating Cherenkov optical parametric oscillations with a tunable frequency in a "double" Fabry–Perot interferometer placed inside a laser resonator. A theory of such oscillations is given. Expressions are obtained for steady-state radiation intensities and for the efficiency of conversion of laser radiation into a coupled parametric oscillation mode. The high (of the order of unity) efficiency and the low threshold of parametric oscillations (corresponding to a laser output power of the order of 10⁻⁴ W) should make it possible to use low-power cw lasers as pump sources.

An analysis is made of a light-induced drift observed under cascade excitation of atomic levels. A detailed description is given of the drift in the case of absorption of monochromatic and quasimonochromatic radiation as a result of a transition between the excited states. Preliminary excitation is provided either by broad-band radiation or by traditional nonlaser methods. It is shown that the drift is possible also when the transport cross sections of the particles which are at the active levels (involved in a transition under the action of resonance monochromatic or quasichromatic radiation) are identical: a difference between the transport cross sections for any pair of levels is sufficient for the occurrence of the drift.

A study was made of the multiphoton absorption spectra of a molecular SF₆ beam at various vibrational and rotational temperatures, and also of molecules of the same compound in a cell kept at room temperature but at a fairly low gas pressure (8.5 mTorr). An increase in the excitation energy density (in the range 0.1–3.0 J/cm²) did not increase the "red shift" of the multiphoton absorption spectra. The position of the maximum and the width of the spectrum were governed mainly by the vibrational temperature, i.e., by the initial distribution of molecules between the vibrational states.

An experimental investigation was made of the dependence of the microstructure and morphology of the surfaces of copper–tin alloy films on their phase composition, thickness, formation conditions, and heat treatment.

A theoretical study is made of the propagation of coupled plane monochromatic first (ω₁) and second (ω₂) harmonics in a nonlinear active medium combining the functions of generation of laser radiation of frequency ω₁ and of doubling of this frequency. The wave mismatch is not restricted. The cases of homogeneous and inhomogeneous broadening of a luminescence line are discussed. Equilibrium (asymptotic) solutions are found and their stability is investigated. One or two stable equilibrium solutions can be obtained, depending on the wave mismatch and on the pumping rate. Transient conditions and establishment of asymptotic solutions are studied. It is shown that wave mismatch can be used to optimize the generation or amplification of a second harmonic in a nonlinear active medium.