Table of contents

A review is given of the structure of phosphate glasses, thermooptic distortions in laser active elements, dependence of the thermooptic properties on the phosphate glass composition, and nonlinear optical characteristics of the glasses. An analysis is made of the criteria governing changes in the structure, spectral-luminescence, and lasing characteristics of the glasses in systems utilizing monovalent and bivalent metal metaphosphates with different additives. A comparison is made between the lasing parameters of phosphate and silicate glasses activated with Nd³⁺ions. Data are given on excitation energy transfer processes in phosphate glasses, and on the spectral-luminescence and lasing properties of phosphate glasses activated with Yb³⁺+Er³⁺ and Nd³⁺+Yb³⁺ ions. The potential usefulness of phosphate glasses in different laser systems is demonstrated.

A method is developed for measuring the vibrational temperatures of the asymmetric vibration mode T₃ and both symmetric modes T₂ of the CO₂ molecules under gasdynamic laser resonator conditions. The method involves simultaneous measurement of the gain at the wavelength 10.6μ and the spontaneous emission intensity in the 4.3μ band of carbon dioxide using the vibrational-rotational band theory in the calculations. The results of measurements of the vibrational temperatures T₃ and T₂ in a supersonic stream of 0.05CO₂ + O.O5N₂ + 0.2CO + 0.7He laser active mixture are presented and the errors of the method are analyzed.

An analysis is made of the feasibility of constructing a plasma laser utilizing a transition in the He₂* molecule to the ground repulsive state (81.5 nm) pumped by an electron beam. The results of numerical calculations based on a detailed self-consistent model of relaxation of a helium plasma are used. In the case of a favorable (so far unknown) ratio of the He₂* photodissociation and photoionization cross sections the self-excitation threshold may be attained under conditions close to the limit of present-day technical capabilities.

Theoretical and experimental investigations were made of the effect of partial locking of "domain"-type axial and transverse modes on the characteristics of second harmonic generation. An analysis was made in the constant field approximation using a second-order correlation function formalism and the reverse effect of the second harmonic was allowed for by a numerical technique using a digital computer. It was shown that partial locking of these modes increases the conversion efficiency by a factor of 8–14 compared with single-frequency conditions for nonlinear crystals having small anisotropy angles. It is noted that in nonlinear crystals having large anisotropy angles the efficiency of second harmonic generation by a fundamental-frequency beam with partially locked transverse modes is reduced substantially.

It is shown that a dye laser with forced mode locking and intracavity neon-filled discharge tube may be used for selective intracavity laser spectroscopy. Locking, stabilization of the position, and narrowing of the locked mode band were observed near the neon absorption line λ₀. It was found that one of the possible mechanisms for these effects may be diffraction by an optically induced refractive index grating in the material placed in the laser resonator. It was established that intracavity interaction between the laser radiation and this grating results in self-locking of the radiation to two wavelengths symmetric with respect to λ₀. As a result of competition in the active medium, only one of these frequencies may be observed in the emission. It was shown that the spectral position of the locked frequencies may be altered by altering the absorbing atom concentration and also by means of a longitudinal magnetic field applied to the intracavity discharge cell.

An analysis is made of a model problem of optical creation of a steady-state cleared channel in a cloud under free convection conditions. A dimensional approach is used as the basis of classification of possible types of thermal and concentration convection. Three specific types of free convection (weak, moderate, and advanced) are established. The ranges of the parameters of optical beams needed to ensure the formation of a cleared channel are determined.

An experimental investigation was made of the gain of a lasing medium based on CO₂ in a pulsed transverse discharge preionized by auxiliary wire electrodes as a function of the active mixture pressure and composition, and the energy supplied to the discharge. A model was developed for the kinetic processes in the active medium, based on kinetic equations which allow accurately (within the harmonic approximation) for energy exchange between the CO₂ modes. A comparison of the results of calculations, made using this model and the most reliable data on the vibrational relaxation constants, revealed satisfactory agreement between theory and experiment.

Boundary layer equations are used in calculations of a self-contained diffusion-type HF chemical laser. Some characteristics of the calculation technique are discussed. An investigation is made of the dependences of the laser energy characteristics on the system parameters. Attention is focused on the influence of factors which control the mixing process of the oxidizer and fuel streams. The results may be used for practical optimization of the energy characteristics of a cw HF chemical laser.

The saturation parameter of a waveguide CO₂ laser is investigated theoretically. Its radial distribution in a capillary tube is considered for different pressures of the active mixture. The dependences of the saturation parameter and of the maximum radiation power density on the active medium and laser tube characteristics are calculated.

A study was made of wavefront reversal, accompanying stimulated Brillouin scattering in various media, and of the way it was affected by the geometrical, temporal, and polarization characteristics of the exciting radiation. In an oscillator-amplifier system, practically complete compensation was achieved for phase distortions of the oscillator signal introduced by the amplifier components. The output energy of the light beam was then 3 J, its diameter was 4 mm, and its divergence (diffraction limited) was 0.3 mrad. The results of the experiments indicated a practical possibility of applying the wavefront reversal phenomenon in high-power multichannel lasers.

An electron-beam-controlled cw CO₂ laser was provided with gas-mixture regeneration, which enabled radiation at a wavelength of 10.6 μ to be obtained with a power of 10 kW and an overall laser efficiency of about 5%. Experimental investigations were made of the electrical characteristics of a nonselfsustaining discharge in a CO₂+N₂+He+Xe+CO gas mixture subjected to various heating regimes. It was established that the addition to the mixture of small quantities (about 1%) of the gases Xe and CO substantially (by 30–50%) increased the limiting power of the nonself-sustaining discharge and, therefore, the output power and efficiency of the laser. An analysis was made of the dependence of the laser output power on the pump power, pressure, composition, and flow rate of the gas mixture.

A proposal is made for a new type of free-electron laser amplifier based on recent experimental results which have confirmed the long-predicted nonlinear interaction between intense electromagnetic fields and charged particles. The proposed free-electron laser amplifier is very efficient and capable of generating multimegajoule output pulses at a rate required for economical energy production by laser fusion.

An experimental and theoretical study was made of the influence of the scanning velocity on the average output power and efficiency of a semiconductor laser excited by a fast-electron beam. A study was made of the dynamic changes in the spectrum and output power during an excitation pulse. It was found that the laser action mechanism changed during stimulated emission and this reduced the laser efficiency at low scanning velocities.

Results are presented of an experimental investigation of the radiative characteristics of single-channel semiconductor injection lasers operated under cw conditions (77°K) with different active region widths (1.5–11μ). It is shown that two-sided horizontal confinement of the active region, using Ge-doped layers of an Al_xGa_1–xAs solid solution, makes it possible to produce a structure exhibiting good waveguide properties and low leakage currents. A study was made of the threshold, spectral, and watt-ampere characteristics of the radiation and the field distribution in the near- and far-field zones. An investigation of a large number of single-channel lasers showed that these emit a single axial mode over a broad range of the injection current. The maximum radiation power under single-frequency conditions was greater than 100 mW (in both directions). It was established that single-frequency emission terminated and a transition to multimode emission took place at a radiation power density of ~(1–2)×10⁵ W/cm² in the channel.

Results are presented of an experimental investigation of the nonlinear perception of infrared radiation in the 800–1355 nm range with the unaided human eye. In this range, the infrared radiation is perceived as visible radiation at a wavelength close to but not coinciding with the second harmonic wavelength. The eye sensitivity curve for infrared radiation has an irregular profile with the envelope maximum near the 1113 nm wavelength. It is shown that the mechanism for nonlinear perception of infrared radiation is apparently localized in the retinal nerve layers. A comparison of these results with established data on the vision mechanism of vertebrates suggests the existence of a new phenomenon — two-photon photoisomerization of the visual pigment molecules in the retinal cones, as the basis of the nonlinear perception of infrared radiation.

Factors limiting the pulse repetition frequency in pulse-periodic CO₂ lasers are discussed and an analysis is made of their operating efficiency. Experiments are reported on model systems with different laser mixture flow rates. A description is given of a pulse-periodic CO₂ laser having an average radiation power of 1.2 kW designed to obtain weighable amounts of isotopically pure substances.

An experimental investigation was made of isotope separation by multiphoton dissociation of molecules on an industrial scale. The sulfur isotopes ³⁴S and ³²S were separated by dissociation of the ³²SF₆ molecules with CO₂ laser radiation. It was shown that pulsed lasers having a high (1 kW) average radiation power can be used for isotope separation. The resultant heating of the gas was negligible and did not influence the parameters of the separation process. A system was built for the enrichment with the sulfur isotope ³⁴S. The yield obtained was sufficient for this process to have practical applications.

The time, temperature, and impurity-concentration dependences of the formation and relaxation of phase holograms in iron-doped lithium niobate were determined. A volume phase holographic grating with a period of 0.69μ and a thickness of 14 mm was characterized by an angular selectivity of 22" and a spectral selectivity of 1.36 Å.

A study was made of the action of high power laser pulses on the surface of an optical waveguide. It was established that the threshold for surface damage is mainly dependent on the laser pulse parameters and on the condition of the surface. A yellow coloration was found to appear in the irradiated zone. A mechanism is proposed for the formation of colloidal silver particles, which are responsible for the coloration.

Single-frequency stimulated emission (λ = 635 nm, line width 0.3 nm) was obtained from a laser utilizing a dye-activated single-mode planar waveguide. Distributed feedback was provided by a rectangular grating having a pitch of 0.433μ along both coordinates located on the waveguide surface. This grating was used to extract the radiation along the normal to the waveguide plane. Using this structure, it was possible to achieve extremely low radiation divergence (~20'×20') since the total surface of the excited waveguide section emitted coherently (the area of the excited section was approximately 1 mm²). The use of different types of two-dimensional gratings in similar structures is discussed.

A theoretical analysis is made of existing experimental data on the stimulated emission spectrum of an optically pumped CF₄ laser. It is found that weak transitions with changes in the rotational angular momentum (ΔR=±1, ±2) play an important part in the excitation and emission of the CF₄ molecule. A model is proposed for the operation of a CF₄ laser based on the concept of coupled transitions and explaining the negligible role of rotational relaxation in the formation of the laser stimulated emission spectrum.

An investigation is made of saturation resonances in the absorption of the SF₆ molecule found on interaction between strong saturating and weak probe radiation waves from two CO₂ lasers, propagating in an absorbing cell either in the same direction or in opposite directions. For the SF₆ pressure range of 20–350 mTorr, the resonance widths increase linearly with pressure and, within the limits of the experimental accuracy, the resonance width for the opposite waves is twice the resonance width for the unidirectional waves but in both cases the resonance profile for weak saturation is Lorentzian. An interpretation of these results using a model of three relaxation constants for a two-level system with highly degenerate levels suggest that the upper and lower vibrational-rotational levels of SF₆ have the same collisional quenching probability and very few phase-shifting collisions occur.

An investigation was made of the parameters of a passively mode-locked cw dye laser utilizing a jet of a rhodamine 6G and DODCI solution in ethylene glycol. A continuous train of pulses of 0.4 psec duration was obtained. The tuning range under mode-locking conditions was 590–610 nm. The subpicosecond pulses were amplified by a factor of 100 using an amplifer comprising a rhodamine 6G solution in ethanol pumped by N₂ laser radiation.

A solid-state ring laser is described in which electrooptic negative feedback and a special law for the Q switch were used to obtain rectangular radiation pulses of miscrosecond duration. In order to compensate for the delay in the negative feedback signal, the transit times of the light pulse half way round the ring resonator and of the electrical pulse around the feedback loop were matched, so enabling the spike on the leading edge of the radiation pulse to be eliminated and the fluctuations of its top to be reduced. The use of a traveling-wave lasing regime made it possible to obtain stable rectangular microsecond pulses with a small number of transverse modes, and to reduce the radiation divergence to 3–4'.

An analysis is made of the combined effects of random inhomogeneities of the medium and thermal nonlinearity on the spatial coherence of an optical beam. It is shown that a thermal defocusing lens increases the spatial coherence radius of the field.

Theoretical and experimental investigations were made of the transient self-diffraction of neodymium laser radiation in an absorbing liquid. Calculations carried out without assuming a constant pump field showed good agreement with the experimental results. The high energy efficiency of the conversion process and the virtual absence of distortion effects in the liquid suggests that this process may be used to compensate for laser radiation wavefront deformations. The possibility of increasing the axial brightness was demonstrated in a model experiment by introducing appreciable phase distortions into a strong beam.

It is shown that the laser self-emission of a medium may have a substantial influence on the time behavior of the various plasma components of a nonself-sustained gas discharge exciting excimer lasers.

A theoretical investigation is made of the influence of the gas temperature on the kinetic cooling of a CO₂–N₂ mixture by CO₂ laser radiation. The dependences of the cooling parameters on the initial temperature and compositions of various mixtures subjected to laser radiation of various intensities have made it possible to determine the temperature range of the existence of the cooling effect.

An analysis is made of the influence of inhomogeneities of the gain and refractive index of an active medium of a double-pass amplifier with an ideal wavefront-reversing mirror on the accuracy of reproduction of spatial distributions of the amplitude and phase of the input signal. It is shown that if inhomogeneities of the refractive index are sufficiently small and an amplifier can be regarded as a thin lens (phase corrector), the wavefront of the output radiation is reversed relative to the wavefront of the input signal. If these conditions are not satisfied, reversal does not take place.

An analytical investigation is made of the profile of an emission line of ions in a low-temperature plasma which are resonantly excited with a high-intensity traveling light wave. It is shown that the line shift caused by the acceleration of the ions in the electric discharge field decreases on increase in the saturation parameter.

It is demonstrated experimentally that an individual spike in a regular series of intensity pulsations from a glass-fiber laser having a core doped with neodymium ions, consists of a train of short pulses having a duration of 2–8 nsec and a period of ∼15 nsec. The spectrum of stimulated emission from a fiber laser with a total half-width Δλ ≲ 1.8–5.6 nm, governed by the pump energy, takes the form of discrete narrow lines with the half-width Δλ ≤ 0.075–0.3 Å. This character of the stimulated emission is attributed to the self-mode-locking in a fiber laser due to the effect, at the emission wavelength of λ=1.06μ, of short-lived color centers which are produced in the glass by the ultraviolet part of the pump radiation spectrum.

An experimental investigation was made of the control of a dye laser emission spectrum by direct application of an external optical signal to a selection element in the resonator. Switching occurred as a result of nonlinear response of this element to the control radiation.

The mechanism for the onset of small-scale optical inhomogeneities in the active zone of iodine photodissociation lasers is identified. It is found that in iodine lasers, the radiation undergoes a nonlinear defocusing self-interaction as a result of the closed chain radiation-chemical reaction-gasdynamic perturbation-radiation. As a result of the slow response of the medium, the parametric buildup of small-scale optical inhomogeneities is possible. It is noted that under such conditions, a similar effect may be observed in other types of lasers. It is shown that under inversion storage conditions and short pulse amplification, small-scale optical inhomogeneities should not be observed.