Table of contents

The degree of coherence of the laser radiation transmitted by a single optical fiber was determined by the Young interference method. It was found that the modulus of the complex degree of coherence decreased with increasing fiber length and also with decreasing diameter (in the case of large diameters). However, for fibers up to 10 cm long this modulus did not fall below ~0.7. The results obtained were explained by a difference between the paths of different rays of the laser beam which appeared during the passage through the fiber; the imperfections in the fiber also played an important role. The main factors responsible for the path difference were considered and their relative importance was estimated.

A description is given of the apparatus for measuring the thermal expansion coefficients of materials using a Fabry–Perot interferometer. The expansion coefficient is deduced from the number of resonance transmission peaks of a Fabry-Perot interferometer (these peaks are the result of coincidence with the frequency of a stabilized laser when the temperature of the sample is varied). A simple method is used in the determination of the sign of the expansion coefficient. Hermetic sealing of an inner cavity drilled through a sample dispenses with the need for evacuation of the Fabry–Perot interferometer and makes it possible to use various heat carriers in the thermal stabilization of the sample. The expansion coefficient over a range of 10°C can be determined to within ±1×10^–7 deg^–1.

A numerical solution is obtained for the problem of the steady-state oscillation in a transversely pumped Raman laser with a telescopic resonator. It is shown that the generation of the first Stokes component can be stabilized throughout the resonator. The transverse distributions of the densities of the pump and Stokes radiation fluxes are given and the quantum efficiency of the laser is calculated for different parameters. A comparison is made of the generation under single- and double-sided pumping conditions. Some analytic estimates are given of the flux densities and of the quantum efficiency. The influence of the inhomogeneity of the reflection coefficients of the mirrors on the laser action characteristics is discussed.

An investigation is made of the kinetics of a single-mode solid-state laser with an additional moving mirror characterized by a low reflection coefficient. Conditions are derived for the "soft" excitation of the radiation pulsations because of the finite delay of the light traveling between the main resonator and the additional mirror.

An analysis is made of the influence of a change in the diameter of the reconstructing beam and of its displacement relative to the center of a hologram on the signal/noise ratio of a holographic memory. The optimal ratio of the diameters of the hologram and reconstructing beam, corresponding to the maximum signal reaching the photodetector, is estimated.

An analysis is made of the efficiency of the excitation of molecular laser states in the vacuum ultraviolet by a beam of fast electrons. The efficiency is calculated for the whole laser cavity during one pulse. It is assumed that the electron energy is utilized optimally in a system in which the electrons are completely decelerated. Characteristics of specific collision processes are assumed and allowance is made for the secondary electrons of different generations. The total excitation efficiency of the laser states by the primary and secondary electrons is about 30%, and it approaches the limit for the quantum-mechanical system of a hydrogen laser (~50%).

It is shown theoretically that an increase in the length of the resonator of a laser with a bleachable filter enhances the nonlinearity of the amplifying medium in the course of bleaching of the filter. This results in more efficient growth of ultrashort pulses from the original noise. It is demonstrated that there is an optimal resonator length which corresponds to the maximum contrast of ultrashort pulses.

The thresholds of the appearance of opacity and damage in the bulk and on the surface of some materials used in the infrared range were determined. A laser cleaning effect was observed for materials whose surface optical strength exceeded the breakdown threshold of air near the surface. This made it possible to raise the opacity threshold to the bulk damage threshold. The opacity threshold of such a cleaned surface fell as a result of adsorption of contaminants from air.

Measurements were made of the temperature of laser plasmas formed by irradiation (with a flux of ~5×10¹⁴ W/cm² density) of targets containing deuterium and heavy-element impurities and of homogeneous targets with the nuclear charge in the range Z = 12–23. The temperature was deduced from the relative intensities of dielectronic satellites and resonance lines of heliumlike/and hydrogenlike ions. The dimensions of various regions of a laser plasma jet, characterized by temperatures in the range 200–800 eV, were estimated. The absolute intensities of the spectral lines in the 2–10 Å range indicated that the plasma radiation power ranged from 10⁴ to 10⁸ W.

An investigation was made of the main characteristics of a liquid-nitrogen Raman laser pumped longitudinally by a neodymium laser. Pumping lasting 200–600 μsec and it consisted of a series of ~30 pulses of 0.1–0.3 μsec duration each. This made it possible to increase considerably the pump energy compared with the case when only one pulse was used. A study was made of narrow-band excitation (in this case the pump spectrum Δν_p≈0.05 cm^–1 was narrower than the spontaneous Raman scattering line of liquid nitrogen Δν_l= 0.067 cm^–1) and of wide-band excitation (Δν_p = 80 cm^–1Δν_l). The Raman laser emitted a series of 30 pulses with a total energy up to 40 J. The average efficiency was ~15%. The output power exceeded 15 MW. The output beam diameter was ~1 cm and its divergence was (0.5–1.5)×10^–3 rad. A great proportion of the output energy was contained in the first Stokes component (λ_s = 1.41 μ).

A theoretical analysis is made of the convective evaporation of a water drop in the field of strong laser radiation of λ = 10.6 μ. wavelength. A system of integrodifferential equations describing the evaporation process is obtained in the quasisteady approximation. The time dependences of the radius, surface temperature, and temperature at the center of a drop are obtained for the whole of the evaporation process. The upper and lower limits of the convective evaporation regime are derived for drops of different initial radius R₀. It is shown that if R₀ = 10 μ, the temperature of explosive boiling at the center of the drop is reached when the radiation power density is ~1.3×10⁴ W/cm².

The rate equations are used to consider the response of the emission intensity of a neodymium garnet laser when the resonator losses are modulated. Models of a laser emitting one and two longitudinal modes are considered. The results are given of an experimental investigation of the relationship between the modulation of the resonator losses (because of natural instabilities) and pulsations of the emission intensity of a cw neodymium garnet laser. It is shown that the main physical mechanism of the spiking is the quenching of weaker modes as a result of modulation of the resonator losses.

The kinetics of multimode free oscillation is analyzed on the basis of rate equations allowing for an inhomogeneous distribution of the population inversion. The model of two overlapping modes is considered. It is shown analytically and confirmed by computer calculations that transient pulsations (spikes) of the radiation emitted by such a system should be damped out and the steady-state emission should be stable. In the case of external modulation of the losses, undamped oscillations should appear only if the modulation coefficient is sufficiently large and the modulation frequency is close to the pulsation frequency. It is shown that undamped pulsations appear when the mode composition is altered by slow changes in the losses or stimulated emission cross sections. Such conditions are encountered in pulsed lasers when the hearing causes the mode frequencies and center of the gain profile to shift with time at rates which differ from one mode to another. Criteria for the appearance of damped or undamped pulsations are established. The theoretical results are supported by experimentally obtained slit scans of the free-oscillation emission of ruby at 100°K which exhibits undamped regular pulsations.

A calculation is made of the kinetics of the vibrational relaxation in CO₂+H₂O+N₂ and CO₂+He+N₂ mixtures during their flow from a slit into vacuum. Allowance is made for the most important energy exchange mechanisms vibration-vibrational and vibration-translational processes. The dynamics of a nonequilibrium gas is considered in the approximation of an adiabatic motion of a medium with an effective adiabatic exponent (ratio of the specific heats) corresponding to a particular degree of "freezing" of the vibrational component of the specific heat of the gas. A comparison is made of the calculated values of the gain with the experimental results.

An investigation was made of pulsed stimulated emission due to vibrational transitions in the ground electronic state of CO excited by discharges in pure CO and in mixtures of CO with helium, argon, and xenon. It was found that the output power, shape and duration of the output pulses, and spectral composition of the stimulated radiation were different when discharges took place in pure CO from the corresponding parameters obtained for mixtures. Moreover, the discharge conditions had a strong influence on all these parameters. The experimental results indicated that different mechanisms of the population inversion of the active levels took place in the investigated mixtures. Allowance for the excitation and deexcitation of the vibrational levels by electrons, collisional exchange of the vibrational quanta (ν-ν exchange), and excitation via higher electronic states of the molecules explained the experimental observations.

An analysis is made of the spatial structure of the fluctuations of the reflected optical field in a turbulent medium. A more accurate functional relationship is obtained for the determination of the altitude profiles C_n²(h) from the optical amplitude measurements. The mathematical analysis of the experimental results is discussed. Examples of the determination of C_n²(h) are given.

An investigation is made of the behavior of a single-mode laser in the case when the steady-state emission is unstable. This instability makes the laser emit π pulses, which are transformed into 2π pulses when the pump power is increased. The amplitude and duration of the output pulses are calculated. The influence of the spontaneous radiation on the establishment of a statistical limit cycle is considered.

The results are reported of an experimental investigation of high-current (~100 kA) discharges in nitrogen and argon at pressures up to 11 atm over distances of ~1 m. Such discharges are used as laser optical-pumping-sources. It is shown that the self-similar theory of discharges predicts correctly the changes in the discharge parameters with increasing pressure: The brightness temperature is practically unaffected, whereas the plasma expansion velocity decreases.

The shadow photography method was used in a study of the dynamics of, the development of filamentary damage regions which appeared in a KDP crystal under the action of a single ultrashort laser pulse. These damage filaments appeared in the focal region, and they grew in the direction of propagation of the pulse and in the opposite direction. In the former case the growth of the damage region ended earlier. A comparison was made of the damage threshold of various transparent materials (K-8 glass, fused quartz, and KDP crystals) at two wavelengths (1.06 and 0.53 μ).

An experimental investigation was made of a multielement semiconductor laser pumped longitudinally by an electron beam. When the inequality d < (2Lλ)^1/2 (d is the distance between the laser elements, L is the distance to the external mirror, and λ is the stimulated emission wavelength) was satisfied, mode-locked operation was possible and this reduced the angular divergence of the ouput radiation. Amplified spontaneous radiation, reflected from an external mirror, had a considerable influence on the laser operation: It gave rise to a complex field pattern in the far zone and reduced the coherent output power until the laser action stopped altogether. The influence of the amplified spontaneous radiation increased with increasing total size of the multielement system.

An investigation was made of the nonlinear interaction between spectral modes in an injection laser with external resonator which had two resonant feedback channels. These channels were provided by diffraction gratings used as tunable selective elements. The emission occurred at two wavelengths and, in addition to the usual competition due to the interaction of the fields via the population inversion, there was an interaction which depended on the magnitude and sign of the spectral separation between the modes. When this separation was 1 nm or less the long-wavelength mode was emitted preferentially. The anomalous competition was attributed to the parametric interaction between the modes.

The characteristics of a point-contact W–Ni diode were investigated under conditions of direct and heterodyne detection of infrared radiation (λ = 10.6 μ) and direct detection of visible radiation (λ = 0.63 μ). The detectivity of the Schottky barrier in this diode was found to have an upper limit at infrared frequencies. Methods for increasing the detectivity of metal-oxide-metal diodes (additional oxidation and ion implantation) were investigated.

An investigation was made of the spatial resolution achieved in optical recording of information using a metal-nitride-oxide-semiconductor (MNOS) structure with a continuous electrode. A resolution of at least 10 μ in photoelectric reading was achieved only when an additional dielectric-film grid was deposited below the continuous electrode. Six bits of information were recorded experimentally as separate points.

Double-sided heterostructures were prepared from four-component solid solutions Ga_xIn_1–xAs_yP_1–y(λ=1.02 μ) and Al_xGa_1–xSb_yAS_1–y(λ=0.945 μ). In the former system the problem of compatability of the lattices was solved by adding Ga and As to InP in amounts which ensured that the lattice period was the same (because of the opposite influence of these impurities). Indium phosphide was used as the wide-gap layer. The resultant laser heterostructure was the first system not based on the mutual replacement of Al and Ga, in contrast to earlier heterolasers and to the second investigated system (Al_xGa_1–xSb_yAS_1–y).

An experimental investigation was made of the influence of back reflections (scattering), due to the presence of inhomogeneities in the optical elements of the resonator, on the difference frequency of a helium-neon ring laser (λ=0.63 μ). The parameters describing back reflections in the resonator were deduced from the modulation (at the difference frequency) of the intensities of the opposite waves in the laser. The changes in these parameters, which occurred when the tuning of the resonator was altered, were compared with the corresponding changes in the difference frequency. The experimental results were found to be in satisfactory agreement with the theoretical predictions of the influence of back reflections on the difference frequency in a ring laser.

An experimental study was made of the reaction of a detector, utilizing the photon drag effect, to high-power (500 kW) CO₂ laser pulses. The responsivity (1×10^–7 V/W) was determined and the power at which a spark appeared on the surface of germanium (7×10⁶ W/cm²) was measured. This spark produced an additional voltage pulse which was superimposed on the useful signal.

An experimental investigation was made of the resonant photoactivation of chemical processes in complex condensed polymer systems irradiated with a definite wavelength. Weak monochromatic illumination of a previously heated film of ML-12 enamel raised the degree of polymerization to practically 100%, instead of the usual 85–90%. Moreover, such illumination increased considerably the rate of drying so that the drying temperature could be lowered. The results obtained suggested that laser illumination could be of practical use in the technology of drying of microelectronic devices.

An investigation was made of the influence of laser radiation on the degree of completion of the imidization of a polyamide acid, prepared from dianhydride of pyromellitic acid and 4,4''-diaminodiphenyl ester in dimethylformamide. The influence of the laser radiation was attributed to a "resonant situation" in the vibrations of the end groups.

The Sixth All-Union School on the Physical Basis of Holography was held in Erevan on February 11–17, 1974. A brief report is given of the lectures presented and some comments are made about the history and purpose of this school.

Reference 15 on p. 520 in the current volume (4) should read: ¹⁵Laser Focus, 8, No. 7, 34 (July 1972) Reference 4 on p. 528 in the current volume (4) should read:⁴V. P.Veiko and M. N. Libenson, Fiz. Khim. Obrab. Mater., No. 4, 44 (1968).