Table of contents

An experimental investigation was made of the performance of IFP-8000 flashlamps in compact illumination enclosures. It was established that the electrical conductivity of the gas-discharge plasma increased by ~30% and the discharge temperature by ~ 15%, compared with a flashlamp operated in the open, but the maximum safe output energy decreased by a factor of ~ 1.5.

An analysis is made of modes of arbitrary order in resonators containing elements with square-law dependences of the refractive index on the transverse coordinates. It is shown that a calculation of the field in such resonators reduces to the application of two transformations. One of them represents the wellknown ABCD law and describes the transformation of the beam variance. The other describes the transformation of the complex argument of Hermite polynomials which occur in the expression for the resonator field. This transformation may be omitted if the medium is loss-free but its use is essential in the case of media characterized by inhomogeneous gain. The proposed calculation method makes it possible to determine uniformly all the mode characteristics for a wide range of resonators, including unstable ones.

An analysis is made of reference-beam-free holographic systems from the point of view of the required mechanical stability. It is shown that the stability requirements are easier to satisfy for these systems than for conventional holographic systems.

A new method was used to investigate the laser damage on the basis of nonlinear scattering of light by the zone interaction between laser radiation and an optical material. This method revealed irreversible changes in the bulk of sapphire crystals and several glasses exposed to laser radiation below the threshold of visible damage. It was found that such irreversible changes were accumulated in a series of laser pulses which eventually caused visible damage. This effect was interpreted and its possible use as a test of the influence of defects on laser damage was suggested.

Measurements were made of the intensity of acoustic waves generated in water by YAG laser radiation with monochromatic intensity modulation. The results obtained were in good agreement with theoretical calculations.

The semiclassical approximation is used in an analysis of the nonlinear shift of the frequency of an electric dipole transition in a high-power radiation field. It is shown that the shift can only increase the frequency. The magnitude of the shift is proportional to the field intensity but independent of its frequency. Secondapproximation relationships are derived for the calculation of the parameters of the interaction of the field with an active medium. Numerical estimates of the Stark effect are obtained for some gaseous active media.

An analysis is made of the influence of the recoil effect on the position of the emission frequency of a gas laser stabilized by the frequency-modulation method using an arbitrary modulation frequency.

A description is given of a light modulator in which radiation travels along a waveguide and is modulated by variation of the refractive-index discontinuity at the boundary between the waveguide and an electrooptic material. A derivation is given of the dependence of the depth of modulation of the radiation on the change in the refractive index and the number of reflections at the waveguide boundary. It is shown that this method makes it possible to increase significantly the modulation frequency.

Increments are calculated for growing perturbations traveling at small angles with respect to the direction of propagation of a high-intensity plane wave in a periodic system composed of layers of a cubic nonlinear medium and layers of free space. It is shown that these perturbations, traveling at certain angles, may be suppressed by a suitable selection of the geometric dimensions of the system.

An experimental analysis is made of the possibility of using a thin-film interferometer as a calorimeter and a refractometer.

A theoretical analysis is made of the parametric excitation of the higher Stokes components of stimulated Raman scattering by amplification of the first component in a dispersion-free medium. It is shown that the excited Stokes components are locked and this can be used in the generation of ultrashort light pulses.

A theoretical analysis is made of the spontaneous Raman scattering of a test wave by coherent surface polaritons, which are created by the difference frequency in biharmonic pumping. Formulas are obtained for the intensity of the scattering of the test wave. Numerical estimates show that this type of scattering produces much higher intensities than the scattering by noise surface polaritons.

Commercial silicon surface-barrier diodes of the DKPs type, with a sensitive surface area of 0.5–2.5 cm in diameter, are suggested as simple and convenient laser radiation detectors. This type of detector can operate in the wavelength range 0.27–1.2 μ with a time constant not exceeding 10 nsec. The absolute responsivity, measured for a batch of diodes, is 0.1–0.3 and 0.005–0.01 A/W at wavelengths of 0.63 and 1.15 μ, respectively. The responsivity of different points on the surface varies by no more than 5%.

A report is given of the properties and parameters of waveguides formed by irradiation of fused quartz with H₂⁺, He⁺, Ar⁺, and Xe⁺ ions of energies up to 3 MeV and doses up to 5×10¹⁶ cm^–2. The lowest losses (~3 dB/cm) are reported for waveguides prepared by irradiation with Xe⁺ ions; subsequent annealing (up to 500°C) can reduce these losses to ≲ 0.3 dB/cm. The implantation of H₂⁺ is not recommended because waveguides prepared in this way exhibit spontaneous annealing.

Stimulated emission was obtained from a CS₂-O mixture utilizing dissociation of oxygen behind a reflected shock wave. An oxygen-argon mixture heated in a shock tube expanded in a supersonic nozzle and was then mixed with CS₂ in an ejector. An output power of 10 W was obtained in the form of 1.8 msec pulses when the active region in the resonator was 7 cm long.

A double sided Al_xGa_1–xSb_1–yAs_y–GaSb–Al_xGa_1–xSb_1–yAs_y heterostructure was used in an injection laser which emitted stimulated radiation of 1.5–1.8 μ wavelengths at room or lower temperatures. The threshold current density was 220 A/cm² at 77°K and 6.2 kA/cm² at 300°K.

Part I of the review (Secs. 1–5) classifies the optical (photochemical and photophysical) isotope separation methods, gives the history of their development, and demonstrates the advantages of laser radiation. The problem of isotopically selective excitation and isotopic shifts in the spectra of atoms and molecules, and also limitations of the selectivity and methods for increasing it are considered in Sec. 2. The method of selective multistep photoionization of atoms, general relationships applying to this method, principal experiments (on Rb, U, etc.), methods for increasing the photoionization cross section of excited atoms, and limitations on ionization selectivity are discussed in Sec. 3. The method of selective two-step photodissociation of molecules and limitations (bottleneck effect due to the rotational structure, steepness of the photoabsorption edge, and nonselective thermal excitation), and the first experiments (on NH₃, BCl₃) are analyzed in Sec. 4. The method of selective photopredissociation of molecules, its limitations and advantages, first experiments (on H₂CO and I₂), and its potential applications in isotope separation are discussed in Sec. 5 (Secs. 1, 3, and 4 are by V. S. Letokhov and Secs. 2 and 5 are by C. B. Moore).

An investigation was made of the photoconductivity induced in metallo-organic semiconductors (copper organoacetylenides) by neodymium laser radiation and the damage thresholds of these compounds were determined. These thresholds increased monotonically with increasing forbidden band width and an induced photoconductivity from local levels in the forbidden band of copper phenylacetylenide appeared at energies which were three orders of magnitude lower than the damage threshold.

The nonlinear interaction force between intense laser fields and cold plasma shells efficiently transforms radiant energy into mechanical energy of implosion. This transfer of energy has been considered before in numerical experiments and it is treated here analytically in a didactic example starting with an inhomogeneous Rayleigh density profile. Up to 50% of the laser energy can be transformed into the energy of compression if a single "untailored" pulse of 2.5×1O¹⁶ W/cm² intensity and of only a few picosecond duration is used for spherical illumination of a shell. If the pulse is short enough to reduce collisiona) thermalization, then the collapse and compression of the plasma can remain at the threshold of Fermi degeneracy and still be adiabatic. This results in nuclear reaction gains G, based on the deposited energy, E₀, and without a-particle reheating, of G = 400 for E₀ = 2.25 kJ (D–T reaction), 900 kJ (D–D), 13 MJ (H–B). About 1000 times less laser energy is necessary than in the case of gas dynamic ablation resulting in the same nuclear reaction yields.

Estimates are obtained of the various components of the pulse response of an optical fiber. These components are associated with the angular distribution and nonzero width of the radiation source, and also with inhomogeneities at the interface between layers in the fiber. Some numerical expamples are given.

An investigation was made of the action of ultrashort laser pulses on linear KDP crystals. The damage was found to be discrete and independent of the orientation of the crystals or the focal length of the lenses employed. The threshold damage energy increased with the focal length of the lenses. This energy decreased severalfold when λ₁ = 1.06 μ. radiation was replaced with λ₂ = 0.53 μ pulses. Two-frame pulse holography was used to demonstrate that the damage spread in both directions from the focal plane. The velocity of the damage front in the direction toward the laser was found to be 1.6 × 10¹⁰ cm/sec.

Stimulated emission was obtained from tellurium-doped Cd₃P₂ single crystals in the temperature range 10–150°K. The properties of the stimulated radiation were investigated. This radiation grew out of a spontaneous cathodoluminescence band at λ_max = 2.12 μ at 90°K and the threshold electron current density was ≲ 50.5 A/cm². Cathodoluminescence of Zn₃P₂ was investigated for the first time and the results were used to find more accurately the forbidden band width of this material. The cathodoluminescence of cadmium and zinc phosphides was located in the region of the fundamental absorption edge and the position of the cathodoluminescence maximum was practically independent of temperature.

The possibility of using metastable electronic states of molecules in a gas laser emitting in the visible range is analyzed. Consideration is given to several types of the transfer of excitation from metastable molecules to electronic levels of atoms of an impurity gas with the aim of achieving stimulated emission. Potential mechanisms for the pumping of metastable molecular levels are discussed.

An expression is obtained for the nonlinear susceptibility governing the addition of frequencies under resonance conditions. Allowance is made for the Stark shift, changes in populations, and transient response of the medium. Analysis is made of the stimulated emission of the third harmonic as a result of two-photon resonance in parallel and focused beams. It is shown that the saturation effect limits seriously the conversion coefficient of a nonlinear medium of fixed length. Optimal intensities of the fundamental-frequency radiation are found. It is shown that when a beam is focused sharply in a nonlinear medium, the saturation effect and Stark shift make it possible to add frequencies in media with normal dispersion. The relationship between four-photon parametric effects and coherent resonant processes such as self-induced transparency is established for ultrashort pulses.

A description is given of apparatus capable of providing time scans on selected parts of the luminescence spectra of semiconductors with a time resolution of ~3 × 10^–11sec. The results are given of an investigation of the kinetics of luminescence spectra of CdSe and Cd_xSe_1–x, layered single crystals generated by one-photon excitation with ultrashort pulses of the second harmonic of neodymium laser radiation. Measurements are reported of the lifetime of the first phonon replica of excitons in CdSe (190 ± 30 psec), CdS_0.4Se_0.6(140 ± 30psec), and CdS_0.7Se_0.3(230 ± 100 psec) crystals.

An investigation was made of the dynamics of a high-current discharge in indium vapor. The average values of the discharge temperature and plasma density were determined. The spectral brightness of the discharge was measured with photocells in the visible range and with an open ionization chamber in the vacuum ultraviolet range. The experimental results were compared with the theoretically calculated approximate spectral distribution. The investigation demonstrated that a high-current discharge in indium vapor was an efficient source of vacuum ultraviolet radiation.

It is demonstrated for the first time that the formation of spatially coherent laser beams is influenced strongly by interference effects in the laser resonator and by amplification. Lasers with lumped and distributed feedback are considered. A relationship is established between the natural angular broadening Δθ of beams and the natural laser line width. The theoretical estimates of Δθ are in agreement with the experimental data for He-Ne lasers.

A theoretical analysis is made of the superradiance and photon echo when the exciting radiation frequency does not coincide with the central frequency of an inhomogeneously broadened line. It is found that the distortion of the profile of a nonresonantly excited superradiance signal depends strongly on the spectral width of the exciting pulses.

An investigation was made of the possibility of using ozone as an oxidant in a chemical CO laser based on oxidation of carbon disulfide. It was found that the efficiency of SF₆ as a diluent was considerably higher than that of He or Ar. A specific energy of 4.5 mJ/cm³ was obtained: this was twice as high as that obtained for a CS₂+O₂ mixture under optimal conditions.

A description is given of a method for measuring the relative abundances of isotopes in molecules with the aid of laser spectrophones. The basis of the method is the use of laser radiation of two frequencies and a periodic comparison of the signals produced by two spectrophones. The apparatus employed is described. The results are given of an experimental determination of changes in the ratio of the abundances of the B¹⁰ and B¹¹ isotopes. The question of the maximum sensitivity of the proposed method is considered. In principle, the use of lasers with output powers of about 1 W should make it possible to measure deviations down to 10^–4% from the standard isotopic composition of a molecule.

An experimental investigation was made of the amplitude and frequency characteristics of ring gas lasers. The first determinations were made of the dependences, on the resonator tuning Δ, of the additional frequency shift of beats (averaged over the total phase ψ of the coupling coefficients) and of the phase shift ϕ₀ between the variations (due to the change in ψ) of the beat frequency and the wave intensities. A comprehensive investigation was made of the dependences of the beat frequency Ω_b and of the intensities of the opposite waves on the ring resonator tuning and on the total phase ψ of the coupling coefficients. These dependences made it possible to compare several modifications of the theory of ring gas lasers. Far from the frequency locking region the dependence Ω_b(Δ) was found to be mainly due to the difference between the resonator losses experienced by the two opposite waves. When the pump intensity was increased, the deviation of the beat frequency Ω_b(Δ) increased. The experimental results were compared with the theory of ring gas lasers.

A low-toxicity PBr₃-AlBr₃-SbBr₃ neodymium-activated liquid medium with an absolute quantum efficiency of 83% and an excited-state lifetime of 230 μsec was developed. The energy characteristics were determined of lasers using this medium in sealed quartz cells with an internal diameter of 18 mm and 100 mm long. The thermo-optic constant of the medium was measured and interferograms of thermal stresses in the cell after a pump pulse were obtained.

An investigation was made of the population inversion lifetime, intensity rise time, spectral composition, divergence, and shape of light pulses emitted by an iodine laser with active Q switching. The laser beam diameter was 8 mm, output energy was 0.2 J, and the duration of single spikes in a pulse was ~ 5 nsec.

A. theoretical analysis is made of the steady-state single-frequency Intraresonator generation of second optical harmonic under wave mismatch conditions. It is assumed that at the fundamental frequency both resonator mirrors are totally reflecting but the second harmonic escapes without hindrance from the resonator. The process of wave amplification in the active medium is described by a system of rate equations and the interaction of waves in the nonlinear medium by a system of reduced equations for the real and imaginary parts of the complex amplitudes. It is assumed that the transverse structure of the waves is homogeneous. An analytic solution of the problem is obtained near the threshold. Numerical solutions are obtained for the case of strong pumping in a wide range of parameters of the problem. It is shown that, in particular, the optimal harmonic generation conditions can be achieved even in the presence of a wave mismatch provided it does not exceed a certain critical value; it is also demonstrated that there is a multitude of optimal regimes. The nonmonotonic nature of the dependence of the optimal length of the nonlinear crystal on the pump rate is established. The results of an experimental investigation of the harmonic generation are in agreement with the theoretical predictions.

It is shown theoretically that, under certain conditions, a considerable supersaturation may occur in the vicinity of a large water drop being evaporated by heating with CO₂ laser radiation. This supersaturation may result in the condensation of the. vapor and formation of a large number of fine particles. The possibility of heterogeneous and, under conditions of substantial overheating ( > 70°C), homogeneous condensation is considered. The growth of drops in a laser radiation field under supersaturation conditions is discussed. It is shown that the total attenuation cross section of the new drops may exceed, in the visible range, the attenuation cross section of the original drop and this may result in optical "dimming" of the interaction zone. Experimental results support the formation of a train of fine particles alongside evaporating large cloud particles at subzero temperatures. "Dimming" as a result of the interaction of CO₂ laser radiation with a crystalline cloud medium is reported and the dependences of this dimming on the radiation intensity, size of cloud particles, attenuation coefficient, and temperature of the medium are accounted for. The experimental results are in qualitative agreement with the theory.

An experimental investigation was made of the dependence of the frequency shift of a stabilized He-Ne-CH₄ laser on the degree of saturation of an absorbing medium and on its gas pressure. When the saturation parameter I_8,7[0] was increased from 0 to 1, the emission line, whose homogeneous width was 75 kHz, shifted by 400 ± 40 Hz. The dependence of the shift on the saturation parameter was nonlinear. A theoretical calculation was made of the shift of the frequency at the tip of the dip which appeared in the absorption line due to the magnetic hyperfine structure of the molecular transition.

An analysis is made of the scattering of photons by random optical inhomogeneities of a medium. The probabilities of photon transitions to guided and undamped (in vacuum) steady-state modes are determined. Characteristics of undamped modes associated with semitransparent walls result in almost periodic peaked frequency and angular dependences of the intensity of the scattered light in vacuum. The results are used to explain the properties of the radiation emitted from a thin-film ZnS:Mn laser.