Table of contents

A study was made of a tunable laser with a new active medium in the form of a chrysoberyl crystal activated with trivalent titanium ions (BeAl₂O₄:Ti³⁺). Lasing due to the ²E–²T₂ electronic-vibrational transition in Ti³⁺ was observed in the wavelength range 0.7–0.9μ when the active medium was pumped by the second harmonic of a YAG:Nd³⁺ laser (0.5μ).

Generation of the sum and difference frequencies ω_vis + 2ω_ir as a result of coherent four-wave frequency mixing of quasiresonance (relative to vibrationai transitions) CO₂ laser radiation and of visible radiation in the form of the second harmonic of a YAG:Nd laser was studied in media consisting of various molecules (SF₆, BCl₃, CF₃Br, CF₃I). The signal was very sensitive to tuning of the infrared radiation frequency relative to the Q branch of a two-photon vibrational transition. A theoretical model was developed which ensured a good agreement with the experimental dependence of the intensity of the signal on the infrared radiation frequency in the case of gaseous CF₃Br. It was predicted theoretically that the investigated process should cause a considerable narrowing of the Q branch of a two-photon transition compared with the usual thermal width.

A description is given of the construction of a two-pass CO₂ amplifier system with a mirror utilizing a degenerate four-wave interaction in SF₆ gas and having an output energy of about 30J with an angular divergence of 4.4×10⁻⁴ rad at 50% of the maximum energy. A partial compensation of simulated inhomogeneities introduced into the signal beam was observed.

A study is made of the influence of phase inhomogeneities of mirrors in a Fabry–Perot interferometer on its pass band, average intensity (transfer function) and variance of the intensity fluctuations. A study is made of phase inhomogeneities with a Gaussian statistics and also in the form of a random stop process. In the latter case the role of phase fluctuations increases on expansion of the beam. The results obtained are of interest when a Fabry–Perot interferometer is used as a resonator. An expression is obtained for the Q factor of the resonator and this expression allows for phase inhomogeneities of the mirrors.

An experimental investigation was made of the divergence of radiation from a copper vapor laser with an unstable resonator. It was found that the beam from this laser could be regarded as consisting of two components with divergences greater and smaller than a certain optimal value. The relative contributions to these components varied with the pulse repetition frequency. It was found that when a dye laser was pumped with radiation from this copper vapor laser, the energy contributed by the component with the greater divergence to the dye lasing did not exceed 1%.

An estimate is obtained of the time of appearance of optical breakdown in air near a rough surface of a solid. The various stages of breakdown are considered: heating of surface irregularities to the evaporation temperature, equilibrium heating of the vapor to the ignition temperature, and transformation of local spherical breakdown regions into a homogeneous planar breakdown in air. An analysis is made of the variance of the time of appearance of breakdown as a function of the degree of roughness of the surface.

Theoretical and experimental investigations are reported of an unstable resonator with a perforated mirror used to couple out information on the phase structure of a radiation wavefront over the whole of its cross section inside the resonator. Estimates are obtained of the precision of determination of the wavefront which, for a given experimental error, makes it possible to determine the permissible size of perforations in the mirror. A numerical investigation is reported of the structure of the field inside a resonator with a perforated mirror when the active medium is subject to low-order aberrations. A report is given of an experimental study which confirms that, in principle, it is possible to use perforated mirrors in unstable resonators for obtaining information on the field structure necessary for the operation of systems with intracavity adaptation.

An analysis is made of the problem of widening the band of continuous tuning of the emission frequency of a CO₂ laser with an open resonator. Tuning throughout the gain profile is possible by selection of longitudinal modes of the laser resonator. This can be done using a combined resonator with two selection components: a diffraction grating and a diffraction reflecting interferometer with a Troitskii selector. The sharpness factor of a selector needed to select a single longitudinal mode is found. A report is given of experimental attainment of tuning in a 300 MHz band (c/2L=200 MHz) for the P(18) 00⁰1–10⁰0 laser emission line of the CO₂ molecule.

A study is made of the problem of the possibility of adaptive formation of images of objects in coherent light transmitted by a randomly inhomogeneous medium. A comparison is made of two sharpness functions of similar type related to the spectrum of the spatial frequencies of the images. It is shown that in the absence of apriori information on the object and on the distorting medium, maximization of the sharpness function does not result in image reconstruction, whereas in the case of an arbitrarily assumed a priori information and illumination of an object by a radiation field with a known spatial distribution of the amplitude it is possible to reconstruct images unambiguously. A specific amplitude distribution in the form of mutually orthogonal spatial harmonics is suggested for the investigated sharpness function.

Resonance multiphoton parametric forward scattering of CO₂ laser radiation was observed in gaseous SF₆. Studies were made of different self-diffraction orders, up to the third inclusive. The small-signal gain due to stimulated parametric processes was 8. The experiments were used to estimate the effective nonlinear susceptibilities (nonlinear interaction coefficients) of the third, fifth, and seventh orders. The investigated effect could have a significant influence on the nonlinear optical processes, including wavefront reversal.

A theoretical investigation is made of the growth of acoustic vibrations in the active gas stream of a CO₂ laser with an unstable resonator. The heat release associated with a perturbation of the light intensity accompanying the influence of an acoustic wave on the lasing conditions in the unstable resonator leads to modulation of the radiation. An analysis is made of the dependences of the instability growth rates on the parameters of the laser system. The threshold conditions and the growth rate of the acoustic vibrations are determined.

The methods of measuring the cutoff wavelength in real single-mode fiber-optic waveguides are compared and analyzed. There is a good qualitative correlation between the systematic differences exhibited by the data and the physical characteristic features of the methods of measurement. Recommendations are given for choosing a method for practical use in determining the cutoff wavelength.

Wavefront-shearing interferometric methods for measuring the energy of the scattered component of a light wave were tested experimentally. A method is proposed for measuring the transverse spatial correlation function of a random phase of a homogeneous scalar light field.

The variational principle is used to derive a system of equations describing the four-wave parametric oscillation in a medium with a reactive cubic nonlinearity. In the case of a transversely pumped configuration the integrals of the system can be used to obtain an analytic solution of the problem. The steady-state above-threshold amplitude of the signal waves is limited by their phase self-modulation and it is close to the amplitude of the reference waves.

A study was made of the spectral, luminescence, and lasing properties of an yttrium scandium gallium garnet crystal activated with chromium and erbium. The lifetimes of the Stark sublevels of some of the levels of Er³⁺ were determined and the schemes of these sublevels were constructed. It was established that, in addition to the "forward" energy transfer, there was also the reverse process Er³⁺→Cr³⁺. The mechanism of the interaction and its microscopic constants were determined. The chromium and erbium concentrations could be varied within wide limits without significant deterioration in the conditions for the transfer of energy from Cr³⁺ to the ⁴I_11/2 level of Er³⁺. A study was made of the lasing properties of this crystal (λ_l = 2.80 ± 0.01μ) under free-running and Q-switching conditions.

Mechanisms of energy losses in laser-active media due to summation of electronic excitations are discussed and classified. Theoretical ideas are illustrated by the results of experimental studies.

The results of experiments on using hopcalite to stabilize the gas mixture composition in pulse-periodic and single-pulse CO₂ lasers are reported. A study was made of the reasons for a fall in the activity of the catalyst with time under typical CO₂ laser conditions and a catalyst regeneration regime was selected. The use of hopcalite ensured prolonged operation of a high-power pulse-periodic CO₂ laser without replenishment of the gas mixture in a closed loop. Certain characteristic features concerning the use of hopcalite are described.

An analysis is made of laser heating of a bulk metal sample when an exothermal reaction takes place on its surface. The limit of stability is found in the form of a critical curve P=P_cr(r₀), where P is the power and r₀ is the radius of the radiation beam, such that for P>P_cr (r₀) the combustion affects the whole surface of the sample. It is shown that in the case of oblique incidence of laser radiation on the surface the ignition threshold P_cr(r₀) of a sample depends on the polarization of the radiation and can be much lower than in the case of normal incidence. The theory developed in the present paper is in agreement with experimental results.

A laboratory version of a pulsed laser vacuum ultraviolet spectrometer was designed and made. It is based on the principle of mixing the frequencies of two dye lasers in a nonlinear medium (magnesium vapor). The spectrometer operates in the spectral range 141–164 nm with a spectral resolution of Δλ=0.5nm (0.2cm⁻¹) and an average intensity of 10⁹ photons/ sec.

A cw high-power MIL-05 ion laser is described. Its output powers are ∼50W and ∼10W respectively from argon ion transitions in the visible and ultraviolet regions. It has a long service life. The results are given of investigations of mode selection and mode locking of its radiation.

The Stark representation of the resonator field and the first order of perturbation theory of the fluorescence field are used to study the fluorescence spectrum of one and two two-level atoms in an ideal resonator as a function of the initial excitation of the system. It is shown that in the absence of resonator photons (n_c = 0) the spectrum obtained for initial excitation of one atom consists of two lines located symmetrically relative to the central frequency ω_α. In all other cases the spectrum consists of the central harmonic ω_α and of ω_α ± 2Ω satellites (Ω is the Rabi frequency). The intensity of collective higher harmonics ω_α ± 4Ω is low and it falls on increase in the resonator field as I/n_c². The nonlinearity of the interaction Hamiltonian results in parametric amplification of the ω_α + 2Ω harmonics and in attenuation of the ω_α–2Ω harmonics. In the case of two atoms it is possible to achieve stimulated fluorescence at the harmonics ω_α ± 2Ω.

A description is given of the first photodissociation laser utilizing the B–X electronic transition in mercury iodide (λ = 442, 443, 444 nm) with wide-band optical pumping of HgI₂ vapor by thermal radiation from a plasma. The pump source was a linearly stabilized surface discharge with a brightness temperature T_b ≈ 25kK in the ultraviolet part of the spectrum. Laser pulses of 0.5J energy and 3.5μsec duration were generated.

A system for cascade compression of near infrared pulses is proposed. Mathematical modeling is used to determine the ultimate capabilities of this approach and to find the optimal values of the parameters.

An experimental study demonstrated the selective nature of the interaction of ultraviolet laser radiation with some nitroaromatic compounds directly in the condensed phase when (π,π*) electronic transitions took place in molecules. This occurred in molecular condensed systems with a strong (τ_ic ∼ 10⁻¹²sec) intercombination conversion and a large absorption cross section σ_π,π* ∼ 5×10–¹⁷cm². Therefore, in spite of the high rates of relaxation processes in the condensed phase, it was possible to saturate the triplet levels by resonance laser radiation and to observe selective dissociation of molecules by single- and double-stage mechanisms, so that the dissociation yield was an order of magnitude higher than the evaporation yield.

An investigation was made of spontaneous Raman scattering of YAG:Nd³⁺ laser radiation with the wavelength λ=1.064μ by the metastable state 2¹S of the helium atom and the 2p⁵3p[5/2]₂ state of neon, both excited in a hollow-cathode discharge. This scattering produced radiation at the wavelengths of 56.9 and 62.8 nm, respectively. The spectral brightness of the source was 1.5×10¹⁵ and 1.0×10¹⁴ photons·sec⁻¹·cm⁻²·sr⁻¹·cm when the population of the He(2¹S) state was 1.4×10¹¹cm⁻³ and the population of the Ne(2/p⁵3p[5/2]₂) state was 3.5×10⁸cm⁻³. It was found that the Raman scattering by the atomic or ionic levels excited in a gas-discharge plasma made it possible to generate strong far ultraviolet radiation and could be used for plasma diagnostics.

An analysis is made of the difference magnetooptic signals in fluorescence induced by anisotropic collisions discussed in an earlier paper of the authors [Sov. J. Quantum Electron. 14, 703 (1984) ]. It is shown that different variants of the signals are described by different sets of anisotropic relaxation constants. Variants of signals optimal from the point of view of obtaining information separately on each of the anisotropic relaxation constants are determined.

Approximate analytic expressions are obtained for the calculation of the propagation constants of the fundamental modes and for the determination of the critical wavelength of the first higher mode in a three-layer fiber waveguide with an anisotropic core and an elliptic inner cladding. A comparison is made of the contribution made to the birefringence of such a waveguide by departures of the transverse cross section of the cladding from the circular form and by a transverse anisotropy of the insulating core material induced by thermoelastic stresses. It is shown that the difference between the propagation constants of two orthogonally polarized modes is influenced mainly by the core anisotropy. Therefore, in the case of large dimensions of the inner cladding the departures of its transverse cross section from the circular form can be ignored and the propagation constants as well as the critical wavelengths or frequencies of modes in such a waveguide can be calculated using expressions derived for a two-layer fiber waveguide with a transverse anisotropy of the core insulator.

An investigation was made of the efficiency of the deposition of energy in a plasma of a pulse discharge pumping a copper vapor laser operating under self-heating conditions. It was found that the efficiency of energy deposition reached 50–70% when a thyratron power supply system was used. An analysis was made of the characteristics of stimulated emission on the energy characteristics of the excitation under various operating conditions.

Oscillation was observed in linear and ring resonators with an amplifying medium when gratings were formed holographically in a cerium-activated barium strontium niobate crystal. Threshold-free oscillation was observed as a result of pumping with argon laser radiation of the 488 nm wavelength. It was found that the main nonlinear mechanism responsible for the oscillations was diffusion.

An experimental study was made of the influence of the pressure of air in the range 25–743 Torr on the dynamics of the appearance of an optical discharge plasma initiated on a thin tungsten filament by λ=1.06 laser radiation. A study was made of the transmission by the discharge plasma and of the threshold of its formation in an aerosol medium as a function of the air pressure.

The results are given of the first theoretical and experimental investigations of the distribution of the density of wavefront "dislocations" in a coherent optical field at the exit from a multimode waveguide. A formula is obtained for description of the dependence of the dislocation density on the transverse coordinate. Experimental measurements are shown to be in qualitative agreement with theoretical calculations.

The Born approximation is used to solve the problem of the influence of inhomogeneities of the refractive index of a medium filling a resonator on the resonator parameters. The attention is concentrated on finding the effective optical path (phase fluctuations) allowing for diffraction, which should give the expression for the Q factor of a resonator with an inhomogeneous medium. It is shown that diffraction has a positive influence reducing the role of distributed homogeneities and thus increasing the Q factor.

It is shown that a 2.5-fold improvement in the efficiency of a laser with a gadolinium scandium gallium garnet crystal activated with Cr and Nd (GSGG:Cr:Nd), compared with a YAG:Nd laser crystal in a plane-plane resonator, was retained up to average pump powers of ∼500 W. The efficient utilization of the energy stored in a GSGG:Cr:Nd crystal during formation of a giant pulse was achieved by reducing the pump duration to 100μsec and reducing the delay of the operation of an electrooptic switch to 60–75μsec.

A study is made of the influence of phase inhomogeneities of resonator mirrors on the efficiency of nonlinear optical processes such as optical frequency doubling and parametric amplification of light waves. The results obtained make it possible to estimate the nonlinear frequency conversion efficiency of a resonator with real mirrors.

An experimental study was made of conversion of pulsed radiation of the 0.53μ wavelength to the 0.63μ radiation by stimulated Raman scattering in methane enclosed in an optical delay line. Selective coating of the mirrors used in the delay line made it possible to reduce the losses due to the generation of the second and higher Stokes components. The quantum efficiency of conversion to the first Stokes component achieved in these experiments was ∼0.52.

An investigation was made of the field distribution in LiNbO₃:Ti waveguides at the wavelength of 1.3μ. The field distributions with the depth and across the waveguide were compared with theoretical calculations. These calculations were carried out using the effective refractive index and transverse resonance methods. The experimental results agreed well with the calculations. Waveguides with the minimal asymmetry of the field distribution were fabricated.

A study was made of reconstruction of the wavefront of CO₂ laser radiation in the course of forward resonance parametric scattering by gaseous SF₆ and a reverse passage after reflection from a mirror.

Transmission of an optical signal with a frequency-switched optical carrier and optical channel separation was demonstrated experimentally. The radiation source was a pulsed injection cleaved-coupled-cavity (CCC or C³) laser.

A lithium iodate crystal, placed inside the resonator of a continuously pumped YAG:Nd³⁺ laser, was used to achieve for the first time stimulated Raman scattering (STRS) generating wavelengths 556.46 and 582.99 nm with an average power in the mode-locking regime of about 50 mW and 7 mW, respectively. In the Q-switching regime at the 582.99 nm wavelength the average power was 320 mW in one direction. It was also found experimentally that weak radiation of wavelengths 509.78 and 612.33 nm was emitted. The conditions for the observation of these effects were analyzed. The wavelengths of the observed radiation were attributed to STRS of the fundamental-frequency radiation accompanied by simultaneous generation of the second harmonic, and also of sum frequencies in the lithium iodate crystal.

A study was made of the dependence of the duration of emission from an atmospheric-pressure H₂–F₂ laser on the composition of the mixture and on the degree of initiation. The initiation source was a neodymium glass laser system with a harmonic generator emitting 10-nsec pulses of the third and fourth harmonics which reached energies of 5 and 2J, respectively. The duration of emission from the H₂–F₂ laser was 45 nsec when the initial concentration of the fluorine atoms was [F]_o=3×10¹⁷cm⁻³ and the composition of the mixture was F₂:O₂:H₂:He=300:30:100:320 Torr. The divergence of the radiation emitted by this chemical laser was 1.1 mrad when the specific output energy was 40J/liter; this divergence was only 1.6 times greater than the diffraction limit.

Experimental studies showed that the output energy of a pulsed chemical oxygen-iodine laser depended weakly on the partial water vapor pressure in the resonator right up to values which were three times higher than the oxygen pressure, which corresponded to thermally insulated operation of a generator supplying singlet oxygen to the laser.

An investigation was made of the width of the emission line of single-frequency GaAlAs heterojunction lasers of various types. It was found that the absence of lateral optical confinement resulted in considerable broadening of the emission line. The observed broadening of the emission line of lasers exhibiting a gain-induced waveguiding effect was attributed to an increase in the level of spontaneous background of the modes generated in such lasers.

An electric-discharge F₂ laser emitting at 157.6 nm was constructed. A study was made of its spectral, energy, and temporal characteristics. High values of the output power W and efficiency of an F₂ laser were obtained by increasing the pressure in the He–F₂ active medium to 10 atm, which made it possible to achieve W = 3 MW, efficiency of 0.17%, and specific output energy in excess of 1J/liter. Each of the two components of the output spectrum was ∼ 0.02 nm wide.

A review is given of the papers presented at a plenary meeting of the Scientific Council of the Academy of Sciences of the USSR on Coherent and Nonlinear Optics, held on May 27–29, 1985 in Tbilisi. The current status of research on and development of excimer and waveguide lasers was discussed.