Table of contents

This review deals with the photoinduced generation of the second harmonic in centrosymmetric media. A brief account is given of the main results of the reported investigations. The models proposed to describe the effect are discussed. The principles for experimental separation of the contributions of the various mechanisms are put forward and the results of the relevant experiments are described. A detailed phenomenological description of the photogalvanic mechanism is given and an experimental confirmation of this mechanism is reported.

The conditions for energy deposition in a pulse-periodic XeCl laser with a 9 cm × 6 cm aperture were investigated and analysed. A new power supply system in the form of two series-connected LC inverter circuits, combined with a magnetic pulse-compression stage, ensured the formation of a homogeneous gas discharge at charging voltages not exceeding 30 kV. An output energy of 10 J per pulse was obtained at a pulse repetition rate of 100 Hz.

An investigation was made of the efficiency of a number of flashlamp systems designed for electrically and spectrally matched pumping of dye lasers. The optimal system for pulse-periodic lasers with an average output power up to 10 W consisted of one tubular flashlamp and an elliptic pump enclosure with polished internal walls. An efficient pulse-periodic dye laser was designed and constructed. It operated at a pulse repetition frequency up to 10 Hz, the average power was ~ 6 W, and the efficiency was in excess of 0.5%.

A three-electrode discharge laser was developed. The discharge is ignited by the application of a high-voltage pulse to the middle control electrode. A storage capacitor for the main discharge is connected directly to the other two electrodes. The examples of excimer XeCl and KrF lasers, and also of an Ar—Xe laser, are used to show that such excitation of the active medium can reduce by tens of times the load on the switches in the power supply system.

The feasibility of using commercial semiconductor diodes for interrupting the current in a pulse generator with inductive energy storage, designed for pumping gas lasers, was demonstrated experimentally. The efficiency of stimulated emission as a result of the C³ Π_u − B³ Π_g (λ = 337 nm) transition in nitrogen was 0.2% relative to the energy deposited during an emission pulse. Such pulse generators should be useful in pumping incoherent excimer and exciplex light sources ('excilamps') and pulsed dense-gas lasers.

The operation of a chemical oxygen—iodine laser with a drop-jet generator of singlet oxygen, operating at pressures up to 90 Torr, was demonstrated. When the active-gas pressure in the generator was 70 Torr, the output power was 90 W, the chemical efficiency was up to 10%, and the specific power obtained from a unit area of the generator was 80 W cm^-2. The interaction of drop jets with a gas flowing at a high velocity resulted in partial drag of the gas by the jets and in a considerable fall of the pressure in the reaction zone. A counterflow generator ensured the removal of the drop aerosol from the active gas and the transfer of heat to the solution during relaxation of high-pressure O₂(¹Δ).

Comparative investigations were made of the characteristics of two compact KrF lasers with different geometries and different gas flow velocities. The same electric-discharge system, optimised to reach a high KrF laser efficiency (~ 3.5%), was used in both lasers. It was established that at a high pulse repetition frequency the specific nature of acoustic perturbations, which appeared in the gasdynamic channel, influenced the homogeneity of the discharge and of the laser radiation.

Measurements were made of the gain in a disk amplifier module containing neodymium-activated phosphate glass active elements with an aperture of 24 cm. When the pump energy was 580 kJ, the small-signal gain experienced in the module was 0.015 cm^-1 for pulses of 2.5 ns duration and 40 mJ cm^-2 energy density.

An analysis is made of the factors influencing the degree of the spatial coherence γ of miniature chip lasers with longitudinal semiconductor-diode pumping. Estimates are obtained of the ultimate spatial coherence of the radiation emitted by such lasers. It is shown that the value of |1-γ| can reach 10^-7 in the limit set by the beam width.

The gain spectrum of a long-pulse ( ~ 0.5 μs) XeCl laser was investigated in a master oscillator—single-pass amplifier configuration. One Fabry—Perot etalon with a free spectral range ~200 cm^-1 and a finesse F = 30 was used to tune the frequency of the XeCl master oscillator. This made it possible to generate frequency-tunable radiation with a linewidth ~1 cm^-1 and to record, with approximately the same precision, the gain spectrum of the XeCl laser in the range 307.5–308.8 nm. The time dependence of the small-signal gain of the active medium of the XeCl laser was investigated, with a resolution of ~10 ns, for the 0–2 transition in the XeCl molecule.

A method is proposed for the description of ring cavities with a nonplanar axial contour. This method makes it possible to find the stability regions and the characteristics of a Gaussian mode with general astigmatism, which is formed in such cavities. Numerical calculations are reported of the field characteristics and stability regions of a four-mirror ring cavity with a nonplanar axial contour.

An experimental study established the feasibility of improving the spatial and energy parameters of the radiation from an electron-beam-controlled CO₂ laser, emitting pulses several tens of microseconds long, by lowering the active-mixture pressure from 1 to 0.5 bar.

A numerical model is developed of an XeCl laser with an additional power supply and a saturable inductor. The model is checked against experimental results obtained for lasers with different energy parameters. In a wide range of output energies the best fit to experiments is obtained on the assumption of dissociation of NeXe⁺ ions in the discharge and of equality of the discharge and electrode areas. Optimisation of the composition of the active medium of a 10-J XeCl laser, as well as of the cavity parameters, discharge area, and interelectrode distance (or the initial voltage across a storage capacitor) of this laser is carried out either for a fixed initial voltage across the storage capacitor (fixed interelectrode distance) and for given parameters of the circuit. It is shown that the lasing efficiency, relative to the energy stored in the capacitors, can reach ~6.5% both in the presence and absence of dissociation of NeXe⁺ ions, but in these two cases there is a major difference between the optimal conditions (composition, discharge area, etc.) under which the maximum efficiency is reached. Under optimal conditions the excitation is aperiodic, i.e. all the energy is deposited in the active medium during the first half-period of the discharge of the storage capacitor.

An experimental investigation was made of the feasibility of phase conjugation of a pump beam with a spectral line width 26 cm^-1 generated by an XeCl laser. Spectral and spatial selection of the pump beam by a nonlinear medium was observed when the intensity of this beam was ~100 MW cm^-2. This effect was related to the characteristics of the spectral profile of the XeCl molecules. Phase conjugation of a part of the incident beam was achieved and the spectral width of the Stokes line was 0.3–0.5 cm^-1.

An experimental investigation was made of a XeCl laser with a cavity formed by a semitransparent plane mirror and a stimulated-Brillouin-scattering (SBS) medium. Spatial and spectral selection by the SBS mirror resulted in the formation of narrow-band highly directional radiation representing a superposition of converging and diverging beams. Each beam was amplitude-modulated at intervals of time corresponding to two round trips through the cavity and it consisted of a set of spectral lines 0.3 cm^-1 wide, distributed over an interval of two SBS frequency shifts. About 30% of the energy of each beam was within the diffraction angle. The total output energy was 10 mJ when the pulse duration was 120 ns.

An investigation was made of the conversion of radiation from multijoule XeCl* lasers in solutions of organic compounds. The advantages of using circularly pumped liquid-filled cells in conversion of high-power ultraviolet radiation were demonstrated. In cells with axicon reflectors the output energy obtained from a solution of coumarin 102 was 3.2 J and the conversion efficiency was 31%. The feasibility of conversion of radiation with the pump energy up to 100 J was analysed.

The quenching buffer method was used in measurements of the rate constants of the principal reactions involved in flash photolysis of the iodide C₃F₇OI (RI) and of its mixtures with buffer gases M (CF₃H, H₂O): R+R → R₂; R+I → RI; I^*+RI, M, I₂ → I+RI, M, I₂; I + I + RI, M → I₂ + RI, M. The broadening coefficients were determined for the line due to the active transition in a photodissociation laser when the broadening was due to collisions with C₃F₇OI, CF₃H, and H₂O molecules. The probabilities of formation of excited iodine atoms by photodissociation of the iodides C₂F₅I and C₃F₇OI with broadband ultraviolet radiation were also found. The low recombination efficiency (found for the iodides investigated earlier) was confirmed for the C₂F₅ and C₃F₇O radicals interacting with excited iodine atoms.

Experimental and theoretical investigations were made of turbulent diffusion in an industrial CO₂ laser with crossed electrodes. The particle diffusion reduced considerably the spatial inhomogeneity of the gain. The results obtained were used to propose a new method for the determination of the turbulent diffusion coefficient.

A theoretical investigation is made of the feasibility of improving the quality of phase conjugation by four-wave interaction (mixing) in media with a noncubic nonlinearity. It is shown that distortions of the angular spectrum of the reflected wave can be weakened considerably by expanding the angular spectrum of the signal radiation.

A stochastic theory of relaxation in optical bistable systems is developed. The Fokker—Planck equation for the amplitude distribution function of an electromagnetic field in a passive medium is derived from fundamental principles in the adiabatic approximation. An analysis of the lowest eigenstates is used to construct a theory of relaxation of metastable and unstable states.

The effect of a distributed feedback on stimulated Brillouin scattering (SBS) is considered. The feedback is due to linear scattering by random inhomogeneities or microinclusions in the medium. A theoretical analysis is made taking account of focusing, in this medium, of laser pump radiation with different degrees of spatial coherence. It is shown that, in spite of the beam attenuation on increase in the scatterer concentration, the SBS threshold may decrease because of the distributed feedback. The range of conditions under which this effect can appear is determined. A well-defined maximum of the SBS efficiency, considered as a function of the scatterer concentration, can be used to obtain information on the scattering inhomogeneities.

A theoretical investigation is reported of different variants of a modulator based on coupled waveguides with a Y coupler at the entry. A calculation is made of the parameters of three-section modulators (two rows of electrodes of identical length separated by an electrode-free region) ensuring that the third-order intermodulation noise does not exceed −100 and −60 dB in the range of the modulation depths of the output optical signal of at least up to 50 and 80%, respectively. A method is proposed for the compensation of technological errors contributing to the coupling constant of the waveguides. This method should make it easier to construct the investigated modulator structures.

An analysis is made of the mechanisms of frequency conversion of the carrier of an optical wave which propagates in a waveguide with a refractive index modulated at a frequency up to 10⁶ less than the carrier frequency of light.

An analytic investigation is made of a nonlinear directional coupler based on second harmonic generation from constant-intensity fundamental-frequency radiation. The parameters of the coupler are found and the intensity of light needed to induce a nonlinear shift of π is calculated. Single-frequency output of a directional coupler favours its use in optical data processing systems.

An account is given of the design characteristics of a transversely excited CO₂ laser with automated tuning over the lines in regular (00⁰1-10⁰0, 00⁰1–02⁰0) and nontraditional (00⁰2–10⁰1, 00⁰2–02⁰1, 01¹1–11¹0) bands, developed specially for sounding of the atmosphere. Measurements of the small-signal gain and an experimental determination of the vibrational temperatures T₃ and T₂ were used to optimise the active medium. About 200 stimulated emission lines were excited by regular and nontraditional transitions in the range λ = 9.1–11.3 μm. The output energy per pulse was in excess of 2 and 1 J for the strong regular and nontraditional lines, respectively.

An experimental investigation was made of the radiation emitted by pulsed coaxial excimer light sources ('excilamps') pumped by barrier and longitudinal discharges. Stable operation of excilamps was demonstrated at pulse repetition frequencies up to 12 kHz. An average output radiation power was of the order of 0.2 W at λ ~ 308 nm. Excitation by a longitudinal discharge generated, for the same pump pulse energies, pulsed output radiation powers several times higher than excitation by a barrier discharge.

The damage thresholds of bulk high-purity chalcogenide glass samples and of fibre waveguides made from them were determined for YAG : Er laser radiation (λ = 2.94 μm). A cumulative effect was observed: the optical strength decreased during repetitive application of laser pulses and this was true both of the bulk samples and of the fibre waveguides. A correlation between the damage threshold and the concentration of heterophase inclusions was established experimentally. When the inclusion concentration was less than 10⁴ cm^-3, the damage threshold exceeded 1 kJ cm^-2 for single pulses and was approximately half that in the pulse-periodic regime. The measured energy density of the λ = 2.94 μm radiation, needed for efficient cutting of the crystalline lens biotissues in the human eye, amounted to 50 J cm^-2. The feasibility of constructing a scalpel with an Er laser and transmission of its high-power radiation (up to 1 kJ cm^-2) along a chalcogenide fibre waveguide was demonstrated in principle.

An investigation is made of the efficiency of repeated excitation of a sample by laser pulse pairs when data are stored by the photon echo method. Fluctuations of the difference between the pulse phases in different pairs, resulting from instability of the optical path length in the delay line, are shown to prevent cophasal superposition of a set of level-population gratings. The possibility of weakening the negative influence of this effect on the intensity of the read echo signal is considered.