Table of contents

The advantages of fiber waveguides over bulk nonlinear media are demonstrated. A review is given of the main nonlinear effects which occur during propagation of laser radiation in fiber waveguides. The most interesting applications of nonlinear fiber optics are discussed.

Experimental and theoretical investigations were made of the angular distribution of amplified external radiation injected off-axially into an unstable resonator containing an active medium. Feasibility of angular selection of the external signal was demonstrated. The calculations agreed with the experimental results and made it possible to estimate the role of the frequency effects and also to identify the requirements in respect of the parameters needed to maximize radiation selection.

Equations describing changes in the slowly varying amplitudes and phases of the field, taking into account the slow response (inertia) of the active medium, are derived for a two-mode gas laser. It is found that allowance for the inertia does not influence steady-state lasing, but the conditions for the attainment of a steady-state value depend strongly on the degree of inertia of the medium and the intermode interaction. Theoretical and experimental studies were made of the influence of the scanning rate of the resonator modes on the lasing regime. It was found that when the medium had a long response time, the characteristics of the two-mode regime depended strongly on the scanning rate. A comparison was made between the lasing characteristics when single-mode and two-mode regimes were established. Numerical calculations and experimental investigations were made for a CO₂ laser.

An active medium was formed from lithium fluoride single crystals with the maximum concentration of the active centers (F₂, F₃⁺), reaching the range of concentration quenching. The coloration method made it possible to reduce greatly the concentration of parasitic aggregate centers and, consequently, the optical losses in the medium. The lasing threshold for the F₂ and F₃⁺ centers was 4 and 60 kW/cm², respectively.

An analysis is made of a distributed analog of a multielement phased laser comprising a broad injection laser with periodic modulation of various parameters parallel to the mirrors. The case when the modulation period is much greater than the wavelength of light is discussed. The single-mode approximation is used to show that the laser characteristics, in particular the field structure and the angular distribution, are governed by the effect of a spatial parametric resonance of a smooth envelope of the laser field. The lasing threshold, frequency, and amplitude of the field are determined as a function of the injection current and the modulation parameters. It is found that the resonance may be suppressed by varying the modulation. The form of the laser angular distribution is discussed.

The small-signal gain of the active medium in a pulse-periodic CO₂ laser was determined. The resonator parameters were optimized. The use of a germanium Fabry–Perot etalon as the exit mirror made it possible to increase the output energy from 0.4 to 1.2 J per pulse. An analysis was made of various ways of constructing selective resonators with diffraction gratings. It was found that when radiation was coupled out using the zeroth diffraction order and the resonator was optimized by rotating the direction of the polarization of the radiation in the course of tuning of the emission frequency, there were additional losses in a Brewster window due to a change in the polarization azimuth on reflection from a diffraction grating.

For the first time, a theoretical and experimental analysis has been made of the contribution of combination tones to the instability of radiation from a two-mode gas laser. It was found that these tones resulted in regular oscillations of the mode intensities at twice the intermode beat frequency. The level of these oscillations and the law governing the behavior of their amplitude were determined as a function of various operational parameters in a two-mode He–Ne laser (λ = 3.3922 μm).

An experimental investigation was made of the interaction between stimulated emission lines of a CO laser by varying the losses and the frequency of one of the lasing transitions under cw and Q-switched conditions. Mutual enhancement of the lines was observed, which was an effect opposite to competition between the transitions. Theoretical estimates of the effect were in qualitative agreement with the experimental results. The influence of cascade and relaxation processes on the characteristics of the laser radiation was analyzed.

The first results are reported of an experimental investigation of the energy parameters of a nitrogen laser (λ = 337.1 nm) excited by high-power nanosecond microwave pulses. The measurements were carried out for a specific pump power of 0.4–1.6 MW/cm³ at a gas pressure of 3–30 Torr when the length of the active medium was 20–100 cm. The lasing efficiency was determined as a function of the specific power deposited in the microwave discharge and of the length of the active medium. The maximum efficiency did not exceed 0.06% and the peak radiation power was 12.5 kW.

The Stark effect was used to reproduce in the cw regime six stimulated emission lines of a submillimeter PH₃ laser pumped by CO₂ laser radiation. Previously, these had only been obtained in the pulsed regime. Stark tuning of the output radiation frequency was observed for five of these lines. Detailed investigations were made of the character and scales of frequency tuning of the λ = 195 μm output radiation from the PH₃ laser.

An investigation was made of a YSGG:Cr:Nd laser using a resonator and a polarization method for coupling out radiation. The Q factor was switched electrooptically (pulse duration 10 ns) and the repetition frequency was up to 50 Hz. The differential efficiency was 5.6% and the absolute efficiency was 3.6% when the output energy per pulse was 0.46 J. The highest average output power of single pulses of linearly polarized radiation was 23 W when the average pump power was 640 W. The results indicated that in the pulse-periodic regime the polarization method of coupling out the output radiation ensured near-optimal conditions so that the efficiency was high, approaching that of a YSGG:Cr:Nd laser with a conventional resonator and coupling out of radiation via a transparent mirror operating in the single-pulse free-running regime.

Calculations are made of the gain of the active medium and the specific output power of a pulsed laser utilizing a mixture of O₂(¹Δ) and an iodine aerosol. It was found that, in principle, lasing could be achieved without external initiation due to self-heating of the mixture accompanying relaxation of some of the stored energy. However, as a result of scattering of light by iodine particles and energy losses due to heating of the mixture, the specific output energy is low (~ 4 J/liter). The laser energy characteristics can be improved appreciably (up to 11 J/liter) by rapid evaporation of iodine particles using external radiation. An estimate of the electrooptic efficiency of a laser utilizing evaporation of iodine particles by flashlamp radiation gives a value of ~ 800%.

A theoretical analysis is made of the influence of doping of the active region on the threshold and temperature characteristics of GaAs lasers. A comparison is made with the experimental data obtained for electron-beam-pumped lasers. A satisfactory description of the experimental results obtained for active elements with the full range of dopant concentrations requires an allowance for the influence of the absorption by carriers and of the Auger recombination of nonequilibrium carriers on the laser characteristics. Doping of the active region of a GaAs laser makes it possible to reduce the threshold pump current only in the case of short nonradiative lifetimes τ_nr. If τ_nr ≈ 1 ns, the optimal acceptor concentrations in p-type GaAs are 4 × 10¹⁸–10¹⁹ cm⁻³ and the corresponding optimal concentrations of donors in n-type GaAs are (1–2) × 10¹⁸ cm⁻³.

An investigation is reported of the influence of the profile of the reflection coefficient of mirrors on the selectivity of dispersive resonators with different parameters. It is shown that in the range where the Fresnel number exceeds unity the dependence of selective properties on the profile becomes considerably stronger and there is an optimum, which is a function of the nature of the laser operation when the selectivity can be much higher than for a homogeneous aperture.

The absorption and emission of the CO₂ molecule in the 4.3 μm range was used to study experimentally the dependences of the specific stored vibrational energy, the specific output energy, the maximum output power, and the gain on the input energy, pressure, and CO₂ concentration in an N₂–He–CO₂–O₂ mixture excited by a self-sustained glow discharge. The distribution of these parameters over the length of the discharge chamber of a continuous-flow commercial CO₂ laser was also studied. A comparison was made between experimental values of the vibrational and translational temperatures and theoretical values determined from the gasdynamic and vibrational kinetics equations using measured electrical characteristics of the discharge. The influence of the unexcited layers of the active mixture on the parameters of the active medium was investigated.

An analysis is made of the amplification and stimulated emission of coherent radiation by an electron beam having an appreciable angular and energy spread in an undulator with a transverse magnetostatic field. It is found that the amplification mechanism is due to the inhomogeneity of the electron phase distribution as the resonance point is approached. As a result, the efficiency is almost insensitive to the initial spread. The optimal "nondipolarity" parameter for an undulator giving the maximum efficiency is determined. Under lasing conditions, it is possible to achieve an appreciably higher radiation power than that in a traditional free-electron laser system due to a shift of the saturation level toward higher powers.

A theoretical analysis is made of the kinetics of electronic–vibrational relaxation of excimer molecules in the active medium of a KrF laser. Equations are derived to describe the dependence of the lasing quantum efficiency on the photon flux in the resonator, on the mechanism of formation of electronically excited molecules, and on the composition of the active gas mixture.

The operational characteristics of a Ne laser (λ = 585.3, 724.5, and 703.2 nm) were determined during the afterglow stage (Δt ~ 50–120 ns) of a plasma created by a short (τ < 10 ns) pulse of an electron beam of j ~ 20–100 A/cm² current density, which was used to pump longitudinally or transversely dense (p ~ 0.5–3 atm) gas mixtures He–Ne–As (Kr) and Ne–H₂. It was found experimentally that a preliminary heating of the gas resulted in deterioration of the lasing characteristics. Calculations were made of the kinetics of relaxation of an He–Ne–As mixture and of the lasing characteristics of the yellow line of neon. The results of these calculations were in good agreement with the measured characteristics.

The characteristics of an electric-discharge XeCl* laser were improved by the addition of hydrogen to an He–Xe–HCl mixture. The energy of the output radiation pulses increased by 15% and the operation of the laser became stabilized. A study was made of the dependence of the output energy on the hydrogen concentration.

Low-threshold wide-band lasing was achieved for a YAG crystal with optically and thermally stable color centers. The absorption and luminescence spectra were determined, and the gain was measured in the range of wavelengths corresponding to the color-center luminescence. The cross section of the lasing transition was estimated (8 × 10⁻¹⁹ cm²) and the efficiency of conversion of the pump radiation from a neodymium laser into the output radiation was determined (10%).

The output power and the gain were determined for the 5.4 μm (3p₁–3s'₁) laser transition in a compact laser containing pure neon. The length of the active medium was 60 cm. The measurements were made at different neon pressures and for different discharge currents. The results obtained were used to draw the conclusion on the suitability of a pure neon laser emitting at λ = 5.4 μm for the detection of traces of nitric oxide.

The method of coherent anti-Stokes Raman scattering (CARS) was used in shock-wave diagnostics. CARS spectra were obtained, for the first time, for nitrogen heated in the equilibrium zone of a shock wave in the temperature range 1.6–3.2 kK. The experimental data were analyzed using a model which took into account collisional transfer in the Q band of N₂ and the effects of two-photon Raman saturation.

Vacuum ultraviolet radiation was generated in the range 124–126 nm by the method of four-wave mixing in a magnesium plasma produced in a pulsed electric discharge. This nonlinear medium was found to exhibit a high spatial homogeneity which made it possible to ensure phase matching. An experimental investigation was made of the characteristic features of parametric oscillation under conditions of two- and three-photon resonances in strong fields. It was found that the presence of an intermediate single-photon quasiresonance can lead to multifrequency vacuum ultraviolet oscillation. A conversion coefficient of ~ 10^− 3 was obtained for a pump input power of ~ 10 W.

The results are given of analytic and numerical investigations of the amplification of laser radiation under conditions of strong saturation of resonant transitions induced by optical pumping. Wave-like solutions of propagation of light pulses in a three-level medium are found. Characteristic dependences of the radiation conversion length on the parameters of the radiation and medium are determined.

An analysis is made of the amplitude and polarization effects which accompany the propagation of laser radiation in a ring resonator filled with a nonlinear gyrotropic medium. The ranges of the parameters of the radiation, resonator, and medium are found for which there are periodic and chaotic changes in the intensity, degree of ellipticity, and angle of rotation of the polarization ellipse of the output radiation. Analytic expressions are obtained for the frequency of changes in the output characteristics.

It is found that thermal self-defocusing of the exciting beam near its focal constriction is the main factor that increases the stimulated Brillouin scattering threshold in a field of long laser pulses and reduces the fraction of the Stokes wave reproducing the focused pump beam. The universal dependence of the excess over the stimulated Brillouin scattering threshold and the reproduction fraction on the ratio of the laser pulse energy to the critical energy of thermal self-defocusing is determined.

The approximation of a constant intensity of reference waves is used in the case of low values of the reflection coefficients (less than unity) to investigate numerically the efficiency of optical phase conjugation of a wave from a pulsed chemical HF laser based on a chain reaction. It is shown that the reflection coefficient for the three strongest emission bands P_1–0, P_2–1, and P_3–2 of a pulsed chemical HF laser can be 0.1.

An investigation is made of the possibility of compensation of small-scale inhomogeneities with dispersion not exceeding 1.5 rad² in a laser resonator characterized by a Fresnel number ~ 1 when a phase-conjugate mirror is in the form of a Zernike cell. An analysis is made of the factors responsible for reduction in the quality of compensation of distortions.

The solution of a nonlinear parabolic equation is used in a demonstration of the effectiveness of diverging (or converging) beams in suppression of small-scale self-focusing in high-power laser systems and also in control of such self-focusing. Characteristic radii of curvature of the wavefront of diverging beams are found as a function of the radiation power density and of the nonlinear coefficient of the medium in a laser. In the case of two-dimensional diverging beams the regular spatial distribution of the amplitude in the transverse direction is subject to distortions.

Holographic interferometry was used in a study of the dynamics of growth of perturbations in the medium of a pulsed self-sustained discharge with preionization by ultraviolet radiation. The discharge zone was probed along two mutually perpendicular directions. The inhomogeneities of the gaseous medium typical of such a discharge were classified. The sources of gasdynamic perturbation were identified and their temporal and spatial characteristics were determined. It was found that acoustic waves were generated because of a more rapid evolution of energy in side regions of the discharge and in the electrode layers. It was demonstrated experimentally that the periodic system of perturbations of the medium in the central part of the discharge gap was due to inhomogeneous preionization of the working zone due to the presence of a grid electrode in front of the ultraviolet radiation sources.

An experimental investigation was made of the spatial spectrum of waves emitted by an anisotropic optical LiNbO₃:Ti waveguide when a guided mode was converted into a leaky wave by interaction with a surface acoustic wave (SAW). The spectrum of the emitted field was modified greatly by the propagation of the diffracted radiation along a part of the anisotropic waveguide which was free of the SAW. The efficiency of the interaction with the SAW increased by two orders of magnitude for the radiation modes characterized by the maximum coupling coefficient.

An analysis is made of the configuration of the field and losses experienced by normal oscillation modes of a conjugate resonator with an array-type controlled transparency. It is shown that the presence of gaps between the active areas of a transparency results in deformation of the normal modes in such a resonator, compared with a resonator which has a continuous aperture. Computer calculations are reported of the distribution of the field of normal modes on a resonator mirror and of the dependence of the losses experienced by the modes on the Fresnel number of the resonator. It is shown that the dependence has certain singularities corresponding to qualitative changes in the structure of normal modes, resulting in a considerable increase of the angular divergence. These features of the behavior of normal oscillation modes are supported by experiments carried out as part of the present study.

An experimental study was made of the suppression of phase noise during propagation of giant ruby laser pulses in short (up to 0.1 m) multimode fiber waveguides. A segment of a fiber waveguide, in which stimulated Brillouin scattering took place, acted as a mirror performing phase conjugation. The dependences of the parameters of the corrected signal on the lengths of both waveguide segments were determined.

A laser mixing method was used in preparation and investigation of GaInAsSb solid-solution films. The crystal lattice parameters and the compositions of the films were determined. The corresponding band gaps agreed well with the values obtained from optical investigations. The spectral dependence of the observed quantum size effect in a film of thickness d = 29 nm was in good agreement with an analytic expression derived by the authors.

An experimental investigation was made of the feasibility of lasing as a result of transitions in the Be II ion in an expanding laser plasma. Formation of a shock-compressed zone in the path of the expanding plasma resulted in stimulated emission due to 4d–3p, 4f–3d, and 4s–3p transitions in the Be II ion when the plasma was enclosed in an optical resonator. Measurements were made of the gain, divergence, and duration of laser pulses emitted by the plasma.

A theory of nonlinear fluorescence of phytoplankton is developed on the basis of a model of the structure of its photosynthetic unit. It is shown that the main role in fluorescence saturation is played by singlet–singlet annihilation. An analysis of the expressions obtained can be used to identify the parameters proportional to the concentration of chlorophyll a. Theoretical predictions (nature of the saturation curve, proportionality of the identified parameters to the concentration of chlorophyll a found by conventional extract method) are supported by experiments carried out on three pure seawater algae cultures.

An analysis is made of the task of measuring and phase conjugation of the optical field against the background of noise in the case of a priori indeterminacy of the statistical properties of the received signal. Mode expansion of a useful signal is employed to show that the optimal algorithm may be realized in at least two variants: either on the basis of multichannel heterodyne reception and phase conjugation of the filtered signal at a radio frequency or with the aid of "physical" phase conjugation methods directly at an optical frequency. Expressions are obtained for estimating the quality of the measurements and phase conjugation. It is shown that practical realization of the latter variant of the optimal algorithm may result in a significant deterioration of the potential characteristics.

Theoretical and experimental investigations were made of the influence of the power of longitudinal resonator modes adjacent to the fundamental mode on the width of the emission line of injection lasers made from GaAlAs heterostructures with lateral optical confinement. The width of the emission line observed in the course of cw emission of real lasers was within the limits set by the homogeneous and inhomogeneous broadening of the gain profile. When the output power was ~ 5 mW from a face an increase (by up to 10%) in the relative power of the longitudinal modes adjacent to the fundamental one could broaden the line right up to the inhomogeneous limit. This effect was attributed to spatial depletion of the population inversion along the resonator axis.

A method is proposed for controlling the intensity distribution over the cross section of a laser beam. In this method the laser exit mirror is an inhomogeneous interferometer whose transmission profile is continuously variable by varying its length over a range of λ / 2. The results are reported of a modification of the profile of an He–Ne laser beam from close to Gaussian to a beam having a dip on the axis, with almost no reduction in the laser output power. It is noted that an inhomogeneous interferometer can be used to control the phase distribution in a beam and to modify the mode structure of the laser radiation.

A differential scattering absorption lidar was constructed utilizing a TEA CO₂ laser. Some atmospheric parameters were determined. Measurements were also made with an ammonia cell. The results made it possible to estimate the operating characteristics of the lidar: the operating range was 10 km for topographic reflection and the maximum sensitivity (for gases with absorption coefficients of 10–30 atm⁻¹·cm⁻¹ in the differential scattering absorption regime) was 0.5 parts per million with a spatial resolution of 100 m. Fundamental difficulties were noted in using the differential scattering absorption method to probe atmospheric pollutants.

A theoretical analysis is reported of the dependence of the frequency of a signal obtained from a laser Doppler anemometer on the size of scattering particles. This analysis is made for a differential optical system and a noncoherent model is employed. It is shown that in general the Doppler frequency shift is a function of two parameters: the ratio of the size of the scatterers to the spatial period of the interference pattern and the number of interference fringes in the investigated region. It depends also on the nature of the intensity distribution function in the scattering region and on the particle scattering function. The results apply to large scatterers in a small scattering region. Although an increase in the Doppler frequency shift caused by the influence of the finite size of the scatterers is accompanied by a reduction in the depth of modulation of the signal, under normal conditions the measurement error may reach 1–2%.

The mass spectrometric method was used in an experimental investigation of the fast ions in a plasma generated using moderate laser radiation power densities (q = 10¹⁰–10¹² W/cm²). It was found that, for power densities of (q 0.5 TW/cm² and p-polarized laser radiation incident at angles of Θ = 18 ± 5°, two groups of ions appeared and these differed in both energy and charge multiplicity. For q = 1 TW/cm² the ion velocity was ~ 10⁸ cm/s.

A study was made of the possibility of using thermal sensitization of a silver halide photographic emulsion for recording YAG:Er³⁺ laser radiation (λ = 2.94 μm). Using an FT-101 photographic film it was possible to record the radiation down to energy densities of ~ 1 mJ/cm² in a dynamic range of ~ 100 with a resolution of at least 50 lines/mm when the laser pulse duration was ~ 150 μsec. This method was employed to determine for the first time the spectral width of a line emitted by a YAG:Er³⁺ laser operating in the free-running regime.

A study was made of additional optical losses in the range from – 195 °C to + 85 °C observed in a fiber-optic cable containing different types of waveguides. The use of waveguides with noncrystallizing primary and fluoroplastic secondary claddings made it possible to ensure that the additional losses in this cable were less than 1 dB/km at temperatures in the range from – 120 °C to + 85 °C.

A calculation is reported of the reduction in the degree of linear polarization of a beam on increase in the length of a waveguide with a parabolic refractive-index profile when the propagation in this waveguide is accompanied by a Rytov rotation of the plane of polarization. It is shown that the degree of circular polarization remains constant.

Simultaneous emission of two orthogonally polarized fundamental EH₁₁ modes was observed for a waveguide electric-discharge HCN laser (λ = 337 μm). The difference between the frequencies of these modes was continuously tunable in the range from 100 kHz to 4.5 MHz. A change in the beat frequency was observed as a result of a change in the discharge current.