Table of contents

An investigation was made of the influence of the input energy, gas flow rate, active mixture composition, and discharge zone length on the energy characteristics of a longitudinal-discharge cryogenically cooled gas-flow CO laser. A high efficiency of ≳ 30% was obtained in a wide range of experimental conditions, with a laser output power reaching hundreds of watts. The influence of the input energy on the spectral composition of the laser radiation was investigated. Qualitative explanations are given of the dependences obtained.

A description is given of several models of industrial kilowatt CO₂ lasers developed at the Scientific-Research Center. These lasers differ from their prototypes because they meet the requirements for commercial production and industrial use, particularly in respect of the parameters describing the quality of laser radiation beams, stability, and ease of control by microprocessor-based automatic control systems.

A numerical analysis of the rate equations is used to find the requirements which must be satisfied by the parameters of a glass laser. A report is given of the development of a glass laser with an optomechanical switch capable of emitting pulses of microsecond duration with a smooth bell-shaped profile and highly reproducible from one shot to the next. The laser has a very simple configuration without any negative feedback components and the duration of the output pulses can be varied within the range 250–2300 ns.

Optimization of the output parameters of a transversely excited CO₂ laser, emitting in the 4.3 μm wavelength range as a result of 10⁰1(02⁰1)–10⁰0(02⁰0) transitions, was carried out. A specific output energy of ~ 50 μJ/liter was achieved in a simple configuration with ultraviolet preionization. It was found experimentally that the key role in the lasing mechanism could be played by nonlinear optical eifects. Several new transitions in the region of 4.3 μm were observed.

An investigation was made of the spatial characteristics of radiation from a CO₂ laser with InAs in a stable resonator. Inhomogeneities of the spatial distribution of the radiation intensity observed in the near-field zone were due to diffraction of the radiation by the smallest aperture inside the resonator because of formation of a nonlinear diverging lens in InAs. The beam emerging from the laser had the properties of a converging wave. In spite of the characteristics of the spatial distribution of the laser radiation, the system with InAs inside the resonator ensured a nearly 100% phase conjugation, i.e., the fronts of the waves counterpropagating in the resonator were conjugate.

An investigation was made of the characteristics of a cw rf-excited CO₂ laser with a diffusively cooled active mixture and a resonator formed from a coaxial waveguide with two plane reflectors. An output power of 14 W was obtained with an efficiency of 3.3%. The possibilities of simultaneously increasing the laser output power, efficiency, and frequency tuning range are discussed.

A planar laser is proposed and investigated theoretically. Its resonator consists of diffraction-coupled regular active and corrugated passive waveguides. An analysis is made for optical waveguides with arbitrary smoothly inhomogeneous transverse distributions of the permittivity. The condition for the generation of surface H and E waves is formulated. Calculations are reported of the dependence of the threshold gain and of the phase mismatch parameter on the resonant coupling coefficient and on other quantities.

Numerical and experimental studies were made of the feasibility of high-contrast regenerative amplification of ultraviolet radiation in an XeCl* laser. Narrow-band (δν≈0.015 cm ^{− 1}) tunable radiation pulses were obtained with an energy of ~ 1 J and a duration of ~ 60 ns. For weak transitions (0–0,0–3) in the B–X band, a contrast of ~ 15 was achieved, whilst for the strong transitions (0–1,0–2) the contrast was higher than 10⁴.

An experimental investigation was made of the stability of the output radiation power obtained from an industrial CO₂ laser with a toroidal resonator. The dependence of the amplitude of pulsations of the output power on the resonator parameters was determined in the frequency range 0–1 MHz. Technical measures reduced these pulsations to 10%.

An investigation was made of picosecond optical pulses generated by an injection semiconductor laser excited by an optoelectronic switch. The switching was performed by a picosecond YAG laser. The use of Cr:GaAs switches with a gap partly closed by a dye made it possible to generate electrical pulses shorter than 40 ps at midamplitude. They corresponded to single (free of relaxational oscillations) optical pulses of 40 ps.

A considerable improvement in the pump parameters of the existing gas-discharge CO₂ lasers with a self-sustained discharge in a dense gas was achieved by replacing traditional metal electrodes with anisotropically resistive electrodes.

An experimental study revealed the existence of an optimal specific energy of initiation of a fluorine–hydrogen mixture by a high-power relativistic electron beam. In the presence of a good catalyst, the relaxation of SF₆ in the active mixture increased greatly the output radiation energy and the duration of lasing.

The results are presented of an experimental investigation of a solid-state ring laser with a stimulated Brillouin scattering mirror and lasing initiated by a series of ~ 200–300 ns pulses of 1.06 μm wavelength. It is shown that this laser may be useful for the development of a source with radiation parameters controlled by an external signal (energy, transverse and time structure) and also of a low-threshold mirror for phase self-conjugation of radiation.

A comparative analysis is made of the time characteristics of the radiation emitted as a result of visible transitions in Hg II and the results are given of mathematical modeling of the active medium of an ionic He–Hg laser. A good agreement is reported between the experimental and calculated data. The results of an analysis show that, under discharge conditions typical of lasing, the contribution of the multistage Penning reaction (involving collisions of metastable mercury and helium atoms) to the pumping of the active levels of Hg II during the afterglow is negligible compared with the contribution of impact-radiative recombination at λ = 567.7 nm and charge transfer at λ = 615 nm. The contribution of Hg²⁺ ions, formed as a result of thermal collisions of singly charged mercury ions and metastable helium atoms, is calculated to be ~ 5%.

Experimental investigations were made of the threshold pump energy of a YSGG:Cr³⁺:Er³⁺ crystal as a function of the filtering wavelength of the pump radiation and the concentration of erbium ions. The contributions of two mechanisms of erbium ion excitation to population inversion of the ⁴I_11/2 → ⁴I_13/2 Er³⁺ transition in a YSGG:Cr³⁺:Er³⁺ crystal were nonadditive, as predicted previously. These mechanisms involved Cr→Er energy transfer and cross-relaxational depletion of the ⁴S_3/2 state of Er³⁺.

An analysis is made of the probability of formation of an active medium capable of emitting radiation as a result of 4–3 transitions in the Be IV ion when pumped with continuous-spectrum radiation from a high-current electrical discharge.

An investigation was made of the luminescence spectra and lasing properties of a calcium lithium niobium gallium garnet crystal activated with chromium and erbium. The lifetimes of the upper (⁴I_11/2) and lower (⁴I_13/2) active levels of the Er³⁺ ion were determined. A study was made of the kinetics of decay of the Cr³⁺ luminescence and it was concluded that energy was transferred from the Cr³⁺ to the Er³⁺ ions. Lasing properties of the crystal (λ_l = 2.707 μm) were studied under free-running conditions at room temperature.

Suppression of amplified spontaneous emission and a 1.5-fold increase in the energy efficiency were achieved in YSGG:Cr³⁺:Nd³⁺ crystals when saturable Cr centers absorbing at a wavelength of 1.06 μm were formed.

An investigation was made of the emission spectrum of a cw dye laser with an intracavity lens-like absorbing medium. The emission spectrum of the frequency-locked laser was obtained as a function of the oscillator strength of the absorption line and the resonator tuning, and measurements were also made of the selective reduction in the losses near the absorption line (10 ^{– 4}–10 ^{– 5} cm ^{– 1}). It was confirmed experimentally that the reason for the frequency locking of the laser radiation was a reduction in the diffraction losses in the resonator under the action of the lens-like properties of the absorbing medium.

Low-frequency self-modulation of the average radiation power, observed in actively mode-locked solid-state lasers, was found to be excited when the glancing angle of a beam incident on an acoustooptic modulator (switch) corresponded to minima of the diffraction characteristic of the modulator. A theoretical model of the mechanism of excitation of such self-modulation is put forward. Synchronization of a sequence of giant pulses can increase the stability of their repetition frequency and reproducibility of the peak power by more than one order of magnitude.

It is shown that the use of a T-shaped resonator and a stimulated Brillouin scattering mirror outside the resonator ensures generation of single-mode high-power giant pulses of YAG:Nd laser radiation.

A laser with a nonlinear resonator is proposed. Phase self-modulation in an optical fiber makes it possible to realize effective passive mode locking. Numerical calculations are used to show that the proposed system is feasible irrespective of the sign of the dispersion and steady-state solutions are found. Since the radiation parameters depend on the nonreciprocal phase introduced into the resonator, such a laser can not only emit short pulses, but can also be used to measure magnetic fields and the rotation velocity.

The results are given of an experimental study of the stabilization of the emission frequency of a cw dye laser with the aid of the pass band of an external thermally stabilized Fabry–Perot interferometer. Two acoustooptic modulators were used to form a dispersive profile of the error signal and this profile had a linear region much wider than the slope of the pass band of the interferometer, which made it possible to achieve stable operation of the laser with a line width not exceeding 100 kHz and continuous tuning in the range ~ 3 GHz throughout the range of stimulated emission of the R101 dye (620–670 nm).

A theoretical investigation is reported of the deformation of the density profile of a plasma by a ponderomotive force under transient conditions. Initially, the structure of the density profile near the critical point coincides exactly with the solution of the steady-state problem. Plasma expansion is accompanied by growth of a spiky instability in the form of stimulated Brillouin scattering.

It was established that when laser radiation, intensity modulated at a frequency of 2.2 GHz, interacted with an optical breakdown plasma which it had created, a microwave component appeared in the thermal emf of the plasma. The amplitude of the microwave thermal emf reached 0.7 V for a laser radiation intensity of 6 GW/cm². Laser radiation with λ_L = 1.06 μm was converted to the microwave range with λ_mω = 13 cm in the optical breakdown plasma. A microwave signal power of ~ 0.5 W was obtained from a laser power of ~ 5 MW.

An experimental investigation was made of the emission of charged particles due to the irradiation of moving steel and graphite targets with cw CO₂ laser radiation. The characteristics of the emission current signals were determined for different laser irradiation regimes. The maximum emission current density from the surface of a melt pool ( ~ 1.1 × 10 ^– 2 A/cm²) and the average temperature of the liquid metal (~ 2040 K) were measured for an incident radiation power density of 550 W and for horizontal and vertical target velocities of respectively ~ 1.5 mm/s and ~ 0.17 mm/s. The authors propose to utilize this phenomenon for monitoring the laser processing of materials.

Thermally induced optical damage (TIOD) was observed in undoped barium–sodium niobate (BSN) crystals as a result of changes in their temperature. This damage was deduced from the behavior of YAG:Nd³⁺ laser radiation when a BSN crystal was inserted in the resonator and also using a helium–neon laser probe beam. The experimental results were satisfactorily explained by the familiar pyroelectric model of TIOD and, in the crystals studied, an inhomogeneity of the conductivity rather than an inhomogeneity of the pyroelectric constant played the main role.

Calculations are made of the divergence of amplified spontaneous radiation in a laser plasma allowing for refraction by free electrons. An analysis is made of the divergence of the radiation generated due to a 3p→3s' transition in neon-like ions. Calculations are made of the divergence of the radiation due to the 4→3 transition in the O VIII ion allowing for refraction during expansion of a Formvar plasma.

An investigation was made of the interaction between high-power cw and pulse-periodic CO₂ laser radiation and a low-threshold optical breakdown plasma near a metal surface. Characteristics of the breakdown plasma were studied as a function of the experimental conditions. A qualitative analysis was made of the results using a simple one-dimensional model for laser combustion waves.

An investigation was made of the enhancement in the efficiency of generation of currents when a target in air was subjected to two consecutive CO₂ laser radiation pulses. Preliminary interaction with a low-energy (1.5–5 J) pulse increased by more than one order of magnitude the currents generated by the second pulse and this was true in a wide range of energies of the latter pulse. The energy conversion efficiency was practically unaffected. The results were in qualitative agreement with the proposed pattern of plasma formation and propagation of shock waves near a target.

A method proposed for investigating the statistics of bulk optical breakdown relies on multifrequency lasers, which eliminates the influence of the laser radiation intensity statistics. The method is based on preliminary recording of the peak intensity statistics of multifrequency laser radiation pulses at the caustic using the optical breakdown threshold of K8 glass. The probability density distribution function was obtained at the focus for the peak intensities of the radiation pulses of a multifrequency laser. This method may be used to study the self-interaction under conditions of bulk optical breakdown of transparent dielectrics.

An investigation was made of some characteristics of the formation of the structure of Al–Si alloys containing 10%, 12% and 20 % Si, and also of the commercial alloy V124 under conditions of surface fusion by laser-arc and laser sources. It was established that as a result of local fusion there was a change in the silicon deposition morphology, the α solid solution became oversaturated, and the eutectic point was shifted toward high silicon concentrations. It was found that the hardened layer retained its high hardness when treated at temperatures up to 250 °C. The commercial alloy V124 was used as an example to show that an alloyed layer with a controlled silicon concentration can be obtained on the surface by using a laser-arc or laser source.

A theoretical investigation is reported of the behavior of optical pulses under quasiresonant adiabatic tracking conditions. It is shown that dispersion of the group velocity and saturation of the populations of the resonant levels may give rise to soliton propagation of light. The exact solution for the envelope of a soliton is obtained for the first time subject to the saturation effect. Expressions are derived for the critical energy density in a soliton and for its duration. A study is made of the influence of the resonant saturation on the shape of a soliton envelope and its parameters. An analysis is reported of the behavior of light pulses of energies higher and lower than the critical value. It is shown that these pulses exhibit three types of quasisoliton propagation: weak, strong, and steady-state. The results for the pulses are generalized to two-dimensional light beams in a nonlinear waveguide.

An analytic solution is obtained of the problem of nonlinear p-type surface polaritons, which appear near an interface between isotropic media, one of which is linear and the other has a cubic polarization nonlinearity. Expressions are obtained for describing one-dimensional (along the normal to the plane interface) spatial evolution of the field amplitudes of steady-state nonlinear surface polaritons and also their energy characteristics. An analysis is made of the dispersion relationship for such polaritons and of the properties and conditions under which they exist.

Single crystals of p,p'-dihydroxydiphenyl sulfone were grown. The components of the nonlinear susceptibility tensor and the dispersion of the refractive index were measured. Promising applications of these crystals in nonlinear frequency conversion were analyzed.

An analysis is made of the influence of thermal self-interaction of long pulses (of duration greater than 100 μs) on the efficiency of phase conjugation in stimulated Brillouin scattering. It is found that thermal self-defocusing of the pump radiation results in a mismatch between the Stokes wave formed at the constriction and the pump wave during propagation toward the entry end of the cell. This increases the divergence of the Stokes beam and reduces the phase conjugation coefficient. A reduction in the peak pump power in the constriction results in strong attenuation of the Stokes output beam.

An analysis is made of resonant self-focusing of high-intensity laser radiation in a medium consisting of two-level atoms and exhibiting a homogeneously broadened transition. An ordinary nonlinear differential equation with exponential factors is used to describe the change in the average radius of the beam. It is shown that in the absence of absorption this average radius oscillates periodically. The spatial period of the oscillations is determined. Calculations are made of the transverse and longitudinal dimensions of a caustic of nonlinear foci. It is shown that the dependence of these dimensions on the detuning from a resonance is V-shaped, whereas the dependences on the density of the medium, on the radius of curvature of the beam, and on the average radius of the beam at the entry to the medium are monotonic.

An investigation was made of parametric generation of high-quality (τΔν ≤ 1) picosecond light pulses of wavelengths in the range 0.672–2.56 μm by pumping an optical parametric oscillator with a dispersive resonator by the second harmonic of radiation from a cw actively mode-locked and Q-switched YAG:Nd³⁺ laser. The duration of the parametric oscillation pulses was 15–45 ps and the width of the emission spectrum did not exceed 1 cm ^− 1 throughout the working range of wavelengths. The maximum energy efficiency of the investigated optical parametric oscillator was 8.1%.

Time scanning with an image-converter camera revealed fluctuations of the phase of the conjugated wave. Shifts of the focusing spot of the output beam were observed for a multipulse amplifier with phase conjugation: these shifts were due to phase fluctuations of the conjugated wave.

Theoretical and experimental investigations of the characteristics of matching of a planar active waveguide to an external axially symmetric resonator are reported. The dependences of the threshold current on the external resonator length are obtained for stripe injection lasers with the waveguide amplification effect. These dependences are shown to be opposite for a resonator with a parallel beam in the external part and for a system with beam focusing by an external mirror. In the case of lasers with a dispersive external resonator the slopes of the dependences are sensitive to the spectral tuning of a selective element. It is shown that the observed characteristics of the dependences of the threshold pumping rate on the length of the external resonator are related to the change in the curvature of the wavefront of the field during the passage through the external part.

Experimental and theoretical investigations are reported of the response function of a deformable adaptive mirror. The operation of this mirror is described using a model of a thin plate. A numerical modeling is made of the formation, by this adaptive mirror, of surfaces conjugate to primary wavefront aberrations.

A new method for analysis of non-Gaussian optical systems involves their replacement with approximately equivalent Gaussian systems. An algorithm for deriving an equivalent system is proposed. The method is used to analyze the diffraction of a Gaussian beam by an aperture with a partly smoothed edge and the behavior of the fundamental mode of a resonator containing such an aperture. The conditions under which the fundamental mode of a resonator with a finite aperture can be regarded quite accurately as Gaussian are formulated.

Analytic and numerical investigations are reported of the mode structure in an anisotropic resonator composed of two corner-cube total-internal-reflection prisms. It is shown that polarization aberrations transform significantly the mode structure and increase the diffraction losses. In the range of moderately high Fresnel numbers the lowest resonator modes form a group which is quasidegenerate in respect of the losses. The eigenstates of the modes are spatially inhomogeneous and differ from those predicted by geometric optics.

An investigation was made of a working prototype of a continuous elastic mirror intended for modal control based on first- and second-order phase aberrations. A system of six lumped (concentrated) loads acting along the perimeter of a circular mirror clamped at the center produced tilts, resulted in focusing, and induced astigmatism of the wavefront with a satisfactory precision. The use of electromechanical drives to produce the loads made it possible to displace the reflecting surface of the mirror by up to 300 μm.

A new type of transverse mode selector is proposed. It is in the form of an interferometric stop representing a tunable inhomogeneous interferometer in which one of the mirrors contains a variable-thickness dielectric layer. A threefold change in the effective size of the interferometric stop is achieved by control with the aid of a piezoelectric ceramic. In experiments on an He–Ne laser this made it possible to vary continuously the number of the transverse lasing modes, select the fundamental TEM₀₀ mode, and excite axisymmetric higher-order modes. Such an interferometric stop could be used in gas lasers with internal mirrors and in solid-state laser resonators where an iris stop would cause difficulties.