Table of contents

An investigation was made of the energy, angular, spectral, and coherence characteristics of high-power radiation from a neodymium laser, scattered by a plasma corona on the surface of an irradiated solid target. The high homogeneity of the beam formed from the scattered radiation was due to the loss of coherence on a reflecting surface which acquired a rippled structure. An analysis was made of the possibility of using plasma filters in laser fusion experiments to provide the required homogeneity of irradiation of spherical targets.

Quantum-mechanical equations are derived for the amplitudes of the probabilities of transitions in a free-electron laser when transverse electron oscillations are due to a static potential, which is homogeneous in the direction of motion of the beam, but has a transverse gradient. The resonance frequency of the system is found and the linear gain is derived for the odd harmonics of this frequency. Averaging of the gain is carried out over the initial distribution of electrons in a transverse cross section of the beam. Estimates are obtained of the maximum lasing frequency and of the gain at this frequency.

A high-pressure ammonia laser pumped transversely by the 9R(30) line of a TEA CO2 laser was constructed and investigated. Lasing was observed at frequencies 828 and 868 cm−1 when pressure in a 14NH3−14N2 mixture was up to 4 atm and at a frequency 777 cm−1 when the pressure in a 15NH3–14N2 mixture was up to 1.5 atm. The maximum output energy of the 14NH3 and 15NH3 isotopic species was 21 and 5 mJ, respectively, when the volume of the active region was 4 cm3 and the divergence of the output radiation was 20 mrad. At pump intensities of 1 GW/cm2 it was possible to achieve lasing as a result of P(J,K) transitions, where J ≥ 5 and K = 0–4 from the 14NH3–14N2 mixture at pressures up to 10 atm, so that continuous tuning of the emission frequency of the laser was possible within 6.3 cm−1 .

The characteristics of a thin-film Bragg transducer with an opposed-comb electrode system and a period of 8μ, deposited on the surface of a diffused cadmium sulfide waveguide, were calculated and studied experimentally. The characteristics were obtained for the acoustooptic and electrooptic interactions. The control voltage of an electrooptic modulator which ensured the maximum amplitude of the diffracted signal was 17.5 V and the bandwidth at the 3 dB level was Δƒ=660 MHz. The control acoustic power ensuring 100% transfer of the optical power from the zeroth diffraction order was 15 mW and the operating frequency range of the acoustooptic modulator was 10 MHz.

The characteristics of XeF and XeCl lasers stabilized by an external low-power ionizer were considered. Two laser operating regimes were investigated, one with a buildup of negative ions and the other with a discharge stabilized by an external low-power ionizer. The temporal, spectral, and energy characteristics of these lasers were investigated. The minimum ion concentration in the negative-ion buildup regime was found to be [F⁻]=10¹¹cm⁻³ and [Cl⁻]=10¹²cm⁻³, while the use of a discharge stabilized by a low-density electron beam made it possible to reduce the current density of the electron beam to ∼0.1mA/cm² in the case of the XeCl laser. A value of 10 J/liter was achieved for the maximum specific output energy of the XeCl laser.

A theoretical investigation is made of an essentially nonstationary double resonance excited in a system with a common upper level in the field of a 2π pump pulse and of constant low-intensity signal radiation. The conditions are found for the formation of an exponentially amplified signal pulse of asymptotically constant profile moving at the velocity of the pump pulse. A study is made of the relationship between the signal pulse parameters and the parameters of the medium and of the pump pulse. It is shown that competition between the local and wave transient processes is responsible for the existence of an optimal pump pulse energy which depends on the values of the detuning from a resonance in the pump and signal channels. When this optimal energy is exceeded, the output signal pulse energy decreases. It is demonstrated that when the initial values of the detuning in the two channels are unequal the formation of a signal pulse is accompanied by pulling of the signal frequency to the value which satisfies the conditions of equality of the detuning values. During the early stages of formation of the signal pulse this results in splitting into a train of several pulses which are shorter than the pump pulse.

An experimental investigation was made of the use of liquid-crystal phase correctors of wavefronts. Prototype correctors were prepared with independent addressing of the elements and these were found capable of compensating distortions of wavefronts of radiation of wavelengths 0.5–6μ.

It is shown that the asymmetric nature of the radiation emitted by a self-terminating neon laser (λ = 614.3 nm) is a consequence of the propagation of ionizing potential gradient waves that ensure the traveling nature of the excitation of a gas. A solution of the Maxwell-Bloch equations yields formulas relating the delay between the appearance of superradiance pulses from the opposite ends of a source to the velocity of propagation of these waves. Estimates obtained of the velocity of propagation of ionizing potential gradient waves in neon are in agreement with the available experimental data.

A study is made of the problem of stimulated Raman scattering (STRS) in a resonant medium pumped by a strong pulse satisfying the adiabaticity condition. It is shown that in the absence of relaxation the maximum STRS gain is reached during the tail (trailing edge) of the pump pulse, which is the reason for the absence of the Stark shift. If the pump pulse is considerably longer than the relaxation time, the lag of the STRS pulse behind the pump pulse is short and then the Stark shift is manifested in the spectrum and a characteristic interference structure is formed. Noise and monochromatic "seeding" variants are considered.

Theoretical and experimental investigations were made of the dependence of the absorption of infrared radiation by ozone on the intensity of such radiation in the range from 1 kW/cm² to 3 GW/cm². The nonlinear absorption spectra of ozone were recorded by switching between the CO₂ laser emission lines in the range 970–1090 cm⁻¹. A comparison of the theory with the experimental results was made on the basis of an absorption mechanism typical of triatomic molecules. The effects of lifting of saturation of the rotational (bottleneck effect) and vibrational absorption transitions were studied. Two-photon and cascade (multistage) processes were considered.

The results are given of numerical modeling of the dynamics of growth of a self-sustained discharge in a mixture of the He:He:HCl = 100:1:0.2 composition (p = 2 atm). The fullest possible allowance is made for the great variety of all the elementary processes which occur in the plasma of an excimer laser. It is shown that the duration of emission in the case of a low photon density resonator is limited by the growth of a multistage ionization instability in the presence of a spatially inhomogeneous perturbation of the electron density across the current. At high values of the photon flux density the duration of emission is governed by the depletion time of the halogen carrier.

A theoretical study is reported of the possibility of simultaneous lasing as a result of two transitions corresponding to the wavelength of 10.6μ in the case of the CO₂ molecule and 38.3μ in the case of the CS₂ molecule when an N₂–CO₂–CS₂ mixture is pumped by transfer of energy from N₂ molecules excited in an electric discharge. The level populations and the gains are calculated for the vibrational transitions 00⁰1–10⁰0(CO₂) and 10⁰0–01¹0 (CS₂). The predicted gains are of the order of 0.1m⁻¹, indicating that two-wave lasing is possible.

Theoretical and experimental investigations were made of the intensity kinetics and of the emission spectrum of a laser operating in the regime of transverse mode locking. A study was made of the role of phase relationships between the modes and the symmetry of the spatial-temporal scanning of the intensity in the near-field zone was analyzed. It was found that the spatial coherence of a superposition of degenerate modes is complete irrespective of deformations, amplitude-phase relationships, and mode parity. It was found experimentally that locked generation of transverse modes is the most probable operating regime of a neodymium laser with a positive electrooptic feedback.

A new absolute method was developed for the determination of the quantum efficiency of the fluorescence of organic dye solutions. The method is based on determination of the fraction of the excitation energy converted into heat from the efficiency of diffraction of probe radiation on a thermal grating of the refractive index induced by laser pulses in a dye solution. The efficiency of diffraction was practically unaffected by the processes of absorption and reemission of the fluorescence, so that in principle it should be possible to determine the fluorescence quantum efficiency more accurately than by traditional calorimetric methods. The new method is best suited to strongly fluorescing substances. A report is given of measurements of the fluorescence quantum efficiency of solutions of several dyes used widely in lasers and the results are in good agreement with the published data. The error in such measurements is 1.5% for substances with high quantum efficiencies.

A theoretical model is proposed for the active medium of a chemical oxygen-iodine laser containing H₂O, H₂O₂, and Cl₂ as quenching impurities. The main rules of the behavior of the gain and of the amplification length in the downstream direction are determined. Numerical calculations are used to show that values of the gain amounting to 0.1-1m⁻¹ can be achieved if the oxygen pressure in the resonator is 4–8 Torr and the partial pressure of iodine vapor is up to 20–40 mTorr. Degradation of the active medium as a result of an increase in the oxygen pressure can be prevented by maintaining the relative concentrations of the quenching impurities and aerosols at suitable levels.

An analysis is made of two methods for phase correction of the wind refraction of a light beam traveling in a medium containing droplets which can be evaporated because of interaction with the beam. It is shown numerically that programmed or adaptive control of the phase makes it possible to improve considerably the quality of focusing of the beam. It is demonstrated that convergence of the phase conjugacy algorithm in adaptive focusing of a beam deteriorates on increase in the water content of the medium.

Reproducible generation of steady-state ultrashort pulses (of ∼7.4 psec duration) was achieved for the first time in a pulsed ruby laser with passive mode locking. The lasing was stabilized by a slow-response negative feedback established by an electrooptic switch. An experimental study was made of the formation of steady-state ultrashort pulses from initial radiation fluctuations. The proposed method should be suitable for the generation of reproducible pulses of ∼100 fsec duration in wide-band amplifying media.

The results are reported of the optimization of a TEA CO₂ laser emitting the lines of the P branch in the 01¹1–11¹0 hot band. A maximum gain of 0.7 m⁻¹ was obtained in a CO₂:N₂:He=1:0.8:1 mixture at a total pressure of 220 Torr and with a specific input energy (into all the vibrations of the CO₂ and N₂ molecules) of ∼265J·liter⁻¹·atm⁻¹. The output energy of 2.3J obtained in the P(19) line of the hot band was ∼40% of the energy in the strong lines of the conventional 00°1–10°0 band in this system under optimal conditions.

Frequency-modulation resonances were used for the first time as reference points in stabilization of the emission frequency of ring lasers. These resonances appeared when the ring laser perimeter was modulated at a frequency greater than that corresponding to the homogeneous width of a line of an intracavity absorbing gas. The use of frequency-modulation resonances of the saturated dispersion made it possible to eliminate practically completely the influence of instability of pressure in the active medium on the reproducibility of the ring laser frequency.

An effective method for increasing the spectral brightness of a tunable organic dye laser was based on the use of two coupled resonators with electrooptic switching of the Q factor during the process of pumping. When the width of the lasing spectrum was 2.5 pm and the output radiation was strongly polarized, the near-maximum (for this laser) efficiency was achieved and its value was 8.5 times higher than the efficiency of a similar laser with a single dispersive resonator.

A report is given of the first operation of a copper vapor laser with a self-pumped wavefront-reversing mirror based on a four-wave interaction with the active medium and free of additional devices for the self-excitation of the laser. The steady-state energy characteristics are calculated and an analysis is made of the condition for the occurrence of cw lasing. It is shown that in the case of the degenerate four-wave interaction such cw lasing is possible only in the case of some saturation of the medium at a frequency coinciding with the center of the gain profile of the medium.

A study was made of the evolution of the spectral composition of the green emission lines λ=534.1 and 542.0 nm of a self-heated manganese vapor laser during an output pulse. The total average output power of these lines was 200 mW, the line at 534.1 nm consisted of four spectral components, and the line at 542.0 nm consisted of two. The half-widths of the individual components of the lines varied during a lasing pulse in the range 0.02-0.04 cm⁻¹ and the sequence in which they appeared was directly related to the relative intensity of the corresponding hyperfine structure components of the active transitions in manganese.

A study was made of the mechanism of inactive losses in γ-irradiated LiF crystals in the spectral region of the absorption by the F₂⁻ centers. The inactive losses were not determined by the processes of nonlinear absorption and transfer of energy from an excited state of the F₂⁻ centers to impurity ions. The results obtained indicated that the main contribution to the inactive losses was due to attenuation of light by colloidal lithium particles.

A theoretical investigation is made of parametric amplification of light pulses in nonlinear crystals under conditions of group velocity mismatch and dispersion spreading. It is shown that dispersion spreading has a strong influence on the amplification. As the role of such spreading increases, the saturation of the gain due to the group velocity mismatch gradually disappears and a steady-state amplification regime is established when the shape of the envelope, and the duration and position of the peak of a signal pulse are all independent of the length of a crystal, whereas the intensity rises exponentially with the distance.

The six-beam Progress laser facility was used in a study of compression and heating of glass microbubbles filled with a DT gas and coated outside by a thin (~0.1μ) aluminum film. The power density of Nd laser radiation (λ=1.054μ) reaching the target surface was (1–2)×10¹⁵ W/cm² and the specific energy deposited in the target was 0.1-0.25 J/ng. Two-dimensional images of a laser plasma were recorded using x-ray lines of multiply charged Si and Al ions. Strong Al lines observed at distances close to the target center were evidence of a strongly unstable nature of compression of the laser plasma. This was confirmed also by a weak dependence of the neutron yield on the specific input energy, which did not agree with the results of one-dimensional calculations carried out using the Zarya program.

A theoretical analysis is made of steady-state backward stimulated Brillouin scattering (STBS) in an optical waveguide under conditions such that the attenuation length of hypersound is less than, of the order of, or greater than the length of diffraction spreading of the hypersonic wave. The STBS gain, as well as the line width and profile, are found on the assumption that the acoustic properties of the medium are homogeneous. A relative reduction in the diffraction length l_d increases the line width Δω in a cylindrical waveguide proportionally to l_d⁻¹, whereas the static gain g falls proportionally to l_d ; in a planar single-mode waveguide the corresponding dependences are Δωl_d^−1/2 and gl_d^1/2 .

A study was made of the selective properties of diffused optically controlled waveguides made of FKhS4 and FKhS7 photochromic gasses. The induced attenuation in these waveguides was 30 dB/cm when the control radiation power was 6×10⁻⁴W. It was found that colored diffused waveguides acted as spatial and polarization selectors of waveguide modes, both under dynamic and steady-state conditions.

A simple CO₂ laser with combined electrical and optical excitation was constructed. It emitted in the region of 4.3μ (10⁰1–10⁰0 band). Suppression of lasing in the usual 10–μ band was the result of selection of the composition of the gas mixture and not due to the use of an intracavity cell containing heated CO₂. Tuning of the radiation between the lines of the P and R branches with values of J from 8 to 40 was performed by a diffraction grating. A peak output power of 60 W was achieved for the P(26) line when the pulse duration was 300 nsec.

The first experimental observations were made of the frequency shift and of the change in the spectral composition of picosecond light pulses scattered by optically induced gratings in semiconductors. The observed effect is attributed to the time dependence of the refractive index and to the formation of a nonequilibrium electron-hole plasma.

A discovery was made of a large irreversible change in the refractive index (Δn≈–0.2) occurring in a surface layer about 30 nm thick under the action of high-power ultraviolet laser radiation (wavelength 249 nm, pulse duration 5 nsec, power density 20 MW/cm²).

A theoretical study is made of the possibility of reversal of the wavefront of a light beam in respect of the phase and polarization by degenerate four-wave interactions in photorefractive ferroelectric crystals. It is shown that the polarizations of the signal and reversed waves can be conjugate only under certain conditions which must be satisfied by the intensity, phase, and polarization of the pump beams, and also by the orientation of the optic axis of a crystal. Specific situations when these conditions are satisfied are considered in the case of photorefractive ferroelectric crystals of the 3m and 4mm symmetries.

A study was made of the influence of unstable defects on the generation of laser radiation in YAG:Nd³⁺ crystals. It was found that F⁺ centers and iron impurity ions were the main unstable defects. The influence of these defects on lasing was manifested in different temperature ranges. At temperatures 287-312 K the F⁺ centers predominated, whereas above 312 K the effect was mainly due to iron impurity ions.

The example of stimulated Raman backscattering of picosecond pulses in a CaCO₃ crystal confirmed experimentally the principal estimates of the optimal conditions for pulse compression (shortening) in focused beams. The duration of the second harmonic of a YAG:Nd³⁺ laser was reduced by a factor of 10–20 and at the same time the power was increased by a factor of 4–8. The minimum duration of the Stokes pulses formed as a result of stimulated Raman scattering compression was ∼2 psec.

An experimental investigation was made of the characteristics of the radiation emitted by a laser with an unstable ring resonator containing an angular selector. The conditions were found under which such a laser could be used as a single-mode master oscillator in compact-aperture lasers.

It is shown that the reception of an echo signal by a frequency modulated laser gives rise to beats of the laser radiation intensity. The peak-to-peak amplitude of the beats is governed by the characteristics of the propagation path and the frequency is proportional to the difference between the instantaneous frequencies of the transmitted and reflected signals. The results are given of experimental investigations carried out over atmospheric paths of different length using a CO₂ laser. Measurements are reported of the threshold sensitivity to an external signal in the case of coherent laser reception.

A study was made of the operation of a copper vapor laser and a resonator with a dynamic wavefront-reversing mirror induced in the active medium by a four-wave interaction with pump waves excited in an auxiliary resonator. Radiation with a diffraction-limit divergence was generated in a resonator with a convex second mirror. The energy, time, and spectral characteristics of lasing were determined.

Thirty-two new emission lines were observed in the 178-1345μ range when a submiiiimeter molecular CD₂Cl₂ laser was pumped optically with cw CO₂ laser radiation. The wavelengths, relative intensities, and polarizations of the emission lines were determined.

It is shown that defocusing in plasma layers and filaments results in reduction of the intensity, which can be compensated by amplification in a plasma medium. Defocusing increases somewhat the amplification threshold of a plasma layer or filament, but when this threshold is exceeded, the process of defocusing optimized in respect of its magnitude plays a positive role giving rise to single-mode emission, providing an opportunity for distributed coupling of energy out of a layer, and lifting limitations on the length of the active medium.

Electron stimulated Raman scattering in atomic cesium vapor was used in efficient conversion of the freqeuncy of the second harmonic to the infrared range in the case of picosecond YAG and neodymium phosphate glass lasers. The tuning range of ultrashort pulses was 2.21-2.55μ, the spectral width of the output pulses did not exceed 2 cm⁻¹, and the quantum efficiency of the conversion process was 10%. In addition to tunable ultrashort pulses, picosecond pulses at fixed wavelengths of 3.1 and 0.88μ were generated.

An analysis is made of the threshold conditions for parametric oscillation as a result of frequency degenerate pumping in a semiopen linear resonator. It is shown that the threshold is highly sensitive to an unbalance of the pump wave intensities; in the case of media with a local nonlinear response the optimal parametric oscillation conditions correspond to exact equality of the pump wave intensities, whereas in the case of a nonlocal response the equality of pump wave intensities prevents parametric oscillation altogether. The calculated results are confirmed by experiments on parametric oscillators made of cadmium telluride (local response) and of barium sodium niobate (nonlocal response).

A high-power Ar–Xe laser (λ = 1.73μ) was pumped by an electron beam alone (j ∼ 1A/cm²) and by a combination of an electron beam and an electric field acting on a large volume (30×30×300 cm). An output energy Eout ≈ 9.5J was obtained for the beam pumping (pulse duration τ_0.5 ≈ 2.1/μsec) and Eout = 11.5J for the combined pumping. The efficiency, measured relative to the energy deposited by an electron beam, was 0.4%. It was found that additional pumping by an electric field became significant at the onset of nonthermal ionization of the medium and electron multiplication.