Table of contents

A method for measuring the target potential was used to determine the peak electron temperature of a plasma produced by the action of a laser pulse (of 0.8 ps duration and of contrast in excess of 10¹²) on a metal target. The electron temperature increased linearly with the radiation intensity, reaching 1 keV at 2.3 × 10¹⁶ W cm^-2. This temperature was independent of the polarisation of the optical field incident obliquely on the target. The observed effect was interpreted by inverse-bremsstrahlung absorption in a plasma of density approaching that of a solid.

A four-channel pulse-periodic (25 Hz) laser was investigated. Its main features were compensation of aberration and polarisation distortions in Nd : YAG crystals (15 mm in diameter) and summation of radiation from four-pass channels in a single diffraction-limited beam.

Calculations are made of lasing resulting from the 3 — 2 transition in the C ion VI under the same conditions as in an experimental investigation of the VULCAN recombination x-ray laser at the Rutherford Appleton laboratory. A carbon filament target with a diameter of 7 μm and up to 5 mm long was irradiated with Nd laser pulses (λ =1.053 μm) of ∼ 2.4 ps duration and ∼ 20 J energy. The calculated space-and-time average value of the gain coefficient at the 18.2 nm wavelength was ∼18 cm^-1 for the specific absorbed energy. This was in satisfactory agreement with the experimental results. The optimal lasing conditions for the investigated target irradiation conditions were estimated.

A theoretical and experimental investigation was made of the temporal and spectral characteristics of a tunable LiF crystal laser with F₂^- colour centres. The temporal profiles of the output pulses overlapped considerably the pump laser pulses because of a large amplification cross section. A study was made of the influence of a change in the pump laser wavelength on the spectral and oscillation characteristics of the LiF : F₂^- laser.

An analysis is made of the published data on the discharge and lasing characteristics, and on the level population kinetics of pulsed lasers based on vapours of metals and their halides mixed with hydrogen and HBr additives. The influence of dissociative attachment, particularly to the HBr molecule, on the population inversion kinetics of these lasers is considered. Such attachment makes it possible to explain all the observed features of the investigated lasers, including an increase in the efficiency and power of a CuBr laser with added hydrogen and of a 'HyBrID' laser. The addition of molecules with a large dissociative-attachment cross section and with a suitable position of the maximum of this cross section provides new opportunities for improving the characteristics of pulsed lasers based on r — m transitions between resonance (r) and metastable (m) levels.

A numerical investigation is reported of the energy and spectral characteristics of the generation of the first vibrational overtone of the CO molecule in a high-power supersonic electron-beam-sustained CO laser operating under various conditions. It is shown that the electro-optical efficiency of this laser can exceed 30% for the first vibrational overtone. A study is also described of the efficiency of intracavity selection of the emission lines generated as a result of the fundamental and overtone transitions in the CO molecule. It is shown that the output power of the selected lines can be many times higher than the power of the corresponding lines generated by a free-running laser.

A Co : MgF₂ crystal laser was pumped with radiation (λ =1.35 μm) from a neodymium glass laser. This resulted in generation of radiation of 1.6 J energy with a quantum efficiency of 67%. The Co : MgF₂ crystal could thus be used for efficient conversion of λ =1.3 μm laser beams, such as those from iodine photodissociation and chemical oxygen — iodine lasers.

A method for detection of laser damage to the surfaces of optical materials and coatings was investigated. The damage was detected with the aid of a gas discharge induced by the products of a laser ablation plume. A suitable selection of the parameters of the detection system made it possible to investigate laser surface damage at radiation intensities W>130 MW cm^-2 (λ = 1.06 μm, t_p=10 ns). A special feature of the electric response was that the result was determined solely by the occurrence of damage and not by its nature or by the laser damage threshold. This feature makes the method suitable for automatic damage detection systems used in statistical investigations.

An investigation was made of changes in the dissociation yield of UF₆ molecules from pulse to pulse in the radiation field of a pulse-periodic CF₄ laser. The energy per pulse was 25 — 130 mJ and the UF₆ pressure was 0.3 — 0.6 Torr. The dissociation yield was proportional to the volume of the focused-radiation zone inside a cell where the photon energy density Φ exceeded the threshold value Φ₀ throughout the investigated ranges of the energy per pulse and of the UF₆ pressure. The first pulse dissociated 28% ± 3% molecules in this zone for Φ₀=1 J cm^-2 and 53% ± 5% of molecules for Φ₀=1.5 J cm^-2. A steep fall of the UF₆ photodissociation yield from pulse to pulse was observed for the first time. An analysis was made of the processes which accompanied dissociation and estimates were obtained of the influence of the relevant reactions on the observed fall of the UF₆ photodissociation yield from pulse to pulse.

The MNDO quantum-mechanical method and the framework of a model of an isolated molecular cluster were used in a study of the properties of a defect representing a doubly-negative charged ≡ Ge — O — Ge ≡ bridge in glassy silica. It was found that such a defect has optical absorption bands near 600 and 400 nm, and that absorption in these bands excites fluorescence near 650 nm. The defect also has an anomalously high polarisability, which is at least two orders of magnitude higher than the local polarisability of the glass network. A model describing a change in the refractive index of glass under the action of UV radiation is proposed on the basis of the calculated properties of the defect.

An analytic solution is obtained of the wave equation describing the propagation of a femtosecond laser pulse with a Gaussian temporal profile in a nonlinear dispersion-free instant-response medium, considered in the approximation of a weak nonlinearity and taking account of the wave reflected at the input.

A study is reported of the dynamics of mutual transformations of transverse field structures, associated with optical bistability, in a thin wide-aperture Fabry — Perot interferometer filled with a Kerr nonlinear medium.

Picosecond pumping was used to excite for the first time multifrequency parametric Raman oscillation in a crystal of leucosapphire α-Al₂O₃ based on the SRS-active vibrational mode A_g1 with ν_R ≈ 419 cm^-1. The recorded Stokes (up to seven) and anti-Stokes (up to six) SRS components were identified.

The high-frequency temporal structure of probe-laser and phase-conjugated signals, generated in the course of degenerate four-wave mixing of long pulses from CO₂ and CO lasers in their own inverted media, was investigated experimentally with nanosecond resolution throughout the whole pulse. Single-frequency (corresponding to one vibrational — rotational transition) and multifrequency pulses of ∼10 — 20 μs and ∼ 200 — 300 μs durations, generated respectively by electron-beam-controlled discharge CO₂ and CO lasers, had a complex temporal dynamics with a depth of modulation up to 100%. In one round-trip period (∼100 ns) the pulses from these CO₂ and CO lasers consisted of several spikes (free-running mode) or of one spike (self-mode-locking with the aid of a plasma mirror) of τ_0.1 ∼10 ns duration. The temporal dynamics of the phase-conjugated signal also had a complex structure, differing from the probe signal structure over longer (in excess of 100 ns) and shorter time intervals. The relative influence of the amplitude and phase mechanisms of the formation of transient diffraction gratings in the active medium, and also the influence of large- and small-scale diffraction gratings, and of the temporal synchronism on the dynamics of the phase-conjugated signal were considered.

Theoretical and experimental investigations were made of the frequency of relaxation oscillations and of the operational dynamics of a solid-state ring laser when the frequency of self-modulation oscillations was varied within a wide range by altering one of the feedback coefficients of counterpropagating waves. The generally accepted formula for the relaxation frequency ω_r = [(ω/Q) η/T₁]^1/2 is valid only in the limited range of self-modulation oscillation frequencies; here, (ω/Q) is the cavity bandwidth, η is the excess above the pumping threshold, and T₁ is the relaxation time of the population inversion. A strong interdependence of the frequencies of self-modulation and relaxation oscillations, as well as mutual locking of these frequencies in the region of a parametric resonance were observed. Period doubling of self-modulation oscillations and dynamic chaos were investigated in the region of a parametric resonance between self-modulation and relaxation oscillations.

A method for generating multiband tunable radiation in TEA CO₂ lasers is described. Simultaneous lasing in all four branches of the 10⁰0← 00⁰1 and 02⁰0← 00⁰1 vibrational — rotational bands of CO₂ was achieved. Frequency tuning in each of the bands in multiband lasing was ensured by introduction of selective absorbers (SF₆, CF₂HCl, CF₃I) and/or parallel CaF₂ or BaF₂ plates into a two-mirror laser cavity. The frequency in a selected band was tuned with the aid of an additional cavity formed by a diffraction grating, a beam-splitting plate inside the main cavity, and one of the laser mirrors. This additional cavity made it possible to control also the energy of a pulse at the resonance frequency and to reduce (by no more than 50 ns) the delay time of this pulse (relative to a discharge-current pulse), compared with the delay times of pulses at other frequencies. The results obtained were used to propose schemes for two-frequency isotopically selective dissociation of molecules inside a nonselective cavity of a TEA CO₂ laser (without a diffraction grating).

Experimental and theoretical investigations were made of dynamic chaos in a solid-state ring laser with periodically modulated pumping. Synchronised dynamic chaos in a bidirectional ring laser was demonstrated for the first time. Regions of existence of strange attractors of two types, synchronised and nonsynchronised, were found in the plane of the system parameters (pump frequency and modulation depth). Competition between these attractors was discovered. The spectra of the Lyapunov exponents and the information dimensionality were calculated for the attractors of both types.

An investigation was made of the cause of the instability of the self-modulation frequency ω_m of a monolithic Nd : YAG ring laser. One of the main reasons for this instability was a strong temperature dependence of the effective coupling coefficients of counterpropagating waves characterising extracavity components. A method for weakening such coupling was proposed and implemented experimentally. This method made it possible to reduce the instability of the self-modulation frequency by over two orders of magnitude.

A strong (by a factor of almost 100) narrowing of the spectrum was observed in quasi-stady operation of a solid-state laser with neodymium-doped potassium gadolinium tungstate when the cavity contained a negative feedback loop and a slow-response saturable absorber. The role of a nonlinear narrow-band selector, which reduced the width of the spectrum, was played by a periodic saturated-absorption grating formed by a standing intracavity laser radiation wave in a lithium fluoride crystal containing F₂^- colour centres and used as a saturable absorber.

The eigenfrequencies of a loop stimulated-Brillouin-scattering (SBS) laser and their dependence on the loop length were determined experimentally. The conditions under which a change in the loop length resulted in continuous frequency tuning over the whole gain profile were found. A theoretical model based on a mode theory of SBS with spatially inhomogeneous pumping was found to be in qualitative and quantitative agreement with the experimental results.

An investigation is made of the geometric phases in a ring laser with counterpropagating waves. It is shown that the frequency splitting of the counterpropagating waves that appears in the case of radiation pulsations or is induced by displacement of an external mirror can be regarded as a manifestation of a geometric phase.

An experimental investigation was made of the dependence of the optical density of a layer of a strongly scattering medium on its thickness in the case of a model aqueous milk solution. The radiation sources were IR lasers generating pulses of various (from nanosecond to femtosecond) durations. The dependences of the attenuation coefficients of such pulses on the solution concentration were determined for low and high optical densities of the solution layer. Estimates were obtained of the absorption and scattering coefficients of the medium. A modification of the two-flux Kubelka — Munk model was used to derive an expression describing the dependence of the transmission of a solution layer on its parameters when radiation detectors with a finite angular aperture are used. A comparison of the calculations and experiments revealed a forward scattering anisotropy of short-duration laser radiation in an aqueous milk solution.

An interferometric method was used for the first time to detect, under surface-plasmon resonance conditions, phase changes in a reflected beam caused by changes in the refractive index of the medium being diagnosed. The threshold of the sensitivity to changes in the refractive index was estimated to be 4 × 10^-8. The proposed interferometer can be used successfully in bio and chemical-sensor systems.