Table of contents

A new laser based on an orthorhombic Er³⁺ : BaLu₂F₈ crystal was developed. This laser generated cw λ =3 μm radiation at 300 K when pumped with an InGaAs laser diode.

It is shown for the first time that consecutive three-frequency interactions of waves with multiple frequencies ω, 2ω, and 3ω in periodically inhomogeneous and quadratically nonlinear crystals, such as lithium niobate, can result in highly efficient conversion of the energy of a pump wave of frequency 2ω into the energy of a wave of frequency 3ω.

UV laser irradiation and hydrostatic compression of germanosilicate glass resulted in different changes in the glass structure. Modifications of the glass structure by UV irradiation were localised near dissociated germanium oxygen-deficient centres. Destruction of these defects was assumed to stimulate formation of threefold rings containing Ge — O — Ge bonds.

The mass of a photon beam, equivalent to its energy, forms a gravitational potential well (an induced waveguide) in vacuum and this leads to the appearance of a self-sustained photon jet in free space. A calculation of this effect, which is analogous to the familiar self-focusing of light in nonlinear material media, is made in the paraxial approximation.

A theoretical investigation is reported of the interaction of light with a system of two films which contain two-level centres and whose thickness is less than the incident radiation wavelength. It is shown that when light of the same intensity is incident from both sides of the films, symmetry-breaking bifurcation is possible, i.e. the transmitted fields are different for the opposite directions. The relationship between symmetry-breaking bifurcation and optical bistability is determined. If the distance between the films represents an odd number of the optical half-wavelengths symmetry breaking occurs at a lower value of the parameter representing the interaction nonlinearity than that corresponding to the onset of bistability. The opposite is true when an even number of half-wavelengths can be fitted between the films. The supression of symmetry-breaking bifurcation is illustrated for various intensities of the incident optical field.

An investigation was made of a laser (Nd : YLF/Nd glass) laser system generating pulses of 1.5 ps duration and up to 0.4 TW power at the 1053 nm wavelength. Chirped pulse amplification in a multipass amplifier was used. The amplifier had a misaligned conjugate cavity and a net gain of ∼ 4 × 10⁶ , as well as two additional amplifiers with an output aperture 20 mm in diameter. After optimisation of the parameters of this multipass amplifier the angular divergence of the output radiation was close to the diffraction limit and the average intensity was up to ∼ 10¹⁷ W cm^-2 in a focused beam. This laser system was developed as the frontend component of a multiterawatt laser facility under construction for research on the interaction of ultrastrong fields with matter.

An experimental investigation was made of the energy and gasdynamic characteristics of a supersonic cw chemical HF laser with three radial-expansion nozzle arrays (nozzle — sonic injector configuration) of 35 cm × 10 cm dimensions, but differing in respect of the geometric expansion ratio of the nozzles. The distributions of the Mach number, of the static pressure, and of the total pressure behind a plane shock wave were determined in the flow field of the active medium. An increase in the geometric expansion ratio of the nozzles, subject to a constant pressure at the oxidant exit, i.e. an increase in the Mach number of the oxidant flow did not increase significantly the total pressure in the flowing active medium because of large pressure losses during flow through the nozzles and in the course of mixing with hydrogen jets. An increase in the calculated Mach number of the oxidant flow from 4.2 to 5.4 reduced by 26% the specific output energy of the laser, tracking the concurrent increase of the static pressure in the lasing zone.

It is shown that a self-sustained volume discharge is possible in mixtures of SF₆ with hydrocarbons (or deuterated hydrocarbons) without special devices for the preionisation of the gas. This can be achieved with an extremely compact electrode system. Wide-aperture nonchain HF (DF) lasers with a high output radiation energy can be constructed on the basis of the discharge. It is reported that output energies of 144 J for HF and 115 J for DF were achieved in a system with a 21-litre active medium and an aperture of ∼15 cm. The electric efficiencies were 2.8% and 2.2%, respectively.

An analysis is made of the results of an experimental investigation of a high-power electric-discharge KrF laser with UV preionisation. It is shown that the average output power of the laser increases with increase in the pulse repetition rate up to 620 Hz, at which the maximum (for the investigated class of laser) average power of 630 W is reached. The results demonstrate a realistic promise of constructing an industrial KrF laser which has a stabilised output power of 300 — 500W. This laser can operate continuously when the stabilised average output power is 300 W: there is no need to replace the gaseous mixture for 8 h (12 × 10⁶ pulses). The pulse-to-pulse instability of lasing is then ~ 2%.

The problem of the operational stability of a KrF laser with an average output power of at least 600 W was investigated. An experimental study was made of the dependences of the rms deviation σ of the output energy on the charging voltage, on the pulse repetition rate, and on the operating time. The value of σ varied from 1.2% to 6.0%, depending on the experimental conditions. For an average power of ∼ 600 W, the deviation σ did not exceed 3.2%.

Lasing of ethylene oxide (C₂D₄O) molecules was achieved for the first time by optical pumping. This resulted in the emission of 26 submillimetre lines in the 244 — 1063 μm range. The submillimetre laser was pumped by a cw CO₂ laser of up to 20 W power. The results are given in the form of data on the pump lines, the corresponding submillimetre emission wavelengths, their output intensities, and their polarisation relative to the pump-radiation polarisation.

The absorption spectra of titanosilicate glasses with the titanium concentration ∼ 0.1 — 3% by weight were investigated in the wavelength range 225 — 460 nm. These glasses were synthesised in a steady laser plume in an atmosphere of air or nitrogen at normal pressure. Synthesis in these two atmospheres gave rise to Ti ions of different valence in the glasses. For the same Ti concentration the intensity of the characteristic absorption bands of titanium in its reduced states (Ti²⁺, Ti³⁺) with maxima at λ ∼ 278 and 306 nm were 3 — 6 times higher when the synthesis took place in nitrogen. The enhancement of the 278 nm band was approximately half that of the band at 306 nm. Clear-cut strong absorption in the UV range at 260 — 280 nm as well as strong absorption in the visible range at 400 — 500 nm, observed for the first time for titanium-doped silica glass, were the results of a record-high concentration of Ti³⁺ ions, compared with the concentrations found in bulk glass samples prepared by other technologies such as melting, reduction of Ti⁴⁺ ions, and plasma hydrolysis.

The luminescence spectra of titanosilicate glasses, grown in a steady laser plume in an atmosphere of air or nitrogen at normal pressure, were investigated in the spectral range 260–500 nm. The luminescence was excited by the second harmonic of a cw Ar⁺ laser (λ =257nm) and was observed for glasses synthesised in air and nitrogen atmospheres throughout the investigated range of the titanium concentrations by weight (from ∼0.1% to ∼6%). When the titanium concentration was less than 1%, the luminescence spectrum of glass grown in a nitrogen atmosphere had a band with a maximum at λ ∼ 280 nm associated with silicon oxygen-deficient centres and a correlated new strong band with a maximum at λ ∼ 380 nm. An increase in the Ti concentration from ∼0.1% to ∼0.4% resulted in an exponential fall (by a factor of ∼4) of the intensity of the 280 nm luminescence band accompanied by a rise of approximately 30% in the intensity of the luminescence in the 380 nm band. Luminescence in the range 350–500 nm was observed in glass grown in a nitrogen atmosphere when the titanium concentration was ∼1%—2%. Glass grown in air there had a wide band in the luminescence spectrum and its maximum was near ∼ 440 nm (titanium concentration ∼2%) and a band attributed to the Ti⁴⁺ luminescence (with a maximum near ∼420 nm when the titanium concentration was 3%—6%).

Experimental and theoretical investigations showed that dynamic gain saturation in the presence of self-phase modulation shifts the emission wavelength of ultrashort pulses to the red or blue part of the spectrum of lasers mode-locked by self-focusing. Selection of the ratios of the rates of saturation of the gain and losses in the presence of such a shift makes it possible to increase mode-locking stability.

Analytic expressions for the parameters of relaxation oscillations in a cw 2-μm, Cr,Tm,Ho : YSGG crystal laser are derived and analysed. They are compared quantitatively with the corresponding parameters of a 2-μm Cr,Tm : YSGG crystal laser. Significant differences are found between the damping decrements. The experimental and theoretical results are compared and the different operational dynamics of the Ho³⁺ and Tm³⁺ ion lasers explained.

A model is proposed of passive Q switching in a laser by a crystal switch with phototropic centres. Expressions for the output energy and duration of a giant pulse are derived taking account of the geometric factor representing the distribution of the orientations of these centres relative to the crystal lattice of the passive switch. The calculated results are in agreement with those obtained in an experimental investigation of passive Q switching in a neodymium laser with a switch in the form of a doped LiF :F₂^- or Cr⁴⁺ : YAG crystal.

An investigation was made of the causes of the low precision of phase locking of two orthogonally polarised low-energy beams. Configurations capable of at least 95% phase locking of collimated beams with an overall stimulated Brillouin scattering increment of 2 — 3 and of focused beams with a total energy equal to 3 — 4 thresholds of stimulated Brillouin scattering were proposed and implemented experimentally.

A CO₂ laser emitting two longitudinal modes and containing a saturable absorber in its cavity is considered. Numerical simulation is used to predict regular generation of intensity pulsations and it reveals the possibility of occurrence of transitions between three equilibrium states.

A description is given of a repetitively pulse source of narrow-band coherent radiation with continuous frequency tuning in the ranges 1.5–1.85 μm and 2.5–3.7 μm, and with energies per pulse up to 8 and 3 mJ, respectively. The source is based on parametric amplification of a weak IR signal from a difference-frequency generator or from a single-mode optical parametric oscillator. This source was used to determine the spectra of IR absorption and of degenerate four-wave mixing, as well as the spectra of excitation of stimulated resonance Raman scattering of light in some molecules under static or supersonic jet conditions.

An experimental investigation was made of stimulated Raman scattering in leucosapphire (α-Al₂O₃) pumped at the wavelength λ = 532 nm by pulses of τ =30 ps duration (the second harmonic of an Nd³⁺ : Y₃Al₅O₁₂ laser). The Raman gain coefficient was determined for the first vibrational Stokes component with the wavelength λ = 544 nm, corresponding to a Stokes shift of 419 cm^-1. This gain coefficient varied from 2.5 × 10^-10 to 4.6 × 10^-10 cm W^-1, depending on the relative orientation of the leucosapphire crystal axes relative to the plane of polarisation and to the axis of the pump beam. The energy conversion efficiency was η =1% when the pump intensity was 100 GW cm^-2.

A method for elimination of the background (resulting from direct four-photon processes) from polariton CARS spectra is proposed and demonstrated experimentally. The method is based on the ability of polaritons to propagate in a medium over macroscopic distances and it requires a geometry of the interacting waves in which the regions of biharmonic excitation of polaritons and probing of the excited polaritons by a probe field are spatially separated in a crystal.

Mathematical simulation methods are used to optimise of three-stage isotopically selective photoionisation of atoms of the ytterbium-168 isotope. The influence of nonlinear deformation of laser beams is taken into account. It is shown that exact tuning of the laser radiation frequencies to the frequencies of the lines of radiative transitions in the ytterbium-168 atom results in absorption of the laser radiation by atoms of other isotopes and in self-focusing of the laser beams. Introduction of a two-photon detuning reduces considerably the influence of the absorption and self-focusing. A determination is reported of the two-photon detuning, and of the dimensions and the spatial distribution of the power in the laser beam required to ensure that the absorption and nonlinear self-focusing of laser beams do not reduce significantly the ionisation efficiency.

A report is given of the methodology and of the results of measurements of the relative spectral reflection coefficients for separate emission lines of a DF laser in the range λ=3.6–4 μm reflected by five typical topographic objects. Estimates are given of possible errors in the measurement of hydrocarbon concentrations in air by a lidar system based on a DF laser and utilising topographic objects as retroreflectors.

A two-dimensional model framework is used to study the effects associated with a redistribution of an internal electric field under the influence of a space charge induced by a nonuniform rate of separation of photo generated carriers at high energies of optical excitation of a metal — semiconductor — metal (MSM) photodiode. This results in a considerable distortion of the pulsed response of the photodiode, and in reduction of the efficiency and of the pass bandwidth of the photodiode. The conditions are found under which the influence of screening of the internal electric field on the response signal profile of the photodiode is unimportant.

A study was made of the amplifying properties of LiF crystals with stabilised F₂⁺ colour centres with a view to possible use of these crystals as amplifiers of weak IR radiation pulses. A single-pass system pumped with the second harmonic of a pulsed Nd : YAG laser (λ =532 nm) made it possible to achieve 20-fold optical amplification of broad-band nanosecond IR radiation from a laser diode (λ=900 nm).

It is shown that the main equations of a deformation instability model, the solutions of these equations, and the results given in the paper of F Kh Mirzoev are contained in earlier published papers of other authors.

Comment on F Kh Mirzoev 1996 Quantum Electron.26 806.