Table of contents

Self-mode locking is achieved in a F₂^-:LiF laser pumped by a cw fibre laser using a new passive switch based on single-wall carbon nanotubes.

A hypothesis is proposed that explains qualitatively a strong absorption of light in quartz. The hypothesis is based on an analogy with the so-called chemical radiative collisions (RC reactions), which were extensively studied in the 1970–1980s. In these reactions, individual chemical bonds dissociate and new bonds are formed. An estimate is made for a reaction in which a free (non-bridge) oxygen atom bounds with a bridge atom, while another oxygen atom undergoes a reverse transition. The estimates show that the limiting absorption coefficient can achieve ten hundreds inverse centimetres.

A repetitively pulsed high-frequency UV nitrogen laser based on magnetic pulse compression (MPC) ciruit is developed. The special characteristics of laser operation at pulse repetition rates up to 11 kHz are studied. Lasing with an average power of 1.4 W for a beam divergence of less than 0.5 mrad is obtained.

The energy and spectral characteristics of an HF laser operating by using a new method of obtaining the oxidising gas based on a two-region mixing are studied experimentally. The investigation involved redistribution of the overall amount of the rare-gas diluent (helium) delivered to the laser between the gas generator (the first mixing region) and the subsonic parts of the nozzles of the nozzle unit (the second mixing region). In this case, the temperature in the gas generator was maintained constant and the experiments were conducted under conditions when either the mass flow of atomic fluorine or the pressure in the gas generator were maintained constant. It was shown that upon a nearly full transfer of helium from the first mixing region to the second one in the constant-pressure regime in the gas generator, the laser output power increased by 70%, while in the regime of a constant mass flow of atomic fluorine, the output power increased by 14% along with a simultaneous increase in the specific energy extraction by 40%. The lasing region was found to lengthen by no less than 35%.

A 308-nm XeCl laser with an active volume of 600 L is studied experimentally. The laser is pumped by a radially convergent electron beam from an accelerator with vacuum insulation. The output energy of 1.9 kJ and power of ∼7 GW are achieved upon pumping the Ar—Xe—HCl mixture. The laser pulse FWHM is ∼250 ns.

The parameters of 0.98-μm laser diodes with a narrow ridge waveguide fabricated by low-energy ion — chemical etching using an inductively coupled plasma source are studied. It is shown statistically that, when the geometrical parameters of ridge waveguides correspond to the results of calculations, the production of acceptable 150–200-mW diodes can achieve 78%. It is found that the optical properties of a waveguide in the perpendicular direction substantially affect the percent of production of diodes with a specified output power. When conventional SiO₂—ZrO₂ optical coatings were used, the maximum single-mode power (220–230 mW) was limited by the degradation of mirror faces.

The prototype of a passively Q-switched, longitudinally diode-pumped erbium glass laser is fabricated. A slope efficiency of 38% is achieved in the free-running lasing regime. It is shown that a pump energy of only 42 mJ is required to generate a 9-ns, 1.8-mJ output pulse with efficiency 4.2%.

It is shown that the finite thickness of the second-harmonic crystal distorts the results of measurements in nonlinear autocorrelators intended for measuring the durations and fields of femtosecond light pulses mainly due to dispersive broadening (or compression) of the pulses being measured, as well as due to the group velocity mismatch between the fundamental and sum-frequency pulses. The refractive index dispersion of the crystal, scaled by half its thickness, distorts the pulse duration to a certain extent depending on its initial chirp and thus determines the width of the energy distribution recorded in the autocorrelator. As the crystal thickness increases, the group velocity mismatch leads to a transformation of the recorded distribution from the correlation function of intensity to the squared modulus of the field correlation function. In the case of Gaussian pulses, such a transformation does not affect significantly the recorded distribution. Errors of pulse duration measurements are estimated.

The schematic model of a resonator with a reflection diffraction grating, providing a high frequency selectivivty, a wide spectral range, and a low loss level at a selected frequency, is studied theoretically and experimentally. The frequency-selective parameters of the resonator are calculated and analysed. It is shown that the dependence of the loss increment on the diaphragm diameter for a fixed frequency detuning Δν is nonmonotonic. Lasing at 100 lines of conventional bands and 36 lines of sequential and hot transitions is obtained. The minimum separation Δν_L between resolvable lines is found to be ∼0.2 cm^-1.

The temporal characteristics of coherent stimulated radiation at the ³P₀— ³H₆ transition in the Pr³⁺ ion in a LaF₃ matrix are studied by tuning the pump frequency in the vicinity of the ³H₄— ³P₀ transition. It is found that in the case of the exact tuning to the resonance, a laser pulse, consisting of a train of picosecond spikes of total duration about 10 ns, was delayed by 3–4 ns with respect to the pump pulse onset. As the pump pulse detuning was increased, the shape of the coherent laser pulse changes and its delay increased up to 10 ns. The experimental results are interpreted theoretically.

Phase and amplitude — phase corrections of laser beam distortions during their propagation in a turbulent atmosphere under conditions of strong intensity fluctuations are compared. The effect of wavefront dislocations and the possibility of controlling the amplitude and phase of an optical wave are studied. Two approaches are analysed: phase correction using amplitude control and two-mirror phase correction. The efficiency of both methods is demonstrated.

The problem of coexistence and lateral self-mode locking in the resonator of an injection laser with a parabolically inhomogeneous permittivity is considered. The stability of the self-mode-locking process and factors causing its disturbance are studied. It is shown that the appearance of nonequidistant lateral modes due to the dependence of the refractive-index profile in a medium on the concentration of nonequilibrium carriers is the main cause of disturbing the self-mode-locking regime.

The dependence of polarisation characteristics of radiation of a linear Nd:YAG laser on polarisation of radiation of a pump diode laser is studied experimentally and theoretically. It is shown that, in the case of polarisation isotropy of the optical resonator, the polarisation of radiation of the Nd:YAG laser is completely determined by the polarisation of pump radiation. Based on the vector model of this laser pumped by polarised radiation, an analytic solution describing stationary lasing is obtained.

Absorption and emission cross sections are measured for the ²F_5/2 → ²F_7/2 transition in Yb³⁺ ions in silica fibres doped with Al₂O₃ and P₂O₅. The measurements were performed by methods based on spectroscopic data and by more direct methods based on laser-transition saturation. Possible lasing ranges of ytterbium-doped double-clad fibre lasers are calculated from the measured spectral dependences of stimulated transition cross sections.

A fibre attenuator is fabricated for the telecommunication wavelength range near 1.5 μm in which a single-mode silica fibre with side polishing is used. The fibre surface is covered by a layer of fluorine-containing polymer with a large thermooptic coefficient. The principle of attenuator operation is based on a change in the conditions of total internal reflection for a guided mode in the polished region due to thermally induced variation in the refractive index of the fluoropolymer layer. The attenuator is insensitive to light polarisation, it has a continuously variable attenuation coefficient in the range 0.2–27 dB, and can be easily incorporated into fibreoptic links.

The dynamics of radiation pulses is studied in a fibreoptic system with amplification and an arbitrary dependence of the real and imaginary components of dispersion parameters on the longitudinal coordinate. The dependence of compression regimes of a wave packet on the chirp and imaginary components of dispersion parameters related to the frequency dependence of the gain increment is analysed.

A flat-top format is proposed for the transmission of digital signals with the sinc-shaped profile of a single pulse resonantly located along several bit intervals. Due to a rapid decrease of the spectral power, such a format reduces the interaction between two adjacent frequency channels. Numerical simulation showed the possibility of error-free transmission at a rate of 40 Gbit s^-1 over 1200 km in the link with the 0.8-bit s^-1 Hz^-1 spectral efficiency without using polarisation wavelength-division multiplexing.

The effect of pulsed radiation from a 2.92-μm, 130-ns Cr³⁺:Yb³⁺:Ho³⁺:YSGG holmium laser and a 510-nm, 20-ns copper vapour laser on an aqueous suspension of ultradispersed diamond is studied. X-ray photoelectron spectroscopy and electron energy loss spectroscopy revealed that exposure of the suspension to holmium laser radiation reduces the concentration of nondiamond carbon impurity in it. This is due to the dissolution of carbon impurity in supercritical aqueous solution, caused by radiation absorption. Dissolution of the nondiamond fraction may serve as an indicator of the solution being in the supercritical state as a result of laser irradiation of liquids. This process can be used for efficient purification of ultradispersed diamonds.

The electrical and spectroscopic characteristics of electron-beam-sustained discharge (EBSD) in oxygen and oxygen-containing gas mixtures are studied experimentally under gas pressures up to 100 Torr in a large excitation volume (∼18 L). It is shown that the EBSD in pure oxygen and its mixtures with inert gases is unstable and is characterised by a small specific energy contribution. The addition of small amounts (∼1%-10%) of carbon monoxide or hydrogen to oxygen or its mixtures with inert gases considerably improves the stability of the discharge, while the specific energy contribution W increases by more then an order of magnitude, achieving ∼6.5 kJ L^-1 atm^-1 per molecular component of the gas mixture. A part of the energy supplied to the EBSD is spent to excite vibrational levels of molecular additives. This was demonstrated experimentally by the initiation of a CO laser based on the O₂ : Ar : CO = 1 : 1 : 0.1 mixture. Experimental results on spectroscopy of the excited electronic states O₂(a¹Δ_g) and O₂(b¹Σ_g⁺), of oxygen formed in the EBSD are presented. A technique was worked out for measuring the concentration of singlet oxygen in the O₂(a¹Δ_g) state in the afterglow of the pulsed EBSD by comparing with the radiation intensity of singlet oxygen of a given concentration produced in a chemical generator. Preliminary measurements of the singlet-oxygen yield in the EBSD show that its value ∼3% for W ∼ 1.0 kJ L^-1 atm^-1 is in agreement with the theoretical estimate. Theoretical calculations performed for W ∼ 6.5 kJ L^-1 atm^-1 at a fixed temperature show that the singlet-oxygen yield may be ∼20%, which is higher than the value required to achieve the lasing threshold in an oxygen—iodine laser at room temperature.

A new method of modulated light reflection — below-bandgap photoreflection, is considered. Unlike the conventional photoreflection method, the proposed method uses optical pumping by photons of energy smaller than the bandgap of any layer of a semiconductor structure under study. Such pumping allows one to obtain the modulated reflection spectrum for all layers of the structure without excitation of photoluminescence. This method is especially promising for the study of wide-gap semiconductors. The results of the study of semiconductor structures used in modern high-power multimode semiconductor lasers are presented.

The AlGaAs/GaAs laser heterostructure with the InGaAs quantum well is studied by the contactless photoreflection method upon local optical pumping. Unlike the conventional photoreflection method, regions on the sample surface illuminated by the probe and pump light are spatially separated. The method allows one to separate photoluminescence and photoreflection signals and to construct the three-dimensional energy band diagram of the laser structure.