Table of contents

A compact optical clock based on a double-mode He—Ne/CH₄ optical frequency standard and a femtosecond Er³⁺ fibre laser is realised and its stability against a commercial hydrogen frequency standard is measured.

A bismuth-doped fibre laser emitting at wavelengths within the second transparency window of silica fibres is fabricated. It is shown that Bi-doped aluminium-free phosphogermanosilicate fibres pumped in the regions of 800 and 1200 nm have the amplification band at 1275—1375 nm.

It is shown that the model of the defect-deformation instability describes the recently found linear dependence of the period of ordered surface relief microstructures, produced on brass in liquid upon multipulse laser ablation, on the radius of the spot on the target surface.

The electrodynamic problem of propagation of light in a fibre with a cladding made of coaxial dielectric layers with alternating values of the refractive index is solved. The fibre core is a dielectric, in particular, air with the lowest permittivity in the fibre structure. A method is described for determining the structure of the multilayer cladding of a fibre having the minimal optical loss of the guided radiation for a particular mode. Losses in a fibre with a cladding with quasi-periodic layer thicknesses are calculated and the dispersion properties of the fibre are analysed. The analysis is performed for the lowest TE and TM modes and for the lowest hybrid mode.

Two systems of cross-polarisation optical coherence tomography based on polarisation-maintaining and polarisation non-maintaining fibres intended for in vivo endoscopic studies of biological objects are described. The sensitivities of the systems detecting scattered light with the initial and orthogonal polarisations in media with local microscopic optical anisotropic inhomogeneities are compared.

The thermal and optical properties of polyacrylamide hydrogels and cartilages are studied by the method of IR laser radiometry. The thermal diffusivity, heat capacity, and the effective absorption coefficient at a wavelength of 1.56 μm measured for polyacrylamide gel with 70% water content and the degree of cross-linking 1:9 and for the nasal septum cartilage proved to be close. This allows the use of polyacrylamide hydrogels as equivalents of cartilages in simulations of laser-induced temperature fields.

Terahertz absorption spectra and dispersion of biologically important substances such as sugar, water, hemoglobin, lipids and tissues are studied. The characteristic absorption lines in the frequency range of a terahertz spectrometer (0.1—3.5 THz) are found. The refraction indices and absorption coefficients of human tooth enamel and dentine are measured. The method of terahertz phase reflection spectroscopy is developed for strongly absorbing substances. Simple and reliable methods of time-resolved terahertz spectroscopy are developed.

The efficiency of a master oscillator—amplifier scheme and the possibility of its using in multichannel nuclear-pumped laser systems are studied. The small-signal gain and saturation parameter are measured for the He:Ar:Xe = 380:380:1 mixture at a pressure of 1 atm at a wavelength of 2.03 μm. It is shown that the small-signal gain increases linearly with the specific pump power density and achieves 1.1—1.2 m^-1 at a pump density of 40 W cm^-3. The saturation parameter is almost independent of the pump power and is equal to 70—90 W cm^-2. It is found that at the pump-pulse maximum (40 W cm^-3) the laser radiation power increases after amplification by 100%.

Two types of semiconductor optical amplifiers (SOAs) based on a double-layer quantum-well (InGa)As/(GaAl)As/GaAs heterostructure are investigated. The optical gain of more than 30 dB and saturation output power of more than 30 mW are achived at 1060 nm in pigtailed SOA modules. These SOAs used as active elements of a tunable laser provide rapid continuous tuning within 85 nm and 45 nm at output powers of 0.5 mW and more than 30 mW, respectively.

The luminescence and laser characteristics of a synthetic opal matrix filled with organic dyes are studied upon excitation by nanosecond laser pulses. The appearance of stimulated emission in a partially ordered scattering medium is investigated. It is shown that if the luminescence spectrum of a dye (oxazine-17) is located far outside the photonic bandgap of the opal matrix, stimulated emission along a preferential direction in the (111) plane is observed when pumping exceeds a threshold even without an external optical cavity. The stimulated emission spectrum is considerably narrower than the luminescence spectrum and consists of several narrow lines located within the dye luminescence band. If the luminescence spectrum of a dye (rhodamine 6G) overlaps with the photonic bandgap of the opal matrix, a different picture is observed. The loss of radiation in the matrix leads to the red shift of the luminescence spectrum, while the stimulated emission as in the case of oxazine-17 lies is observed within the luminescence band.

The results of studies of radiation parameters of a chemical non-chain DF laser (emitting in the range from 3.5 to 4.1 μm) with an intracavity control of the radiation pattern with the help of spatiotemporal modulators based on PLZT electrooptic ceramics are presented.

The possibility of reconstructing a singular wave front of laser beams by the local tilts of the wave front measured with a Hartmann sensor is considered. The accuracy of the reconstruction algorithm described by Fried is estimated and its modification is proposed, which allows one to improve the reliability of the phase reconstruction. Based on the Fried algorithm and its modification, a combined algorithm is constructed whose advantages are demonstrated in numerical experiments.

The effect of two main regimes of the anisotropic Bragg acoustooptic (AO) diffraction on the coherence degree of an optical field from the point of view of the number of acoustic grating layers intersected by an optical beam is analysed. It is shown based on the AO interaction in a TeO₂ single crystal that despite a substantial difference (approximately by five times) between acoustic frequencies used in the regimes of large and small angles of light incidence with respect to the acoustic wave front, the character of the speckle pattern is almost the same.

The mode structure of the Stokes component of stimulated Raman scattering (SRS) in a multimode fibre excited by a pump field with statistics corresponding to a narrowband Gaussian noise model is theoretically analysed. The relation is obtained between the number of spatially coherent radiation modes of the Stokes wave and fibre parameters and SRS excitation conditions. It follows from this relation that the degree of spatial coherence of the Stokes radiation at the fibre output is determined by the number of spatial modes of pump radiation and the averaged gain of the Stokes radiation over the interaction length. It is established that there exists the threshold gain at which the number of spatially coherent radiation modes of the Stokes component increases infinitely and, therefore, its spatial coherence tends to zero. The estimates of the number of spatially coherent radiation modes of the Stokes component showed that already at the Raman scattering excitation threshold their number is so great that the degree of spatial coherence of the Stokes component radiation corresponds to the degree of coherence of a thermal source radiation. The measured dispersion of spatial intensity fluctuations of the Stokes component sepa-rated by using an interference filter (with the transmission bandwidth ≈1 nm at 620 nm) proved to be approximately three times higher than the dispersion of radiation intensity fluctuations of a luminescent lamp. The large dispersion of spatial intensity fluctuations of the Stokes component is caused most likely by individual intensity spikes in a small-scale (unresolved by a detector with a spatial resolution of ≈14 μm) speckle pattern produced in the detection plane by monochromatic Stokes radiation.

The problem of influence of the diffusion process of nonequilibrium carriers on the efficiency of the dynamic grating photoinduced in a semiconductor material is studied theoretically and experimentally. An analytic expression is derived, which allows one to estimate the outlooks for applications of different semiconductor materials as active media for writing dynamic gratings.

The combined influence of the resonator parameters and spatially nonuniform axially symmetric amplification distribution in a four-level active medium on the spatial structure of the fundamental mode of laser radiation is studied numerically by the Fox—Li method. The model of two beams interacting due to the spatial amplification nonuniformity is used to obtain simple analytic estimates. The transition of the spatial structure of the fundamental mode from the Gaussian one to the structure formed by a set of Laguerre—Gaussian beams is studied. It is shown that near the degenerate configurations of the resonator a decrease in the diameter of the pump beam leads to an increase in the number of Laguerre—Gaussian beams forming the fundamental mode. It is found that the range of detunings from the strong degeneracy within which substantial differences of the spatial structure of the fundamental mode from the Gaussian structure are realised, increases with increasing the gain.

The general properties of photodissociation of carboxyhemoglobin (HbCO) in buffer solutions of whole human blood are studied by the flash photolysis method on a setup with intersecting beams. It is shown that the efficiency of photoinduced dissociation of the HbCO complex virtually linearly depends on the photolytic irradiation intensity for the average power density not exceeding 45 mW cm^-2. The general dissociation of the HbCO complex in native conditions occurs in a narrower range of values of the saturation degree than in model experiments with the hemoglobin solution. The dependence of the pulse photolysis efficiency of HbCO on the photolytic radiation wavelength in the range from 550 to 585 nm has a broad bell shape. The efficiency maximum corresponds to the electronic Q transition (porphyrin π—π* absorption) in HbCO at a wavelength of 570 nm. No dissociation of the complex was observed under given experimental conditions upon irradiation of solutions by photolytic radiation at wavelengths above 585 nm.

The spectral and energy characteristics of multiband barrier-discharge coaxial KrBr excilamps are studied experimentally at pressures from a few tens of Torr to 0.4 atm. It is shown that an increase in the Br₂ concentration reduces the emission intensity of KrBr* molecules with respect to the emission intensity of Br₂* molecules and reduces the total emission power of the excilamp. This can be explained by the nonradiative decay of exciplex KrBr* molecules caused by their quenching by molecular bromine. The emission power and efficiency in the Kr:Br₂ = 400:1 mixture at a pressure of ≈230 Torr and a discharge gap of 8.5 mm were 4.8 W and 2.4%, respectively.