Table of contents

Basic technological and physical aspects of injection lasers based on arrays of self-organised InAs/InGaAs quantum dots on GaAs substrates for optical communication systems in the 1.2–1.3 μm spectral range are considered. The possibility of simultaneous lasing at a great nimber of longitudinal modes at a high power level and low noise is demonstrated. The use of these lasers in wavelength-division-multiplexing systems based on the spectral separation of the laser output spectrum is substantiated.

The near- and far-field emission patterns of GaAs/AlGaAs heterostructures are studied in the regimes of lasing and generation of femtosecond superradiant pulses. It is explicitly demonstrated that, unlike lasing, superradiant emission does not exhibit such phenomena as self-focusing, instability and deformation of near- and far-field emission patterns. The observed phenomena can be explained by the properties of the coherent cooperative electron—hole state, which has been found earlier.

The efficiencies of different fuel compositions used in the combustion chamber of an autonomous cw chemical HF—DF laser for obtaining high specific energy parameters during simultaneous lasing in HF and DF molecules in two spectral ranges are theoretically analysed. It is shown that mirrors with the reflectance above 99% in these spectral ranges can be manufactured in principle.

To improve the planning of experiments, the statistical analysis of a large amount of experimental data obtained for a copper bromide laser emitting at 510.6 nm and 578.2 nm was performed. Various statistical methods such as factor analysis, method of principal components, multiple regression and others were applied for studying the influence of the ten basic input laser parameters on the output laser power. It was found that the most important parameters are the inside diameter of the laser tube, the diameter of internal rings, the length of the active area and the input electrical power.

The propagation of an optical discharge (OD) through optical fibres upon interference of LP₀₁ and LP₀₂ modes is studied. Under these conditions after the OD propagation through the fibre, the formation of an axially-symmetric group sequence of voids with a spatial period equal to that of mode interference (200—500 μm depending on the parameters of the fibre) is observed. The groups of voids are formed near the sections of the fibre with a minimal diameter of the intensity distribution of laser radiation. Large spaces between voids in the fibre have allowed us to measure accurately the difference Δn of refractive indices of the fibre core and cladding and distribution of dopants in different cross sections of the fibre after the OD propagation. A substantial increase in Δn (up to ten times) is observed. Approximately half this increase is caused by compression and densification of the fibre material after the propagation of the optical discharge.

A new type of optical waveguides in silicon-on-insulator (SOI) nanostructures is proposed and studied. Their optical properties and the possibility of their application in tunable optical filters and reconfigurable multiplexers are discussed based on the results of numerical simulation by the BPM and FDTD methods. A new design of heterogeneous waveguide structures containing additional regions with a high concentration of free charge carriers in the form of a p—n junction, which are located at the edges of a multimode strip waveguide (the cross section of the silicon core being ≈0.22×35 μm), is proposed. This doping provides single-mode behaviour of the heterogeneous waveguide due to low optical losses in the fundamental mode and to enhanced losses in highest modes. Heterogeneous waveguides can be used for the fabrication of different photonic elements including new types of tunable optical filters and reconfigurable multiplexers based on the multireflection technology.

The problem of light propagation in a layered periodic medium with a step refractive index profile is considered. The exact solution of this problem is presented in the form of a nonuniform wave, for which the field amplitude distribution is written in an analytic form and the shape of its wave surfaces is determined. The reflection coefficient is obtained for a plane wave incident from the homogeneous medium at the boundary of a semi-infinite layered periodic medium and exciting a Floquet—Bloch wave. Critical conditions are found in which the Floquet—Bloch wave is infinite in the semi-infinite layered medium and exponentially decays in the adjacent homogeneous medium. Dispersion equations and field distributions of surface waves (modes) localised near the boundary of the semi-infinite layered medium are derived and conditions of their appearance are determined. The boundaries of admissible values of the refractive index of the adjacent medium depending on the parameters of the layered periodic medium are established. Dispersion relations for the surface modes in the semi-infinite layered periodic medium (bounded by a system of coupled waveguides) are obtained upon changing the thickness of the boundary layer.

Experimental studies of a laser-induced spark produced in air by 1.05-μm, 100-ns pulses with spatial TEM₀₀, TEM₀₁ and TEM₀₂ modes are described. It is found that when the spark is observed at an angle of 90° to the laser beam axis, the scattered radiation has the maximal intensity outside the beam waist. The intensity ratio of the scattered laser radiation for two orthogonal polarisations is ≈100, and the spatial structures of their depolarisation considerably differ. These properties are explained by using a model of the Fresnel reflection from the spherical front of the plasma—undisturbed gas interface.

The possibility of using Young—Michelson and Brown—Twiss interferometers for measuring the angular dimensions and parameters of the surface shape of remote passively scattering and self-luminous nonplanar rough objects by optical radiation propagating from them is substantiated. The analysis is based on the properties of approximate transverse functions of field coherence B_t and B_t' and intensity coherence B_ti and B_ti' formed by the time averaging of the products of fields and intensities taken at two points of a receiving aperture (the prime denotes self-luminous objects). The averaging time is set to be much longer than the coherence time of radiation propagating from an object. It is shown that for the radiation coherence length much smaller than the depth of the visible region of the object, the functions B_t and B_t' are proportional to the Fourier transform of the intensity distribution in the image of a remote object, which is the generalisation of the Van Cittert—Zernicke theorem to the case of a nonplanar object, while functions B_ti and B_ti' are proportional to the squares of the modulus of the Fourier transform of this distribution. It is also shown that the recording of functions B_t and B_t' with a Young—Michelson interferometer gives only the angular dimensions of the visible region of objects, whereas the recording of functions B_ti and B_ti' with a Brown—Twiss interferometer allows one to find these dimensions and the radius of curvature of the object surface.

A mathematical model of laser radiation scattering from a remote rough surface illuminated by a focused laser beam is proposed. The model can be used to estimate the mean number and size of speckles and their mean and maximum intensity. Fluctuations of the speckle intensity were observed upon focusing radiation in the object plane. The magnitude of fluctuations achieves 50%—70% and depends on the object roughness and the laser beam parameters.

The dynamics of the phase difference of counterpropagating waves is studied theoretically and experimentally in the self-modulation oscillation regime in a solid-state ring laser. It is found that in the case of a small enough frequency nonreciprocity of the ring resonator, the phase difference of counterpropagating waves changes within a limited range, performing periodic oscillations with the intensity self-modulation frequency. The instant frequency difference of counterpropagating waves also changes periodically in time; however, its mean value is zero (the frequency locking for counterpropagating waves takes place). The width of the frequency locking region is measured. It is shown that the phase difference of the coupling coefficients considerably affects the phase dynamics. This opens up new possibilities for determining the phase difference of coupling coefficients of counterpropagating waves.

The efficiency of using the time-of-flight (TOF) method at a wavelength of 820 nm for detecting the changes in the optical properties of multilayer light scattering medium in connection with the problem of the glucose level detection in the human tissue is discussed. Pulses scattered from a three-layer biotissue phantom consisting of two skin layers and a blood layer between them, are calculated with the help of a program code based on the Monte Carlo algorithm for different glucose concentrations. Relative changes in the recorded signals caused by variations in the glucose content are analysed for different source – detector separations. It is shown that the maximum relative change in the total pulse energy is 7.2% and 4.8% for the anisotropy factor of the layer mimicking skin g = 0.9 and 0.7, respectively, and the change in the glucose concentration from 0 up to 500 mg dL^-1. The use of time gating leads to the increase in these values up to 12% and 8.5%, respectively. The sensitivity maps are obtained which can be used to determine the optimal duration and the time delay of the time gate relative to the probe pulse for five values of the source – detector separations.

The influence of glucose on the diffuse reflection of near-IR femtosecond laser radiation from single- and three-layer media simulating biological tissues is studied experimentally. Based on a 800-nm femtosecond Ti:sapphire laser emitting 40-fs pulses and a VUV Agat streak camera, a setup is built for time and spatially resolved detection of radiation diffusely reflected from the volume of a strongly scattering medium. A multichannel fibreoptic system is developed for detecting pulses simultaneously at several fixed distances between a radiation source and detector. It is shown that the peak intensity and total energy of detected pulses are sensitive to variations in the glucose concentration in the medium under study from 0 to 1000 mg dL^-1. The relative sensitivity in our experiments achieved 0.030% mg dL^-1.

The propagation of light through a planar layer of a cholesteric liquid crystal doped with dye molecules is considered. The features of the emission spectra of the crystal are studied both in the absence and presence of dielectric boundaries. The increase in the emission intensity is investigated for different layer thicknesses and different concentrations of dye molecules. It is shown that an anomalously strong increase in the emission intensity with the diffraction intrinsic polarisation takes place in the case of a comparatively small crystal thickness and a relatively low concentration of dye molecules. The obtained results can be used for the development of miniature lasers with the circular polarisation of the fundamental radiation mode.