Table of contents

The use of optical fibres with a high concentration of active ions in the fibre core allows the reduction of the active fibre length and the increase in the threshold powers of various nonlinear effects in the fibre, thereby increasing the maximum output power of fibre lasers. For this purpose, we manufactured a highly concentrated ytterbium-doped (∼1.0×10²¹cm^-3) phosphate glass for the active fibre core and a pure glass of a similar composition for the fibre cladding. A single-mode fibre is fabricated from these glasses and generation is obtained in an ytterbium laser based on this fibre with the slope efficiency of no less than 60% at a wavelength of 975 nm.

The state of the art in the nonlinear dynamics of cw solid-state ring lasers with the homogeneously broadened gain line is systematically analysed. Diverse lasing regimes appearing upon variation of laser parameters are considered and physical mechanisms determining the conditions of their development and stability are analysed. Relaxation processes and temporal and spectral characteristics of radiation are studied.

The results of thermophysical investigations of semiconductor lasers are reported, which underlie the formulation of the optimal requirements on the materials, design parameters, and technological assembling conditions for high-power semiconductor lasers and their one- and two-dimensional arrays with the goal of most efficient heat removal. The methods are outlined for calculating the residual post-assembling mechanical stress and determining the assemblage conditions under which the attendant stress is insignificant and its effect on the laser quality is minimal. Also the methods are given for calculating the thermal resistance for different heat sinks, including heat sinks with forced cooling, and of determining the design requirements on the heat sink arising from specific service conditions.

The polarisation dynamics of a single-mode bipolarisation chip Nd:YAG laser with a weak phase anisotropy of the cavity is studied experimentally and theoretically. It is shown that the three types of relaxation oscillations exist in the bipolarisation lasing regime, the two of which are responsible for the antiphase dynamics of orthogonally polarised modes. It is demonstrated that pumping by linearly polarised radiation induces the gain anisotropy. It is found that the main properties of the low-frequency polarisation dynamics and the induced gain anisotropy are almost independent of the orientation of crystallographic axes of the active element. The model of a single-mode bipolarisation solid-state laser, which was developed earlier and takes into account the phase-sensitive interaction between orthogonally polarised modes, well describes the main properties of the low-frequency polarisation dynamics observed in experiments.

Low-threshold lasing (E_th = 0.94 mJ cm^-2) is obtained in crystalline ZnO powders at room temperature. The stimulated emission spectra exhibit a distinct mode structure. The powders consisting of nanocrystals with the tetrapod-shaped morphology were synthesised by high-temperature pyrolysis of organic zinc salts.

A high-pulse-repetition-rate electric-discharge HF laser with inductive—capacitive discharge stabilisation in the active H₂—SF₆—He mixture is studied. The multisectional discharge gap with a total length of 250 mm is formed by pairs of anode—cathode plates arranged in a zigzag pattern. The width of the discharge gap between each pair of plates is ∼1 mm and its height is ∼12 mm. The laser-beam cross section at the output cavity mirror is ∼9 mm × 11 mm. The maximum laser pulse energy and the maximum laser efficiency for the H₂—SF₆ mixture are 14.3 mJ and 2.1%, respectively. The addition of He to the mixture reduced the laser pulse energy by 10%—15%. The maximum gas velocity in the gap between the electrodes achieves 20 m s^-1. The limiting pulse repetition rate f_lim for which a decrease in the laser pulse energy is still not observed is ∼2kHz for the H₂—SF₆ mixture and ∼2.4kHz for the H₂—SF₆—He mixture. The average output power ∼27 W is obtained for a pulse repetition rate of 2.4 kHz.

The relation between the correlation and coherent properties of light fields scattered by rough surfaces and the geometrical parameters of these surfaces is analysed. It is shown that, if the coherence time of illuminating radiation is τ_c>10ω₀ (where ω₀ is the central frequency of the emission spectrum), then, by averaging the field intensity scattered by a rough surface over time T>10τ_c , we can determine the stationary regions and coherent parameters of the scattered field and the conditions imposed on the narrowband and chromatic (spectral) parameters of illuminating radiation. These regions, parameters, and conditions are specified by the following parameters: the coherence length L_c=τ_cc of illuminating radiation, the transverse size d of the backscattering region of the surface, the depth L_s of the backscattering region, the distance r_c from a receiving aperture to the surface, the size d_ρ of the receiving aperture, the central wavelength λ₀=c/ω₀ of illuminating radiation, and the root-mean-square deviation σ of the height of irregularities of the surface. The obtained results give the relations between L_c , L_s , and σ at which illuminating radiation behaves as monochromatic, quasi-monochromatic or polychromatic radiation and the scattered field behaves as coherent, partially coherent or incoherent field.

The influence of photons of different scattering orders on the formation of a detected signal in optical low-coherence tomography (OCT) is considered. The scattering orders are estimated by analysing the spatial distribution of the probability density for the effective optical paths of detected photons calculated by the Monte Carlo method. The influence of photons with different scattering orders on the formation of a signal is estimated quantitatively depending on the optical properties of the medium under study. The results of numerical simulations are interpreted within the framework of possible applications of OCT for non-invasive diagnostics of the human skin and other highly scattering random media. It is shown by the example of calculation of OCT signals from model biological tissues that the OCT method gives reliable information on their internal structure from optical depths up to 0.3 mm.

The results of the analysis of coherence of nonmonochromatic radiation propagating in a turbulent atmosphere are presented. It is shown that the spatial and temporal coherence of the field of a nonmonochromatic spatially limited partially coherent Gaussian beam depends on the position and orientation of radius vectors of observation points in its cross section. Upon passing to unlimited plane and spherical waves, the temporal and spatial coherence of laser radiation becomes homogeneous over the entire transverse plane.

The parameters of a dark magnetooptical trap for rubidium atoms are measured. The rubidium atoms captured and cooled in the trap occupy a hyperfine level of the ground electronic state, which does not interact with cooling laser radiation. The pulsed filling of the trap is produced due to desorption of rubidium caused by irradiation by a short (1 ms) light pulse. The trap captures and cools 2.5×10⁸ rubidium atoms approximately for 0.2 s. The absorption spectra of a weak probe field by cold atoms are obtained, which demonstrate a good spectral resolution of the hyperfine structure in the excited state. This structure is completely hidden by the Doppler broadening in rubidium vapours at room temperature.

The spatiotemporal characteristics of nonlinear optical adaptive processes with the use of the effective reflectance of a phase-conjugate (PC) mirror based on the stimulated scattering of light or three-wave mixing are considered. The efficiency of the use of this physical parameter for studying the characteristics of PC mirrors in the case of inertialless nonlinearity is demonstrated by the example of phase conjugation of magnetostatic waves.

It is shown by the example of a loop self-pumped phase-conjugate (SPPC) mirror based on a photorefractive crystal (PRC) BaTiO₃ that formation of a phase-conjugate (PC) wave in a SPPC mirror can be considerably accelerated by using a preliminary training of the mirror. For this purpose, it is necessary to direct preliminary an auxiliary (training) optical field on the SPPC mirror, which contains some information on the properties of the input signal whose wave front will be conjugated later. This procedure provides the writing of static refractive-index gratings in the PRC already at the training stage. The presence of these gratings ensures a much more rapid (by 6–20 times) production of volume refractive-index gratings required for the efficient conjugation of the signal radiation. Several variants of static and dynamic SPPC mirror training procedures are simulated and their efficiencies are compared.

The nonlinear optical properties of a number of polymethine dyes (PD 7005, 7006, 7031, and 7098) with a fixed polymethine chain are studied upon their excitation by the focused second-harmonic radiation from a nanosecond Nd:YAG laser. It is found that PD 7098 has the minimum linear absorption over a greater part of the visible spectrum and the strongest nonlinear reverse saturable absorption. The attenuation coefficient of the 100-MW cm^-2 laser radiation in the ethanol solution of this dye is K=14. The characteristics of a single-stage high-power-laser-radiation limiter consisting of two confocal lenses with a focal distance of 5.5 cm are optimised. The maximum values of attenuation coefficients measured in experiments are 420 (for the initial transmission T₀≈50%) and 170 (T₀≈70%). The singlet—singlet absorption cross sections of the dyes are estimated from experiments. The efficient laser radiation limiter considered in the paper features a broad spectral range in the visible region and a high service time.

The photodetection of weak light signals preliminary amplified in optical light amplifiers is analysed quantum-mechanically. A linear amplifier and a four-wave-mixing amplifier are considered. The parameters of the signal-to-noise ratio are calculated and analysed for light sources with various quantum-statistical properties. The signal-to-noise ratio for the detector photocurrent is studied as a function of the quantum-statistical properties of the signal and idler waves involved in amplification by using the four-wave-mixing scheme. The four-wave-mixing scheme with the idler wave in the coherent or Fock (squeezed, sub-Poisson) state has a higher signal-to-noise ratio compared to the linear preamplification regime.

The polarisation properties of a Spun optical fibre are studied in connection with their applications in fibreoptic current sensors based on the Faraday effect. A model of this fibre is proposed which represents it as an anisotropic medium with the spiral structure of the fast and slow birefringence axes. A sensor is developed based on an all-fibre low-coherence linear interferometer with a threshold sensitivity of 70 mA Hz^-1/2, a maximum measured current of 3000 A, and a scale-factor reproducibility of ±0.6%. It is found that for a given diameter of the fibre contour, the normalised sensitivity is independent of the fibre length. The experimental results confirm the theory.

The possibility of developing a photochemical setup on the basis of an optically pumped ammonia laser with an intracavity photoreactor is proved. The obtained values of the cavity 'implication' factor γ are comparable with those of intracavity systems based on a CO₂ laser. The conditions for achieving the maximum energy in the focusing cavity are determined and the ways to control the shape of its caustic are indicated.