Table of contents

A brief review is presented of the state of the art of fibre optic communication based on materials of the conferences held in 1999. The main attention is paid to fibre optic systems with the spectral multiplexing of channels and the bit rate of ~Tbit s^-1.

The dependence of the mode optical gain on current in InAs/InGaAs quantum-dot structures grown by the method of molecular-beam epitaxy is obtained from the experimental study of ultra-low-threshold laser diodes. The record lowest inversion threshold at room temperature was about 13 A cm^-2. A theoretical model is proposed that relates the optical gain to the ground-state transitions in quantum dots. The effective gain cross section is estimated to be ~7 × 10^-15 cm^-2.

It is found that inserting aerosol in a cavity of a multimode (wide-band) Co:MgF₂ laser can strongly affect its emission spectrum, provided the typical size of aerosol particles is comparable to the laser wavelength (~2 μm). In the presence of aerosol, intensity spikes are formed in the spectrum at the initial lasing stage, which develop in time. At moments of time ~100 μs, condensation of the spectrum was observed, which manifested itself as a sharp (more than two orders of magnitude) narrowing of the laser emission spectrum down to the value determined by the spectral resolution of a spectrograph (not better than 0.04 cm^-1), with an insignificant decrease in the output energy.

A powerful, repetitively pulsed 1444-nm Nd:YAG laser is studied. It is shown that the efficiency of this laser is mainly limited by the absorption at 1444 nm in the active element, the UV radiation of the pump lamps, the lasing at the internal mode frequencies of the active elements, and the amplified luminescence at 1444 nm. An optimisation procedure resulted in the creation of a powerful laser producing 1.4-J pulses at a repetition rate of between 1 and 10 Hz at 1444 nm.

Simultaneous lasing on two lines having a common upper level is analysed. Within the framework of the two-level model, a formula for the gain in each line is obtained. Two possible types of line competition are found: symbiosis and quenching. It is shown that competition in the quenching regime can be used to study the rates of population of the lower levels. The results are illustrated by competition diagrams for the 2.65-μm and 2.03-μm of the Xe atom in the Ar—Xe mixture and the 703.2-nm and 724.5-nm lines of the Ne atom in the Ne—Ar mixture.

The kinetic model of relaxation in the Ne—Ar—Kr mixture pumped by a hard ioniser is constructed in connection with the analysis of the possibility of lasing at the Ne⁺Ar→NeAr⁺ transition of the inert-gas ionic exciplexes. The calculations based on the typical rates of plasmachemical reactions demonstrate that the lasing is possible but difficult to realise: One needs high pressures (greater than 16 bar) and high pumping densities (~ 1 MW cm^-3). In the most favourable cases, the laser efficiency lies between 0.05 and 0.25%.

The comparative analysis of the processes responsible for dissociation of molecular iodine in the oxygen-iodine medium is made. The rate of relaxation of vibrationally excited oxygen in the O₂—I₂—H₂O medium is shown to be limited by the VV energy exchange between oxygen and water molecules, and the fraction of vibrationally excited O₂ (ν=1) in the active medium of the oxygen-iodine laser can reach several tens of percent. It is noted that the VV exchange in O₂(¹Δ,ν=1)+I(²P_3/2)↔O₂(³Σ,ν=1)+I(²P_1/2) reaction may be of considerable importance in the kinetics of formation of the active medium in the oxygen-iodine laser.

Complex spectral structures located between the resonance lines of H- and He-like MgXII and MgXI ions were recorded in experiments on plasma heating by the radiation of a low-power short-wavelength excimer XeCl laser (12-ns pulses with an energy of 2 J). The above spectral structures were shown to arise from transitions in the so-called hollow multicharged ions, i.e., in ions with an empty 1s-shell, which were previously observed in laser produced plasmas only with ultrahigh-power femto- and picosecond laser facilities having extremely high-contrast laser pulses.

Two models of interaction of UV and VUV laser radiation with MgF₂, CaF₂, and BaF₂ crystals are considered. In the first version, the laser radiation wavelength falls into the absorption region of F-centres, in the second version it is located in the short-wavelength region of the H-centre absorption wing. The photodissociation of F₂^--complexes in crystals into two holes is shown to be an efficient mechanism of defect formation in the crystal structure. The consideration of these and other processes of relaxation of electronic excitation allows one to explain many experimental effects produced by ionising and laser radiation in ionic crystals.

The appearance and development of large-scale self-organising microstructures on the surface of monocrystalline Si and several other materials (Ge, Ti) are studied upon their evaporation by 20-ns pulses from a copper vapour laser at 510.6 nm. The structures were formed upon repeated pulsed laser irradiation (~10⁴ pulses with an energy density of 1–2 J cm^-2) in the absence of optical breakdown of the medium above the target surface in the 1–10⁵ Pa pressure range in a wide range of angles of laser radiation incidence on the surface. The structures are cones with an apex angle of ~20–25°, which grow towards the laser beam. It is shown that the spatial period of the structures developing during laser evaporation is determined by the period of the waves arising on the melt surface and is equal to 10–20 μm. The x-ray diffraction analysis showed that the modified substrate region has a polycrystalline structure and consists of crystallites with dimensions ranging from 40 to 70 nm, depending on the pressure of the ambient atmosphere.

The α-particle imaging of a compressed core of microtargets using a multi-pinhole regular diaphragm is proposed. The image reconstruction technique is described. The results of the α-particle imaging of a compressed core of microtargets obtained at the 'Iskra-4' laser facility are reported.

The effect of electrical diaphragm-target coupling on the formation of ion component characteristics of a laser-produced plasma is considered. When the target and the diaphragm are insulated from each other and are under a floating potential, a high-energy ion peak is observed and the integral number of ions and their multiplicity increase compared to the case when the target and the diaphragm are connected and grounded.

The first observation of x-ray emission spectra of multiply charged ions in the plasma produced by a 35-fs laser pulse with an intensity up to 10¹⁷ W cm^-2 in CO₂ and Kr gas jet targets is reported. The emission in the wavelength ranges of the 1snp—1s²(n=3–6) transitions of O VII ions and the Ly_α line of O VIII ions, as well as of the (2s_1/22p⁶3p_3/2)₁—2s²2p⁶¹S₀ and (2s_1/22p⁶3p_1/2)₁—2s²2p⁶¹S₀ lines of Ne-like KrXXVII ions testifies that the highly ionised plasma is formed by collision processes in clusters. Modelling the shape of the spectral lines of oxygen ions by including the principal mechanisms of broadening and absorption in optically dense plasmas reveals that the main contribution to the time-integrated intensity is made by the plasma with the parameters N_e=(2–20)×10²⁰ cm^-3 and T_e=100 — 115 eV.

A comparative analysis of the dependences of the refractive index of Mg:LiNbO₃ single crystals on the Mg concentration is performed in the visible and IR transparency regions. The results of measurements at the wavelengths above 4 μm suggest the four-step mechanism of incorporation of the Mg impurity to the LiNbO₃ crystal in the range of molar concentrations of Mg from 0 to 7.1 %. The resonances observed between 1 and 2 μm and between 2.8 and 3.5 μm in the transparency region are interpreted as contributions from polaron states. It is shown that the resonance in the region between 1 and 2 μm is strongly enhanced in crystals with the molar concentration of Mg above 5.1%.

The electric parameters of the volume discharge and the transition of the discharge from the volume combustion to the channel stage are experimentally studied. It is shown that cathode spots of a hemispherical shape appear at the stage of the discharge formation. Over them, plasma plumes are developed which are combined to form a volume column of the discharge plasma. The discharge homogeneity increases with increasing density of the cathode spots. The distortion of the shape of cathode spots initiates the development of plasma channels. The development of the plasma channel is computer simulated. The results of calculations are discussed and compared with experimental data.

Isotopically selective IR multiphoton dissociation of molecules (SF₆, CF₃I) in a pulsed gas-dynamic flow interacting with a solid surface was studied for the first time. A noticeable (severalfold) increase in the yield of products (compared to excitation of molecules in an unperturbed flow) without a substantial decrease in the selectivity of the process was observed. Possible reasons for the effect are discussed.

A technique for two-dimensional imaging of an x-ray source with a continuous time-base sweep is described. The technique is based on the use of an x-ray image converter tube in combination with a pinhole camera, which has a linear pinhole array. The basic relationships for determining the parameters of the recording system are given. Setting up the procedure is described and the results of two-dimensional microtarget imaging with a continuous time-base sweep in an experiment at the 'Iskra-4' laser facility are presented.

Oscillations and hysteresis in the behaviour of the director of a nematic liquid crystal were observed upon its light-induced hydrodynamic reorientation caused by direct volume expansion. The light propagated through the liquid crystal placed between crossed polarisers provides the feedback. This light falls back on the liquid crystal and is absorbed by producing the volume expansion. A theory is suggested that describes the observed behaviour of the director of the nematic liquid crystal.

The echolocation range and the remote sensing of ethylene in the atmosphere are simulated for a differential-absorption lidar based on TEA CO₂ lasers. The dependence of the lidar echolocation range on the energy and the peak power of probe pulses is shown to be close to logarithmic. It is demonstrated that the use of narrow-band spectral filters is justified only for low-noise detectors and viewing angles of the receiver exceeding 5 mrad. The relative measurement error of the ethylene concentration in the atmosphere is estimated for various detection modes.