A brief description of the theory of mode locking of a semiconductor laser when the modulating generator frequency is close to the intermode frequency is given. It is shown that during mode locking there is phase locking, due to which the dispersion of summed intensity in the interpulse interval tends to zero.

High-efficiency harmonic generation is observed in a semiconducting polymer RR-P3HT [regioregular poly(3-hexylthiophene)], which exhibits a high nonlinear susceptibility [ χ (2) > 10 −6 m V −1 ] under pumping by a pulsed fibre laser. The harmonic generation efficiency in RR-P3HT is comparable or higher than that observed for liquid crystals, which are used in reference experiments to estimate the susceptibility [the quadratic susceptibility of a nematic liquid crystal NLC 1289 is χ (2) ≈ 2 × 10 −6 m V −1 ]. No generation is observed in polythiophene with a random structure, RRa-P3HT [regiorandom poly(3-hexylthiophene)], at the same pump power. It is experimentally demonstrated that a necessary condition for generating the second and third harmonics in a polymer, along with a high pump power density, is the presence of a large power density gradient (exceeding 10 13 W m −3 ). Based on a preliminary theoretical analysis, we can suggest that the quadrupole mechanism, which is a consequence of the regularity of RR-P3HT structure in the thin near-wall layer, may contribute significantly to the nonlinear radiation conversion in semiconducting polymers.

We investigate the regime of active mode synchronization in a three-mirror diode laser obtained by current modulation at the intermode beat frequency of the external laser resonator. The current modulation is achieved using electrical feedback. It is shown that under certain conditions, a stable mode-locking regime is observed, and when the length of the external resonator changes, the corresponding intermode beat frequency tuning occurs. The main conditions, for which this regime is achieved, are established. The ~300 Hz width of the intermode beats spectrum at a 300 MHz intermode frequency is obtained experimentally. The broadening of the optical spectrum at the 0.5 level is ~4 nm.

This paper presents the results of experimental studies of active mode-locking in an external-cavity semiconductor laser. Two methods of obtaining active mode-locking regimes have been studied: by modulation from an external generator with a frequency close to the intermode frequency of the external laser cavity and by laser current modulation at the frequency of intermode beats of the external laser cavity through an amplification feedback loop. In the first case, a stability of intermode beats of 10 -14 is experimentally achieved, and in the second case, it is shown that semiconductor laser current modulation by an amplified intermode beat signal from the output of a photodetector provides a stable mode-locking regime also with mode frequency tuning, i.e., a mode-locking regime involving intermode frequency tuning with optical cavity tuning. The effects that accompany mode locking, in particular, the pulling effect and the stability of the mode-locking regime, and possibility of obtaining highly stable intermode beats by suppressing the amplitude noise component in the modulating signal have been studied. A phenomenological study of the pulling and phase locking of the intermode frequency is attempted.

A set of equations governing field amplitudes and phases that describe the single-mode regime of oscillation of a semiconductor diode in a long cavity is derived. The set of equations is generalized to a multimode regime, taking into consideration modulation of the diode current with frequency close to the mode beat frequency. The exact solution obtained for a simple case reveals the existence of an ultrashort-pulse generation regime. The shape of the pulse, its width, and the number of modes involved in its formation are determined.

Summary form only given: The temperature impact on the optical harmonic generation in a fiber-coupled nematic liquid crystal (NLC) pumped by femtosecond laser radiation was explored. A strong thermal enhancement of the third harmonic generation (THG) was revealed. Upon heating to a certain temperature (near the upper limit of the mesophase), the NLC-based convertor features a spike of the THG efficiency in excess of 10%. This effect may be attributed to noncritical phase matching. Also appreciable forth harmonic generation in the ultraviolet occurred upon the thermal enhancement of THG.

In article it is considered some characteristics of the photon and crystal wave guides created from polymeric fiber wave guides. The method of formation of periodic structure of a photon and crystal wave guide by a way of convolution of a film with the fibers fixed on it with the set step is offered.

The article discusses some of the characteristics of materials used in the THz range. First registered thermo orientation effect in centrally symmetric liquids, leading to structural changes that violate the original symmetry of nematic liquid crystals under the action of heat flow.

The article deals with the attenuation of terahertz radiation in politetraftoretilene during heating.

Highly efficient lasing at difference frequencies is obtained in a nematic liquid crystal excited by several visible lines from a cw argon laser with a total power of 0.08—1.5 W. The maximum conversion efficiency was ≈1% and the quadratic susceptibility was ≈2×10-6 m V-1. The field of application of the approximate mechanism of quadratic nonlinearity and frequency conversion considered in the paper requires specification. The nonlinear conversion of radiation in a nematic crystal has specific features compared to lasing at difference frequencies in solid crystals.

Experimental results of investigations into metal-oxide-metal (MOM) diodes based on W-Ni, Pt-Ti, and Pt-W contacts in the infrared and microwave bands are presented. The schematic diagram for setting the bias current for the MOM diode with protection of the diode during transient switching-on and switching-off processes is given. This diagram is intended for measuring the volt-ampere characteristics of the MOM diode and ensuring an output band of up to 100 MHz.

A theoretical and experimental description is given of the director orientation dynamics in nematic liquid crystals placed in variable electric fields comprising several components and having different directions (crossed fields) and frequencies up to 5 MHz. The calculations show that the system of interest is a physical object governed by non‐linear dynamics. Depending on numerical values of governing parameters, the following director state regimes are obtained: stationary, periodic, quasi‐periodic (multi‐modal), and stochastic strange‐attractor‐type. In the calculations, all the states are obtained by solving a determinate system of two time‐dependent first order non‐linear equations, ignoring the electrohydrodynamic effect. Preliminary verification is performed, and qualitative agreement with the mathematical model under consideration is obtained in the range of frequencies (approximately 100 Hz and higher) that allows electrical conductivity to be ignored. The influence of 2D and 3D rotating electric fields with one frequency, and two‐ or three‐component fields with different frequencies, in the interval from 10 Hz to 5 MHz on the orientation of a nematic liquid crystal is considered.

Various oscillation regimes of an actively mode-locked semiconductor laser are studied experimentally. Two types of regimes are found in which the minimal spectral width (∼3.5 kHz) of intermode beats is achieved. The width of the optical spectrum of modes is studied as a function of their locking and the feedback coefficients. The maximum width of the spectrum is ∼3.7 THz.

Five new emission lines in the wavelength range of 70 — 140 μm are obtained from a CH3OH submillimeter laser pumped by high-power periodic (500 Hz) quasi-stationary 70-μs pulses from a CO2 laser. The wavelengths (with an error of 0.5 %), the threshold pump powers, and the optimal pressure are measured. It is shown that no saturation is present at pump powers of 1000 — 3000 W.