Sergey I. Mikhailov

Theoretical study of coherent reflection imaging at grazing angles

The theory of reflection image formation at coherent slant illumination is specified for the case of confined objects. The approach is based on the parabolic wave equation with a boundary condition posed on an object surface that is tilted in respect to the beam k-vector. The result is obtained in the form of a new propagation integral. 3D simulation of coherent reflection imaging at grazing angles of illumination in a lens optical scheme is performed and discussed.

Small signal SBS: amplification

The amplification of small signals was investigated both experimentally and theoretically. The theoretical analysis was provided in terms of unsaturated regime. It is shown, that it is necessary to take into account the temporal shapes of interacting waves. When pump pulse duration is of the order of hupersound phonons lifetime, the Stokes pulse has duration in two times lower then pump. It is necessary to have time delay between input Stokes pulse and pumping pulse of the order of life time of hupersound phonons to have amplification maximum. Energy amplification ~ 2•109 was obtained experimentally when the energy of input signal was about 10-11 J. The affect of the active medium SBS - amplifier parameters on SBS threshold is investigated. The universality of expression for a threshold increment of amplification, valid in a big range of change of parameters is shown. The threshold increment of amplification in the light guide in which spontaneous noise can amplify in big solid angle is experimentally confirmed. Experiments on SBS amplification in the light guide of small signal images are carried out. Amplification coefficient ~106 is obtained without essential change of picture quality.

Transformation of the ytterbium laser spectrum with pulsed selective pumping

The Yb-doped crystals and glasses present promising active media for high power lasers. These media are typical of broad gain linewidths -- near 10 nm for Yb:YAG and 100 nm for glasses. The experiments have been performed on pumping Yb- doped YAG and glass samples in a cavity by Nd:YAG laser at 0.94 micrometer. The pumping intensity was 105 divided by 106 W/cm2, stored energy density, 1 - 2 J/cm3. For Yb:YAG simultaneous oscillations have been produced at transitions near 1.03 and 1.05 micrometer. The spectral width for both bands reached 3 nm. There was established a competitive character of lasing at these transitions displayed in opposite amplitude modulation of the corresponding oscillation envelopes. The laser spectrum transformation using the interconnection of 1.03 and 1.05 micrometer transitions is discussed. When pumping the Yb-doped silicate glass there were registered the spectra near 1.03, 1.04 and 1.06 micrometer with the total width over 40 nm. A method of the laser spectrum transformation based on the stored energy transfer over inhomogeneously broadened emission line is proposed for Yb glasses. The expected range of the spectrum transformation makes several tens of nm, the transfer rate 103 - 104s-1. The obtained results are of interest for the creation of pulse-repetitive laser sources with the spectrum varying from shot to shot.

Absolute instability of oppositely directed waves without a frequency detuning and a high-reflectivity phase-conjugate mirror

An absolute instability achieved at a zero-frequency detuning and at an input-signal energy of 10 to the -10th J is described, which has a high weak-signal reflection coefficient of 5 x 10 to the 6th. Absolute instability of the interacting waves is demonstrated both with and without having an external signal applied. The wave detuning can be varied smoothly by means of angular detuning of the oppositely directed pump reference waves, which also permits angular selectivity in the reflection of an external signal.