Novel features of phase conjugation at SBS of beams passed through an ordered phase plate

The phase conjugation (PC) at steady-state stimulated Brillouin scattering (SBS) of an aberrated laser beam is numerically investigated in the case of a PC-mirror composed of an ordered phase plate, main focusing lens and SBS-cell, The ordered phase plate has the form of a raster of identical small lenses. Our calculations show that in the far field the Stokes beam has an ordered discrete structure including one central mode and several modes with angular shifts that corresponds to experimental data obtained previously. Furthermore, we optimise the PC-mirror arrangement and find a new effect of extremely low noising of Stokes beam. Its essence is in the fact that the angular selection of central Stokes mode leads to nearly perfect PC when the input window of SBS-cell is placed in a certain intermediate region between the focal plane of rasher's lenses and the focal plane of the main lens. The PC coefficient h after angular selection reaches 95%, whereas prior to the selection it does not exceed the value of similar to 30%. The excellent PC fidelity is thus achieved at the expense of some energy loss due to cutting off shifted Stokes modes. Moving the SBS-cell towards the main focusing lens increases the selector transmission coefficient, but leads to a rather appreciable decrease of PC coefficient after the selection. It is shown also that the usage of the Gaussian random phase plate instead of ordered one, which was traditional in the PC research, results in the substantially poorer PC fidelity with h less than or equal to 30% and unprincipled role of angular selection. (C) 1999 Published by Elsevier Science B.V. All rights reserved.