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 wit h angular shifts that corresponds to experimental data obtained previously. Furthermore, we optimise the PC-mirror arrangement and find a new effect o f 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 t he input window of SBS-cell is placed in a certain intermediate region betw een 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 t o the selection it does not exceed the value of similar to 30%. The excelle nt PC fidelity is thus achieved at the expense of some energy loss due to c utting off shifted Stokes modes. Moving the SBS-cell towards the main focus ing lens increases the selector transmission coefficient, but leads to a ra ther appreciable decrease of PC coefficient after the selection. It is show n also that the usage of the Gaussian random phase plate instead of ordered one, which was traditional in the PC research, results in the substantiall y 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 righ ts reserved.