Beam cleanup by stimulated Brillouin scattering phase conjugation for a KrF laser at high repetition rate

A stimulated Brillouin scattering (SBS) phase conjugation with a KrF amplifier is used to obtain the high-pointing stability and small beam divergence necessary for precision micromachining. The goal is to achieve a beam divergence 1.5 times the diffraction limit, 0.1 cm^-1 of a bandwidth, and the high-pointing stability at a pulse rate of 1-kHz. An experimental scheme that provided preliminary results of beam cleanup by SBS phase conjugation up to 100 Hz of the pulse rate is described. The method used to analyze the spectrum distribution in the beam cross section is presented, and examples of the Fabry-Perot fringes and calculated shift (in wavenumber) mapped onto the beam cross section are show. This example indicates that the spectrum in the band-narrowed KrF laser can be nonuniform, and it can cause poor reflectance in the phase conjugation. Calculation of the energy distribution associated with the narrowband and broadband portions showed that the narrowband seed beam contained a considerable amount of the broadband component, which is mostly distributed in the outer part of the beam.