Improvement of CuBr laser coherence properties

Abstract

Great improvement of CuBr laser beam spatial coherence was made by a special design of the laser resonator, the generalized diffraction filtered resonator. Utilizing it diffraction-limited beam divergence can be easily obtained throughout the laser pulse. Since the spatial coherence is in inverse relation with the beam divergence, decreasing the latter we increase the former. The temporal evolution of beam divergence for the more intense green (λ=510nm) laser line was measured within laser pulse of MO (master oscillator) CuBr laser system fitted with a stable plane-plane resonator (PPR), a confocal unstable resonator of positive branch (PBUR) and a generalized diffraction filtered resonator (GDFR). With the MOPA (master oscillator power amplifier) system only GDFR was used. The estimations were verified by direct coherence measurements by means of a reversal shear interferometer that was a modified Michelson interferometer. The estimations as well as the direct measurement of spatial coherence show that coherence degree increases from PPR through PBUR to GDFR. Moreover, with GDFR it is time-independent. With MOPA system the coherence degree goes up further. So the degree of coherence measured interferometrically with MO is: for PPR - 0.16, for PBUR - 0.28 and for GDFR - 0.36. For MOPA the measured degree of coherence reaches 0.65. The estimated and the measured coherence trends show similarity. Based on the Michelson interferometer and having just four optical components (a spherical lens, an optical wedge and two plane mirrors), a new rigid instrument for spatial coherence analysis of optical beams was introduced as well.