TIME-RESOLVED DIVERGENCE MEASUREMENT OF AN EXCIMER-LASER BEAM BY THE KNIFE-EDGE TECHNIQUE

We present a simple method to measure the time-evolution of the diverg ence of pulsed laser beams, joining the time-resolution capability of photodetectors and the space-resolved information of the knife-edge te chnique. We measured the time-integrated and the time-resolved diverge nce of a long pulsewidth, high output energy XeCl laser (lambda = 308 nm), equipped with positive branch unstable resonators having either m agnification factor M = 5 or M = 9. In both cases, the time-resolved b eam divergence was smaller than the time-integrated one for most of th e 100 ns full-width-at-half-maximum laser pulse duration. A minimum di vergence of (40 x 23) mu rad(2) at 86.5% energy content and a peak rad iance of 9 x 10(14) W/cm(2)/ster were achieved a few ns before the pea k of the laser pulse, for the case of M = 9. The corresponding time-in tegrated values were (56 x 38) mu rad(2) and 5 x 10(14) W/cm(2)/ster, respectively.