Positive-branch unstable resonators with thermal lens compensation

In order to compensate for thermal lensing in Nd : YAG rod active media, placed in unstable resonators with a super-Gaussian reflectivity profile of the output mirror, a deformable thin glass plate was used as the rear mirror. Unstable resonators with a magnification of M = 1.8 and with one and two Nd : YAG rods inside were investigated. By a proper variation of the radius of the thin glass plate, the magnification of the resonator was maintained constant by varying the refractive powers of the rods. For compensated resonators, the output energy of the laser working at 10 Hz repetition rate and pump energies up to 50 J per pulse was close to those obtained for a 1 Hz repetition rate.