Iterative reconstruction of thermally induced phase distortion in a Nd3+:YVO4 laser

The thermal distortions in a continuous-wave end-pumped Nd:YVO4 laser are experimentally investigated by use of an external TEM00 probe beam (TEM standing for transverse electric magnetic). After a round-trip of this probe through the active medium, three irradiance profiles are recorded along its propagation axis. The distorted phase pattern of this probe beam is then reconstructed from these profiles by means of a Gerchberg-Saxton-like iterative algorithm. To identify the contribution of each kind of aberration in the phase profile, we have computed a Zernike polynomial expansion, and have shown that the main terms constituting the aberration are (in order) defocusing (optical path difference (OPD) 0.7 lambda FWHM (full width at half-maximum)), spherical aberration (OPD 0.3 lambda FWHM), and first-order astigmatism (OPD of +/-0.14lambda(p) FWHM). Moreover, all higher-order terms result in a significant phase peak (lambda/4 FWHM over 400 mum).