Transition intensity calculation of Yb: YAG

The Yb:YAG is an excellent high-average power and ultra-short pulse laser crystal. Transition intensity parameters A_tp^k and Huang-Rhys factors are fitted to its emission spectrum by the full-profile fitting method. Calculated results indicate that the emission spectrum of Yb:YAG at cryogenic temperature consists of three pure electron state transitions and two phonon-assisted transitions, one vibronic transition releases one-phonon of 3 cm^-1, and the other vibronic transition absorbs one-phonon of 22 cm^-1. At 300 K, the phonon assisted transition of 3 cm^-1 turns into two-or more-phonon assisted transitions. The procedure absorbing phonon can reduce the thermal load of Yb:YAG and improve the laser efficiency, which may be one of the reasons why Yb:YAG has excellent performance. The emission bands of Yb:YAG are broadened thermally, and the peak values decrease by several times. The emission cross sections of Yb:YAG determined by Fuchtbauer-Ladenburg (F-L) formula are remarkably different from those calculated with A_tp^k, which indicates that it is necessary for a laser material to determine its transition intensity parameters A_tp^k in order to reasonably evaluate the laser performance.