Transition intensity calculation of Yb:YAG**Project supported by the National Natural Science Foundation of China (Grant Nos. 61405206, 51502292, and 51702322), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant Nos. CXJJ-16M251 and CXJJ-15M055), and the National Key Research and Development Program of China (Grant No. 2016YFB0402101)

Abstract

The Yb:YAG is an excellent high-average power and ultra-short pulse laser crystal. Transition intensity parameters 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 , which indicates that it is necessary for a laser material to determine its transition intensity parameters in order to reasonably evaluate the laser performance.