Evidencing the Resonant and Non-Resonant Contributions to the Index of Refraction of a Nd:YAG Rod Amplifier

An index gradient is generally induced in an amplifier medium (AM) upon its longitudinal or transverse optical pumping. This index gradient acts as lens that needs to be properly compensated to avoid the degradation of the beam shape quality at the exit of the AM. Two contributions are responsible for this index gradient. One is mainly assigned to the heating of the AM amplifier [1]. However, besides this thermal lens effect, another phenomenon often overlooked, is linked to the change of the AM complex refractive index
$n(\lambda)=n^{\prime}(\lambda)+in^{\prime\prime}(\lambda)$
upon the population of the different electronic states of the doping ions [2]. It was found to be comparable with the thermal effect in Nd-doped materials [3]. Hereafter, using a wavefront analysis method, we show that excited state population and thermal lenses can be easily revealed in Nd:YAG rod amplifier under quasi-continuous side pumping by laser diode bars centered around 808 nm. This contribution resonant around 1064 nm for Nd
³
+ ions is evidenced in Nd:YAG rod amplifier upon its optical pumping.