RESUMEN
Using a wavefront sensor, we have measured the temporal evolution of the lens induced in a Nd:YAG rod amplifier under side pumping by laser diode bars centered around 808 nm in a quasi-continuous wave regime. The evolution of the induced lens is drastically different when measured with a probe pulse centered at 532 nm or 1064.5 nm. To explain this evolution, we developed a model that accounts for both the excited state population of the Nd3+ ions and the thermal contribution to the refractive index of the amplifier. This model, which takes into account amplification and wavelength shift of the probe pulse at 1064.5 nm, makes it possible to quantitatively describe the spectral and temporal evolution of the amplifier focal length. It also shows that the excited state population contribution is more important around 1064.5 nm and can partly compensate for the thermal lens induced in the amplifier.