Pump-induced lensing in Nd:YVO₄ using transient wavefront measurements

Understanding pump-induced lensing in laser amplifiers is important for power scaling of solid-state lasers with maintenance of high beam quality, however, there is still incomplete knowledge of the relative role of different lensing processes, particularly between thermally-induced lensing and excited-state population-induced lensing. In this study, we directly measure the transient dioptric lensing in the diode-pumped neodymium-doped yttrium vanadate (Nd:YVO₄) gain medium using a time-gated wavefront sensor. Pulsed pump-probe measurements of the transient growth and decay of pump-induced lensing clearly distinguish thermal and population lensing mechanisms by their different time signatures. Comparison of experimental transient pump-induced lensing results to analytical theory provides quantitative information on the absolute and relative thermal and population contributions to the amplifier lens under both lasing and non-lasing conditions. Observation of Nd:YVO₄ samples with different doping concentrations indicates higher heating in higher-doped sample due to increased energy transfer upconversion (ETU) leading to higher thermal lensing. From the measured population-induced lens and comparison to theory, quantitative estimate is made of the difference in polarizability of the excited and ground state in the Nd:YVO₄ Δα_p ≈ 7.7 10^-32 m³.