Two-photon absorption in potassium niobate

We report measurements of thermal self-locking of a Fabry–Perot cavity containing a potassium niobate (KNbO3) crystal. We develop a method to determine linear and nonlinear optical absorption coefficients in intracavity crystals by detailed analysis of the transmission line shapes. These line shapes are typical of optical bistability in thermally loaded cavities. For our crystal we determine the one-photon absorption coefficient at 846 nm to be α=(0.0034±0.0022) m-1, the two-photon absorption coefficient at 846 nm to be β=(3.2±0.5)×10-11 m/W, and the one-photon absorption coefficient at 423 nm to be (13±2) m-1. We also address the issue of blue-light-induced infrared-absorption and determine a coefficient for this excited-state absorption process. Our method is particularly well suited to bulk absorption measurements where absorption is small compared with scattering. We also report new measurements of the temperature dependence of the index of refraction at 846 nm and compare with values in the literature.