Experimental determination of the nonlinear refractive index in an operating Cr:forsterite femtosecond laser

The characteristics of a femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd:YAG laser are reported. Transform-limited femtosecond pulses of 60-fs (FWHM) duration with average TEM 00 output powers of 200 mW at 1.25 μm were generated. Experimental studies of power stability and self-starting regime are described and a new experimental method to determine the nonlinear refractive index of the chromium-doped forsterite crystal (which is either the laser rod) is reported. We infer a value of 6 × 10 −16 cm 2/W at 1.25 μm, making possible Kerr-lens mode-locking.