Z-scan measurements of the nonlinear refractive index of a pumped semiconductor disk laser gain medium

We measure the nonlinear refractive index of an antiresonant 1050 nm semiconductor disk laser (SDL) gain medium using a reflection-type z-scan system, with a 1064 nm, 10-ps-pulse laser as a probe, and a fiber-coupled 808 nm diode pump laser for carrier injection. Empirically, the nonlinear refractive index is found to depend approximately linearly on applied pump intensity, having a value of −1.5(0.2) × 10⁻¹² cm²/W at zero excitation but increasing to take on positive values at typical SDL operating conditions. The focal lengths of corresponding Kerr lenses calculated using typical SDL pulse intensities and spot sizes are sufficiently short to be comparable to SDL cavity mirrors, implying that Kerr lens modelocking may be responsible for the behavior described in recent reports of self-mode-locked SDLs.