Optical bistability and differential gain between 85 and 296 °K in a Fabry-Perot containing ruby

Bistability, differential gain, discriminator, clipper, and limier actions were observed using a plano-concave Fabry-Perot cavity containing ruby. Input powers were approximately 20 mW from a cw ruby laser. Operation wa anticipated near 77 °K where the R1 transition was nearly resonant with the laser. However, the device functioned at room temperature. The refractive index of ruby depends on the fractional ground-state population. Driving the R lines leads to a nonlinear refractive index due to dispersive contributions from nonresonant pump and charge-transfer bands, which explains the room-temperature operation.