Characteristics of alexandrite lasers

Alexandrite, BeAl2O4:Cr3+, is a tunable solid-state laser material similar in physical characteristics to ruby but which is capable of four-level vibronic lasing at and above room temperature. The lasing band extends from below 700 nm to over 800 nm and shows a marked dependence of gain on temperature. The vibronic lasing in alexandrite involves a relatively low-emission cross section, which depends strongly on both temperature and wavelength. As a result the optimum optical output coupling is low in comparison with most solid-state insulator lasers. The low-emission cross section implies a high saturation flux, and consequently efficient energy extraction depends on high intra-resonator power densities. As a result optimum energy storage for Q-switched operation is effectively determined by the optical damage limits of resonator components, and there is an increased dependence on the oscillator in contrast to amplifier stages in the design. In the design of the alexandrite laser, optimum departures from conventional Nd:YAG and ruby performance depends on certain relatively minor systems. A broadband pump chamber reflector is required to cover the pump bands of alexandrite which extend from 380 out to 650 nm, yet operation at high average power can tax the chemical stability of many broadband metallic reflectors. The temperature-dependent gain leads to designs that permit the rod temperature to be varied. The highest energy extraction efficiencies are achieved when the rod temperature is near and above 100°C.