Stable narrow-linewidth VECSEL operating for sodium guide star generation

In the development of laser guide star technology, Vertical External-Cavity Surface-Emitting Lasers (VECSELs) are attractive because of their simplicity and compactness. VECSELs operating at 1178 nm may readily be frequency doubled to the sodium D2 resonance at 589 nm. The output power of VECSELs can be scaled to multiple tens of watts by expanding the spot size on the gain medium, or “lateral scaling”. It can also be scaled by using multiple VECSEL devices in one cavity, or “longitudinal scaling”. In the case of longitudinal scaling, at least one VECSEL device must be at a fold of the cavity. This typically causes problems in longitudinal mode stability. In this paper, we present the results of successful experiments to demonstrate a technique to deliver stable single-frequency operation of a multi-VECSEL cavity at 1178 nm. Single-frequency output of more than 10 W at 1178 nm was demonstrated in a standing-wave VECSEL cavity with two gain mirrors. In comparison, the output power for operation with one gain mirror only was 7.4 W when operating in single frequency. The new technology paves the way for sodium guide star lasers delivering tens of watts that are more compact and substantially cheaper per watt than any existing technology.