Second-harmonic generation and optical stabilization of a diode laser in an external ring resonator

The second harmonic of the 842-nm output of a GaAlAs diode laser is generated in a KNbO3 crystal in a resonant, external ring cavity. The diode laser is optically stabilized to the ring cavity through feedback from the counterpropagating fundamental wave, which is weakly excited in the resonator. We have produced 6.7 mW of tunable, narrowband radiation at 421 nm and have used that light to perform saturation spectroscopy on narrow transitions in rubidium.