Abstract

Frequency doubling of a GaAlAs diode laser using a build-up cavity with a KNbO₃ crystal is an attractive approach for developing an efficient, compact blue light source.^1-3 If the laser and resonator frequencies are precisely matched the 856-nm fundamental radiation builds up to a high intensity which results in efficient second-harmonic generation of 428-nm light. Small changes of the diode-laser wavelength due to current (~5 GHz/mA) or temperature (~40 GHz/°C) variations and shifts of the cavity resonance induced by self-heating phenomena² result in substantial fluctuations of the infrared power inside the resonator and thus in the blue output. At low laser powers, passive self-locking based on optical feedback has been demonstrated.¹ Here, have used a simple, effective, electronic frequency locking technique that resulted in the generation of 9.5 mW of stable blue output from a frequency-doubled GaAlAs diode laser. In order to lock the GaAlAs diode laser frequency to the monolithic ring KNbO₃ resonator of ~200 MHz linewidth, we modified the Pound-Drever technique often used for stabilizing dye and Argon lasers⁴ by exploiting the convenient modulation and tuning properties of the semiconductor laser. To produce small sidebands around the ~10 MHz-wide laser emission line, a weak 291 MHz current was superimposed on the dc injection current by capacitive coupling. By using a double-balanced mixer for heterodyne detection of the light reflected from the input surface of the resonator, a strong servo signal indicative of the match of the laser and resonator frequencies was generated.⁴ This servo signal was amplified and used to control the dc-current of the diode laser so that frequency stabilization was obtained by tuning of the laser frequency so that it tracked the cavity resonance. With the servo loop closed, the GaAlAs laser frequency remained locked on resonance for several hours, producing a stable 428-nm output power of 9.5 mW from the 63 mW of near-infrared power incident on the KNbO₃ resonator.

© 1989 Optical Society of America