Abstract

Generation of frequency correlated laser beams is of importance in various applications, in particular, for extending the frequency of the laser Raman microwave clock¹ into the millimeter region where acoustooptic methods are not available. We propose and demonstrate an all-optical technique for such a purpose. The technique is based on the recently observed Raman-shifted oscillation² that occurs when a three-level atomic system of Λ type is optically pumped and placed in a ring cavity. Our initial experiments are performed in sodium vapor pumped by a cw ring dye laser. Raman-shifted oscillation, detuned from the pump laser by 1.772 GHz which is the ground state hyperfine splitting of sodium, occurs in the wings of the Doppler broadened D₁ line. This Raman-shifted oscillation is demonstrated to be highly frequency correlated with the pumping laser when the two are heterodyned on a fast photodetector. In our experiments the heterodyne beat signal at 1.772 GHz had a linewidth as low as 17 kHz, whereas the pumping laser has a rms frequency fluctuation of 300 kHz. The beat signal stays narrowband when the pumping laser is unlocked from the reference cavity increasing its rms frequency fluctation to ~10MHz.

© 1985 Optical Society of America