Compact frequency stabilization system for a master laser diode in optical phase locked loop using a single reference cavity for simultaneous application of optical feedback along with Pound-Drever-Hall method

For the purpose of heterodyne optical phase-locked-loop (OPLL), a compact and efficient frequency stabilization system has been developed using a single reference cavity for simultaneous application of optical feedback along with electrical feedback. Using the identical reference cavity for each feedback technique makes the system compact. The phase margin of optical feedback loop can be maximized by stabilizing the center frequency of a laser diode (LD) to the reference cavity mode, which is identical to the optical injection locking mode based on negative electrical feedback. To improve the long-term frequency stability relative to the reference frequency, the optical feedback path length has also been controlled by using frequency error signal obtained by Pound-Drever-Hall technique. With present system, the linewidth of a 35mW visible LD has been reduced to less than 40kHz. Using this frequency stabilized LD as a master light source, heterodyne optical phase-locking has also been carried out with Littorow configuration extended-cavity slave LD in order to evaluate applicability of our system to the OPLL. The achieved minimum square root of the Allan variance of the beat frequency fluctuation was 3.8×10 -13 at integration time of 10 sec. Throughout this phase-locking experiment, we could confirm that our master system maintained good time base stability, not showing any system instabilities such as mode hopping or disturbances.