Highly coherent RF signal generation by heterodyne optical phase locking of external cavity semiconductor lasers

Two narrow-linewidth (<50 kHz) external cavity semiconductor lasers are successfully offset phase-locked to a reference radio-frequency (RF) signal by using a discriminator-aided optical phase-locked loop. By incorporating a phase advance circuit to increase the loop bandwidth, a highly coherent optically heterodyned RF signal with a total phase variance of 1.19/spl times/10/sup -4/ rad/sup 2/ is demonstrated. The measured noise level is -116 dBc/Hz close to the carrier (200-kHz offset), increases to -110 dBc/Hz at an offset of 5.5 MHz, and then decreases to less than -120 dBc/Hz at offset frequencies larger than 10 MHz.