HIGHLY COHERENT RF SIGNAL GENERATION BY HETERODYNE OPTICAL-PHASE LOCKING OF EXTERNAL-CAVITY SEMICONDUCTOR-LASERS

Two narrow-linewidth (<50 kHz) external cavity semiconductor lasers ar e successfully offset phase-locked to a reference radio-frequency (RF) signal by using a discriminator-aided optical phase-locked loop. By i ncorporating a phase advance circuit to increase the loop bandwidth, a highly coherent optically heterodyned RF signal with a total phase va riance of 1.19 x 10(-4) rad(2) is demonstrated. The measured noise lev el 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.