Coherent Optical Generation of a 6 GHz Microwave Signal with Directly Phase Locked Semiconductor DFB Lasers

Experimental results of a wideband heterodyne second order optical phase locked loop with 1.5 μm semiconductor lasers are presented. The loop has a bandwidth of 180 MHz, a gain of 181 dBHz and a propagation delay of only 400 ps. A beat signal of 8 MHz linewidth is phase locked to become a replica of a microwave reference source close to carrier with a noise level of -125 dBc/Hz. The total phase variance of the locked carrier is 0.04 rad
²
and carriers can be generated in a continuous range from 3 to 18 GHz. The loop reliability is excellent with an average time to cycle slip of 10
¹¹
seconds and an acquisition range of 640 MHz.