1.55-μm optical phase-locked loop operation with large loop delays and monolithically integrated p-i-n/HBT photoreceivers

Abstract

The optical phase-locked loop (OPLL) provides a method of generating channel offsets for use in dense wavelength division multiplexed (WDM) systems. The development of monolithically integrated OPLLs will make their widespread application more feasible. Typical approaches to making hybrid OPLLs use either very narrow linewidth lasers or reduce the loop delay as much as possible. The use of semiconductor lasers with their moderately large linewidths has encouraged the development of very compact but inflexible loop designs. Previous designs using semiconductor lasers have avoided the use of fiber couplers to minimize loop delay. A hybrid, fiber-based system allows design, layout, and testing flexibility which is necessary in a test bed for the development of components for an integrated OPLL. In spite of the large loop delay that comes from using fiber components, we demonstrate that phase- locking can be achieved using semiconductor lasers of moderate linewidths and fiber components. We achieved a very wide hold-in range of 1.558 GHz and possible locking frequencies ranging from 100 to 20.75 GHz, both of which are among the best reported values. As the first step towards integration, a monolithically integrated p-i-n/HBT photoreceiver was successfully employed in the test bed.