Heterodyne locking of a fully integrated optical phase-locked loop with on-chip modulators

We design and experimentally demonstrate a highly integrated heterodyne optical phase-locked loop (OPLL) consisting of an InP-based coherent photonic receiver, highspeed feedback electronics, and an RF synthesizer. Such coherent photonic integrated circuits contain two widely tunable lasers, semiconductor optical amplifiers, phase modulators, and a pair of balanced photodetectors. Offset phase-locking of the two lasers is achieved by applying an RF signal to an on-chip optical phase modulator following one of the lasers and locking the other one to a resulting optical side-band. Offset locking frequency range > 16 GHz is achieved for such a highly sensitive OPLL system which can employ up to the third-order-harmonic optical sidebands for locking. Furthermore, the rms phase error between the two lasers is measured to be 8 degrees. (C) 2017 Optical Society of America