High-Resolution Optical Frequency Synthesis Using an Integrated Electro-Optical Phase-Locked Loop

Optical frequency synthesizers have widespread applications in optical communication, sensing, and ranging systems. These synthesizers typically consist of an electro-optical phase-locked loop (EOPLL) that phase locks a widely tunable laser (TL) to the teeth of a highly stable optical frequency comb. In this paper, a partially integrated optical frequency synthesizer is reported, where a stable and low-noise frequency comb is used as the optical reference. In the proposed architecture, frequency synthesis is done in two phases. In a coarse tuning phase, a widely tunable distributed Bragg reflector laser is continuously tuned by adjusting its mirrors and phase section currents. While the laser is being tuned, an indexing system detects and counts the number of passing comb teeth. Coarse tuning is concluded when the TL reaches the target comb tooth. Following the coarse tuning phase, a heterodyne integrated EOPLL phase locks the TL to the selected comb tooth. Under the phase lock condition, fine frequency tuning is performed by adjusting the frequency of an RF local oscillator. The proposed synthesizer has the capability of synthesizing optical frequencies over a 5-THz range with a coarse tuning speed of 0.5 THz/s and coarse and fine tuning resolutions of 20 MHz and less than 1 Hz, respectively.