Towards Integrated Wideband High Resolution Optical Synthesizers
In the absence of optical frequency dividers, stable optical frequency combs are often used to provide a direct frequency relationship between a laser and an RF source in an optical frequency synthesizer. Here, an optical frequency synthesizer is presented where an integrated electro-optical phase-locked loop (EOPLL) is used to phase-frequency lock a tunable laser (TL) to the teeth of an optical frequency comb. Frequency synthesis is performed in two phases. In the coarse tuning phase, the TL is continuously tuned, passing through reference comb teeth, while a tooth indexing system selects the desired tooth as the reference frequency to the EOPLL. Once the TL is phase-locked to the selected comb tooth, fine tuning is done using a heterodyning RF local oscillator. The implemented synthesizer is capable of optical frequency synthesis over a 5 THz range with sub-Hz resolution and a tuning speed of 0.5 THz/sec at 20 MHz coarse resolution.