A 40 GHz clock with low jitter is recovered from a 640 Gbit/s optical time-division multiplexed (OTDM), single-polarised, return-to-zero PRBS data signal. The clock recovery circuit is an injection-locked loop which contains a 40 GHz Gunn oscillator, a 40 GHz pulse source, and a semiconductor optical amplifier (SOA) assisted by a filtered chirp as optical phase comparator.
Mulvad, H. C. H., Tangdiongga, E., Waardt, de, H., & Dorren, H. J. S. (2008). 40 GHz clock recovery from 640 Gbit/s OTDM signal using SOA-based phase comparator. Electronics Letters, 44(2), 146-147. https://doi.org/10.1049/el:20083045