Digital system for keeping optical frequency comb in long-term stable operation

The presented work deals with a novel application of digital signal processing and software-defined radio algorithms for keeping an optical frequency comb in long-term stable operation. We are using a two-stage stabilization scheme. A frequency-locked loop (FLL) pre-stabilizes the offset frequency and steers it into the capture range of a phase-locked loop (PLL) that consequently takes control. Such combination of FLL and PLL together with additional control logics is also able to handle situations when the PLL unexpectedly goes out of lock and so it is capable of keeping the optical frequency comb in a long-time stable operation. The stability of the resulting system was investigated. Allan deviations from data measured during 8 days of operation were computed. The resulting long-term stability of the setup is better than 1.6 × 10^-11.