Closed-loop polarization mode dispersion mitigation for fiber-optic time and frequency transfer

A polarization-switching pulse interleaver is shown to be effective in reducing timing noise due to polarization mode dispersion in time and frequency transfer based on mode-locked lasers and standard single-mode (SM) fibers. In closed-loop time transfer over a 30-km dispersion-compensated fiber link with 300 fs of differential group delay, polarization interleaving reduced the delay variations to <20 fs. The results indicate that the remaining drift is caused by polarization-dependent loss and by AM-to-PM noise conversion in the photodiodes, suggesting the need for a "double-balanced" phase detector in the receiver, i.e., a phase detector balanced in power and polarization. By mitigating the polarization dependence, this work demonstrates a simple approach that can potentially yield sub-femtosecond-level, long-term time transfer in long-haul fiber links utilizing standard single-mode fibers.