Out-of-Band Fiber-Optic Time and Frequency Transfer Using Asymmetric and Symmetric Opto-Electronic Repeaters

Signal repeaters for fiber-optic communication can be realized with back-to-back connected transceivers. This configuration can provide high gain ( $\approx 30$ dB) at low cost, and the needed semiconductor lasers and modulators can be realized for practically any relevant wavelength. Unfortunately, for time and frequency (TF) transfer the uncorrelated wavelength drifts in the transceiver lasers can compromise transfer stability, and device replacement due to failure may result in large time offsets that have to be measured via global navigation satellite services (GNSS). This work demonstrates that good results can nevertheless be obtained with standard telecom dense wavelength-division multiplexing (DWDM) transceivers over long time periods (years). More importantly, a simple wavelength-symmetric repeater is proposed that can be used to cancel the detrimental effects of wavelength drifts and which lessens the need for link recalibrations after transceiver replacements. In a proof-of-concept test setup, timing drift due to wavelength drift of a repeater laser is reduced by approximately two orders of magnitude.