Digital Instrumentation for Phase-Coherent Frequency Transfer over 300 km Fiber Link

This paper presents a digital instrumentation for coherent frequency transfer over fiber links. It detects the phase and the amplitude of the beat notes at the two ends and uses them for compensating the fiber phase noise and the polarization rotation. The system is implemented on the commercial platform Red-Pitaya, which integrates the Zynq System-on-Chip with fast Analog to Digital and Digital to Analog Converters, and provides great flexibility for on-site reconfiguration and remote operation. The system we propose can be used for Doppler and two-way compensations, both in open and closed loop. Additionally, it features a detection bandwidth of 10 MHz which is of particular importance for minimizing cycle slips in thousand kilometer links. The electronics exhibits residual frequency stability of the order of 10-18 at 1 s exceeding 10-21 at 10000 s. We verified the system performance on a 300 km loop link for the two-way scheme.