Performance of digital servos in an optical frequency transfer network

We demonstrate the use of three kinds of flexible digital servos for the stabilization of the optical fiber link, the interferometer temperature, and the polarization of the transmitted light at the remote site, respectively. The main fiber noise cancellation digital servo provides a large phase detection range (similar to 2(10)pi radians), automatic relock function, and low cycle-slip rate over a 62 km field-deployed fiber link achieved by utilizing a feedback optical actuator of an acousto-optic modulator fed by a voltage-controlled oscillator. The temperature control and polarization control digital servos enable that the temperature of the interferometer can be stabilized at a stability of 0.01K and the data uptime is enhanced from 85.5% to 99.9% by implementing the polarization controller. The results demonstrate that the performance of the three digital servos is sufficient for high-precision optical frequency transfer applications and indicates comparable performance to existing analog optical frequency control systems. The full digital controlled optical frequency transfer method demonstrated here provides guidance for the development of a low-cost, low-complexity, and high-reliability optical frequency transfer system. Published under license by AIP Publishing.