High-Accuracy Optical Fiber Transfer Delay Measurement Based on Comb-Enhanced Phase Detection and Phase-Derived Ranging

We propose and experimentally demonstrate an approach for high-accuracy optical fiber transfer delay (OFTD) measurement based on comb-enhanced phase detection and phase-derived ranging. To achieve a high measurement accuracy, two comb lines selected from an optical frequency comb (OFC) with a large frequency interval are transmitted through a fiber under test and used for OFTD measurement. With the use of the dual-heterodyne phase error transfer scheme, the phase change of the two comb lines induced by the OFTD variation can be mapped to the phase change of a lowfrequency RF signal, which can be measured using a highaccuracy phase detector. To extend the measurement range, a phase-derived ranging approach is employed, in which the frequency interval of the OFC is swept within a small range and the integer ambiguity caused by phase detection can be removed. In the experiment, an ultrahigh OFTD measurement accuracy of ±0.025 ps and a large measurement range over 20 km have been achieved. The proposed system holds unique advantages of high measurement accuracy and large measurement range, which is potential to be widely used in distributed coherent radar, optical fiber communication and fiber-optical sensing systems.