High-Accuracy Optical Fiber Transfer Delay Measurement Using Fiber-Optic Microwave Interferometry

Optical fiber transfer delay (OFTD) measurement with high accuracy and stability is an urgent demand for many applications such as fiber-optic sensors and fiber-based distributed systems. In this article, we propose a novel method using fiber-optic microwave interferometry to meet the above practice demand. Two incoherent optical carriers with different wavelengths are coupled into an intensity modulator driven by a microwave signal. The intensity-modulated signal is then divided into two portions through a dense wavelength division multiplexer. One portion directly passes through the reference path while the other undergoes the transfer delay of a fiber under test (FUT). After photo-detection, two probe signals that undergo different delays are recovered and superimposed. By sweeping the microwave frequency, periodic microwave interference fringe is generated. Then, OFTD measurement is achieved by measuring the frequency of the last valley in the interference fringe. Experimental results show that a system stability of +/- 0.02 ps, an accuracy of +/- 0.07 ps, and a measurement range of at least 500 m are achieved.