Coherent Optical Fiber Sensing Based on a Frequency Shifting Loop

In this paper, the proof-of-concept of an optical fiber sensing technique based on a frequency shifting loop (FSL) is proposed. The system combines both a refreshing rate of the sensing measurement in the MHz range and a centimetric spatial resolution. The FSL is used as a versatile source of broadband optical chirped waveforms, with controllable chirp rate. Digital pulse compression is applied to optimize the system's signal-to-noise-ratio. The sensor system is used to monitor the phase perturbation applied to a localized transducer (an electro-optic phase modulator) placed in an optical fiber. A preliminary and promising sensitivity <; 4 μrad/Hz^(1/2) in a 20 kHz-2 MHz bandwidth is obtained without any stabilization of the FSL. The sensor system is also tested on an optical fiber containing two transducers, emulating a quasi-distributed antenna. The system enables the accurate measurement of a phase shift between the two modulators, making it suitable for geolocation operation.