Single-Arm Frequency-Shifted Interferometry Using a Bidirectional Electro-Optic Modulator

Multiplexing fiber-optic sensors is an effective way to perform large-area quasi-distributed sensing. Among many sensor multiplexing and interrogation methods, frequency-shifted interferometry (FSI) emerges as a unique low-cost technique that is capable of resolving sensor locations using a continuous-wave broadband source, a bidirectional optical frequency shifter, and a slow photo detector. An acousto-optic modulator is typically used as the frequency shifter in FSI due to its high modulation efficiency and bidirectional operation. But its relatively low modulation bandwidth results in low spatial resolution, on the order of meters. To improve spatial resolution, single-arm FSI (SA-FSI) that employs an electro-optic modulator (EOM) as the frequency shifter was proposed, but the EOM's broad modulation bandwidth cannot be fully utilized because conventional EOMs cannot support bidirectional modulation at gigahertz frequencies due to the speed mismatch between the optical wave and the applied RF wave, especially when the two are counter propagating. In this paper, we modified a commercial EOM to increase the modulation efficiency of the counter-propagating optical wave and used it to interrogate 15 weak fiber Bragg gratings (FBGs) in an array, achieving a spatial resolution of ~3 cm and a sensing range of >1 km. A signal-to-noise ratio as high as ~18 dB was obtained with a 15-mW broadband amplified spontaneous emission source without any optical amplification. This is the first demonstration of both spatially resolved and spectrally resolved SA-FSI, and it is the first time that FBGs are interrogated with the SA-FSI technique.