Interrogation Sensing Scheme Based on a Figure-of-Eight Fiber Loop Mirror

A fiber optic interrogation sensor scheme based on a “figure-of-eight” configuration created from a single directional 3 $,\times,$ 3 fiber optic coupler is proposed. Two loops are formed in each arm and one of them contains the sensing head and the other is used as reference signal. A theoretical study based on Jones matrix analysis of this fiber loop mirror combination is reported. The optical configuration is tested as an interrogation scheme for a fiber strain sensor where the spectral response arises from the combination of the reference signal modulated by the sensor signal. The strain sensor configuration shows a phase sensitivity of $6.7\pm 4.38\times 10^{-2}~{\rm mrad/}\mu{\rm stra\in}$ by linear regression.