Investigation on the Impact of Hi-Bi Fiber Length on the Sensitivity of Sagnac Interferometer

Optical fiber Sagnac interferometers (OFSIs) constructed by a 3-dB coupler and a high-birefringence (Hi-Bi) fiber has been developed into various sensors in recent decades. However, the discussion on the length of Hi-Bi fiber in this setup remains limited. In this paper, the sensing length of the Hi-Bi fiber is separated from the total length of the Hi-Bi fiber for the first time in analysis and the impact of Hi-Bi fiber length on the sensitivity of the Sagnac interferometer is investigated. It is found that the sensitivity of OFSI is linearly proportional to the ratio of the sensing length over the total length of the Hi-Bi fiber. This dependence is confirmed by the experimental results of an OFSI-based strain sensor. The highest sensitivity achieved is about 20 times of the lowest sensitivity achieved using the same setup but different sensing length to total length ratios. This paper provides a general guideline for the sensitivity enhancement and optimization of OFSI-based sensors.