Hybrid structure Mach-Zehnder interferometer based on silica and fluorinated polyimide microfibers for temperature or salinity sensing in seawater

An all-fiber hybrid structure Mach-Zehnder interferometer (MZI) based on silica fiber and fluorinated polyimide microfiber (FPMF) for temperature or salinity sensing in seawater is proposed. Theoretically, transmission characteristic and sensing sensitivity of the MZI are derived for this kind of hybrid structure, and typical interference spectrum is observed around optical communication band. Experimentally, by tracking the shift of interference peaks, temperature and salinity sensing are demonstrated with sensitivities of about 140 pm/degrees C and 64 pm/parts per thousand, respectively, which are much higher than those of existing fiber Bragg gratings and microfiber knot resonators. In addition, dependences of sensitivity on sensor parameters, such as fiber diameter and fiber lengths, are also investigated. Unlike the other microfiber based sensors that higher sensitivity usually comes from the thinner fiber, sensors demonstrated here show that sensitivities can be tuned by changing the path-length difference of the two arms, which greatly decreases the difficulty in component fabrications. Temperature or salinity sensor demonstrated here show advantages of simple fabrication, low cost, tunable sensitivity and dynamic range, relatively high sensitivity and compatible with standard fiber optics system, which may have potential applications in developing low cost and compact size optical sensors in ocean or other liquid surroundings. (C) 2018 Elsevier Ltd. All rights reserved.