Investigation On Micro-structure Fiber Optic Sensing Technology Based On Mode Coupling Interference

In recent decades, due to the development of optoelectronic technique and opticalcommunication, the obtaining of information is becoming more and more important. Fiberoptical sensing technology is one of the most important technologies to sense, measure,monitor and convert information, which transmits the information of optical wave throughthe fibers. Compared with the traditional sensors, fiber sensors have many advantages,including small size, easy fabrication, high stability, immune to electromagnetic interference,adapt to work under hostile environment, high sensitivity and the convenience for senornetworking, etc. it has been widely used in several tens of parameters sensing applications,including the fields of defense and military, earthquake, transportation, electricity, energy,aeronautics and astronautics, sanitary and medical services, etc.Our work is supported by the National Natural Science Foundation of China:“BasicResearch for Key Technology of Fiber Earthquake Wave Monitoring in Major ProjectsDisaster Warning”, and focuses on the investigation of novel optical fiber sensor structures.The main research achievements are described as follows:(1) Generally summarize the research status of fiber optical sensing technology, anddescribe the principle and classification. Introduce the development of fiber optical sensorsthat based on Mach-Zehnder interferometer, multimode interference, and fiber Bragggratings in detail.(2) We propose and demonstrate a SMTS（Single mode-Multimode-Thinned-Singlemode） fiber structure based Mach-Zehnder interferometer for multi-purpose sensingapplications，including the measurements of refractive index, liquid level, temperature andaxial strain.(3) A simple and compact multimode interferometer structure based on No-Core fiber(NCF) and Fiber Bragg gratings (FBG) is proposed for simultaneous measurement ofsurrounding refractive index (SRI) and temperature. Two measured methods are novel andvarious. Moreover, experimental results indicate that the SRI sensitivity of the FBG reflectedcore mode is up to160.23dB/RIU, which is the highest value to our knowledge.(4) Based on the optical waveguide coupling equation, we describe and analyze thethree co-directional guided modes coupled in the two long period gratings using the supermodes theory and the transfer matrix method. This kind of design can form the basisfor a new generation of mode converting, multi-wavelength-tunable band-pass opticalfiltering and optical switching.