Signal Demodulation Methods For Hi-Bi FLM Sensors:Research And Application

High-birefringence fiber loop mirror (Hi-Bi FLM) sensors are attractive devices in the optic fiber sensing areas due to their ease of fabrication, good stability, high sensitivity and the input polarization independence. In this dissertation, Hi-Bi FLM sensors and the Hi-Bi fiber sensing characteristics are analyzed, which include the demodulation method of sensing, the measurement method of Hi-Bi fiber beat length, the temperature and the refractive index sensing characteristics. The purpose of our work is to obtain high resolution, high sensitivity, wide range, absolute measurement. The main content of the dissertation are summarized as follows:(1) We propose a novel signal demodulation method, which is based on two adjacent resonance dip wavelengths of the spectral, call "the order of interference measurement method". In this method, the orders of the resonance dips can be calculated by any two adjacent resonance dip wavelengths, the calculate error is eliminated because of a basic property that the order of resonance dip is an integer. The absolute phase can be measured by the resonance dip, an explicit equation to describe the transfer functions between phase and external parameter can be obtained. We theoretically analyze the principle and the error of this method, and propose a criterion for this method.(2) We propose and experimental demonstrate the measurement method of beat length in Hi-Bi fiber by using the order of interference measurement method. Compared to traditional methods calculated by the broadband light source and interference spectrum, this method have higher precision, stability and easier the built. The measurement precision of beat length is only affected by the precision of Hi-Bi fiber length. In the experiment, we measure two section Panda fibers with different length, the measurement accuracy was less than 0.001 mm and have a great agreement with the theory.(3) A fiber temperature sensor based on stress-induced Hi-Bi FLM with absolute measurement and wide range is proposed and experimentally demonstrated. The comparison of the traditional signal demodulation methods and the order of interference measurement method show that the novel method can obtain wide range, absolute temperature measurement and has the same precision with the spectral shift method. Experimental measured the changes of these parameters versus temperature in a range of 35-350?, with a measurement precision of?0.04? and a temperature accuracy of?0.09?, the results verify the theoretical model well.(4) An optical fiber refractive index (RI) sensor fabricated by etched-stress applying parts (SAPs) Panda fiber is researched and experimentally demonstrated. An etched-SAPs structure is designed by using selective etching technique because the SAPs are etched faster than the cladding layer in Hydrofluoric acid (HF) solution. A high precision method for controlling the etching process is proposed and a section of Panda fiber is chemically etched, high sensitivity of 456 nm/RIU (refractive index unit) and a resolution of 4.4×10-5 RIU are obtained for the proposed liquid refractive index sensor.The sensing signal demodulation method proposed in this dissertation show advantages over Hi-Bi FLM sensing in many aspects such as high resolution, high sensitivity, wide range, absolute measurement, its engineering application value may be further improved.