Research Of Curvature And Low-frequency Acoustic Wave Sensor Based On Few-mode Fiber And Long-period Fiber Grating

As a new type of sensor, optical fiber sensors have been widely researched with many distinct advantages of high sensitivity, immunity to magnetic interference, corrosion resistance, small size, light weight and distributed measurement. Among them, optical fiber curvature and acoustic sensor has potential applications in structural health monitoring, earthquake early monitoring and so on. Under the support of National Natural Science Foundation of China, this thesis is devoted to study the related technology of optical fiber curvature and acoustic sensor based on few-mode fiber and long-period fiber grating, including theoretical analysis and experiment research. Main researching work and achievements of this thesis are listed as follows:(1) We demonstrate a symmetrical core-offset splicing fiber structure based on the Single mode-Few-mode-Single mode fiber(SFS) to measure curvature. The interference fringe visibility changes with the radius of the splicing structure, which can be real-time monitoring. Through optimizing design of relative offset value and the length of few-mode fiber, we analyze the interference principle and bending sensing principle of offset splicing structure. In the curvature range of 0.5-16m-1, the interference fringe visibility has been measured, a sensitivity 0.107/m-1 can be achieved in the range of 11-16 m-1 with a good linearity. So far we resolve the problems existing in curvature sensors-the curvature range is small and cannot be real-time monitoring.(2) As LPG is sensitive to bending, a curvature sensor based on SFS offset structure cascaded with a LPG is proposed. According to the principle of LPG for acoustic measurement, we also study the sensing characteristics of low-frequency acoustic wave. The principle of optical intensity demodulation has been analyzed and employed to demodulate the curvature and acoustic wave. Using the method of Jones matrix, the spectral characteristic of cascaded structure is simulated. Experimental results show that in the curvature range of 0.124 m-1 to 0.304 m-1, the transmitted optical intensity of LPG attenuation peak is linear and the sensitivity is about 93.01dB/m-1. Moreover, in the ambient of different curvature, LPG indicates clear sensitivities to acoustic frequencies between 110 Hz and 230 Hz with sensitivity of ~15mV/Pa at 110 Hz and ~4.5mV/Pa at other frequencies.(3) We improve the cascaded structure in previous chapter and propose a Sagnac interferometer based on the SFS structure cascaded with LPG. The proposed sensor can also be realized for measurement of curvature and acoustic wave. Employing the method of Jones matrix, the spectral characteristic of Sagnac interferometer is calculated. Optical intensity demodulation is also adopted and the experiment results demonstrate that in the curvature range of 0.119 m-1 to 0.349 m-1, the transmitted optical intensity of interference fringe at 1552 nm is changed from-70.255 dB to-59.712 dB. The curvature sensitivity is about 45.84dB/m-1. When the LPG is extremely straight, the sensor has a high sensitivity of ~70mV/Pa at 110 Hz, and ~35mV/Pa at other frequencies. In addition, the sensitivity of acoustic wave is tunable with the intensity of optical source. The Sagnac structure has solved a problem that in the straight cascaded structure, light source and detector are at different ends.