| Fiber cavity ring-down?FCRD?sensing technique has attracted more and more attention from domestic and foreign research scholars due to its advantages such as high stability and high sensitivity,and it has obtained preliminary application in pressure and strain detection.However,the traditional FCRD sensing technique has the problem of high cost due to the use of pulsed light,fast detectors,and high-speed oscilloscopes,which makes it difficult to obtain a wide range of practical applications.Therefore,this thesis combines frequency-shifting interferometry?FSI?and FCRD sensing technique,and a FSI-FCRD pressure/strain sensing technique was proposed.This technique not only reduces the cost by using continuous-wave laser,slow detector and slow acquisition card,but also improves the stability of the sensing system by using differential detection.The main research contents and achievements of this thesis are as follows:1.A pressure sensing method based on FSI-FCRD was proposed and verified by experiments.The pressures were measured in the range of 0-10.4 MPa by using a section of single mode fiber with buffer layer stripped as the sensor head.By performing Fourier transform on the detected differential signals,the ring-down signals in the space domain under different pressures were obtained.The experimental results showed that the reciprocal difference of the ring-down distances was in a good linear relationship with the pressure.The slope of linear fitting curve indicated that the sensitivity of the sensing system can reach 0.024(km-1?MPa-1),and the sensitivity can be further improved by increasing the length of the sensing head.The stability experiment showed that the system stability can be up to 0.1%,and the corresponding pressure detection limit can be up to 0.05 Mpa.2.A multichannel pressure sensing scheme based on FSI-FCRD was proposed,and a dual channel FSI-FCRD pressure sensing system was built to experimentally verify it.By measuring the ring-down distances under different pressures,the position and pressure information of each sensor were obtained.The sensitivity of the two sensor heads were 0.030(km-1·MPa-1)and 0.042(km-1·MPa-1)respectively,and the detection limits were 0.126 MPa and 0.403 MPa respectively.By power budge analysis,the maximum sensor number was predicted to be 37 over a 50-km distance under the same experimental settings.The experimental and simulation results showed that the system has the advantages of low cost,high sensitivity,good linear response,good stability and so on,which indicated that it has a certain application prospect in the measurement of multipoint micro pressure.3.A strain sensing method based on FSI-FCRD was proposed and experimently verified.A single-mode–multimode–single-mode?SMS?structure was utilized to construct the sensor head for large strain measurement range and a dynamic range of up to 5000??was achieved.The strain sensitivity of up to 0.34 km-1·m-1was obtained within such a large strain measurement range;the corresponding strain detection limit was 48.2??.The experimental results show that the system has the advantages of easy manufacture,low cost,good linear response,good stability and large dynamic range.Therefore,it is attractive and promising for practical applications,such as for the large-dynamic-range detection in concrete structures.4.A multipoint strain sensing method based on FSI-FCRD was proposed,and a dual-point FSI-FCRD strain sensing system in series was experimentally investigated.In order to obtain a large dynamic range and a high sensitivity,we use a biconical multimode fiber as the sensor head.By measuring the ring-down distances under different strains,the corresponding cavity loss was deduced.The experimental results showed that the strain sensitivity of the two sensor heads can reach 0.137 dB/m?and0.197 dB/m?respectively,and the corresponding detection limit was 0.012 dB and0.036 dB respectively.What's more,both sensor heads can realize the detection of dynamic range up to 6 m?and have a good linear response. |
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