Study On The SMPS And SMPMS Fiber Structure-based Sagnac Loop Optical Fiber Sensors

Compared with traditional optical fiber sensor,the optical fiber sensor based on the modes interference has the advantages simple structure and low cost,which make it has a wider application prospect.In this thesis,we propose and fabricate two types of Sagnac loop sensors by using single mode fiber-multimode fiber-polarization-maintaining fiber-single mode fiber(SMPS)and single mode-multimode-polarization-maintaining-multimode-single mode(SMPMS)fiber structures based on intermodal interference principle.The temperature and strain characteristics of the two sensors are studied in detail by experiments.The main work of this thesis is as follows:1.The state of the art of optical fiber sensor and the research background and significance are firstly introduced.And then the basic principles of optical fiber sensor and the four classical interferometers are reviewed based on Maxwell's equations.Besides,the principles of fiber model interferometric sensors and Sagnac loop has also been briefly described,which will provide an important theoretical basis for designing and fabricating the sensors in the following work.2.In order to improve the sensing performance of the sensor,we proposed SMPS and SMPMS fiber structure-based Sagnac loop sensors,and the sensing mechanisms of the sensors are theoretically analyzed.The effects of polarization maintaining fiber(PMF)length and multimode fiber(MMF)length on the sensitivity and the free spectrum width of the sensors are studied in detail.The experimental results show that the two kinds of fiber lengths have great influence on the temperature sensitivity but have little effect on the strain sensitivity.The free spectrum width of the sensor systems has a negative correlation with the length of the PMF,and the length of the MMF has no obvious influence on the sensors.To investigate the influence of the PMF length on the sensitivities of the sensors,the MMF length in these two structures are kept in 15 cm.The maximal temperature sensitivity of 1.73nm/ °C and strain sensitivity of 34.40 pm/?? are achieved for SMPS sensor when the length of the PMF is 8cm.when the PMF length is 6cm,the sensor displays the low temperature and strain sensitivities of 1.30nm/ °C and 25.83pm/??,respectively.With regard to SMPMS sensor,the highest and lowest temperature sensitivity of 1.98 nm / °C and 1.40 nm / °C is obtained when the length of PMF is 6cm and 10 cm,respectively.The maximum and minimum strain sensitivity is 38.98 pm/?? and 32.77 pm/?? when the length of the PMF is 12 cm and 8cm,respectively.In experiments for studying the impact of the MMF length on the sensitivities of the sensors,the length of the PMF remains 10 cm.Both the SMPS and SMPMS sensors respectively reach the maximum strain sensitivities of 37.96pm/?? and 37.03m/?? when the MMF is with the shortest length of 6cm.And achieve the highest temperature sensitivity of 1.93nm/ °C and 1.71nm/ °C when the MMF length is 12 cm and 15 cm,respectively.3.In order to reduce the cost of the system,we modified the two sensors with the power demodulation scheme by using a power meter to instead the optical spectrum analyzer(OSA).In the experiment,the length of PMF and MMF in the two kinds of sensors are kept at 6cm and 15 cm.The experimental results show that the obtained strain and temperature sensitivities of the SMPS sensors are 0.014 dBm/?? and 0.866dBm/°C,respectively.The strain and temperature sensitivities of the SMPMS sensors are 0.040 dBm/?? and 1.283 dBm/°C,respectively.The experimental results confirmed the feasibility of the power demodulation scheme and the SMPMS sensor shows a higher sensitivity comparing with the SMPS structure.