| Besides the advantages of general fiber optic sensors,the hybrid interferometer that combine modal interference and Sagnac ring are with the superiorities of high sensitivity,low requirement of light source,and can be used for accurately measuring various physical parameters such as temperature,humidity,curvature,refractive index,displacement,magnetic field and so on.Resently,it is an important research direction in the field of optical fiber sensing technology.The modal interference type optical fiber sensor is usually constructed by spliced different kinds of optical fiber together or core-offset spliced the same kind optical fiber.For modal interference,the changes of external environmental parameter such as the temperature or strain loading on the sensing fiber will result in the changes in the interference pattern,which is different from the interference pattern when there is no environmental disturbance.By measuring the variation of the output interference spectrum,it can demodulate the external environmental parameters as well as their changing laws,and realize the sensing function.The modal interferometer has the advantages of more conpact structure,more simple light path,low requirements of light source,small power consumption,easy installation and so on.However,there are some problems such as the low sensitivity and the low extinction ratio of the output of sensor,which directly limit the practical application of this kind of sensor.Therefore,the investigations on how to improve the extinction ratio,as well as design of higher sensitivity inter mode interference structure to compensate the defects of the current modal interferometer is very important for practical application.In this thesis,the improvements of the extinction ratio of the interference pattern and sensitivity of sensor are studied based on the basic theories and sensing principle of modal interference.the modal interference structures of single-mode-polarization maintaining-single-mode optical fiber(SPS),single-mode-multimode-polarization maintaining-single-mode optical fiber(SMPS)and single-mode-multimode-polarization-maintaining-multimode-single-mode optical fiber(SMPMS)is respectively inserted into the Sagnac ring to construct the hybrid interferometers,which can effectively improve the extinction ratio and sensitivity of the sensors.The main research are as following:(1)Two kinds of hybrid interferometers respectively combined single-mode-polarization maintaining-single-mode optical fiber(SPS)and Sagnac ring,or core-offset single-mode structures and Sagnac ring are proposed and experimentally demonstrated.The influences of splitting ratio for the coupler on the interference pattern of sensor are theoretically and experimentally studied.The results show that the introduction of SPS and core-offset single-mode structures in Sagnac ring can improve the extinction ratio of sensor interference pattern and the sensitivity of sensor.(2)The temperature and strain characteristics of the hybrid interferometer based on SPS and Sagnac rings,as well as the effect of the length of the polarization maintaining fiber(PMF)on the strain and temperature sensitivity of the sensor are experimental studied in detail.The experimental results show that the length of the PMF has little effect on the strain sensitivity.The strain sensitivity of the sensor composed of different length of PMF is kept at 21pm/??.However,the length of the PMF shows a great impact on the temperature characteristics of the sensor.when the PMF length of 15cm,the sensor obtained maximum temperature sensitivity of 1.63 nm/?.(3)The crosstalk of strain and temperature for the hybrid interferometer combined SPS structure and Sagnac ring are experimentally studied.The experimental results show that the temperature and strain crosstalk of the interferometer are not obvious.The strain sensitivity of the sensors is about 21pm/?? when the temperatures at 34?,36?,38?,40?,42?,44?,46?,48?,50? and 52?.The temperature sensitivity is maintained at 1.63under the strains of 400??,800??,1200??,1600??,2000??,2400??,2800?? and 3200??.(4)In order to achieve a higher sensitivity,the hybrid interferometers combined SMPS or SMPMS structure and Sagnac ring are proposed,the temperature and strain characteristics of the interferometers are experimentally investigated.The results show that The sensitivity of hybrid interferometers with SPMS structure can reach 33.68pm/??,and the temperature sensitivity of 1.78nm/?.The obtained sensitivity of hybrid interferometers with SMPMS structure is 37.17pm/??,and the temperature sensitivity of 1.4nm/?. |
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