The Research On Distributed Sensing Technology Based On Measuring The Beat Length Of Polarization Maintaining Fiber

With the wide application of fiber optic technology, the demand for distributedoptical fiber sensor which have a high sensitivity, high spatial resolution(millimeterlevel)become more and more urgent. Due to the features of high receiving sensitivityand spatial resolution is inversely proportional to the receiver bandwidth, sensingsystem based on optical frequency domain reflectometry probing become a new hotspotin the field of fiber optic sensing.In this thesis, a distributed pressure sensing method that polarization maintainingfiber beat length measurement combined with the OFDR system probing which canachieve ultra-high spatial resolution is proposed.The changes of the beat length ofpolarization maintaining fiber with external force is analysized, and the relational modelof beat length with an external pressure is built based on polarization coupled modetheory. Swept narrow linewidth light source is designed and made, and OFDR system isbuilt. The main contents are as follows:1.Based on the coupled-mode equations and the elastic-optic theory, the relationalmodel of the beat length with external force is built, and the expressions are given. Themethod of measuring the beat length of polarization maintain fiber base on Sagnacinterferometer is analysized and simulated. The results indicates that the changes of thebeat length with external force angle showing a sinusoidal variation which the cycle isπ when the external force is a constant, but it is an approximately linear relationshipwhen the angle is a certain. This conclusion has laid a theoretical foundation for thesensing system with beat length.2.The method of measuring the beat length of polarization maintain fiber base onSagnac interferometer is proposed and verified by experiments. Experimental resultsand theoretical analysis conclusions are matched.3.Based on optical heterodyne detection principle, the system composition andforce sensing principle combined with measuring the beat length of polarizationmaintain fiber and OFDR are researched. A kind of distributed optical fiber sensorwhich can measure the external force positioning and quantitative with high spatial resolution is achieved theoretically.4.The experiments of swept narrow linewidth light source and OFDR system arecarried out. Swept narrow linewidth light source with500GHz frequency sweep rangeand less than10KHz linewidth is made by using a method of a saturable absorbercombined with a F-P filter. OFDR sensing system is built.