Research And Application Of Polarization Effect In BOTDA System

Fiber optic sensing technology has become a research hotspot in recent years.Among them,the Brillouin Optical Time Domain Analysis(BOTDA)technology has turned to be one of the topical issues in the field of fiber sensing technology depending on its ability to realize distributed temperature and strain sensing with high spatial resolution over long distances.Previous studies on the polarization effect of BOTDA sensor systems have focused on polarization inhibition.Therefore this paper proposes a vibration monitoring method of BOTDA system combined with polarization detection based on the study of relevant polarization effect.Firstly,this paper has studied the use of birefringence change law in single-mode fiber to realize vibration signal monitoring along the fiber.Then a single mode fiber vibration monitoring model was established under Jones matrix and Stokes vector polarization model analysis method.Meanwhile the feasibility of the model was verified by simulation analysis of the response to different vibration signals.A long-distance single-mode fiber vibration sensing experimental system was built in the laboratory environment,and vibration monitoring was carried out on 21.5km and 46.5km sensing fiber respectively.The polarization analysis of the output light at the receiving end has showed that the system could identify the sinusoidal and square wave vibration signals as well as the interference signals of the cable scour and friction vibration conditions.Then the Jones matrix polarization vector model for the optical path of the BOTDA system was established according to the study on the relevant polarization effect.Besides the relationship between Stokes light and vibration signal was simulated and analyzed,then a Brillouin vibration sensing experimental system combining polarization detection was designed and set up.In the experiment,the detection light carrying SBS information was monitored simultaneously by the system's high-speed data acquisition equipment and polarization analyzer.Finally the experimental results have implied that the high speed data acquisition equipment in BOTDA system has no obvious effect on vibration signal recognition.By contrast,the polarization monitoring module can identify the presence of vibration and the different frequencies of vibration signals,then the sinusoidal and square wave vibration signal waveforms as well as the interference signal waveforms under the actual conditions of Marine cable scour can be reproduced,thus realizing the dynamic strain signal identification with vibration frequency in the range of 0.1Hz?100 Hz.