Research On Multi-channel Matching Demodulation Method Of Optical Fiber Fabry-perot Sensor For Low-coherence Interference Demodulation System

Optical fiber Fabry-Perot(F-P)sensor technology has advantages of high measurement precision,large dynamic range,anti-electromagnetic interference and so forth.Therefore,it is now widely used in deep sea pressure monitor,aviation,high temperature oil well and other fields.Optical fiber F-P sensor also can be used to implement distributed multi-parameter measurement and is suitable for multi-probe sensor heads multiplexing demodulation system.However,the practical application of multi-channel time division multiplexing(TDM)technology confronts the challenge of channel inadaptability of F-P sensor because the sensor can only be demodulated accurately in the pairing channel.In this case,the flexibility of system and the reusability of sensor are very poor.In this paper,we establish a position-compensation model to illustrate the cause of position deviation between different channels.Based on this,a position-compensation method is proposed to enable F-P sensor to be demodulated in all channels without repetitive calibrating process.Air pressure measurements are performed for demonstrating the effectiveness of the proposed method.The measured maximum measurement pressure error is no more than 0.13 kPa in a 4-channel polarized LCI,which indicated that the method is reliable for improving the channel adaptability of F-P sensors.The specific research contents of this paper include:1)We constructed the optical beam propagation path model about the multichannel polarized low-coherence demodulation interferometry and derived the position distribution formula of the low-coherence interference fringes on the CCD.After simulation,an explanation is given about the position deviation between different channels under the same optical path difference(OPD)caused by demodulation interferometry.Based on this,a position-compensation method is proposed,which consists of an envelope peak position(EPP)retrieval process and a position compensation process.Thereinto,the EPP retrieval process uses the phase information to recover the EPP with high precision;the position compensation process compensates the position deviation between sensing channel and reference channel.2)We have carried out pressure experiments within 100~200 kPa in a 4-channels polarized low coherence demodulation interferometry to verify the effectiveness of the proposed method.The results showed that the maximum measurement pressure error and standard deviation of demodulation error are 0.13 kPa and 0.076 kPa respectively,which indicate that our approach can effectively enable F-P sensors to be demodulated in any channels without repetitive calibrating process while keeping the same high accuracy.3)Considering it is hard to obtain the accurate structural parameters of demodulation interferometry,we proposed a method which is based on standard optical path difference array linear fitting to calculate the distribution of optical path difference of the demodulation interferometry.The proposed method was tested by the pressure experiment within 3~283 kPa.The results showed that the maximum measurement pressure error is 0.287 kPa and proved the effectiveness of the methods.