High speed and high precision demodulation method of fiber grating based on dispersion effect
Authors: Li Zheng-Ying  , Zhou Lei  , Sun Wen-Feng  , Li Zi-Mo  , Wang Jia-Qi  , Guo Hui-Yong  , Wang Hong-Hai  ,   Key Laboratory of Optical Fiber Sensing Technology and Information Processing, Ministry of Education, Wuhan University of Technology, Wuhan 430070, China;,   National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China
Published: December 2020

Fiber Bragg grating sensor is widely used in military, construction, transportation, aviation and other fields due to its advantages in high sensitivity, high precision, high multiplexing and small volume. However, in some special fields such as ultrasonic flaw detection, high-speed vibration and aeroengine monitoring, the signals are rapidly changing, thus requiring high speed sampling. But the demodulation speed of traditional fiber Bragg grating demodulation techniques is hardly to satisfy the requirements, which seriously limits the application of fiber Bragg grating sensor in these fields. To solve this problem, in this paper we propose a dispersion compensation fiber(DCF)-single mode fiber(SMF) dual-channel demodulation method. Based on the SMF and the DCF with the characteristics of positive and negative dispersion coefficients in the anomalous dispersion region respectively, and combining with the optical time domain reflection technology, high speed and high precision demodulation of fiber grating can be realized. This system adopts the whole fiber structure without wavelength scanning, and the grating wavelength and position information can be obtained according to the pulse delay difference under a single optical pulse. There are three factors that quite influence the system accuracy and need to be solved: the grating space disturbance which is caused by the temperature change of the sensor network fiber; the dual-channel length disturbance caused by the DCF-SMF dual-channel temperature change; the dispersion disturbance caused by the inaccurate dispersion difference of the DCF-SMF. By constructing the DCF-SMF dual-channel, adopting the reference grating and introducing the dispersion difference correction model, these influence factors are solved. The case of temperature disturbance elimination is tested by the 5-75℃ temperature experiments. And the results are as follows: when the temperature of the sensor network fiber changes, the standard deviation of this dual-channel demodulation system is 16.8 pm, while only using the DCF single-channel to form the demodulation system, the standard deviation is 3614 pm. And when the DCF-SMF dual-channel is disturbed by temperature, the standard deviation is 11.9 pm. For a long time demodulation under constant temperature, the standard deviation of this system is 6.4 pm. Thus the influences of the sensor network fiber temperature change and the dual-channel temperature change on the system demodulation accuracy are effectively reduced. The feasibility and accuracy of this method are also verified by the strain experiment. Experimental results show that the highest demodulation rate of this method is 1 MHz, while the linearity can be up to 0.9998, and the accuracy is about 8.5 pm. So the system with the dispersion difference correction model has a high precision. Therefore, this novel demodulation method has advantages of high speed and high precision, good stability and large dynamic range, and it is very applicable to quasi-distributed fiber Bragg grating sensing system.