Research On Quasi-distributed FBG Demodulation Technique Based On Chaotic Light Source

Fiber Bragg Grating(FBG)sensors are widely used in such fields as power transportation,humane meteorology,aerospace,and building structure inspection due to their advantages of small size,light weight,stable structure,and strong anti-electromagnetic interference.In quasi-distributed sensing applications,fiber gratings are composed of sensor arrays through different multiplexing methods.The demodulation technology requires measurement of changes in the position and physical parameters of individual gratings,as well as the ability of locating breakpoints in fiber transmission to facilitate system maintenance.In order to meet the above conditions,this paper combines the correlation characteristics of the chaotic light source,conducts a theoretical analysis,proposes a demodulation scheme for the weak inverse grating array,and conducts experimental research.The scientific research results obtained on this subject can be summarized into the following three points:(1)In order to improve the problem of the long fiber ring cavity length which affects the demodulation sensitivity in the traditional pulse light-based fiber loop ring-down(FLRD)system,the autocorrelation characteristics of chaotic light and the FLRD system are combined,and the autocorrelation characteristics of the system were theoretically studied.The effects of different cavity lengths and different intra loop losses on the ring down curve were explored,which provided theoretical support for the subsequent system construction and optimization.(2)Because the Bragg grating is a wavelength-sensitive sensor,the wavelength tunable characteristics of the light source in the grating demodulation technology are very important.The common wavelength tunable chaotic light sources are mostly based on optical narrowband filters,whose output power is low,and the wavelength tuning range is limited.In order to improve this defect,a wavelength-tunable chaotic light source based on external light injection was built to achieve a stable chaotic light output of-10 d Bm power and 40 nm wavelength tunability.This light source is applied to the FLRD system,and combined with the dual-wavelength differential demodulation algorithm,it is used to demodulate the cascaded identical ultra-short fiber gratings.The demodulation technology realizes the quasi-distributed strain sensing of three cascaded gratings.The measured dynamic range is 0 ～ 1200με,and the maximum measurement error is 5.26με.At the same time,the system has the ability to locate fiber breakpoints.(3)Aiming at the problem that the demodulation results in the current grating demodulation system based on chaotic cross-correlation characteristics are susceptible to optical power jitter,combined with the dual-wavelength differential demodulation algorithm,a demodulation technique for quasi-distributed ultra-short FBGs based on tunable chaotic light is proposed.The positions of the sensors and the wavelength shifts can be accurately extracted.The experimental results of quasi-distributed strain sensing show that in the dynamic range of 0 ～ 1200με,the error of stress sensing is less than 1με.The demodulation result is not affected by optical power jitter.In addition,the system has realtime monitoring capabilities for fiber breakpoints,with a spatial resolution of 8.2cm.