Research On Fiber Bragg Grating Sensor Based On Tapered SMS Structure

With the continuous development of single-mode-multimode-single-mode(SMS)all-fiber structure sensors in the field of optical fiber sensing,the SMS structure has received extensive attention due to its advantages such as simple structure,convenient fabrication,and high sensitivity.However,the sensor of the all-fiber structure has problems such as the spectrum is susceptible to interference and the peak fluctuation is large.Aiming at this problem,this paper proposes a high strain sensitivity sensor combining SMS and Bragg fiber grating(FBG).Because of the coarse and easy coupling characteristics of multimode fiber core in SMS structure,the multimode fiber is melted and tapered to improve the sensitivity of the sensor.Writing FBG on the side of the cone of multimode fiber,so that the sensor can detect the change of external parameters based on the change of resonant wavelength.It has higher strain sensitivity than ordinary FBG,and has higher anti-interference than the full interference type sensor.Based on the study of characteristics and demodulation methods of the sensor,the preparation process and demodulation algorithm of the sensor are improved,and the sensing characteristics of different regions of the cone are analyzed in detail.The main work of this article is as follows:(1)Combining multi-mode interference principle and coupling mode theory to analyze the spectral characteristics of SMS structure,and analyzing the characteristics of mode excitation in refractive index graded multimode fiber grating and refractive index stepped multimode fiber grating,and the impacts of fiber radius and grating length on the spectrum and sensing characteristics.Based on the above analysis,a single mode-tapered multimode-single mode(STMS)fiber grating sensor is proposed.(2)According to the results of theoretical derivation,the numerical simulation of the sensor's spectra is shown.Studying the effect of welding parameters,taper length,taper speed,and grating writing position on the fiber geometry shape and spectral characteristics.Based on the simulation results,the appropriate parameters were selected to product the sensor combined with the fusion taper and phase mask method.The geometric characteristics and spectral characteristics of STMS-FBG sensors with different parameters were analyzed and compared.(3)Based on the principle of optical frequency domain reflection(OFDR)demodulation,a demodulation method for STMS-FBG is proposed.The high spatial resolution demodulation of the gratings in different radius regions of the cone region is realized by demodulating the position information and the spectral information.(4)The sensors with different parameters were prepared for temperature sensing and strain sensing experiments.The whole demodulation and segmentation demodulation methods are used to demodulate the grating.The different sensing characteristics of different parameter sensors and different sensing characteristics of the same sensor corresponding to different positions at the cone are analyzed.The quantitative relationship between temperature,strain sensitivity and cone radius are verified experimentally.Achieving the high sensitivity,high spatial resolution strain sensing measurements and the simultaneous measurement of temperature and strain.