Sensing Study Of Long Period Fiber Gratings In High Refractive Index Media

The long period fiber grating is a fiber grating with a period of several hundred microns.Due to the advantages of low loss,small backscatter,simple fabrication,and low cost,it has a good application prospect in the field of optical fiber sensing and optical communication,which is one of the fastest growing fiber optic devices in recent decades.At present,long-period fiber gratings are more widely used in low-refractive-index media sensing such as water level and temperature sensing,and are rarely used in high-refractive-sensing applications.In practical applications,the refractive index of flammable and explosive materials such as gasoline,diesel and chemical agents is higher than the refractive index of the fiber.Based on this,this paper mainly studies the sensing application of long-period fiber gratings in high refractive index liquids.Firstly,a reflective sensor based on long-period fiber grating is designed and fabricated.The device is mainly composed of a single-mode long period fiber grating(LPFG),the coating layer of the pigtail behind the grating is completely stripped and the end face of the pigtail is coated with a silver film.The structure is simple and easy to manufacture.And the single-mode fiber used is more conducive to light leakage into the medium of the external environment.Secondly,the sensor was applied to a liquid level sensing experiment.Based on the guided mode-leakage mode effect formed by long-period fiber grating(LPFG),the theoretical model of reflective LPFG guided mode-leak mode is established.The simulation analysis is carried out by MATLAB,and the relationship of reflection spectrum with liquid level is simulated.In the experiment,after selecting the appropriate parameters for the liquid level sensor and the measured liquid,a sensor sensitivity of-0.77 dB/mm was obtained at a liquid level of 0-20 mm,and the linear correlation coefficient was 0.99594.Finally,the sensor was applied to an experiment in temperature sensing.The sensing sensitivity of 0.01 nm/°C was obtained in the temperature sensing experiment of low refractive index medium,and the linear correlation coefficient was 0.99754.The sensing sensitivity of-0.15 dB/°C was obtained in the temperature sensing experiment of high refractive index medium with a linear correlation coefficient of0.99672.It can be seen that in high refractive index media,the sensor is a power type sensor.In addition,the device is simple in structure,easy to manufacture,easy to package and practical,and can be widely used in the field of high refractive index liquid sensing.