Research On Reflective Long Period Fiber Grating Refractive Index Sensor

This article begins with the simulation of a single long-period fiber grating’s spectral characteristics and refractive index sensing characteristics, and we found that in conditions of ambient refractive index less than the refractive index of the cladding, the resonance peak of LPG drifts toward the short wavelength with the ambient refractive index increase. However, the drift velocity of the resonant peak also depends on the radius of the cladding and the level times of the cladding mode. The higher level times of the resonant peak and the smaller radius of the cladding lead to the faster resonant wavelength shift. Based onthis, we started theoretical and experimental research on the phase sensitivity of modal interference refractive long period fiber grating index sensor. Through the study we found that the best way to test is keeping the LPG away from the test solution, because it impacts the resonance peak and reduces the phase sensitivity. During the measurement, we got the results that the longer wavelength, higher level times of the resonant peak, smaller radius of the fiber cladding of the interferometer arm and longer length of the interferometer arm will result in the greater phase sensitivity of the sensor. However, in the experiment, if the radius of the cladding layer is reduced, it will directly affect the mechanical strength of the sensor head, also the length of the interferometer arms cannot be set too long for the operation of the experiment. So the experiment is realized by using the1510-1590nmband lights, and choosing order9cladding mode resonance areas as the interference area, selecting the same radius of the cladding of the optical fiber and making the interferometer arm be14.2cm long as the sensor structure. And the relationship between phase sensitivity and ambient refractive index is approximately linear when refractive index is in the range of1.33-1.38and its sensitivity averages to-1911.96°/RIU; and the relationship between phase change with ambient refractive index showed a higher-order curve when ambient refractive index is greater than1.38, and its highest sensitivity is-74108.01°/RIU.This paper proposes a new refractive index demodulation method by using the cross-correlation demodulation algorithm. The demodulation algorithm is applied to the experiment to get a resolution of6.8×10-6RIU around the refractive index of1.369, and a resolution of3×10-6RIU around the refractive index of1.4228.