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Highly sensitive long period fiber grating refractive index sensor based on thin cladding

An ordinary long period fiber grating(LPG) whose cladding diameter is 125μm is less sensitive to external refractive index. The cladding diameter of LPG has effects on transmission spectrum of each cladding mode to varying degrees. In this paper, the changes of resonant wavelength with the external refractive index(RI) of different cladding diameters based on LPG and the changes of the refractive index sensitivity with the cladding diameter of each cladding mode are calculated and analyzed. The calculation results show that we can achieve highly sensitive long period fiber gratings refractive index sensor by reducing the cladding diameter for appropriate cladding mode. LPG with 80μm and 125μm cladding diameters are used to measure the refractive index of NaCl solution respectively in this paper, temperature-compensated measurement is also performed in our experiment. Experimental results show that the sensitivity of ultrafine LPG with 80μm cladding diameter is more than 3 times higher than the sensitivity of ordinary LPG with 125μm cladding diameter.