Sensing Characteristics of Long-Period Fiber Gratings Written in Thinned Cladding Fiber

Abstract

We demonstrate the fabrication of the long-period fiber gratings (LPFGs) in the thinned cladding fiber by CO2 laser and the characteristics of the gratings is experimentally investigated for the sensor application. The cladding modes are excited in the thinned cladding fiber, which is found to be highly sensitive to the surrounding index changes. A high index sensitivity of 7366.6 nm/refractive index unit (RIU) was achieved, which is much higher than that of the same order cladding mode of the common LPFGs written in the conventional single-mode fibers. In addition, the over-coupled LPFGs obtain a higher grating contrast in the index sensitive region (1.447-1.457), which is helpful for the wavelength detection. The temperature and bending sensitivities of this promising refractive index sensor were measured to be 70.85 pm/°C and 2.32 dB/$\text{m}^{-1}$ , respectively.