Refractive Index Sensing Characteristics of Long-Period Fiber Gratings Near Dispersion Turning Points at 2 μm Waveband

In this paper, we demonstrate the surrounding refractive index (SRI) sensing characteristics of long-period fiber gratings (LPFGs) near dispersion turning points (DTP) working at 2 $\mu$ m waveband. The dependence of contrast and wavelength shift of LPFGs in different SRI range has been investigated. The contrast varies gradually from 4.5 dB to 12 dB in lower SRI region of 1.000-1.330, and an SRI sensitivity of 22.5 dB/RIU can be obtained. With the increasing SRI, the dip at DTP spilt into dual resonant dips. A high sensitivities of 3780 nm/RIU and 8233.3 nm/RIU can be achieved in the SRI regions of 1.320-1.420 and 1.420-1.450, respectively. The proposed LPFGs based SRI sensor has potential application in the field of fiber sensing.