Temperature-independent gas refractometer based on an S-taper fiber tailored fiber Bragg grating

A fiber Bragg grating (FBG)-based gas refractometer is proposed and demonstrated experimentally. The configuration consists of a short section of S-type taper incorporated in the upstream of a FBG. The S-taper is capable to couple the core mode to cladding modes into the downstream single mode fiber (SMF), and the low-order cladding modes can be reflected back to the fiber core via the FBG. Because of the recoupling efficiency depending on surrounding refractive index (SRI), the reflection power of the device presents high response to gas RI change with the sensitivity of 172.7 dB/RIU. This power-referenced RI measurement and wavelength-referenced temperature measurement have been achieved via selective cladding modes monitoring.