Temperature compensated refractometer based on a cascaded SMS/LPFG fiber structure

A hybrid optical fiber structure for refractive index (RI) measurement with the temperature compensation capability is reported. The structure consists of a CO2 laser-induced long period fiber grating (LPFG) and a section of single mode–multimode–single mode (SMS) fiber structure. The LPFG serves as a refractometer with a temperature compensator using a simple SMS fiber structure. The sensing mechanism of this device has been investigated and experimentally demonstrated. The SMS structure was insensitive towards the change in the ambient RI but was sensitive towards the changes in the ambient temperature whereas the LPFG was sensitive towards both. A sensitivity matrix was built to compensate the temperature-RI crosstalk. The capability of simultaneous measurement of temperature and RI of the sensor was experimentally validated.