Refractive Index and Temperature Sensor Based on Double-Pass M–Z Interferometer With an FBG

In order to measure the refractive index (RI) and temperature simultaneously, a double-pass in-line Mach-Zehnder interferometer (MZI) is proposed. The in-line MZI was formed by inscribing a pair of long period gratings in a single mode fiber. A fiber Bragg grating (FBG) was inscribed next to the output port of the MZI. Part of the MZI interference light was reflected by the FBG and the fiber end, and then a double MZI interference pattern was formed. A sensitivity of -21.07 nm/RIU is obtained in the RI range of 1.33-1.43. In addition, the FBG is also used as a temperature measuring and compensating section. This kind of RI and temperature simultaneous measurement optical fiber sensor could be used in chemical or biochemical sensing fields.