Simultaneous measurement of external refractive index and temperature based on long-period-grating-inscribed Sagnac interferometer and fiber Bragg grating

A compact Sagnac interferometer with a long-period grating (LPG) inscribed in a polarization-maintaining fiber (PMF-LPG) and a fiber Bragg grating (FBG) is proposed. The PMF-LPG is intrinsically sensitive to the external refractive index (ERI) and temperature, whereas the FBG is only sensitive to temperature. Simultaneous measurement of the ERI and temperature can be achieved by detecting wavelength shifts in the interference patterns of the Sagnac interferometer and the FBG. Because a Sagnac interference structure is used, a high ERI sensitivity of -21.07 nm∕refractive index unit in measurement range 1.33-1.43 is obtained, which is about three times higher than for the normal LPG ERI sensor.