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Simultaneous measurement of temperature and refractive index using a simplified modal interferometer composed of tilted long-period fiber grating and fiber bitaper

A simplified long-period fiber-grating-based Mach–Zehnder interferometer (MZI) for simultaneous temperature and refractive index (RI) measurement is presented. The sensing element is constructed by connecting a waist-enlarged fiber bitaper after a tilted long-period fiber grating written by high-frequency CO2 laser pulses, which has higher writing efficiency than the conventional long-period fiber grating (LPFG). By employing the proposed MZI, the transmission bandwidth for each resonant wavelength of the fiber grating could be conveniently reduced without the presence of the LPFG pair. Experimental results indicate that the MZI has good temperature sensitivity (70.42 pm °C−1) and RI sensitivity (−37.43 nm/RIU, in the RI range from 1.3425 to 1.3750). Besides, the extinction ratio of interference fringes could reach 20 dB at the resonant wavelength. With several advantages such as narrow bandwidth, large extinction ratio, simple structure and ease of fabrication, the proposed device would be a promising candidate for precise large-capacity dual-parameter sensing application.