Simultaneous Measurement of Dual Parameters Based on Periodically Embedding MMF-Induced Ultralong-Period Fiber Grating

In this article, an ultralong period fiber grating (ULPFG) based on periodically embedding multimode fiber into single mode fiber (PEM-ULPFG) is proposed to simultaneously measure the liquid refractive index (RI) and temperature with high sensitivity. Five resonant dips belong to different cladding modes, and diffraction mode orders appear in the transmission spectrum. The responses of the dips to RI and temperature are different. Therefore, high sensitivity and dual-parameter measurement are realized. Numerical simulation based on the beam propagation method (BPM) is also carried out to design and optimize the parameters of the device. Experimental results show that the maximum RI sensitivity of −1230 nm/RIU in the range of 1.4021–1.4383 RIU is achieved by tracing the target dip (dip 4). The temperature response of dip 4 is also characterized to be as low as 52.6 pm/°C. Another dip (dip 5) is used for measuring temperature with the temperature sensitivity of 92.3 pm/°C. Different responses to the external changes of the dips obtained by the PEM-ULPFG make the proposed sensor competitive in the field of high sensitivity and dual-parameter sensing.