Multiparameter Sensor Based on DCF Composite Tapered Structure Cascaded LPFGs for the Simultaneous Measurement of Refractive Index, Temperature, and Axial Strain

In this article, a fiber optic sensor based on double-cladding fiber (DCF) composite tapered structure cascaded long-period fiber gratings (LPFGs) for measuring refractive index (RI), temperature, and axial strain is designed. The two ends of a Section of DCF are, respectively, fused with single-mode fiber (SMF) and LPFG1 to form two up-tapers, and then the DCF is formed into a down-taper. Another end of LPFG1 is cascaded with LPFG2, and the center wavelengths of the two LPFGs are different. LPFG1, LPFG2, and tapered DCF generate three resonance peaks, namely, dip1, dip2, and dip3, respectively, whose sensitivity in response to different parameters is different. When external parameters change, the dips will have spectral shifts. The experimental results indicate that, when the RI is changed within 1.3355–1.3715, the sensitivity of dip1, dip2, and dip3 is –52.12, – 132.58, and 37.61 nm/RIU, respectively. When the temperature varies from 30 °C to 80 °C, the sensitivity of dip1, dip2, and dip3 is 74.05, 98.02, and 26.33 pm/°C in turn, respectively. When the axial sensitivity of dip1, dip2, and dip3 is – 1.95, – 2.58, and –3.24 pm/ $\mu \varepsilon $ , respectively. The resolutions for RI, temperature, and axial strain are 0.0005 RIU, 1.3375 °C, and $14.0886 \mu \varepsilon $ , respectively. The three parameters can be monitored simultaneously using the coefficient sensitivity matrix. The designed sensor has high repeatability and good linearity, and it is practical in the field of multiparameter detection.