In recent years, with the improvement of sensor accuracy and performance requirements in biomedicine, chemical sensing, and environmental monitoring, the demand for multi-parameter simultaneous monitoring is growing. For example, in marine environmental monitoring, it is necessary to detect the refractive index, pH, and temperature simultaneously. However, most current sensors have complex structures, poor mechanical strength, and small dynamic measurement ranges, making it difficult to meet the simultaneous measurement needs in complex environments. Therefore, it is essential to develop a kind of optical fiber sensor with simple structures, high performance, and large measurement ranges, which can realize multi-parameter simultaneous detection in complex environments. We propose an optical fiber sensor based on surface plasmon resonance (SPR) and Mach?Zehnder interference (MZI) effects combined with empirical mode decomposition (EMD) to simultaneously detect the refractive index, pH, and temperature.

We combine the SPR and MZI effects in the multimode optical fiber-hollow core optical fiber-multimode optical fiber (MMF-HCF-MMF) structure with EMD to achieve simultaneous detection of the refractive index, pH, and temperature. First, the principles of the two effects are analyzed, the performance parameter formulas of SPR and MZI are derived, and simulation is adopted to verify the feasibility of the theory. Meanwhile, we analyze the principles and steps of EMD and its effectiveness in enhancing the free spectral range (FSR) of the interference spectrum. Then, an optical fiber sensor based on the MMF-HCF-MMF structure is fabricated, and the side of HCF is divided into region I and region II by silver nanofilm and polyacrylic acid/chitosan (PAA/CS), which are employed to measure the refractive index and pH respectively. Additionally, the MZI interference spectrum generated by the ambient temperature change in the region-Ш is decomposed into intrinsic mode functions (IMFs) via EMD to achieve temperature measurement. Finally, a refractive index, pH, and temperature experimental test platform is built to conduct performance testing on the system.

The Refractive index response of the sensor without a pH-sensitive film is first tested, and the corresponding refractive index sensitivity is 2680.26 nm/RIU (Fig. 6). Secondly, the refractive index and pH sensing performances of the sensor are tested. The sensitivity of region I to the refractive index is 2381.71 nm/RIU [Fig. 9(b)], and that of region II to pH is -14.62 nm/pH [Fig. 10(b)]. Then, the effect of temperature on refractive index and pH detection is tested. The sensitivity to temperature in region I and region II is obtained to be -0.14 nm/℃ and -0.17 nm/℃ respectively [Fig. 11(b)]. To reduce the influence of temperature changes, we introduce EMD to obtain the sensitivity of region III to temperature as 85.27 pm/℃ [Fig. 13(f)]. Subsequently, the temperature compensation is realized by the sensing matrix, and the error analysis is performed via adopting the sensing matrix. The results show that the refractive index, pH, and temperature change errors are 2.5%, 2.1%, and 1.8% respectively. Finally, the stability and repeatability of the sensor are verified. The experimental results indicate that the maximum sensitivity errors of refractive index, pH, and temperature are 68.34 nm/RIU, 0.23 nm/pH, and 0.59 pm/℃, respectively. The sensor proposed in our study has better performance and lower cost than other sensors.

We propose and fabricate an optical fiber sensor based on SPR and MZI effects combined with EMD to detect the refractive index, pH and temperature simultaneously. The sensor adopts the structure of MMF-HCF-MMF, and divides the HCF into two independent sensing regions by silver nanofilm and polyacrylic acid/chitosan (PAA/CS) composites, which measure the refractive index and pH respectively. Meanwhile, the interference spectrum generated by MZI is decomposed into IMFs by EMD, and FSR is extended to realize temperature compensation. The experimental results show that the sensor has sensitivity of 2381.71 nm/RIU for the refractive index, -14.62 nm/pH for pH, and 85.27 pm/℃ for temperature. The proposed sensor not only shows high sensitivity and a simple fabrication process, but also has low cost and the ability to detect multiple parameters simultaneously. These characteristics make the sensor have great application potential in marine monitoring, biochemical analysis, and other fields.