Refractive index (RI) measurement plays an important role in many fields, especially in medical diagnosis, industrial manufacturing, and food safety. The fiber-optic surface plasmon resonance (SPR) sensor has attracted much attention from researchers owing to its advantages of small size, compact structure, high sensitivity, and strong anti-electromagnetic interference. It is highly sensitive to changes in the external environment due to its high sensitivity. The cross-sensitivity of the fiber-optic SPR sensor to the RI and temperature limits the accuracy of the sensor. Therefore, research of a RI sensor with temperature compensation has certain practical significance. Some researchers used different grinding angles to produce different SPR phenomena and achieved temperature compensation by combining different grinding angles. Nevertheless, the sensors that they adopted were complicated to manufacture and could not be mass-produced. Others have proposed temperature compensation through fiber Mach-Zehnder (M-Z) interference, fiber Fabry-Perot (F-P) interference, and the combination of fiber Bragg Gratings (FBGs) with the SPR effect. However, this kind of SPR sensor with temperature self-compensation often needs two demodulation systems, and the demodulation process is relatively complicated. This study proposes and implements a cascaded fiber-optic SPR RI sensor with temperature self-compensation readily available for the simultaneous measurement of the RI and temperature to achieve the purpose of temperature compensation.

In the cascaded fiber-optic SPR RI sensor with temperature self-compensation, thin-core fiber is used to obtain a multimode fiber-thin-core fiber-multimode fiber (MMF-TCF-MMF) structure, and the cascade mode is adopted to achieve dual-channel sensing. When the light is transmitted from the MMF to the TCF, part of the light leaks to the TCF cladding due to fiber core mismatch. The evanescent wave generated immediately penetrates the metal film and reaches the interface between the metal and the medium to be measured, triggering electronic oscillation on the surface of the metal film. Surface plasma is thereby generated. In this process, a kind of transverse magnetic wave (p-polarized light), namely, a surface plasmon wave, propagates along the interface of the medium. As a result, the SPR phenomenon occurs. Channel 1 is obtained by coating the TCF with a silver film, while channel 2 is composed of another section of TCF coated with a composite film (Ag-ITO) and a thermosensitive polydimethylsiloxane (PDMS) film. The two channels are cascaded together by welding technology. The PDMS coating not only prevents the ambient RI from contacting the metal film but also has a thermo-optical effect. When the external temperature changes, the RI of the PDMS changes accordingly, causing a resonant wavelength shift. Temperature measurement can thereby be achieved. In summary, channel 2 is insensitive to changes in the RI, while channel 1 is sensitive to changes in both the RI and temperature. Finally, the sensitivity matrix is used to calculate the changes in the RI and temperature and study to achieve the purpose of temperature compensation. To verify the accuracy of the sensor matrix, this ultimately changes temperature and the RI simultaneously and obtains their changes from the changes in the two resonant wavelengths. The set standard values are used to analyze the errors in the experimental results. The analysis shows that the error of channel 1 under RI changes is 0.2%, and that of channel 2 under temperature changes is 1.3% (Fig. 7). Clearly, the sensor matrix has certain practicability.

At the same temperature of 40 ℃, the ambient RI ranges from 1.333 to 1.357 RIU, and the transmission spectrum changes are tested (Fig. 6). As the ambient RI increases, the resonant wavelength corresponding to channel 1 gradually red-shifts, while the one corresponding to channel 2 is almost constant because the temperature remains unchanged. The RI sensitivities of channel 1 and channel 2 are 3 141.85 nm/RIU and 0 nm/RIU, respectively. After deionized water is dribbled onto channel 1 of the sensor, the sensor was placed on a small heating table to increase its temperature from 40 to 80 ℃, and the transmission spectrum is recorded every 10 ℃ (Fig. 6). Due to the high thermo-optical effect of the PDMS, its RI decreases as temperature rises. As a result, the resonant wavelength corresponding to channel 2 blue-shifts significantly. Moreover, since the RIs of water and optical fiber change with temperature, the resonance wavelength corresponding to channel 1 blue-shifts slightly. The temperature sensitivities of channel 1 and channel 2 are -0.07 nm/℃ and -1.74 nm/℃, respectively.

This study proposes and experimentally verifies a cascaded fiber-optic SPR RI sensor with temperature self-compensation. The proposed sensor can measure the RI and temperature simultaneously to achieve the purpose of temperature compensation. It has a dual-channel cascaded MMF-TCF-MMF-TCF-MMF-MMF structure. Channel 1 is obtained by coating TCF with a 50 nm Ag film and is sensitive to the ambient RI and temperature. Developed by coating TCF with a composite film of 50 nm Ag and 30 nm ITO and another layer of PDMS film, channel 2 is only sensitive to ambient temperature. The sensor measures RI and temperature simultaneously to achieve temperature compensation and reduce the temperature crosstalk in RI measurement. The length of the TCF is about 10 mm. The RI sensitivity is as high as 3141.85 nm/RIU in the RI range from 1.333 to 1.357 RIU, and the temperature sensitivity can reach -1.74 nm/℃ in the temperature range from 40 to 80 ℃. In addition, the experimental results reveal that the crosstalk between the two sensing channels is negligible. With the advantages of simple manufacture, low cost, and stable structure, the proposed sensor has potential practical value in the fields of environmental monitoring and biochemistry.