The refractive index (RI) of a material that varies with temperature is quantified as the thermal-optic coefficient (TOC), which is crucial for characterizing materials and performing chemical and biochemical analyses. The key challenge in TOC measurement is the simultaneous measurement of changes in temperature and RI. Traditional TOC measurement systems involve the separate detection of temperature and RI changes, resulting in inconsistent spatial and temporal data. This leads to measurement errors and low accuracy in the TOC due to nonuniform thermal conduction and convection in materials. Fiber-optic sensors have gained significant attention and research interest owing to their ability to measure multiple parameters, such as corrosion resistance, and their compact size, high sensitivity, and strong immunity to electromagnetic interference. With the increasing applications of fiber optic sensing, various temperature- and RI-sensitive fiber optic sensors have been developed to measure the TOC of liquid materials, including sensors based on surface plasmon resonance (SPR), interference, and fiber Bragg grating. However, achieving high-precision measurements remains challenging because of issues such as crosstalk and cross-sensitivity. A cascaded Fabry-Perot interferometer (FPI)-SPR all-fiber liquid TOC sensor is proposed to address these challenges. This sensor uses a single-mode fiber (SMF) with one end coated with polydimethylsiloxane (PDMS) and a no-core fiber (NCF) embedded in a capillary to construct a PDMS-air-PDMS nonintrinsic FPI structure. In addition, a silver-coated nanofilm on the side of the NCF forms an SPR sensing unit for high-sensitivity temperature and RI measurements. The FPI compensates for the temperature in the SPR sensing unit, eliminating the cross-sensitivity of temperature and RI in SPR.

First, two segments of multimode and single-mode optical fibers are prepared. One segment of the multimode fiber is fused with a 15 mm long coreless fiber using a fiber-optic fusion splicer. Subsequently, the SMF and fused coreless fiber are sequentially dipped vertically into a PDMS solution with a depth of approximately 2 mm and slowly lifted to deposit the PDMS with a semi-ellipsoidal shape on the fiber end face. The fiber is then placed on a heating table at 80 ℃ for 2 h to cure the PDMS, resulting in an end-face thickness of approximately 30 μm. Subsequently, a quartz capillary tube with approximately 10 mm length is prepared. The fibers are inserted into the quartz capillary tube, and the distance between the PDMS end faces is controlled to be 50 μm using a micro-stage displacement platform. An ultraviolet-cured adhesive is used to secure the capillary tubes at both ends. Finally, the sensor assembly is placed in a magnetron sputtering machine and the 50 nm thick silver film with 10 mm length is deposited on the surface of the coreless fiber-optic cable.

The proposed sensing structure exhibits high accuracy and sensitivity for temperature and RI sensing. Within the range of 1.333?1.371 RIU (refractive index unit), the SPR peak drifts with the change in the environmental RI, and the drift amount demonstrates a linear relationship with the RI variation (Fig. 6). The FPI is sealed in a capillary tube, isolated from the external environment, and sensitive only to temperature. The wavelength shift of the FP interference peak shows a linear relationship with temperature variation within the range of 20?80 ℃ (Fig. 7). In addition, temperature compensation is applied to the RI channel to eliminate cross-sensitivity effects. In the measurement of the ethanol TOC, as the environmental temperature increases from 25 ℃ to 55 ℃ in increments of 5 ℃, the SPR interference peak shifts towards shorter wavelength direction, while the FP interference peak shows a linear shift (Fig. 8). These characteristics demonstrate a sensitive response to changes in ethanol temperature and RI.

This study proposes and validates a full-fiber liquid TOC sensor based on a cascaded FPI-SPR structure. The temperature-sensing unit of the sensor adopts a sealed PDMS-air-PDMS structure, which has higher temperature sensitivity and stability. The RI sensing unit is realized by coating silver nanofilms on the side of an NCF to form a high-sensitivity SPR sensing unit. The crossover sensitivity between temperature and RI is eliminated by temperature compensation for the RI channel. The sensor has an RI sensitivity of 2913.13 nm/RIU and a temperature sensitivity of 597 pm/℃ within the RI range of 1.333?1.371 RIU and temperature range of 20?80 ℃. Linearity reaches 99.7%. Finally, the TOC of anhydrous ethanol is tested, obtaining a measurement error of 0.82%. The experimental results show that, compared with similar fiber liquid TOC sensors, the proposed sensor effectively solves the inherent crosstalk and crossover sensitivity problems and has higher sensitivity and measurement accuracy. This sensor has a broad range of applications in biomedicine and biochemistry.