Temperature and relative humidity sensors play an important role in various processes. Compared with traditional sensors, optical fiber sensors have the advantages of simple structure, strong corrosion resistance, and strong anti-electromagnetic interference capability. Functional sensors reduce manufacturing difficulty and cost compared to non-functional sensors because they do not require other sensitive components. Among them, interferometric sensors have become a topic of interest in recent years because of their simple manufacturing process and low cost. In particular, the Mach-Zehnder interferometer (MZI) plays an indispensable role in the measurement of basic physical quantities, such as temperature, relative humidity, refractive index, and stress. However, indicators such as the sensitivity and stability of MZI sensors need to be improved. To further improve the sensitivity and stability of such sensors, we propose and demonstrate a temperature and relative humidity sensor based on a tapered thin-core fiber coated with a graphene quantum dots-polyvinyl alcohol (GQDs-PVA) solution. The sensor consists of a 2 cm segment of tapered thin-core fiber and two 2 mm multi-mode fibers. The two multimode fibers are located between two single-mode fibers, in which the thin-core fiber constitutes the sensor cone after distributed taper processing. When the external temperature, relative humidity, and other physical quantities vary, the effective refractive index difference between the cladding and core layers changes, resulting in the movement of the central wavelength of the spectral line, which can be monitored in real time on the spectrometer. Therefore, this optical fiber sensor can monitor changes in these external physical quantities. The proposed sensor exhibits higher temperature sensitivity after coating with the GQDs-PVA solution than traditional sensors. In addition, owing to the unique properties of PVA, the sensor exhibits high sensitivity to relative humidity. Compared to other sensors intended to measure these quantities, this sensor is more effective and less expensive. The sensor is characterized by high sensitivity and stability, and thus, it is a suitable candidate for research and practical applications.

First, two segments of multi-mode fiber (MMF) with a length of 2 mm were spliced onto both sides of a 2 cm segment of thin-core fiber (TCF) using a fiber fusion machine. Single-mode fibers (SMFs) were then fused to both sides of the TCF. The thin-core fiber was then placed in the special optical fiber fusion machine (FSM-100P +), the taper mode was selected to provide a distributed taper, the center position of the thin-core fiber was aligned with the center of the special optical fiber fusion machine, and the diameter of the thin-core fiber was initially tapered from 125 μm to 100 μm and finally to 70 μm; discharge amounts were -20, -50, and -70 bits. Finally, graphene quantum dots (GQDs) and polyvinyl alcohol (PVA) were thoroughly mixed and the GQDs-PVA solution was evenly applied to the sensor cone. The sensor was placed horizontally in a vacuum oven and heated at 80 °C for 30 min. After the heating process, the GQDs-PVA mixed solution adhered to the sensor surface in the form of a film. The particle size and distribution of the GQDs were examined using scanning electron microscopy (SEM). The structure was placed in a programmable constant temperature and relative humidity experimental chamber before and after taper pulling to test the effect of temperature and relative humidity on the sensor spectra. The stability and repeatability of the structure were also tested, and its sensitivity and linearity, based on several experiments, were evaluated.

First, we performed temperature experiments on the experimental samples before and after tapering the thin-core fiber, and the temperature sensitivity of the sensor was measured to be 31 pm/℃ and 72.7 pm/℃, respectively (Fig. 5). The GQDs-PVA mixture solution was then evenly applied to the sensor cone, and the temperature sensitivity increased to 288.3 pm/°C after application of the solution (Fig. 6). In addition, the refractive index of the PVA solution changed according to the ambient relative humidity, and the sensitivity of the sensor to ambient relative humidity was 131.7 pm/% after coating with the GQDs-PVA solution (Fig. 8). We also tested the repeatability and stability of the sensor after coating. As the temperature (Fig. 6) and relative humidity (Fig. 8) varied, the sensitivity showed little deviation and the sensor exhibits good repeatability. When the temperature was 48 ℃, the peak at 1515.5 nm of the central wavelength of the sensor exhibited minimal shift (Fig. 7). When the relative humidity was 52% and 58%, the center wavelength of the sensor drifted (1442.2 nm and 1445.2 nm, respectively), indicating that the sensor was sensitive and exhibited good stability and repeatability with respect to both temperature and relative humidity.

In summary, we propose and demonstrate a temperature and relative humidity sensor based on tapered thin-core fibers coated with GQDs-PVA. The experimental results indicate that the temperature sensitivity of tapered thin-core fibers can be increased to 72.7 pm/°C, and it can be further increased to 288.3 pm/°C after surface application of the GQDs-PVA mixed solution, which makes the sensor 9.3 times more sensitive than the uncoated device. The relative humidity sensitivity of the proposed sensor is also higher than that of other types of Mach-Zehnder interferometer (MZI) sensors. Because PVA absorbs water and changes its refractive index when the ambient relative humidity changes, it changes the effective refractive index difference between the cladding and the core, which in turn causes a beneficial spectrum drift. Therefore, the coated sensor exhibites an ambient relative humidity sensitivity of 131.7 pm/% instead of the complete insensitivity at the beginning. The GQDs-PVA sensor has a simple structure, good stability, high sensitivity, and is a good candidate in the field of temperature and relative humidity sensing.