Accurate measurement of temperature and relative humidity plays an important role in many fields, such as health care, environmental testing, and safety monitoring. Various sensor devices have been developed to meet various needs, but traditional mechanical hygrometers and thermometers, as well as temperature and humidity sensors based on capacitance and resistance, have disadvantages, such as large space occupation and deterioration from aging. Fiber sensors have become a popular research topic because of their small size, light weight, resistance to electromagnetic interference, corrosion resistance, and many other advantages. Researchers have designed optical fiber temperature and humidity sensors that use grating structures, interference structures, and surface plasmon resonance (SPR) effects by combining optical fibers with temperature- and humidity-sensitive materials. The SPR effect has attracted considerable attention because it improves the sensitivity of optical fibers in sensing the change in the refractive index of the surrounding medium. We propose an SPR-based single-mode fiber (SMF) temperature- and humidity-sensing structure. It is made of a single-mode fiber coated with a silver film that partially removes cladding and senses the changes in temperature and relative humidity via a sensitive material, which is on the outer side of the silver.

The coating layer with a length of ~10 mm was first removed from the middle of a single-mode fiber, after which an HF acid solution with a volume fraction of 37.6% was applied dropwise to the surface of the fiber. The cladding was submerged in HF, and corrosion was observed several times with a microscope. The diameter of the cladding was corroded to ~15 μm after 1 h and 20 min. The treated fiber was then fixed on a glass slide and placed in a magnetron sputter coater to plate a silver film with a thickness of 40 nm. At the end of the first silver plating, the fiber-optic sensing structure was flipped to the back side, and silver plating was repeated to ensure uniform coating of the entire fiber-optic sensing structure. In the next step, an appropriate amount of liquid PDMS was applied dropwise to one side of the fiber-sensing structure, and a small amount of PVA solution was applied to the other side of the fiber-sensing structure. Finally, the structure was placed on a drying table at 60 ℃ until it completely solidified and was left to cool to room temperature. The sample drop-coated PVA solution with mass fraction of 1%, 2%, and 3% was recorded as s-1, s-2, s-3, respectively.

The proposed sensing structure has advantages in temperature and humidity sensing. With an increase in temperature and relative humidity, the double-resonance absorption peaks are significantly shifted. The SPR peak has good stability with the change in time, the resonance peak gradually shifts to the short wavelength direction when the temperature increases, and the central wavelength shift is basically linear with the temperature change within 20-70 ℃ (Fig.6). Owing to the hydrophobicity of PDMS, its refractive index is barely affected by relative humidity; therefore, the material has a weak cross-sensitivity (Fig.7). In the humidity test, samples s-1, s-2, and s-3 produce resonance peaks at different central wavelengths (Fig.9). The resonance peak of sample s-2 at a certain relative humidity was selected as the measurement sample, and the shift in the resonance peak was recorded under a change in temperature from 20 to 90 ℃ (Fig.10). It can be observed from the figure that the wavelength of the center of the resonance peak shifts to the left as the temperature increases.

In this study, a temperature and humidity sensor based on SPR is designed. The outer side of the fiber, with part of the cladding removed, was coated with a nano-silver film. This method excites the cladding modes of the fiber core light with silver to produce an SPR effect. Then, temperature- and humidity-sensitive materials were coated on silver film to achieve a highly sensitive measurement of temperature and relative humidity. The experimental results show that the central wavelength of the humidity resonance peak is blue-shifted as the thickness of PVA decreases. When the relative humidity varies in the range of 30%-78%, the average sensitivity of the sensing structure with 2% (mass fraction) PVA solution drop-coated reaches -4.36 nm/%. When the temperature varies in the range of 20-70 ℃, the average sensitivity of the sensor reaches -2.01 nm/℃, and the linearity reaches 99.5%. The temperature and humidity cross-sensitivity tests demonstrate that PDMS is barely affected by changes in relative humidity, whereas PVA undergoes smaller refractive index changes under temperature changes. The temperature and humidity sensor proposed in this study has the characteristics of high sensitivity, wide range, good stability, and low cross-sensitivity and can be applied to the field of temperature and humidity sensing.