As society and the economy continue to advance, there is an increasing demand for real-time monitoring of temperature and strain in fields such as oil extraction, aerospace, and renewable energy. Traditional methods, which rely on thermocouples and resistance strain gauges, often suffer from low accuracy, environmental sensitivity, and limitations in measurement (typically only at a single point). Fiber optic sensors are widely used in engineering due to their corrosion resistance, immunity to electromagnetic interference, and ease of integration into networks. However, traditional fiber optic sensors tend to be bulky, which limits their ability to meet the increasing demand for smaller, lighter designs in modern applications. In addition, addressing temperature-strain cross-sensitivity in these sensors often requires complex fabrication processes, and the specialized structures involved may not be practical for real-world use. To address these challenges, we propose a dual-parameter sensing structure for temperature and strain, combining indium selenide and fiber Bragg grating (FBG) technologies.

The D-shaped fiber in this structure is prepared by side-polishing a standard single-mode optical fiber. Due to the small distance between the polishing surface and the core, along with the flatness of the polishing area, a large evanescent field is generated. This facilitates the integration of the two-dimensional material, indium selenide, enabling efficient fluorescence excitation and collection. Using a femtosecond laser, the FBG structure is inscribed point by point in the polished area of the D-shaped fiber. The mechanically stripped indium selenide material is then transferred onto the polished surface of the fiber, which is inscribed with the FBG, via a dry transfer technique. When the fiber core is excited by 532 nm light, the evanescent field of the D-shaped fiber stimulates indium selenide to emit fluorescence at approximately 1000 nm. The central wavelength of the FBG is primarily influenced by changes in the effective refractive index and the periodic spacing of the grating. As the temperature changes, thermal expansion and thermal optical effects alter both the refractive index and the period of the grating, resulting in a shift in the central wavelength. Similarly, when strain is applied to the FBG, both the periodicity of the grating and the effective refractive index in the grating region are modified due to the elastic-optic effect, causing a shift in the central wavelength. By establishing the relationship between environmental parameters and the Bragg wavelength drift, it is possible to monitor a variety of environmental conditions. Moreover, experimental results indicate that as temperature increases, the bandgap energy of the fluorescent material decreases, leading to a linear redshift in the fluorescence peak. This redshift is much larger than the Debye temperature of the material in the temperature range of interest, allowing for real-time temperature monitoring through the linear relationship between the fluorescence peak wavelength and temperature. Importantly, the side-polished D-shaped fiber disrupts the structural symmetry of a conventional cylindrical fiber, but the axial strain within the mechanical strength range (0?5000 με) is insufficient to significantly alter the bandgap of indium selenide. Thus, it can be concluded that the fluorescence peak of indium selenide in this composite structure is primarily sensitive to temperature.

In the temperature test, the sensor is subjected to a temperature range from room temperature to 350 ℃. As the temperature increases, the reflection peak of the D-shaped FBG exhibits a redshift. The temperature sensitivity of the D-shaped FBG is found to be 13.5 pm/℃ [Fig. 9(a)]. For strain testing, the D-shaped FBG is subjected to strains ranging from 0 to 3000 με. As strain increases, the reflection peak also redshifts, with the strain sensitivity measured at 1.28 pm/με [Fig. 9(b)]. The fluorescence temperature response is tested between room temperature and 150 ℃. As the temperature increases, the fluorescence peak wavelength redshifts and the peak width broadens. The temperature sensitivity of the fluorescence signal is 207.4 pm/℃ [Fig. 11(a)]. Next, the effect of axial strain on the fluorescence wavelength of indium selenide is examined within the range of 0 to 3000 με. The results show that the fluorescence wavelength of the composite structure remains largely unaffected by strain, demonstrating that its axial strain sensitivity is negligible (approximately 0 pm/με) within this range [Fig. 11(b)]. Based on these findings, a sensor matrix can be developed, enabling dual-parameter monitoring of temperature and strain. By tracking both the Bragg reflection wavelength shift of the D-shaped FBG and the fluorescence wavelength drift of indium selenide, the system can effectively distinguish and measure temperature and strain independently, resolving the issue of cross-coupling between the two parameters.

In this paper, we present a composite structure combining indium selenide and a D-shaped FBG, fabricated using PMDS-assisted transfer and femtosecond laser point-by-point inscription. This structure leverages the fluorescence properties of the two-dimensional material and the temperature and strain response characteristics of the D-shaped FBG, enabling simultaneous detection of both parameters. Experimental results demonstrate that the composite structure achieves a temperature sensitivity of 207.4 pm/℃ and a strain sensitivity of 1.28 pm/με. Future work will focus on the packaging and material protection of the composite structure to expand the temperature detection range of the indium selenide fluorescence signal. The proposed composite structure effectively resolves the cross-sensitivity issues between temperature and strain commonly encountered in traditional optical fiber sensors. Compared to conventional devices, it offers superior sensing performance and shows promising potential for practical engineering applications.