Optical Fiber Sensor for Simultaneous Measurement of Temperature, Humidity, and Strain Based on Hollow Core Fiber

Objective In recent years, with the development of fiber optic sensing technology and increasing environmental monitoring demands, simultaneous multiple parameter measurement has caught more attention. However, currently various available sensors can only achieve measurement of 1-2 parameters, making it difficult to meet the requirements for simultaneous multiple parameter measurement in complex environments. For example, in structural health monitoring of large buildings such as bridges, it is necessary to simultaneously monitor strain, temperature, and humidity. However, existing sensor solutions mostly rely on multiple sensors for simultaneous measurement or adopt cascaded structures for three-parameter measurement, which is generally complex and costly. Therefore, the utilization of a single structure to achieve simultaneous monitoring of temperature, humidity, and strain in a multi-parameter fiber optic sensor has research significance. We propose a fiber optic sensor for simultaneously measuring temperature, humidity, and strain based on a single-mode-hollow-core-single-mode (S-H-S) structure. By utilizing the S-H-S structure to excite the coexistence of Fabry-Perot (FP), Mach-Zehnder (MZ), and anti-resonance (AR) effects, the simultaneous measurement of temperature, humidity, and strain is achieved within a single structure. We hope that our research can provide a more stable, low-cost, and compact solution for bridge health monitoring. Methods We achieve simultaneous measurement of three parameters by utilizing the coexistence of three sensing mechanisms in the S-H-S structure. Firstly, the coexistence principles of the three mechanisms are analyzed, and the performance parameter calculation formulas for FP, MZ, and AR effects are derived in this structure. Meanwhile, we analyze the generation principle of the AR effect, fabricate S-H-S structures with different structural parameters, and test the influence of structural parameters on sensing performance. Then, S-H-S structures with optimal parameters are fabricated and GO-PVA thin films are coated on the air-core fiber to enhance humidity sensitivity. The sensor performance changes before and after sensitivity enhancement are tested. In response to the humidity sensitivity varying with humidity changes, the humidity sensitivity range has been divided into two segments based on the application scenario, which leads to a higher linear correlation of humidity sensitivity. Finally, a temperature, humidity, and strain testing platform is set up to conduct performance tests for the three parameters. The matrix method is employed to eliminate cross-sensitivity among the three parameters, enabling the simultaneous measurement of the three parameters. Results and Discussions The proposed S-H-S structure achieves the sensing mechanism coexistence of FP, MZ, and AR effects. In this structure, when the air core size of the hollow core fiber (HCF) is smaller than 10 mu m, the reflection and transmission spectra of the S-H-S structure coexist with these three sensing mechanisms. The reflection spectrum of this structure exhibits FP interference and the envelope of AR fringes, while the transmission spectrum shows the superposition of MZ interference and AR (Fig. 3). S-H-S structures with different structural parameters are fabricated (Fig. 4), and the changes in reflection and transmission spectra under different parameters are compared (Figs. 5 - 7) to determine the optimal parameters for yielding the desired spectral fringe effects. The S-H-S structure is coated with GO-PVA to enhance sensitivity, improving the relative humidity sensitivity by 32%-715% (Fig. 9). Sensitivity tests for temperature, relative humidity, and strain demonstrate that the highest temperature sensitivity is 22.4 pm/degrees C (Fig. 10), the highest relative humidity sensitivity is 37.5 pm/% (Fig. 11), and the highest strain sensitivity is 1.22 pm/mu epsilon (Fig. 12). The temperature, relative humidity, and strain sensing of the sensor exhibit good linearity and stability within the target range. Comparison reveals that the proposed sensor outperforms traditional approaches in terms of smaller size, better performance, and lower cost. Conclusions We analyze the coexistence of FP, MZ, and AR effects in the S-H-S structure. The modulation effects of different parameters on the three mechanisms are discussed to highlight the flexibility of this structure in multi-parameter sensing. To meet the needs of bridge structural health monitoring, we design a optical fiber sensor capable of simultaneous measurement of temperature, humidity, and strain, and utilize GO-PVA hybrid sensitization. Experimental results demonstrate that the sensor achieves a maximum temperature sensitivity of 22.4 pm/degrees C, a maximum humidity sensitivity of 37.5 pm/%, and a maximum strain sensitivity of 1.22 pm/mu epsilon, all with good linearity. By adopting transfer matrix techniques to eliminate cross-sensitivity, the simultaneous measurement of the three parameters is achieved within a single structure. The proposed sensor ensures performance and enables the simultaneous measurement of multiple parameters, while the single structure reduces the sensor's size and cost. It has significant application potential in areas such as long-distance bridge monitoring.