Objective Optical fiber sensors are a significant branch in the sensor field due to their compact structure, remote monitoring capabilities, and resistance to electromagnetic interference. As sensor technology advances and application demands increase, there is a growing need to monitor multiple parameters simultaneously. For instance, in marine ecological monitoring and the petrochemical industry, simultaneous measurement of strain, temperature, and refractive index is essential. Using single or dual- parameter sensors requires deploying multiple different sensors, which complicates the sensing system. Existing optical fiber sensors typically measure only two parameters, and three- parameter sensors are rare and complex, posing challenges for simultaneous multi- parameter measurement in complex environments. Therefore, designing a compact and flexible three- parameter sensor for simultaneous measurement of strain, temperature, and refractive index is both significant and promising. In this paper, we propose a hybrid structure fiber sensor that integrates a tapered phase- shifted Bragg grating and a Mach - Zehnder interferometer. By leveraging the complementary sensing characteristics of these two components, the sensor can simultaneously measure strain, temperature, and refractive index, offering a novel solution for multi- parameter measurement scenarios in environmental monitoring and the petrochemical industry. Methods The hybrid sensor utilizes the complementary sensing characteristics of the tapered phase- shifted Bragg grating and the Mach - Zehnder interferometer to simultaneously measure strain, temperature, and refractive index. The sensing principles for strain and temperature of the tapered phase- shifted Bragg grating and strain and refractive index of the Mach-Zehnder interferometer are analyzed, demonstrating their complementary nature. A Mach-Zehnder interferometer based on multimode fiber (MMF) and photonic crystal fiber (PCF) is constructed. The influence of PCF length on the spectrum is tested to determine the optimal length. The tapered phase- shift grating is fabricated using arc discharge from a fusion welding machine, with precise control of the minimum radius to enhance strain sensitivity while maintaining structural strength. Silver deposition is used on the fiber end face to enable reflective monitoring, reducing sensor length and easing device layout. Finally, a sensing test system for strain, temperature, and refractive index is established, and the performance of the three parameters is evaluated. The matrix method is used to eliminate cross- sensitivity between the parameters, enabling simultaneous measurement. Results and Discussions The reflection spectrum of the sensor is monitored using a spectral analyzer and a broadband light source. After tapering the initial fiber Bragg grating (FBG), a transmission window is created in the middle of the reflection peak (Fig. 2). The spectral change in this window characterizes the response of the tapered phase- shifted grating to strain and temperature, while the intensity change at 1560 nm indicates the Mach-Zehnder interferometer's response to refractive index and strain. Strain sensing experiments show that with strain increasing from 0 to 300 tie, the central wavelength shift of the conical PS-FBG transmission window is 0.59 nm, with a reflection peak intensity difference of 6.42 dB, corresponding to strain responses of 1.88 pm/tie and 0.0213 dB/ tie. The intensity change at 1560 nm is-0.664 dB, with a sensitivity of 0.0022 dB/tie (Fig. 4). Temperature experiments show no change in the phase shift of the phase- shifted grating as the temperature rises from room temperature to 120 degrees C, with a wavelength modulation temperature sensitivity of 12.2 pm/degrees C and an MZI temperature sensitivity of 0.0014 dB/degrees C (Fig. 5). Refractive index experiments demonstrate no change in the conical PS-FBG spectrum as the refractive index increases from 1.3330 to 1.3707, with an MZI refractive index sensitivity of-344.81 dB/RIU (Fig. 6). The sensor exhibits good linearity in strain, temperature, and refractive index measurements across the experimental range. Conclusions A hybrid structure fiber three- parameter sensor based on a tapered phase- shifted Bragg grating and an MZI has been proposed and experimentally validated. The tapered PS-FBG is created through arc discharge, and the MZI is constructed using MMF and PCF. These components are cascaded and silver-plated on the end face. By leveraging the advantages of the tapered PS-FBG for strain and temperature sensing and the MZI for refractive index sensing, the sensor achieves simultaneous measurement of strain, temperature, and refractive index. Experimental results indicate that the sensor's temperature sensitivity is 12.2 pm/degrees C , strain sensitivity is 0.0213 dB/tie, and refractive index sensitivity is - 344.81 dB/RIU, all demonstrating good linearity. The proposed hybrid structure sensor offers a compact design, low manufacturing cost, and flexible use, making it a valuable tool for various applications.
