Multi-parameter measurement sensor based on no-core fiber

Abstract

Aiming at the phenomenon of single measurement parameters and low sensitivity of most Mach-Zehnder sensors based on fiber core mismatch, in this paper we design and build a Mach-Zehnder sensor based on single-mode-no-core-single-mode-no-core-single-mode fiber structure, which can be used to measure refractive index and temperature simultaneously. In this sensor, two no-core optical fiber serve as input and output couplers, the intermediate single-mode is used as a sensing arm. Using finite element simulation and theoretical analysis, the optimal length of the coupler and the sensing arm are determined to be 15 mm. High-order modes excited by no-core optical fiber propagate through the cladding of single-mode fiber, which is affected by the ambient refractive index and temperature because of the influence of the evanescent filed. Trough of different interference orders of transmission spectrum is selected as a research object to realize the simultaneous measurement of refractive index and temperature by using sensitivity coefficient matrix. After the further Fourier transform of the transmission spectrum, the frequency of the main mode that interferes with the fundamental mode is analyzed from the spectrogram to be 0.00098 nm<sup>–1</sup>. Because of the influence of temperature on the refractive index of water during temperature sensitivity measurement, temperature sensitivity formula and water temperature coefficient are introduced to perform temperature compensation to eliminate the cross sensitivity. In this paper, the 10 mm and 15 mm sensing arms are selected for refractive index comparison experiment, and the temperature experiment is focused on the sensing arm with an optimal length of 15 mm. The experimental results show that the transmission spectrum is blue-shifted with the increase of refractive index in a refractive index range of 1.333–1.397, and the transmission spectrum is red-shifted with the increase of temperature in a temperature range from 30 ℃ to 70 ℃. The refractive index and temperature sensitivity of the interference valley near 1545 nm are –153.89 nm/RIU and 0.166 nm/℃, respectively; the refractive index and temperature sensitivity of the interference valley near 1570 nm are –202.74 nm/RIU and 0.183 nm/℃, respectively. The experimental results are consistent with the theoretical analyses. Compared with the sensor of the same type, this sensor can still maintain high sensitivity while achieving simultaneous measurement of refractive index and temperature, and has a simple structure, which has a good application prospect in biomedical and other aspects.