Simulation of liquid gas and displacement optical fiber sensing system based on vortex beam

Abstract

This paper attempts to propose an optical fiber sensing system based on the vortex beam, which can be applied to measure liquid refractive index (RI), gas concentration and displacement. Its sensing structure consists of a silver-coated microfiber for surface plasma resonance (SPR) sensors, a fiber aligned with it, and a interference structure for Gaussian beam interfering with the transmitted vortex beam. Due to SPR effect, when the RI of liquid changes, the wavelength of resonance spectrum will shift accordingly, thus the sensitivity of the RI can be achieved. When the gas refractive index n and distance L between two fiber end face change, phase difference will be generated between the transmitted vortex beam and the bypass Gaussian beam. Thereupon, the interference pattern of the two beams will rotate, the measurement of the gas RI and displacement can be achieved. In comparison with the traditional optical fiber SPR sensor, the simulation result shows that SPR excited by the vortex beam is more explicit with higher sensitivity. When the RI of the external environment is 1.385, the sensitivity of 4600 nm/RIU under the condition of Gaussian beam, the sensitivity of sensor based on vortex beam increases to 6800 nm/RIU, which is 47.8% higher. When the RI of the external environment is 1.365–1.385, the average sensitivity of the sensor increases from 3950 nm/RIU to 5100 nm/RIU. Meanwhile, the gas RI sensitivity is 36°/0.0001 RIU and the displacement sensitivity is 36°/ 100 nm. In terms of the CCD angle resolution, the theoretical resolution of gas RI measurement is 1.54e-10 RIU, and the resolution of displacement is 1.54e-4 nm.