Optical Fiber Sensor with Novel Structure and Its Applications in Oil and Gas Exploration

Abstract

Compared to electronic sensors, Optical Fiber Sensors (OFSs) have received increasing attention due to their advantages, such as small size, light weight, anti-electromagnetic interference, easy reuse, and remote sensing. This paper proposes a novel all-fiber mode interferometer. This interferometer excites higher-order cladding modes through mode mismatch between standard single-mode fibers and thin cores. There is interference between higher-order cladding mode and core layer mode, resulting in interference fringes used as sensing signals. Sensitivity testing is conducted on the designed OFS to demonstrate whether it meets the application requirements for petroleum exploration. Transmission-type Thin-Core Fiber Modal Interferometers (TCFMIs) with different lengths of Thin-Core Fibers (TCFs) (20 mm, 40 mm, and 60 mm) are produced in the experiment. Among them, TCFMIs at 60 mm TCF length can obtain ideal sensing signals. The TCFMI (with a length of 20 mm TCF) is encapsulated in a self-made aluminum groove in rosin to test its Refractive Index (RI). The results show that as the RI increases, the central wavelength shifts towards the long wavelength direction. Its sensitivity reaches 146 nm/R.I.U. When the temperature is increased, the central wavelength shifts towards the long wavelength direction, resulting in lower temperature sensitivity. In the sensitivity test, the phase change obtained by designing OFSs is proportional to the vibration acceleration. It is fixed on the vibration table to keep the acceleration of the vibration table constant and adjust the vibration frequency of the vibration table. The results show that the vibration spectral line of the sensor is relatively flat within 100 Hz, and resonance occurs within the range of 200 Hz to 350 Hz. Through phase demodulation, sensors loaded with different oscillator masses increase linearly in the low-frequency range. When the vibration frequency approaches the resonance frequency of the sensor, the phase sensitivity of the sensor increases nonlinearly.