Directional Bending Sensor and Multiparameter Sensor Based on D-Core Multimode Fiber

For the sensor with interference between fiber core mode and cladding mode, the directional bending sensor can be realized by processing asymmetric structure on the cladding or using eccentric fiber, but the temperature and strain crosstalk of the sensor with interference between fiber core mode and cladding mode is large. In this article, a kind of fiber intermode interference sensor with asymmetric structure is proposed, which can realize vector bending sensing and reduce temperature and strain crosstalk. The designed D-core multimode fiber has cladding and air on both sides of the fiber core. When 0° and 180° bending are applied to it, the interference spectrum moves in different directions, thus realizing the recognition of bending direction. For curvature detection, in the curvature range of 0–0.62 $\text{m}^{-{1}}$ , the sensitivity of 0° bending is 11.00 nm/ $\text{m}^{-{1}}$ , and that of 180° bending is −11.40 nm/ $\text{m}^{-{1}}$ . For liquid level detection, the sensing sensitivity is 234 pm/mm with the range of 0–4 cm, and the linearity is 0.993. In addition, the temperature sensitivity is as low as 10 pm/°C, and the strain sensitivity is as low as −1.28 pm/ $\mu \varepsilon $ . The proposed sensor realized the directional bending sensing and liquid level sensing of intermodal interference. Its low sensitivity to temperature and strain can be used to avoid the cross-sensitivity problems. In the field of water flow detection, the simultaneous detection of water level and water flow direction can be realized by the proposed sensor.