A Novel Wearable Knee Joint Angle Monitoring Sensor Utilizing Mach–Zehnder Interferometer With Triple-Clad Fiber

We propose a novel wearable knee joint angle monitoring sensor based on the Mach–Zehnder interferometer (MZI) using a triple-clad fiber (TCF), which is fabricated by directly fusing a TCF between two single-mode fibers (SMFs). The varying motion angle of knee joint will cause various optical path differences between the fiber core mode and the cladding modes, which will lead to the wavelength shifting of the MZI. Experimental results show that the sensor can measure the bending angles of the knee joint in the range from 40° to 90°. When the knee joint angle changes from 65° to 90°, the angle sensitivity can reach 569 pm/°, and the correlation coefficient can achieve 0.98. In addition, the temperature sensitivity of the proposed sensor is 69 pm/°C in the temperature range from 40 °C to 90 °C, which shows that this sensor is hardly sensitive to temperature. Thus, the cross-sensitivity effect between the temperature and curvature can be avoided. These advantages provide a potential application prospect in the areas of sports medicine rehabilitation and robot monitoring.