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 triple-clad fiber 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/degree, 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.

We present a high-sensitivity curvature and strain Mach–Zehnder interferometer (MZI) fiber sensor based on a configuration of no-core fiber (NCF) and four-core fiber (FCF). We used an optical fiber fusion splicer to directly splice a segment of FCF between two segments of NCF, with both the FCF and NCF made of SiO₂, where the FCF exhibits multi-path interference characteristics that allow for higher sensitivity. The NCF, with its self-focusing property, excites higher-order modes, which split and transmit it into the four cores of the FCF. The experimental results show that within a curvature range of 0.0104 m⁻¹–0.1515 m⁻¹, the maximum sensitivity can reach −78.04 dB/m⁻¹ with a high linear value of ~0.99. Additionally, the strain response is also experimentally studied. In the range of 0–600 με, the maximum strain sensitivity is −6.49 pm/με. The sensor demonstrates high curvature and strain sensitivity, indicating its potential applications in sensing measurements.

We propose a Mach-Zehnder Interferometer (MZI) sensor fabricated by a triple-cladding fiber (Tr-CF) and a coreless fiber (CLF). The sensor comprises a segment of Tr-CF sandwiched between two segments of CLF, with single-mode fibers (SMFs) connected at both ends, forming a SMF-CLF-Tr-CF-CLF-SMF structure. The mode mismatch between SMF and CLF efficiently excites higher-order modes, leading to increased cladding modes in Tr-CF. Enhanced sensitivity is achieved as light interacts with the external environment through the cladding layer of Tr-CF. Experimental results demonstrate a maximum curvature sensitivity of 122.83098 dB/m ^-1 as the curvature varies from 0 m ^-1 to 0.09142 m ^-1 . These findings highlight the sensor’s exceptional curvature sensitivity and its potential applications in engineering measurement, health detection, and other fields.

We have proposed and presented a hybrid mini-two-path Mach–Zehnder interferometer (MTP-MZI) sensor based on four-mode fiber (FMF), where the reference path comprises of a section of a single-mode fiber (SMF), and the sensing path adopts a structure of SMF-FMF-SMF (SFS). Using arc discharge technology, the two paths are effectively fused and coupled, resulting in a robust MTP-MZI structure sensor. In the curvature detection, the maximum intensity sensitivity of curvature reaches 168.41 dB/m⁻¹ when the curvature ranges change from 0 m⁻¹ to 0.091 m⁻¹. To the best of our knowledge, it is the highest curvature sensitivity in the MZI fiber sensor with intensity modulation. Furthermore, we also conducted a temperature-sensing experiment. The experiment results show that the maximum temperature sensitivity is only 78 pm/°C with a temperature range of 30–65 °C. The diverse exhibition of sensing performance for curvature and temperature enables us to effectively mitigate cross-sensitivity challenges. These results provide the experimental basis for developing a high-sensitivity sensor by employing the mini-two-path structure combined with specialty fibers.

A knee-flexion angle sensor based on spherical fused triple-clad fiber (TCF) is proposed and demonstrated. The optical path difference between the core and cladding modes in the angle sensor changes as the knee joint flexion angle changes, resulting in deviations in the interference spectrum. We conducted comparison experiments on various TCF lengths. When the TCF length is 3 cm, the maximum sensor’s angle detection range is from $0~^{\circ }$ to $110~^{\circ }$ . The angle sensitivity of the sensor is −0.35 nm/°, and the linear regression coefficient (R2) is 99.6 %, respectively. Furthermore, in the temperature range of 30- $90~^{\circ }$ C, the maximum temperature sensitivity is 0.08 nm/°C, and the R2 is 99.8 %. The sensor has the advantage of being compact, easy to manufacture, and highly repeatable, making it suitable for real-time human health monitoring and rehabilitation.

A high-sensitivity optical fiber curvature sensor based on eight-core fiber (ECF) is proposed and demonstrated. The sensor is fabricated by directly fusing a segment of ECF between two single-mode fibers (SMFs), forming a Mach-Zehnder interferometer (MZI) in an SMF-ECF-SMF structure. Due to the mode field mismatch between ECF and SMF modes, the fundamental mode of SMF is excited to higher-order modes and coupled into ECF to detect the external environment. The cores of the ECF present a $2\times 4$ arrangement, which can play the role of multipath MZI due to the existence of multiple cores. In the curvature detection, the maximum curvature sensitivity of the sensor can be up to −53.32 dB/m−1 within the curvature range of 0.3006 m−1–0.5483 m−1. A wavelength dip with a maximum extinction ratio of 22 dB is obtained in the transmission spectrum, which indicates that the ECF effectively equalizes light energy. In addition, the strain response is also measured. When the strain range changes from 0 to $900~\mu \varepsilon $ , the maximum strain sensitivity is −1.83 pm/ $\mu \varepsilon $ . The proposed sensor, with high sensitivity, simple structure, and good repeatability, has potential applications in many fields, such as medical devices and aerospace.