Abstract

We proposed and experimentally demonstrated a high-resolution omnidirectional vector bending sensor based on 7-core fiber Mach-Zehnder interferometers (MZIs), which is interrogated by the microwave photonics technique. The sensor is composed of three MZIs in the 7-core fiber, and each MZI consists of two symmetric outer cores. Instead of using the traditional wavelength demodulation method, which has the drawback of very limited resolution due to the used optical spectrum analysis devices, e.g., optical spectrum analyzer (OSA), the proposed sensor is interrogated by high-resolution microwave photonics filter (MPF) system, which is capable of offering up to KHz level measurement resolution. As a result, curvature measurement with resolution of 0.0213 m⁻¹ is demonstrated when the used resolution of vector network analyzer (VNA) is 500 kHz. Thanks to the ultra-high measurement resolution of VNA (∼KHz), the curvature resolution can achieve 4.27 × 10⁻⁵ m⁻¹ in theory, which is 2 orders of magnitude higher than the traditional wavelength demodulation-based fiber bending sensors. The measured curvature sensitivity of the MZI exhibits highly bending direction dependence, attributed to different strain variation under distinct bending angle. In addition, owing to the different angular positions of cores of the MZI in the 7-core fiber, the MZIs have differential response to fiber bending. This feature has been used to develop vector bending sensing, which is manage to determine the bending direction by using any two MZIs, and the obtained average relative errors for the retrieved curvature and bending direction are 2.57% and 0.86%, respectively. The proposed sensor possesses the unique advantages of high sensitivity and high resolution over the state-of-the-art technology, which is very promising for omnidirectional vector bending sensing in the future.

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