Comparative Analysis of MMF and NCF Mode Couplers in SCF-MZI for Curvature Sensing

Abstract

Optical fiber curvature sensors based on Mach-Zehnder interferometers (MZI) incorporating multicore fibers offer high sensitivity. However, the influence of the beam splitter and combiner fibers on overall sensor performance remains underexplored. This work systematically investigates three seven-core fiber (SCF)-MZI curvature sensor configurations employing different splitter/combiner fibers: multimode fiber (MMF) with 105/125 μm diameter, MMF with 110/125 μm diameter, and no-core fiber (NCF) with 125 μm diameters. The sensors are evaluated under bidirectional curvature (+x and-x directions) to assess both wavelength-based and intensity-based sensitivities. Experimental results demonstrate that the choice of splitter/combiner fiber critically determines sensitivity and sensor’s response trend. The MMF (105/125 μm) coupler delivers superior overall performance, achieving the highest wavelength sensitivity of-149.63 nm/m−1 (in the-x direction) with a low standard deviation of 0.30 nm and a minimal limit of detection (LOD) of-0.0060 m−1. For intensity modulation, the sensor provides a robust sensitivity of 430.38 dBm/m−1 (in the +x direction) and achieve its finest LOD of 0.0028 m−1 while maintaining excellent spectral dip visibility. In contrast, the MMF (110/125 μm) configuration shows moderate sensitivities, and the NCF-based sensor exhibits the poorest performance, particularly in the-x direction where wavelength sensitivity drops to-34.65 nm/m−1. These findings identify the MMF (105/125 μm) as the optimal splitter/combiner fiber for SCF-MZI curvature sensors, offering the best trade-off between high sensitivity and overall sensor response, with minimal measurement error and exceptionally low limit of detection.