High-Performance Bending Sensor Based on Femtosecond Laser-Inscribed in-Fiber Mach–Zehnder Interferometer

A novel bending sensor based on an in-fiber Mach–Zehnder interferometer (MZI) has been fabricated by femtosecond laser. The MZI is formed by an off-axial straight waveguide inscribed in a no-core fiber spliced between two standard single mode fibers. The in-fiber MZI is formed due to the interference of a fundamental mode and a higher-order mode which excited in the no-core fiber. Analysis results of the structure agree well with the measured transmission spectra. Experimental results show that the device has low temperature crosstalk and high curvature sensitivity of 14.91 nm/m−1 and 12.86 nm/m−1 for the 0° and 180° orientations in the range of 0–3 m−1, respectively. We can distinguish the bending orientation of the device by the shift direction of the spectrum. The advantages of high sensitivity, low temperature crosstalk, compact structure, and easy fabrication make the device attractive for actual engineering bend sensing.