Femtosecond laser-inscribed in-fiber Mach-Zehnder interferometer for ultra-sensitive small-angle torsion measurement

• Published in: Optics and laser technology 2025-02, Vol.181, p.112058, Article 112058
• Main Authors: Huang, Zhiwei, Zhao, Rong, Xu, Zhenxiang, Miao, Qi, Shu, Xuewen, Yu, Benli, Lu, Liang, Yu, Qi
• Format: Article
• Language: English
• Subjects: Femtosecond laser fabrication; Fiber sensing; Mach-Zehnder interferometer
• ISSN: 0030-3992
• DOI: 10.1016/j.optlastec.2024.112058

•We propose a new technique approach for the ultra-sensitivity in-fiber MZI torsion sensor for small twist angles, which is constructed with two parallel waveguides inscribed along both sides of the core-cladding boundary of a single-mode fiber using a femtosecond (FS) laser.•The key breakthrough of this paper lies in inducing higher geometric birefringence and mode coupling efficiency without increasing the structure's size. Both simulations and experiments demonstrated that constructing a second straight waveguide (WG2 ) on the opposite side of the fiber core significantly enhances the interference fringe contrast more than four times, superior to the improvement achieved by merely increasing the waveguide length•Besides, by finely adjusting the waveguide space between the WG2 and the fiber core during the FS fabrication process, a large tuning range of ~ 200 nm for the designable interference wavelength with maximal fringe contrast is achieved.•The proposed in-fiber MZI performs ultra-sensitivity with 17417.92 dB/(rad/mm) over small twist angles from -10° to 0° for the sample “WG1-4/WG2-4” and 19480.57 dB/(rad/mm) over small twist angles from 0° to 10° for the sample “WG1 -4/WG2 -6”, which shows the highest sensitivity up to now, to the best of our knowledge.•The sensor also exhibits superior performances of high repeatability and low crosstalk for temperature and strain. We propose and demonstrate a novel in-fiber Mach-Zehnder interferometer (MZI), using a femtosecond (FS) laser to inscribe two parallel straight waveguides along both sides of the core-cladding boundary of an optical fiber. Both simulations and experiments demonstrated that constructing a second straight waveguide (WG2) on the opposite side of the fiber core significantly enhances the mode coupling efficiency in fiber, resulting in ultra-sensitive small twist-angle torsion measurement by intensity interrogation. By finely adjusting the waveguide spacing between the WG2 and the fiber core during the FS fabrication process, a large tuning range of ∼ 200 nm for the designable interference wavelength with maximal fringe contrast has been achieved. Through comparative experiments, the proposed in-fiber MZI performs ultra-sensitivity with 17417.92 dB/(rad/mm) over small twist angles from −10° to 0° for the sample “WG1-4/WG2-4” and 19480.57 dB/(rad/mm) over small twist angles from 0° to 10° for the sample “WG1-4/WG2-6”. Besides, the device is insensitive to temperature and strain. The exhibite