Sensing Characteristics of a Compact All-in-Fibre fs-Laser Inscribed MZI and Grating Dual Structure

This work investigates the sensing capabilities of a compact, all-in-fibre Mach–Zehnder interferometer (MZI) fabricated using femtosecond laser inscription. The device was created within the cladding region of a single-mode optical fibre, in close proximity to the cladding–air interface, enabling light extraction from the core and enhanced evanescent-field interaction with the surrounding environment. The sensing structure integrates three collocated optical elements: a cladding-based MZI, a fibre Bragg grating (FBG) inscribed within the MZI sensing arm, and a conventional FBG inscribed in the fibre core, providing distinct and complementary sensing responses. All components were fabricated using the plane-by-plane (Pl-by-Pl) femtosecond laser inscription technique, ensuring high fabrication flexibility, repeatability, and reliability through the use of common inscription parameters. The sensing performance of the compound device was experimentally evaluated under variations in temperature, relative humidity, axial strain, bending, and surrounding refractive index. The three integrated sensing elements exhibited complementary responses, enabling multiple parameters to be monitored using the same interrogation equipment while providing opportunities to mitigate cross-sensitivity effects through the selection and combination of appropriate sensing signals. The proposed compact all-in-fibre sensing platform demonstrates the potential of femtosecond laser micromachining for the development of robust, multifunctional optical fibre sensors.