Femtosecond-laser-written microstructured dual hole flat fibre pressure sensor

Interest in silica flat fibre-based sensing technology has been fuelled by their flexibility, defined orientation, and ease of integration with fibre composites, in particular for structural health monitoring. However, the solid flat fibre strain sensitivity remains largely dictated by bulk material properties. We demonstrate here the fabrication and use of a silica microstructured flat fibre with a dual hole cross section for pressure sensing, wherein two sealed air holes concentrate strain caused by external pressure changes to a central glass region containing a waveguide core and Bragg gratings, thus enhancing their sensitivity. Integrated sensing elements were inscribed by femtosecond laser λ = 515 nm 200 fs pulses in the central region, including a waveguide and two identical 4^th order 3 mm long gratings at λ = 1.55 μm Bragg wavelength separated by 11 mm to form an interference sensor. The laser inscription provides an intrinsic waveguide birefringence leading to a Vernier effect in the reflected spectrum. When external pressure is increased, the birefringence changes resulting in sensitive peak and dip level changes. The sensor was interrogated using a broadband erbium doped amplifier source while recording the back-reflected spectrum as pressure was varied from 0 to 5 bar using a nitrogen pressure chamber enclosing the sensor. The peak-to-dip extinction showed a linear pressure sensitivity of 0.83 dB/bar over the 0 to 3 bar range, before plateauing at 4 bar and decreasing slightly at 5 bar, as well as a high consistency under repeated pressure ramping.