Type I fiber Bragg gratings (FBG) written through the coating of various off-the-shelf silica fibers with a femtosecond laser and the phase-mask technique are reported. Inscription through most of the common coating compositions (acrylate, silicone and polyimide) is reported as well as writing through the polyimide coating of various fiber cladding diameters, down to 50 µm. The long term annealing behavior of type I gratings written in a pure silica core fiber is also reported as well as a comparison of the mechanical resistance of type I and II FBG. The high mechanical resistance of the resulting type I FBG is shown to be useful for the fabrication of various distributed FBG arrays written using a single period phase-mask. The strain sensing response of such distributed arrays is also presented.

This paper reports on an experimental investigation into the differences between directly and remotely bonded optical Fibre Bragg Grating (FBG) sensors when measuring Lamb waves in an aluminium plate. A sensor array comprising 16 individual short gauge-length FBG sensing elements is used to decompose the wave into its constituent modes for the directly bonded case where the array is attached directly to the plate and for the remotely bonded case where the same array is located in a suspended fibre 250 mm downstream from the fibre/plate contact region. The experimental results were consistent with model predictions and show that while the directly bonded fibre array can resolve the full multimodal Lamb wave spectrum in the plate, the remotely bonded sensor measures only a single non-dispersive longitudinal acoustic mode predicted by the analytical solution for a cylindrical waveguide. The inability of remote sensing to distinguish between different modes of Lamb wave propagation represents a significant limitation with respect to the diagnostic utility of this sensing approach.

An array of ten distributed femtosecond fibers Bragg gratings (FBG) was written through the polyimide coating of a standard silica fiber using a single period uniform phase-mask. All FBG wavelengths are spectrally distributed over 40nm.