In-fibre directional transverse loading sensor based on excessively tilted fibre Bragg gratings

We report a distinctive polarization mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with a tilted angle exceeding 45 degrees. The ex-45 degrees TFBGs exhibit pronounced polarization mode splitting resulted from the birefringence induced by the grating structure asymmetry. We have fabricated TFBGs with a tilted structure at 81 degrees and studied their properties under transverse load applied to their equivalent fast and slow axes. The results show that the light coupling to the orthogonally polarized modes of the 81 degrees-TFBGs changes only when the load is applied to their slow axis, giving a prominent directional loading response. For the view of real applications, we further investigated the possibility of interrogating such a TFBG-based load sensor using low-cost and compact-size single wavelength source and power detector. The experimental results clearly show that the 81 degrees-TFBGs plus the proposed power-measurement interrogation scheme may be developed to an optical fibre vector sensor system capable of not just measuring the magnitude but also recognizing the direction of the applied transverse load. Using such an 81 degrees-TFBG based load sensor, a load change as small as 1.6 x 10(-2) g may be detected by employing a standard photodiode detector.