Analysis of FBG reflection spectra under uniform and non-uniform transverse loads

Loads applied transversely on the external surface of waveguides change their circular cross-sectional geometry generating birefringence. Due to this effect the reflected spectrum of a Fibre Bragg grating (FBG) undergoes a splitting of the single peak of the Bragg wavelength. In this work, we employed the Transfer Matrix Method (TMM) for x- and y-polarized wave-modes to model the uniform FBG reflection spectra for uniform and non-uniform transverse loads. We also performed experimental measurements for two different transverse load scenarios. The load profiles chosen for these experiments were applied on the FBG sensor through a block of steel and a roll bearing pin. Then, the modelled and experimental results were compared resulting in good agreement of 85% (on average). Finally, during the roll bearing pin loading test, different responses were observed depending how the FBGs were surface mounted. To investigate this, the glue layer influence on the reflected spectrum was further studied experimentally.