Eccentrical Bragg gratings in multimode polymer optical fibres for the 3D detection of bending

Abstract

We report on a novel bend sensor with high flexibility and elasticity based on Bragg grating structures in polymer optical fibres. The concept is very simple and relies on the inscription of eccentrical Bragg gratings into multimode graded-index polymer optical fibres via contact exposure with a krypton fluoride excimer laser in the ultraviolet region and an optimized phase mask. Depending on the polymer optical fibre deformation, the lattice constant of the inscribed Bragg grating is strained or compressed due to its position relative to the fibre core. This in turn results in a shift of the Bragg wavelength of up to 1.3 nm to the red or blue wavelength region, respectively. Therefore, deformation along one axis can be observed with a single Bragg grating with a sensitivity of 50 pm/m⁻¹. Moreover, multiple Bragg gratings inscribed into the same polymer optical fibre at different positions would allow to determine the shape deformation of the fibre relative to a reference frame. Consequently, this technology could form the basis for new applications in the areas of medical diagnostics, robotics or augmented reality in the future.