Studies on optical properties of multimode Core-Only silver halide fibres under bending

Abstract

In this paper, we investigated the influence of bending on some functional characteristics of core-only multi-mode mid-infrared fibres placed into a polymer protecting shell. As a test fibre, we chose AgBr0.87I0.13 polycrystalline fibre of 525 µm in diameter with in-plane bends. Using an analytical equation, we calculated the critical bending radius of the fibre within the entire operating wavelength range of 3–27 µm with the increment of 1 µm. Then, we estimated total energy output through numerical simulations by COMSOL Multiphysics over the bend radii range of 1.5–52.5 mm. Further, we acquired mid-infrared transmission spectra of the fibre bent to various radii of curvature and compared the data to the analytical results. Moreover, we validated the simulations by measuring power output from the bent fibre guiding the radiation of a CO2 laser. It was revealed that both analytical and modelling results are in good agreement with the experiments. Then, we determined cut-off values of the bending radius which allow for maintaining optimum working conditions for the fibres and showed that stripping out higher-order modes due to bending leads to significant improvement of the quality of the laser beam outgoing from the test fibre.