Reduction of the overall coupling loss using nonuniform tapered microstructured optical fiber

We report the possibility of using nonuniform tapered microstructured optical fiber (NTMOF) to reduce the overall coupling loss from an upstream MOF to a downstream fiber being another MOF with different geometry. The NTMOF is supposed to be drawn at the output end of the upstream MOF, with no coupling loss between the two. The propagation loss along the NTMOF is calculated using the Finite-Difference Vector Beam Propagation Method (FD-VBPM) versus the shape and the length of the NTMOF. Numerical simulations are conducted in order to optimize the profile and the cross section of the NTMOF. Then, the splice loss between the NTMOF and the downstream fiber is taken into account. As an example, a decrease of more than 3.9 dB of the overall coupling loss between an upstream MOF (hole spacing = 12 μm, hole diameter = 3.6 μm) and a downstream MOF (hole spacing = 4 μm, hole diameter = 1.2 μm) is obtained compared to a splice between the same non-tapered fibers. The effect of the polar-angle misalignment between two identical MOFs on the splice loss is also studied. We show that the splice loss due to polar-angle misalignment depends on the geometrical parameters of the fibers and on the wavelength.