Effect of Coupling between Fundamental and Cladding Modes on Bending Losses in Single-Polarization Single-Mode Photonic Crystal Fiber

In this paper we study the effect of bending on the performance of a single-polarization single-mode (SPSM) photonic crystal fiber previously reported in the literature. The optical fiber consists of a photonic crystal fiber with hexagonal lattice of circular air-holes with an elliptical core formed by eight enlarged holes surrounding the central region of the fiber. Numerical analysis based on full-vector finite element method (VFEM) with a cylindrical perfectly matched layer (PML) was applied to evaluate the bending loss and the confinement loss of the fiber. For the straight fiber only the fundamental slow axis mode (x-polarization) exist because of the very high confinement loss of the fundamental fast axis mode (y-polarization). However, for the bent fiber the fast axis mode and the slow axis mode couple with the gallery of the cladding modes. These coupling prevent the fast axis mode to reach the cutoff. As a consequence, the bending loss curve as a function of the wavelength presents oscillations related with these coupled modes. Our simulations demonstrate that the analyzed fiber does not maintain its single-polarization single-mode characteristics when it is bent with a small curvature radius of 10 mm. However, for a small wavelength range (1.50 pm to 1.53 pm) the fundamental slow axis mode presents values of bending loss lower then 0.1 dB/km while the fast axis mode reach losses of about 150 dB/km, what can be useful for some application.