Theoretical investigation of vector modes in a conventional photonic lantern

Vector mode properties in a 3×1 conventional photonic lantern (CPL) are theoretically investigated. Analytical solutions of vector modes in fiber cores at the weak tapering region of a 3×1 CPL are derived based on coupled mode theory and linear combination method. While at the strong tapering region of a 3×1 CPL where core modes cut off, vector modes in cladding can be derived by ignoring the impact of original fiber cores. These analytical solutions are verified by numerical calculations by a fully vectorial finite element mode solver. Further investigation shows that although there is no linear polarization selection mechanism in the tapering region of a CPL, linear polarization (LP) modes are still suitable for CPLs only if fiber modes finally evolve into LP modes in the output fiber. This conclusion can be very useful for simplifying the mode analysis of CPLs.