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.
The beam quality factor (M2 parameter) of output laser in a three-core and seven-core conventional photonic lantern excited by incoherent sources is analyzed based on analytical and numerical method. Theoretical results show that the limitation of output laser beam quality is M2=1.75 for a three-core photonic lantern and M2=2.70 for a seven-core photonic lantern. Both the mode evolution process and beam quality factors of these two kinds of photonic lanterns are verified by numerical calculations. It is shown that good beam quality of the theoretical limitations can be realized only if the adiabatic conditions are satisfied very well. These results are very meaningful for practical application of high brightness incoherent beam combining based on conventional photonic lanterns.
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