Mode evolution in photonic lanterns and requirements for achieving good beam quality and mode control

Abstract

The photonic lantern is an all-fiber-based linear optical element that couples light efficiently and evolves modes functionally between a set of single-mode waveguide and a multimode waveguide. Our study is based on different core photonic lanterns, which are fabricated by using the ‘ferrule technique’ method, with the single-mode array in a certain geometric arrangement. The process of the light propagation through the lanterns is simulated and the experiments according to the simulation results are conducted. The mode combining and evolution in the 3-core lanterns is simulated to study the necessary conditions for achieving the fundamental mode with good beam quality in large-mode area (LMA) fiber. Appropriate input (the amplitude ratio of each channel is 1:1:1, the phase and polarization state are the same) is injected at the single-mode (SM) end with some random disturbance on amplitude and polarization (the relative change is 20%). The M2 factor at the multimode (MM) end has a standard deviation of 0.0001 orders of magnitude. However, the M2 factor varies from 1.05 to 2 or even more with the changes of the phase difference at the SM end. Thus, the adaptive optics (AO) technique is used in our experiment, which can adaptively determine the appropriate phase to be applied to the input fibers. In addition, the modes behavior in 6- and 7-core photonic lanterns is simulated to obtain the ideal input (including the amplitude, phase and polarization of each SM channel) for achieving the six lowest order modes output. And these two kinds of photonic lanterns are compared from the aspects of drawing difficulty, mode coupling efficiency, transmission loss, and application feasibility in module multiplexing.