Analysis of mode-selective coupling between few-mode fibers and waveguides with lateral misalignment

We present an analysis of the effects of lateral misalignment on the performance of butt-coupling between a 6-mode step-index circular-core fiber and a matched 6-mode square-core waveguide. For a typical 6-mode fiber with a core index of 1.4540, a cladding index of 1.4440, and a core diameter of 13.1 μm, low modal crosstalk (–20 dB) butt-coupling can be achieved with a lateral misalignment smaller than about 0.4 μm.