Mode-selective coupling between few-mode fibers and buried channel waveguides.

With intensive computation, we analyze in detail butt-coupling between few-mode fibers and buried channel waveguides and discuss, in particular, the conditions to achieve mode-selective coupling for the first 6 spatial modes. In the case of coupling a buried rectangular-core waveguide to a step-index or a parabolic-index circular-core fiber, a modal crosstalk smaller than -20 dB for all the 6 spatial modes can be achieved, when the shape of the waveguide core is sufficiently close to a square (within ± 0.7% variation in its aspect ratio) and the waveguide and the fiber have close mode volumes. In the case of coupling a buried rectangular-core waveguide to a step-index elliptical-core fiber, it is possible to achieve a modal crosstalk smaller than -20 dB with many combinations of waveguide and fiber parameters. The aspect ratios of the elliptical core and the best matched rectangular core can be very different (e.g., 0.85 for the elliptical core and 1.05 for the rectangular core) and an elliptical core that has a moderate ellipticity and an area not close to the upper limit allowed for supporting 6 spatial modes is preferred. The use of a parabolic-index profile in the elliptical core can further improve the mode selectivity with greatly relaxed tolerances on both the aspect ratio and the area of the rectangular core required. In general, when the fiber and the waveguide core are matched for low-crosstalk performance, the butt-coupling losses for all the 6 spatial modes as well as the mode-dependent loss are also small (typically well below 1 dB). Our results are useful for the design of mode-selective waveguide devices, such as mode (de)multiplexers, mode converters, and mode switches, for fiber-based mode-division multiplexing applications.