Spatial mode multiplexer/demultiplexer based on tapered hollow waveguide

A spatial-mode multiplexer/demultiplexer using a Bragg reflector tapered hollow waveguide is proposed. Depending on each spatial mode, vertical radiation takes place at a cut-off condition while the radiation position is dependent on the order of each spatial mode. Modeling of our structure is carried out by using a mode-matching method. The result shows low wavelength dependence and no noticeable polarization dependence while a low insertion loss is expected. Also a tapered hollow waveguide consisting of highly reflective multilayer mirrors with a wafer bonding process is fabricated. Clear vertical radiation could be observed, depending on the order of spatial modes. Our proposed device enables us to integrate surface-normal optical devices such as photo-detectors and VCSELs. The device will be useful for ultra-high capacity optical interconnects using spatial mode multiplexing.