Tapered velocity mode-selective couplers

In this paper tapered velocity mode-selective couplers are presented. These tapered couplers are approximately adiabatic devices which unlike standard mode-selective couplers, do not rely on precise phase conditions to be satisfied over an extended length, to achieve their mode-selective functionality. Therefore these mode couplers permit ultra-wideband mode-division multiplexing of few-mode waveguides. Moreover, their behavior is largely independent of parameters such as index contrast and the precise coupler dimensions. The mode-selective functionality is demonstrated in slab geometry, with the principles generalizing to planar or fiber-based devices.