Non-reciprocal waveguide grating with blocked spurious output mode

A non-reciprocal waveguide grating is one in which the modal coupling strength is not equal in both directions, in stark opposition to the case of the standard index grating, which is necessarily reciprocal. In all previous designs of non-reciprocal gratings, the desired coupling mode is accompanied by a spurious non-coupling mode, which is detrimental to some applications. In this work, we introduce a variation on the design of the non-reciprocal waveguide grating in which the spurious mode is prohibited from propagating through the output waveguide, in a process mediated by a third mode. An additional benefit of this design is that it provides the ability to achieve any desired power splitting ratio in the allowed coupling direction, independent of the total power in the device.