Higher-Order Loss Processes and the Loss Penalty of Multimode Operation

Pulse spreading caused by the different group velocities of the guided modes of a multimode waveguide can be reduced by providing intentional coupling between the modes. Coupling among the guided modes inevitably leads to radiation losses. This loss penalty is discussed for two types of loss processes. We consider that the highest-order mode loses power by second-order coupling to the continuous spectrum of radiation modes. We also consider a loss process that is caused by nonresonant coupling of guided modes to lossy neighboring modes. Both loss processes can cause a substantial loss penalty. However, the loss penalty can always be reduced by limiting the intentional coupling to fewer of the guided modes, allowing the highest-order modes to die out. The discussion is based on a slab waveguide model.