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2008) Novel approach for design of low index contrast multimode interference devices, Journal of Modern Optics, 55:12, 1913-1921 To link to this article: http://dx.Self-imaging theory is widely accepted as a good method in designing multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 Â N MMI couplers. The average effective width of the MMI waveguide and the average effective propagation constant of the MMI waveguide are used in the improved self-imaging theory. An approach is given to find the average effective width. We use this approach in the optimal design of a 1 Â 4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures.