# Modal spectrum engineering of multilayer optical waveguides: towards digital vertical multimode interference couplers

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## Abstract

A new technique is proposed for improving the coupling efficiency of vertical multimode interference couplers. The technique replaces conventional multimode waveguides with multilayer waveguides (MLWs) in the design of these couplers. The separation between the high refractive index layers of the MLWs is modulated to fit the propagation constants of their guided modes to the ideal parabolic profile required for perfect coupling. Numerical computations of the proposed couplers demonstrate the ability to transfer 86 % of optical power between 220-nm thick SOI waveguides at a vertical separation of 2.3 μm.

## Keywords

Multilayer waveguides Optical coupling Optimization methods## References

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