L. B. Soldano, M. K. Smit, B. H. Verbeek, E. C. M. Pennings
As opto-electronic integrated circuits (OEICs) become a reality, there is an increasing need for optical signal routing and signal processing devices with smaller dimensions, improved fabrication tolerances, and polarization-independent operation. Recently, several papers reported on multi-mode interference (MMI) 2X2 directional couplers that, in contrast with conventional two-mode interference (TMI) couplers, can fulfill all of the above requirements. Insertion losses as low as 0.5 dB and extinction ratios better than 30 dB have been obtained2,3 with very compact, polarization-independent fabrication tolerant devices. The key feature of the high performance of these couplers is the self-imaging effect by multi-mode interference. Self-imaging is a property of multimoded waveguides by which the exciting field at the entrance will be reproduced (either replicated or mirrored, single or n-folded) at periodic intervals along the propagation direction of the guide.
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