In this paper, a simple effective index model of directional coupler (DC) with coupling gap refractive index, different from cladding region, has been presented. Using this model, we have shown for the first time the transition from DC to two mode interference (TMI) coupler by decreasing coupling gap and transition from DC to multimode interference (MMI) coupler-based general interference by enhancing refractive index of coupling gap towards refractive index of waveguide core. It is seen that DC with coupling gap ∼0.02µm is equivalent to TMI coupler and DC with coupling region refractive index ∼0.43% less than that of core region is equivalent to MMI coupler. It is also seen that the reduction of beat length is more in TMI coupler than that for MMI coupler, during transformation of DC to TMI and MMI coupler.
Directional coupler Two mode interference Multimode interference SEIM Coupled mode theory
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