Reflection properties of multimode interference devices

We report on the reflection properties of multimode interference (MMI) devices: we distinguish between reflection back into the input waveguides and internal resonance modes due to the occurrence of simultaneous self-images. Because of self-imaging, reflection can be extremely efficient, even in the case of MMI devices with optimized transmission. This conclusion is confirmed by the observed spectral behavior of InP-based ring lasers incorporating MMI 3 dB couplers and MMI power splitters. Several techniques are proposed to minimize the influence of these reflections.<
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