Compact 2×2 tapered multimode interference couplers based on SU-8 polymer rectangular waveguides

A 2×2 tapered multimode interference (MMI) coupler is designed and fabricated by using air-cladded SU-8 rectangular waveguide, which is effective in reducing the device size and improving the self-imaging quality. The parabolically tapered MMI section is used to reduce further the size of the MMI coupler. A compact 2×2 coupler with a total MMI size of about 5.3×34.2μm2 is demonstrated and characterized. The measurement results show that the fabricated 2×2 MMI coupler has relatively small excess loss and nonuniformity for both polarizations in a broad wavelength range (from 1480to1630nm).