Compact 2x2 tapered multimode interference couplers based on SU-8 polymer rectangular waveguides

A 2x2 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 2x2 coupler with a total MMI size of about 5.3x34.2 mu m(2) is demonstrated and characterized. The measurement results show that the fabricated 2x2 MMI coupler has relatively small excess loss and nonuniformity for both polarizations in a broad wavelength range (from 1480 to 1630 nm).