Ultracompact multimode interference 3-dB coupler with strong lateral confinement by deep dry etching

Extremely short InP-InGaAsP MMI 3-dB couplers (L similar to 15-50 mu m) with conventional structure have been successfully fabricated. The significant size reduction was achieved by deep dry etching and tapered access waveguide with large S-bend design. Optical transmission measurements on these devices have been performed. An optical bandwidth about 80 nm with a limit of 2-dB excess loss was obtained for most of the couplers. During the whole tunable wavelength range (lambda similar to 1485-1580 nm), the imbalance is within +/-0.5 dB. Fabrication tolerance and polarization independence are also investigated and the results demonstrated the applicability of these ultracompact devices in high-density photonic integrated circuit.