Polarization independent integrated mode transformer for uncladded InGaAsP/InP rib waveguides without epitaxial regrowth

A polarization insensitive fibre port for integration in complex PICs has been demonstrated. The spot-size transformation is performed by a thickness taper integrated with an uncladded InGaAsP/InP rib waveguide in cooperation with guiding layers in the waveguide buffer. No epitaxial overgrowth is necessary to achieve polarization independent coupling. The thickness taper is structured by using shadow mask ion beam etching. The first fabricated integrated spot size transformers show a polarization independent coupling loss of 2.3+or-0.2 dB (butt coupled cleaved fibre). The radiation loss along the waveguide taper amounts to 0.6 dB. The -1 dB-alignment tolerances are larger than +or-2.4 mu m. A theoretical coupling loss of 1 dB is predicted for an optimized waveguide shape. The high yield fabrication process is suited for integration of the spot size transformer in complex PICs.