Vertically tapered waveguide spot size converters fabricated via a linewidth controlled grey tone lithography for InP-based photonic integrated circuits.

We report a novel and simple fabrication process to realize vertically tapered spot size converters (SSC) on InP photonic integrated circuits. The vertical tapering was achieved via a linewidth controlled local optical dose variation, leading to a grey tone photoresist profile. The fabricated SSCs are compact, polarization insensitive and demonstrate a very high mode conversion efficiency of 95%. Integrated SSCs improved the overall loss by 5 dB giving a coupling loss as low as 1.3 dB/facet, for a lensed fibre with a mode field diameter of 3.0 µm. A good agreement was found between the fibre-to-fibre optical loss measurements and those predicted from simulations.