A Feasible Method for Fabricating Vertical Taper Applied to Spot Size Converter

Spot size converter (SSC) can significantly reduce the coupling loss between single mode fiber and waveguide. Therefore, it has been widely used in photonic integrated circuits (PICs). In this letter, an InGaAs/InP vertically tapered SSC has been proposed and fabricated with a length of
$1000~\mu \text{m}$
and a height of
$1~\mu \text{m}$
. We proposed a feasible way of fabricating the SSC by contact lithography. According to the relationship between the exposure dose and the residual photoresist thickness. Choose the appropriate exposure dose, multiple overlays to form a step shape on the photoresist, and the thermal reflow makes it form a slop. This method can be applied to fabricate vertical tapers of various sizes. 1 dB alignment tolerance is enhanced by this vertical taper spot size converter, which achieves 1 dB alignment tolerances
$\pm 2.5~\mu \text{m}$
(horizontal) and
$\pm 3.0~\mu \text{m}$
(vertical). The insertion loss is reduced from 1.8 dB to 1.0 dB.