A new monolithic integration scheme of fabricating optical spot-size converter (SSC) is realized in this work.
High-speed electroabsorption modulator (EAM) is used to integrate such SSC. By laterally tapering the active region of
an optical waveguide through undercut active region, a vertically asymmetric waveguide coupler can be defined to form
an SSC, where the top is a tapered active waveguide, and the bottom is a large core of passive waveguide mode-matched
to single-mode fiber (SMF). Through the top tapered active waveguide, the effective index can be gradually varied in the
propagation direction, momentarily matching the bottom low-index passive waveguide. It not only performs the resonant
coupling in such asymmetric waveguide coupler, but also locks the transferred power by the tapered structure.
InGaAsP/InP multiple quantum wells are used as active region of active waveguide. Based on the highly
selective etching properties between InGaAsP and InP, the tapered active waveguide can be fabricated by a method,
called selectively undercut-etching-active-region (UEAR), enabling the processing a narrow waveguide structure (up to
submicron) by general wet etching from a large waveguide ridge. It also leads to good microwave performance of
waveguide. By taking this advantage, a SSC-integrated EAM can perform high-speed electrical-to-optical (EO) response
as well as low-insertion loss properties. A mode transfer efficiency of 70% is obtained in such SSC. By narrowing
waveguide by UEAR, over 40 GHz of -3dB electrical-to-optical (EO) response is obtained from this device. The high
efficient SSC integrated with high-speed EAM suggests that the UEAR technique can have potential for applications in
high-speed optoelectronic fields.