Vertically integrated spot-size converter in AlGaAs-GaAs.

We report on the demonstration of a spot size converter (SSC) for monolithic photonic integration at a wavelength of 850 nm on a GaAs substrate. We designed and fabricated a dual-waveguide AlGaAs chip. The design consists of a lower waveguide layer for efficient end-fire coupling to a single-mode fiber, an upper waveguide layer for high refractive index contrast waveguides, and a vertical SSC to connect the two waveguide layers. We measured a SSC conversion efficiency of 91% (or -0.4  dB) between the upper and lower waveguide layers for the TE mode at a wavelength of 850 nm.