Abstract

An easy-to-fabricate and efficient fiber-to-waveguide adiabatic coupling on a high-index-contrast (HIC) InGaAs/InP quantum well (QW) waveguide platform has been theoretically analyzed and experimentally demonstrated through a 3D spot-size converter (3D-SSC). The 3D-SSC has three monolithically integrated sections: a HIC and a fiber-matched section, coupled using a 3D-adiabatic mode converter (AMC). Vertical tapering of 3D-AMC is achieved by $\text{ZrO}_{2}$ based selective area impurity-free QW intermixing (IFQWI) on the photo-lithographically delineated horizontally tapered region. The 3D-AMC has a compact length of 30 $\mu$ m and adiabatically coupled the expanded mode size to a compressed mode size from input to the output of 3D-SSC. A better than 7 dB improvement in the coupling loss is obtained for the vertical tapered waveguide over a non-tapered waveguide of 4 $\mu$ m rib width. An additional $\sim$ 1 dB improvement in coupling loss is obtained for the vertical and horizontal taper for a bare fiber input. These finding matches very well with the simulated analysis. The fabricated 3D-SSC reduces the insertion loss by 6 dB from that of the non-SSC waveguide for a bare fiber coupling, with a facet coupling loss of 3.33 dB and a large misalignment tolerance. A low coupling loss of 1.85 dB is measured for a lensed fiber. The total propagation loss for a 100 $\mu$ m integrated device is as low as $\sim$ 0.1 dB, making it very suitable for monolithic integration with other optical devices.

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