Abstract
An ultralow coupling loss and broad bandwidth fiber-to-waveguide mode-size converter is demonstrated for nano-scale waveguides on SOI platform using CMOS technology in this paper. The mode-size converter consists of a cantilevered PECVD SiO(2) waveguide and a-Si nano-tapers by removing the adjacent SiO(2) layer and underlying substrate Si. The a-Si waveguide is located at the center of the cantilevered SiO(2) waveguide. We characterized the cantilevered mode-size converter using cleaved optical single mode fiber with 10.5 µm mode field diameter. With refractive index (1.375) matching oil, the measured coupling efficiencies between the cleaved optical fiber and this converter are higher than 80% per facet and 70% per facet for TE and TM modes at 1600 nm, respectively. The polarization dependent loss and the coupling loss variation of this converter are less than 1.0 dB at the wavelength range of 1520~1640 nm. The 1-dB bandwidths for both TE and TM modes are more than 120 nm. The alignment tolerances for TE and TM modes are ± 2.8 µm and ± 2.1 µm at 1-dB excess loss in horizontal direction and vertical direction, respectively.
Highlights
Silicon photonic has been developed rapidly in the past decade, due to the perfect material properties and the mature fabrication process which is compatible with complementary metaloxide-semiconductor (CMOS) technology
We have demonstrated a high performance mode-size converter for silicon photonic devices using CMOS compatible technology on SOI platform
The cantilevered mode-size converter was designed and fabricated on the above SOI wafer. This mode-size converter has a low coupling loss and it is independent on the operated wavelengths
Summary
Silicon photonic has been developed rapidly in the past decade, due to the perfect material properties and the mature fabrication process which is compatible with complementary metaloxide-semiconductor (CMOS) technology. The coupling loss is low with tapered fiber, but high with normal cleaved fiber This kind of converter is a good choice for characterization in Labs, but not for commercial products. The latest reported coupling loss for vertical grating coupler is 1.6 dB/facet; and it has a good alignment tolerance with cleaved fiber. The similar converter is reported by Bell Lab [18] These converters are fabricated on the commercial silicon-oninsulator (SOI) wafer (single crystal Si on the top layer) with 2 ~3 μm buried oxide (BOX). The coupling loss is ~1.5 dB/facet with index matching oil In principle, this kind of converter should have a lower coupling loss if using a commercial SOI wafer with a thicker BOX.
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