Fiber-chip edge coupler with large mode size for silicon photonic wire waveguides

Abstract

Fiber-chip edge couplers are extensively used in integrated optics as one of the key structures for coupling of light between planar waveguide circuits and optical fibers. In this work, a new fiber-chip edge coupler concept with large mode size for coupling to submicrometer silicon photonic wire waveguides is presented. The coupler allows direct coupling to conventional SMF-28 optical fiber and circumvents the need for lensed fibers. We demonstrate by simulations a 95% mode overlap between the mode at the chip facet and a high numerical aperture single mode optical fiber with 6 µm mode field diameter (MFD). We also demonstrate a modified design with 89% overlap between the mode at the chip facet and a standard SMF-28 fiber with 10.4 µm MFD. The coupler is designed for 220 nm silicon-oninsulator (SOI) platform. An important advantage of our coupler is that large mode size is obtained without the need to increase buried oxide (BOX) thickness, which in our design is set to 3 µm. This remarkable feature is achieved by implementing in the SiO 2 upper cladding two thin high-index Si 3 N 4 layers. The high-index layers increase the effective refractive index of the upper cladding layer near the facet and are gradually tapered out along the coupler to provide adiabatic mode transformation to the silicon wire waveguide. Simultaneously, the Si-wire waveguide is inversely tapered along the coupler. The mode overlap at the chip facet is studied using a vectorial 2D mode solver and the mode transformation along the coupler is studied by 3D Finite-Difference Time-Domain simulations. The couplers are optimized for operating with transverse electric (TE) polarization and the operating wavelength is centered at 1.55 µm.

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“…The coupling loss at the oxide mode converter-nanotaper interface is dominated by the nanotaper design and is weakly dependent on the oxide cladding thickness. The 1dB penalty misalignment tolerance between the fiber and the oxide mode converter is +/-2.5µm and +/-2.4µm in the horizontal and vertical directions, respectively ( figure 5); which compares favorably with other edge coupling methods [1,12,20,21,25,30,41].…”

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