Efficient dual-band grating coupler for 10 Gbit passive optical networks fabricated by 193-nm deep-ultraviolet lithography

The large mode size mismatch between standard single-mode optical fibers and silicon-on-insulator (SOI) waveguides poses a significant challenge to efficiently couple light from the optical fiber to the chip, and vice versa. Surface grating couplers are often used for this purpose, however, their operational bandwidth is limited to a few tens of nanometers, as a consequence of the wavelength-dependent radiation angle. This constraint seriously hampers the use of surface grating couplers for next-generation passive optical networks (PONs), in which the wavelengths used for the upstream and downstream channels are separated more than 150 nm. In this work, we present a dual-band grating coupler for 10 Gbit symmetric PONs. Our device operates as a wavelength multiplexer/demultiplexer, simultaneously coupling and combining/splitting two optical signals at the wavelengths of λ_1=1270 nm and λ_2=1577 nm. The coupler is based on engineering a surface grating coupler to obtain opposite radiation angles for the two respective wavelengths. To achieve a higher coupling efficiency, the material platform thicknesses were optimized as a tradeoff between the waveguide propagation loss and the substrate reflectivity. By judiciously choosing the period (Λ=500 nm) and the duty cycle (DC=55%) of the grating section, an efficient dual-band grating coupler is designed with a minimum feature size of 225 nm. The coupler was fabricated in ST Crolles using their 300 mm SOI platform and 193-nm deep-ultraviolet lithography, demonstrating that large-scale fabrication is feasible. Measured fiber-chip coupling efficiencies were -4.9 dB and -5.2 dB with a 3-dB bandwidth of >27 nm and 56 nm at λ_1=1270 nm and λ_2=1577 nm, respectively.