Research team publishes breakthrough in Si-based all-optical ICs

APRIL 14, 2009 -- A new paper published in Nature Photonics details results from a collaboration between the University of Karlsruhe, Germany; research center IMEC, Leuven, Belgium; Lehigh University, Bethlehem, PA; and ETH Zürich, Switzerland. The April issue of Nature Photonics features what the researchers say is the first experimental proof of all-optical ultrafast communication signal processing with silicon (Si)-based devices for transmission speeds faster than 100 Gbps. The achievements are expected to be a key step toward the development of complex Si-based photonic ICs.

A crucial element to enable all-optical processing is optical waveguides with highly nonlinear and ultrafast performance. Researchers from University of Karlsruhe, IMEC and its associated laboratory INTEC at Ghent University, Lehigh University, and ETH Zürich fabricated an optical waveguide structure by combining deep-ultraviolet lithography, standard CMOS processing, and organic molecular beam deposition. The so-called silicon-organic hybrid (SOH) approach enables the fabrication of waveguides which could pave the way toward all-optical processing, where photons do not need to be converted to electrons. This is considered to be one of the most promising ways to handle the rapidly increasing global communication traffic.

A 4-mm-long SOH waveguide with a record nonlinearity coefficient of γ ≈ 105(Wkm)-1 in the 1.55-µm telecommunication window proved the capability of the SOH concept. Based on these waveguides, all-optical demultiplexing of a 170.8-Gbps telecommunication signal to 42.7 Gbps was transmitted using four-wave mixing. This is reportedly the fastest silicon photonic optical signal processing demonstrated to date.

With the SOH approach, some inherent limitations of silicon could be overcome. Si-based technology, in particular silicon-on-insulator (SOI) technology, has already proven successful for the fabrication of passive linear optical devices such as filters. The development of ultrafast active Si-based functionalities, such as all-optical switching, remained challenging due to the slow dynamics caused by unwanted nonlinear effects in silicon. So far, the data rate achieved by using bare silicon waveguides was limited to 40 Gbps. The SOH approach overcomes the intrinsic limitation -- thus enabling data rates faster than 100 Gbps -- by combining mature CMOS processing used to fabricate the waveguide and organic molecular beam deposition used to cover it with organic molecules. The molecules efficiently transfer all-optical interaction without introducing significant absorption. The ability of the organic material to homogeneously fill the slot between the waveguides is a key feature of the deposition process.

The silicon circuits were designed by researchers of the University of Karlsruhe in a fabless way and were fabricated through the ePIXfab service on IMEC's 200-mm silicon photonics platform. ePIXfab is a European-funded initiative coordinated by IMEC to allow cost-effective fabless prototyping in wafer-scale silicon photonics technology for R&D. ePIXfab runs multiproject wafer shuttles in which designs from worldwide users share mask and processing costs.