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EU group demonstrates first optical interconnect layer on silicon

22 Jun 2007

European researchers have demonstrated an integrated device that could form the basis for future on-chip optical interconnects.

A European consortium known as PICMOS has integrated electrically-pumped microdisk lasers with a nanophotonic silicon wire waveguide for the first time. PICMOS says that the result forms an essential step towards demonstrating a full photonic interconnect layer to be integrated on top of next-generation CMOS chips.

The team has also released preliminary results for a full optical link connecting a microsource to a microdetector through a nanophotonic silicon waveguide. Led by IMEC/Ghent University, Belgium, the seven-strong group also involves CEA-LETI and the Lyon Nanotechnology Institute, both in France.

The PICMOS device incorporates microdisk lasers, made from indium phosphide and with a diameter of just 7.5 µm, with a silicon wire waveguide. "Silicon wire waveguides allow for highly dense optical interconnect circuits (with a bend radius of 2 µm and pitch of 1.5 µm) and they can be fabricated using the advanced equipment that also makes high performance electronic processor chips," said project leader Dries van Thourhout.

"The main problem is that there is no straightforward way to integrate compact sources with these silicon wire waveguides," he continued. "We developed a new process based on a rapid die-to-wafer bonding technique that allows the integration of InP-based gain material with the silicon wire waveguides."

The microdisk lasers showed a threshold current of just 500 µA under CW operation and a side-mode-suppression ratio of 30 dB. About 200 lasers were integrated on a single electronic processor chip for the demonstrator, and the team showed that more than 100 µW was coupled to the silicon waveguide under pulsed operation.

"Due to the small size of the microdisk lasers, we expect that they can be directly modulated at high speed, although this still remains to be tested," said van Thourhout. "The threshold current is low, about 500 µA, which is important if one wants to minimize total power consumption for the optical links."

On-chip optical interconnects are seen as a possible solution for the data-transfer bottleneck in future generations of computer chips. By 2015, bandwidths of 30-80 Tbit/s will be needed for the high-performance chips used in games consoles and high-end desktop computers, which most people believe can only be handled by optical links. Intensive research in this domain is now also starting in the US and Asia.

However, the availability of a generic platform for electro-photonic integration could have a much broader impact, says van Thourhout. "Since the microlasers can be fabricated using CMOS-compatible processes they can be incorporated in cheap chips, such as for building powerful optical sensors or for off-chip optical links in consumer electronics," he said.

As for commercialization, van Thourhout expects the large IC manufacturers to develop applications for on-chip optical interconnects, while smaller companies and spin-offs will commercialize the smaller scale applications in sensing and consumer electronics.

"The target market is large so the impact of this development could be widespread," he continued. "The electronics people are also developing new solutions, such as those based on microwave circuits. Within the optics world, the big 'alternative' solution is to use a powerful off-chip laser and on-chip modulators."

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