High performance transceivers for optical interconnects in data centers

Our society is increasingly hungry for data. In the near future, data centers will have to transport hundreds of Gigabits of data per second within their networks. Since copper wires cannot cope with these gigantic data rates, researchers from the ECO group of the department of Electrical Engineering at Eindhoven University of Technology develop photonic alternatives. PhD student Chenhui Li designed, produced and tested different cost-effective solutions to enable high bandwidth data transport at the lowest possible energy expense. Through his outstanding work, he has obtained the PhD title Cum Laude.

The PhD student came up with novel packaging concepts which enable powerful electro-optical transceivers that convert electrical signals used in data processing applications into an optical format used for high bandwidth transport, and vice versa. A transceiver consists of a transmitter unit, containing the laser driver and laser diode, and a receiver unit composed of a photodiode and data recovery circuits. Both of these units are combined in a package that establishes the electronic and optical connections to the outside world. Li developed new ways to package these transceivers in such a way that they consume much smaller footprint and thus enable considerable size reduction of the data server boards, thus yielding much higher data processing power in the same server volume.

The main challenge was to develop a packaging solution that is small, relatively cheap and is able to deal with large bandwidths, Li explains. ‘We had to ensure the quality of both the electronic and optical connections, and develop solutions to disperse the heat that is generated in such a way that the transceiver can operate under optimal conditions.'


Silicon as multitasker
The PhD student proposed to make use of a silicon interposer: a silicon layer that enables both electronic connections and optical access, and at the same time acts as a heat sink. He developed a wet etching process to realize well-defined metal patterned areas in the silicon for positioning and light-to-chip coupling. With this process, he produced and tested three types of packaging.

In the first scheme he proposed, the optical and electronic components are placed side by side, embedded within the interposer. In the second scheme, the elements are also put inside the silicon, but this time on top of each other. Electrical information comes in on the top, and light is coupled out of the back via an array of lenses into 12 fibre channels. Li’s most advanced scheme is based on this 3D-structure, but incorporates four optical components instead of one, leading to a smaller sized chip (similar sized module) that can handle four times as many optical channels, with specially designed two-dimensional optical access.

‘We have developed a technology to package transceivers efficiently with higher channel counts and higher port- and bandwidth-density, while taking up less space. We demonstrated that with these packaging schemes it is possible to realize ultra-compact receivers and transceivers that offer up to 10 gigabits per second per square millimeter bandwidth density,’ Li concludes. ‘Since we deliberately made use of commercially available chips, silicon that is widely used in CMOS technology, and existing technology for flip-chip bonding, our modules are close to being ready for production.’ The PhD student has patented parts of the technology, and will stay in the ECO group as a postdoc to commercialize the platforms.

Chenhui Li defended his thesis entitled ‘Silicon-Based Opto-Electronic Integration for High Bandwidth Density Optical Interconnects’ on Wednesday June 27th at Eindhoven University of Technology. Promotoren prof.ir. A.M.J. Koonen and dr. O. Raz. He obtained the PhD degree Cum Laude.