Madeleine Glick on Polymer waveguides for high speed board-level optical interconnects

The continued growth of data rates in servers, routers and high-bandwidth computing systems has led to an increased interest in optical backplanes for these applications. Data rates in the backplane are increasing to several Gbps/channel and higher. The trend to multi-core and many core processors is an additional factor contributing to increasing bandwidth demands. Electrical interconnects pose serious challenges at bit rates at 10’s of Gb/s, optical links can alleviate many of these difficulties by improving bandwidth-length products and eliminating electromagnetic interference. There is a growing research effort to enable the move from the electrical to the optical domain.

View poster from recent lab open house event.

Polymers are seen as promising optical materials for transmission and even active devices because of their relative ease of manufacture and processing and potential cost effectiveness. At Intel and elsewhere in academia and industry researchers are investigating polymers for applications at both the chip and board levels. Compared to copper, optical technology has proven capabilities for ultra high data rate transmission, while compared to standard optical fiber, board level polymer waveguides offer the possibility of lower cost and a more compact technology that is compatible with electronic manufacturing technology. At the board level, to enable high data rate transfer over longer distances, the main research challenges are to achieve low optical losses in a robust, stable material and to develop low optical loss, low cost connectors to transfer the light to and from the polymer guides.

I have been working with the Centre for Photonic Systems at the University of Cambridge and Dow Corning to demonstrate the use of polymer waveguides at the board level in backplanes for high speed optical networks. My collaborators had previously developed novel polymer low loss devices that can be integrated with printed circuit boards. The siloxane polymers being used have been demonstrated to have low optical loss at data communications wavelengths (~0.04 dB/cm at 850 nm) and possess excellent mechanical and thermal properties compatible with lead-free solder reflow processes [ref 1,2]. The technology is now ready for research and demonstrations at the system level.

We are presenting a paper at CLEO 2008 entitled “Terabit Capacity Passive Polymer Optical Backplane” which describes a novel, compact optical backplane featuring a scalable architecture using a planar array of multimode polymer waveguides. We have fabricated a proof of concept passive optical backplane enabling full non blocking communication between 10 cards (Fig. 1). The fabricated backplane is patterned from siloxane polymer by photolithographic techniques on a FR4 substrate, so it can also be used for mounting electronic components. The waveguide cross-section is 50 x 50 ?m with a lateral separation of 250 ?m to match conventional ribbon fiber and standard VCSEL and photodiode arrays. In this architecture we use bends for the 90° waveguide turns rather than corner reflectors to simplify the fabrication process. This also ensures that all crossings occur at 90° minimizing loss and crosstalk. The bend radius is 8 mm which, according to previous measurements, should induce additional loss of approximately 1 dB for a multimode fiber (MMF) input, while crossing losses are approximately 0.01 dB/crossing [ ref 3]. The low intrinsic waveguide loss together with the low crossing loss and low crosstalk values imply that this architecture has considerable scaling possibilities.


Full line-rate Gigabit Ethernet data transmission between a pair of computers was achieved over the backplane with no packet losses while using both the highest-loss link together with the corresponding link most susceptible to crosstalk. We also performed bit error rate measurements showing that each of the 100 on-board links can achieve error free transmission at 10 Gb/s indicating an aggregate interconnection capacity potential of a terabit per second.

This is an exciting interdisciplinary research collaboration. Polymer waveguides have the advantage of being bit rate transparent and therefore scalable to higher data rates. We intend to examine various system architectures for data rate scaling including using multiple wavelengths to further increase the capacity. A high data rate backplane with a switchless architecture has the potential to lower the cost of the initial purchase of the blade server to incremental costs associated with purchasing additional cards.

Madeleine Glick is a principal research scientist at Intel Research Pittsburgh where she leads the optical systems work. Her research interests include signal processing and coding for optical links and optical switching for high performance computer interconnects. Madeleine has published over 100 articles. She is a Fellow of the Institute of Physics, a member of the UK EPSRC Peer Review College, an associate editor of the OSA Journal of Optical Networking and on the editorial advisory board of OSA Optics and Photonics News. She received a PhD in physics from Columbia University and is an adjunct professor in the ECE Department of Carnegie Mellon University.

Ref [1] J. D. Ingham, N. Bamiedakis, R. V. Penty, I. H. White, J. V. DeGroot Jr., T. V. Clapp, in Proc. CLEO 2006, [2] N. Bamiedakis et al. ICSO 2006 [3] N. Bamiedakis, J. Beals, R. V. Penty, I. H. White, J. V. DeGroot Jr., T. V. Clapp, in Proc. CLEO 2007

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