Polymer waveguides for board-level optical interconnects
In-system bandwidth densities driven by interconnect speeds and scalable input/output (I/O) within data storage and server enclosures will continue to increase over the coming years, thereby severely impacting cost and performance in future data centre systems. This is fuelling the migration of optical connectivity into the system enclosure itself and will need to be addressed by disruptive new embedded optical interconnect technologies.
Electro-optical printed circuit boards based on polymer waveguides and supporting technologies can provide the cost viable embedded optical interconnect ‘eco-system’ to mitigate this impending bottleneck. In addition the technology could have a wide range of other potential applications in several large markets such as vehicle wiring looms and data distribution for displays and consumer goods.
‘Our industrial partner Dow Corning believed that its siloxane materials could be used to make polymer waveguides but wanted the materials to be proven in this application,’ explains Professor Richard Penty in the Centre for Photonic Systems in the Department of Engineering. Siloxane polymers exhibit stable optical performance with very low loss maintained over long lifetimes and at high temperatures. The materials can be formed into waveguides using simple photolithography/wet chemical and embossing processes and potentially with printing.
The polymer material can be processed at low temperature, in contrast to the more conventional high temperature processing required for the formation of glass waveguides. Unlike other polymers, the siloxane system is compatible with high temperature lead-free soldering processes so it can be used in standard printed circuit board processes.
Funding enabled Penty and his team to demonstrate high quality basic building blocks, including waveguides, bends, couplers and crossings, with excellent performance. The team was then able to build the first Terabit/s capacity passive router, with 100 waveguides, with up to 90 crossings (which remains world leading in terms of passive router functionality).
An optical waveguide layer on one side of a standard circuit board and electronic components on the other was combined by developing a simple ‘L connector’ where surface-emitting lasers and photodiodes are slotted through the circuit board and connected to both the electronic components and the waveguides. This enabled the demonstration of a 10Gb/s ONU transceiver circuit with error free operation in Tx and Rx modes and the realisation of an on-board 4x10Gb/s parallel interconnect.
A multi-channel scalable waveguide layout comprising a wide range of passive multimode components was then designed and implemented with Dow Corning’s materials on low-cost FR4 substrates. As a proof-of-principle, 4-channel 3-card optical bus modules have been fabricated and interconnected via a prototype 4 x 10 Gb/s opto-electronic 3R (reamplifying, reshaping and retiming) regenerator.
The final step of the project integrated polymer waveguides, electronics and optoelectronics onto a standard circuit board using a low cost interconnect technology with integrated transceivers operating at data rates up to 10Gb/s.
The main outcome of the work, to date, has been the development of a low-cost manufacturing approach using materials compatible with the high temperature steps in conventional circuit board manufacture and a novel solution for high performance computer interconnect interfacing issues, which has also led to new photonic systems concepts. The next step is to take the technology to the stage where it can be readily transferred to production.
‘We see the optical waveguide PCB activities being carried out at the CIKC as a very strong contender to satisfy the requirements of next generation system embedded optical interconnect solutions, given the disruptive optical design activities, successful deployment into hybrid electro-optical PCB and the quality of the Dow Corning optical polymer itself,’ says Richard Pitwon, lead photonics engineer at Xyratex, a global supplier of data storage technologies with facilities in Havant, Southampton.
Xyratex is leading a collaborative project bid for EU funding to develop such an embedded optical ‘eco-system’, which includes companies in the PCB, transceiver and connector industry.