Light in the box
Printed circuit boards lie at the heart of all modern electronic systems, from computers to planes, to phones, to satellites, to washing machines, to cars, and in all areas of embedded electronics. The rate at which data is transmitted along the printed circuit board – its bandwidth – is increasing.
The rise in data communication speeds exposes electronic systems to some of the fundamental physical constraints implicit in the conveyance of high frequency electronic signals along the copper channels in conventional printed circuit boards. Dielectric absorption and resistive loss mechanisms will attenuate more strongly the higher speed signals conveyed along a copper channel, while reflections, signal skew and interference from other electronic channels will distort the data. Furthermore, the environmental effects of system operation, such as temperature and humidity, will cause changes in the circuit board substrate, thus altering the carefully balanced characteristics of the electronic channels.
Many of these constraints can be mitigated to some degree, albeit at ever mounting cost to the overall system design and with higher power consumption. It is for this reason that the projected increases in capacity, processing power and bandwidth density in future information communication systems must be and are being addressed by the migration of optical channels into the system enclosure itself. The introduction of optical circuit boards is a critical part of this migration.
Optical interconnect and the Cloud
One area in which this migration is apparent is in modern data centres. One consequence of the widespread adoption of smaller mobile data devices (smart phones and tablets) over fixed larger computer terminals (PCs and laptops) is that a dramatic shift is now occurring in where customers want to store their information. Until recently, data was mostly stored locally on the hard drive of a desktop or laptop; now the amounts of data generated "on the fly" are so large that the storage available on mobile devices is rapidly becoming insufficient for long term accumulation and retention of data.
So-called "Cloud" services are therefore emerging to meet swelling customer demand to store data remotely and securely. Data centres provide the dedicated computer, storage and server equipment needed to meet the remote data processing and storage requirements of these emerging Cloud environments. However, in order to cope with rapidly changing customer demand, the fundamental architectures underlying the data centres themselves need to evolve. A critical part of that evolution is the deployment of optical connections at all levels of the data centre environment.
This migration is reflected by the emergence of a new technology eco-system including board-mountable optical transceivers and very high density parallel optical interfaces. Major international collaborative research and development initiatives such as the European PhoxTrot project and the US HDPuG Optoelectronics project are also helping evolve the approach.
Optical circuit boards
Embedded optical interconnect technologies, whether deployed at the cable level, circuit board level or chip level offer significant performance and power advantages over conventional interconnect. These include higher channel density, higher data rates, no electromagnetic interference, reduction in power consumption and reduction in materials.
Optical circuit board technologies provide a platform which can accommodate hundreds of times the volume of data in comparison with conventional circuit boards. They are therefore seen as a key enabler in modern data communication systems.
Many types of optical circuit board technology exist today including fibre-optic laminate, polymer waveguides and planar glass waveguides.
Laminated fibre-optic circuits, in which optical fibres are glued into place on a substrate, benefit from the reliability of conventional optical fibre technology. However these circuits have some shortcomings. They cannot accommodate waveguide crossings in the same layer; i.e. fibres must cross over each other and cannot cross through one another. With each additional fibre layer, backing substrates must typically be added to hold the fibres in place, which increases the thickness of the circuit significantly. This limits the long term usefulness of laminated fibre-optic circuits in PCB stack-ups. At best they can be glued or bolted onto the surface of a conventional circuit board.
Embedded polymer waveguides provide a mid-term – and potentially very low-cost – solution to short range optical links within the system, such as inter-chip connections on a board or board to board connections. They are also suitable for applications in which thermo-optic, electro-optic or strain-optic coefficient properties of the polymer can be used to support advanced switches or long range interconnections.
Embedded planar glass waveguide technology combines some of the performance benefits of optical fibres, such as lower material absorption at longer operational wavelengths and lower modal dispersion with the ability to fabricate dense complex optical circuit layouts on single layers and integrate these into PCB stack-ups. The Fraunhofer Institute of Reliability and Microintegration (Fraunhofer IZM) in Germany is one producer of planar glass waveguide based optical circuit boards.
Widespread adoption of optical circuit boards will herald substantial performance, cost and environmental benefits for the data communications industry, but there are still a number of inhibiting factors that need to be addressed. Though optical circuit board technology has advanced considerably over the past decade, commercial maturity will also be constrained by the availability of additional conformity standards for forging future design, performance and measurement procedures.
International standardization can accelerate commercial adoption
Standardization of optical circuit board technologies will be instrumental in speeding up commercial adoption of the technology and the IEC is leading in this area. IEC TC 86: Fibre optics, develops standards for fibre-optic technologies and IEC TC 91 does the same for electronics assembly technology.
TC 86 and TC 91 formed IEC TC 86/JWG 9, a joint working group, the purpose of which is to prepare International Standards and specifications for optical circuit boards. This JWG has already published a number of Standards in this area and is developing more. In particular, it is strongly active in developing new Standards for measurement, assembly and connectorization . These are crucial to helping optical circuit boards mature and bringing the substantial benefits of this technology to the market.
This standardization work will be central to bringing optical circuit boards to communications systems, allowing the data gap to be bridged.