Light and PCB Design: The Future of Optical Interconnects Is BrightMarch 27, 2020 by Robin Mitchell
How do optical interconnections help defy Moore's law while pushing toward optics?
With Moore’s law increasingly called into question, engineers are looking at alternative technologies to increase data bandwidth and processing power. One such technology is optical interconnect technology.
Researchers Sumita Mishra, et al. observe that there has been a wave of research on optical interconnect technology, which essentially focuses on integrating optical systems with electrical systems to create high-speed data links.
In this article, we will discuss the place of light in PCB design and assess one new branch of optical interconnections that promises higher bandwidth and low-noise signal connections.
Addressing a Myth About Light
Light, which is already replacing traditional copper wires in telecommunications, is one very tangible candidate for replacing electrical connections for several factors, according to a chapter on optical communication systems from the University of Colorado Boulder. This is especially relevant given the high demands of 5G networking. But before we explain these factors, there is one myth that needs to be clarified regarding light.
Some believe that light is better than electrical current because light is faster than electricity and therefore can transport information from one place to another faster. This, in some cases, is factually incorrect because light travels slower in fibreoptic cables than electricity travels through copper wires.
A Major Pro of Light: High Bandwidth
The advantage of using light in data connections is that light has a higher bandwidth, allowing more data bits to be carried per second—a result of the higher signal-to-noise ratio (SNR) that light has when compared to electrical signals.
A diagram comparing the electrical and optical interconnect density. Image (modified) used courtesy of IBM
One contributing factor to the higher SNR is that fiberoptic cables themselves are virtually immune to their environment whereas classic electric cables can pick up external electric and magnetic fields (i.e. EMI). Other factors include the ability to send multiple frequencies of light (red, blue, and green) down the same optic cable without them interfering with each other, effectively increasing the overall bandwidth. Light can also travel longer down a cable than electricity before requiring a signal boost, therefore removing sources of potential noise injection.
To learn more about the difference between RF, optics, and a third option—RF Over Fiber, check out Marie Christiano's technical article on the subject.
Optics and PCB Design
So far, optical systems have been commercialized primarily through fibreoptic cables for internet technologies.
But there are multiple technologies that have used optical layers on a PCB to transmit data around the board between components. This they achieve through specialized 90° grooves that can create optical vias to reflect the light up to the component layer. Researchers from the Malaysia-Japan International Institute of Technology (MJIIT) proposed a 90° light-path conversion device, which depicts this concept below.
Proposed 90° light-path conversion device. Image used courtesy of MJIIT
But while inter-optical connections on a single PCB are possible, it will only provide local bandwidth increases on a board. What we really need is the ability to connect individual PCBs to an optical backplane where large computer systems can transmit data across many hundreds of boards. Such a system would reduce the complexity of interconnecting boards without the need for many fibreoptic cables. This also allows for direct optical communication between ICs.
To solve the problem with optical interconnection, a team of researchers at Tokai University has developed an optical interconnection system as well as a fabrication technique to easily connect two optical channels together while minimizing signal degradation.
The system relies on the technology developed by another research team that was able to create optical plugs similar to typical electrical pin headers. These plugs are incredibly small—only a few 1,000 microns in length and 10 μm in diameter—and are grown using a UV curing resin and photomask.
While these plugs are able to transmit optical information, they are incredibly fragile, which may be problematic when attempting to connect them into receiving holes. This issue was addressed by fabricating a micro-hole array that used tapered holes to accept the optical wire plugs. The image below shows the fabricated plugs and the resultant tapered holes that accept the plugs.
Fabricated plugs (left) and tapered holes that accept the plugs (right). Image used courtesy of Tokai University
Tapered holes are easily constructed by exploiting a common problem with general lithography: undercutting. Lithographic processes often require that the artwork placed on top of a material be as close as possible, otherwise edges are blurred and the UV light can cure underneath the artwork.
However, in this instance, researchers used a spacer, so when UV light was shone through the mask, it produced tapered holes.
Diagram of how a spacer allowed UV light to shine through in such a way that it produced tapered holes. Image used courtesy of Tokai University
The resulting connection between the plugs and the tapered holes was a signal degradation of 1.5 dB with a 0.03 mm gap and 4.8 dB with a 0.11 mm gap. What is crucial in this setup is that the tight spacing of the plugs resulted in no cross-talk between the channels, which would otherwise be a major concern if electrical signals were used.
Will Optimal Layers Make the Cut for Future PCB Design?
Normally, research on new components and materials warrants a level of skepticism. As engineers, it's natural for us to ask, Will this new technology actually have a significant effect? But when we size up this new research on optical interconnections, we might see some extra promise—merely by virtue of the fact that optical components have already been successfully built.
As Zachariah Peterson from Altium explains, embedded optical interconnects for ultra-high-speed designs in PCBs may even become a necessity with 5G around the corner. It's not a far leap, then, to consider the possibilities of future PCBs including optical layers that are easy to design and provide high-bandwidth and low-noise signal connections.