Embedded Optical Interconnects in PCBs for Ultra High Speed Design

August 20, 2019 Zachariah Peterson

Fiber optic cables on a server rack

5G will be here sooner than you think, creating plenty of new opportunities for PCB designers, manufacturers, and companies that provide optical networking equipment. The huge data transfer rates in 5G networks and in specialized applications like aerospace requires greater use of optical interconnects throughout electronic systems, ultimately requiring conversion to electro-optic systems and fully photonic systems. What can PCB designers do to keep up with this trend? The fact is, PCB designers may soon be incorporating optical interconnects into their standard board.

 

Why Optical Interconnects?

Most PCB designers—except those that work on optical transceivers—are probably not aware of the coming revolution in silicon photonic integrated circuits (PICs), electronic-photonic integrated circuits (EPICs), and greater proliferation of embedded optical systems outside of telecom. Applications outside of telecom (for example, military and aerospace systems) that require massive data transfer rates already make use of fiber optics for embedded computing.

In the realm of telecom, more of the electronic infrastructure will need to be replaced with equivalent optical infrastructure in order to enable progressively faster data transfer rates required in 5G networking. As electrical signals switch at faster rates, signal integrity problems such as crosstalk and radiated EMI become more severe, and losses on standard substrates increase at higher frequencies. While over-the-air transmission from a cell tower to a user’s mobile device will still be wireless, the networking and tower equipment itself will need to interface with a fiber network in order to accommodate the huge amount of data moving through the network.

Replacing the electrical infrastructure in PCBs for networking equipment with optical interconnects relieves many signal integrity problems. With multi-mode fiber, the number of channels can be increased in a single interconnect without increasing routing density. This allows data rates to scale without significantly scaling board sizes or component sizes.

If you think that all this sounds like it came from an episode of Star Trek, rest assured that this technology is closer to becoming commercialized than you may think. Companies like AIM Photonics companies are developing photonic microprocessors, electronic-photonic integrated circuits are being developed by the research community and private companies, and many in the community have already created proof-of concept boards that include optical interconnects for interfacing between optical and electronic components.

At some point, the greater use of optical signalling alongside standard electronic equipment will take up so much space that placing cables inside a chassis is simply impractical. Think about the space required to form 50 or more optical connections inside a chassis with fiber optic cables, and all without bending cables past the minimum bend radius...it’s simply not possible to meet form factor and performance requirements. This means electronics manufacturers will need to accommodate direct printing of optical waveguides directly on PCBs.

 

Optical Interconnect Options for PCBs

The two best options for optical interconnects in PCBs are to embed glass fibers in the interior layers of a multilayer PCB. The other option is to deposit polymer waveguides on the interior layers or surface layers. Glass fibers could also be placed on the surface layer, but using polymers allows greater control over geometry.

This becomes important for interfacing with optical components as the geometry and coupling optics must be precisely defined on the surface layer. Regardless of which method is used, the design process will not change considerably as optical interconnects do not suffer from the same signal integrity problems as interconnects.

Glass optical interconnects will likely be easiest to integrate into standard multilayer PCB manufacturing processes as they can be embedded in the core or prepreg layers. The right material between FR4 laminates can act as a cladding layer for glass waveguides. There is no reason that glass or polymers could not be used simultaneously; standard glasses for optical fibers can be used in the inner layer, while polymers would be easiest to deposit on the outer layer.

Embedded optical interconnects in standard PCB

Circuit board with embedded glass optical interconnects. Image credit.

Glass or polymer waveguides placed in the interior layers of a PCB require transmission back to the surface layer and coupling optics for EPICs/PICs, or using a photodetector with fast response time (normally a PIN photodiode). Especially in optical BGAs for EPICS and PICs, an optical interconnect requires some type of coupling optics in the form of 45-degree mirrors. This requires extremely precise micromanufacturing. Otherwise, the laser diode and receiver in an optical interconnect will need to be embedded in the substrate.

 

Looking to the Future: Manufacturing Optical Interconnects

The remaining challenge is mass manufacturing and greater integration of optical components and waveguides into PCBs for electro-optical modules and backplanes. This requires scalable printing techniques for manufacturing dielectric waveguides directly on PCBs for interfacing between various optical components, electronic ICs, EPICs, and PICs. Using polymers for optical interconnects directly on an FR4 PCB is preferable as they can be patterned using standard lithography techniques.

As data rates scale higher, these optical interconnects will need to scale smaller in order to accommodate shorter wavelengths, although modal dispersion will become a problem with continued scaling and as more modes are packed into a given fiber. This doesn’t mean that copper will go the way of the dinosaur; copper will still be necessary to isolate optical interconnects, particularly with radio over fiber applications. The research community and some companies have already produced proof-of-concept boards with multimode waveguides that operate at 12.5 Gbps and higher data rates.

Optical fibers on blue background

These fibers might be found in the core of a multilayer PCB in the future

Anyone that works with optical components and who is looking to commercialize new products with optical interconnects can benefit from the full suite of PCB design tools in Altium Designer. The industry-standard layout and simulation tools are ideal for applications in electro-optical embedded computing and high speed networking. The data management and documentation tools can help you quickly prepare your new designs for manufacturing.

Contact us or download a free trial if you’re interested in learning more about Altium Designer. You’ll have access to the industry’s best layout, simulation, and data management tools in a single program. Talk to an Altium expert today to learn more.

About the Author

Zachariah Peterson


Zachariah Peterson has an extensive technical background in academia and industry. Prior to working in the PCB industry, he taught at Portland State University. He conducted his Physics M.S. research on chemisorptive gas sensors and his Applied Physics Ph.D. research on random laser theory and stability.

His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental systems, and financial analytics. His work has been published in several peer-reviewed journals and conference proceedings, and he has written hundreds of technical blogs on PCB design for a number of companies.

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