Fiber Optics Move into Military Backplanes
The recent Embedded Tech Trends conference highlighted a trend: Boards and subsystems for military applications are increasingly employing optical cable.
Fiber optic technology is continuing its push into high-end military systems as engineers increasingly choose it to meet accelerating speed demands. This marks a change from its traditional role as a technology for long-distance communications. Board and system suppliers are responding to this demand with a variety of optical products for backplanes and short-distance applications.
Though copper remains viable at high speeds, optical interconnects are expanding across the industry. Recently, board and subsystem developers highlighted new fiber-based products at the 2020 Embedded Tech Trends (ETT) conference, where VITA members and military technologists discussed trends in boards and module technologies.
Boards have traditionally used copper to communicate over backplanes, where distances are far shorter than in the infrastructure applications where long fiber optic cable has typically been employed, but that is changing. The products introduced at ETT continue a transition to systems that use fiber optics in the card racks that house boards used in applications such as drones and many other military systems.
Room for Growth
Although there are solid reasons to stick with copper for now, many engineers are using fiber optics to set the stage for forthcoming enhancements. Risk-averse military planners often start with time-tested technologies before they adopt higher-performance techniques that have been in use in other industries. For example, data centers have used fiber optics for some time, but are using techniques that haven’t yet migrated to the demanding applications addressed by many of the products made by VITA members.
“Data centers are further along with fiber optics than the military is. They’re using larger connector packages and different technologies, like pulse amplitude modulation (PAM4), which doubles the data rate over the same cable,” said David Givens, senior standards director at Samtec. But the proven performance in these applications can make it easier for military systems to advance to faster products too, and moving to more advanced encoding techniques like PAM4 can provide significant improvements in mil/aero applications. Once military users are comfortable with fiber optic technology, they can migrate to advanced encoding like PAM4 with minimal design work. In addition, the rapid speed improvements in semiconductors like field programmable gate arrays (FPGAs) can help further hasten the changeover to fiber.
“We’re doing 10Gb/s on fiber optics now. Nothing changes when you go to 28Gb/s,” said Michael Munroe, principal backplane architect at Elma Electronic. “Xilinx FPGAs are now going up to 50Gb/s, but those fast signals can’t go very far on copper traces. If Xilinx put direct fiber output on the chips, that would be a game changer.” [Xilinx invented FPGAs and created the first fabless manufacturing model.]
Location, Location, Location
ETT speakers discussed a range of techniques that use short fiber optic links to replace copper traces on circuit boards. Arlen Martin, product line director at Reflex Photonics, a Smiths Interconnect Company, noted that the placement of fiber optic connectors can change the performance of the board.
If optical connectors are on the edge of the board — the usual spot for connectors — the signal traces on the board will add some latency. Moving fiber optic connectors to the center of the board can eliminate those copper traces and increase performance. As such, that configuration is likely to become more common as chip speeds rise and the volume of data from sensors soars.
“Some FPGAs are providing speeds that can’t use copper interconnects,” said Patrick Mechin, CEO at Techway. “In some systems, even 100Gb/s is not sufficient; for example, when there are a lot of sensors and things like phase array antennas. We developed systems that have up to 24 optical connections.”
Connected Systems Drive Demand
Higher frequencies and communication demands associated with high-resolution sensors are primary drivers in the changeover to fiber optics. Mercury Systems’ John Bratton estimated that between 10 and 30% of Mercury’s programs currently include fiber and that it’s rapidly growing, as many systems now include higher-resolution cameras, lidar, and radar sensors and connect to systems that merge sensor data and analyze inputs. Others at the conference noted that pricing and size reduction can also be benefits.
“It’s a big investment to go from 10Gb/s copper to 25Gb/s copper. It’s a lot cheaper to go to fiber optics,” said Xavier Marchand, field application engineer at Interface Concept. “Another advantage of replacing copper with fiber is that you have a lot more communication lanes. In the space for 16 lanes with copper, you can get 48 lanes with fiber optics.”
“We just announced our first computer board with optical interconnects,” Nigel Forrester, director of business development at Concurrent Technologies, announced at ETT. “There’s a significant amount of sensor data coming into the CPU board directly via optical fiber.”
Copper Remains Relevant
While a number of companies touted the benefits of moving to fiber optics, others noted that the move must be determined by operating conditions and other factors. Rugged application environments, particularly in battlefield scenarios that involve sand and dust debris, may make copper a better choice, at least for now. The challenge of transitioning between electronic and optical signals is another issue.
“In military and aerospace, there are lots of reasons for picking fiber optics. It’s light and it has high bandwidth,” said Tom Smelker, vice president and general manager for Mercury Systems’ Custom Microelectronics Solutions group. “However, one problem, especially for ground vehicles, is cleanliness. Another is the task of translating from copper and back again.”
Those factors led some of the companies at ETT to stick with copper. Its performance has continued to rise far beyond speeds that were once thought to be a realm that would be dominated by fiber. Clever engineers continue to push the limits of copper, meaning fiber optics is an option but not an imperative at this point.
“We just debuted an FPGA board that runs 100G Ethernet over copper,” said Noah Donaldson, CTO at Annapolis Micro Systems. “People previously assumed that could only be done using fiber optics. You need very good connectors and you need to be very careful how you lay out the board.”
TE Connectivity noted that as data rates double every two or three years, optical connectivity adoption will increase steadily in the 2020s. The VITA 87.0 standard, currently in working group, will address circular connectors with optical mechanical transfer. Although fiber usage is expanding steadily, however, young engineers still need to learn how to use copper. Its demise isn’t found anywhere except in the longest of road maps.
“Fiber optics is not something that will replace copper,” said Samtec’s Givens. “It complements it.”
Miss getting out to shows? Bob Hult will cover another recent show, the Optical Networking and Communication Conference & Exhibition (OFC 2020), in our April 14 Datacom issue. Click here for a sneak preview.