Samtec out-geeks PCIe’s OCuLink

-August 15, 2012

Rather than specifying minimum compliant cable lengths, like “Long Reach,” off-the-rack PCIe 3.0 specifies that a cable’s output has to be a compliant signal, regardless of its length. Typically, cables of a few meters or more require repeaters to bolster signal strengths.

Enter OCuLink, PCIe’s contribution of the geekiest acronym in the history of technology. OCuLink stands for Optical Copper Link (get it? Cu = copper. Nice). It’s a new cable/connector agreement and, as the name implies, it will support both optical and electrical signaling and specify both external and internal connectors with independent reference clocks that support spread spectrum clocking in a small cable form factor. It’s targeted for product adoption in the latter half of 2013.

Initial OCuLink technology will operate at 8 Gb/s per lane with up to four lanes – a sweet 32 Gb/s – and the incorporation of optical signaling opens the door to arbitrary cable lengths. But it’s not ready and the first OCuLink technology will probably be Cu not O.

So what if you need to connect two PCIe ports at each end of a football field and hate repeaters?

Check this out. It’s a cable, it’s a fiber, it leaps tall buildings, it reaches a city block with change. It’s Samtec’s PCIe x8 Active Optical Cable Assembly (a somewhat less creative name than OCuLink).

If you attend tradeshows, you’re accustomed to seeing the booth with Samtec’s big orange tiger. Since I believe that cables are engineering entitlements, like dirt to a gardener the only time they make it onto my radar is when I get burned by a lesser cable or don’t have one that reaches as far as it should. So, except for the legendary Samtec Meet-and-Geek parties, I’d have passed right by. Instead, I went up to the Samtec booth thinking there might be food and drink in it for me. There wasn’t, so I turned to walk away. But then I saw what looked like a broken connector, shown in the photo. I did an honest double-take.

“Really? Those are fibers? And you can just plug it in?”

The photo is a cutaway view of the connector showing eight multi-mode optic fibers connected to a transceiver. The transceivers use Fabry-Perot lasers which consist of a simple resonant cavity, not unlike the laser pointers you’re entitled to get at tradeshows, and basic PIN diodes connected to a TIA (trans-impedance amplifier) all incorporated in a single chip that fits inside the connector. They connect to standard PCIe iPass connectors the same way as copper cables and draw power from the PCIe connection so you never have to think about cleaning fibers or laser safety. The only observable differences are the power draw of about 1.5 W from each PCIe port and the really long cables.

Their length is limited by the use of multi-mode fibers. Multi-mode fibers have larger diameter cores than the more expensive long-haul single-mode fibers. The larger core fibers are easier to couple to lasers but are limited  in distance because the different modes interfere. PCIe 2.0 signals at 5 Gb/s can make it 300 m before modal dispersion impairs the signal below spec and 8 Gb/s PCIe 3.0 signals get 100 m. Not bad.

I have now been impressed by a cable. It’s sort of embarrassing, really.

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