Optical active cables find new niches

17 Jan 2008

Soon after unveiling the first 40Gbit/s optical active cable for data centers, Luxtera claims another first with a cable for video networks.

Components vendor Luxtera spent five years being a startup, but is now throwing off that mantle and talking about products and applications, not just the underlying technology. Founded in 2001, the company is betting its future on optical active cables (OACs), which could find applications in consumer video links as well as data centre connectivity. In fact, anywhere that copper is struggling with high data rates over long distances represents an opportunity for fiber-optic cabling in general and Luxtera in particular.

Luxtera is claiming a couple of firsts with its OAC products. At SC07, a high-performance computing conference held last November, the vendor said it was the first company to sample a 40 Gbit/s OAC targeting Infiniband, Fibre Channel and Ethernet applications, called Blazar. And at this year's Consumer Electronics Show in Las Vegas, it showcased what it claims is the first OAC to meet the DisplayPort standard for transmitting audio and video signals in home networks.

The need for fiber in interconnects has emerged as data rates hit 10 Gbit/s. Infiniband, which is mostly targeted at high-performance computing clusters, has been widely adopted at 10 Gbit/s, and is already transitioning to 20G double data rate (DDR) with 40G quad data rate (QDR) on the horizon. At those data rates, bulky cables containing multiple pairs of shielded copper wires are required, and some data centres are literally falling apart under the weight of all that copper.

In consumer electronics, higher screen resolution and faster refresh rates have resulted in a new standard for connecting computers to home cinema system or monitor equipment. DisplayPort consists of up to four channels running at up to 2.7 Gbit/s for aggregate throughput up to 10.8 Gbit/s. Copper cabling is limited to distance of about 2 m at this speed, whereas fibre cables can reach up to 4 km.

Simply put, fast data rates need optics. But the traditional telecom way of doing things, using optical fiber that plugs into a rack-based transceiver, isn't ideal for connecting computers. Data centres and homes are not clean rooms, and fiber connections can run into problems with dust and moisture. There are also issues with interoperability between the transceivers at either end of the link, which need to be made to tight tolerances to ensure they can talk to each other.

As noted, the solution, which has appeared on the market in the last 12 months from vendors such as Intel, Finisar and Zarlink as well Luxtera, is active optical cable – a fiber-optic cable with permanently attached optical modules at the ends. "The cable is fully enclosed so the user is only exposed to the electrical interface – the optical interface is sealed inside," explained Marek Tlalka, vice president of marketing for Luxtera. "It's a true point to point solution just like a copper cable."

Because the optics is hidden, the only standard that any OAC manufacturer or user needs to worry about is for the connector. Blazar is compliant with quad SFP (QSFP) form factor, and has an SFP+ electrical interface.

Historically, optical modules have been expensive, but Luxtera says its technology allows it to offer products at an attractive price. Blazar is being offered at a similar price to DDR cables, and the long term goal is to match the price of copper cable.

The company says lower costs come from integrating all the transmit and receive electronics with most of the optics onto a single silicon chip fabricated using 0.13 µm CMOS processing. Instead of requiring dozens of separate components, the optical module is condensed down to just two chips: the silicon chip, and a laser chip – generating light is the one thing that CMOS can't do.

"We call ourselves a fiber-to-the-chip company," said Tlalka. "We can literally take a single fiber or array of fibers and couple directly to the CMOS die."

Blazar, which is available in a range of lengths from 1 m up to 300 m, is currently sampling and will be available in volume in the third quarter 2008. The DisplayPort cable will sample towards the end of the year.