Active Optical Cables (Part 1)

Active Optical Cables (AOC) are typically defined as a fiber subsystem intended for reaches of 3 to 300 meters, but Luxtera and Finisar both promise products stretching a kilometer or more for campus-network solutions. However, I don’t believe AOCs beyond 300 meters will get much traction in the market due to issues with trying to pull these delicate transceiver ends through kilometers of pathways and spaces (conduit or tray), around all types of obstacles. AOCs main applications in high-speed networks are in the data center, including (and probably most relevant) high-performance computing (HPC) clusters.

Intel (AOCs now part of Emcore) and Luxtera were among the first to promote AOCs for consumer and data-center markets. Zarlink (its optical products group is now part of Tyco Electronics) launched its AOC effort in 2007, Finisar introduced three varieties of vertical-market AOCs in 2009, and Avago announced its QSFP+ AOC in late 2009. Other participants include Lightwire, MergeOptics/FCI and Reflex Photonics. And, of late, we’ve even seen structured cabling companies like Siemon introduce some of these products, albiet by the looks of it, it is partnering with Luxtera to do so.

The QSFP+ form factor continues to be an enabler for 40G AOCs and in fact, was the first “form factor” released for this data rate. Since the QSFP+ supports Ethernet, Fibre Channel, InfiniBand and SAS, it will be an economic solution for all protocols. This AOC combines the QSFP physical module with management interfaces extendable to 40G, common protocols to support multiple physical layers in a single module and operates at 10G per lane producing a cost-effective solution. A significant ramp in quad data rate InfiniBand and 40G Ethernet will start to accelerate volume applications for these products. QSFP+ AOCs also give an easier path to market for tranceiver vendors, as they allow them to control both ends of the optical link, which is much easier to design - there are two less compliance points.

A summary of some of the product implentations of AOCs for high-data-rate networks:

Emcore has incorporated its existing technology into a pre-terminated active assembly using the current InfiniBand and 10GBASE-CX4 copper connector. So, what is presently being connected by copper can be replaced immediately by an active optical cable assembly. For 40G InfiniBand, this will turn into the CXP connection. The QDR 40 cable from Emcore was announced in mid-June 2008 and according to the company, has been shipping to select customers since early 2009. Yet, it does not seem to be a released product since the only reference to it on the Emcore Web site is its initial press release - no specifications are available there.

Luxtera is addressing the data center market with both InfiniBand- and Ethernet-compliant AOCs. It uses its CMOS photonics at 1490nm wavelength and a high-density QSFP+ directly attached to multi-fiber (ribbon or loose-tube) SMF. This is suitable for 40G applications and has proven a cost-effective solution for data centers that have discovered the difficulty with copper cables. Although the specifications for copper interconnects support 10m of cable, in reality there are both performance issues and mechanical problems with them.

To be continued in my next post...