Blades of Light:

HP Labs puts optical connections inside the server

This design further reduces the need for signal repeaters and server
cooling. And because it makes possible direct optical connections
between individual blades, it also eliminates the need for the
traditional server backbone.

By Simon Firth, November 2008


This is the first in a two-part series looking at recent work in photonics being pursued by researchers in HP’s Information and Quantum Systems and Exascale Computing labs.

Photonics – the science of transmitting information via light – found its first great commercial use in fiber optic cables spanning the world’s oceans.

These optical cables formed the backbone of national telecommunications networks, eventually carrying voice and data into homes and businesses. Lately, they’ve been connecting networks of computers together within those business locations. 

“Now what we’re trying to do at HP Labs,” says researcher Mike Tan, “is to break through to the next level to bring photonics into the computer itself.”

Better than metal

HP photonics researchers, from left to right, Paul Rosenberg, Sagi Mathai and Michael Tan
HP photonics researchers, from
left to right, Paul Rosenberg,
Sagi Mathai and Michael Tan

In particular, Tan explains, that means using light to transmit data between the separate circuit boards, or blades, that typically make up a modern server.

Today those connections are made by copper wires and pins. But as blade circuitry gets ever faster, copper won’t be able to keep up.

Blade electronics currently run at 5 gigabits per second, and copper can only go to 9.6 . Optical connections already run that fast, and in the future will go to at least 20 gigabits per second. 

Photonics, however, promises to do more than simply keep up with the speed of blade circuitry.

“Not only will optical connections work faster,” explains Tan, “but we’ve shown that they’ll use less power and they’ll provide new connectivity between blades that’s not available today.” 

Cost and other obstacles

Despite its potential, both technical and financial hurdles have so far kept photonics outside the server box.   

Cost is perhaps the biggest issue. Optical connections are expensive, and each server requires hundreds.

“At the same time we're developing new concepts in connecting blades,” says Tan, “we’re also using the telecommunications industry's existing technology to try and do it in a very low-cost manner.”

Connecting the blades

One great advantage to building servers out of blades is that each blade can be directly and independently plugged into the server’s shared backbone, or mid-plane – a design that minimizes cable usage and the labor required to connect those cables every time you change your arrangement of blades.

Any switch from copper to optical connections would need the same ease of connectivity, so HP photonics researchers first set their sights on creating flexible optical connectors that would still allow blades to be hot-swapped into and out of a server rack.

The researchers' innovation: a way to link two optical connectors without having to connect them manually.

The secret lies in outputting the blade’s data via standard optical converters -- vertical cavity surface emitting lasers (VCSELs) -- to standard optical connectors that are attached to magnets. Clever engineering ensures that when the magnets are anywhere near each other, they first attract each other and then align their complex sets of light beams perfectly.

These magnetically coupled interconnects not only get data off the blade at much faster speeds than conventional copper connections, they also allow the entire server backbone to be optically wired. That in turn saves huge amounts of power, because electrical signals fade quickly on copper lines, requiring frequent (power-hungry and heat-generating) repeaters to boost their signal.

Going cable free

But what if you could eliminate the need for optical cables altogether?  You’d have the same increased speed and power-efficiency, but gain an extraordinary level of flexibility in how you connected the blades in a server.

A second major innovation from HP Labs – the Free Space Optical Interconnect – does just that. 

“We’re basically shooting optical beams across and between blades," Tan says.

Here's how it works: Researchers set up standard VCSELs so that they slot in no more than 5cms apart from each other. The optical beams from the transmitting and receiving VCSELs are then focused in such a way that they’re automatically detected and aligned – allowing tolerance for initial misalignment and adjusting for rotation and tilt of the light beams using a set of actuators. 

This design further reduces the need for signal repeaters and server cooling. And because it makes possible direct optical connections between individual blades, it also eliminates the need for the traditional server backbone. That could create faster-running, and thus more powerful, computers – even without increasing the processing power of any individual blade.

“Essentially, you’ve enabled a more flexible computer architecture,” suggests Tan, “because you now have connections which were previously unavailable to you.” 

Running light on the board itself

High Speed Optical Multidrop Bus
High Speed Optical Multidrop Bus

A third recent innovation from HP photonics researchers uses light to connect the circuitry of the blade itself.

As computer chips work at ever higher frequencies, the laws of electronics make it harder to connect them in a parallel bus configuration – because doing so requires ever more power, which adds ever more capacitance, which in turn reduces transmission speed, which means the components can’t ever connect as fast as they operate.

Because of this, the industry has turned to serial point-to-point connectivity, which itself has problems with latency and the need for ever more power to retransmit signals.  

But HP’s new High Speed Optical Multidrop Bus allows a return to parallel bus connectivity – even at very high speeds – by sending multiple light beams along narrow hollow metal tubes called waveguides between the components on a blade, and splitting or tapping  a small portion of the light so that it always has the same power wherever it is in the circuit.

Innovation at the speed of light

To keep costs low, researchers split-off the light with the same techniques used to put coatings on a pair of eyeglasses.

Potential benefits of the optical bus include making it possible to add more memory to multi-core systems on a blade, without the power and latency issues that come with serial point-to-point connectivity.

It’s not a bad set of innovations for a project that has been running less than 15 months. But this is a team on the fast track.

“Within three to five years,” says Tan, “we want to have photonics inside the rack.”