GigaTera Goes Multiwavelength
GigaTera unveiled its pulse-generating laser, called ERGO, last year. In a nutshell, the device produces a train of RZ (return-to-zero) pulses at 40 Gbit/s without requiring a separate modulator (see GigaTera Sends Pulses Racing). It turns out that the same technology, with only a minor modification, can be used to make a device that delivers 32 channels of light simultaneously.
"All we have to do is alter the pulse repetition rate of the laser," explains Michael Brownell, the company's VP of product development. "The rest is automatic."
It's not immediately obvious how a device that can produce pulses can also produce multiple continuous-wave (not pulsed) channels of light. The answer lies in a mathematical tool-cum-physical phenomenon called "Fourier transform." Understanding this is not easy or intuitive, but here's a quick explanation for the physicists among you.
Ultra-fast pulses of light can be split up -- both theoretically and literally -- into a number of frequency components. When the pulses have a fixed repetition rate, then those components consist of a series of individual frequencies with the same spacing as the repetition rate. Therefore, a laser that pulses at 25GHz will give rise to channels spaced by 25GHz, which happens to match the International Telecommunication Union (ITU) grid.
Whether the user sees pulses or multiple channels depends on how the light is measured, says Brownell. If the laser beam is observed as is, without splitting or changing the output in any way, then it consists of pulses. If the different frequencies are separated, by sending the laser beam through a demultiplexer, then each output of the demultiplexer yields a separate channel.
GigaTera doesn't claim to be first with this idea. In fact, researchers at Bell Labs demonstrated something similar six years ago, although it doesn't appear to have commercialized the work (see Bell Labs' press release for details).
"We can't claim credit for inventing this," says Brownell. "But the key here is that our device is super-simple, stable, small, and cheap."
The device can be small and cheap because it contains considerably fewer parts than 32 fixed-wavelength light sources. An equivalent fixed-wavelength solution would require 32 lasers, 32 sets of driver electronics, and 32 wavelength lockers. A multiwavelength light source comprises just four components: a single pulse-generating laser, one set of driver electronics, one wavelength locker, plus something extra -- namely a dynamic gain equalizer (DGE), which demultiplexes the channels and then balances the output power to ensure it is the same across all channels. GigaTera is using a DGE from Silicon Light Machines for this purpose.
The ERGO laser is currently available as a chassis-based instrument, which potential customers can use to evaluate the technology. GigaTera says it can bring the multiwavelength laser source to market quickly by building it into the same chassis.
In the future, the company plans to shrink the instrument down to something that will fit onto a line card. This should be feasible, given that the laser alone comes in a tiny package measuring approximately 15 mm per side.
Analysts think it's an intriguing idea, but many wonder if it will catch on. "Yes, it is very interesting," says Tom Hausken, director of optical communications components research at Strategies Unlimited. "However, I think the fact that Bell Labs didn't pursue it says a lot."
There are a number of reasons for potential customers to be wary of the new technology, he says. The main one is that they would be putting all their eggs in one basket: If the device fails, a carrier loses 32 channels in one fell swoop, instead of just one.
The idea of putting 32 channels on a single line card may also be a bit ahead of its time, in the sense that there's a long way to go before systems vendors can cram the electronics associated with 32 channels onto a single line card. Most systems today still have only a single channel per line card.
Nevertheless, GigaTera's Brownell insists that people are interested in the multiwavelength light source. "We've pushed ahead with its development as a result of customer feedback," he says. One of the early applications is likely to be in test and measurement, which wouldn't require the device to be shrunk down onto a line card.
It's worth pointing out that at least one other startup -- Israel's KiloLambda Technologies Inc. -- is developing a multiwavelength light source (see Startup Invents Laser Alternative). So far, however, KiloLambda hasn't revealed much about its technology.
— Pauline Rigby, Senior Editor, Light Reading