Cable Tech

Essex Claims 4000-Channel DWDM

Last night (Monday), Essex Corp. (OTC: ESEX.OB) created a bit of a sensation when it announced an advance that could deliver 4000 channels down a single fiber (see Essex Demos 1-Gig Channel Spacing). But in the cool light of day, things look rather different.

In yesterday's press release, Essex claimed that it has a technology that will dramatically increase fiber capacity. At first glance, it seems obvious that more channels equals greater capacity. But while this is true in some situations, the issue gets a whole lot more complicated when channels are squashed really close together.

Essex is cramming more channels down a fiber by squeezing each channel into a narrower range of wavelengths. It's managed to get down to 1 GHz, which is approximately equal to 0.008 nanometers in wavelength terms (at 1550 nm). That's 50 to 100 times closer together than in today's optical networks.

However, as a rough guide a channel can only accommodate a data rate equal to its bandwidth. In other words, it's not possible to force 40 or even 10 Gbit/s down a system with 1 GHz spacings. The maximum will be about 1 Gbit/s, and in fact, this is the bit rate in Essex's demonstrator, according to Len Moodispaw, Essex's president and CEO.

Do the sums, and it turns out that an Essex system with 4000 channels and 1 Gbit/s per channel will be no better in terms of capacity than a 100-channel system at 40 Gbit/s per channel. So, if capacity is not the big issue, what is?

"We don't need thousands of wavelengths for bandwidth, we need it for connectivity," says Simon Cao, chief technology officer at Avanex Corp. (Nasdaq: AVNX), a company that's also developing high channel-count systems.

Cao figures that the availability of thousands of channels in combination with tunable lasers will make it possible to eliminate complex optical switches, by using wavelength routing instead (see Researchers Unveil All-Optical Advances). That idea is the driving force behind all of Avanex's developments, he says.

Despite its careless capacity talk, Essex has also thought out how to take advantage of the extra connectivity. Moodispaw reckons that Essex's solution will be perfect for customer-owned wavelengths in the access network. About 1 Gbit/s would be adequate to supply a gigabit Ethernet line to a business. Each customer pays less for its wavelength, but the service provider is able to sell to a lot more customers, so everybody is happy.

It's worth pointing out that Avanex seems to have made faster progress than Essex. Cao, who invented Avanex's WDM technology, claims that the company has achieved channel spacings of 0.4 GHz (0.0032 nm). What's more Avanex has already demonstrated a channel count of 1000 in laboratory experiments, it claims. Essex, on the other hand, has a demonstrator that only does 16 channels.

However, comparing Essex and Avanex simply in terms of channel spacings and channel counts is not totally fair, because the underlying technologies are totally different.

Essex's device takes a single lightstream and splits it into 16 different channels. Each channel can be encoded with a separate data signal, and then all the channels are recombined onto the same fiber. Moodispaw won't say how the device that divides up the light actually works -- it’s the subject of a patent. All he will say is that it's solid state.

The advantage of Essex's widget is that it's cheaper per channel -- another reason why it might be suited to the access network. To make a system with more channels, several widgets are used in parallel and then the outputs are combined. Essex is working on a 50-channel version that will further reduce the overall component count and slash costs, Moodispaw adds.

Avanex, on the other hand, uses something that's more like an interleaver. This kind of device overlays two groups of channels accurately. Both channel groups have wide channel spacings but are offset from each other, and so combining them creates a system with half the channel spacing. The process can be repeated many times to get to ridiculously small channel spacings. This technique requires one transmitter per channel -- a big reason that Avanex is looking at using tunable lasers. -- Pauline Rigby, senior editor, Light Reading http://www.lightreading.com

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