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Phaethon Embarks on Tunability

Light Reading
NFOEC News Analysis
Light Reading

Startup Phaethon Communications emerged from stealth mode today, claiming a unique solution to a troubling problem that's emerged in optical networks (see Phaeton 'Unveils Its Vision').

That problem is how to ensure that wavelengths stay strong across fibers that have different physical characteristics. It's a problem that arises when carriers are designing or installing DWDM (dense wavelength-division multiplexing) gear and when wavelengths are switched. When this happens, wavelengths are often driven across fibers of different types and lengths.

At the root of the problem is a technical issue called dispersion -- the degradation of light signals when they're sent over fiber. There are different types of dispersion. "Chromatic dispersion" happens because a light pulse isn't just a single wavelength; it's a bunch of wavelengths representing slightly different colors. The longer wavelengths travel at slightly different speeds than the shorter ones, so over distance the pulse spreads until it bumps into the adjacent one, at which point the signal is lost.

This problem gets worse with speed. At 40 Gbit/s, for instance, the problem is 16 times worse than it is on 10-Gbit/s links.

To compensate for the inevitable effects of chromatic dispersion, Phaethon has designed a tiny device for use by system vendors. It's a fiber Bragg grating (FBG) that has been etched in a special fashion, then microscopically stretched. The end result is a component that can change the compensation values for different bunches of wavelengths in a single fiber. Hence, the term "tunable" compensator.

Today's chromatic dispersion compensators use a different tack. Most are small modules containing dispersion compensating fiber, which are inserted at the receiver on a DWDM system and in the network itself, right before the amplifiers on the fiber -- typically every 80 to 100 kilometers.

These modules, exemplified by the products of Corning Inc. (NYSE: GLW), are preset with so-called "compensation values" that match the type and length of fiber on which they reside. Accommodating different types or lengths of fiber, or switching to different speeds, requires another module.

Phaethon says its solution is useful because one component will be able to automatically adjust to different compensation values.

"Say you had a 100-channel DWDM system with channels organized in groups of ten," says Bruce Barton, VP of marketing at Phaethon. "You can set different compensation values for all of the channel groups, using one device. There's no need to drive out and reinstall the fiber and modules."

Analysts say that if Phaethon can live up to its promise, it's got something good. "I haven't heard anyone offer remote tunability yet," says Tom Hausken, director of optical communication components at Strategies Unlimited, referring to Phaethon's capability to build a component that can make dynamic adjustments.

There are drawbacks. For one thing, Phaethon won't have automatic tunability in its products right off the bat. Instead, for a few months, customers will be offered the ability to tune the compensation values by hand.

Another problem: Phaethon will offer tunable compensation for only small groups of wavelengths at first. It plans to unveil 4- and 8-channel modules at the upcoming NFOEC show in Baltimore.

Phaethon's devices also don't solve all the problems associated with chromatic dispersion. LaserComm Inc., a startup selling dispersion compensating modules based on higher-order fiber, says its devices will take care of "dispersion slope," which refers to the ability to set different degrees of dispersion compensation for different channels. Phaethon's devices can't tackle this problem right now, although the vendor says that capability is on the drawing board.

Phaethon also doesn't yet handle (but plans to handle) polarization mode dispersion (PMD), a problem caused by the fact that light can travel along slightly different paths inside a fiber, depending on how it bounces around as it travels. Vendors such as Yafo Networks have started to tackle PMD (see Can Yafo Lift Speed Limits?).

Some naysayers hold that fiber Bragg gratings aren't the way to handle chromatic dispersion problems, since they tackle a low number of frequencies at once. Not so, says Phaethon, which insists its FBGs are key to the tunability feature and offer low insertion loss and low cost ("less than $10,000 per 4- or 8-channel module," according to Barton).

Phaethon also points to its other assets, which include more than 15 patents from the University of Southern California, thanks to founder Alan Wilner, a professor at USC. The company also has right of first refusal on any new photonic technologies from USC, and it can opt to license them exclusively for a ten-year period.

Still, the complexity of the problems Phaethon hopes to solve ensures it's going to be a while before full success is achieved. Meantime, its competitors, including the likes of LaserComm and Yafo, also are at work on comprehensive solutions for chromatic problems. And there's always a chance they may get there first, given the headstart they've got.

It's also not unthinkable that some as-yet unimagined combinations may result from all these developments. "We're looking at adaptive optical networks, having a way to sense the quality and respond with a standalone system that works with multiple DWDM devices," says Henry Yaffe, founder, chairman, and CTO of Yafo Networks. "A company such as Phaethon could be partners with us. Or, depending on their viewpoint, they could be competitors. Or a bit of both."

- Mary Jander, Senior Editor, Light Reading
http://www.lightreading.com For more information on NFOEC, please visit the Light Reading NFOEC Site.

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12/4/2012 | 8:11:10 PM
re: Phaethon Embarks on Tunability

Please remove the line "It's a problem that's exacerbated when fiber is touched, bending the core subtly out of shape so that light signals aren't kept intact" for the sake of technical sanity.

Chromatic dispersion is caused because different frequency components of light travel at different group velocities within a fiber (yes, even a straight untouched fiber) due to a) material properties, i.e., index of refraction of silica is a function of wavelength and b) waveguide properties, i.e., part of the mode is propagating in the core and a small part is traveling within the cladding and these two parts of the fiber have different properties.

It would be worthwhile to know which other startups are building tunable dispersion compensators since this is of tremendous interest for 40 Gb/s systems. Can lightreading look into this ?

Also, what are the technical properties of the tunable dispersion compensators from Phaethon - dispersion range, insertion loss, available bandwidth, PMD, temperature dependence.

12/4/2012 | 8:11:07 PM
re: Phaethon Embarks on Tunability
You might take a stab at defining PMD as well. Chromatic dispersion is, as mentioned, a result of different specific wavelengths within the band traveling at different speeds.

Polarisation Mode Dispersion results because light travels on two "orthogonal" axes (the light wave is cross sectionally three dimentional). Polarization of the waveband varies as it travels down the fiber. A device sensitive to PMD will detect birefringent noise due to the fiber having a refractive index that varies with polarity.
12/4/2012 | 8:11:01 PM
re: Phaethon Embarks on Tunability
Hi Mary,

You might want to take a look at the discussion that has been going on for the past week or so on the boards for two articles: The Marconi ULH article and the Nortel 40G one. There has been much discussion of both chromatic and polarisation mode dispersion compensation.

The two dominant ways of compensating for chromatic dispersion are fibre and gratings. Other solutions such as high order mode (HOM)fibre and phased arrays (VIPA), but these are not nearly so common. In today's systems dispersion compensating fibre (DCF) made by Corning and Lucent is the dominant solution, with LaserComm coming in somewhere lower. The market is extremely closed since Corning and Lucent make the bulk of the world's transmission fibre, and therefore are best able to make the compensating fibre.

At 10G, standard dispersion compensating fibre is fine. For 40G and 10G ULH, you need to match both the dispersion, and the slope of the dispersion (DSCF). The accuracy of the slope compensation is expressed as a percentage - 0% is a unsloped compensator, 100% is perfect slope matching. Typical DSCF is about 60% today and is rising rapidly.

Dispersion compensating gratings (DCG) are not used much in today's networks. The problem is the ripple that inaccuracies in the grating cause to the dispersion. The grating does a reasonable job of correcting the dispersion, but the Q penalty that the ripple produces makes the overall performance lower than that of DCF. The masks that you use to write the gratings are quite short, so you need to 'stitch' several mask sections together. This stitching process can be the cause of a lot of the ripple, and it gets worse the longer the grating you try to write. A longer grating is better as it allows you to compensate for a longer span, or over a greater bandwidth. If Phaethon (note spelling) can't do more than 8 channels, then that suggests that they have a stitch problem.

Tunable gratings cause you all sorts of problems. A key issue is how you use them - since they are designed to be easy changed, they have a nasty habit of drifting over a long period of time. That means that you need to operate them closed loop. So how do you do that without getting highly integrated into a system vendors product? By now most of the system houses have already solved these problems and are well on the way to shipping products. Are Phaethon too late?

As a parting shot, let's have a go at this bit:
"Analysts say that if Phaethon can live up to its promise, it's got something good. "I haven't heard anyone offer tunability yet," says Tom Hausken, director of optical communication components at Strategies Unlimited, referring to Phaethon's capability to build a component that can make dynamic adjustments."
I strongly suggest that Mr Hausken actually attends a trade show once in a while. He might have noticed the 6-10 different companies offering tunable compensators at OFC 2001 alone. As a free first hint - try Avanex. And then try other large component manufacturers.


PS: Hi Aaron.
12/4/2012 | 8:10:57 PM
re: Phaethon Embarks on Tunability
The two other companies that are into PMD compensation are:

Yafo Networks
Santel Networks

Yafo is generally perceived as the leader in the field.

12/4/2012 | 8:10:48 PM
re: Phaethon Embarks on Tunability
Thanks to Petabit for his posting providing technical accuracy that the article lacks.

Let me add a marketing twist: why on earth is Phaethon entering the chromatic dispersion compensation market? They will sink loads of resources just to qualify a product that will face stiff competition from other Bragg vendors in addition to other non-Bragg solutions.

To Phaeton's VP Marketing: focus on PMD compensation. There is only one other player there - Yafo. Alan Wilner's work at USC focussed on that and believe me, what he did on PMD was real good. Burn your money on the big bet!!! Not a me-too product.

12/4/2012 | 8:10:45 PM
re: Phaethon Embarks on Tunability

Have you heard of a tunable CD compensator from a company called Jemez Optics?


12/4/2012 | 8:10:43 PM
re: Phaethon Embarks on Tunability
I don't care who did what regarding the PMD issue in the fiber. I have worked on this problem over several years and believe me the theories describing PMD in the fiber is totally off the wall. This problem has not been fully understood and to say there will be a product soon that can addresses the problem is simply not realistic. The best solution to PMD is, just lower the bit rate while maintaing your spectral efficiency
12/4/2012 | 8:10:42 PM
re: Phaethon Embarks on Tunability
"This problem has not been fully understood and to say there will be a product soon that can addresses the problem is simply not realistic. The best solution to PMD is, just lower the bit rate while maintaing your spectral efficiency"

I was not suggesting that there will be a product anytime soon, but rather that Phaethon should burn their cash on that problem rather than competing head to head with JDS, Avanex, etc. on tunable Bragg for DC.

No VC will fund them for long to increase their knowledge of qualification and Telcordia specs. They will if - like Yafo - they prove they are making progress on PMD compensation.

12/4/2012 | 8:10:38 PM
re: Phaethon Embarks on Tunability
exphoton EP:

"Also, what are the technical properties of the
tunable dispersion compensators from Phaethon -
dispersion range, insertion loss, available
bandwidth, PMD, temperature dependence."

No, no, no, no, no, you just don't understand,
at all. This is all a problem of C. P. Snow's
'Two Cultures'.

The next thing you will want is derivations
with Maxwell's equations.

Now, let me explain it to you so that even a
technical person can understand:

The authors on Light Reading along with Light
Reading, such sites on the Web, the Media, are
part of the classical tradition of literature
going back to the Greeks. Here the main
interests are communication, interpretation of
human experience, emotion. Science,
engineering, technology, and CERTAINLY not
physical optics or Maxwell's equations, have no
role here at all. The goal is to grab people,
by the heart, the gut, or lower still, and
certainly to avoid anything above the

In particular, you are missing the human
aspects so clearly conveyed in this story: To
the author's great credit, here we have
classical high drama. The name, Phaethon is
apropos. The people are struggling with nasty
photons and have problems. They are suffering
severe insertion losses from incompatibilities
from coupling of dissimilar media, and if you
can't understand this, then we will have to
revise your 9th grade grade in Shakespeare.

Now, our hero in this story is Phaethon who
solves the problem with a compatibility device.

And, the little photons move across, the
dissimilar couplings are successful, and
everyone lives happily ever after.

Feel better now?

12/4/2012 | 8:10:37 PM
re: Phaethon Embarks on Tunability
"Have you heard of a tunable CD compensator from a company called Jemez Optics?"

Nope. Tell me about it.

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