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DWDM

Fiber Bragg Gratings on Speed

New developments in fiber Bragg gratings (FBGs) -- devices used to split and recombine light in dense wavelength-division multiplexing (DWDM) systems -- were announced today by Southampton Photonics Inc. (see 'First FBGs with 25 GHz Spacing').

On the face of it, the big news is that Southampton Photonics has shipped the first commercial FBGs with 25-GHz channel separation. This promises to enable vendors' DWDM gear to handle 160 wavelengths per channel, enabling many of them to honor promises to their customers.

However, the real significance of the announcement might not be in the FBGs themselves but in the way in which they were made. Southampton Photonics has automated the process of setting up the production equipment to make its FBGs to particular customer requirements, shrinking the time required to do this from an average of six to eight weeks to a mere 24 hours.

The company has also automated the manufacturing process, enabling each production line to churn out as many as 5,000 FBGs a month. That’s an average of an FBG every nine minutes, assuming 100 percent yields -- a figure that’s “impressive” (if a little hard to believe), according to Tom Mock, director of product management at Ciena Corp. (Nasdaq: CIEN). Ciena makes FBGs for use in its own equipment and has already announced 25-GHz and 12.5-GHz channel spacing (see Ciena Expands CoreStream Offerings).

Southampton Photonics already has two production lines and plans to build a third one -- facilitating production of 15,000 FBGs a month. This prompts more raised eyebrows. “That’s a very big number,” says Mock. Victor Mizrahi, Ciena’s former chief scientist, doubts whether there’s demand for that quantity of FBGs.

Ciena and Southampton Photonics appear to make FBGs in similar ways and reap similar benefits. Both outfits avoid the need to design and make special phase masks for placing over lengths of doped fiber before blasting it with ultraviolet light to create gratings (barcode-like patterns of stripes) -- the way most other vendors make FBGs. Instead, both Ciena and Southampton Photonics have developed computer programs that are able to make gratings from a set of standard phase masks.

Eliminating the purpose-designed phase mask avoids lengthy manufacturing delays. It also enables engineers to make longer, more complex gratings that can isolate wavelengths more cleanly than is possible with other processes. This equates to being able to reduce the spacing between wavelengths.

As the fiber can be of any length, Southampton says there’s no real limit to channel spacing. “We can do 12.5 [GHz], we can do six, we can do one,” says David Payne, Southampton Photonics’ chairman and the director of Southampton University’s Optical Research Center. Cramming even more wavelengths into a fiber isn't a technical challenge: "It's a cost equation," he notes.

Being able to create different FBGs by simply tinkering with a computer program also means that Southampton Photonics can shift production from one grating design to another at the drop of a hat. That’s handy, of course, because vendors need a different FBG for each wavelength. In other words, a 160-channel DWDM system will need 160 different FBGs.

All of this automation leads to lower costs, but Southampton Photonics isn’t keen on talking about how that might effect its prices. A spokesman says that high-performance FBGs like theirs sell for $1,000 apiece. At that rate, the startup could make a cool $15 million a month from its three production lines, if they worked flat out.

Mizrahi is dubious. “People aren’t willing to pay $1,000 for an FBG,” he says. If they would, he’d get into the business of making them himself, he says.

-- Peter Heywood, international editor, Light Reading http://www.lightreading.com

Ace 12/4/2012 | 8:54:28 PM
re: Fiber Bragg Gratings on Speed
I'm surprised that this article doesn't mention the packaging of the gratings. It is one thing to write the bragg grating on fiber, but another to then package it so that there is insignificant wavelength shifting over temperature. My guess is that it is more expensive to stabilize the grating in a mechnical package then it is to write the fiber.

One other point; a grating alone does nothing but reflect wavelength selective light. It has to be used with a 2nd component (coupler, circulator, etc) to become a useful device, which adds to the cost. So far, many DWDM systems do not use gratings, and instead AWGs and thin films, because of this. I have heard DWDM prices below $300 per port; so what's up with a $1000 grating?
Peter Heywood 12/4/2012 | 8:54:21 PM
re: Fiber Bragg Gratings on Speed Ace, Southampton Photonics' FBGs end up in packages a bit less than half an inch square and about four inches long. It's a "2 part athermal package" which means that it compensates for differences in temperature so that it ends up expanding and contracting at exactly the same rate as the fiber. The whole thing is hermetically sealed against moisture.

Developing this, and developing the process by which it's made, is considered a major achievement by Southampton Photonics. But I don't know enough about packaging in general to be able to say how this stacks up with what other folk do.

Another thing I didn't mention...In order to print the grating, the outer coating of the fiber has to be removed - and exposing the innards to air like this can make it go brittle and crack. Southampton Photonics reckons that it's perfected a way of annealing the fiber after printing the grating, in a way which solves the brittleness problem without disturbing the grating.

On the price issue, there are a bunch of different technologies for multiplexing light in DWDM systems, each of which has different pros and cons and different costs.

The main options include thin film filters, arrayed waveguide gratings, bulk diffraction gratings and FBGs. (What have I missed?).

Anyhow, FBGs tend to be used in high performance gear - ie high channel counts, high data rates and long transmission distances.

The other technologies come into their own for other applications - such as supporting lots of wavelengths carrying low datarates, which might suit access equipment. In this case, the cost per wavelength would be a lot less. Avanex, for instance, says it could support 800 wavelengths in a fiber, and talks about equipment costing a mere $100 a wavelength.

Peter
pingu 12/4/2012 | 8:54:14 PM
re: Fiber Bragg Gratings on Speed Hi Peter,

Since you brought up Avanex, I thought I'd offer this link to a slide in their corporate presentation which compares competing DWDM technologies:

http://media.corporate-ir.net/...

The price/wavelength of the current PowerMux is <$500, but the PowerMux NxG (in beta now, with revenues expected to be realized by the September quarter) will bring the cost down to $100 per wavelength.

Why? Because it will be utilizing holographic gratings from Avanex's Holographix acquisiton last year instead of the PowerFilter (Avanex's own TFF).

PowerMux, as you might know, is a combination of a "back-end" consisting of a TFF (original PM) or holographic grating (PM NxG) and a "front-end" consisting of tunable, non-linear interferometer. Here's a link to the patent, which was filed in August 1998 and granted just last October:

http://164.195.100.11/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm&r=1&f=G&l=50&s1='6130971'.WKU.&OS=PN/6130971&RS=PN/6130971

Avanex had apparently considered AWG's for the "back-end" portion of PowerMux, but decided that they were too susceptible to environmental factors (temperature, etc.).

It may be interesting to look into the holographic grating technology as Avanex may capitalize on its purported advantages in other products. CEO Walter Alessandrini said as much in the last conference call.

Maybe holographic gratings could be perhaps be not only used in PowerMux's high channel-count DWDM system, but as a product to go head-to-head against lower-channel count technologies such as TFF, FBG, and AWG.

Regards,
pingu
Ace 12/4/2012 | 8:54:13 PM
re: Fiber Bragg Gratings on Speed
pingu,

> I thought I'd offer this link to a slide
> in their corporate presentation which
> compares competing DWDM technologies

The slide has some inaccuracy. Thin film WDMs are not $1000 per channel; they are less than $400 today, and with Asia starting to OEM output you know these will drop lower.

Also, can you clarify what you by "supporting multiple bit rates?" A WDM component operates on an optical layer, and does not care about bits.

Thanks.

pingu 12/4/2012 | 8:54:04 PM
re: Fiber Bragg Gratings on Speed Re "supporting multiple bitrates",

PowerMux is the ultimate slicer and dicer of light: it can cut it up into multiple lamdbdas down to megabit proportions, or even less if desired. Or it can multiply fewer channels at higher bitrates. The ability to adjust channels, spacing and size gives the PowerMux a high level of scalability and adaptability.

Regards,
pingu
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