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DWDM

Tunable Filters Go Solid State

A laser lighting company, StockerYale Inc. (Nasdaq:STKR) announced today that it was setting up a subsidiary called Optune Technologies to develop “a new class of tunable optical filter” (see Tunable Filter Venture Launched).

The tunable filters will differ from existing ones in three key respects, according to StockerYale’s president and CTO, Alain Beauregard. They’ll switch from one wavelength to another at much higher speeds, they’ll be tunable over a much wider range, and they’ll be much more reliable. Beauregard declines to give specific figures or say what technology the filters will be based on until patents have been filed.

Although this doesn’t give much to go on, one thing is clear. The development of tunable filters is following in the footsteps of tunable lasers (see Tune In!). Solid state ones will emerge and will play a key role in making optical networks far more flexible.

Here’s the score. Right now, there’s a tendency to lump together all types of optical switch, when, in fact, they’ll have to address a wide range of applications (see Optical Switching Fabric).

One group of switches will be called on to connect all of the wavelengths from one fiber to all of the wavelengths in another fiber -- and for such applications arrays of tiny tilting mirrors or bubbles may be the best bet.

However, another group of switches will be called on to pluck out individual wavelengths and connect them to different fibers. And in this case, mirrors and bubbles simply won’t cut it. Tunable filters will be needed to pull out individual wavelengths from a big bundle in the fiber.

The problem is that current tunable filters aren’t up to the job on a number of counts, according to Beauregard. Some of them are based on fiber Bragg gratings (FBGs), lengths of fiber that are treated in a special way so they reflect back a specific wavelength (see FBGs: Key to DWDM's Future?). This wavelength can be changed by stretching or heating the FBG so that the spacing of the grating in the fiber alters. However, this results in relatively slow switching speeds and limited tuning ranges, according to Beauregard. Putting fiber through repeated stretching or heating and cooling cycles also raises questions about reliability, he adds.

Other tunable filters are mechanical devices. They can have wide tuning ranges but they’re very slow and very expensive, according to Frank Tooley, CTO of Terahertz Photonics, a Scottish startup that’s also aiming to make next-generation tunable filters (see Scotland Spawns Component Startups).

Terahertz is more forthcoming about the technology it will use to make its tunable filters. They’ll be solid state and will be formed using a special polymer coating process. It’s still in an early stage of development, but Tooley reckons manufacturing costs will be very low.

StockerYale has different priorities. “We’re definitely shooting for performance. We’ll see about cost,” says Beauregard. One clue to StockerYale's technology might be that the company makes phase masks, which are used to create Bragg gratings. This doesn’t mean that StockerYale’s developments are fiber based, Beauregard adds.

The advent of these new types of tunable filter could spawn a new wave of startups developing equipment that takes advantage of their properties. One such startup, Sweden’s Lumentis AB was announced earlier this week (see Metro DWDM: Another Leap Forward?). Lumentis aims to make metro DWDM (dense wavelength-division multiplexing) equipment based on tunable lasers and tunable filters, contending it will give carriers unprecedented flexibility to provision bandwidth where and when it’s needed.

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

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