Lots of startups are developing tunable lasers, and quite a few are developing tunable filters, but Sparkle Optics Inc. is working on something else that’s tunable -- a tunable demultiplexer.

A demultiplexer, in this context, is a widget used in wave division multiplexing (WDM) systems to split light into different colors, or wavelengths, so that a single fiber can actually carry multiple streams of data, each on a slightly different wavelength. Making it tunable, as Sparkle is doing, means that the band of wavelengths it's dealing with can be changed, almost on a whim.

Opinions differ on whether demultiplexers really need to be tunable-on-a-whim in this way. But Santanu Basu, who heads up Sparkle Optics, a tiny contract R&D firm, clearly has applications up his sleeve. For the time being, however, he’s fobbing people off with a niche one -- saying that such a device could be a boon for systems integrators that want to buy a single demultiplexer for their labs, one that could work anywhere in the C, L, or S bands.

Anyhow, Basu figures his tunable demultiplexer has a number of advantages over arrayed waveguide gratings (AWGs), one of the existing ways of splitting white light into different wavelengths. In particular, it’s practical for small channel counts, it doesn’t require temperature control (which consumes power), and, as noted, it’s tunable.

How does it do all this? Because it's based on a diffraction grating, says Basu, and these properties are inherent to the technology.

It's important to note that Sparkle isn't the only company developing grating-based demultiplexers. In fact, there's a growing band of players, including APA Optics Inc. (Nasdaq: APAT), Highwave Optical Technologies, Lightchip Inc., Photonetic (now part of GN Nettest), and Zolo Technologies Inc.. These companies already have products out.

Sparkle, on the other hand, has quite a bit of work to do. Basu says he's built a six-channel prototype out of bulk optical components, like lenses and ruled gratings. The prototype has a channel spacing of 50 GHz, which is good. It has an optical loss of 10.7 dB, which is bad. And the dimensions are 11.3 x 3.7 x 1.3 inches, which is laughably huge.

"It's a small beginning," acknowledges Basu, without irony. "Once we have completed the technology licensing, we will make a second generation in small quantities. We will improve the size, the crosstalk loss, and polarization-dependent loss."

It should be possible to reduce optical losses to less than 6 dB, which is lower than commercially available AWGs, he adds.

Sparkle's competitors are not easily impressed, however. Mike Wearsch, VP of business development at Zolo says that what's needed is reconfigurability, not band tuning. "If you're making an add/drop multiplexer, you need to be able to drop any six channels, not just the six channels next to each other," he says. Zolo, he notes, is working on a way of doing this using MEMS (micro-electro-mechanical system) technology.

AWG vendors are quick to point out that grating-based devices in general could have a big drawback: reliability. "How do you expect it to stand up to vibration when it's going to be hand-made in Asia with cheap labor?" says John Midgley, CEO of Lightwave Microsystems Corp.

Basu counters this criticism by saying that he's planning to make the demultiplexer smaller and more robust by miniaturizing and integrating fewer components onto a silicon optical bench.

All of this work is being undertaken by Sparkle under R&D contracts with Silicon Valley companies, Light Reading has learned.

Basu -- a graduate of Stanford University and former IBM Corp. (NYSE: IBM) researcher -- didn't want to reveal the identities of the companies sponsoring his work, but did give a few hints.

Both companies are in the Bay area, he said. One is an established manufacturer of lasers, not just for telecommunications but for a wide range of applications. The most likely candidate is Coherent Inc. (Nasdaq: COHR), which has its headquarters in Santa Clara. This also fits with Coherent's recent decision to refocus the company on telecom components (see Coherent's Strategy Shift Explained).

The other vendor is a "strong telecom player," and it's not JDS Uniphase Inc. (Nasdaq: JDSU; Toronto: JDU), which has decided to back AWGs (arrayed waveguide gratings) all the way, says Basu.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.comWant to know more? The big cheeses of the optical networking industrywill be discussing this very topic in a session at Opticon 2001, LightReading's annual conference, being held in San Jose, California on Aug13-16. Check it out on Opticon2001.