Which Amp is the Champ?

AMSTERDAM -- ECOC 2001 -- Here at the ECOC exhibition, a couple of startups are showing amplifier developments that could bring down metro network costs considerably.

Both companies -- Teem Photonics of Grenoble, France, and Cisilias A/S of Lyngby, Denmark -- are showing multichannel EDWA (erbium-doped waveguide amplifier) arrays. In essence, they’re much lower cost, lower power alternatives to the EDFAs (erbium-doped fiber amplifiers) already used extensively in long-haul networks (see Erbium Doped-Fiber Amplifiers (EDFAs)).

Vast numbers of low-power amplifiers are likely to be required in metro networks, to boost light after it’s passed though optical crossconnects and to balance the strength of light in adjacent wavelengths in DWDM systems. So getting amplifier costs down as low as possible, as EDWA arrays promise to do, is a big deal.

Another big plus point: Space is also likely to be a key consideration in metro networks -- and EDWA arrays are a tiny fraction of the size of EDFA arrays.

So far, Teem Photonics has been a leader in this field. It showed a prototype 4-channel EDWA array at ECOC last year, and this year it’s showing a commercial version with improved performance (see Teem Ships Improved EDWA Array). It’s also previewing an 8-channel EDWA array.

Cisilias hasn’t been around for so long but appears to be catching up. It’s also showing a prototype 8-channel EDWA array at ECOC, and, on the face of it, it has better performance than Teem’s array (see Startup To Display Amplifier Array). However, making like-for-like comparisons in this field is tricky.

Before digging into the details, here’s some background on Cisilias. It was spun out of the Center for Communications, Optics and Materials (COM) at the Technical University of Denmark in August 2000 and is 100 percent owned by NKT Holding, which has long-standing links with COM. Other Danish startups in this field, including Ionas A/S and Crystal Fibre A/S, were founded in a similar way and are also part of the NKT group.

Cisilias’s CEO, Jacob L. Philipsen, hired last June, was previously head of amplifier development at Teem Photonics.

Cisilias is using a process called plasma enhanced chemical vapor deposition (PECVD) to make its EDWA arrays. This lays down a thin film of silica on top of a silicon substrate. Philipsen says this has some fundamental advantages over Teem’s approach, which uses a process called ion exchange to dope pieces of phosphate-based glass with erbium.

With Cisilias's approach, the erbium doping can be concentrated on just the waveguides, creating the equivalent of the doped fiber core of an EDFA, according to Philipsen, who claims this delivers better performance and makes it easier to integrate passive devices on the same chip because the rest of the silica isn’t contaminated with erbium. In addition, he adds, the silicon substrate holds out the promise of being used to integrate the lasers needed to pump light into the EDWAs, to re-energize the signals passing through the core of the waveguides.

Teem Photonics points out that its approach enables it to boost performance by using a lot more erbium. “We can dope the medium 100 times more than you can with glass,” says Andrew Benn, Teem's product line manager. Benn also notes that longer waveguides are needed with the PECVD process, which equates to larger chips.

Philipsen says Cisilias gets over this by arranging waveguides in a spiral, rather than making them straight. “With our process we can do sharper turns, so at the end of the day we end up with about the same chip size,” he says. Benn, however, notes that curved waveguides ring alarm bells with system designers because they often equate to bigger problems with polarization mode dispersion and polarization dependent loss.

Philipsen says that Cisilias’s 8-channel EDWA array delivers 12 decibels of gain over the whole C band when it’s pumped with a 100 milliwatt, 980 nanometer laser. This compares with 10 decibels quoted by Teem for both its commercial 4-channel EDWA array and its prototype 8-channel array. (Companies often quote much higher peak powers because things sound better that way, but peak power is far less important than the power over the whole range of wavelengths.)

Philipsen says that Cisilias delivers 9 dBm of output power, while Teem quotes 6 dBm (dBm refers to decibels relative to a milliwatt -- see Optical Units Reference for more details). Philipsen also cites noise below 5 dB, compared to Teem’s 6 dB.

Whether or not this is comparing like with like is a moot point. For a kickoff, Cisilias doesn’t have a commercial product yet. It hopes to start shipments in the second quarter of next year. “There’s a real difference between something that engineers can build and one that’s in production,” notes Teem’s Benn. Philipsen agrees, acknowledging that Teem is ahead in this respect.

Teem’s commercialization effort has included the integration of its EDWA arrays with various passive devices like splitters to divide light into different channels, taps to bleed off a little light to monitor power, and isolators to prevent reflections causing problems, Benn notes. “All of the passive technology has been incorporated in the EDWA,” he says. Teem makes many of these passive components and has a lot of skills in this area, he adds. “We’re head and shoulders above anybody in coming up with higher levels of integration."

It’s unclear how far Cisilias has got with integrating passive components. The performance figures quoted by Teem allow for light lost by taps, isolators, and so on. It’s unclear whether Cisilias’s figures do.

As the race between Teem and Cisilias heats up, other players are likely to jump into the EDWA array market. Northstar Photonics Inc. says it will announce a single-channel EDWA in the coming months and says multichannel EDWA arrays are on its development roadmap. It’s using a process technology similar to Teem's. Philipsen says he knows of “a few” other startups developing EDWAs, all in stealth mode at present.

— Peter Heywood, Founding Editor, Light Reading
http://www.lightreading.com