Once considered a key piece of the all-optical network, tunable filters have begun targeting more modest applications, such as optical performance monitoring (see Ondax Celebrates First Order). But component makers continue to pursue the technology, asserting that it's still crucial for next-generation networks, whenever they emerge.

Quite a few vendors at last month's OFC Conference were demonstrating tunable filters. Thin-film specialist Aegis Semiconductor Inc., having altered its business plan to make tunable filters, showed its products for the first time. Auxora Inc. and Micron Optics Inc. likewise showed tunable filters based on thin films, while Iolon Inc. introduced a filter based on micro-electro-mechanical systems (MEMS).

Aegis had started out making thin films for VCSEL arrays to monitor power consumption (see Aegis Semiconductor Inc.). But company officials decided that tunable filters presented a richer market, "especially if you want to move up the chain to subsystems," says CEO Matthias Wagner. The change in strategy could be paying off, as Wagner claims Aegis' tunable filters are in five beta trials, with general availability of its parts expected to come midyear.

Aegis' trick is to use two types of thin films: semiconductor thin films, more commonly used for flat-panel displays, and dielectric thin films, the type regularly used in optics. Multiple layers of the films are stacked to produce the filter.

The filter is currently built on a quartz substrate, but the manufacturing process is compatible with CMOS, meaning the parts could be built on commonplace semiconductor manufacturing lines. Wagner hopes to eventually develop CMOS parts that could integrate some control circuitry.

The semiconductor thin films helped Aegis produce a thermally tunable part -- one without moving parts, in other words. That runs contrary to the linear tuning that others have tried. In those cases, the thin films are nonuniform, so that shifting the filter's position results in different wavelengths being blocked.

Yet another method is angular tuning, where the filter is tilted with respect to the beam of light. That's the method chosen by Chameleon Optics, which was acquired by Micron Optics in October. Micron Optics used OFC to announce commercial availability of the Chameleon filter (see Micron Optics Intros Filter and Micron Optics Acquires Chameleon).

Angular tuning has been attempted before, but it tends to require additional optics to compensate for polarization, says Kevin Hsu, Micron vice president of R&D. Micron Optics claims to have developed a filter impervious to polarization, allowing the design to stay simple.

At OFC, Micron company showed three- and four-port filters, the former able to add or drop a wavelength, and the latter able to add and drop wavelengths. Micron also sells a simple two-port device.

Still another thin-film contender is Auxora, which introduced its technology at NFOEC last fall and demonstrated it in two-and four-port devices at OFC (see Auxora Demos Tunable Filter and Agilent Invests in Big Bear). Auxora is keeping quiet about the tuning mechanism it's chosen, however. Both devices are "almost ready to ship," says Jack Latchinian, manager of sales and marketing.

Outside the thin-film realm, Iolon unveiled plans for a tunable filter based on the MEMS actuator used in its tunable lasers (see Iolon Unveils Tunable Filters).

Like those in the thin-film camp, Iolon says its filters can be tuned over the complete C- or L- band, for 50GHz or 100GHz channel spacings. Using MEMS technology reduces the size of the component and also reduces losses, according to Saeid Aramideh, Iolon's VP of product management. Insertion loss is less than 2dB across the band, significantly less than existing products in the market, he says.

Using MEMS also holds out the promise of enabling Iolon to use mass manufacturing techniques to reduce costs. Aramideh cites an average price of "less than $2,000." He adds that Iolon is "in the process of securing major design wins" for its tunable filters, which have been shipping for revenue since last December.

Existing manufacturers of tunable filters aren't bowled over by Iolon's announcement. A spokesman from Santec Corp., who requested anonymity, acknowledges that Iolon would have an edge if there was a mass market for tunable filters, but he says only small volumes are being sold right now. "Even then, we're close to $2,000." He claims Santec's tunable filter also has an insertion loss of "below 2dB".

JDS Uniphase Corp. (Nasdaq: JDSU; Toronto: JDU) makes similar comments. It has a "Voltage Controlled Filter" (VCF) that appears to have 3dB maximum insertion loss and 0.5dB typical insertion loss over its tuning range. A spokesperson declines to give the price.

The thin-film manufacturers are quoting insertion loss figures in the same ballpark; Micron Optics, for example, says its angular-tuned filter hits 2dB to 3dB of insertion loss, "within the expectation of most applications," says Micron's Hsu. -- Craig Matsumoto, Senior Editor, and Peter Heywood, Founding Editor, Light Reading