Tunable filters show real promise

By MEGHAN FULLER

Tunability appears on many analysts' lists as one of the few bright spots in the current economy. Tunable or wavelength-agile network elements promise to reduce network complexity and cost by eliminating unnecessary optical-electrical-optical (OEO) conversions. However, the high cost and immaturity of such devices, coupled with a lack of demand for dynamic networking, have delayed their widespread deployment.

Today, tunable filters are used primarily to "probe what's going on in the system," explains Jerry Bautista, director of technology and strategic marketing, business development, at Intel's photonics operation in San Jose, CA. They are used in optical power monitors and channel monitors for scanning the wavelength range to garner information about optical signals. Tunable filters are also used to mitigate amplified spontaneous emission (ASE), or noise generated by amplifiers in a system.

In the future, though, tunable filters will find themselves right in the optical path, says Bautista. The most promising application may be in reconfigurable optical add/drop multiplexers (ROADMs).

"One of the key problems that the carriers are having right now is transporting information, or loss of information, from, say, New York to San Francisco," observes Maurice Tremblay, product-line manager for voltage-controllable tunable filters at JDS Uniphase (San Jose, CA, and Nepean, Ontario). "Typically, the backbone network will require that the traffic go through Atlanta, for example, and two or three other cities in between. The way it is presently configured, everything goes into the electric domain in Atlanta, and then gets reconverted back into the optical domain and then goes on to another city. That's a very expensive way of doing things," he says, "especially when you know that maybe 25%, 35%, 40% of the traffic might be dropped at Atlanta to go on to Miami. So what you want to do is just have access to some, not all, of the traffic that's going through in Atlanta."

The ROADM enables carriers to selectively drop wavelengths without interfering with the express traffic. Carriers are in field trials of ROADMs now, says Tremblay.

While the high costs and lack of demand have and will continue to keep tunable filters out of the optical path for at least the next 12-18 months, there is another key obstacle to their widespread deployment. "I think people are still concerned about reliability issues," admits Bautista. "Sometimes the filter materials can be exotic or they may be mechanical-potential reliability concerns. People are reluctant to send their transmission bit streams through relatively new devices without some kind of history of reliability."

Much of the progress made on tunable filters has been driven by startups, all of which seem to champion different tuning mechanisms, materials, and technologies. Some of these will pan out, some will not, but by all accounts, it's still too early to predict which technology or vendor will come out on top.

At OFC in March, CiDRA (Wallingford, CT) unveiled its AgileWave tunable bandpass filter for use in test and measurement applications and wavelength selection. It is based on the company's proprietary optical-waveguide technology and Bragg grating packaging technology. Also using fiber Bragg gratings is Bragg Photonics (Montreal), whose FlexFilter employs three tuning mechanisms: a piezoelectric stack actuator, dynamic bi-metal structure/heat source, and stepper motor.

Fellow startup Ondax (Monrovia, CA) has developed the FASMA 100-1D tunable bandpass filter based on three-dimensional Bragg grating technology recorded in a photorefractive bulk material. It features what the company calls "hitless tuning," or the ability to tune to specific channels without affecting the express traffic passing through the device.

Optune Technologies (Montreal), a Stocker Yale (Salem, MA) company, manufactures a tunable optical filter for wavelength-selection applications. According to the company's Website, the filter "is a proprietary multiple-beam interferometer without a resonant cavity." Planar-lightwave-circuit technology enables Lambda Crossing (Caesarea, Israel) to develop 25-, 50-, and 100-GHz tunable data filters. The company also manufactures the LambdaFlow tunable OADM and LambdaWatch optical performance monitor.

Solus Micro Technologies' (Westlake Village, CA) TFM series of tunable Fabry-Perot interferometers employs the company's proprietary Compliant MEMS (CMEMS) filter design. Micron Optics (Atlanta) also relies on Fabry-Perot lasers for its tunable filter (FFP-TF) and tunable notch filter (FFP-TNF).

Santec (Komaki, Japan), meanwhile, has a range of tuning products in the works, including a tunable WDM module for reconfigurable OADMs; the OTF-655, which tunes via stepper motor actuation, for wavelength selection; and the OTF-610, a voltage-controlled high-speed optical tunable filter. Also boasting a range of tunable filters is JDS Uniphase. The company's VCF050/100 series of voltage-controlled tunable bandpass filters is frequently used for ASE suppression of optical signals, single-channel demultiplexing from a multichannel DWDM stream, and dynamic wavelength optical add/drop applications. The device is tuned using a stepper motor driven by an external IC driver.

"Some of the technologies being marketed today were started in R&D labs maybe three years ago," says Tremblay, "and since then, requirements have changed drastically-notably that of cost. Two years ago, you could have sold something at $5,000 and customers would have bought it because they had the money. These days, they come to us and say, 'Look, there's no point in even talking to you if you don't come down to this price.'"

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