DWDM drives design changes in embedded OSAs

TECHNOLOGY

By MEGHAN FULLER

The performance requirements of embedded optical spectrum analyzers (OSAs)-and embedded test equipment in general-have changed over the past year, driven by the widespread deployment of sophisticated DWDM systems. Previously responsible for measuring channel wavelength, power, signal-to-noise ratio, channel spacing, and wavelength drift, today's embedded OSAs are now responsible for conducting complex performance monitoring functions, as well.

Performance monitoring on WDM systems has always been somewhat primitive, asserts Lawrence Gasman, president of market reasearcher CIR Inc. (Charlottesville, VA), because the systems themselves were primitive. "They were point-to-point links that were easy to monitor, because you had a box at one end and a box at the other and maybe a few little boxes along the way," he explains.

DWDM systems are a great deal more complex now, and this complexity has changed the requirements of embedded equipment. To increase bandwidth capacity, systems designers have in creased data rates, increased the spectrum utilization, and decreased the channel spacing-all of which necessitates more sophisticated analysis. "There is just more that can go wrong," says Gasman, "and when it does go wrong, it's harder to find out what's wrong."

According to Jim Nershook, vice president of marketing at NetTest (Utica, NY), today's embedded OSAs, which are located within the transmission systems themselves, must cover both the C- and L-bands and monitor up to 1,000 channels. In the past, they were responsible for 10 to 16 channels in the C-band only. They must also feature a dynamic range of +10 to -70 dBm, says Nershook. The power accuracy must be 0.5 dB, and the wavelength accuracy must be 20 pm. In addition, they must work on both modulated and unmodulated signals.

The new embedded OSAs are capable of far more than traditional spectral analysis, however. They also serve as a feedback mechanism for dynamic gain equalization, explains Nershook. "That means if you drop or add channels, the OSA will tell you about channel power and OSNR [optical signal-to-noise ratio]. It will enable you to adjust the associated electronics-the amplifiers and input powers-to improve your system's performance," he says.

Embedded OSAs can also be used for wavelength routing. "If you have a switched network where you are switching wavelengths in and out, you can actually identify which wavelengths have gone in which direction," he explains.

The basic design of the devices also has been altered in response to the need for greater functionality. In the past, the majority of OSAs featured a single- or multiple-pass grating element to separate the wavelengths before detection. Their performance was limited to channels separated by 50 to 100 GHz, however. At SuperComm last June, several companies debuted DWDM systems operating at 12.5-GHz channel spacing or less. To handle such high-channel-count systems, OSAs are now employing other technologies, including tunable filters.

For the folks at NetTest, Fabry-Perot filters make sense because they are both small and rugged; there are no moving parts. "You have to remember you are taking an OSA that normally would occupy a benchtop," says Nershook, "a big OSA that would weigh 40 lbs or more, and you are shrinking it down to an area that is 11x7x2 inches. It's very small."

JDS Uniphase (Ottawa, Ontario) employs a fiber Bragg grating filter in one of its embedded OSA product lines. "We adopted this [technology] because it provides inherently better optical signal-to-noise ratio monitoring," says Wei Loh, director of new-product development. "People use fiber Bragg gratings for add/drop applications because of the very good spectral quality of these filters." The company's other line of embedded OSAs employs a dispersive element that spatially separates the light out into different wavelengths that simultaneously impinge upon an indium gallium arsenide photodiode array, which takes the spectral measurements.

"I think [the increased functionality] is what's driving a lot of interest right now in embedded optical-channel monitors and embedded performance monitors and embedded spectral analyzers," asserts Gasman. "There's a whole movement toward all of these things, and I think there's a real market opportunity there."

The complexity of the new generation of embedded OSAs has led to some question about its classification, however. "A lot of folks in the industry are having a difficult time trying to determine if this embedded test equipment is actually a component and should, as such, be tested as one, or if it is an optical module, which has a different set of testing requirements altogether," explains Nershook. That presents a problem, he says, because Telcordia Technologies outlines separate specifications for qualifying components and modules.

According to Gasman, a component is a discrete device that is single-minded in its functionality-like a laser, which is simply a little chip that lases. A transmitter, by contrast, is a module because it contains at least the laser and a modulator and perhaps even an isolator and an amplifier.

"A lot of embedded equipment, by this definition, is probably a module," he asserts. "Now, the problem with going down that route is that there is a movement toward integrated optics, where you may well get an optical-channel monitor on a chip. But that's a few years off, and when that happens, those categories simply break down and we move on to other things."

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