In optics, one-by-n devices remain very popular. Demultiplexers can be 40 channels, 80 channels, and in the future, maybe 100 or 200 channels. In the past, the way those were measured was with a broadband source with a spectrum analyzer and a switched chassis. What they're doing now, Chappell says, is taking a tunable laser source, putting it into the demultiplexer, and then hanging inexpensive power meters on the output of every single channel.
With a single sweep of a tunable laser source, every power meter is recording data at trigger points. At the end of one sweep, all n channels are characterized, whatever n may be. As device capacity is raised from 20 to 40 to 80 channels, more hardware is needed. But their total test time remains the same. "The throughput is amazing, and that's something that is just now making it into manufacturing," says Chappell. "It's a different solution from a spectrum analyzer."
A lightwave measurement system like the one just described is manufactured by an Agilent division in Germany. It's a tunable laser source and a chassis for power-meter modules. The user can slide in n number of power-meter modules and trigger them all to make measurements at the same point. All of the data is dumped back to a PC, and the results are shown on the screen.
Chappell expects optical dispersion measurements to become a huge area of interest. This includes both chromatic dispersion and polarization-mode dispersion, commonly referred to as CD and PMD. As data rates increase from 2.5 and 10 Gbits/s to 40 Gbits/s, CD can potentially limit the transmission distance. So designers are characterizing not only the fiber, but every component that goes into the system for dispersion.
Right now, this kind of testing is accomplished with a tunable laser source, an RF network analyzer, and a novel test set. That matches a product recently announced by Agilent: the 86037C chromatic-dispersion test system.
Getting back to oscilloscopes, though high-speed sampling oscilloscopes are needed to test the fastest communications signals, high-end general-purpose DSOs still can test slower optical signals. LeCroy Corp. introduced a system about a year and a half ago for testing optical signals. It looks at OC-3/STM1 and OC-12/STM4 optical telecom systems.
Called the MT03 Mask Test Kit, it lets the user do all of the things that are normally done with an oscilloscope. In addition, the incoming signal can be taken in and compared to a standardized test mask. The OE325, which is part of the kit, performs the very important job of converting the optical signal to an electrical signal—not an easy device to design. "That's a problem I think every test-equipment manufacturer has, converting the optical signal to an electrical signal," says Mike Lauterbach, director of product management at LeCroy.
The kit also contains a reference receiver that performs the filtering with a four-pole Bessel-Thomson filter, as specified by the standard. After Conversion and filtering, you can look at these signals as if they were any other electrical signal.
Paul Fowler, product manager for communications at LeCroy, notes that as you move up the line from OC-3 and OC-12 to OC-48 and beyond, the speed gets high enough whereby you start moving away from the realm of a general-purpose oscilloscope. If you want to see signals in the time domain, you end up in the sampling-oscilloscope arena.
A lot of the research is being done at the higher-speed data rates in the R&D labs. But it takes some time for that technology to migrate its way down to an actual product that's offered to customers, Fowler points out.
"What we're seeing right now is that although there is a lot of work being done at OC-48 and speeds faster than that, there's a tremendous amount of interest at this point in OC-12 and its counterpart in Europe, STM4, which is going into production at this point," says Fowler. "We have a lot of demand for our products in production environments, particularly in STM4."