Strife begins at 40Gbit/s

Test equipment firms supporting 40Gbit/s developments face the same technology hurdles as their customers.

By Stephen Hardy, Lightwave

The challenges of overcoming the effects of chromatic and pulse-mode dispersion, as well as nonlinearities get even tougher for developers of 40Gbit/s equipment if they cannot measure how they're performing.

As Larry DesJardin, high-bandwidth program manager at Agilent Technologies, puts it, "To build a BERT (bit-error-rate tester) at 40Gbit/s requires the very same advanced high-speed technology that will be used in next-generation 40Gbit/s systems - sometimes even better. So, this remains challenging, because we need the very technology that is being tested, but we need it earlier."

Since 40Gbit/s can, in some ways, be thought of as an extension of previous work in 10Gbit/s technology, DesJardins says that many of the same test-system types - such as BERTs, tunable lasers, optical spectrum analysers, dispersion analysers, digital communication analysers, jitter test sets, and SONET/SDH testers - are necessary when designing at the new, higher speeds.

However, this equipment may need new capabilities. BERTs have traditionally been electrical nonreturn-to-zero (NRZ) instruments. With the use of return-to-zero (RZ) signalling, mentioned later, such equipment must also handle wavelength-tunable RZ and NRZ optics.

Dispersion and jitter are a greater problem at 40Gbit/s, so a wider range of components must be tested. New types of test equipment that measure both chromatic and polarisation-mode dispersion simplify such tests.

The unique aspects of 40Gbit/s design also lead to unique test approaches. For example, "Raman amplifiers require test equipment throughout the entire component chain to be different: very low polarisation-dependent loss (PDL) for each of the passive components within the amp, S-band measurements of the Raman pump sources, and new noise figure measurement techniques of the entire amplifier, which includes the fibre," says DesJardins. "Also, the Raman pumps are of extremely high power, so new power heads had to be invented to measure the power."

RZ problems
The use of RZ techniques makes testing even more difficult. "First of all, there is no standard 'eye-diagram' for RZ, so we've had to create RZ analysis ourselves. And, as if multiplying the bandwidth needs by four times wasn't enough, RZ modulation doubles the bandwidth used again, making 40Gbit/s RZ signals require eight times the bandwidth to analyse more than 10Gbit/s NRZ signals. This pushes measurements to the limit that standard optoelectronic (OE) technology can work at."

Finally, the frequent use of radio-frequency (RF) signal modulation techniques further complicates the test process. "Many equipment manufacturers are developing exotic modulation techniques, such as suppressed carrier RZ and related single-sideband transmission formats. Soon there may even be duo-binary formats - four levels," says DesJardins. "Modulated spectrum analysis is one of the key techniques used in the microwave world for analysing similar methods and will be essential in the optical domain as well. However, standard grating-based optical spectrum analysers do not come close to analysing the spectrum of individual transmissions." So, new test equipment that uses microwave techniques in the optical domain, coupled with advanced digital signal analysis, has been developed.

Microwave lessons
Optical systems developers must approach jitter testing in a different way when they stray into the RF domain. The digital wireline world has a model of jitter that is measured in unit intervals and frequency. The microwave world has a similar concept to jitter, known as phase noise. Fortunately, there is a direct mathematical relationship between phase noise and jitter, and Agilent has applied microwave phase noise techniques to measure jitter as well.

As a result, test equipment is now available to help engineers develop 40Gbit/s systems. "Test equipment is getting better," said Dr Katherine Hall, chief technology officer at 40Gbit/s system start-up PhotonEx. Hall reports that, when PhotonEx engineers first started, they"borrowed" test equipment and strategies from her previous experience with high-speed transmission at MIT Lincoln Labs.

Hall offered optical pulse characterisation as an example of the difficulties of testing in a 40Gbit/s environment. A way to characterise an optical pulse is to use a nonlinear technique called "auto-correlation". An engineer would split the pulse into two arms, recombine them on a nonlinear crystal, and vary the length of one arm. This creates a signal at twice the frequency. So, the engineer maps something out in time by changing its delay in space.

"That's a more difficult thing to measure cleanly electronically, because the response of the electronic equipment has to be fast enough to resolve all the details of the optical pulse," Hall said. "If a photodiode only has a 30GHz bandwidth and my scope plug-in has a 40GHz, 3dB bandwidth, then I can take a guess at the waveform I'm measuring and deconvolve it. But the accuracy of that would not be as high as an auto-correlation measurement." However, electronic test systems are now providing results with an accuracy closer to those obtained via auto-correlation, concludes Hall.

'First' integrated 40Gbit/s jitter testing system
Spirent Communications ( claims to have launched the first totally integrated solution with SONET framing and jitter testing for 40Gbit/s optical kit.

The company believes that its OTA-4400 system puts it in "pole position" for the emerging 40Gbit/s test market. It offers an architecture that will support both OC-768 SONET functional testing and 40Gbit/s jitter testing - both essential to accelerate the introduction of reliable 40Gbit/s components and systems over the next couple of years.

Spirent's marketing director Jason Nutt says, "Unlike existing products, which first de-multiplex the 40Gbit/s stream into lower rate channels before performing analysis, the OTA-4400 works at the 40Gbit/s stream, allowing for extensive capabilities beyond Layer 1 40Gbit/s BER (bit error rate) testing.

Nutt said this particular feature will help accelerate the deployment of next generation optical network systems in a more cost effective way. Antony Savvas

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