Agilent Technologies today unveiled a test platform that should make life a lot easier for developers of 40 and 100 Gbit/s optical modules and systems. The N4391A Optical Modulation Analyzer is a new type of test instrument designed specifically for in-depth analysis of optical signals based on advanced modulation schemes.
The problem with traditional 10 Gbit/s test equipment is that it has been designed to measure amplitude-modulated signals only, whereas 100 Gbit/s long-haul optical transmission will be based on some form of phase modulation.
Advanced modulation schemes are needed to overcome limitations due to fibre impairments such as dispersion (see New modulation formats: what's the best solution?).
Although widely deployed in the wireless industry, phase-based modulation schemes are relatively new to the optical industry, and there isn't a whole lot of equipment available to test them.
"What we hear from a lot of our customers is that they are dealing with measurement problems that they don't have the equipment for," commented Oliver Funke, product manager for digital photonic test at Agilent.
Developers of high-speed optical equipment have been forced to use workarounds, he says. They can use interferometers to convert phase into amplitude, and then measure the resulting signal with a traditional instrument like an sampling oscilloscope or digital communications analyser. But this approach is far from ideal because it requires considerable in-house expertise to design and build the phase-amplitude converter and to write the software needed to run the test. What's more, the converter is bit-rate specific, so a new test bed would need to be designed for each new modulation scheme.
Agilent says that the N4391A Optical Modulation Analyzer is a completely new kind of instrument that fills this gap in the market. The new tester is based on a combination of three technology platforms: the 90000 Infiniium high-speed oscilloscope, a special version of the 89600 Vector Signal Analysis software (originally developed for the company's RF signal analyser products), and a polarization-sensitive optical coherent receiver.
The most common application for the N4391A will be to test optical transmitters for 40 and 100 Gbit/s. In this application the output of the transmitter is simply connected to the N4391A, and the instrument can perform a full analysis of the optical signal in real time.
It can output, for example, an optical vector diagram, showing the I (in-phase) and Q (quadrature-phase) components of either the x or y polarization signals. Alternatively, there is the constellation diagram of the signal, which provides a visual image of any signal distortion in much the same way that an eye diagram shows distortion of an amplitude-modulated optical signal with time.
The N4391A can provide quantitative data as well as visual information. The software translates images into numerical values representing modulation accuracy such as phase error, frequency error, IQ offset and gain imbalance.
One big advantage of this type of tester is that is it flexible and hence fairly future-proof. It can characterize data rates up to and beyond 100 Gbit/s, depending on the speed of the data acquisition unit and the modulation scheme being used, and can measure any kind of advanced modulation format. The software library currently includes 34 modulation formats, including the most popular ones such as DSPK and DQPSK, with or without polarization multiplexing. New modulation formats will simply require the development of a new library software module, the company says.
The one thing the customer must be aware of, says Funke, is that because there are no standards for high-speed receivers, it is impossible to obtain definitive bit-error rate (BER) measurements. Data processing in a different optical receiver may well generate a different value of BER than the one measured by the tester. Once standards appear, they can be incorporated into the software, of course.
Mintera, a developer of 40 and 100 Gbit/s optical modules, is impressed enough to give the product a wholehearted endorsement. "This is exactly the kind of test instrument that fills the gap for the optical industry, as engineers can now gain deep insight into the behaviour of transmitters and links operating with advanced optical modulation schemes," said Niall Robinson, the vendor's vice-president of product marketing.
The starting price for the N4391A is $190,000 (€148,389) for the basic set-up, rising to around $400,000 for a full configuration. The customer could save money, for instance, by adding their own local oscillator, a tunable laser in other words.
A prototype will be on display at OFC/NFOEC in San Diego later this month with general availability in April.