Accurate DWDM measurements demand multiple instruments and capabilities

Ghislain Lévesque

Sophisticated test and measurement equipment is essential to support the development of advanced DWDM networks. Although the OSA is the primary tool of choice, the future ideal instrument will combine many new functions.

FIGURE 1. The question is what will engineers expect their test equipment to do in two to three years?

After discussions with a variety of different sources in the fiberoptics industry—ranging from operators to research scientists—a number of trends and needs become evident. Also, research experts conducted a survey based on a simple question: what do you anticipate your testing equipment needs to be in the next two to three years?

Our results are presented here as a condensed version of the anticipated test and measurement requirements for the DWDM industry. The optical spectrum analyzer (OSA) is the main instrument discussed as the best representative of DWDM test equipment. It is clear that there is no do-it-all test instrument. Therefore, the needs of the individual customer must be defined and prioritized (see Fig.1).

SOFTWARE
Software needs to be as easy to use as possible with no confusing submenus or labyrinths of infinite choices. Engineers who wish to program software parameters, acquisition settings, result tables, and reports need configurable software designed to fit their needs. At the other end of the spectrum, users need fully automated high-level software capable of building and analyzing a comprehensive database collected from the test results of any instrument. Most importantly, software should be able to notify the user if there is a problem, where it is, and how to resolve it.

PORTABILITY
Another important consideration is portability. Units, including their cases and accessories, should be able to fit into an airplane luggage bin. Instruments also need to hold their calibration for long travel periods. Customers are requesting handheld equipment—a definite challenge for OSAs as well as polarization-mode dispersion (PMD) analyzers, protocol analyzers, optical time-domain reflectometers (OTDRs), or more specialized instruments used during installation and commissioning. Situations in which external power sources are not available require battery-powered equipment. Basically, customers want smaller, tougher, user-friendly cordless equipment (see Fig. 2).

Fabry-Perot filters and MEMS are rugged. In addition, wavelength accuracy is often better than for other types of OSAs, but the power dynamic range and wavelength range are limited. The small dimensions, low costs, and ruggedness of the MEMS are particularly interesting for handheld OSAs because of the industrial potential of the technology (see Fig.3).

Gratings do have moving parts and are less rugged. Despite this factor and their higher cost, they are the technology of choice for high-performance instruments such as benchtop OSAs.

Interferometers use a mathematical algorithm to determine wavelength and power measurements. They are capable of very accurate wavelength measurements because the interference patterns that result from the interferometer are wavelength-dependent. On the other hand, the power accuracy and dynamic range of the interferometer-equipped instrument can be two to three times worse than an OSA equipped with gratings technology.

In large companies, each application has its own independent development team. Levels of knowledge and expertise existing within these teams are all different, so their needs are different. For this reason, you might see different test equipment vendors within the same company, depending on the application. Test equipment manufacturers often have two to three different products to fulfill the different requirements of different applications.

Market studies indicate that everything is gearing up for the development of DWDM metro in the next two to five years. Even though we cannot say that metro (and shorter distance) will dominate near-term developments, short-distance applications are definitely something to consider. Cost is one reason why WDM is not yet fully implemented. Metro will require equipment to be inexpensive, universally easy to use, small, and portable, while offering the best possible specifications.

SPECIFICATIONS
Instruments should be chosen based on clearly identified, well-defined needs. It is also very important to compare apples to apples. A portable modular unit cannot be compared to a big benchtop unit. It is becoming harder to meet user specifications while trying to take into account the software, portability, and applications needed. Unfortunately, small size usually does not indicate high performance. Ease of use and configurability also tend to be polar opposites. In addition, the component manufacturer's scientific requirements are very different than requirements of the CATV maintenance team, but both need the best possible specifications.

The most important specifications for an OSA are its optical rejection ratio (ORR), dynamic range, wavelength range, and wavelength accuracy. ORR is the ability to measure signal-to-noise ratio (SNR) at a certain distance from a channel central wavelength. ORR is always given at a certain distance (for example, 40 dBc at 0.2 nm from the peak). The "c" in dBc stands for carrier and therefore represents the SNR measurement from the carrier peak power to the noise at the given distance. System vendors are asking for very high SNR measurement while, at the same time, channels are getting closer and closer. Being able to measure higher SNR close to the peak is crucial. Most system vendors require a minimum ORR of 40 dBc at 0.2 nm (see Fig. 4).

Dynamic range represents the ability to measure full range from low power to very high power. With new technology such as Raman amplification and the use of higher power sources, the maximum input power is becoming important. Total power can easily surpass 20 or 25 dBm. Sensitivity is important when measuring insertion loss of optical devices such as filters, couplers, multiplexers/demultiplexers, and wavelength locker.

Wavelength range is a necessity. If you want to test a WDM system that runs channels in the C band only (1530 to 1565 nm), then your OSA needs to cover 1530 to 1565 nm. If the system runs 1310 nm with 1550 nm (S and C bands) and 1625 to 1650 nm (L band), then your OSA needs to cover 1300 to 1650 nm. Taking into account that most vendors are expanding their bandwidth capacity, an OSA that covers all three bands is a must for future deployment.

OSAs are not known for their wavelength accuracy. Wavelength accuracy is the strength of the multiwavelength meter (MWM) based on interferometric technology. You can expect an OSA to have an accuracy of 0.02 nm. Under similar circumstances, an MWM will have an accuracy of 0.001 nm, although measurements of ORR and dynamic range will be diminished. Because an OSA is used for its power capacity, 0.02 nm is sufficient for most applications. New technology such as MEMS could change this need for compromise between good power measurement (ORR and dynamic range) and wavelength accuracy.

Most platforms can accept only one or two modules at a time, working with independent software. The most advanced platform will offer the possibility of multiple modules working together with a single, integrated software solution. Sophisticated software capable of processing the data of every module to create a single database with a single interface will be required to minimize test time and maximize data management. Software will be based on applications instead of test modules. For example, software for fiber characterization will include bidirectional OTDR and optical loss test set (OLTS) measurement, ORL, PMD, chromatic dispersion, nonlinearity problems, and more.

MORE TEST EQUIPMENT COMING
To test your system or component in depth, you need a variety of test equipment and measurement capabilities: OSA, MWM, PMD analyzer, chromatic dispersion, OTDR, OTDR vs. wavelength, polarization-dependent OTDR, OLTS vs. wavelength, PDL vs. wavelength, tunable laser, ASE broadband source, filter, power meters, chirp analyzer, and all types of electrical analyzers. Some of this equipment is now commercially available; some is in development, and the rest is still on the drawing board.

Obviously, it can be difficult to identify which equipment you really need. Not everyone needs everything—probably not even half of the test equipment mentioned above. But one thing is certain: the more complex systems and components become, the more complicated the test equipment will become as well. We strongly recommended that test and design teams keep their testing requirements in mind (e.g., maintenance requires less testing than production or development) and confer with a vendor's representative to be sure they make the best purchase for the best price based on their needs.

Ghislain Lévesque is a product manager at EXFO Electro-Optical Engineering, Inc., 465 Godin Avenue, Vanier, (Québec) G1M 3G7, Canada. He can be reached at (418) 683-0211.