Accurate measurements in the L-band require added features

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Graham Sperrin

Equipment makers have opened the L-band for WDM transmission. This additional capacity creates challenges for taking L-band measurements with an optical-spectrum analyzer to ensure wavelength accuracy and resolution.

Capacity demand has created the need to open other wavelength windows beyond the conventional or C-band (1525 to 1562 nm). The long or L-band (1565 to 1620 nm) is the next obvious choice. Working in this window requires greater management of chromatic dispersion. In addition, for transmission in the L-band to be successful, optical networks require advanced test capabilities.

Achieving high transmission rates in the L-band is accomplished through a variety of technologies. The problems of chirp arising from direct modulation of lasers and polarization-mode dispersion in the fiber have been addressed by using external modulators and scrambling, respectively. External modulators, which will play a key role in the development of high-speed transmission, are being integrated with distributed-feedback lasers.

Techniques such as gain pre-distortion and pulse shaping are used to equalize optical-amplifier response (passband) and nonlinearity in the L-band. The next challenge is to optimize optical-fiber design to widen the window of transmission, as well as to address the issue of nonlinearity caused by the high powers resulting from multiple channels. The latter requires the use of special large mode-field fibers.

Recently, special fibers have been produced that have a specific fixed chromatic dispersion rather than a design that sets the minimum limit. Such a technique solves the problem of four-wave mixing, which results from multiple channels with equal channel spacing. The issue of dispersion can be addressed by dispersion management or compensators distributed or localized at the transmit or receive ends of terminal equipment.

INSTRUMENTATION AND TESTING
Technological advances have addressed many network capacity issues, but a significant challenge to expanding into the L-band is testing the network. Typical test instrumentation used to analyze optical fiber includes power meters, optical time-domain reflectometers (OTDRs), chromatic-dispersion analyzers, and optical-spectrum analyzers (OSAs; see Fig. 1).0601feat6 1

Wavelength-division-multiplexing transmission technology requires quantitative measurement of the signal quality and wavelength-

transmission characteristics of each channel. Instruments for this purpose require highly accurate wavelength and level measurements. Furthermore, accurate measurement of a fiber amplifier's noise figure requires extremely good polarization-dependent loss characteristics, and level linearity specifications. With the extended spectral reach into the L-band, higher performance is also required from OSAs, including increased wavelength accuracy, a resolution bandwidth accuracy of 3%, level flatness (±0.1 dB), and level linearity.

An OSA can detect and identify faults on a DWDM channel. It measures peak power, total power, optical signal-to-noise ratio (OSNR), channel wavelength, and the redistribution of erbium-doped-fiber-amplifier gain in the event of a single channel or multiple channels going down. With close channel spacing, wavelength stability and accuracy are important because small variations can cause a loss of a channel or more.

There are testing limitations when working in the L-band. Much of the traditional optical test instrumentation will not function at all in some cases. Other instrumentation will have inaccuracies, noise, instability, poor sensitivity, no traceability or calibration, and temperature drift. Power meters need to be checked for performance in this region.

Several techniques can be applied to existing OSAs to improve performance for testing channels in the L-band. The testing challenges are created by macro bending, the detector material, and the need for a different wavelength-scanning technique. Fortunately, several instruments have been optimized to provide high performance in the L-band for accurate and reliable measurements.

A number of criteria must be considered when selecting an OSA for L-band analysis. The dispersion technique—also known as the grating technique and packaged in a double-pass monochrometer—should provide a tuning range that covers the extended bands with dynamic range greater than 70 dB and high accuracy. Such high dynamic range is important for resolving channel separation and measurement of OSNR between close channel spacing. This dispersion technique for spectral analysis is extremely versatile and can be made portable. Other techniques, such as Fabry-Perot filters, have a restricted wavelength range. OSAs utilizing Fabry-Perot filters generally exhibit a tradeoff between dynamic range, resolution, and wavelength span.

PHYSICAL LAYER TESTING
It's important to note that instruments designed to cover only the C-band (in terms of wavelength coverage as well as accuracy) are not adequate to support the new generation of DWDM networks. New demands are put on the network at the L-band by the level of dispersion, macro bending, enhanced optical amplifiers, and dispersion management or dispersion-compensating fiber.

To support DWDM applications, advanced test instruments are needed to determine the health of an optical network. This capability will become even more important as we move into all-optical networking with wavelength routing, in which true optical transparency will exist. Optical transparency may not be deployed for several years, but wavelength routing exists now.

From the maintenance perspective, there are two types of testing at the physical layer to identify minor problems: testing the health of individual channels and testing active components. Long-term testing of channel health challenges network managers who are using remote fiber testing because they must look for trends in the parameters that indicate the capability of the network to carry digital traffic. For example, bit-error rate checks will identify a fault before results can be reported by physical measurements, but an accumulated history of these measurements can indicate the likelihood of possible service interruption.0601feat6 2

The Q measurement is a quality measurement of the network and can support live traffic monitoring when using an autocorrelation technique (see Fig. 2). The Q measurement is based on the measurement of bit errors and can provide up to 15-dB advanced warning of system performance degradation. Software has been developed in addition to an autocorrelation technique to accurately make Q measurements using a bit-error-rate test system. Previously, such tests were made with an oscilloscope and were not accurate, nor were they capable of monitoring live traffic.

In addition to bit-error rate or Q measurement, physical parameters to be monitored include optical channel spacing, optical power level, crosstalk, central wavelength drift, channel drift, amplifier noise, OSNR, gain, and gain tilt (flatness of optical-amplifier gain with wavelength). The deviation from a reference recorded earlier or at installation can be used to identify such a change.

The second type of testing is troubleshooting faults. A portable instrument that can be taken to a remote site must be able to measure many of the physical parameters mentioned above with a minimal amount of expertise required by the field personnel. The deployment of OSAs into the field over recent years to work alongside OTDRs has become a common occurrence. Both of these instruments are designed specifically for such applications and combine light weight with simpler operation.

There is a need for one-button operation that brings full characterization of the network, automated measurements with preset thresholds for pass/fail, and long-term testing modes with recording of variations with statistics over time, specifically for wavelength and OSNR. The importance of the measurement performance and range of an OSA should not be sacrificed at the expense of weight and size. There are instruments on the market that offer a portable package with high performance and range, which are required to characterize next-generation networks.

Graham Sperrin is product-marketing manager, North American region operation, Anritsu, 1155 East Collins Blvd., Richardson, TX 75081; he can be contacted at graham.sperrin@anritsu.com or 1-800-267-4878.

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