DWDM Systems — field test equipment

Mar 1st, 2001
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Robert McMahon

Selecting the appropriate laboratory, manufacturing, and field test equipment depends upon application requirements. All equipment must provide optical-power measurements, but lab equipment includes options for experimentation while field equipment is mostly used for specialized measurements and documentation, and must be portable and easy to use.

Field testing is a requirement for those implementing dense wavelength-division multiplexing (DWDM) technology. Compared to typical laboratory test equipment, the field test equipment for this technology is designed for specific requirements while being easy to operate and adapted to the outdoor environment in which portability, size, and ruggedness are required. This article expands on last month's article (see WDM Solutions, Februay 2001, p.85) and will discuss the equipment and field testing of DWDM systems as compared to typical laboratory and manufacturing test equipment.

DWDM optical-fiber communications systems have been developed using new or older optical fiber, and systems assembled from components and subassemblies. These transmission systems were developed and tested in laboratories and manufacturing facilities to meet ITU, Telcordia (Bellcore), vendor, and user specifications. Testing the systems in the field requires that specific measurements be made during the installation to verify the specifications. Personnel must accomplish these tasks using existing or new generations of field test equipment, and new monitoring systems for maintenance applications are being developed.

One of the challenges associated with DWDM technology is anticipating potential maintenance problems. For example, component aging may be responsible for most of the anticipated system maintenance issues, while fiber-related problems still affect the overall system performance and reliability.

DIFFERENCES IN EQUIPMENT
Several types of test equipment are necessary and each has its own set of requirements.

Laboratory equipment. Laboratories responsible for developing DWDM systems are constantly upgrading and buying new- generation test equipment with the best performance to develop the highest quality components. Laboratory test equipment must also be able to handle complex R&D experiments. The wide range of research experimentation indicates that some of the more useful laboratory test equipment is complicated to operate because of all of the versatility required.

Clearly, laboratory equipment provides levels of measurement and complexity not needed (for typical test requirements) by field technicians and maintenance personnel. Therefore, an evolutionary set of test equipment has been established to provide test equipment for use in networks and by outside plant technicians (see Fig. 1).

Manufacturing test equipment. Manufacturing test equipment is primarily used to determine if the components and systems are within the desired specifications. These tests are geared for high-volume production, and the test equipment needs to be semi-automated or totally automated to perform consistent measurements. As an example, the Telcordia qualifications require that the components be tested under specific environmental conditions. Telcordia's requirements are standardized to ensure the instruments will perform at optimal levels in the field. The Telecommunications Industry Association (TIA) has standardized fiberoptic test procedures so that the methods of measurements and the results are repeatable.

Field test equipment. Over the last several years test equipment has been developed for use in the field (such as the optical spectrum analyzer). Important specifications can now be verified during the installation and troubleshooting of DWDM systems. Field test equipment must be maintained in the outdoor environment, which can range from very hot to very cold, with humidity reaching 100%. This equipment must be able to make repeated measurements that are stable, accurate, and consistent and are independent of changes in temperature and humidity

During installation or troubleshooting, the field test equipment's measurements are required for verification that the optical-fiber systems components are performing to their specifications after deployment. Field test equipment is typically designed for simple operations by performing a narrower range of measurements. Most field test equipment is equipped with a "hard copy" function to record results (see Fig. 2).

UNIVERSAL TEST EQUIPMENT
Some test equipment used to maintain DWDM systems is designed to perform measurements not only in the laboratory but in manufacturing facilities and in the field. The measurements are usually very simple. For example, one of the universal test instruments—called the fiberoptic backreflection meter—is used to measure fresnel reflection most often caused by connectors. A laser is pulsed and the magnitude of the light being reflected back is measured. This instrument is used not only in the field but in the laboratory as well. On the other hand, an optical-spectrum analyzer (OSA) is a complicated measurement tool with many adjustments, such as resolution, frequency range, bandwidth filter, and sweep time.

The OSA and the wavelength meter both measure the wavelengths versus amplitude of the optical power. The OSA is quieter and the noise floor is lower than the wavelength meter. The wavelength meter has a measurement accuracy of approximately 0.003 nm, and the OSA has a measurement accuracy of approximately 0.5 nm. Consequently, the measurement will be more accurate with the wavelength meter.

Optical-power meters used for testing DWDM systems need to be calibrated for the wavelengths being measured (980, 1310, 1480, 1550, 1625 nm). The calibration is necessary because of the nonlinear effects of the output current versus the wavelength of light. The power meter also requires a detection range that is able to measure the high power levels (plus 17 dBm) of the erbium-doped fiber amplifiers. As a result, the 980- and 1480-nm calibration points are critical for accurate measurements (see table).

IN-LINE DWDM MONITORING
One network-testing arrangement currently available is the automatic in-line optical fiber testing system. The DWDM system can use a tunable laser in the monitoring system that would monitor the path of the different wavelengths of lasers. The DWDM system can also have a single wavelength laser, which could monitor the long distances of optical fiber. Optical networks can now be continuously monitored to quickly recognize and resolve potential problems using remote test unit (RTU) connected to the optical-fiber system. The RTU contains a computer, memory, data lines, optical switches, and an internal optical time-domain reflectometer (OTDR). The program has the capability and the information necessary for testing without the need for personnel to be present to test the many optical fibers and cable spans.. The OTDR is connected to the system through a 1XN optical switch that is connected to either dark fibers or uses the 1625-nm wavelength on fibers in use via a DWDM coupler. The network is usually configured with the OTDR at centralized hubs or switch sites so it can access as many cable runs as possible.

A computer inside of the RTU is programmed with the original measurement data. The OTDR takes new measurements that are then compared to the original measurements to determine if any problems exist. If any of the new measurements are out of specification, the RTU can determine the location of the mismatch, and relay the new data and alarm to the selected maintenance facility.

In the future, new generations of test equipment with improved measurement capabilities will be introduced. This equipment will be more sophisticated as a result of the increased complexity of the DWDM system compared to other communication networks. Yet the equipment still must address issues including increased performance while maintaining simplicity and reliablility in the service provider's environment. The constant advance of DWDM systems will drive the need for better test equipment with greater performance requirements as a result.

Next month's article will discuss optical-spectrum analyzers, wavelength meters, and testing of semiconductor optical amplifiers and erbium-doped fiber amplifiers.

Robert McMahon is a research engineer with The Light Brigade. The Light Brigade develops fiberoptic training material and teaches fiberoptic training courses. For more information, contact www.lightbrigade.com.

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