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Software tools improve fiber-optic network testing: fundamental test tool gains new functionality. (Testing and Diagnostics).

Network operators are being challenged to deploy sophisticated new fiber-optic communication systems quickly and keep them running at peak performance in order to meet intense demands for reliable high-bandwidth services. There are more links to install and maintain, new and more complex parameters to check, and more potential disruptions to track down.

New functionality in the fundamental test tool for fiber-optic cable plants--the optical time domain reflectometer (OTDR)--is proving to be a significant source of help for technicians in light of today's market challenges. Software enhancements are transforming OTDR testing with powerful new data-processing tools and techniques that allow even the novice operator to analyze the fibers quickly and completely, and to detect subtle features with ease.

While basic OTDR concepts are simple, specific measurements can be complicated. Reflected optical power is a tiny fraction of transmitted pulse power (typically one-millionth)--and it varies greatly with wavelength, cable length, fiber backscatter coefficients, and splice and connector characteristics.

In order to optimize fiber measurements with an older, manual OTDR, measurement parameters had to be carefully selected based on the mode, length and attenuation of the fiber under test. For all but the shortest fibers, the optimal parameters also vary in relation to the distance of the event from the instrument. Newer OTDR instruments incorporate software programs that automatically sense and configure the optimum test parameters, and present results in simple formats.

Most fiber-optic cables require multiple OTDR measurements using different parameters to completely and accurately characterize their proper ties. These tests take time, and time can be a precious commodity during a network emergency or a lengthy commissioning process.

To address the problem of close-range resolution vs. long-range visibility, acquiring several sets of waveforms, using different OTDR settings, often is necessary. After completing the first scan with a short-duration optical pulse, a longer-duration pulse is selected to provide additional optical power to test further along the fiber. When the testing is completed--after two, three or more acquisitions with successively longer pulses--the traces must be laid out next to each other to get a complete picture of the fiber.

Newer OTDRs incorporate built-in testing programs that automatically characterize the fiber in a sequential manner, starting at the instrument-to-fiber connection and working outward. Such programs automatically determine which parameters need to change, based on criteria like signal-to-noise ratio, length, total loss and elapsed time. They may increase the number of averages, change the filtering, or adjust the gain of the detection circuitry in order to optimize the test results for each specific cable segment.

Slight bends, fusion splices and closely spaced connections can be difficult to locate and measure, especially when they occur further down the cable. Background noise often masks the reflections from these subtle events, even after many averages.

Waveshape-analysis software can improve the ability of an OTDR to characterize events when they are masked by noise, and can identify closely spaced events in the dead zones following previous events. This method of data processing uses algorithms that discover inflection points in the data.

As a rule of thumb, line-fit algorithms can only locate and measure events whose magnitudes are at least twice that of the noise in the data. Waveshape-analysis algorithms can accurately locate and measure events whose magnitudes are as low as one-half that of the noise. This improvement is the result of using inflection points to detect the shape of the whole data and calculate the shape of the curve. Because noise is random, it converges to an envelope around the proper shape after a minimal amount of averaging. This technique also yields more accurate amplitude measurements.

Waveshape analysis improves the precision of location (distance) measurements over line-fit techniques by clearly identifying the leading edge (distance to event) between sample points. For previous OTDR's, best-case distance resolution is limited to the sample spacing. An OTDR with waveshape analysis can locate events to within one-tenth of the sample spacing.

Many other software enhancements have been introduced to the acquisition, analysis and archiving of optical test data, making the OTDR an even more valuable asset for technicians to meet the challenges of supporting fiber-optic cable plants.

Circle 254 for more information from Tektronix


The optical time domain reflectometer is an instrument that uses the inherent backscattering properties of an optical fiber to detect and categorize its condition. The OTDR sends high-power pulses of laser light down the fiber and captures the light that is reflected back.

By measuring the time of flight and power level of the pulses, the instrument correlates the reflected information with physical locations along the fiber and displays a trace that shows backscattered optical power vs. distance. Attenuation of the fiber is detected as the slope of the trace. Interruptions, such as splices, connectors or flaws in the fiber, appear as transitions (events) that represent their nature and location.

Because it operates from a single location, the OTDR eliminates the expense and complication of separate transmitters/receivers at each end of the fiber, saving test time and personnel.

The OTDR is used in fiber-optic networks to commission (map and document) the network at installation: verify quality-of-service operating parameters, such as attenuation, losses and reflectance: detect problems, such as cuts, bends and defective splices or connectors: pinpoint the location and nature of events to facilitate repairs; and verify that network repairs have been made properly.

Reunert is product line manager, optical installation and maintenance, at Tektronix, Beaverton, OR,
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Comment:Software tools improve fiber-optic network testing: fundamental test tool gains new functionality. (Testing and Diagnostics).
Author:Reunert, Michael
Publication:Communications News
Geographic Code:1USA
Date:Feb 1, 2002
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