Testing fiber-optic premises networks certifies proper installation

Sept. 1, 1995

Testing fiber-optic premises networks certifies proper installation

A newly installed fiber-optic premises network must be measured for end-to-end operation to ensure overall integrity and long-term performance

todd jennings

siecor corp.

Reliable, established and field-proven test procedures are necessary to certify that a fiber-optic premises network is correctly installed. Proper testing ensures system longevity, minimizes downtime and maintenance, and facilitates upgrades and reconfigurations. Documented test results quantify network quality, identify cable and component faults, and prescribe accountability when multiple vendors are involved.

The recommended tests should be made in accordance with Annex H, Optical Fiber Link Performance Testing of the American National Standards Institute/Telecommunications Industry Association/Electronic Industries Association-standard 568A. Because the annex addresses only an end-to-end attenuation test, additional information is provided on proven test methods and common field-test practices.

End-to-end attenuation and optical time-domain reflectometer tests accurately measure the performance of an installed fiber-optic cable system. In preparing for field premises network tests, the following general guidelines are important to ensure accurate test results:

Make sure the test jumpers (for end-to-end attenuation testing) or the test fiber box (for optical time-domain reflectometer testing) are of the same fiber core size and connector type as the cable system--for example, 62.5-micron core test jumpers should be used for testing a 62.5-micron multimode cable.

Ascertain that optical sources are stabilized and have center wavelengths within 䔸 nanometers of the 850/1300-nm multimode and 1310/1550-nm singlemode nominal wavelengths. In accordance with TIA/EIA-526-14A, multimode light-emitting diode sources should have spectral widths from 30 to 60 nm at 850 nm and 100 to 140 nm at 1300 nm.

Ensure that the power meter is calibrated at each of the nominal test wavelengths and traceable to the National Institute of Standards and Technology calibration standard.

Set the network test tools, such as the power meter and the light source, to the same wavelength.

Clean all system connectors, adapters and jumpers before making measurements.

End-to-end attenuation test

Attenuation, defined as the optical power loss measured in decibels, is the primary parameter field-tested in fiber-optic premises networks. Cables, connectors, splices and patch cords all contribute to a network`s overall attenuation. Additional losses might be induced by tight bends or excessive forces placed on fiber-optic cables while they are being transported or installed. Testing must be done after installation to ensure that the fiber-optic network meets the attenuation specifications established by the end user.

The most important test of an installed fiber-optic link is end-to-end attenuation. This test measures the optical power loss between cable termination end points. Acceptable loss values depend on cable length, signal wavelength, and number and type of connectors and splices.

End-to-end loss should always be less than the link-loss budget calculated during the network system design phase. The best way to verify that fiber-optic cable meets the loss limit is to measure each link segment from one patch panel to another. Because of the stress and bending that fiber-optic cables undergo during installation, the attenuation of each connectorized link should be certified after installation.

Attenuation is measured by comparing the difference in optical power level tests. The power levels are measured in decibels relative to milliwatts. By definition, 0 dBm is equivalent to 1 milliwatt of power. Virtually all optical power levels in fiber-optic premises applications are negative dBm values, indicating levels less than 1 mW. For example, a typical light-emitting diode has an output power of approximately -20 dBm. Loss values in decibels are determined by subtracting two absolute power levels in dBm.

End-to-end attenuation testing of installed fiber-optic cable systems is performed using a three-step insertion-loss method. A stabilized light source and an optical power meter are used to compare the difference in optical power levels.

In the first step, the light source transmits a reference power level over a test jumper cable to the power meter. Next, the light source delivers a check power level via its test jumper connected to an adapter and a second test jumper connected to the power meter. This check power level must be within 0.5 dB of the reference power level. Otherwise, do not proceed to the third step until all connectors, except the light source connector, have been cleaned.

In the last step, disconnect the two jumpers at the adapter. Attach the optical light source test jumper to the input end of the network`s fiber-optic cable to be tested. Attach the optical power meter test jumper to the exit end of the network`s fiber cable. Record this level as the test power level. Calculate the system attenuation as the reference power level minus the check power level.

Best results are achieved with factory-terminated test jumpers. The test should also be conducted in accordance with relevant TIA/EIA specifications. Multimode fiber is tested using TIA/EIA-526-14A, Method B; singlemode fiber uses TIA/EIA-526-7, Method A.1.

The test procedure includes the representative connector loss at the connecting hardware associated with the mating of patch cords. It does not include the connection at the transmission equipment interface, which is normally accounted for in the acceptable loss budgets of the transmission electronics.

End-to-end attenuation tests should be performed at both specified wavelengths for every connectorized fiber in backbone fiber-cable segments. Multimode fibers should be tested in one direction at 850 and 1300 nm, and singlemode fibers should be tested in one direction at 1310 and 1550 nm to account for attenuation differences due to wavelength. The attenuation of a system at one wavelength is not necessarily related to that of the other. Some end users require bidirectional loss testing to meet transmission equipment certification or site standards.

Acceptable link attenuation relies on the backbone length and the number of splices. Attenuation figures are based on fiber-optic cable loss, two connector pairs and no splices per TIA/EIA-568A. If the link contains splices or additional connector pairs, add 0.3 dB per splice point and 0.75 dB per connector pair.

For horizontal fiber cabling, the short lengths of the multimode segments--typically less than 300 feet--result in connector-dominated loss that is independent of wavelength. End-to-end attenuation testing is typically required in only one direction at a single wavelength, typically 850 nm, with loss less than or equal to 2.0 dB for each fiber link.

Test tools for attenuation testing

The primary tools used to perform end-to-end attenuation tests are an optical power meter and an optical light source. Optical power meters measure optical power levels in dBm and losses in dB. Although some models measure either multimode or singlemode fibers, most devices can measure both fiber types at 850-, 1300-, 1310- and 1550-nm wavelengths.

Optical power meters capable of storing multiple reference values suit dual-wavelength, end-to-end attenuation testing. Interchangeable connector adapters allow adaptation to a variety of connector types. Other features include ruggedness, handheld packaging for field use, backlit graphic display for use in dark wiring closets and radio-frequency shielding to prevent error-causing interference from electronic equipment.

Optical light sources are used to launch a stabilized light of steady output power and known wavelength into the fiber. Light-emitting diodes at 850 and 1300 nm are used for multimode testing; 1310- and 1550-nm lasers are used for testing singlemode fiber systems.

When performing bidirectional attenuation testing, a pair of optical testers that combine an optical meter and an optical source in the same unit are commonly used to save test and travel time. A pair of optical testers can also be used on duplex connector systems to test both fibers simultaneously and ensure proper polarity. A single optical tester can be used to inspect cable on the reel, test cable assemblies and measure looped-fiber attenuation.

To determine what causes end-to-end attenuation loss and where it occurs in the fiber-optic cable system, an optical time-domain reflectometer is needed. This instrument can locate fiber events and measure the losses attributable to cables, connectors, splices and other components. The graphical display of loss over a length of fiber cable provides analysis and documentation and is commonly referred to as the link`s signature trace.

Because the reflectometer can provide a detailed analysis of the installed components with instrument access to only one end of the fiber, it can be used in a variety of scenarios:

Cable acceptance. The device evaluates the integrity, overall length and fiber attenuation in decibels per kilometer for fiber cables before and after installation. It is useful for checking a cable against specification, uncovering point defects that result from handling during transportation or installation, and measuring unterminated fibers.

Signature trace documentation. The instrument provides useful documentation for fiber-optic cable system acceptance, network planning and maintenance as the "as-built" fiber blueprint.

Connector and splice loss. The reflec- tometer measures and documents field-installed connector and mid- span mechanical or fusion-splice losses. This measurement is used to determine whether a splice or connector is acceptable or needs to be reworked.

Troubleshooting. The instrument provides a benchmark of initial system performance for comparisons over time and a tool for identifying and locating cable problems or breaks. Fiber discontinuities and localized losses become visible when compared to original signature traces.

Although attenuation testing confirms the end-to-end loss, optical time-domain reflectometer analysis can document the integrity of the fiber-optic cable system, locate and measure each event or component, and uncover faults throughout the cable.

Before fiber is cut from a storage reel into sections and various cable segments are installed, the tester can provide proof of an optical fiber cable`s initial quality and integrity. With a splice or bare-fiber adapter to access one cable end, an optical time-domain reflectometer can be used to verify the length and attenuation of each fiber. The instrument can also detect point faults or discontinuities that develop during shipping and handling. This test inspection provides important cable information when several vendors are involved in an installation.

After installation and termination of a fiber backbone network, the testing device can test each fiber at one wavelength (850 or 1300 nm for multimode, 1310 or 1550 nm for singlemode) on all backbone segments longer than 300 feet. For point-to-point connectorized links, a reflectometer inspection can verify installed cable integrity and length. For more complex links that include a splice or mid-span connection point, the instrument should be used to document the signature trace of each fiber as a computer file or a printout. Some end users require dual-wavelength (850/1300 nm for multimode, 1310/1550 nm for singlemode) or bidirectional testing to meet transmission equipment certification or site standards.

In addition, for backbone segments, measurements should be made of each field-installed connector and mechanical or fusion splice at one wavelength to ensure they meet acceptable loss values. Unless otherwise specified, acceptable losses are less than or equal to 0.75 dB per mated connection and less than or equal to 0.3 dB per splice.

To measure near-end connector loss, a test fiber box of sufficient length should be used to connect the reflectometer and the patch panel. The fiber length in such a box is typically greater than or equal to 90 meters for multimode or greater than or equal to 200 meters for singlemode. Use of a test fiber box also allows simultaneous testing of a link`s signature trace and near-end connector loss. These test results can be documented together on an optical time-domain reflectometer that provides event tables.

Based on the short length of and easy access to horizontal fiber segments, reflectometer testing is not required for certification. When the end-to-end attenuation of a horizontal link exceeds the 2.0-dB loss limit, the device can be used to inspect and troubleshoot links of 100 feet or more to isolate a faulty connector or cable.

Bandwidth and dispersion are measures of the information-carrying capacity of multimode and singlemode fibers, respectively. Actual system bandwidth or dispersion is a function of fiber quality and length and system transmitter characteristics. Because this capacity is not adversely affected by installation, field testing of these parameters is not required. It is common practice to specify cable bandwidth and dispersion performance to meet the requirements of TIA/EIA-568A, ensuring compatibility with transmission electronics without field testing. The manufacturer`s bandwidth and dispersion performance should be documented on a specification sheet and saved for future reconfigurations and upgrades. u

Todd Jennings is product manager of optical test equipment at Siecor Corp., Hickory, NC.

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