Testing basics

As fibre installations increase, it is necessary for contractors to equip their technicians with better ways to test fibre sections. Harley Lang III, marketing manager at Fluke Networks writes on the benefits of both OLTS and OTDR testing and how they complement each other.

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By  Harley Lang III Published  November 14, 2007

OTDRs display results using a plot or trace of reflected and backscattered light power versus distance along the length of fibre. The Y axis displays the power level and the X axis shows distance. When you read the plot from left to right, the backscatter values decrease because the loss increases as the distance increases on the cable.

OTDR traces have several common characteristics. Most traces begin with an initial input pulse that is a result of Fresnel reflection occurring at the connection to the OTDR. Following this pulse, the OTDR trace is a gradual curve sloping downward and interrupted by gradual shifts. The gradual decline results from Rayleigh scattering as light travels along each fibre section. This decline is interrupted by sharp shifts that represent a local deviation of the trace in the upward or downward direction. Loss events appear as a step down on the plot. These shifts or point defects are usually caused by connectors, splices or breaks. The end of the fibre can be identified by a large spike after which the trace drops dramatically down the Y axis. Finally the output pulse at the end of the OTDR trace results from Fresnel reflection occurring at the output of the fibre-end face.

An OTDR trace makes it possible to certify that the quality of installation meets the design and warranty specificiations for applications.

An OTDR trace makes it possible to certify that the workmanship and quality of the installation meets the design and warranty specifications for current and future applications. If an individual splice or connector does not meet the design specification, the installer can correct it while still on-site with the help of the OTDR.

This explains why Tier 2 testing is becoming a requirement of many installation projects. A complete Tier 1 and Tier 2 fibre certification provides the most comprehensive picture of the fibre installation and proof of a quality installation. If there are any problems later the contractor normally will not be obligated to fix them without charge.

Are both OLTS and OTDR tests needed to test fibre?

This raises the question: if an OTDR is used is an OLTS still necessary? The answer is that an OLTS measurement is still required in nearly every application because it provides a direct measurement of the fibre plant losses and length while these values can only be inferred from an OTDR measurement.

Traditionally, OTDR testing has been performed with stand-alone instruments that cost a significant amount of money and have their own unique and often complex user interface. These stand-alone instruments provide obstacles to equipping technicians to perform Tier 2 fibre certification.

New OTDR modules are available that enable complete Tier 2 testing of fibre links using the same instrument and interface normally used for copper testing. These greatly simplify the task of providing Tier 1 and Tier 2 testing of fibre links. The new generation of OTDR modules enables contractors who are familiar with copper certification to perform extended fibre certification. Users see the familiar copper tester interface, test command, stored setup values and expert diagnostics. This shortens the learning curve and extends the value of the existing copper tester.

Developing a comprehensive fibre certification strategy

Datacom contractors should develop a testing strategy based upon the requirements set by the consultant or network owner and the contractor's available resources, equipment and tolerance for risk. Some end users will require only basic testing and others will require both basic and extended testing.

Inspection and verification tools should be used during installation to minimise simple problems such as dirty or poorly terminated connectors that slow down certification testing.

Technicians should perform certification testing with tools that are easy to use and capable of delivering the needed information in an easy to understand format. Performing basic Tier 1 certification with a light source and power meter ensures that the system meets the loss budget for the immediate applications. Extended Tier 2 certification proves that the cabling and connections were done correctly. It is a good practice to perform both of these tests in both directions and at multiple wavelengths on the fibre.

The increasing volume of fibre installation as well as the higher margins usually associated with fibre installation provides a tremendous opportunity to contractors. Contractors now have the opportunity to generate additional revenues by equipping the same technicians that are now performing copper certification to perform fibre certification as well. Technicians can leverage their existing knowledge of the instrument so relatively little training is required to certify the fibre plant.

Bringing it together

The increasing proportion of network installation jobs involving fibre make it critical for contractors to understand the technologies involved in fibre testing and develop an appropriate certification strategy. Contractors, network owners and fibre system designers need to understand the difference between OLTS and OTDR testing. These technologies serve different purposes and perform a complementary rather than mutually exclusive role in the fibre certification process.

Pq: Rather than being competitive, OLTS and OTDR are actually complementary tools that both play a role in the majority of fibre installation projects.

An OTDR trace makes it possible to certify that the quality of installation meets the design and warranty specificiations for applications.

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