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Advanced OTDR Analysis

Seasoned professionals along with new fiber technicians are choosing OTDRs that automatically select most testing parameters and test against pre-assigned limits based on industry or job-specific requirements.


September 1, 2008  


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The knowledge and skills to manually set up tests, interpret and understand OTDR traces is becoming as obsolete as film has with the advent of digital photography. The latest OTDRs utilize embedded processors running sophisticated software to help users acquire traces and interpr et the results.

Like the best digital cameras, these instruments can usually take a better picture than legacy OTDRs that relied on expert users to set them up and subjectively “process” the results.

An Optical Time Domain Reflectometer (OTDR) is the only tool that can give detailed visibility of loss and reflective instances, which are often called “events.”

An OTDR’s ability to detect events such as connectors, splices, and faults in an optical fiber run depends mainly on its dead zone, dynamic range, maximum range, distance accuracy, loss threshold, linearity, and sampling resolution specifications.

In order to accurately acquire and assess these events, the instrument must be set to the most appropriate pulse widths, averaging time, wavelengths, loss threshold and distance range.

Once a trace is acquired, the determination must be made whether the result is acceptable or not.

Seasoned professionals along with new fiber technicians are choosing OTDRs that automatically select most testing parameters and test against pre-assigned limits based on industry or job-specific requirements. As a result, they are relying on their OTDR to compare event characteristics and pass or fail the trace based on these results. This saves both novices as well as experts an inordinate amount of time and eliminates subjectivity from the testing process.

However is the automatic capability in today’s OTDRs making advanced OTDR analysis a lost art? What are some of the advanced OTDR techniques that expert technicians used to perform during testing?

Similar to a digital photographer’s film camera experience, an understanding of the knowledge and skills used for manual OTDR testing can enhance a technician’s fiber testing when using the latest OTDR with automatic capabilities.

Some tips and tricks of advanced OTDR analysis include:

Keeping connectors and the OTDR port clean. The launch fiber and OTDR port should be cleaned with approved fiber optic cleaning solvent (preferably not isopropyl alcohol) and lint free swabs or dry wipes before connecting -every time. Remember to clean in a straight line or an N shape to avoid recontamination of the end face. A dirty OTDR port will use up valuable dynamic range and make testing difficult and inaccurate. Similarly, a dirty connector anywhere in the fiber channel being tested increases Deadzones.

Starting with Automatic OTDR Settings and use Pass/Fail limits. This may seem contradictory to all the discussion about the power of manual settings. But in most cases, the OTDR will still acquire and analyze traces better and faster than the best technician. The trick is to use stringent pass/fail limits, then use manual to enhance the troubleshooting experience when a failure is identified.

Testing at multiple wavelengths. Always test at both the shorter and longer wavelengths, even if the job or design specification does not require it. This allows comparison of traces for the same fiber at different wavelengths and faster identification of problems such as dirty connectors.

Use bi-directional averaging for increased accuracy. It is common to see a gainer or negative loss value due to a mismatch in backscatter coefficient between the launch fiber and the fiber that is being tested.

The solution and most accurate OTDR testing procedure is to test in both directions on the same fiber, then use software to average the losses recorded in opposing directions.

Using qualified launch and receiving fibers and utilize launch fiber compensation. To accurately measure the first and last connector on a fiber a launch and receive fiber must be used. The launch fiber must be longer than the attenuation dead zone for the maximum pulse width. Keeping the end faces of launch and receive fibers clean and protected from damage is key.

Choosing a results management software package that is easy to use for reporting and analyzing after testing is long over. Test results should be quickly and easily downloaded to a software program on a computer. Customized professional reports should be easy to create, and analysis of tests should be possible. The ability to email trouble traces is also useful.

OTDRs are an important documentation and troubleshooting instrument used by organizations to install and maintain optical fiber.

In addition to troubleshooting, they can examine the performance of each connection, as opposed to an optical loss test set which only shows the sum of all losses. In this way it can improve the quality of an installation and ensure that poor connections are detected and not masked by other very good connections in the fiber link.

New technology has enabled a wide range of automatic capability in today’s OTDRs, but that does not mean a manual approach to advanced OTDR analysis is a lost art. Understanding OTDR traces and how to use manual settings is still beneficial for all testing and required in rare cases where an OTDR can misinterpret events, or miss them altogether, due to improper test parameter set up or improper choice of an OTDR for the application.

In many cases, technicians are limited by their inability to correctly interpret OTDR traces without the aid of the instrument’s software. Understanding how the OTDR and its analyzer work, how an OTDR’s specifications affect its performance and how to properly set it up can help users get maximum performance from their device.

David Green, P. Eng., is Director of Marketing for Fluke’s AmPac Region, including Canada, Australia and Latin America, and has been involved in technical support, sales and marketing of various technologies for communications, automation, testing and troubleshooting of industrial and commercial systems for over 30 years. He can be reached by e-mail at david.green@fluke.com.

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Editor’s Note: This article is the second in a two-part series on OTDR tools and practices and provides an overview of advanced OTDR analysis.