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OTDR CHRONICLES

A critical tool in the certification process, it is the only instrument that allows technicians to perform Tier 2 certification -- a must in today's testing continuum.


July 1, 2008  


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As the use of fiber in premise networks continues to grow, so do the requirements for fiber testing and certification. With the increased usage of fiber networks, there is more demand on technicians and installers to offer certification services.

In today’s competitive environment, it is important that contractors develop a test strategy based upon the requirements set by the consultant, system designer or network owner and their own resources, equipment and tolerance for risk. This demands tools that are easy to use and capable of delivering test results and reports in an easy-to-understand format.

A critical tool in the certification process is the Optical Time Domain Reflectometer (OTDR). It is the only instrument that allows technicians to perform Tier 2 certification — a must in today’s testing continuum.

Tier 1 certification, which is performed using an Optical Loss Test Set (OLTS) is a mainstay for testing fiber optic cabling. An OLTS tests for loss budgets on the fiber link. (An Optical Loss Test Set 1 certification is described in standards such as Telecommunications Industry Association’s TSB140 bulletin entitled “Additional Guidelines for Field- Testing Length, Loss and Polarity of Optical Fiber Cabling Systems.”)

Tier 2 certification is necessary for proving the cabling and connections were done correctly. This goes beyond overall loss budgets to look more specifically at loss budgets for individual splices and connectors. Because OLTS products cannot test at this level, standards organizations are recommending “Extended” or “Tier 2” fiber certification.

OTDRs certify the performance of new fiber links and detect problems with existing fiber links. An OTDR measures optical fiber characteristics; presents a graphical plot of reflected optical power along a fiber; and provides a table listing optical event characteristics discovered in a fiber.

Measurements include total loss, segment, length, as well as characterization of events in the fiber including breaks, connections, splices and tight bends. An OTDR can also measure overall fiber optical return loss and the reflectance of connectors.

An important advantage of an OTDR is its ability to perform single-ended testing and troubleshooting. This means it is needed at only one end of the fiber to run the test. An OTDR can also identify and characterize loss and reflectivity of events not detected by other types of fiber test equipment.

This ability to pinpoint the distance to a fault means you can quickly find their location and make rapid repairs. Some OTDRs include other useful troubleshooting capabilities, such as end-face inspection or a power meter.

An OTDR works by using special pulsed laser diodes to transmit high- power light pulses into a fiber. As the pulses travel down the fiber, most of the light travels in that direction. High-gain light detectors measure any light that is reflected from each pulse.

The OTDR uses these measurements to detect events in the fiber that reduce or reflect the power in the source pulse.

For example, a small fraction of the pulse light is scattered in a different direction due to the normal structure of fiber and small defects in the glass. This phenomenon of light scattered by impurities in the fiber is called Rayleigh backscattering. A certain amount of backscatter is expected based on a fiber’s attenuation coefficient specification.

When a pulse of light meets connections, breaks, cracks, splices, sharp bends or the end of the fiber, it reflects due to the change in the refractive index.

These reflections are called Fresnel (pronounced frA-NEL) reflections. The amount of light reflected (not including the backscatter) relative to the source pulse is the called reflectance.

It is expressed in units of dB and is usually expressed as a negative value for passive optics, with values closer to 0 representing larger reflectance, poorer connections and greater losses.

OTDRs display trace results by plotting reflected and backscattered light versus distance along the fiber as shown in figure 1 on p. 22. The Y axis represents 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.

OTDR traces have several common characteristics. Most traces begin with an initial input pulse that is a result of a Fresnel reflection occurring at the connection to the OTDR. Following this pulse, the OTDR trace is a curve sloping downward and interrupted by gradual shifts. The gradual decline results from backscattering as light travels along the fiber. This decline may be interrupted by sharp shifts that represent a 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 fiber 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 reflection occurring at the output of the fiber-end face.

An OTDR trace is valuable because it makes it possible to certify that the workmanship and quality of the installation meets the design and warranty specifications for current and future applications. For example, common requirements are that the loss associated with a splice should be no larger than 0.3 dB and that associated with a connector should be no more than 0.75 dB.

While these event losses are completely invisible to an OLTS, the performance of each splice and connector can be measured with an OTDR. If they do not meet specification, they can be corrected during the installation process before the network is live. Many contractors perform Tier 2 certification as preventative maintenance and to document their workmanship on a completed installation.

Another recent development in fiber optic testing is the availability of OTDR modules for copper cable analyzers. OTDR modules greatly simplify the task of performing Tier 2 testing of fiber links.

Anyone familiar with copper certification can now easily perform Tier 2 fiber certification because they see a familiar user interface, commands, and diagnostics. This shortens the learning curve and extends the value of the existing copper tester.

Choosing the right OTDR for the job is critical. A variety of OTDRs are available, each claiming high performance, fast testing, and ease of use. However, OTDR specifications, interpretation, and measurement methods can be inconsistent. Informed buyers must not only understand OTDR specifications, but also what is important for their intended application.

Following are some guidelines on selecting an OTDR:

Understand the primary use — Contractors performing extended certification will want a simple, easy-to-use OTDR that has the same user interface as their other fiber and copper certification testers. Features such as test limits, report generation, and efficiency are very important. Network owners performing maintenance and troubleshooting on the other hand will want an OTDR that has multiple functions for keeping the network running and performing moves, adds, changes. In particular, since the fiber lengths in premises’ fiber networks and data centers are short (and they include patch cords), it is essential to choose an OTDR with short “dead zones.”

Understand your user — If only the most experienced technicians or installer will use the OTDR, a complex unit is appropriate. If a wider range of people will be using it, choose one that is easier to learn and operate.

Factor in productivity needs — The time that it takes to operate an OTDR varies from instrument to instrument. An OTDR may meet performance specifications and be less expensive, but not deliver the expected return on investment once it is put into use. The tester should not waste a technician’s ti
me on complicated set-up and operation. During critical troubleshooting, you do not want to have users waste time trying to remember how to make the OTDR work or to set it up correctly.

Consider the design and ease-of-use Ensure that the device has specifications for shock, vibration and drop testing. Handle it to see if it is easy to handle and carry. Also confirm the modularity to ensure that you can upgrade the unit as technology evolves. Take the unit for a “test drive” to make sure the buttons and menus are easy to navigate; as well as to determined which processes are automated vs. manual (e. g. event analysis, test parameter selection, etc.).

As fiber network speeds increase, more extensive cable testing is required. Specifications for new installations demand extended fiber certification in addition to insertion loss testing order to ensure the quality of the cabling infrastructure. And many technicians need a quick, accurate way to troubleshoot problems and get a complete picture of the network that they maintain.

Choosing the right OTDR for your needs is important. Some OTDRs are optimized for contractors performing certification testing.

Others are great for network engineers and technicians maintaining fiber cabling on corporate campuses and in enterprise datacenters. And many OTDRs are designed only for testing long-distance optical fibers, which makes them a poor match for testing and troubleshooting premises fiber networks.

Therefore it is important to perform the research into available options so you can make an informed decision on the OTDR that’s right for your needs.

David Green 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.

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Editor’s note: This article is the first in a two-part series on OTDR tools and practices. The next will provide an overview of advanced OTDR analysis.