Network-Passive

You can use an OTDR Tester to identify Optical Fiber Cabling faults

If you use optical fiber cables for network connectivity, you ought to know about OTDR Tester! In this article, let us see what an OTDR tester is, why use OTDR testers & some key functionalities, capabilities and parameters about OTDR – Optical Time Domain Reflectometer.

What is an OTDR?

OTDR refers to Optical Time Domain Reflectometer. OTDR is basically used to identify and analyze reflective & non-reflective events that cause disturbance in an Optical Fiber Cable link. An OTDR can identify both expected events (like connectors, fiber ends, splices, etc) and unexpected events (like cable cuts, etc). So, basically you can use an OTDR Tester to test and identify any connectivity problems with Optical Fiber Cable links.

How do OTDR Testers work?

When high power light pulses are sent in to an Optical Fiber Cable (Core), a small portion of this light is reflected back to the source due to small defects in the glass that fibers are made of. This is called backscatter. Reflections can also be caused inside an optical fiber cable (core) due to sharp bends, cracks, splices, cable terminations, cable cuts, etc.

So, an OTDR Tester sends a series of short but high intensity light pulses in to the core of an optical fiber cable and records the power of the reflected light (at various points) as each pulse travels across the fiber cable. It uses specialized pulse laser diodes and high gain light detectors for this purpose.

How are OTDR Test results represented/measured?

The OTDR Trace results are represented as a graph which typically has the distance traveled by the light pulses (in meters) on the X axis and their corresponding power levels (in dB) on the Y axis. This graph can be seen live either on the OTDR Tester screen (or) on a computer screen connected to the OTDR for further analysis and conclusions.

What are OTDR Testers used for?

OTDR testers can be used for some (or) all of the following:

  • Locating fiber cable cuts/ breaks. OTDR can be used to predict the distance (in meters from the source) where the optical fiber cable has been disconnected.
  • Measuring the performance of optical fiber cable connections by detecting various types of signal losses (like point of high loss/ reflectance, end to end link loss, optical return loss, etc).
  • Measuring the distance between two points on a fiber cable link.
  • Identifying unclean/broken connectors.
  • Identifying Sharp bends/ cracks in optical fiber links that affect the signal power.
  • Identifying dissimilar and mismatched fibers (for example, 50 micrometer fiber & 62.5 micrometer fiber) that are connected together affecting the intensity of the light signal.
  • Identifying usage of fiber patch cords with a different/incorrect core size that affect signal strength.
  • Incorrect fiber laying methods which result in some light leaking from the optical fiber cables.

Some Salient points you need to know about OTDR Testers:

There are separate OTDR models for Single Mode (Operating in 1310 nm, 1550 nm predominantly) & Multi Mode (Operating in 850 nm, 1300 nm predominantly). Some vendors offer both in the same model. With some vendors, its possible to make multiple wavelength measurements in a single test.

Its important for an OTDR to support minimum event dead zone value (minimum distance of a segment that can be recognized and tested). This value could range from 0.5m, 0.7m. 1m, 1.3m, etc depending on the vendor. The minimum event dead zone parameter is useful in recognizing short network segments like fiber patch cords etc, individually so that the administrator can have a complete picture of all the components that make the optical fiber link.

Every OTDR comes with a maximum value of distance up to which it can take accurate measurements. For example, for Multi mode, this might be around 3-7 KM & for Single Mode this might be 60-400 KM. This varies with the vendor as well as application (for enterprise networks, the distance might be less and for Metro Ethernet, the distance might be more). The pulse-width of the source signal can be altered based on the distance to be measured. Generally, more the pulse-width, more the distance.

Some OTDR’s come with USB ports so that external hard disks/ storage modules can be attached to it for storing the test results. Some vendors offer RJ-45 Network ports/ Wi-Fi connectivity so that the OTDR can directly connect to the Local Area Network. With a network controlled OTDR, once the initial connectivity to the fiber cable has been made, the OTDR can be controlled from a computer attached to the LAN and the output can be seen from the computer monitor, instead of the smaller OTDR screen.

There are hand held OTDR’s (with a small screen) and some special purpose OTDR’s that are made for specific applications (like identifying cable cuts). These might cost less, and sometimes sufficient for enterprise fiber testing applications. OTDR’s can also be rented for a specific time.

There are some OTDR vendors who offer simple/ easy to use interfaces for OTDR analysis (either through a software module in the computer or in the OTDR itself). They offer interfaces with human understandable results (in a table format) about what the various graphical waveform outputs (events) mean. With some vendors its also possible to overlap an older/averaged waveform with real time waveform for getting a better understanding of the events.

Most of the OTDR vendors offer an ‘Auto Test mode’ where tests can be performed using default test parameters/ results can be viewed using default template values. This enables their easy and simple usage even for inexperienced personnel. The more experienced users who want detailed test reports, might use the ‘Advanced/Manual Test mode’ which allows them to set the individual test parameters according to their requirements.

Generally, its sufficient if OTDR/ Operator is present only at one end of the fiber cable to be tested. But it is advisable to conduct the same test from both ends in order to achieve more accurate results.

OTDR’s can be used for testing/identifying faults in an installed fiber optic cable network as well as certifying/testing new fiber optic cable installations.

Here is a Video that shows How to use OTDR Tester:

 

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