Definition: Optical Time Domain Reflectometer

Optical fibers have opened up gateways to very high speed long-distance communications. As with any technology however there are limitations and various complications. One such complication has to do with the frailty of the materials used in the cables.

When dealing with optical fibers one must worry about lack of integrity of the cable and the subsequent data loss. This is where a piece of equipment known as an Optical Time Domain Reflectometer (OTDR) comes in. The OTDR is designed to test optical fiber lines for faults. By sending a pulse of light into the fiber optic cable and measuring the intensity of reflected light, an OTDR can show a user where faults, connectors, splices, and other such ‘events’ occur. Since the speed of the pulse is known, an OTDR can pinpoint how far along the line an event occurs. One could even use the OTDR to measure the length of a fiber optic cable by looking for the event where reflection drops to zero.


Figure 1: from http://www.techoptics.com/pages/OTDR/How%20it%20works.html

The image above shows an example of an output from an OTDR. The spikes in the output reveal ‘events’ such as a connector or splice. Connectors are characterized by a spike (reflection), and then a drop in overall power. A splice is characterized by the short drop in output intensity. To actually get the correct value here, one would take the drop in power and divide it by two to get the overall power drop at that event.

Something important to keep in mind when reading the output of the OTDR is differences in scatter coefficients of different fiber optic cables. When splicing two cables together, the cables could potentially have different scatter coefficients (a different amount of light will be scattered) which means that one might reflect more light than the other.


Figure 2: from http://www.jimhayes.com/OTDR/otdrs_c.htm

As shown in the image above, the output can be affected by this difference in scatter coefficients. One way to account for this and also some other similar errors is to take measurements from both ends of the cable and average the results. Doing so cancels out the error, giving you filtered output.

There are yet more quirks to take into consideration when using an OTDR. These include, but are not limited to ghosts, resolution limits, and factors involved in multimode fibers. An example of a ghost in the output is shown below.


Figure 3: from http://www.jimhayes.com/OTDR/otdrs_c.htm

A ghost occurs when light reflects from the end of the fiber and “reflects back and forth in the fiber until it is attenuated to the noise level” (Lennie Lightwave's Guide To Fiber Optics). These generally occur at multiples of the length of the cable.


Figure 4: from http://www.jimhayes.com/OTDR/otdrs_c.htm

Resolution is limited with the OTDR. An OTDR cannot distinguish ‘events’ if they occur within a pulse length. This means that errors can exist in the cable which the OTDR cannot detect distinctly. A drop would be detected either way, but the OTDR would see the two events as just one event.


Figure 5: from http://www.jimhayes.com/OTDR/otdrs_c.htm

In the case of multi-mode fibers, the OTDR laser has problems detecting certain events since the laser sent out travels along the inside of the fiber and therefore does not interact with the edges where distortion would normally occur.


  1. "How Does OTDR Work? – The Blogs at HowStuffWorks." The Blogs at HowStuffWorks. Web. 21 Oct. 2011. <http://blogs.howstuffworks.com/2009/04/09/how-does-otdr-work/>.
  2. "Lennie Lightwave's Guide To Fiber Optics - OTDRs -Home." Jim Hayes' Home Page. Web. 21 Oct. 2011. <http://www.jimhayes.com/OTDR/index.html>.
  3. "Optical Time Domain Reflectometers." TechOptics.com. VDV Works. Web. 21 Oct. 2011. <http://www.techoptics.com/pages/OTDR/The%20trace.html>.

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