What is an OTDR and how to read its measurements

December 10, 2019.

The OTDR is an instrument used in optical fiber networks to locate losses and breakages, finding out the distance to these failures (so called events) and thus allowing finding out the physical point of the network where it is needed to perform the appropriate maintenance task.

Several OTDR models from PROMAX used in optical fiber networks


Why to use an OTDR?

In the internal optical fiber network within a facility is relatively easy to locate the failure point. But when a broader optical network comes into play it is almost impossible to locate the street, façade, tunnel or conduction where a breakage has occurred if we don’t use an OTDR that shows up the distance between the measurement point and the fault.

How to read the OTDR measurements

Graphic representation of an optical fiber line analyzed using an OTDR
Graphic representation of an optical fiber line analyzed using an OTDR. By looking into the events it is possible to discover fusion splices, connectors, splitters or macrobend and it is possible to physically locate the point of the network there these events are taking place.

What does the OTDR measure

OTDR stands for Optical Time Domain Reflectometer because it is a tool that emits a light pulse, analyzes its reflection, measures the time it has taken to occur and this way it can calculate how far away the events have occurred –as well as other measurements such as:

  • Measurement of the length of the optical fiber.
  • Measurement of the distance between two events of the fiber.
  • Locating faults and ruptures of optical fiber.
  • Graphically displaying the events in an optical fiber line.
  • Measuring losses between two points on an optical fiber.
  • Measuring loss of tie-ins.
  • Measuring reflection events of optical fibers.

Keep this in mind!: While working with an OTDR and taking measurements from the fiber runs, it is recommended to select a measurement scale at least twice the expected distance (plus the launch coil). This is done to allow a clear identification of the point where the optical fiber end is located and not to confuse the noise floor with other events.



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PROMAX is a leading manufacturer of test and measurement systems, broadcast and TV signal distribution equipment. Our product lines include measuring instruments for cable TV, satellite TV, broadcast, wireless and fibre optics networks, FTTH and GPON analyzers. DVB-T modulators, IP streamers or IP converters (ASI, DVB-T) are among the company’s latest developments.