How OTDR Works: A Layman’s Definition


An Optical Time Domain Reflectometer (OTDR) is used to check if long distance optical fiber are installed and working correctly by using backscattered light signals. OTDRs are best used in the case of Outside Plants (OSPs) that are mostly outside buildings and can go on for very long distances (from thousands feet to hundreds of miles). OSP installations have very high fiber counts (which can go up to 288 fibers) and are all single mode fibers and for that to happen, the fibers have to be spliced (or joined) together. This can cause problems because of abnormal behavior in the regions with splices.

This is why OTDRs are very useful. They are very essential in measuring the power loss at each splice so that we can maintain the cables properly and be aware of the condition of our plant. They can also be used to troubleshoot other problems and anomalies that we might have in out plant.

How does an OTDR work?

Power meters measure the power of the fiber optic cables and their losses directly at the source whereas an OTDR uses a more indirect approach. The light that enters an optical fiber scatters in all directions and an OTDR is used to see the light that gets backscattered or moves back towards the source to test for the condition of the optical cables and splices. The OTDR works by first sending a high power laser light pulse through the fiber and then observing the backscattered light from the fiber or light reflected from splices. 

Since we can control the speed of the light that goes through the cable, an OTDR can use the time and position of pulse in the fiber to find the exact location of light in the fiber. Since energy(light) and power dissipates as it moves along the cable, we can calibrate the other measurements such as speed and intensity of laser beams that enter the cable to make sure we can find the correct position of the light along the optical fiber for longer distances.

An OTDR comprises of a laser pulse transmitter, a coupler and a photo detector. The transmitter transmits a laser pulse through the coupler into the optical fiber and for every single pulse there are multiple backscattered light pulses that are read by the photo detector which tell us about the condition of the fiber optic cables at different points in the cable. Hence, they are used to measure for power, distance and any irregularities. All these measurements can be used to correlate positions of light through time and create an OTDR trace.

How to analyze the information provided by an OTDR?

Since a very small percentage of the light emitted comes back for testing when using an OTDR (about 1%), we need to make sure that the receivers are very sensitive to get accurate information over long distances. But this also means that any big reflections can quickly cause the receiver to become saturated and create an area in which the reflector takes some time to recuperate and is thus caused the dead zone. A lot of the time, we can quite easily observe power loss in the OTDR trace in the regions associated with splices or connectors, which help us analyze the information easily. But at times, we can’t always locate a big power loss at an area with a splice and these can be mistaken for “ghosts”, which are just anomalies. These errors can be reduced by trying to minimize reflections that come back from the fiber.

When we have trouble with OTDR in a dead zone, we can use visual fault locators(VFL’s) which focus on finding loss of light in the dead zone. It has a visible laser and a universal adapter and a VFL is important because it can be used with an OTDR and its high power helps to go through regions in high fiber cables where there are major losses or reflections or wear and tear and it can help us find accurate calculations in all regions of the optical fiber cable.

What You Should Take Away From This

To conclude, an OTDR works on the principle of radar that sends out blasts of laser pulses and measures the backscattered light pulses. An OTDR is used during the installation and maintenance of optical fiber cables to check for proper behavior and to make sure there are no abnormalities. It is used to check how much loss of light occurs across each intersection (connector or splice). We can use OTDR’s for solving problems too in case we see odd behavior, as this is a very quick and efficient way to do so, considering we know what we are doing first. Basically, if you want to invest in an OTDR, it’s always good to know how to do read an OTDR trace first and understand it’s basic workings properly and hopefully this post will help you do that.



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