OTDR - Optical Time Domain Reflectometer

OTDR - Optical Time Domain Reflectometer


OTDR is an abbreviation for Optical Time Domain Reflectometer. Did the name confuse you? Yeah, it does the same to everyone when you get to listen to it for the first time.

But don't worry. In this article, we will explain OTDR thoroughly to you. Make sure you read it till the end so that there's no ambiguity left.

What is OTDR?

An OTDR is a fiber optic instrument used for characterizing, building, certifying, troubleshooting, and maintaining optical communication networks.  

A virtual image of the fiber cable is built by OTDRs for determining the performance capability and condition of the fiber optic cable.

Apart from testing the components, the path of cables such as bends, connection points, or splices can also be tested by using OTDR to check the capability of the cable from start to finish.

Looking To Buy OTDR Tester? Click Here!

Working principle of OTDR:

A pulse is generated inside a fiber that has to be tested for defects or faults by the OTDR.

A Rayleigh backscatter is created by different events in the fiber.

These pulses then return to OTDR. We measure and calculate their strengths as a function of time. It is then plotted as a fiber stretch function

These returning signals tell about the intensity and location of the fault within the cable. Apart from maintenance, OTDRs are utilized by installation services.

Another type of reflection that takes place is Fresnel reflection. It is a peak of high reflection at the connection points. It detects the physical events that take place throughout the link. When there is an abrupt change in refraction index, a reflection of a higher amount of light takes place. This reflection is Fresnel Reflection. it is a thousand times bigger in comparison with Rayleigh's backscattering.

Dead zones

Dead zones are an important OTDR specification that results due to Fresnel reflection.

There are two types of dead zones



Both are expressed in meters.

We simply define a dead zone as the time length in which the detector gets blinded temporarily by a high reflected light till the time of its recovery to read the light again.

Event Dead Zone:

The minimum distance for detecting another event by the OTDR after a Fresnel reflection is called the event dead zone. The minimum fiber length is required between two events of reflection. Just like we can hardly identify an object moving on the road after we get blinded temporarily by a car

Attenuation Dead Zone:

The minimum distance where the loss of a consecutive event can be measured easily by an OTDR  after a Fresnel reflection is called the attenuation dead zone.

Just like after some time on the road, you can identify the object clearly, same.is the case with the detector to detect and measure the loss of the event.

Moreover, the telephone exchanges and poles across the nation use OTDRs to function smoothly.

The understanding of the physics involved in this instrument provides insight into the OTDR working principles

The theory that led to the development of the first operational laser in the early 1960s was given by Einstein according to whom a particular waveform could be emitted by electrons. The development of the OTDR testers has been revolutionized over the years.

As we will be dealing with pulse width so let’s see what a pulse width is

Pulse Width:

The time duration during which the laser is in the ON state is called the pulse width. Due to the conversion of time into the distance, the pulse width consists of a length. The shorter the length of the pulse, the lesser the energy carried by it and the shorter the distance it travels. A great amount of energy is carried by a long pulse and can be used in extremely long fibers.

If the pulse is short, the information will be lost at the noise floor level due to the loss of energy before reaching the end of the fiber.

Structure of OTDR:

An OTDR consists of a photodiode detector, a laser diode, and an accurate time base (timing circuit). A pulse of light is emitted at a specific wavelength. This light pulse travels through the fiber under test. While traveling, some light portion is either reflected/refracted or scatters back to the photodetector inside OTDR. The loss value is calculated by the returning light's intensity and the time it took to travel back to the diode. It also tells about the location of any event in the fiber cable

Testing Parameters of OTDR:

OTDR testing serves a lot of purposes. The parameters of OTDR must be set correctly to ensure that the measurements obtained are accurate

Auto-test function may be enough to get accurate results in some cases. Other cases require that the OTDR parameters are set manually. The setting is based on the type of cable used, length of fiber cable, and system's complexity.

These parameters will adjust the width of the pulse, dead zones, averaging tome, and the distance range of fiber cable for providing the most accurate results.

Problem Detection using OTDR:

They can be used to troubleshoot the performance of fiber plants. It helps in mapping the cable and to find the locations of faults that tend to hinder the performance of a network.

The network engineers must assure that the selected OTDR has a certain functionality.

It may include a map view of the event/channel, loss-length certification, capabilities of a power meter, a user-friendly interface, and remote options. For documenting the results, the means provided by OTDR must be reliable.

Certifying new links through an OTDR:

A lot of customers know the Tier 1 fiber certification. It measures

-Attenuation (insertion loss)



The purpose of this test is to ensure that the exhibited loss is lesser than the maximum loss budget

This test can be performed with simple power meters or test sets of optical loss.

Tier 2 certification is done by adding Optical Time Domain Reflectometer from one end to the other. The trace of OTDR is a graphical signature of the attenuation of fiber along with the length of fiber that provides information about the performance of components like connectors, cables, and splices. Non-uniformity in the trace of OTDR also tells about the quality of installation.

The trace of OTDR helps in characterizing individual events that may be invisible while conducting tier 1 testing

Bi-directional testing:

It is costly and time taking, so technicians try to test all the links from one end only before they move to the other end. Unfortunately, it is not possible. To overcome this, two fibers are tested at the same time and both are connected through a loop. So both fibers can be tested in one shot without the need to move OTDR to the far end.


This was all the information you would need on Optical Time Domain Reflectometer (OTDR)