OTDR Fundamentals you need to know.

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OTDR or Optical Time Domain Reflectometer is a powerful testing instrument for testing fiber optic cables. Read here to find out more.


OTDR and fiber links.

A reliable fiber link is the foundation of a sound network. So, to access the integrity of infrastructure, fast, reliable, and accurate methodologies and testing devices are required.

An OTDR or Optical Time Domain Reflectometer is a powerful testing instrument for testing fiber optic cables accurately.

When used properly, an OTDR simplifies the testing process and helps increase the value and reliability of a fiber-optic network.


What is An OTDR?

Optical Time Domain Reflectometer or OTDR is an instrument used to test the performance of fiber links to analyze or detect the back-reflected or scattered light through a fiber cable. The back reflection or scattering may be due to imperfections or impurities in the fiber.


OTDR - Yamasaki Y34 Quad OTDR

OTDR – Yamasaki Y34 QUAD OTDR https://www.andcorp.com.au/yamasaki-test-measurement/

The device can detect, locate, and the OTDR measures the elements of impurities at any location on a network. The instrument can be used to create an image of the fiber cable when it is installed. And when a problem in the network arises, the original trace test sets and the new trace can be compared. The analysis or detection of a problem can be easier by having original trace documentation during the cable installation.


Components of and OTDR

An OTDR consists of a

*laser light source, the laser diose sends out pulses of light on command from the controller.


*a coupler/splitter has 3 ports 1/ one for the source 2/ one for the fibre under test 3/ one for the sensor. This device allows the light to travel only in specific directions; FROM the laser source TO the fibre under test amd FROM the fibre under test TO the sensor.


*optical sensor, this sensor is a photodetector that measures the power level of the light coming in from the fibre under test. It converts the optical power in the light to a corresponding electrical level.


*controller section, The controller is the “brains” of the OTDR.

  • tells the laser when to pulse
  • gets the power levels from the sensor
  • calculates the distance to scattering and reflecting points in the fibre
  • stores the individual data points
  • sends the information to the display section.


*display section, The display section is either a CRT or LCD screen that shows the data points that make up the fibre trace


*user controls. The keypad and touchscreen controls on the OTDR allow the operator to change the settings to best suit the working environment. Allowing the operator either full control or the OTDR “auto” control.


wave time range


All OTDRs have basic setup requirements. The user needs to key in these basic data parameters in order to get good and accurate fibre trace analysis.
The required data parameters are :-

  1. Testing Range/Distance scope 
  2. Wavelength
  3. Pulse Width
  4. Index of Refraction 
  5. Averaging Time


How does an OTDR work?

An Optical Time Domain Reflectometer works like a radar. It consists of a high power laser transmitter that transmits a pulse down the fiber. The reflected and backscattered light returns to the OTDR.

The returned signal and reflected light creates a display known as “a trace’” or “a signature.” By connecting the device at one end of the cable, it can calculate fiber attenuation, splice, uniformity, and loss of connectors. It provides pictorial traces, a graph of optical power in dB against the fiber length and reflective events .




The amount of light that backscatters to the OTDR is proportional to the length of the pulse, the test pulse’s peak power, and the backscatter of the fiber.

For a good measurement, you need more backscattered light. For this, you can:

  • Increase the pulse width
  • Increase the peak power of the pulse
  • Send out more pulses and average the signal returned

Most OTDRs use these three ways with a user control feature for more accurate measurement. The testing instrument can be used with a launch cable and a receive cable.

These launch cables are also called “pulse suppressors.” The launch cable allows the device to settle down after transmitting the pulse into the fiber. A receiver cable can be connected to the other end of the fiber cable under test to allow the measurements of the connector on end.


Back Box


The ability of the OTDR to locate and measure the loss and reflectance makes the device an efficient fault locating equipment.


Where to Use an OTDR?

There are many confusions about how and where to use an OTDR effectively. With different applications of fiber optic cables in Premises cabling and Outside Plant (OSP) cabling, the functionality of the OTDR also varies.

In Outside Plant cabling with multiple splices, the testing device is essential and frequently used to ensure that the cable is not damaged and ensure that all the splices are made properly. It also helps troubleshoot problems like locating the breaks in the cable.

On the other hand, premises cabling includes short cable runs and rarely consists of splices, so Optical Time Domain Reflectometer can be regarded as a substitute for insertion loss testing with a power meter and light source. However, the price of OTDR is roughly ten times the insertion loss testing equipment.


How to Use an OTDR to test Fiber cables?

The testing using the device can either use a launch cable along with a receive cable or only the launch cable. However, the testing results in both scenarios will be different.

  1. Testing with Launch Cable





The high power test pulse used in the testing overloads the receiver of the instrument, so no measurements can be made at this point. This will make the device blind for that period. The device will take some time to recover from this, causing a dead zone.

A dead zone is an important parameter in OTDR. It is the distance in the cable at which the fault or defects can’t be measured efficiently.


Why Dead Zones Occur?

If a major portion of the signal is reflected, the received power is greater than the backscattered power level. This floods the OTDR with light, making it ‘blind.’ The Optical Time Domain Reflectometer requires some time to recover from this blind phase or saturation. During the recovery phase, the reflectometer is unable to detect the backscattered light, resulting in a dead zone in the trace or signature.

Generally, dead zones are of two types:

  • Event Dead Zone or EDZ
  • Attenuation Dead Zone or ADZ

One can overcome the dead zone by using a launch cable with OTDR. A launch cable will place the required length of fiber between the test cable and OTDR, providing the distance and time required by the device to effectively measure the fiber cable characteristics.

Launch cables do not interfere with the cable under test, making it a secure way to identify faults in the whole length of the optical fiber.


Testing with Receive and Launch Cable

The receive and launch cables contain spools of fiber with specific distances. The cables are usually connected at the ends of the test fiber cable to qualify the front end connectors and far-end connectors. The length of both these cables depends on the length of the cable being tested. For multimode testing, the length is generally between 300-500 m and 1000-2000 m for single-mode testing.

For the very long haul, cables of 4000 m can be used. The length of the cables also depends on the pulse width. The larger the width of the pulse, the longer the length of launch and receive cables. It is important to note that both these cables should be of the same type as the cable under test.



An OTDR is a valuable fiber network testing instrument that can analyze, locate, and measure faults and problems in the optical fiber links to ensure a sound and reliable network. With the right understanding of the function and operation of the device, you can easily detect, locate, and eliminate the faults in the fiber network.