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How to select and use an OTDR

OTDR (Optical Time Domain Reflectometer) Overview


An optical time-domain reflectometer or OTDR is an instrument that characterizes an optical fiber. There are two categories of an OTDR, namely bench-top and hand-held. Depending on the power source of the reflectometer, the two categories differ from one another. In a bench-top OTDR, the power source is direct AC, while in the case of hand-held OTDRs, the power comes from a sourced battery. 


Selecting parameters


Optical time domain reflectometers have an area of usage in the fields and organizations that directly involve the functionalities of optical fiber. They use it to certify the performing ability of the new fiber-optic links and identify any existing problem of the already in use fiber-optic links. The fields requiring the OTDR are - 5G testing, photonic integrated circuit testing in 400G/800G labs. As it performs critical testing tasks, choosing the right OTDR depends on various aspects and parameters that need inspecting before selection.


  1. 1.Measurement range


Various application field needing the instrument to carry their functioning has a requirement of varied ranges of measurement. Depending on the limit of the applicant field, you should choose the right OTDR. The maximum attenuation of the instrument is the highest range in dB units between the placed instrument and the event in measure, which reflects accuracy to understand the acceptable limit of it. 


  1. 2.Dynamic range values


It is essential to be aware of the dynamic range values representing the maximum distance for which the OTDR can provide a display. Depending on the wavelength expressed in decibels, you can identify the number of kilometers to which the OTDR can function. The commonly in use ranges are 35db denoting 80 to 125km, 40dB denoting 95 to 150km, 50dB denoting 125 to 225km, etc. Remember that the actual OTDR measurement actual fiber and event loss happening in the network.


  1. 3.Dead zones


An important characteristic while selecting the correct OTDR for your use is the dead zone that determines its ability to detect and measure the two closely spaced events on the fiber links. The two aspects of dead zone testing carried out by the selectors are the Event dead zone or EDZ and Attenuation dead zone or ADZ. By inspecting EDZ, they identify the minimum distance where OTDR can distinguish two consecutive pairs of connectors. In ADZ, they find the minimum distance after a reflective event that OTDR can measure a splice non-reflective event.


  1. 4.Other factors


Apart from the factors mentioned above, the other aspects worth keeping in mind while choosing an OTDR to exhibit maximum practical usability are


  • Size and weight to ensure ease in using the instrument
  • Display size should be at least 5 inches to obtain a better analysis of the traces 
  • Battery life for hand-held OTDRs to determine the maximum performance
  • Device connectivity; especially wireless connectivity for better result exportation
  • Post-processing software availability for better convenience


Using guide


After choosing the correct OTDR suitable for your use, you need to have the following supplies to get started with the process, namely the reference cable of the same size of the cable plant along with compatible connectors to the plant, mating adapters that are compatible to the connectors and cleaning supplies.  


  • Testing process: First, turn on the OTDR and let it warm up for some time while you clean the connectors and mating adapters. Then attach the launch cable to the OTDR and the receive cable to the far end of the cable. Setup the testing parameters on the instrument and acquire the test.


  • Measuring process: Place one marker on the OTDR just before the reflectance peak between the connection of two cables of launching and under testing. Similarly, place the second marker between the receiving cable and the testing cable. Based on this, the instrument will now calculate the length of the segment between the markers.


  • Good user practices: It is better to follow a few guidelines that ensure the best efficiency in the working process and reduce measurement uncertainty.


  1. 1.Regular cleaning of connectors.
  2. 2.Using highest resolutions while setting up the OTDR.
  3. 3.Non analyzing of traces which are non-linear as they result from high reflectance event or inadequate OTDR resolution.
  4. 4.Proper maintenance of the receiving and launching cables.




Going through the above will help you obtain a basic idea of vital criteria while choosing an OTDR and the working process of it in the practical field of application.