OTDR Basics

What is an OTDR?

An OTDR is a tester for measuring and analyzing the characterization of optical networks. It uses optical time domain reflectometry technology. The purpose of an OTDR is to measure elements at any location of a fiber optic cable. It only needs one end of the cable in order to measure its elements. OTDRs are also capable of generating graphs of a fiber optic cable.


What an OTDR measures

OTDRs measure fiber cables by analyzing reflected light in the cables. Some things an OTDR can measure include:

  • Optical distance to connectors, splitters, faults, etc. and to the end of the fiber cable
  • Loss of splices
  • ORL of link
  • Reflectance of the connectors
  • Total fiber attenuation


Why do I need an OTDR?

Testing fiber cables is an essential process to provide confidence that a network is optimized to deliver its services without errors or faults. OTDR applications include:

  • Outside fiber optic plants like video, data, and telecom service providers. OTDRs would be used to provide accurate information about the cables in use. It is also used to test loss as well as being used to troubleshoot faulty wires
  • Premises testing for enterprises and data centers. These areas have less room for error so testing for loss becomes very important and can help determine whether the splicers and connectors are within appropriate tolerance ranges. It can also locate the exact locations of breaks or faults.

Key Specifications of an OTDR


While testing using an OTDR, the wavelength set on the OTDR should match the wavelength that is used for transmission.

  • For multimode fiber links 850 nm and/or 1300 nm wavelengths should be used
  • For single mode fiber links 1310 nm and/or 1550 nm and/or 1625 nm wavelengths should be used
  • For troubleshooting single mode fiber links 1625 nm or 1650 nm wavelengths should be used
  • For troubleshooting single mode fiber links that carry CWDM transmissions wavelengths of 1270 nm to 1610 nm should be used with a channel spacing of 20 m,
  • Systems that use FFTX architecture should use 1490 nm wavelengths 1490 nm


Dead Zones

A Dead Zone in reference to OTDRs is the distance where the OTDR is unable to find an event in the fiber cable. Dead Zones are usually measured in meters (m). There are two types of Dead Zones;

The Event Dead Zone or EDZ is the minimum distance where two consecutive reflective events can be distinguished by the OTDR.

The Attenuation Dead Zone or ADZ is the minimum distance after a reflective event that a non reflective event can be measured by the OTDR.


Pulsewidths are measured in nanometer  (nm) or micrometer (µm). Testing long fibers requires wide pulses of light allowing for more dynamic range.

Dynamic Range

Dynamic range determines how far the OTDR can measure. It is measured in decibels (dB). Sometimes the distance range or display range is misleading and may refer to the maximum distance the OTDR can display, not what it can measure.

Wavelength 1310 nm 1550 nm 1310 nm 1550 nm 1310 nm 1550 nm 1310 nm 1550 nm
Dynamic Range 35 dB 35 dB 40 dB 40 dB 45 dB 45 dB 50 dB 50 dB
Typical maximum OTDR measurement range 80 km 125 km 95 km 150 km 110 km 180 km 125 km 220 km


The table above is showing estimates of measurement range while the actual range given by an OTDR when testing will depend on the cable in use and the network loss.

Factors to Consider When Choosing an OTDR

With a plethora of OTDRs available on the market, choosing the right one for you can be a difficult decision. Knowing the applications and specifications of OTDRs is a good first step, but before choosing an OTDR, ask yourself these questions:

  • What kind of networks will you test?
  • What is the maximum distance you need to test?
  • What fiber type will you be testing? Single mode or multi mode?
  • What kind of measurements will you perform?

Some more factors to consider are:

  • Size and weight: important for travel and different situations such as on top of a building or tower or inside of a building
  • Display size: A 5” screen is recommended for the display size as it allows for a good range of trace analysis, but the choice is up to you
  • Battery life: An OTDR should last you a full working day
  • Storage: 128 mb should be considered the minimum amount of internal storage
  • Wireless technology/connectivity: Makes exporting results to laptops or computers much easier
  • Upgradability: Less expensive in the long term and allows you to curate your OTDR to your needs
  • Post Processing Software: Makes it easier to analyze and document test results

At GAO Tek Inc, the OTDRs offered range from basic, affordable models to high end, advanced models. If you have any specific needs, GAO Tek Inc also offers custom OTDRs with any specification you desire. Feel free to contact GAO Tek Inc if you are looking to purchase an OTDR that does not appear for sale on the website or if you would like to customize an OTDR that is for sale. Please feel free to contact us.


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