OTDR Optical Time Domain Reflectometer

Optical Time Domain Reflectometer

Optical Time Domain Reflectometer (OTDR) is used to check the performance of fiber optic link and detect problems.

It is  used to create a virtual picture of a fiber optic cable route. It is used to measure:

  • Fiber attenuation.
  • Overall length of the fiber.
  • The distance to faults.
  • Splices and connections. 

Construction

An OTDR contains a laser diode source to insert light in optical fiber, a photo diode detector to detect refracted light and a highly accurate timing circuit.

Working

An OTDR tester works as optical radar.The laser emits a pulse of light at a specific wavelength, this pulse of light travels along the fiber.When the pulse reaches the portion where the fiber is damaged,emitted light refracted back to the photo detector in the OTDR.

The end of the fiber break will be detectable through Fresnel reflection, since a damaged fiber end is also a change in media (glass to air).This reflection is due to the change in refractive index. The intensity of this returning light and the time taken for it to arrive back at the detector tells us the loss type and location of an event in the fiber link.An efficient optical data link must have enough light to exceed the minimum optical power that the receiver requires to operate within its specifications.

OTDR Trace

OTDR trace is the graphical representation of fiber quality

 

In OTDR trace, y-axis represents the optical power level of the reflected signal and x-axis represents the distance between the installation and event in fiber link.

  • There are two types of events 
  1. a) Reflective events : Mechanical splice, fiber break
  2. b) Non reflective events : Fusion Splice 
  • The positive spikes in the trace are the result of Fresnel reflection at the joints of the fiber link..
  • The shifts in the curve are due to losses that occur due to fiber joints.
  • A tail in the curve is the outcome of scattering and is the major reason for the attenuation of the signal inside the fiber.

Dead Zone

This length of fiber close to the OTDR is called the Dead Zone because the faults are masked in this length.

This occurs when trying to look at a very long length of optical fiber.  It is necessary to launch a lot of power to see the end. OTDR may take up to 500 meters or more for this zone.OTDR launch box, a bundle of optical fiber inserted between the fiber under test and OTDR to ensure that the OTDR dead zone happens in the OTDR launch box.

Video Source # 2

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