Similar questions

Short note: OTDR.

Marks: 10M

Year: May 2015, Dec2014, May2014, Dec2013, May2013, Dec2012

• The most popular non-destructive attenuation measurement technique for both laboratory and field use, only requires access to one end of the fiber.

• It is the backscatter measurement method which uses Optical time domain reflectometer (OTDR).

• It is widely used for testing optical fiber cable.

• Common use of OTDR:

  • The overall link length can be determined from time difference between reflections from fiber.

  • Finding faults in fiber.

  • Breaks in cables

  • Losses in splices and connectors.

• OTDR sees reduction in power.

• It relies upon the measurement and analysis of the fraction of light which is reflected back within the fiber’s numerical aperture due to Rayleigh scattering.

• A light pulse is launched into the fiber in the forward direction from an injection laser using either a directional coupler.

• The backscattered light is detected using an avalanche photodiode receiver which drives an integrator in order to improve the received signal to noise ratio by giving an arithmetic average over a number of measurements taken at one point within the fiber.

• The signal from integrator is fed to the fiber with reference to backscatter plot. The backscatter plot is shown below:

• The initial pulse caused by reflection and backscatter from input coupler followed by long trail pulse caused by raileigh scattering.

• Abrupt shifts in the curve caused by optical loss at joints, at connectors because of sharp bends in the fiber line.

• Positive spikes arising from Fresnel reflection at the far end of the fiber, at fiber joints, and at fiber imperfections.