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Discuss a popular non-destructive technique for attenuation measurement.
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  • 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.

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

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  • 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.

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