Mumbai University > Electronics and telecommunication > Sem 7 > optical communication and networks

Marks: 10

Years: MAY 2015

• 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:

$\Rightarrow $ The overall link length can be determined from time difference between reflections from fiber.

$\Rightarrow $ Finding faults in fiber.

$\Rightarrow $ Breaks in cables

$\Rightarrow $ 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:

$\Rightarrow $ The initial pulse caused by reflection and backscatter from input coupler followed by long trail pulse caused by Rayleigh scattering.

$\Rightarrow $ Abrupt shifts in the curve caused by optical loss at joints, at connectors because of sharp bends in the fibre line.

$\Rightarrow $ Positive spikes arising from Fresnel reflection at the far end of the fibre, at fibre joints, and at fibre imperfections.