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.