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Along with the insertion loss, reflection and scattering of signal also occur along fiber that contributes to a degradation of the signal.Return loss of a component is measured as the ratio of optical input power to optical power of the reflected signal and measured in [dB] as
RL = 10 log10(Pin/Pref)
Classically, an optical time-domain reflectometer (OTDR) is utilized to analyze RL through the fiber link. The working principle of OTDR is based on Rayleigh backscattering and Fresnel reflection. An OTDR has a transmitter which launches short duration optical pulses into the fiber and a high-speed receiver which measures the reflected signal.
The measured reflected signal is then traced against time delay. The types of fibers and operation wavelengths impact the profile that is seen in the OTDR display. The OTDR needs to know the type of fiber being analyzed, i.e. the refractive index of the fiber which influences the speed of light in the fiber. Then the OTDR can accurately measure distance from the total time delay of the reflected pulse. Distributed loss seen in the profile comes from Rayleigh backscattering in the fiber. The distributed loss will appear different for different operating wavelengths. Longer wavelengths exhibit less scattering and therefore will have lower loss per km than shorter wavelengths.
A drastic change in the fiber density causes Fresnel reflections. While the fiber itself is homogenous, these discrete peaks occur when an OTDR pulse encounters connectors, ends of fibers or fiber breaks. The magnitude of the peak is influenced by the change in the material density and the connector type (angled or straight). In the event of fiber break, the OTDR displays a huge peak followed by random noise at the location of the fault.
The dips in the profile arise from bends or splices. At these points a part of the signal leaks out thus reducing the signal power. Longer wavelengths are more degraded as they leak out of a fiber easier than shorter wavelengths.