Light scattered by Rayleigh scattering in the glass that happens to be captured by the core heading backward. It is weak, on the order of minus 80 dB relative to a 1 ns pulse, but continuous along the whole fiber, so its slow decline traces the fiber loss. The backscatter level depends on the fiber's composition and mode field diameter, which is why dissimilar fibers produce gainers.
Backscatter's down in the noise past 60 km at that pulse width, go to a longer pulse to see the far end.
Also heard as
- Rayleigh backscatter
- backscattering
Do not mix it up with
- Reflectance: Reflectance comes from a single interface such as a connector; backscatter comes from every point along the fiber.
Related terms
OTDR
Optical Time Domain Reflectometer: a tester that maps a fiber's loss and events by timing light reflected back along it.
Gainer
A splice that shows an apparent gain of light on an OTDR trace, an artifact of mismatched backscatter.
Attenuation
The gradual loss of optical power as light travels along a fiber, expressed in decibels.
Trace
The graph an OTDR produces, plotting returned power in dB against distance along the fiber.