Attenuation in Optical Fiber

The major transmission characteristics for an optical fiber:

1. Attenuation

2. Dispersion

Attenuation: Signal attenuation (fiber dB loss explained) within the optical fibers, as with metallic conductors, is usually expressed in the logarithmic unit of the decibel.  The decibel, which is used for comparing two power levels, may be defined for a particular optical wavelength as the ratio of the input (transmitted) optical power Pi into a fiber to the output (received) optical power P0 from the fiber as:

Number of dB=10log(base 10) (pi/Pout)

In optical fiber communications the attenuation is usually expressed in decibels per unit length (i.e. dB per km) (the second formula discussed in the video)

alpha dB into L is equal to 10 Log base 10 (Pi/Pout)

where alpha dB is the signal attenuation per unit length in decibels (decibel attenuation)which is also referred to as the fiber loss parameter and L is the fiber length.


There are so many things which play the role for the signal attenuation within the optical fibers. These are material composition, the preparation and purification technique, and the waveguide structure. Further, these process includes material absorption, material scattering, curve, and micro bending loss, mode coupling radiation losses and losses due to leaky modes.

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  • difference between material and waveguide dispersion is also responsible for it
  • fiber optic length also contribute but not significantly
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