Module 4 : Signal Distortion on Optical Fibers - Attenuation
Lecture : Signal Distortion on Optical Fibers - Attenuation
What is meant by signal distortion ?
A signal is said to be undistorted if it is delayed in time and scaled version of the original signal. Let a system has input signal and output signal . Then for distortion-less system
-------- (5.1)
Where is a scaling constant and is the time delay.
What is the condition for signal distortion-less transmission ?
To see the condition in the frequency domain let us take the Fourier transform of Eqn (5.1). If is the Fourier transform of and is the Fourier transform of , we get,
-------- (5.2)
That means if a system has a frequency response then it does not produce any distortion of the signal. In other words for a system to be distortion-less, its frequency response must satisfy two conditions.
(1)
The amplitude response must be constant (
should be independent of frequency).
(2)
The phase response should be linear ( should be constant)
Why signal gets distorted while propagating on optical fiber ?
(1)
The loss of optical fiber is wavelength dependent. That makes a function of frequency. This is called the attenuation of the fiber.
(2)
Different wavelengths and different modes of the optical fiber travel with different speeds and therefore have delay which is wavelength dependent. This phenomenon is called the dispersion.
So, in general the signals get distorted on an optical fiber because different frequency components undergo different attenuation and different delays.
and output signal 
. Then for distortion-less system 
is a scaling constant and 
is the time delay. 
is the Fourier transform of 
and 
is the Fourier transform of 
, we get, 
then it does not produce any distortion of the signal. In other words for a system to be distortion-less, its frequency response must satisfy two conditions. 
should be constant) 
a function of frequency. This is called the attenuation of the fiber. 
