Module 4 : Signal Distortion on Optical Fibers - Attenuation
Lecture : Signal Distortion on Optical Fibers - Attenuation
RADIATION OR BENDING LOSS
While laying the fiber the fiber may undergo a slow bend. In micro-bend the bending is on micron scale, whereas in a slow bend the bending is on cm scale. A typical example of a slow bend is a formation of optical fiber loop.
The loss mechanism due to bending loss can be well understood using modal propagation model.
As we have seen, the light inside a fiber propagates in the form of modes. The modal fields decay inside the cladding away from the core cladding interface. Theoretically the field in the cladding is finite no matter how far away we are from the core-cladding interface. Now look at the amplitude and phase distribution for the fibers which are straight and which are bent over an circular arc as shown in Fig.
It can be noted that for the straight the phase fronts are parallel and each point on the phase front travels with the same phase velocity.
However, as soon the fiber is bent (no matter how gently) the phase fronts are no mor parallel. The phase fronts move like a fan pivoted to the center of curvature of the bent fiber (see Fig.). Every point on the phase front consequently does not move with same velocity. The velocity increases as we move radially outwards the velocity of the phase front increases. Very quickly we reach to a distance from the fiber where the velocity tries to become greater than the velocity of light in the cladding medium.
Since the velocity of energy can not be greater than velocity of light, the energy associated with the modal field beyond gets detached from the mode and radiates away. This is called the bending or the radiation loss.
Following important things can be noted about the bending loss :
(1)
The radiation loss is present in every bent fiber no matter how gentle the bend is.
(2)
Radiation loss depends upon how much is the energy beyond .
(3)
For a given modal field distribution if reduces, the radiation loss increases. The reduces as the radius of curvature of the bent fiber reduces, that is the fiber is sharply bent.
(4)
For a given that is for a given fiber bent, if the field spreads more in cladding, the bend loss increases. We know from the modal field analysis that the lower order modes are more confined to the core, that is their fields decay rapidly in the cladding, and the higher order modes have more slowly decaying energy in the cladding . The higher order modes hence are more susceptible to the radiation loss compared to the lower order modes.
(5)
The number of modes therefore reduces in a multimode fiber in presence of bends.
from the fiber where the velocity tries to become greater than the velocity of light in the cladding medium. 
gets detached from the mode and radiates away. This is called the bending or the radiation loss. 
. 
reduces, the radiation loss increases. The 
reduces as the radius of curvature of the bent fiber reduces, that is the fiber is sharply bent. 
that is for a given fiber bent, if the field spreads more in cladding, the bend loss increases. We know from the modal field analysis that the lower order modes are more confined to the core, that is their fields decay rapidly in the cladding, and the higher order modes have more slowly decaying energy in the cladding . The higher order modes hence are more susceptible to the radiation loss compared to the lower order modes. 
