Module 2 : Ray Model
Lecture : Ray Model
 

Bandwidth

So if we make a cladding-less optical fiber, its light launching efficiency is excellent but it has hardly any bandwidth. Even an electrical cable is better than the optical fiber.

Important Conclusion: The cladding is an essential part of an optical fiber. It does not just provide the mechanical support but increases the bandwidth of the fiber.

We can observe from the expression for pulse broadening that keeping all other parameters constant.

Since , we get

 BW .

Important: We can trade in the bandwidth for the length and vice versa. That is, we can send low bit rate signals over long distances and high bit rate signals only over short distances.

2. GRADED INDEX FIBER
(a)
 In a step index fiber since the refractive index is constant inside the core, the velocity of all the rays is constant and
hence there is travel time difference between different rays. If we develop a system where the rays which travel longer distances travel with higher velocities and the rays which travel shorter distances travel with lower velocities, the pulse spread on the fiber can be reduced and consequently the bandwidth can be increased.
(b)

The ray which is at a higher angle, should speed up and the ray which is along the axis of the fiber should travel with

  the slowest possible velocity.
  Since velocity is inversely proportional to the refractive index, it can be manipulated by changing the refractive index of the core. The refractive index of outer layers of the core should be smaller compared to that of the inner layers, so the rays that go in the outer layers, travel faster.
  So we find that for reducing dispersion, the refractive index at the center should be maximum and it should gradually decrease from the center to the core-cladding interface. The rays that go at higher angles speed up and the dispersion gets reduced. 2
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