The impact of fiber optic transmission in multiservices networks in rwanda

2.2.6.2 Multimode step index fiber optic

The figure bellow corresponds to multimode propagation with a refractive index profile that is called step index, when light enters the fiber optic cable on the left, it propagates down toward the right in multiple rays or multiple modes, the light is reflected angularly (in Zig-zag) .

This type of fiber contains : Diameter of the core 50um or 62.5um generally cladding 1,25um Bandwidth of 60MHz.km and Weak attenuation:3dB/km with jump 0,85um.

Figure 9 :Multimode step index fiber optic

For step index fiber the numerical aperture NA the angle which light comes out of or goes into an optical fiber and acceptance angle èA are given by the following formula [10]:

NA= /n -- n~ ~ (2.11)

Where n1 is the index of core and n2 is the index of the cladding èA= sin^-1(NA)

Figure 10 : Acceptance cone and acceptance angle in an optic fiber.

2.2.6.3 Multimode graded index fiber optic

A graded-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the fiber axis because the core parts is closer to the fiber axis and have a higher refractive index than the parts near the cladding then light rays follow sinusoidal paths down the fiber.[10]

It has the following characteristics: Diameter of the core is about 50um or 62,5um, cladding of 125um,Bandwidth is several GHz.km and Attenuation is 3 dB/km with 0,85 um or 1,3um .

Figure 11: Multimode graded index fiber optic