Question

Discuss modal dispersion as it relates to single-mode and multimode cables.

Answer 1

Dispersion- It is defined as phenomenon of spreading of light as it travels from one end of fiber to another end.

A pulse of light having width and amplitude when travels through a fiber, its amplitude decreases and width increases .thus it leads to inter symbol interference (ISI).

Dispersion can be classify into two category.

1. inter modal dispersion

2. Intra modal or chromatic dispersion

1. Inter modal dispersion- This is caused due to different rays make different path in a fibre.

when a light pulse occurs in fibre,it is combination of different rays.if all rays are incident at same time at input of fibre, they will reach at different times.

2. Intra modal dispersion or chromatic dispersion- It is called chromatic dispersion because refractive index(n) depends on wavelength.

This type of dispersion occurs due to pulse spreading that in a fibre as a result of the fact that group velocity is a function of wavelength.

chromatic dispersion can classify into two category.

1. Material dispersion- it arises because of variation of refractive index of core material as a function of wavelength.

2. Waveguide dispersion- This occurs due to guiding characteristics of wave.in optical fibre core contains higher refractive index than clad.hence 80 % power is confined to core while 20 % in cladding because of refractive index so wave travels faster in cladding than core.

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