Question

List and discuss the two major factors involved in the dispersion or broadening in the propagation of optical pulses over long distances. This should be related to fiber optic communications.

Answer 1

Two main factors attributed to the  dispersion of light pulses through optical fibre are as follows

1) Intermodal Dispersion (also called Modal Dispersion or Group Delay)

Modal dispersion is only important in multimode fibers.

Many different modes propagate in a multimode fiber. Each of these modes takes a different path and thus different length when traveling down the fiber.

Another factor is the differing propagation constants associated with each mode (each mode’s group velocity).

Both factors contribute to modal dispersion.

Single mode fiber doesn’t have modal dispersion since there is only one mode propagating in the fiber. As we shall see, single mode fiber’s bandwidth is mainly limited by material dispersion, waveguide dispersion and PMD (polarization mode dispersion).

2)Material Dispersion (also called Chromatic Dispersion)

Material dispersion is caused by the velocity of light (or its refractive index) being a function of wavelength. Each wavelength takes different amounts of time to propagate the same path.

The following figure shows refractive index versus wavelength in silica.

Any given light source (even a laser) emits over a range of wavelengths as shown below.