The effect and the types of dispersion Essay

2.0 INTRODUCTION
This chapter will explain about the fundamental theory and properties needed for this research. Firstly, the dispersion induced broadening such as Group Velocity Dispersion(GVD) and chirping.Then, Gaussian input pulses propagation in optical fiber.Lastly,the use of Fourier Transform during simulation phase.
2.1 Single Mode Fiber
There are three common types of fiber optic cables as listed below -
Figure 2.1 All types fiber optic cable has a specific characteristics for a particular application
The single-mode step-index fiber cable.The single and multimode step-index fiber cables are the simplest types. Single-mode fiber cables have extremely small core diameters, ranging from 5 to 9.5 um[2]. The core is cladding diameter of 125 um.Single-mode fibers can carry signals for long distances with low loss, and are mainly used in communication systems.
The higher the wavelength,the fewer modes the fiber can carry until only one mode left.As,the wavelength approaches the core diameter,it then operates as a single-mode fiber.
2.2 Dispersion
Dispersion in an optical fiber is originated in the material and structural characteristics and results in the fact that the propagation velocity of light is a function of the wavelength. When the beam with finite spectral width is sent to the fiber, each spectral component of the pulse travels throughout with a velocity that depends on its respective wavelength.The difference between propagation velocities of the different spectral components is known as group velocity dispersion (GVD).
In practical terms, dispersion is the spreading out of a light pulse in time as it propagates down the fiber from which it possible to conclude that the inter-symbolic interference or pulse dispersion is the difference between the bandwidth and the entrance and the correspondent bandwidth of the pulse at the exit of the fiber.[12]
There are two types of dispersion in optical fibers intramodal and intermodal dispersion.Intramodal, or chromatic dispersion affects all types of fibers. Intermodal, or modal, dispersion occurs in multimode fibers. Each type of dispersion mechanism is responsible for the bandwidth limitation and also to pulse spreading.
Figure 2.2 The effect of dispersion as it propagates in a dispersive medium shown that the longer the distance traveled the larger the broadening of the pulse
There are different types of dispersion that can cause the signal to broadening such as intermodal dispersion, group chromatic dispersion, polarization-mode dispersion and higher order dispersion. Intermodal dispersion only affects multimode fiber as it will result in a modal delay. The main focus is on single-mode fiber where the intermodal dispersion is absent because the injected pulse is in single mode.
Chromatic dispersion or as known as intramodal dispersion that will spread pulse in a single mode fiber.However,chromatic dispersion related to group-velocity dispersion(GVD) where this phenomenon makes the different spectral components of the pulse travel at slightly different group velocities.This will limit the performance of lightwave system in single-mode fibers.
2.3 Group Velocity Dispersion (GVD)
In a fiber,different spectral component of the pulse travel at slightly different group velocities[5].As a result this phenomenon called as group-velocity dispersion or known as intramodal dispersion.