Dispersion and its types in fiber optics

Dispersion: The speed of light is slower in various materials than it is in a vacuum or outer space. When the light passes into a material at an angle, the light beam is bent or refracted according to Snell’s Law and the index of refraction of the material. But also, the speed of light through a material varies slightly with the wavelength or frequency of the light. Thus, each wavelength is refracted at a slightly different angle when passing through a material at an angle. This spreading out of beam of light is called dispersion.

Types of Dispersions and ways to reduce: Chromatic Dispersion: Chromatic dispersion results from the spectral width of the emitter. It is a term used to describe the spreading of a light pulse as it travels down a fiber when light pulses launched close together(high data rates) spread too much and result in errors and a loss of information. The spectral width determines the number of different wavelengths that are emitted from LED or laser. One way to reduce chromatic dispersion is to narrow the spectral width of the transmitter.

Chromatic dispersion can be compensated for with the use of dispersion-shifted fiber (DSF). Laser has a more narrow spectral width than LEDs. A monochromatic laser emits only one wavelength and therefore, does not contribute to chromatic dispersion. Modal Dispersion: Modal dispersion deals with the path (mode) of each light ray. Most transmitters emit many different modes. Lower order modes travel a shorter distance. Some of these light rays will travel parallel to the center of the fiber while repeatedly bounce off the cladding/core boundary to high order modes their way the waveguide.

The modes that enter at sharp angles are called high-order mode, which having direction towards cladding. These modes take longer path to travel through the fiber than the low-order modes and therefore it cusses of modal dispersion. One way to reduce modal dispersion is to use graded-index fiber. The graded-index fiber’s cladding is doped so that the refractive index gradually decreases over many layers. Graded Index Fiber has a different core structure from single mode and multimode step index fibers.

In Graded index fiber the value of the refractive index changes from the centre of the core. With a graded-index fiber, the light follows a more curved path. The high-order modes spend most of the time traveling in the lower-index cladding layers near the outside of the fiber. These lower index core layers allow the light to travel faster than in the higher index center layers. Modal dispersion can be completely eliminated by using a single-mode fiber. Single mode fiber transmits only one mode of light so there is no spearding of the signal due to modal dispersion.

Material Dispersion: Material dispersion is the phenomenon whereby material causes a bundle of light to spread out as it propagates. We know that a laser pulse contains a continuum of wavelengths in a small range. The index of refraction of a material is dependent on the wavelength, so each frequency component actually travels at a slightly different speed. Material dispersion contributes to group delay, along with waveguide delay distortion, differential mode delay and multimode group delay spread.