Basic Types of Optical Fiber

Optical Fiber Principles
Propagation of light should be totally confined with in the fiber. There are three basic types of Optical Fiber

1. Single-mode step index.
2. Multi-mode step index.
3. Multi-mode graded index.

The term mode means the method by which light is propagate with in the figure, i.e; the various paths in which light travels down the fiber. The optical fiber is also covered by a plastic jacket for protection which is called the cladding.(It is a layer of lower refractive index over the central core of high refractive index)

1. Single-mode step index fiber.

single mode or mono mode step index fiber has a very thin core of about 5um diameter and has a relatively larger cladding of glass or plastic shown in the figure.

As the core is very thin then a strong monochromatic light source (a laser source) is used to send light signals through it. It can carry more then 14 TV. channels or 140000 phone calls.

2. Multi-mode step index fiber.

This type of fiber has a core of relatively larger diameter such as 50um. It is mostly used for carrying white light but due to dispersion effects, it is use full for a short distance only. The fiber core has a constant refractive index. 

3.  Multi-mode Graded index fiber.

The optical fiber in which central core has high refractive index and its density gradually decreases towards its boundary is called multi mode graded index fiber. 
this fiber has core which ranges in diameter from 50 to 1000um. It has a core of relatively high refractive index and the refractive index decreases gradually from the middle to the outer surface of the fiber. There is no noticeable boundary between core and cladding . It is distance for long distance applications in which white light is used. The mode of transmission of light through the fiber is also same,

i.e, continuous refraction from the surfaces of smoothly decreasing refractive index and the total internal reflection from the boundary of the outer surfaces.