Mode of the fiber: As we have discussed in the earlier article about the light propagation through an optical fiber after total internal reflection. The path followed by the light in a fiber is called the mode of the fiber.

The light can pass through one path ( that is mono mode or single mode fiber) or more than one path (that is called multi mode fiber). On the basis of this, the fiber is divided into two types:

1. Step index fiber

2. Graded index fiber

Let us discuss the difference between the two:

1. Step index fiber is of two types viz; mono mode fiber and multi mode fiber.

Graded index fiber is of only of one type that is multi mode fiber.

2. The refractive index of the core of the step index fiber is constant through out the core.

The refractive index of the core of the graded index fiber is maximum at the center of the core and then it decreases towards core-cladding interface.

3. Number of modes for step index fiber N =  V2/2, where V is cut off frequency or normalized frequency or V- number

Number of modes for graded index fiber is N = V2/ 4.

4. V number can be less that 2.405 or more that 2.405 for step index fiber

V number is always more than 2.405 for graded index fiber.