There are two types of OPTICAL FIBERS depending on the refractive index of core and cladding are available ;
a. Step Index Optical Fiber
b. Graded Index Optical Fiber
In step index optical fiber the core and cladding has a uniform refractive index. In the step index optical fibers the rays entering at different angles travel different paths and emerge out at the end of the optical fiber at different times. This time lag between the signals is known as the dispersion.
On the other hand in the Graded Index Optical Fibers, the core has non-uniform refractive index, it is maximum at the central axis of the optical fiber and gradually decreases up to the core cladding interface. But the cladding has a uniform refractive index.
As you know that the refractive index n=c/v; where c is the speed of light in free space and v in any medium. So if n is changing obviously v will be change. Here in the Graded Index Optical Fiber the refractive index of the core is changing and it is maximum near to the central axis so the light ray which enter through this will have lower speed as compared to the light signal which enters near to the co cladding interface, because here the refractive index is lower. So the speed of light will be faster. In this case we found that there is no time lag between the signals, all the signals reach at the same time.
The characteristics of the optical fiber is determined generally by the following parameters, these are;
1. The composition of the material; it indicates to the value of refractive index of the core
2. The nature of light; it means the wavelength of the optical signal.
3. and the diameter of the core and length of the optical fiber.
In case of step index (single mode and multimode), graded index optical fiber (multimode) the diameter of the core and cladding is discussed along with the Band Width, Numerical Aperture, Attenuation and Applications of the Optical fibers.