The fundamental parameter of a single - mode fiber is

There exist two modes for propagating light all along the optical channels:

Single Mode:

Key Points

• In Single mode, light takes a single path through the fiber core.
• The fundamental parameter of a single - mode fiber are core and cladding diameter.
• Step index means sharp step in the index of refraction between core and cladding interface.
• This indicates that in step-index, core and cladding have their own constant index of refractions N1 and N2 respectively. 

Multimode:

In multimode light follows multiple paths through the fiber core. In graded-index, the index of refraction is not constant but varies smoothly across the diameter of the core. The index of refraction is increasing as one goes near the center while decreasing near outer core edges. Index of refraction is maximum at the center of the core. The index of refraction is constant for the cladding part of the fiber.

There are three types of fiber optic cable:

• Multimode step-index
• Single mode step index
• Multimode graded-index