What are the difference between step-index and graded index fiber? I was starting to study optical fibers and I came to the chapter: Classifications of optical fibers. Here I read about singlemode and multimode fibers, regarding the latter he explained the difference between step-index and graded-index. My question is: why do the signals sent in the step take different times while in the graded they don't? I did not understand very well the difference between these two refraction profiles.
 A: 
why do the signals sent in the step take different times while in the graded they don't?

In a ray optics model, if you launch light into a step index multimode fiber at a high angle to the axis, it will bounce back and forth between the "walls" of the core rather than travel straight along the axis. So it will take longer to reach the other end of the fiber than a ray launched along the axis.
If you launch a ray at an angle into a graded-index fiber, it will also bounce back and forth between the "walls". But the higher the angle, the more time it will spend near the walls, where the index is lower and it can propagate faster. If you launch a ray along the axis, it will mostly propagate in the high-index material at the axis, and thus propagate slower. If you get the index profile just right (a perfect parabolic profile) the two effects will balance and the ray launched on axis will take just as long to reach the other end as the one launched at an angle.
A: I recommend you to learn about the speed of light in different mediums, snell's law, total internal reflection and refractive index.

*

*Light travels slower in material with high refractive index

*In step index fiber, differen modes are launched at different acceptance angle causing higher modes to travel longer paths. This causes modal dispersion

*In graded index fiber, refractive index of core gradually lowers towards the cladding. Light travels faster in lower refractive index. The light rays gradually bend and the TIR happens at different points. So the rays travelling longer distance also travel faster and reach on other end of fiber at same time.


