Step index fibers are optical fibers with a uniform refractive index at the core. The refractive index sharply decreases at the core-cladding interface so the refractive index of the cladding is lower than the core. This kind of index profile is mainly seen in single-mode fibers as well as in some multimode fibers.
Step index fibers are generally manufactured by doping high-purity silica glass with different concentrations of materials like titanium, germanium, or boron.
Graded index fibers are optical fibers whose core refractive index decreases as it moves away from the core center, and gradually approaches the cladding refractive index at the interface. The center of the core in such fibers has the maximum refractive index.
Since the part of the fiber core close to the fiber axis has a higher refractive index than the part near the cladding, the light rays follow a sinusoidal path through the fiber. This kind of index profile is seen in multimode fibers.
The main advantage of using graded index fibers over a step-index fiber is to minimize intermodal dispersion. A multimode fiber contains multiple modes and these modes do not necessarily travel with the same velocity due to dispersions. Thus each mode reaches at different times at the end of the fiber. This is known as intermodal dispersion.
In a step-index fiber, since the core has a uniform refractive index, there is no variation in the velocity within the core. So rays either travel through the center or in a zig-zag motion. Thus, the modes which enter the fiber at higher angles take a longer time to travel through the fiber which contributes to modal dispersion.
In graded-index fiber, the light follows a curved path. The modes that enter at large angles travel mostly through the low-index region in the core which allows them to travel faster than the high-index regions. This compensates for the longer paths of these modes and thus reduces the modal dispersion greatly.
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