Step Index & Graded Index Fiber

Topics: Optical fiber, Fiber optics, Dispersion Pages: 2 (325 words) Published: August 27, 2012
step index fiber

It is an optical fiber which has a core of constant refractive index.there are two types of step index fibers:single mode si and multi mode si fiber

single mode step index fiber has only one mode of EM propagation (HE11) an has a core diameter of about 8-12 Um. and multi mode si has many mode of propagation and has a core of diameter of about 50 Um which allow many no of modes through it.

single mode step index has an adv of lower inter modal dispersion as there is only one mode present there is very low chance of dipersion multimode fiber has high intermod dispersion due to large no of propagating modes in it and moreover due to varying group velocities of propagating modes results in dispersion and as a result it restricts bandwidth in multi mode fiber

adv of multi mode over si fibers:

use of spatial non coherent sources i.e. use of led over laser diode

large NA and larger core diameter results in easy optical power transmission and easy connecting of same fibers

lower tolerance of fiber connectors

Graded index fiber

has a core of varying refractive index
it has a high value n at the core which dec radially towards the cladding has a multimode graded index fiber

multimode graded index fiber under parabolic index profile shows a parabolic transmission of rays through it

through geometrical optics,large no of refractions are observed due to varying refractive index from core to cladding

multimod graded index fiber hav a lower intermodal dispersion as compared to multimode step index however there are many modes of propagation but all are normalised through index grading

rays travelling through core hav a shorter path but move with a lower vel due to high n value. thus despite of all this it follows parabolic transmission and provide considerable bandwidth as compared to multi mode step index fiber
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