STEP-INDEX OPTICAL FIBER,types of optical fibers

They are single-mode step-index optical fiber and multimode step-index optical fiber.The refractive index is uniform throughout the core of the fiber.

  STEP-INDEX OPTICAL FIBER

The refractive index is uniform throughout the core of the fiber. As we go radially in this fiber, the refractive index undergoes a step- changes at the core-cladding interface. Again, depending on the propagation of light through the fibre it is further classified into two types. They are single-mode step-index optical fiber and multimode step-index optical fiber.



A. SINGLE MODE STEP INDEX OPTICAL FIBER

The core diameter of this fiber is about 8 to 10µm and the outer diameter of cladding is 60 to 70 µm. 

There is only one path for ray propagation. So, it is called single-mode step-index optical fiber. The construction of single-mode step-index optical fiber is shown in the figure. In this fiber, the transmission of light is by successive total internal reflection.

B. MULTIMODE STEP-INDEX OPTICAL FIBER

The construction of multimode step-index optical fiber is the same as that of single-mode optical fiber except that its core and cladding refractive indices are much larger than single-mode. The core diameter changes from 50 to 200nm and the outer diameter of cladding are changes from 100 to 250 nm. 

The construction of the multimode step-index optical fiber is shown below. The number of possible propagation modes in the core is given by the V- number as,

V = (2π/λ) X a X (N.A), where λ = wavelength of the light

 a = radius of the core

N.A = numerical aperture

II. GRADED INDEX FIBER:

In this type of optical fiber, the refractive index core decreases continuously from the fiber axis to the cladding interface in a parabolic manner. The refractive index is maximum at the centre and minimum at the surface of the core. When a light ray enters into the core and moves towards the cladding interface, then it encounters a rarer and rarer medium due to a decrease of refractive index. As a result, the light ray bends more away from the normal and finally bends towards the axis and moves the core-cladding interface at the bottom. Again, it bends in the upward direction. Thus, the light due to refraction takes sinusoidal paths. This fibre is of refractive type. When two light rays enter into the core with different angles then they adjust their velocities and come to focus at the same point. It results from the two rays that will be received at the other end at the same time. There is no intermodal dispersion in this fiber. 

This fiber is again classified into a single-mode graded index and multimode graded-index optical fibres. The refractive index profile and ray propagation of multimode graded index fiber is shown in the figure. The diameter of core changes from 50 to 200nm and outer diameter of cladding varies from 100 to 250µm.

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