Types of optical fiber


How many types of Optical fiber?

Explain Following:

Single mode fibre (SMF);  Multimode fibre (MMF); Multimode Step index fibre (MMSIF) and Multimode graded index fibre. (MMGIF).

Answer:

types of Optical fiber

The optical fibres may be categorized based on the composition of the materials used in the core and cladding. Most optical fibers are made of glass, although some are made of plastic. There are following three types of fibers can be made using glass or plastic:
i) The core and cladding both are made by glass.
ii) The core and cladding both are made by plastic.
iii) The core is made by glass and cladding by plastic.

types of Optical fiber

On the other hand, optical fibre may also be categorized into following two types based on the size of the core:
i) Single mode fiber (SMF)
ii) Multimode fibre (MMF)
The light rays that enter the fiber within acceptance cone should propagate through the fibre. However, within fibre these waves overlap and interfere with one another. The rays, which interfere destructively with each other will diminish. Thus, light can propagate through the fibre along certain paths only. These allowed paths are called modes.

A single mode fibre carries a single mode at a time because the diameter of its core is very small, typically around 10 microM. Whereas diameter of its cladding is around 125 microM [Fig.(a)]. On the other hand, multimode fibre allows the light to propagate through the core of the fiber along many different paths (modes). The diameter of the core of a typical multimode fiber is around 50 microM and the diameter of its cladding is around 125 microM.

The multimode fibres are further classified into following two categories:

  1. Multimode Step index fibre (MMSIF)
  2. Multimode graded index fibre (MMGIF).

In a step index fibre, the refractive index is constant throughout the core and abruptly falls to a lower value at the core-cladding interface. The refractive index profile and light propagation through a multimode step index fibre is shown in the Fig.


Typically, the core diameter of a MMSIF is 50 – 100 microM. Due to the large core diameter; there are many paths along which light is allowed to propagate through the core of the fibre. In the Fig. three different modes are shown. It is clear that the ray 3 will suffer more total internal reflection than ray 1 and ray 2 thus it will travel comparatively a longer path and reach at the receiving end after the ray 1 and ray 2. Thus, different rays will take different time to reach at the receiving end this is known as intermodal dispersion.

In multimode graded-index fiber (MMGIF), the core is composed of many layers of glass, such that the refractive index of the core is maximum at the fibre axis and decreases gradually to reach a value at the core-cladding interface that equals to the refractive index of cladding. Thus, light in the outer region of the core (covering a longer path) travel faster than the light in the central region of the core. Therefore, all the rays take approximately same time to reach at the receiving end, thus in these fibres the intermodal dispersion decreases considerably. The most common refractive index profile of a typical MMGIF is parabolic profile. The refractive index profile and light propagation through a multimode graded index fibre is shown in the Fig..
Typically, the core diameter of a MMSIF is 50 – 60 microM. In the Fig. three different modes are shown. It is clear that the path of ray 3 is longer than ray 1 and ray 2, but it is travelling away from the fibre axis so it is faster than the other two rays and reaches at the receiving end approximately same time with ray 1 and ray 2.

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