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Module 3
Fibre Optics (L3 & L4)
Classification of Optical Fibre
Optical fibres can be classified based on various parameters such as the materials used for core & cladding, the refractive index profile of the core, the modes of light propagation etc.
1. Classification Based on Material
On the basis of materials used for core and cladding, optical fibres are classified into three categories.
(a) Glass/glass fibres (glass core with glass cladding):
The basic material for fabrication of optical fibres is silica (SiO 2 ). Material having slightly different refractive index is obtained by doping the basic silica material with small quantities of various oxides. For example, if the basic silica material is doped with Germania oxide (GeO 2 ) or phosphorous pentoxide (P 2 O 5 ), the refractive index of the material increase. Such material are used as core material and pure silica as used as cladding material. The glass optical fibres exhibit very low losses and are used in long distance communications.
(b) Plastic/plastic fibres (plastic core with plastic cladding):
In this type of fibres, plastic material such as Poly(methyl methacrylate) (PMMA) and polystyrene are used for core. A fluorocarbon polymer or silicon resin is used as a cladding material. A high refractive index difference is achieved between the core and the cladding materials. Therefore, plastic fibres have large NA of the order of 0.6 and large acceptance angle up 70ยฐ. The main advantages of the plastic fibre are low cost and higher mechanical flexibility.
However, theyโre temperature sensitive and exhibit very high loss. Therefore, they are used in low cost application and at temperature below 80ยฐC.
(c) PCS (Polymer clad silica):
The plastic clad silica fibres are composed of silica cores and surrounded by a low refractive index transparent polymer as cladding. The core is made from high purity quartz. The cladding is made of silicon resin having a refractive index of 1.338. The plastic cladding is less expensive but has high losses. Therefore, they are mainly used in short distance communication.
2. Classification Based on refractive index profile
Refractive index profile of an optical fibre is a plot of refractive index drawn on one of the axes and the distance from the core axis drawn on the other axis. Optical fibres are classified into two categories based on the refractive index profile.
(a) Step index fibre:
In this fibre the refractive index of the core is constant along the radial direction and abruptly falls to a lower value at the core-cladding boundary as shown in Fig. 1(a).
Mathematically, the refractive index profile for a step index fibres is represented as
๐(๐) = ๐ 1 , [๐ < ๐, ๐๐๐ ๐๐๐ ๐กโ๐ ๐๐๐๐]
= ๐ 1 , [๐ > ๐, ๐๐ ๐กโ๐ ๐๐๐๐๐๐๐๐]
Where ๐ 1 and ๐ 2 are the refractive index of the core and cladding respectively. ๐ is the radius of the core.
(b) Graded index fibre (GRIN):
In case of GRIN Fibres, the refractive index of the core is not constant but varies smoothly over the diameter of the core. It has a maximum value at the center and decreases gradually towards the core-cladding boundary. The refractive index of cladding is constant. Figure 1(b) depicts the refractive index profile of a GRIN fibre.
Mathematically, the refractive index profile for a graded index fibres is represented as
๐(๐) = ๐ 1 โ1 โ [2โ(
)๐ผ] , [๐ < ๐, ๐๐๐ ๐๐๐ ๐กโ๐ ๐๐๐๐]
= ๐ 2 , [๐ > ๐, ๐๐ ๐กโ๐ ๐๐๐๐๐๐๐๐]
Where ๐ 1 is the maximum refractive index at the core axis, ๐ the core radius, ๐ 2 is the refractive index of the cladding and โ is the fractional refractive index change. ๐ผ is the grading profile index number which varies from 1 to โ.
(b) Multimode fibre (MMF):
In multimode fibre light follows a number of paths to propagate through the fibre. The core size of multimode fibre is much larger than that in single mode fibre. It can be divided into two categories:
(i) Multimode step index fibre:
Multimode step index fibre allows many zigzag paths of light propagation as shown in Fig. 4. The modes that propagate at angles close to the critical angel are called higher order modes and modes that propagate with angles larger than the critical angle are called lower order modes as shown in Fig. 4. The higher order modes have to traverse longer paths and hence take larger time than the lower order modes to cover a given length of the fibre.
Fig. 4. Propagation of light in a multimode step index fibre.
(ii) Multimode Graded index fibre:
A graded index fibre is a multimode fibre consist of a core having concentric layers of different refractive indices. The refractive index is maxium at the centre and falls of with increasing radial distance from the axis.
Propagation of light in a multimode graded index (GRIN) fibre
when a light ray travels from a region of larger refractive index (central region of the core) to a region of lower refractive index (near the edge of the core), it bent away from the normal till the condition of total internal reflection. Then the ray travels back toawards the core axis, again being continuously refracted as shown in Fig. 2(a). the turning around of the ray may happen even before reaching the core-cladding boundary. Accordingly, continuous refraction is followed by total internal reflection and again continuous refraction towards the axis transversing a sinusoidal path as shown in fig. 2(b).
In Multimode GRIN fibre rays making larger angles with the axis traverse longer path. However, they travel in a region of lower refractive index at a higher speed of propagation. Therefore, all rays travelling through the fibre, irrespective of their modes of travel will have almost the same optical path length and reach the output end of the fibre at the same time.
Fig. 2. (a) Illustration of refraction at the various high and low index interfaces within graded index fibre, giving an overall curved path and (b) light transmission in a graded index fibre. Here, na, nb, nc, nd etc are the refractive indices of different layers of the core with na > nb> nc> nd etc.
Types of rays
The rays that propagate through an optical fibre can be classified into two categories:
(a) Meridional ray:
A ray that propagates through the fibre undergoing total internal reflection is called meridional ray. It passes through the longitudinal axis of the fibre core and cross the optical fibre axis at each reflection as shown in Fig. 5.
Fig. 5. Ray path view of meridional ray (a) along the fibre axis and (b) cross-sectional view of the fibre.
(b) Skew ray:
It describe angular helical path along the fibre. Skew rays do not cross the fibre axis and propagate around the axis on helical path as depicted in Fig. 6. They tend to propagate only in the angular region near the outer surface of the core and do not fully utilize the core as medium. However, they are complementary to the meridional rays and increase the light gathering capacity of the fibre.
