Classification of Optical fibres

Optical fibres are generally classified on the basis of

Material composition

Number of Modes of propagation

Refractive index profile

Material Composition

Optical fibres are available as 1) Glass fibres and 2) Plastic

fibres

1) Glass fibres:

When the fibres are made by mixtures o

E.g.,f metal oxides andsilica glasses, then it is known as glass fibre.

1. GeO2-SiO2 core ; SiO2 cladding

2. P2O5-SiO2 core ; SiO2 cladding

3. SiO2 core : P2O5-SiO2 cladding

2) Plastic Fibres:

Plastic fibres are made of plastics.

E.g., 1. Polystyrene core ; methymetha crylate cladding

2. Polymethy metha crylate core ; polymer cladding

Number of modes of propagation

Based on the number of modes of light, fibres can

be of two types

i) Single mode fibre

ii) Multimode fibre

i) Single mode fibre :

In a fibre, if only one mode of light is transmitted then

it is called single mode fibre as shown in figure 1.

Features :

1.It has smaller Numerical aperture and hence smaller core radius.

2.It requires generally lasers as source to transmit light.

3.As core radius is small in order of 3 microns, splicing the fibres is difficult

4.Attenuation(loss of signal) in this type of fibre is minimum(no intermodal dispersion)

ii) Multimode fibres :

In this type of fibre, more than one mode of light is transmitted through the fibre. It has many advantages over single mode fibre. It is shown in fig.2.

Features :

1. Since many modes are sent, the core radius is larger

2. Numerical aperture of the fibre is also larger

3. Launching of light from source is easier

4. LED can also be used as source in this fibre

5. Splicing (joining) two fibres is easier

6. Attenuation due to intermodal dispersion occurs in this fibre.

Classification based on Refractive Index profile:

The optical fibres are classified on the basis of refractive index as

i) Step index fibre

ii) Graded index fibre

i) Step index fibre

It is based on the comparison of refractive indices of air, cladding, and core materials. In step index fibre, the refractive indices of air, cladding , and core vary in step by step fashion as shown.

Step index fibre can be step index single mode and step index multimode

Step index single mode : (shown in fig.3)

1. In this fibre, only one mode can be transmitted

2. Numerical aperture is small

3. Bandwidth for the fibre is high

4. It has no intermodal dispersion

5. It is used for long haul communication(long distance)

6. It is the most widely used fibre

Application :

1.It used in submarine cabling systems

2. It is used as under-sea cables and as telephone lines

Step index Multi mode:

1. Many modes are transmitted
2. core diameter and NA is larger
3. Bandwidth is comparatively less
4. It has high attenuation (losses)
5. Handling and operating with the fibre is easy

Application :

It is used widely as data link cables.

(refer fig.2 for step index multi mode diagram given above)

i) Graded index fibre

In this fibre, the refractive index of the core varies radially from the axis of the fibre. The refractive index of the core is maximum along the fibre axis and is gradually decreasing towards the core-cladding interface.

Graded index fibre is available only as graded index multi mode . A typical fibre has core diametere of 50 -200 microns . It has indetermediate bandwidth and less attenuation. There is no intermodal dispersion due to varying ref.index at core surface.

Application :

It is used in medium range communications, medical field and in industries.