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Mohamed sheik sirajuddeen: Classification of Optical fibres 1
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Tuesday, December 13, 2011
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. GeO2SiO2 core ; SiO2 cladding 2. P2O5SiO2 core ; SiO2 cladding 3. SiO2 core : P2O5SiO2 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.
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Mohamed sheik sirajuddeen: Classification of Optical fibres
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 (ing) 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
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Mohamed sheik sirajuddeen: Classification of Optical fibres
Application : 1.It used in submarine cabling systems 2. It is used as undersea 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.
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