On completion of this chapter you will be able to understand. The types of optical fibres

Transcription

1 Introduction (Attention Grabber) Learning Objectives On completion of this chapter you will be able to understand. The types of optical fibres 1. Mode of propagation 2. Advantages and disadvantages of each types of optical fibres Types of Optical fibres Optical fibres are classified into three major types based on (a) Material (b) The number of modes and (c) The refractive index profile. A general classification of optical fibres is shown in fig.1. Optical Optical fibres fibres are are based based on on Material No of modes Refractive index profile Glass Plastic Single-mode Multi-mode Step Grade Material prepared by: < Physics Faculty> Topic No: < 2 > Page 1 of 9

2 (a) Material Optical fibres are normally made with two types of materials, namely glass (silica) and plastic. Based on this, there types of fibres, they are Glass fibre Plastic fibre Plastic clad silica fibre Glass fibre Silica is the commonly used materials for the manufacture of a glass fibre. The glass employed in a fibre-optic cable is ultra pure, ultra transparent, silicon, silicon dioxide or fused silica. Glass fibres are made by fusing silica and metal oxides. A pure glass fibre optic cable has a glass core and a glass cladding and is far the most widely used in fibre. During the glass fibre-optic cable fabrication process, impurities are purposely added to the pure glass so as to obtain the desired indices of refraction needed to guide light. Examples: 1. GeO 2 SiO 2 core, SiO 2 cladding 2. SiO 2 core, P2 O 3 -SiO 2 cladding Material prepared by: < Physics Faculty> Topic No: < 2 > Page 2 of 9

3 Plastic fibre Plastic fibre is typically made of plastic. These are low cost optical fibres. They have a large NA and a large refractive index variation between the core and cladding. Plastic fibre-optic cable has the highest attenuation; because of nature of the material they offer larger signal attenuation than glass fibres. However, this drawback is overcome by their ruggedness and durability. Plastic fibre-optic cable has a plastic core and plastic cladding. Examples: 1. Polystyrene (core) methylmethacrylate (cladding) 2. Polymethylmethacrylate (core) copolymer (cladding) Plastic clad silica fibre Plastic clad silica fibre (PCS) is another type of optical cable. It has an attenuation lies between glass and plastic and a cost that lies between their costs as well. PCS fibre-optic cable has a glass core that is made of silica, while the cladding is plastic (low refractive index silicone elastomer). (b) Mode A mode is a path taken by a ray of light through an optical fibre. It is a mathematical concept of explaining the propagation of light waves in an optical fibre. The number of modes of propagation within a fibre depends on the diameter of the fibre core. Normally, narrower of the Material prepared by: < Physics Faculty> Topic No: < 2 > Page 3 of 9

4 fibre core lesser is the number of modes of propagation. Hence on the basis of number of modes, fibres are classified as Single-mode fibre Multimode fibre Single-mode fibre A single-mode fibre has a narrow core (8-10µm) and the refractive index between the core and cladding does not change very much throughout the fibre. Because of the narrower core, light travels almost parallel to the axis, thus creating little or almost no pulse dispersion. Hence, they can carry a higher bandwidth (up to 50 times more distance than multimode fibre) but require a light source with a narrow spectral width. Multi-mode fibre This type of fibre allows more than one mode to travel through it. As it has to allow more number of modes to travel, the fibre has to have a larger core. A typical multimode fibre has a core diameter in the order of µm. Such a large core allows more number of modes to travel but Material prepared by: < Physics Faculty> Topic No: < 2 > Page 4 of 9

5 also makes it easier to launch the light into fibre and to connect two fibres together. On the other hand, larger the number of modes, larger will be the dispersion of light inside the fibre. (C) Refractive Index Optical fibres have two layers namely core and cladding with different refractive indices. The variation of the refractive index within the core determines the nature of the fibre. An optical fibre is characterized according to the variation of the core Step index fibre Graded index fibre Step index fibre In a step index fibre, the refractive index remains constant throughout the core and decreases at the core-cladding boundary and then remains constant throughout the cladding. As the refractive index varies as a step at the core-cladding boundary, the fibre is known as step index fibre. The light rays travel through the fibre as meridional rays i.e., the rays zigzag through the length of the fibre, crossing the fibre axis for every reflection. Material prepared by: < Physics Faculty> Topic No: < 2 > Page 5 of 9

6 Single mode Step Index Fibre Multimode step index fibre Graded index fibre A graded index fibre is different from the step index fibre not only in the core refractive index variation but also in the size of fibre. In a graded index fibre, the refractive index does not remain constant within core, but instead, it gradually decreases on either side from the axis of the core towards the core-cladding boundary. At the boundary, the refractive index of the core is minimum and equals the refractive index of the cladding. Because of the gradual variation of the refractive index along the radius of the core, the fibre is known as graded index fibre. Material prepared by: < Physics Faculty> Topic No: < 2 > Page 6 of 9

7 As the refractive index variation resembles concentric cylinders of equal refractive indices around the axis, any ray of light entering any one of the cylindrical layers get trapped by it remains within layer for the rest of the travel. This means that light does not and will not cross axis of the fibre and merely goes in a helical path around the axis. Therefore, the rays of light in a graded index fibre are known as helical rays. As the path of the light rays is skewed, they are also known as skew rays. Check your understanding 1. For long distance communication (a) graded index fibres are more suitable (b) single mode step index fibres are most suitable (c) step index fibre are more suitable (d) silica fibres are more suitable Material prepared by: < Physics Faculty> Topic No: < 2 > Page 7 of 9

8 2. V-number of the fibre (a) gives the exact number of modes propagation through the fibre (b) is directly proportional to the number of modes propagating through the fibre (c) describes the different modes propagating through the fibre (d) is meant only for the single mode fibres only 3. Self focusing effect is produced in (a) step index fibre (b) step index single mode fibre (c) multimode graded index fibre (d) single mode graded index fibre. Check the correct answers on page: 9 Summary On completion of this chapter you have learned that: 1. Optical fibres are flexible, thin and cylindrical structures and are made of transparent materials such as glass and plastic, which allows light to be guided from one end to other end with minimal loss. 2. A mode is a path taken by a ray of light through an optical fibre and is mathematical concept of explaining the propagation of light waves in an optical fibre. 3. The number of modes (N) that can propagate in a step index fibre is N = V 2 /2 4. The number of modes that can propagate in a graded index fibre is half of that of a step index fibre, which is given by N = V 2 /4 5. On the basis of propagation of number of modes, fibres are classified as single-mode and multi-mode fibre. Material prepared by: < Physics Faculty> Topic No: < 2 > Page 8 of 9

9 6. A single-mode fibre has a narrow core (8-10µm) and the refractive index between the core and cladding does not change very much throughout the fibre 7. A multi-mode fibre allows more than one mode to travel through it and therefore has a larger core. Typically, the core diameter of a multimode fibre is in the order of µm. 8. An optical fibre is characterized according to the variation of the core refractive index as step index and graded index fibre. 9. The refractive index remains constant throughout the core in a step index fibre and decreases at the core-cladding boundary and thereafter remains constant in cladding. 10. The light rays travel through the step index fibre as meridional rays i.e., the rays zigzag through the length of the fibre, crossing on either side for every reflection. 11. In a graded index fibre, the refractive index does not remains constant within the core, but instead, it gradually decreases on either side from the axis of the core towards the corecladding boundary. 12. The ray of light does not and will not cross the axis of the graded index fibre and merely goes in a helical path around the axis and is therefore known as helical ray. Suggested Reading 1. Engineering Physics by Dr. P.K.Palanisamy Answers to Check your Understanding 1. (b) 2. (b) 3. (c) Material prepared by: < Physics Faculty> Topic No: < 2 > Page 9 of 9