Physics presentation(step index and graded index)

Presentation on step index and graded index fibres.
And types of fibres

1. STEP INDEX & GRADED INDEX

2. WHAT ARE OPTICAL FIBRES ?

3. Optical fibre (or " fibre optic") refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic strand or fibre.

4. HOW DO OPTICAL FIBRES WORK?

5. Types of Optical Fibre Optical fibres are generally of two types based upon the size of the core and cladding: Single mode fibre(SMF) Multimode fibre(MMF)

6. 1. Single mode fibre A Single Mode F ibre has a smaller diameter and can support only one mode of propagation. 2. multimode fibre A Multimode F ibre has a larger core diameter and supports a number of modes.

7. Index profile Single mode fibres and Multimode fibres can be distinguished on the basis of index profile. An index profile is a plot of refractive index drawn on horizontal axis versus the distance from the core axis drawn on the vertical axis.

8. Types of index profiles There are two types of index profiles: STEP INDEX PROFILE(SI) GRADED INDEX PROFILE(GRIN) The index profile of a MMF can either be step index or graded index type whereas the index profile of SMF is usually a step index type.

9. STEP-INDEX profile In this type of optical fibre the fibre core has a uniform refractive index n f and cladding also has uniform refractive index n c such that n f > n c . The figure below represents the step-index optical fibre where a and b are the radii of the core and cladding, respectively. n f (fibre core) n c (cladding) Air Air n b a O

10. The following figure shows the path of rays: 2 1 2 The rays entering at different angles, travel different paths and emerge out the fibre at different time along different directions. Two rays are shown in figure. Clearly, an input pulse widens as it travels along the fibre. 1

11. n f (fibre core) n c (cladding) Air Air b n a O Graded-index profile In graded-index optical fibre the core has non-uniform refractive index which increases from axis towards the cladding. The cladding has a uniform refractive index.

12. The path of rays is shown in the following figure: In this type, the entering ray is continuously bent and travels a periodic path along the axis. The rays entering at different angles follow different paths with same period in space and time. Thus there is a periodic self-focusing of the rays. In this type of fibre the pulse-dispersion is less than that in step-index fibre. n c n f

13. Single mode step index fibre A single mode step index fibre has a very fine thin core of uniform refractive index of a higher value which is surrounded by a cladding of lower refractive index. The refractive index changes abruptly at the core-cladding boundary, because of which it is known as step index fibre. n 1 n 2 n 2 n 1 n 2 n 1 n 2 (a) (b) (c) Refractive index profile (b) Cross-section (c) Side view showing mode of propagation

14. The fibre is surrounded by an opaque protective sheath. A typical SMF has a core diameter of 4 µm, which is of the order of a few wavelength of light. Light travels in SMF along a single path that is along the axis. Obviously, it is zero order mode that is supported by a SMF. A SMF is characterised by a very small value of ∆. It is of the order of 0.002. CONTINUED…

15. Multimode step index fibre It is similar to the single mode step index fibre but it has larger core diameter of 100µm which is very large as compared to the wavelength of the light being transmitted. Light follows the zigzag path inside the fibre. Many such zigzag paths of propagation are permitted in a MMF. A typical structure and index profile of a step index MMF are as shown:

16. n 1 n 2 n 2 n 1 n 2 n 1 n 2 (a) (b) (c) Refractive index profile (b) Cross-section (c) Side view showing mode of propagation The NA of a MMF is larger as the core diameter of the fibre is larger. It is of the order of 0.3 . θ CONTINUED…

17. n 1 n 2 r 2a n 2 n(r) n Cladding 2a Core (a) (b) (c) Multimode graded index fibre In this type of fibre, the refractive index of the core varies with distance form the fibre axis. It has a high value at the centre and falls off with increasing radial distance from the axis. A typical structure and its index profile are shown in figures: Refractive index profile (b) Cross-section (c) Side view showing typical ray paths along a GRIN fibre.

18. Such a profile causes a periodic focussing of the light propagating through the fibre. In case of GRIN fibres, the acceptance angle and numerical aperture decreases with radial distance from the axis. For the fibres of the parabolic index-profile, the numerical aperture is given by: NA = n 1 (2∆) 1/2 √1-(r/a) 2 CONTINUED…

19. Thank you By RITESH GOYAL 16103027