1. UNIT-II Optical Fiber ECE – IV SEM Manav Rachna College of Engg.
Manav Rachna College of Engineering

2. Optical Fiber Classification
Can be classified in a number of ways
On the basis of manufacturing
Single component/Multi component
Glass core glass clad
Doped silica core clad
All plastic fiber
On the basis of profile
Step index
Multi mode
Mono mode
Graded index
Manav Rachna College of Engg.

3. Fiber modes
Electromagnetic field propagating in fiber can be described by Maxwell’s equations whose solution yields number of modes M. For a step index profile where a is the core radius and V is the mode parameter (or normalized frequency of the fiber)
Depending on fiber parameters, number of different propagating modes appear
For single mode fibers
Single mode fibers do not have mode dispersion (see the supplementary ‘Mode Theory’ for further details)
Manav Rachna College of Engg.

4. Fiber modes (cont.)
Manav Rachna College of Engg.

5. Inter-modal (mode) dispersion
Multimode fibers exhibit modal dispersion that is caused by different propagation modes taking different paths:
cladding
Path 1
core
Path 2
cladding
Manav Rachna College of Engg.

6. Step Index Fiber
Core and Cladding are glass with appropriate optical properties
Buffer is plastic for mechanical protection
Manav Rachna College of Engg.

7. The Optical Fiber
Fiber optic cable functions as a ”light guide,” guiding the light from one end to the other end.
Categories based on propagation: Single Mode Fiber (SMF)
Multimode Fiber (MMF)
Categories based on refractive index profile Step Index Fiber (SIF)
Graded Index Fiber (GIF)
Manav Rachna College of Engg.

8. Manav Rachna College of Engg.

9. Single Mode Step Index Fiber
Only one propagation mode is allowed in a given wavelength.
This is achieved by very small core diameter (8-10 μm)
SMF offers highest bit rate, most widely used in telecom
Manav Rachna College of Engg.

10. Ray description of different fibers
Manav Rachna College of Engg.

11. Refraction and Reflection Snell’s Law:
Manav Rachna College of Engg.
When Φ2= 90, Φ1= Φc is the Critical Angle
Φc=Sin-1(n2/n1)
Snell’s Law: n1Sin Φ1= n2 Sin Φ2

12. Step Index Multimode Fiber
Manav Rachna College of Engg.

13. Manav Rachna College of Engg.

14. Manav Rachna College of Engg.

15. Step and Graded Index Fibers
Manav Rachna College of Engg.

16. Graded Index Fiber
Manav Rachna College of Engg.

17. Total Internal Reflection
TIR
Manav Rachna College of Engg.
A ray in thinly stratified medium becomes refracted as it passes from one layer to the next upper layer with lower n and eventually its angle satisfies TIR
In a medium where n decreases continuously the path of the ray bends continuously

18. Skew Rays
Manav Rachna College of Engg.

19. Manav Rachna College of Engg.

20. Manav Rachna College of Engg.

21. Manav Rachna College of Engg.

22. Manav Rachna College of Engg.

23. Manav Rachna College of Engg.

24. Manav Rachna College of Engg.

25. Manav Rachna College of Engg.

26. Manav Rachna College of Engg.

27. Higher the Wavelength More the Evanescent Field
Manav Rachna College of Engg.

28. Light Intensity
Manav Rachna College of Engg.

29. Manav Rachna College of Engg.

30. Manav Rachna College of Engg.

31. Manav Rachna College of Engg.

32. Manav Rachna College of Engg.

33. Modal Dispersion
dispersion
Manav Rachna College of Engg.

34. Manav Rachna College of Engg.

35. Manav Rachna College of Engg.

36. Manav Rachna College of Engg.

37. Manav Rachna College of Engg.

38. Manav Rachna College of Engg.

39. Manav Rachna College of Engg.

40. Manav Rachna College of Engg.

41. Manav Rachna College of Engg.

42. Manav Rachna College of Engg.

43. Manav Rachna College of Engg.

44. Manav Rachna College of Engg.

45. Optical fibers - attenuation
Traditionally two windows available:
1.3 mm and 1.55 mm
The lower window is used with Si and GaAlAs and the upper window with InGaAsP compounds
Nowadays these attenuation windows no longer separate (water-spike attenuation region can be removed)
There are single- and mono mode fibers that may have step or graded refraction index profile
Propagation in optical fibers is influenced by attenuation, scattering, absorption, and dispersion
In addition there are non-linear effects that are important in WDM-transmission
Manav Rachna College of Engg.

46. Manav Rachna College of Engg.

47. Manav Rachna College of Engg.

48. Manav Rachna College of Engg.

49. Manav Rachna College of Engg.

50. Manav Rachna College of Engg.

51. Manav Rachna College of Engg.

52. Manav Rachna College of Engg.

53. Scattering loss: from index discontinuity
Scatterers are much smaller than the wavelength: Rayleigh and Raman scattering
Scatterers are much bigger than the wavelength: geometric ray optics
Scatterers are about the same size as the wavelength: Mie scattering
Scatterers are sound waves: Brillouin scattering
Manav Rachna College of Engg.

54. Raman scattering
A small fraction of Rayleigh scattered light comes off at the difference frequency between the applied light and the frequency of a molecular vibration (a Stokes line)
In addition, some scattered light comes off at the sum frequency (anti-Stokes
MIE Scattering
Similar effect to microbending loss
Mie scattering depends roughly on λ-2; scattering angle also depends upon λ
In planar waveguide devices, roughness on side walls leads to polarization-dependent loss
Manav Rachna College of Engg.

55. Couplers and connectors
Metallic system: Wire: Soldering: Lossless: Economical
Fiber system: fiber: Splicing: Loss: Economical
Splicing is the permanent connection of two optical fiber.
Two mechanism of splicing:
Fusion
Mechanical
Splices are of two types:
Midspan: the connecting of two cables
Pigtail: assembly of a fiber that has been factory-installed into
a connector in one end, with the other end free for splicing to
a cable.
The quality of splicing is measured by the insertion and reflection
losses caused by the splice
Manav Rachna College of Engg.

56. Couplers and connectors losses
Connectors losses at any splice stem from the fact that no all light
from one fiber is transmitted to another. The loss results from:
Mismatch: due to fiber’s mechanical dimensions and numerical
aperture. An improvement in splicing technique can not solve
the problem. Also called intrinsic connection losses.
Misalignment: are caused by some imperfection in splicing,
that theoretically can be eliminated. An improvement in
splicing technique can solve the problem. Also called extrinsic
connection losses.
Misalignment Losses in fiber-to-fiber:
Core misalignment and imperfections.
Lateral (axial) misalignment
Angular misalignment
gap between ends contact
Non-flat ends
Cladding alignment
Manav Rachna College of Engg.