Monday, 20 October 2014

Types of Fiber


Optical fiber is classified based on material used, mode & refractive index.



Step Index Optical Fiber:

  • In the step index fiber, the refractive index of the core is uniform throughout.
  • Different light modes in a step-index multimode fiber follow different lengths along the fiber in zigzag way
  • The arrival of different modes of the light at different times causes Inter Modal Dispersion.
  • 50 mm core for step index.











Graded Index Optical Fiber:

  • Graded-index fiber’s refractive index decreases gradually away from its center.
  • Light rays follow sinusoidal paths.
  • Reduces inter modal dispersion.
  • 62.5 mm core for graded index.














Single Mode Fiber:

  • In a single mode fiber only one mode can propagate through the fiber.
  • Core of SM fiber is smaller (8.3 to 10 microns). Common size is 9um.
  • Laser is used as light source for single mode fiber.
  • Used in long distance communication upto100 Gbit/s data in DWDM.
  • 1310 or 1550 nm wavelength is used in SM fiber with higher data rate.










Multimode Fiber:

  • Multimode fibers allow a large number of modes.
  • A multimode optical fiber has larger core (50 to 100um). Most common size is 50um or 62.5um.
  • DWDM is not normally used on multi-mode fiber.
  • 850 or 1300 nm wavelength is used in multimode fiber.
  • LED is used as a light source in multimode fibers .











Practically used Optical Fibers:

  • MM Step index: 62.5/125 µm
  • MM Graded index: 50/125 µm
  • SM Step index: 9/125 µm
  • Single mode graded index is not practically used

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