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