This document discusses different types of optical fibers used to transmit light signals, including single mode fibers and multimode fibers. Single mode fibers only allow one propagation mode and have a small core diameter of 8-10 μm, making them most widely used in telecommunications for their ability to transmit high bit rates. Multimode fibers allow multiple propagation modes and can be either step index fibers, where the core has a uniform higher refractive index than the cladding, or graded index fibers where the refractive index gradually decreases from the center of the core toward the cladding.

1. 1
Birla Vishvakarma
Mahavidyalaya

2. Kardani Dhruvit P.
13008080125007 –B12
Production Engineeering
Birla Vishvakrama maha Vidhyalaya
Mode of Propogation

3. The Optical Fiber
Fiber optic cable functions as a ”light guide” guiding
the light from one end to the other end.
• Single Mode Fiber (SMF)
• Multimode Fiber (MMF)
Fiber categories
based on
propagation:
• Step Index Fiber (SIF)
• Graded Index Fiber (GIF)
Categories based
on refractive
index profile

4. Refraction and Reflection
Snell’s Law: n1 Sin Φ1 = n2 Sin Φ2
When Φ2 = 90,
Φ1 = Φc is the
Critical Angle
Φc=Sin-1(n2/n1 )

5. Single Mode Step Index Fiber
Buffer tube: d = 1mm
Protective polymerinc coating
Cladding: d = 125 - 150 m
Core: d = 8 - 10 m
n
r
n1
n2
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
The cross section of a typical single-mode fiber with a tight buffer
tube. ( d = diameter)
© 1999 S.O. Kasap,O ptoelectronics (Prentice Hall)

6. Step Index Multimode Fiber
• Guided light propagation can be explained by
ray optics
• When the incident angle is smaller the
acceptance angle, light will propagate via TIR
• Large number of modes possible
• Each mode travels at a different velocity
Modal Dispersion
• Used in short links, mostly with LED sources

7. Step Index Multimode Fiber
2
1 1
2 n
2
1
n n
2
1
2
2
n
n
 

 
Fractional refractive-index
profile

8. Ray description of different fibers

9. Thank you