Optical Communication

1. Engr. Abbas Abbasi
Optical Fiber
Theoretical Background

2. Acceptance Angle
 The acceptance angle (qa) is the largest incident angle ray that can be coupled into a
guided ray within the fiber
 The incident angle at air-core interface for which core-cladding interface angle is equal to
the critical angle

3. Numerical Aperture (NA)
1
sin sin
o i r
n n
q q

90
r
q 
 
 
1 1
2
1
2 2
2 2
1 1 1 2
1
2 2
1 2
sin sin 90 cos
cos 1 sin
sin cos 1 sin 1
sin
o i
o a c
o a
n n n
But
For critical angle
n
n n n n
n
n n n
q  
 
q  
q
  
 
 
    
 
 
 
1 2 2
2
2
1
sin sin
90
sin
c
c
n n
n
n
 q
  q


  

2 2
1 2
sin
o a
NA n n n
q
  

4. Relative Refractive Index Difference
 
2 2
1 2
2 2
1 2
2
1
1 2
1
2 2 2 2
1 1 2
1
sin 1
sin
2
1
2
2
o a o
a
NA n n n But n
NA
n n
Binomial Expansion
n
n n
for
n
n n n NA
NA n
q
q
   


 

 
    
  

5. Example
 A silicon optical fiber has a core refractive index of 1.5 and a cladding refractive index of
1.47. Determine:
a) Critical angle at core-cladding interface
b) NA for the fiber
c) Acceptance angle in air for the fiber

6. Another example
 Relative refractive index difference for an optical fiber is 1%. Estimate NA. Also calculate
critical angle at core-cladding interface

7. Meridional Rays Vs Skew Rays
 A meridional ray is a ray that passes through the axis of an optical fiber
 A skew ray is a ray that travels in a non-planar zigzag path and never crosses the axis of
an optical fiber.

8. Mode Coupling
 Irregularities at Core-Cladding Interface
 Fiber Bend
 For both effects, light can not maintain its angle with the fiber axis

9. Fiber Types
 Step Index Fibers
 Graded Index Fibers
 Single Mode Fibers

10. Fiber Types…
 Step Index Fibers
 Graded Index Fibers
 Single Mode Fibers

11. Step Index Fiber
1
2
( )
( )
( )
n r a Core
n r
n r a Cladding


 


Different propagation modes
Different mode velocities
Intermodal dispersion
Dispersion restricts the use of
maximum bandwidth
Preferred at lower bandwidth
applications
Larger diameters and NA
Easier source coupling
Multiple source coupling

12. Graded Index Fibers
 
1
2
1 2 ( )
( )
( )
r
n Core
a
n r
n Cladding
a

 

 


Index Profile

13. Graded Index Fibers…
Refraction
Mechanism
Less Intermodal
Dispersion

14. Single Mode Fiber
No delay differences between different modes because of single mode
All other modes attenuated or absorbed
Most widely used
Greatest bandwidth and lowest loss
Superior transmission quality over other fibers
No modal noise

15. Related Problems
 Chapter 2
 Examples 2.1 & 2.2 (Total = 2)
 End Problems 2.1 , 2.2, 2.3, 2.4 (Total = 4)

16. End of 4th module
 Thank you