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Chapter 5
Laser-Fiber Connection
Content
• Launching optical power into a fiber
• Fiber-to-Fiber coupling
• Fiber Splicing and connectors
Coupling Efficiency
PF
power coupled into the fiber


power emitted from the sourse Ps
Ps
Source
PF
Optical Fiber
[5-1]
Radiance (Brightness) of the source
• B= Optical power radiated from a unit area of the source into a
unit solid angle [watts/(square centimeter per stradian)]
Surface emitting LEDs have a Lambertian pattern:
B( ,  )  B0 cos
[5-2]
Edge emitting LEDs and laser diodes radiation pattern
1
sin 
cos 


T
L
B( ,  ) B0 cos  B0 cos 
2
For edge emitting LEDs, L=1
2
[5-3]
Power Coupled from source to the fiber
As and  s : area and solid emission angle of the source


PF     B( As ,  s )d s dAs 
[5-4]

A f and  f : area and
Af 
 f
solid acceptance angle of fiber
rm 2
2  0 max

      B( ,  ) sin dd  d s rdr

0
0 
0 0
Power coupled from LED to the Fiber
 0 max


 2B0 cos  sin d d s rdr
0 
0



2
rs
P
0
rs
2
0
0
rs
2
 B0 
2
sin
  0 max d s rdr
 B0 
2
 NA d rdr
s
0
0
PLED,step   rs B0 ( NA)  2 rs B0 n1 
2
2
2
2
2
2
[5-5]
Power coupling from LED to step-index fiber
• Total optical power from LED:
2  / 2
Ps  As

0
B( ,  ) sin dd
0
Ps  rs 2B0
2
 /2
2
cos

sin

d



rs B0

2
[5-6]
0
PLED,step
Ps ( NA) 2

  a  2
2


P
(
NA
)
  s
 rs 
if rs  a 


if rs  a 

[5-7]
Equilibrium Numerical Aperture
Examples of possible lensing schemes used to improve optical source-to-fiber coupling
efficiency
Laser diode to Fiber Coupling
Fiber-to-Fiber Joint
• Fiber-to-Fiber coupling loss:
LF [dB]  10 log  F
• Low loss fiber-fiber joints are either:
1- Splice (permanent bond)
2- Connector (demountable connection)
[5-8]
Different modal distribution of the optical beam emerging from a fiber lead to different degrees of
coupling loss. a) when all modes are equally excited, the output beam fills the entire output NA.
b) for a steady state modal distribution, only the equilibrium NA is filled by the output beam.
Mechanical misalignment losses
Lateral (axial) misalignment loss is a dominant
Mechanical loss.
 F ,step
Acomm 2
d
d   d 

 arccos

1   
2

2a a   2a 
a
2



1/ 2
[5-9]
Longitudinal offset effect
Losses due to differences in the geometry and waveguide characteristics
of the fibers
aR
LF (a)  10 log( )
aE
for a R  a E
NA R
LF (a)  20 log(
)
NA E
for NA R  NA E
[5-10]
E & R subscripts refer to emitting and receiving fibers.
Experimental comparison of Loss as a function
of mechanical misalignment
Fiber end face
Fiber end defects
Fiber splicing
Fusion Splicing
V-groove optical fiber splicing
Optical Fiber Connectors
• Some of the principal requirements of a good connector design are as
follows:
1- low coupling losses
2- Interchangeability
3- Ease of assembly
4- Low environmental sensitivity
5- Low-cost and reliable construction
6- Ease of connection
Connector Return Loss