attenuation of optical fiber communication systems.pdf

optical fiber

1. Signal Attenuation

2. Signal Attenuation & Distortion in
Optical Fibers
•What are the loss or signal attenuation mechanisms in a fiber?
•Why & to what degree do optical signals get distorted as they
propagate down a fiber?
•Signal attenuation (power loss) largely determines the
maximum repeaterlessseparation between optical transmitter
& receiver.
•Signal distortion cause optical pulses to broaden as they travel
along a fiber, the overlap between neighboring pulses, creating
errors in the receiver output, resulting in the limitation of
information-carrying capacity of a fiber.

3. Attenuation (power loss)

4. Attenuation (power loss)

5. 






)(
)0(
log
10
]dB/km[
zP
P
z
 Power loss in dB/km
•Where [dBm] or dB milliwatis 10log(P[mW]).
z=0
Z=l
]dBm)[0(P
]km[]dB/km[]dBm)[0(]dBm)[( lPlP  

6. Example 1

7. Solution

8. Computing Power Level in Decibel (dB)
P
1
P
2
Relative Power
dB= Decibels
1
2
RatioPower
P
P

1
210 logdB
P
P
 
 
 
 

P
2
< P
1
,dB < 0 Losses
P
2
> P
1
,dB > 0 Gain

3 210 logdB
1
3
2
10logdB
 
 
 

2 1
10
10
dB
P P
 
 
 
 

1 3 2 1
34 4 2
PP P P
P P P P
 
1 3 2 1
34 4 2
10 log 10 log
PP P P
dB
P P P P
  
   
   
   
 
3 2 1
34 2
10 log 10 log 10 log
PP P
dB
P P P
    
    
     
     
 
4 3 2
dB dB dB dB
System
P
1
P
4
P
2
P
3
System System System

9. P
1
P
4
P
2
P
3
Example 2:Suppose that three elements in the figure have losses of 11, 6 and 3 dBrespectively. Find
the total loss of the commination system. Find the output power if the input power is 5 mW.
Solution

2
10log
mdB P
11 6 3 20
loss
T dB   
10
10
dB
outin
P P
 
 
 
 
Example 3:A Light Emitting Diode radiates 2 mW. Compute the dB
m
value of this radiated power.
This power travels through a group of components having a combined loss of 23 dB.Compute the
output power.
Absolute Power Assume P
1
= 1 mW

2
10 log
mdB P

310 log 2
m mdB dB 
20
10 10
0.0110 10
dB
out
mWP
   
   
   
   

  
The output Power3 23 20
m m
dB dB dB  
10
10
dB
outin
P P
 
 
 
 
or
Solution
1
2
10 logdB
P
P
 
 
 
 

20
10
0.01 0.055 10 5 mW
 
 
 
 

   
23
10
0.012 10 mW
 
 
 
 

 

10. Fiber attenuation (losses) mechanisms
•Material absorption
•Scattering losses
•Bending losses

11. Material absorption losses

12. Fiber Attenuation
A-Material Absorption
1-Intrinsic Absorption by atomic defects in the glass
Crystallography:
an imperfection in
a crystal caused by
the presence of an
extra atom in an
otherwise
complete lattice.
Vacancies
Composition (i.e. missing
molecules, high density clusters of
atoms, group or oxygen defects in
the glass structure
Usually absorption from these defects are small and negligible compared to other sources
2-Extrinsic Absorption by Impurities atoms
Substitutional Impurities
Atomic defectsact like impurities:
they can also contribute to light
attenuation

13. Intrinsic absorption

14. Fiber attenuation characteristics

15. Extrinsic absorption

16. OH absorption peak

17. Scattering losses

18. Scattering losses

19. Example: Optical Fiber Attenuation
WavelengthSMF28 62.5/125
850 nm 1.8 dB/km2.72 dB/km
1300 nm 0.35 dB/km0.52 dB/km
1380 nm 0.50 dB/km0.92 dB/km
1550 nm 0.19 dB/km0.29 dB/km

20. Bending Losses
•MacrobendingLoss
:
The
curvatureofthebendismuch
largerthanfiberdiameter.
Lightwavesufferssevereloss
duetoradiationoftheevanescent
fieldinthecladdingregion.As
theradiusofthecurvature
decreases,thelossincreases
exponentiallyuntilitreachesata
certaincriticalradius.Forany
radiusabitsmallerthanthis
point,thelossessuddenly
becomesextremelylarge.Higher
ordermodesradiateawayfaster
thanlowerordermodes.

21. Macrobending Loss-Example
ForSMF-28fiberalossof0.05dBwillbeintroducedwith100
turnsaroundamandrelofdiameterof75mm.
Losswillincreaserapidlyforasmallerbendingradius.
Typically,abendingradiusshouldbemorethan150timesthe
claddingdiameterofthefiberforlongtermapplicationsand
morethan100timesthecladdingdiameterforshortterm
applications.Forexample,forfiberwith125mmcladding
diameter,thebendingradiusshouldbelargerthan19mm.

22. Bending Losses
•MicrobendingLoss:
Microscopicbendsofthefiber
axisthatcanarisewhenthe
fibersareincorporatedinto
cables.Thepowerisdissipated
throughthemicrobendedfiber,
becauseoftherepetitive
couplingofenergybetween
guidedmodes&theleakyor
radiationmodesinthefiber.
Small-scale fluctuations in the radius of curvature of the fiber axis leads to
microbending losses. Microbends can shed higher-order modes and can
cause power from low-order modes to couple to higher-order modes

23. Bending Losses
Keeping bend radii greater than 10 cm makes bend loss negligible
It is common to wrap optical fiber in communication systems