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Chapter 5
Power Launching and Coupling
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Content
• Launching optical power into a fiber
• Fiber-to-Fiber coupling
• Fiber Splicing and connectors
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Coupling Efficiency
s
F
P
P
source the from emittedpower
fiber the into coupledpower [5-1]
Source Optical Fiber
sP FP
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Fiber-to-Fiber Joint
• 1) Fiber-to-Fiber coupling loss:
• 2) Low loss fiber-fiber joints are either:
1- Splice (permanent bond)
2- Connector (demountable connection)
FFL log10]dB[ [5-8]
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Mechanical misalignment losses
Lateral (axial) misalignment loss is a dominant Mechanical loss.
2/12
2step, 21
2arccos
2
a
d
a
d
a
d
a
AcommF
[5-9]
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Lateral (axial) misalignment loss: Cont…
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Longitudinal offset effect
Losses due to differences in the geometry and waveguide characteristics of the fibers
ERE
RF
ERE
RF
aL
aaa
aaL
NANAfor NA
NA
for
)log(20)(
)log(20)([5-10]
E & R subscripts refer to emitting and receiving fibers.
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Losses due to differences in the geometry and waveguide characteristics of the fibers
ERE
RF
ERE
RF
aL
aaa
aaL
NANAfor NA
NA
for
)log(20)(
)log(20)(
[5-10]
E & R subscripts refer to emitting and receiving fibers.
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Experimental comparison of Loss as a function of mechanical misalignment
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Fiber splicing
Fusion Splicing
1) Fusion splicing involves butting two cleaned fiber end faces and heating them until they melt together.
2) Fusion splicing is normally done with a fusion splicer that controls the alignment of the two fibers to keep losses as low as 0.05 dB.
3) Fiber ends are first prealigned and putted together under a microscope with micromanipulators. 4) The butted joint is heated with laser pulse to melt the fiber ends so can be bonded together.
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V-groove optical fiber splicing
1) Mechanical splices join two fibers together by holding them tightly with a structure or by gluing the fibers together.
2) Mechanical splices may have a slightly higher loss and back reflection. These can be reduced by inserting index matching gel.
3) V groove mechanical splicing provides a temporary joint i.e fibers can be disassembled if required. The fiber ends are butted together in a V – shaped groove
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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
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Connector Return Loss
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Power Coupled from source to the fiber
210
2220
22stepLED, 2)NA( nBrBrP ss
rs is the source radius
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Power coupling from LED to step-index fiber
• Total optical power from LED:
022 BrP ss
arP
r
a
arP
Pss
s
ss
if )NA(
if )NA(
2
2
2
stepLED,
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Example
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Power coupling from LED to graded index fiber
022 BrP ss
Where rs <a
HW: Try rs >a