Chapter 5

Power Launching and Coupling

Content

• Launching optical power into a fiber

• Fiber-to-Fiber coupling

• Fiber Splicing and connectors

Coupling Efficiency

s

F

P

P

source the from emittedpower

fiber the into coupledpower [5-1]

Source Optical Fiber

sP FP

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]

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]

Lateral (axial) misalignment loss: Cont…

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.

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.

Experimental comparison of Loss as a function of mechanical misalignment

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.

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

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

Power Coupled from source to the fiber

210

2220

22stepLED, 2)NA( nBrBrP ss

rs is the source radius

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,

Example

Power coupling from LED to graded index fiber

022 BrP ss

Where rs <a

HW: Try rs >a