Chapter 5

Laser-Fiber Connection

Content

• Launching optical power into a fiber

• Fiber-to-Fiber coupling

• Fiber Splicing and connectors

Coupling Efficiency

s

F

P

P

sourse thefrom emittedpower

fiber theinto coupledpower [5-1]

Source Optical Fiber

sPFP

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:

cos),( 0BB [5-2]

Edge emitting LEDs and laser diodes radiation pattern

LT BBB cos

cos

cos

sin

),(

1

0

2

0

2

For edge emitting LEDs, L=1

[5-3]

Power Coupled from source to the fiber

rdrdddB

dAdABP

s

r

s

A

sssF

m

f f

max0

0

2

0

2

00

sin),(

),([5-4]

source theof angleemission solid and area : and ssA

fiber of angle acceptance solid

and area : and ffA

Power coupled from LED to the Fiber

rdrdB

rdrdB

rdrddBP

s

r

s

r

s

r

s

s

s

22

00

0

2

0

max02

0

0

2

0 0

0

0

NA

sin

sincos2max0

210

2220

22stepLED, 2)NA( nBrBrP ss [5-5]

Power coupling from LED to step-index fiber

• Total optical power from LED:

sincos2

sin),(

2/

0

022

02

2

0

2/

0

BrdBrP

ddBAP

sss

ss

[5-6]

arP

r

a

arP

Pss

s

ss

if )NA(

if )NA(

2

2

2

stepLED, [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:

• Low loss fiber-fiber joints are either:

1- Splice (permanent bond)

2- Connector (demountable connection)

FFL log10]dB[ [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.

2/12

2step, 21

2arccos

2

a

d

a

d

a

d

a

AcommF [5-9]

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

NAlog(20)(

for )log(10)(

[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