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11/25/2011
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4Power Launching and
CouplingTh l b d Ch 5 G KThis lecture is based on Chapter 5, G. Keiser
Topics in this lecture
Launching optical power into a fiber
Fiber-to-Fiber coupling
Fiber Splicing and connectors
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Fiber Splicing and connectors
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Coupling Efficiency
s
F
PP
==soursethefromemittedpower fiber theinto coupledpower η
sp
3
Source Optical FibersP
FP
Radiance (Brightness) of the sourceOptical power coupled into the fiber depends upon the radiance of optical source rather than total output power. (Optical power radiated from a unit area of the source into a unit solid angle) (watts/cm2/steradian)a unit solid angle) (watts/cm2/steradian)→ [spatial distribution of the
optical power of the source]
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Source output pattern
θφθ cos),( 0BB =For surface emitting LEDs
B0 is the radiance along the normal to the radiating surface
For Edge emitting LEDs and lasers
φφ cossin1 22+=
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θθφθ LT BBB coscos),( 00+
Integers T and L are the transverse and lateral power distribution coefficients
For edge emitting LEDs, L = 1
Power Coupled from source to the fiber
rdrdddB
dAdABP
r
sA
sssF
m
f f
θφθθφθθππ
⎥⎤
⎢⎡
=
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡ΩΩ=
∫∫∫∫
∫ ∫Ω
max022
sin)(
),(
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rdrdddB sθφθθφθ⎥⎥
⎦⎢⎢
⎣= ∫∫∫∫
0000
sin),(
source theof angleemission solid and area : and ssA Ωfiber of angle acceptance solid and area : and ffA Ω
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Example: Power coupled from LED to the Step Index Fiber
rdrddBP s
rs
θθθθππ θ
∫ ∫∫ ⎟⎟⎠
⎞⎜⎜⎝
⎛=
2
0 sincos2max0
For rs < a (fiber radius)
rdrdB
rdrdB
s
r
s
r
s
s
θπ
θθπ
π
π
∫∫
∫∫
∫ ∫∫
=
=
⎟⎠
⎜⎝
2
0
2
00
2
0max0
2
00
0 00
NA
sin
7
00
Δ≈= 210
2220
22stepLED, 2)NA( nBrBrP ss ππ
Total optical power from LED:
sin),(2
0
2/
0∫ ∫=ππ
ϕθθϕθ ddBAP ss
sincos2 2/
00
220
2 ∫ ==π
πθθθππ BrdBrP sss
⎪
⎪⎬
⎫
⎪
⎪⎨
⎧
≥⎟⎟⎞
⎜⎜⎛
≤
=arPa
arP
Pss
if)NA(
if )NA(
22
2
stepLED,
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⎪⎭
⎪⎩
≥⎟⎟⎠
⎜⎜⎝
arPr ss
s
if )NA(
Derive equations 5.5 and 5.8 (Assignment)
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Equilibrium Numerical ApertureNon propagating modes will scatter out within a short distance (~ 50 m) and equilibrium is establishedSurface emitting LEDs have max of non propagating modesFl l d l d l l t thi ff tFly–lead coupled lasers are less prone to this effect
eric
al A
pertu
re NA in
NA eq
2
50 ⎟⎟⎠
⎞⎜⎜⎝
⎛=
in
eqeq NA
NAPP
P50 = Power expected 50
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Fiber Length
Num
e
0 50 100 150 meters
at z = 50 m
Examples of possible lensing schemes used to improve optical source-to-fiber coupling efficiency
Collimating
Focuses light onto a line
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LED to Fiber Coupling as a function of Emitting Diameter
In case of:
Fiber NA = 0.2Core Radius ‘a’ = 25 μm
uplin
g E
ffici
ency
(%)
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& Emitting Dia = 25 μmCoupling Efficiency = ….
Cou
Emitting Diameter (μm)
Fiber-to-Fiber Joint
Low loss fiber-fiber joints are either:1 – Splice (permanent bond)p (p )2 – Connector (demountable connection)Fiber-to-Fiber coupling loss LF (dB) = − 10 log ηF
where ηF = Mcomm / ME
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Different modal distributions of the optical beam emerging from a fiber, lead to different degrees of coupling loss
When all modes are equally excited, the output beam fills the entire output NA
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For a steady state modal distribution, only the equilibrium NA is filled by the output beam
Mechanical Misalignment Losses
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Lateral (Axial) Misalignment Losses
2step, =Acomm
Fη
Dominant Mechanical loss
2/12
2p,
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2arccos2
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎠⎞
⎜⎝⎛−−=
ad
ad
ad
a
ππ
π
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Dominant Mechanical loss
Derive the similar expression for GI Fiber (Assignment)
Losses due to differences in the geometry and waveguide characteristics of the fibers 1. Radius
2. Numerical Aperture3 R f ti I d P fil
( ) ( )ER
ERE
RF
ERE
RF
NAL
aaaaaL
αα +
≤⎟⎠⎞⎜
⎝⎛−=
≤⎟⎠⎞⎜
⎝⎛−=
2
NANAfor NANAlog10)(
for log10)(
2
23. Refractive Index Profile
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( ) ( )( ) ER
RE
ERFL αα
αααα
α ≤++
−= for 22log10
E & R subscripts refer to emitting and receiving fibers
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Connector Return Loss
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Important characteristics of few connectors
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Write a brief report on theWrite a brief report on the
Connector parameters
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Experimental comparison of Losses as a function of mechanical misalignment
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Connectors CharacteristicsSome of the principal requirements of a good connector design are as follows:1 l li l1- low coupling losses 2- Interchangeability3- Ease of assembly4- Low environmental sensitivity5- Low-cost and reliable construction
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5 Low cost and reliable construction6- Ease of connection
Fiber End Face
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Fiber end defects
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Fiber Splicing
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Fusion Splicing
V-groove Optical Fiber Splicing
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Exercises
25
