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Electroabsorption
Modulators
R. Sankaralingam
EE 698
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Optical Modulationn Direct modulation
Output frequency shifts with drive signal
n Carrier induced (chirp)
n Temperature variation due to carrier modulation
Limited extinction ratio
n Indirect or External modulation Electro-optic modulation
n Change optical path length with applied electric field
Electroabsorption modulation
n Change amount of light absorbed with applied electric field
Finite insertion loss (6-7 dB)
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Advantages of EA modulator
n Zero biasing voltage
n Low driving voltage
n Low/negative chirp
n High speed
n Lesser polarization dependence
n Integration with DFB laser
n Allows a single optical power source to be used for large
number of information carrying beams
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Electroabsorption modulator
n Mechanisms
Franz-Keldysh effect
n Observed in conventional bulk semiconductors
Quantum-confined Stark effect (QCSE)
n Quantum well structures
n Both of these electroabsorption effects are
prominent near the bandgap of semiconductors
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Franz-Keldysh effect
Tunneling allows overlap
of electron and hole
wavefunctions for photonenergy less than bandgap
a= K(E' )1/ 2 (8b)-1 exp-4
3b3 / 2
where,
E'=q
2E
2h
2
2mr
*
1/ 3
b=e
g-hw
E'
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Excitonic electroabsorption
(Stark effect)
n Excitonic effects gives rise to astep-like rise in absorption
spectran Formation of excitons
manifests themselves as aseries of sharp resonances nearthe bandgap energy
n Formed in very puresemiconductors at lowtemperatures
n Excitons can be very easilyfield ionized
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Quantum Confined Stark Effect
n Quantum well increases theoverlap of electron and holewavefunctions
n Electric field reduces overlapand results in a correspondingreduction in absorption andluminescence
n Exciton absorption peak is not
greatly broadened because ofconfinement
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Device structure
n Primary materials for EAmodulators are III-Vsemiconductors
n PIN structure
n Transmission type does notlead to high enough extinctionratio
n Waveguide type more
commonly used - has higheroptical confinement
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Design considerations
n Operation principle
n Contrast ratio
n Insertion loss
n Modulation efficiency
n Chirp considerations and
efficiency
n Packaging and integration
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Extinction Ratio
n BER directly effected by extinction ratio
n Contrast ratio can be made as large as possible by
increasing the length of the modulator. But propagationloss then becomes an issue.
Ron /off
=P
out(V
on= 0)
Pout(V
off=V)
=e-
a(0)L
e-a(V)L
Ron /off (dB) =10log(Ron /off ) = 4.343 a(V)-a(0)[ ]L
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Insertion loss
n Absorptive loss Longer the modulator, larger the insertion loss.
Trade-off with Extinction Ratio
n Single mode fiber coupling loss Waveguide coupler can be used to reduce coupling loss
Can be as good as 1 dB
Typical numbers are 5-6 dB
L
in
outine
P
VPP )0(1)0(
Loss a--==-
=
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Modulation efficiency
n Modulation efficiency quantifies how much voltage do we need tomodulate the optical signal.
n Smaller detuning will increase the modulation efficiency. However, italso results in a larger insertion loss.
Ron / off
DV= 4.343
a
(V)-a
(0)[ ]LDV
= 4.343DaDF
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Chirp
n Frequency sweep imposed as a result of power change
n Imaginary part of refractive index is related to opticalabsorption coefficient by,
n Kramers-Kronig relation
ae -
2w
c
Dnr
Dg= -
4p
l
Dnr
Dg
a=2w
ck=
4p
lk
Dn =hc
p
PDa
E2 - hw( )
2
0
dE
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Chirp Engineering
DfFWHM
=a
e
2I
dI
dtDn =
hc
p
PDa
E2 - hw( )
2
0
dE
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Integration
n 10Gb/s module, Ith = 20mA, Pmax = 4mW @80mA , extinction ratio = 15dBfor -2.5V
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Acknowledgement
n P. Bhattacharya, Semiconductor Optoelectronic Devices
n S. L. Chuang, Physics of Optoelectronic Devices
n C. J. Chang-Hasnain, UC Berkeley, class notes
n J. S. Harris, Stanford University, class notes
n G. L. Li and P. K. L. Yu, J. Lightwave Tech., Sep 2003