Rajkumar Opt Mod

8/8/2019 Rajkumar Opt Mod

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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

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Rajkumar Opt Mod