lecture: Optical Modulators in Silicon

Photonic Circuits
Prepared by Delphine MARRIS-MORINI,
Laurent VIVIEN, Gilles RASIGADE
Institut dElectronique Fondamentale
Universit Paris Sud FRANCE
delphine.morini@u-psud.fr
Oct 2009
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Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
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Introduction
Optical telecommunications
Higher integration density
Reduced component costs
Optical interconnects
Clock signal distribution
High data rate links
Photodetector
Source
Splitters
90 Bend
Waveguide
Driver
Modulator
Photodetector
Source
Splitters
90 Bend
Waveguide
Driver
Modulator
Photodetector
Source
Waveguides
Modulator
Driver
Modulator
Driver
Modulator
Driver
Photodetector
Source
Waveguides
Modulator
Driver
Modulator
Driver
Modulator
Driver
Example of high speed optical links
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Optical modulation
Simple
Cost-effective
Compact
Chirp: output frequency shifts with drive
signal
Carrier induced (Transient chirp)
Temperature variation due to carrier
modulation (slow chirp)
Limited extinction ratio
Laser is not turn off at 0-bits
Impact on distance . bit-rate product
Additional component
Additional loss
Higher speed
Large extinction ratio
Low chirp
Push pull configuration
Low modulation distortion
Direct modulation of the laser beam External modulation
High performance optical transmission systems are based
on external modulation
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External optical modulator
Bonding technology
Limitation of the number of
modulator on the silicon chip
Cost
Well known
Good performances
Mature device
III-V modulator on Si
Less mature device
Development is required
Physical effect for optical modulation:
more limited in silicon than in III-V material
Less flexibility on material alloys
Compatible with CMOS technology
Low cost
for high production volume
Silicon-based modulator
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External optical modulator
Silicon based modulator: key element to develop high
speed optical links in the telecom wavelength range
Main challenges for optical modulators:
Compatibility with silicon technology
Low bias voltage
High bandwidth
Frequency operation > 10 GHz
High data operation from 10 Gbit/s to 40 Gbit/s
Integration in submicron SOI waveguide
Low insertion loss
Large extinction ratio
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Optical modulation
Optical
modulator
t
Optical intensity
Electrical
driver
t
Modulated optical
intensity
t
Electroabsorption
Phase modulation
Intensity modulation
interferometer
Intensity modulation
Electrorefraction
Absorption coefficient variation
under an electric field
Refractive index variation
under an electric field
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Figure of merits
Basic criterion:
Distinction of the minimum (I
min
) and the
maximum (I
max
) intensity levels
Loss
Output optical
intensity
t
I
min
I
max
Input optical
intensity
t
I
0
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Figure of merits
Distinction between I
min
et I
max
MD : Modulation Depth- %
Extinction ratio (ER) - dB
max
min max
I
I I
MD

=
Example:
If I
max
=0.1mW and I
min
=0.01mW - MD=90% et ER=10 dB

=
min
max
log 10
I
I
ER
Output optical
intensity
t
I
min
I
max
Input optical
intensity
t
I
0
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Figure of merits
Insertion loss
Loss at ON state
Example:
If I
0
=1mW and I
max
=0.1mW then P = 10dB

=
max
0
log 10
I
I
P
Output optical
intensity
t
I
min
I
max
Input optical
intensity
t
I
0
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Modulator speed
Intrinsic speed
Physical phenomenon limitation
RC time constant
Electrical circuit limitation
RF signal propagation
impedance adaptation
Matching of electrical and optical velocities
What are the limitations of the modulator speed?
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Modulator speed
Intrinsic speed: Time constant of the physical
phenomenon responsible of the interaction between the
semiconductor and the EM wave. The intrinsic speed
depends on the physical effect used:
Electro-absorption :
The cut-off wavelength (at the absorption edge) is shifted by applying
an external voltage to the semiconductor: Intrinsically high speed
(f >>GHz).
Electro-refraction: depends on the index variation origin
Thermal variation of the refractive index : very slow
Free carrier concentration variation : time constant from ps (carrier
depletion) to ms (carrier recombination)
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Modulator speed
Bandwidth of the electrical circuit
C
appl appl
f
f
j
V
RCf j
V
V
+
=
+
=
1
2 1
R
C
V V
app
RC time constant should be minimized
Resistance R
Increase of the doped regions of contacts
Decrease of the distance between contacts and active region
Capacitance C
Directly given by the geometry and the modulator optimization process
The equivalent electrical
scheme for MOS capacitor
and pin diode under reverse
bias voltage is a capacitor
RC 2
1
f
c
.
=
when R and/or C
Modulator
electrical scheme
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Modulator speed
RF electrical signal propagation
RF signal at f> GHz is a wave propagating on an electrical
waveguide.
Coplanar electrodes are mainly used. They have to be
defined according to the optical modulator geometry and
the required cut-off frequency
Copropagating electrical and optical waves:
Matching of electrical and optical wave velocities
Impedance adaptation is required to avoid electrical signal
reflection
50 ohms is the impedance of the most RF equipments
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Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
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Absorption in direct bandgap SC
Interaction between semiconductor (SC) and optical wave
By applying an electrical command, the material should be absorbant
or transparent.
Photon absorption in SC :
Energy conservation
Wavevector conservation

G
E
G
=hc/
G
If hc/ < E
G
Transparent
If hc/ > E
G
absorbant
Conduction band
Valence band
E
G
Electron energy
k = wavevector
Photon energy=hc/
Direct bandgap semiconductor
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Absorption in indirect bandgap SC
Electron energy
k = wavevector
Photon energy=hc/
indirect bandgap semiconductor
A two-step phenomenon is required in indirect bandgap semiconductor (SC)
Wavevector is conserved via a phonon interaction
(phonon is a quantum of lattice vibration)
Photon absorption
Phonon emission
Due to this phonon mechanism,
the absorption probability is lower
than in the previous case
E
G
=hc/
G

G
The absorption band edge is less
abrupt than for direct band gap SC
III-V semiconductors (GaAs, InGaAs
et InGaAsP,): direct gap
Group IV semiconductors (Si, Ge,):
indirect gap
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Franz Keldysh effect
The absorption phenomenon is described by solving Shrdinger equations
Determination of wavefunction of electrons and holes
Homogeneous semiconductor under electrical bias
Example: pin diode under reverse bias voltage
P N I
Electron and hole wavefunctions extends in the bandgap:
Overlap between electron and hole wavefunctions
allows absorption of photon with an energy lower than
bandgap energy.
Franz-Keldysh effect: photon-assisted tunnelling absorption effect
Electric field
Band edges are tilted
due do the electrical field
x
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Optical modulator based on Franz-Keldysh effect:
Red-shift of the absorption band-edge
Direct bandgap SC
Absorption coefficient variation
Abrupt absorption band edge
Indirect bandgap SC
Absorption coefficient variation is reduced
Franz Keldysh effect

1
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Electroabsorption in group IV materials
Bandgap Bulk-Si Bulk-Ge
Direct 3.3 eV
0.37m
0.8 eV
1.55 m
Indirect 1.13 eV
1.1m
0.66 eV
1.88m
Silicon is transparent at > 1.1 m, and is an indirect bandgap
material => Electroabsorption is not possible at telecommunication
wavelengths
Germanium direct bandgap energy is at telecommunication
wavelengths. As the direct bandgap energy is not very different from
the indirect bandgap energy, germanium absorption is not so
different from a direct bandgap material
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Franz-Keldysh effect
Liu et al., Nature Photonics 2, 433 - 437 (2008)
Liu et al., Opt. Express 15, 623 (2007)
SiGe modulator using Franz-Keldysh effect has been
demonstrated:
Design of material composition and strain of Ge
1-x
Si
x
to achieve
modulation at 1.55m
Integrated devices using amorphous Si waveguides
Bandwidth = 1.2 GHz
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Heterostructures formed by a thin
layer of a narrower-gap
semiconductor between thicker
layers of a wider-gap material lead
to the formation of potential wells
for electrons and holes.
Possible transition between discrete
energy level
E
0
CB
VB
Ex : GaAlAs / GaAs/GaAlAs
Electroabsorption in QW structures
M
a
t
e
r
i
a
l

1
M
a
t
e
r
i
a
l

2
M
a
t
e
r
i
a
l

1
Absorption in quantum well (QW) structures ?
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E
0
=hc/
0

Homogeneous
material
QW structures

0
Electroabsorption in QW structures
Absorption edge in QW structures is more abrupt than in
homogeneous material
E
0
depends on the quantum well thickness
Adjustment of the wavelength is possible
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Electroabsorption in QW structures
Excitonic peaks ?
When a photon is absorbed: an electron is excited in the conduction band,
leaving a hole in the valence band. Hole and electron are attracted by the
Coulomb force. The exciton results from the binding of the electron with its hole.
As a result, the exciton has slightly less energy than the unbound electron and
hole.
Apparition of peaks in absorption

Excitonic peak
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Quantum Confined Stark effect (QCSE)
How does the absorption coefficient change
when an electrical field is applied on a QW
structure ?
Shift of the energy levels inside the well :
redshift of the absorption coefficient
Decrease of the excitonic peak, because of
hole and electron wavefunction overlap
decrease.
Electrical field
E
t
<E
0

E=0
E0
Quantum confined Stark
effect (QCSE)
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Quantum Confined Stark effect (QCSE)
Direct bandgap SC (III-V materials)
QCSE is more efficient than Franz Keldysh effect (a larger
absorption coefficient is generally obtained.
Well thickness is modified to tune the absorption edge wavelength
Intrinsically high speed process
Integration of QCSE modulator on silicon ?
QCSE in group IV materials ?
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Quantum Confined Stark effect (QCSE)
Integration of III-V QCSE modulator on silicon:
Hybrid silicon evanescent electroabsorption modulator
AlGaInAs quantum wells
Extinction ratio over 10dB, modulation bandwidth of 10
GHz. Open eye at 10Gb/s
Kuo et al., Opt. Express 16, 9936 (2008)
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Quantum Confined Stark effect (QCSE)
QCSE in group IV material:
4.2% of lattice mismatch between germanium and
silicon => growth of Si
1-x
Ge
x
in Si substrate : critical
thickness, depending on x = germanium fraction
Growth of Si
1-x
Ge
x
in Si substrate
Type I heterostructure with a bandgap
energy offset mainly in the valence band with
a conduction band offset lower than 10 meV
Weak electron confinement low
excitonic phenomenon
Si Si
1-x
Ge
x
Si
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Quantum Confined Stark Effect
Qasaimeh et al IEEE JQE, 33 (9), (1997).
QCSE in SiGe/Si structures ?
Modulator based on absorption band edge shift in
SiGe/Si heterostructure:
Limited performances:
Extinction ratio: 1.55 dB
Insertion loss: 28.5 dB for a 100 m long device
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Quantum Confined Stark Effect
QCSE in SiGe/Ge structures ?
Kuo and al, Stanford, Nature (2005)
Kuo et al. IEEE JSTQE 12, 1503 (2006)
Growth of Si
1-x
Ge
x
/Ge on relaxed Si
1-x
Ge
x
buffer
Si
1-x
Ge
x
Ge Si
Demonstrationof quantum confinement at
the direct gap, and strong excitonic
absorption peaks in the spectra.
Demonstration of QCSE with
strength comparable to that in III-V
materials.
Type I alignment with a strong
conduction band and valence
band energy offset
Roth et al., Electronics Lett. 44, 49 (2008)
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Electroabsorption silicon based modulators
Homogeneous GeSi: Franz-Keldysh effect:
10-dB extinction ratio at 1.540 nm
Operating spectrum range of about 1.5391.553 m
3-dB bandwidth of 1.2 GHz
Integration of III-V on silicon
Hybrid silicon evanescent modulator
Extinction ratio over 10dB, modulation bandwidth of 10 GHz. Open eye at 10
Gb/s
Si/Si
1-x
Ge
x
quantum well: Quantum confined Stark effect
weak effect: low electron confinement in the well
Limited performances
Si
1-x
Ge
x
/Ge quantum well : Quantum confined Stark effect
3 dB modulation possible with 1 V swing
Fundamental limit to speed ~1 ps
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Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
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Electrorefraction
Electrorefraction = variation of the refractive index with and
electrical field.
Semiconductor optical properties are modified with the
application of an electrical field :
field dependent permittivity
( ) ...
3 ) 3 ( 2 ) 2 ( ) 1 (
0
+ + + = E E E P

(i)
= i-order component of the susceptibility (tensor)
...
) ( ) ( ) (
+ + + + =
2 3 2 1
R
E E 1
E P E D
r 0 0
. . . = + =
Relative permittivity
P=Polarisation
Electric displacement field
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Electrorefraction
... 1
2 ) 3 ( ) 2 ( ) 1 (
+ + + + = = E E n
R

...
) ( ) (
+ + + =
0
2 3
0
2
0
n 2
E
n 2
E
n n

The refractive index depends
on the applied electric field
Refractive index
...
2 2
2 3
0
3
0
0
=
E g n E r n
n n
4
0
) 3 (
n
g

=
Pockels effect
4
0
) 2 (
n
r

=
r : linear electro-optic
coefficient
Kerr effect
g : quadratic
electro-optic coefficient
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Electrorefraction
Pockels effect:
LiNbO
3
, III-V semi-conductors (GaAs), polymer,
Modulation speed: f >70 GHz
No Pockels effect in silicon (centrosymmetric crystal)
Soref et Bennett IEEE JQE QE-23 (1), (1987).
Kerr effect:
In silicon : low refractive index change at telecommunication
wavelengths
n = 2 10
-5
for E = 5 10
5
V/cm
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Electrorefraction
Is it possible to obtain a linear electro-
optic effect in silicon or germanium ?
Strained silicon
The crystal symmetry can be broken by the deposition of a straining
layer (Si
3
N
4
) on top of a silicon waveguide.
V pm/ 15
) 2 (

0
) 2 (
2n
E
n

=
silicon
SiO
2
silicon
Si
3
N
4
Apparition of Pockels effect
for E = 5.10
5
V/cm, n10
-6
Jacobsen et al, Nature 441, 199 (2006)
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Electrorefraction
2
3
0
E n r
n =
Yu and al, PRB 73, 235328 (2006)
for E = 5.10
5
V/cm, n=10
-4
SiGe superlattice
The intrinsic inversion symetry can be broken in SiGe
superlattices
Superlattice of 34 periods based on a trilayer heterostructure:
Si/Si
0.75
Ge
0.25
/Si
0.5
Ge
0.5
Theoretical demonstration of a linear electro-optic coefficient: r
= 5.8.10
-10
cm/V
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Electrorefraction
Integration of an electrooptic material in Si photonic
crystal waveguide
A modulation bandwidth of 78 GHz and a length of
about 80 m at a drive voltage amplitude of 1 V is
predicted
Slot filled by an
electro-optic polymer
Brosi et al Opt. Express 16, 4177 (2008)
Input
waveguide
Output
waveguide
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Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
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Refractive index variation in silicon
Large refractive index variation in bulk silicon
can also be obtained by:
Thermal variations
thermo-optic coefficient in silicon =
2.10
-4
K
-1
at 1.55m
n= 210
-3
if T=10 K
Effect very slow (time constant
~ms) : cannot be used for high
speed modulation
BUT it can be a parasistic
effect for high speed optical
modulators.
Free carrier
concentration variations
Largely used for optical
modulation (most efficient
way to achieve phase
modulation in silicon)
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Free-carrier concentration variation
Free carrier concentration variations in silicon
are responsible for optical properties variations
(intraband transition : free carrier absortion +)
Refractive index and absorption
coefficient variations
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Free-carrier concentration variation
Free carrier concentration variations in silicon
N = e
-
density variation (cm
-3
)
P = h
+
density variation (cm
-3
)
Example: for P=10
18
cm
-3
n = 1.5 10
-3
at =1.3m
n = 2.1 10
-3
at =1.55m
P 10 4 N 10 6
P 10 6 N 10 2 . 6 n
18 18
8 , 0 18 22
+ =
=


At =1,3 m
P 10 6 N 10 5 . 8
P 10 5 . 8 N 10 8 . 8 n
18 18
8 , 0 18 22
+ =
=


At =1,55 m
Soref et al IEEE JQE QE-23 (1), (1987).
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Free-carrier concentration variation
How is it possible to obtain free carrier
concentration variation in silicon?
Carrier injection in PN/PIN
diode (forward bias)
Carrier plasma shift in bipolar
mode field-effect transistor
(BMFET).
Carrier accumulation in MOS
capacitor
Carrier depletion in PN/PIN
diodes (reverse bias)
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Free-carrier concentration variation
Optical modulator based on free-carrier concentration
variation ?
The available absorption coefficient variation is not sufficient to
be used in an absorption modulator
Most of silicon modulators use refractive index variation by
refraction index variation
Phase modulation
Intensity modulation
Interferometer
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Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
46 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Waveguide integrated optical modulator
Phase propagation in an optical waveguide:
(E
0
,
0
)
(E
1
,
1
)
Amplitude and phase at the
waveguide input
Amplitude and phase after
propagation in the waveguide
L
Silicon optical waveguides:
Silicon on insulator substrate
Rib or strip waveguide
L n
2
eff 0 1

+ =
( ) L
2
0 1
e E E

=
( is intensity absorption
coefficient)
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Waveguide integrated optical modulator
Phase modulation:
( )L n n
2
eff eff 0 1
+

+ =
(E
0
,
0
)
(E
1
,
1
)
Amplitude and phase at the
waveguide input
Amplitude and phase after
propagation in the waveguide
L
Refractive index
variation in the
waveguide
neff depends on:
The refractive index variation
due to free carrier
concentration variation
The overlap between the
optical mode and the refractive
index variation region
=
L n
2
eff

=
eff
n 2
L L

= =

( ) L
2
0 1
e E E

=
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Waveguide integrated optical modulator
To convert phase modulation into intensity modulation:
waveguide integrated interferometric structure
Mach Zehnder Resonators Photonic crystal
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Waveguide integrated optical modulator
Mach Zehnder
The incident beam is splitted in two
beams propagating in both waveguides
At the ouptut combiner, interferences
occur:
Constructive interferences if beams
are in phase
Destructive interferences if beams are
in phase opposition
To go from constructive to destructive
interferences, phase shift is required
Beam splitter
Beam combiner
Active region (phase shifter)
Even for single arm drive modulator, the phase shifter is integrated in
both arms, to have the same loss in both waveguide (totally destructive
interference is possible only with same amplitude beams)
L

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Transmission
n
eff
in the cavity
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Waveguide integrated optical modulator
Resonators
Input
Output
coupleur
Ring resonator
Part of the incident beam is coupled into the ring. After
propagation along the ring, light is partially coupled in
the straight waveguide back Interferences
Phase shifter
integrated in the ring

Transmission
n
eff
in the ring
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Waveguide integrated optical modulator
Photonic crystals
Photonic crystal can be used to tailored dispersion properties
(Photonic band gap, geant and anormal dispersion, etc)
For optical modulator, slow wave in photonic crystals can be used to
reduce the phase shifter length, for example in Mach Zehnder
interferometer
dk
d
v
g

=
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Waveguide integrated optical modulator
( )L n n
2
eff eff 0 1
+

+ =
(E
0
,
0
)
(E
1
,
1
)
Amplitude and phase at the
waveguide input
Amplitude and phase after
propagation in the waveguide
L
Refractive index
and absorption
coefficient
variations in the
waveguide
Refractive index variation goes along with absorption
coefficient variation:
( )

=
L
2
0 1
e E E
This can be a parasitic effect, for example:
when using Mach Zehdner interferometer, if the output field amplitudes
are not the same, interference cannot be entirelly destructive, and Imin 0
at the modulator output)
In resonators absorption increase is responsible for mimimum
transmission increase
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Waveguide integrated optical modulator
Free carrier concentration variation in a silicon waveguide:
The electronic structure has to be integrated in an optical
waveguide
Rib waveguides are required
(in strip waveguide it is not
possible to have the electrical
contacts)
Doped silicon and metal are responsible for large optical loss
Careful design of the electronic structure is required, to
achieve simultaneously:
large refractive index variation,
large overlap of the active region with the guided mode,
low optical loss
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Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
56 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
Optical modulator based on carrier injection in pin diode
(direct bias)
Electron and hole injection in a waveguide
Effective index variation n
eff
~a few 10
-3
P
+
N
+
Si
SiO
2
Optical mode
SiO
2
When bias goes fromV to 0, free carriers in the intrinsic part of
the diode have to recombine.
The modulator speedis limited due to carrier recombinationtime
Commutation time: few10 ns
Cut-off frequency: few100 MHz
Voltage
current
V
metal
57 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
Optical modulator based on carrier injection in pin diode
(direct bias)
To increase the speed:
Direct and reverse biasing
voltage
current
V
1
V
2
Barrios et al JLT 21(10) 2003
Theoretically:
Switching time = 1.3 ns for
V
1
=-1 and V
2
=0.87 V
n
eff
~ 10
-3
58 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
The optical response time can also be reduced thanks
to a nonlinear transfer function of the interferometer
Direct biased PIN diode integrated
in a microring resonator
I
out I
in

T=I
out
/I
in

0
bias increase
For example:
n
eff
T
At
0
:
Xu and al, Nature 2005
59 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
Principle of the optical response time reduction thanks
to a nonlinear transfer function of the interferometer
T
t
n
eff
T
n
eff0
n
eff
t
60 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
Xu and al, Nature 2005
Experimentally:
voltage
current
V
1
V
2
+
Modulation at 1.5 Gbit/s
I
out I
in
Direct and reverse biasing
ring resonator
61 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
To go further: prehemphasis
of the driving signal: pulses
are added to the NRZ signal
Applied voltage
V
2
V
1
V
2
V
1
t
+
I
out I
in
Modulation at 18 Gbit/s
ring resonator
62 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier injection
Carrier injection in PIN diode using prehemphasis of
the driving signal and Mach Zehnder interferometer:
Applied voltage
V
2
V
1
V
2
V
1
t
+
Open eye diagram at 10 Gbit/s
Green et al. Opt. Express 15, 17106 (2007)
Mach Zehnder
63 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
64 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator :BMFET
BMFET (Bipolar Mode Field Effect Transistor)
3 electrodes: source, drain gate
Electron and hole plasma continually injected
Modulation by plasma shift
Sciuto et al IEEE JLT, 21 (1), (2003).
Theoretical evaluation: switching time of a few nanosecondes
operating frequency of a few hundred MHz.
n
p
n
-
n
+
gate gate
source
drain
carriers plasma
Optical mode
n
p
n
-
n
+
gate gate
source
drain
carriers plasma
Optical mode
65 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
66 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: MOS capacitor
Metal workfunction:
M
Insulator electron affinity:
i
Silicon electron affinity:
s
Vacuum level
E
FM
E
C
E
V
E
F SC
q
i
q
M
q
s
Metal
O
x
i
d
e
n-doped
Semiconductor
Metal Oxyde Semiconductor (MOS) capacitor:
gate electrode (metal or polysilicon)
silicon oxide dielectric insulator
doped semiconductor (n in the following)
Potential band diagram of the different parts without contact
67 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: MOS capacitor
Around the oxyde layer:
ionized donors in semiconductor
electrons in metal
E
FM
BC
BV
E
F SC
Depletion
Metal
O
x
i
d
e
n-doped
Semiconductor
(x) charge density
x
oxide SC mtal
x
68 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: MOS capacitor
Flat Band regime : V
g
=V
FB
>0
No charges around the oxide
V
g
E
FM
BC
BV
E
F SC
x
Metal
O
x
i
d
e
n-doped
Semiconductor
(x) charge density
Flat Band
69 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: MOS capacitor
E
FM
BC
BV
E
F SC
V
g
>V
FB
:
Around the oxyde layer:
Electron accumulation in semiconductor
Hole accumulation in metal
Accumulation
Metal
O
x
i
d
e
n-doped
Semiconductor
(x) charge density
x
oxide SC mtal
V
g
70 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: MOS capacitor
Silicon modulator using MOS capacitor in accumulation:
first modulator with GHZ bandwidth
Liu et al, Nature, 427, 615-618 (2004).
L

decreases when the drive voltage

= 7.7 V cm
(n
eff
=4 10
-5
forVd=4V)
Small signal modulation :
-3dB bandwidth >1GHz
PRBS modulation :
data transmission at 1 Gbit/s
n Si
metal
Burried oxyde
SiO
2
gate oxyde
p-polycristalline Si
optical mode
+ Mach Zehnder
71 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: MOS capacitor
Silicon modulator using MOS capacitor in accumulation
improvments
Improvment of the RC time constant and loss tradeoff:
Gate : polysilicon has been replaced by monocrystalline Si
Waveguide: geometry optimization
Modulator biasing:
3.45 mm long phase shifter (C=26, 4pF) has been divided in eleven 315 m-long
sections.
Low impedance driver using HBT technology (70 GHz)
Liao et al, Optics express, 13 (8) (2005)
Experimental results:
V

=3.3V.cm
Loss =10dB
Data transmission:
6 Gbit/s, with extinction ratio =4.5 dB
10 Gbit/s, with extinction ratio =3.8 dB
72 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Outline
I-Introduction :
Optical modulation
Figure of merits
II-Mechanisms for Optical Modulation in group IV materials (Si, Ge)
Electroabsorption
Electrorefraction
Free carrier concentration variation
III-Waveguide integrated silicon optical modulator using free carrier
concentration variations
Waveguide integrated optical modulator considerations
Silicon optical modulators using free carrier concentration variations:
Carrier injection in PIN diode
Carrier shifts in Bipolar Mode Field Effect Transistor
Carriers accumulation in Metal Oxyde Semiconductor capacitors
Carrier depletion in PN/PIN diode
73 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
Carrier depletion in PN diode
In reverse biased pn diode, the space charge region region is widened,
leading to electron and hole concentration variation in the junction region
Theoretical performances:
birefringence free device
Gardes et al Opt. Express 13, 8845
(2005)
V

=2.5 V cm
rise and fall times: 7 ps (drive
voltage =-5 volts)
excess loss: 2 dB for TE and
TM polarizations
P N
Optical mode
Vertical junction
P
N
Horizontal junction
74 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
Demonstrations of high speed modulation using carrier
depletion in PN diode
Vertical PN junction in Mach Zehnder interferometer with integrated driver:
T. Pinguet et al, Group IV Photonics (2007)
Extinction Ratio > 6dB at 10 Gbit/s
Insertion loss: 3dB
Horizontal PN junction in Mach Zehnder interferometer with integrated load
Liu et al Opt. Express 15, 660 (2007)
Liu et al., Semicond. Sci. Technol. 23, 064001 (2008)
Static performances: modulation depth: >20 dB, insertion loss ~7 dB
Dynamic performances: -3 dB bandwidth: > 30 GHz
Data transmission: 40 Gbit/s with ~1dB Extinction Ratio
Vertical PN junction in a ring resonator
F. Gardes et al, Optics express (2009)
-3 dB bandwidth = 19 GHz
75 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
Carrier depletion in PIN diode:
When a PIN diode is reverse biased, the electrical field increase
in the intrinsic region.
If carriers are located in the intrinsic region at equilibrium, they
will be depleted with reverse bias.
Which carriers should we use?
Soref et al IEEE JQE QE-23 (1), (1987).
Free holes are more efficient than free
electrons for refractive index variation for
N, P <10
20
cm
-3
8 , 0 18 22
P 10 5 . 8 N 10 8 . 8 n =

=1.55 m
76 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
non intentionnaly-doped Silicon
Oxide
Metal
Silicon P
+
doped
Silicon N
+
doped
Metal
Carrier depletion in lateral PIN diode
P doped slit in the rib
waveguide
Large modulation efficiency
(overlap with guided mode)
Low optical loss (the
waveguide is not entirely
doped)
Reduced capacitance
(~ 0.2-0.3 fF/m)
Reduction of RC time
constants
Reduction of electrical power
77 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
Operating principle: electrical and optical simulations
(reverse biased)
Hole density
3 2.5 2
0.5
0.7
0.9
x (m)
2 10
17
cm
-3
0
y

(

m
)
1 10
17
cm
-3
Hole density
3 2.5 2
0.5
0.7
0.9
x (m)
2 10
17
cm
-3
0
y

(

m
)
1 10
17
cm
-3
Hole density
3 2.5 2
0.5
0.7
0.9
x (m)
2 10
17
cm
-3
0
y

(

m
)
1 10
17
cm
-3
2.0x10
-4
1.5
1.0
0.5
0.0

n
e
f
f
5 4 3 2 1 0
Voltage (V)
0 V 2 V
4 V
78 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
Theoretical evaluation of device speed
n
eff
= A+B exp(-t/) =1.1 ps
maximum bandwidth:
1/2 = 145 GHz
RC time constant
Compromise between modulation efficiency, optical loss and RC
time constant to be found for each waveguide design
RF signal propagation along the few mms long phase shifter
length
Intrinsic speed
79 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
D. Marris-Morini et al, Optics express, 16, 2008
Experimental demonstration
DC experimental results:
Insertion loss = 5 dB
Contrast ratio up to 14 dB
V

L

= 5 V.cm
Carrier depletion in PIN diode with
p-doped slit in the intrinsic region
Assymetric Mach Zehnder
interferometer
Phase shifter length = 4 mm
80 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
RF experimental benches
81 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Integrated optical modulator: carrier depletion
D. Marris-Morini et al, Group IV photonics 2008
10 GHz
15 GHz
Experimental demonstration: small-signal optical response
Two design for coplanar waveguide electrodes have been compared
design 1
design 2
ground
signal
ground
signal
82 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Conclusion
Intensive research have been made in silicon photonics in the last years.
Silicon based optical modulator is one of the fundamental building blocs
for high performance data transmission systems.
Optical modulation in/on silicon-based device has been proposed and
demonstrated using a large number of physical effect:
Electroabsorption using Franz-Keldysh and Quantun Confined Stark
effects in SiGe and SiGe/Ge structures
Electro-refraction in strained silicon and SiGe supperlattice
Integration of III-V or polymer on silicon
Free carrier concentration variations in silicon using carrier injection,
accumulation, depletion,
83 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Conclusion
Numerous experimental demonstrations of optical modulator
integrated in/on silicon waveguides have been made until the 1
st
demonstration of GHz modulation 5 years ago.
Impressive progress in silicon modulators lets hope that silicon
based 10 and 40 Gbit/s data transmission systems will be available in
the next years, for various applications (telecommunications, optical
interconnect on microelectronics chips, core to core communications
in microprocessors, etc)
84 Silicon Photonics PhD course prepared within FP7-224312 Helios project
Acknowledgements
CEA - LETI (Grenoble, France)
J ean Marc FEDELI
IEF (Orsay, France)
Suzanne LAVAL
Eric CASSAN
Xavier LE ROUX
Paul CROZAT
Daniel PASCAL
Sylvain MAINE
Anatole LUPU
David BOUVILLE
Samson EDMOND
Mathieu HALBWAX
French RMNT project (CAURICO)
http://pages.ief.u-psud.fr/caurico/
IEF/MINERVE
CEA/LETI
Grenoble, France
Clean rooms
Basic Technological Research
European Community's Seventh
Framework Program (FP7)
pHotonics ELectronics functional
Integration on CMOS
European Community's Sixth
Framework Program (FP6)