System applications of silicon photonic
ring resonators
Yikai Su
State Key Lab of Advanced Optical Communication Systems and Networks ,
Department of Electronic Engineering, Shanghai Jiao Tong University, China
yikaisu@sjtu.edu.cn
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Motivation
Electronic processing
Optical processing in silicon
photonics
Complexity (# of units)
High
Low
Line width
10’s nm
>100 nm
Power
mW - W
mW - W
Speed
Gb/s
Gb/s-Tb/s
Optical processing may be
desired in some high-speed
applications
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Parameters of digital differentiator
Filter
A/D
DSP chip
memory
D/A
Filter
I/O
Realization of digital differentiator using DSP
ADC:MAX109
Speed:2.2 Gs/s
Power dissipation:6.8 W
Size:734.4 mm2
DAC:MAX5881
Speed:4.3 Gs/s
Power dissipation:1160 mW
Size:11 mmx11 mm
TMS320C6455 DSP
DSP:TMS320C6455
Speed:
1.2 GHz clock rate; 9600MIPS (16bit)
Size:
0.09-um/7-level Cu Metal Process (CMOS)
BGA package: 24*24 mm2
Power dissipation:1.76 W
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Optical processing using ring resonator
Silicon 250nm
Silica buffer layer 3μm
Silicon handing wafer
525 μm
SEM photos of a silicon microring
resonator
Signal processing functions:
• Slow light (JSTQE 08)
• Fast light (OE 09)
• Wavelength conversion (APL 08)
• Format conversion (OL 09)
• Optical differentiation (OE 08)
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250-nm thickness
450-nm width
Buffer layer: 3-µm silica
Mode area: ~ 0.1µm2
Air gap : ~100 nm
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Outline
 Tunable delay in silicon ring resonators
• Optically tunable buffer for different modulation formats at
5-Gb/s rate
• Optically tunable phase shifter for 40-GHz microwave
photonic signal
 Signal Conversions
• Dense wavelength conversion and multicasting in a
resonance-split silicon microring
• Format conversions (NRZ to FSK, NRZ to AMI)
• Optical temporal differentiator
 Concentric rings for bio-sensing
 Conclusions
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Recent experiments on slow-light delay
in silicon nano-waveguides
Schemes
Footprint
(mm2)
3dB Band
Duration/Delay
width
Max storage
capacity (bits)
Publication
~100GHz
3ps/ 4ps
1.3
Opt. Express
14(2006)
54GHz
--
50ps/510ps
200ps/220ps
10 at 20bps
1 at 5bps
Nature Photonics
1(2007)
photonic crystal
(PC)
~260MHz
1.9ns/1.45ns
<1
Nature Photonics
1(2007)
photonic crystal
coupled
waveguides
(PCCW)
12nm
0.8ps/40ps
SRS
cascaded
microring
resonator
(APF / CROW)
0.09
0.045
LEOS 2007
• Continuous tuning was not demonstrated
• Data format was limited to non return-to-zero (NRZ)
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Tuning signal delay in resonatorbased slow-light structure

Tunable group delay is important for
implementing a practical buffer
Single microring-resonator is a basic building
block of the resonator-based slow-light structure

Tuning methods:
•
•
•
Electro-optic effect by forming a p-i-n structure
Thermo-optic effect by implanting a micro-heater
MEMS actuated structure
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Ring resonator
More
Partial
coupling
coupling
DIInput
Resonance



Incoming light is partially coupled into the ring
The signal in the ring interferes with the input
light after one round-trip time
Only the signal of resonance can be coupled
into the ring
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Slow light
Group delay
Also see the animation
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Tunable slow-light in silicon ring resonator
-2
-4
(a)
5
4
3
-6
-8
-1.5
140
6
2
(b)
120
100
Delay (ps)
0
Phase shift (rad)
Normalizad transmission (dB)
Slow-light principle:
1
80
60
Δθ/Δω
= group delay
=> Slow light
40
20
0
-1.0 -0.5 0.0
0.5
1.0
1.5
Normalized frequency detuning × 10-4
0
-1.5
10
-1.0 -0.5 0.0
0.5
1.0
1.5
Normalized frequency detuning × 10-4
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Temperature tuning
When a pump light is injected into the microring resonator, the
absorbed energy is eventually converted to the thermal energy
and leads to a temperature shift
d T T
PA


dt

 CV
τ- thermal dissipation time
ρ-density of the silicon
C-thermal capacity
V-volume of the microring
Kθ-thermo-optic coefficient
The refractive index changes with the temperature
n  k T
k  1.86 104 K 1
No need of additional procedure in the fabrication, very low
threshold in tuning
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Normalized transmission(dB)
Silicon microring used in the experiment
0
-2
-4
~8-dB notch depth
~0.1-nm 3-dB bandwidth
-6
-8
experimental data
curve fitting
1552.6 1552.7 1552.8 1552.9 1553.0 1553.1
wavelength (nm)
SEM photos of the silicon microring resonator with a radius of 20 μm
250-nm thickness
450-nm width
Buffer layer: 3-µm silica
Mode area: ~ 0.1µm2
Air gap : 120 nm
Silicon 250nm
Silica buffer layer 3μm
Silicon handing wafer
525 μm
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Vertical coupling
Gold grating coupler to couple light between the
single mode fiber (SMF) and the silicon waveguide
 The gold grating coupler is designed to support
TE mode only

Measured fiber-to-fiber coupling loss: ~20dB
The technique was invented by Ghent
SEM photo of the gold grating coupler
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Experimental setup
generation of RZ / CSRZ signal
PRBS
RF
PPG
PC
BPF Attenuator
PC
CW laser
Single drive
MZM
Single drive
MZM
EDFA
PC
A dual-drive MZM is used when
generating RZ-DB and RZ-AMI
DPSK
demodulation
Oscilloscope
Attenuator BPF
EDFA
EDFA
Coupler
PM
Fangfei Liu et al., IEEE JSTQE May/June 2008
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Continuous Tuning of 5-Gb/s Nonreturn-to-zero (NRZ) signal
1G
5G
10G
Delay (ps)
90
60
30
(c)
(b)
0
2.0
1.5
1.0
0.5
0.0
-30
-20
-10
0
10
-30.7dBm
4.8dBm
13.7dBm
2.5
Intensity (a.u.)
(a)
120
20
0
500
1000
1500
2000
Time (ps)
Pump power (dBm)
Delay versus the pump power
Delayed waveforms
Maximum delay of ~100 ps
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Return-to-zero (RZ) signal
(a)
120
1G
5G
Delay (ps)
90
5G RZ
eye diagram
60
30
0
-40
-30
-20
-10
0
10
20
Delay versus
the(dBm)
pump power
Power
Intensity / a. u
Maximum delay of 80 ps
for 5-Gb/s RZ signal
(c)
1.00
0.75
5Gb/s
0.50
0.25
0.00
Qiang Li et al., IEEE/OSA J. Lightw. Technol.,
Vol 26, No. 23, 2008
16
-37.0dBm
3.2dBm
13.6dBm
0
100
200
300
400
500
Time / ps
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5-Gb/s carrier-suppressed RZ
(CSRZ) signal
Maximum delay of 95 ps
0

CSRZ is used in long haul
0
Eye diagrams and waveforms for the 5-Gb/s CSRZ signal
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5-Gb/s RZ-Duobianry (DB) and RZAlternating-Mark-Inversion (AMI) signals
RZ-DB
Maximum delay of 110 ps
RZ-DB is good for dispersion
uncompensated system in
metro
RZ-AMI
Maximum delay of 65 ps
RZ-AMI is tolerant to
nonlinear impairments
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Delay comparisons
Formats
NRZ
RZ
CSRZ
RZ-DB
RZ-AMI
Delays (ps)
100
80
95
110
65
Optical spectra
the narrower,
the larger delay
Qiang Li et al., OSA Slow and Fast Light Topic Meeting, 2008
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Larger delay with cascaded rings
Resonator-based slow-light structures :

Single channel side-coupled integrated spaced
sequences of resonators (SCISSOR)

Double channel SCISSOR

Coupled resonator optical waveguides (CROW)
Single channel SCISSOR
CROW
double channel SCISSOR
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Optically tunable microwave
photonic phase shifter
Operation principle
E(0)
E(L)
(a)
Eout
0
6
10dB
5
-2
4
-3
3
-4
2
-5
(b)
-6
-3
1
Opical spectrum
-1
Phase shift (rad)
Normalized transmission (dB)
Ein
0
-2
-1
0
1
2
3
Normalized frequency detuning × 10-4
20G
20G
(c)
Frequency
The two tones of the microwave optical signal experience
different phase shifts, resulting in group delay change
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Experimental setup
20-GHz microwave photonic signal
Silicon
microring
Temperature tuning
Q. Li et al., ECOC 2008, paper P2.12
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40GHz result – phase shift
Maximum phase shift:
-4.6 rad
Normalized intensity (a.u.)
1.0
-4.6rad
0.8
0.6
0.4
0.2
0.0
0.5
1.0
Time (ns)
1.5
2.0
Qingjiang Chang et al., IEEE Photon. Technol. Lett，vol. 21, no. 1, Jan. 2009
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Phase shift vs. pump power
4
(a)
-1
-2
-3
-4
-5
(b)
3
Phase shift (rad)
Phase shift (rad)
0
6
8
10
12
14
Pump power (dBm)
2
1
0
16
4
6
8
10
12
Pump power (dBm)
14
Continuous tuning based on thermal nonlinear effect
by changing the control light power
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16
Signal conversions in mode-split ring
Side wall roughness in E-beam
results in two resonance modes:
ω0 - the resonance frequency
QE - coupling quality factor
QL – intrinsic quality factor
Qu – coupling quality factor
b
a
st
si
The transmission function of the ring resonator is given by:
st

1
1
 1 0 (

)






si
2Qe j (    0 )  0  0
j (  0  0 )  0  0
0
2Qu
2Qi
2Qe
2Qu
2Qi
2Qe
Mode a is split into two resonance frequencies, ω0-ω0/(2Qu) and
ω0+ω0/(2Qu). The resonance-splitting is determined by the mutual coupling
factor Qu.
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Observation of mode splitting
Resonance-splitting
Motivation: shift the resonance to convert signals by
using free carrier dispersion (FCD) effect
Ziyang Zhang et al., CLEO/QELS 2008
Tao Wang et al., JLT 2009
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Experimental results – dense
wavelength conversion of 0.4nm
nm
 signal
 pump
1. Signal light is originally set at
the resonance -> ‘0’
2. Resonance is shifted when
pump is ‘1’
3. Signal light off resonance ->
‘1’ -> wavelength conversion
4. Inverted case can be realized
Qiang Li et al., App. Phy. Lett., 2008
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Wavelength multicasting
FSR
s1
s2
p
 Conversions of 2 wavelengths ->
wavelength multicasting
 By setting the signal wavelengths
properly, non-inverted and inverted
multicasting can be implemented
Qiang Li et al., App. Phy. Lett., 2008
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Format conversion- NRZ to FSK
5dB/div
0.5nm/div
s1
s2
FSK Spectrum
p
Input NRZ signal
500μW/div
500ps/div
FSK Eye diagram
500μW/div
2.5ns/div
demodulated signal: upper sideband
Fangfei Liu et al., APOC 2008
demodulated signal: upper sideband
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Optical temporal differentiator
In the critical coupling region
(QL = QE), the transfer function
of the microring resonator is:
:
T ( )  j
2Q
0
(  0 )
A typical function for a firstorder temporal differentiator
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Experimental results
10G
Input
5G
Output
Input
Output
(a-i)
50ps/div
(a-ii)
50ps/div (a-iii)
(b-i)
50ps/div
(b-ii)
50ps/div (b-iii)
50ps/div (b-iv)
50ps/div
(c-i)
100ps/div (c-ii)
100ps/div (c-iii)
100ps/div (c-iii)
50ps/div
100ps/div
(a-iv)
100ps/div
Gaussian
Sine
Square
Fangfei Liu, et al., Opt. Express 2008
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Format conversion- NRZ to AMI
A microring is a high pass filter
NRZ + high pass filtering => AMI
10G NRZ
10G AMI
1.0
1.0
Normalized amplitude (a.u.)
Normalized amplitude (a.u.)
(a)
0.8
0.6
0.4
0.2
0.0
(b)
0.8
0.6
0.4
0.2
0.0
0
400
800
1200
Time (ps)
1600
0
2000
400
800
1200
1600
2000
Time (ps)
Qiang Li et al., Chin. Opt. Lett., Vol 7, No. 2, 2009
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How to build an ultra-high-speed alloptical differentiator?
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80-G optical differentiator using a ring
resonator with 2.5-nm bandwidth
Radius: 20 μm
Bandwidth : 2.5 nm
Resonance wavelength: 1551.73nm
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Measurement setup
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80-Gb/s differentiation result
G. Zhou et al., Electron. Lett. 2011
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Future work: 160-G differentiation
Design of new ring resonator: critical coupling, large 3-dB bandwidth
One possible design:
• Large bandwidth: small diameter and high loss
• Critical coupling: long coupling length
B3dB=5nm
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Comparison of optical and
electronic differentiators
Species
Speed
Size
Power dissipation
Silicon ring
80 Gbps or
higher
20 μm (radius)
< 1 mW
Digital
differentiator
a few GHz
mm2
a few W
All-optical differentiator: (1) ultra-high speed
(2) compact structure
DSP based: configurable; can fulfill more than one function
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Differential equation solver
Differential equations are widely employed in virtually any field of
science and technology:
• Physics
• Biology
• Chemistry
• Economics
• Engineering
All constant-coefficient linear differential equations can be modeled
with finite number of:
• Differentiators
1 dy
1 dx
• Couplers/Subtractors
y 

• Splitters
2 dt
2 dt
• Feedback branches
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Optical differential equation solver
y 
1 dy
1 dx

2 dt
2 dt
optical input
signal x
+
optical
differentiator
1
2
optical output
signal y
output port
input port
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Silicon microring for bio-sensing
DNA hybridization
DNA probe
After hybridization:
DNA probe is attached to the ring
The effective index changes around the
waveguide results in resonance shift
Problems with the single ring:

limited sensing area
not easy to control the notch depth (air gap
between the ring and the straight waveguide)

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Proposal: concentric rings
Single ring
concentric ring
Two samples
Field
distribution
The field is evenly distributed among the two concentric rings,
thus increasing the sensing area
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Enhanced notch depth
Blue: single ring
Red: double rings
Enhanced notch depth, easier detection of resonance shift
More rings?
Xiaohui Li, et al., Applied
Optics 2009
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Conclusions
Silicon ring resonators with nano-scale SOI
waveguides can perform many functions:
• Tunable delay
– Digital: different modulation formats at 5 Gb/s
– Analog: 40-GHz microwave photonic signal
• Signal conversions
– Dense wavelength conversion and multicasting
– Format conversions
– Optical temporal differentiator
• Concentric rings for sensitive bio-sensing
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