Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
Lecture 11: Ring Resonator Modulator Transmitters
ECEN689: Special Topics in Optical Interconnects Circuits and Systems
Spring 2016
Announcements
• Exam2 is on Apr. 28 • 2:20-3:45PM (10 extra minutes)• Closed book w/ one standard note sheet• 8.5”x11” front & back• Bring your calculator• Comprehensive, but will focus primarily on
Lecture 5 and beyond
2
Agenda
• WDM Optical Interconnect Motivation
• Silicon Photonic Modulators
• Carrier-Injection Ring Resonator Modulators
• Carrier-Depletion Ring Resonator Modulators
3
Wavelength-Division Multiplexing (WDM) Optical Interconnects
4
• Optical interconnects remove many channel limitations• WDM allows for multiple high-bandwidth (10+Gb/s)
signals to be packed onto one optical channel
[Young JSSC 2010]
Next-Generation 100G Interconnects
5
LinkDistance
TransceiverType
# of I/O Ports(25Gb/s NRZ)
0.5m Electrical PCB Trace × 41m Electrical Backplane × 4
10m Electrical Cu Cable × 4100m VCSEL MMF × 41km MZI / EAM SMF × 4
> 10km MZI / EAM SMF × 4
[Google Datacenter]
Next-Generation 100G Interconnects
6
LinkDistance
TransceiverType
# of I/O Ports(25Gb/s NRZ)
0.5m Electrical PCB Trace × 41m Electrical Backplane × 4
10m
Ring ModulatorWDM SMF × 1
100m1km
> 10km
[Google Datacenter]
Ring Resonator Filter
• Ring resonators display a high-Q notch filter response at the through port and a band-pass response at the drop port
• This response repeats over a free spectral range (FSR)
7
Ring-Resonator Modulator (RRM)
• Refractive devices which modulate by changing the interference light coupled into the ring with the waveguide light
• Devices are relatively small (ring diameters < 20m) and can be treated as lumped capacitance loads (~10fF)
• Devices can be used in WDM systems to selectively modulate an individual wavelength or as a “drop” filter at receivers
8
[Young ISSCC 2009]
Wavelength Division Multiplexing w/ Ring Resonators
• Ring resonators can act as both modulators and add/drop filters to steer light to receivers or switch light to different waveguides
• Potential to pack >100 waveguides, each modulated at more than 10Gb/s on a single on-chip waveguide with width <1m (pitch ~4m)
9
[Rabus]
WDM Photonic Transceiver
10
• High bandwidth density by combining multi-channels on a single waveguide via wavelength division multiplexing
Agenda
• WDM Optical Interconnect Motivation
• Silicon Photonic Modulators
• Carrier-Injection Ring Resonator Modulators
• Carrier-Depletion Ring Resonator Modulators
11
Plasma Dispersion Effect
• The change in refractive index and optical absorption coefficient is induced by free carriers in a semiconductor
N n i N: complex index of refractionn: electro-refraction: electro-absorption
1550 nm
1310 nm 0.822 181.31 6.2 10 6.0 10m e hn n n
18 18 1
1.31 6.0 10 4.0 10 [ ] m e hn n cm
0.822 181.55 8.8 10 8.5 10m e hn N N
18 18 1
1.55 8.5 10 6.0 10 [ ]m e hN N cm
12∆Ne: electrons density∆Nh: holes density
Silicon Photonic Modulators
• Carrier Accumulation• Carrier Injection• Carrier Depletion
13
Figure of Merit Carrier Accumulation
CarrierInjection
Carrier Depletion
Modulation Bandwidth High Low High
Extinction Ratio (ER) Small Large Small
Modulation Efficiency High High Low
Insertion Loss High Low High
+-+ -+ -+ -
+--+
n++ p++
Contact Contact
BOXn++ p++
Contact Contact
BOX
Si n+ p+n++ p++
Contact Contact
BOX
• Electrooptic-polymermodulators have alsobeen developed
• Mach Zehnder Modulator• Microring Modulator
MZM vs Microring Modulators
Figure of Merit Mach Zehnder Microring
Footprint Large Small
Extinction Ratio (ER) Small Large
Insertion Loss High Low
Wavelength Sensitivity Low High
+-
+ -+-+-
14
p+n+
p+n+
n+
p+
Agenda
• WDM Optical Interconnect Motivation
• Silicon Photonic Modulators
• Carrier-Injection Ring Resonator Modulators• Device Operation and Modeling• High-Speed Driver• Wavelength Stabilization Loop
• Carrier-Depletion Ring Resonator Modulators
15
Carrier-Injection Microring Modulator
16
• Ring waveguide is doped as a PIN junction
• Forward-bias voltage injects carriers, changing refractive index for optical modulation via the free carrier plasma dispersion effect
C-I Ring Modulator Speed Limitations
• Speed is limited by long minority carrier lifetimes• Pre-emphasis signaling significantly improves data rates
17
Input
p+ doped
n+ doped
n+ doped
Outputn+ p+Si
V
Waveguide
w/ Pre-emphasis
Simple NRZ 8Gb/s
8Gb/s
Carrier-Injection Ring Modulator Modeling
• Previous Related Modeling Work• P-I-N diode model (only electrical dynamics)• Ring resonator model (only optical dynamics)• Carrier-injection ring modulator model (lack accurate
large-signal behavior)
• Carrier-Injection Ring Modulator Model• Capture nonlinear electrical and optical dynamics
18
[Strollo TPE 1997]
[Smy JOSA B 2011]
[Binhao Wang, Cheng Li, Chin-Hui Chen, Kunzhi Yu, Marco Fiorentino, Raymond Beausoleil, and Samuel Palermo, “Compact Verilog-A Modeling of Carrier-Injection Microring Modulators for Optical Interconnect Transceiver Circuitry Design,” accepted in Journal of Lightwave Technology]
[Binhao Wang, Cheng Li, Chin-Hui Chen, Kunzhi Yu, Marco Fiorentino, Raymond Beausoleil, and Samuel Palermo, “Compact Verilog-A Modeling of Silicon Carrier-Injection Ring Modulators,” IEEE Optical Interconnects Conference (OIC), 2015]
[Cheng Li, Chin-Hui Chen, Binhao Wang, Samuel Palermo, Marco Fiorentino, and Raymond Beausoleil, “Design of an Energy-Efficient Silicon Microring Resonator-Based Photonic Transmitter,” IEEE Design & Test of Computers, 31, 46-54, 2014]
[Cheng Li, Rui Bai, Ayman Shafik, Ehsan Zhian Tabasy, Binhao Wang, Geng Tang, Chao Ma, Chin-Hui Chen, Zhen Peng, Marco Fiorentino, Raymond Beausoleil, Patrick Chiang, and Samuel Palermo, “Silicon Photonic Transceiver Circuits with Microring Resonator Bias-Based Wavelength Stabilization in 65-nm CMOS,” IEEE Journal of Solid-State Circuits (JSSC), 49, 1419-1436, 2014]
[Xu OE 2007, Wu OE 2015]
Model Flow Chart
19
Driving Voltage
Current Response
P+ N- N+
p-i-n diode SPICE model Carrier
Concentration0( )
t
totalQ I t dt q
totalQ freeQ
c RCC
R
0.822 181.31 6.2 10 6.0 10m e hn n n
18 18 11.31 6.0 10 4.0 10 [ ]m e hn n cm
Index and Loss Changes
Plasma dispersion effect
Input
p+ doped
n+ doped
n+ doped
Outputn+ p+Si
V
Waveguide
Optical Power
Dynamic ring resonator model
4
1 11
expnn
n m
E tT t a t j t m
E
input1E
output4E
2E3E
a
Electrical Modeling - PIN Dynamics
• I-V dynamics are based on a moment-matching approximation of the ambipolar diffusion equation
20
[Strollo TPE 1997]
H0 GE G3
Repi
Rlim Gmod
Gpin
Ej Dj
1 5 9 13 17
0
7T
0
11T
0
15T
0
19T
5 9 13 17
10 20
1112
VS2
VS1Anode Cathode
+-
+-
intrinsic region junctions1 0SV
mod 2 2 3111,12 AM AM
M M
L LG V V V VV V
P+ N- N+
VepiVp Vn
IIepi
Ir
Rc Rc
I Iepi
V1 V2 V3
pin epi p n epi jV V V V V V (10,12) (12,20)c epi j cV V V V V V V
1 0( ) 1j TE NVS SI V q I e
0 1 0( )SH I V q pin epi EI G I G
20r E EI G q I
2
03 3 1
3HI j
PT SC
P VTG VV R I
epi rI I I (12,20)j jE V V
Parameters initialization based on an empirical range
Estimation of RC, PHI, IS, N, IE, VM, RLIM and REPI parameters by fitting DC measurement
Estimation of T0, τ, LAM, VPT and RSC parameters by fitting transient measurement
Delivery of the parameter set
Parameters initialization based on an empirical range
Estimation of RC, PHI, IS, N, IE, VM, RLIM and REPI parameters by fitting DC measurement
Estimation of T0, τ, LAM, VPT and RSC parameters by fitting transient measurement
Delivery of the parameter set
Parameters initialization based on an empirical range
Estimation of RC, PHI, IS, N, IE, VM, RLIM and REPI parameters by fitting DC measurement
Estimation of T0, τ, LAM, VPT and RSC parameters by fitting transient measurement
Delivery of the parameter set
Parameters initialization based on an empirical range
Estimation of RC, PHI, IS, N, IE, VM, RLIM and REPI parameters by fitting DC measurement
Estimation of T0, τ, LAM, VPT and RSC parameters by fitting transient measurement
Delivery of the parameter set
Parameter Unit Description ValueRc Ω Contact resistance 50
IS A Saturation current 5.78 1014
N - Emission coefficient 1.46
PHI V Build-in voltage 0.7
T0 s Transit time 1.04610-10
IE A Emitter recombination knee current 1.010-3
VM V High-injection voltage drop on the base 0.12
Rlim Ω Carrier-scattering series resistance 1.810-3
LAM - Forward-recovery coefficient 0.03
τ s Carriers Lifetime in the base 1.010-9
Repi Ω Base region resistance 300
VPT V Reverse-recovery coefficient 10
RSC Ω Reverse-recovery coefficient 18
Parameter Unit Description RangeRc Ω Contact resistance 20 - 100
IS A Saturation current 1 1014 - 1 1012
N - Emission coefficient 1 - 2
PHI V Build-in voltage 0.5 - 1
T0 s Transit time 110-10 - 110-9
IE A Emitter recombination knee current 1.010-4 - 1.010-2
VM V High-injection voltage drop on the base 0 – 0.5
Rlim Ω Carrier-scattering series resistance 110-3 - 310-3
LAM - Forward-recovery coefficient 0 – 0.1
τ s Carriers Lifetime in the base 1.010-10 - 1.010-8
Repi Ω Base region resistance 1102 - 1103
VPT V Reverse-recovery coefficient 5 - 20
RSC Ω Reverse-recovery coefficient 1- 100
Electrical Modeling - Parameter Extraction
21
10Gb/s 0101… excitation
Optical Modeling - Carrier Dynamics
• Carrier Concentration
0
/t
Q I t dt q total free remain recombineI t I t I t I t
total free remain recombineQ t Q t Q t Q t
0.822 181.31 6.2 10 6.0 10m e hn n n
18 18 1
1.31 6.0 10 4.0 10 [ ] m e hn n cm22
High pass filter
The time constant is equal to the carrier lifetime
totalQ freeQ
c RC
CR
10Gb/s 0101… excitation
• Index and Loss Changes
Optical Modeling - Optical Dynamics
• Consider the ring’s cumulative phase shift• Capture non-linear optical dynamics
1E 4E
2E 3E
a
4
1 11
expnn
n m
E tT t a t j t m
E
1
exp
exp
1 exp
n
n
T t a t jn t
a t j t
a t j t
2efft n t L
where
23
2 2 1 [Loannidis OL 1988]
Optical Modeling - Optical Dynamics
• Parameter Extraction
Parameter Unit Description Value
σ - Transmission coefficient 0.9944
a - Loss coefficient 0.9931
neff - Effective index 2.5188
r µm Ring radius 5
24
• Large extinction ratio (ER) ~20dB• High modulation efficiency ~560pm/V
• Ring modulator under test
• 8Gb/s eye diagrams with simple NRZ signal
VoltageSwing DC Bias
Simple NRZ Signaling
1.7V
2V
25-0.3V
0.7V
Parameter Description
Coupling waveguide 350nm (W), 250nm (H), 50nm (slab)
Ring waveguide 450nm (W), 250nm (H), 50nm (slab)
Gap 250nm
Radius 5um
P+ doping BF2+, 5e14 cm-2, 10 KeV, Tilt 8°, Twist 27°
N+ doping As, 5e14 cm-2, 10 KeV, Tilt 8°, Twist 27°
• Pre-emphasis NRZ signal generation
• Proposed model utilized to study the impact of key pre-emphasis parameters• Pulse duration• Pulse depth• DC bias
Pre-Emphasis Signaling
26
Pattern Generator
data
data Combiner MicroringModulatorDelay
Pre-Emphasis Optimization - Duration
• Pulse Duration • Pulse Depth = 0.8V • DC Bias = 0.7V
80ps40ps
27
• 40ps pulse duration allows the eye to partially open• 80ps pulse duration provides optimal eye opening
Duration
Depth
VoltageSwing DC Bias
2V
1.7V
0.9V
0.5V
-0.3V
0.7V
Pre-Emphasis Optimization - Depth
0.9V 0.8V 0.7V
28
• Pulse Duration = 80ps • Pulse Depth• DC Bias = 0.7V
• 0.9V pulse depth results in low amount of charge for logic “1”• 0.7V pulse depth produces excessive charge for logic “1”
80ps
Duration
Depth
VoltageSwing DC Bias
2V
1.7V
-0.3V
0.7V
Pre-Emphasis Optimization - DC Bias
0.75V 0.7V 0.65V
29
• Pulse Duration = 80ps • Pulse Depth = 0.8 V • DC Bias
• 0.75V DC bias produces excessive charge for logic “1”• 0.65V DC bias results in slower carrier injection for logic “1”
80ps
Duration
Depth
VoltageSwing DC Bias
2V
Co-Simulation with CMOS Driver
• Hybrid-integrated CMOS and silicon photonics prototype• Optical transmitter co-simulation schematic• Asymmetric pulse duration pre-emphasis setting• 9Gb/s measured and co-simulated eye diagrams 30
1.45V
1.15V
-0.55V
-0.05V2V
70ps
50ps
0.55V
DAC
Driver500pH
500pH
40fF 40fF
40fF 40fF
PIN RingModulator
Cathode
Anode
High-Swing Pre-Emphasis Driver
31
• Dual-edge pre-emphasis with pulse width controlled by tunable delay cells (30ps~60ps)
• Cascode output stage used to meet high modulation swing requirement
[C. Li JSSC 2014]
Optical Transmitter Assembly
32
Electrical Eye (9Gbps, with Pre-emphasis)
Electrical Eye (9Gbps, w/o Pre-emphasis)
Optical Eye (9Gbps, with Pre-emphasis)
Closed Optical Eye (9Gbps, w/o Pre-emphasis)
* C. Li et. al. IEEE Design & Test, 2014
480fJ/bit
• GP 65nm CMOS 5-channel TX prototype• 130nm SOI carrier-injection ring resonator modulators
Resonant Wavelength Sensitivity
33
• Ring’s resonance wavelength is sensitive to fabrication variations and temperature fluctuations
• Requires tuning schemes to compensate wavelength drifts
Extinction Ratio Impact• Ring devices resonance wavelength can shift
with fabrication and temperature variations• Tuning schemes necessary to stabilize
resonance wavelength
Before Tuning After Tuning
Bias vs Thermal Tuning
35
Bias Tuning Thermal TuningSpeed Fast (~µs) Slow (~ms)
Direction Blue shift Red shiftPower Low HighRange Narrow WideThermal Tuning
Bias Tuning 1311.4 1311.6 1311.8 1312 1312.2-20
-15
-10
-5
0
5
Wavelength (nm)
Opt
ical
Tra
nsm
issi
on (d
B)
Ring Spectrum vs Bias Voltage
Automatic Tuning Loop
36
• Automatic tuning loop sets ring output power to a DAC-generated reference level corresponding to the ring’s resonance point
• Also applicable for thermal tuning
[C. Li JSSC 2014]
Static Tuning Mode37 of 36
Dynamic Tuning Mode• Lock to average power
0 1 2 3 40
2
4
6
8
10
12
Wavelength Shift (Normalized to 0/2Q)
Extin
ctio
n R
atio
(dB
)
Maximum Extention Ratio (Static tuning)Achieved Extinction Ratio (Modulated data)
• ∆ λshift ≥ FWHM to guarantee ER
Bias-Based Tuning Measurements
39
• Extinction ratio dramatically improved after bias-based tuning
• 340W for a tuning range of 0.28nm
Agenda
• WDM Optical Interconnect Motivation
• Silicon Photonic Modulators
• Carrier-Injection Ring Resonator Modulators
• Carrier-Depletion Ring Resonator Modulators• Device Operation and Modeling• High-Speed Driver• Wavelength Stabilization Loop
40
Carrier-Depletion Ring Modulator Challenge I: Output Swing & Biasing
41
• High-speed depletion-mode ring modulator requires: Large swing: >4V Negative DC-bias: -2V
ISSCC2013 This Work
Ring Type Injection Depletion
Doping Profile PIN Lateral
PNQ 8000 5000
Tunability(pm/V) 350 25
Data Rate 9Gb/s 25Gb/s
Swing for>7dB ER < 2Vpp > 4Vpp
1552 1552.6-15
-10
-5
0
- 4V Bias
0V Bias
~8dB ER w/4V Swing
Measured Ring Modulator Spectrum
Opt
ical
Tra
nsm
issi
on
(dB
)1552.2 1552.4
Wavelength (nm)
Carrier-Depletion Ring Modulator Challenge II: Nonlinear Dynamics
42
• Dynamic change of neff → unequal rise/fall times• Asymmetric equalization for non-linearity cancellation
Cathode
Anode
Depletion Ring E-O Model
PhotonLifetime Dynamic
Ring-Index
Change
OutputPower
-80 -60 -40 -20 0 20 40 60 800
0.2
0.4
0.6
0.8
1
Time (ps)
Nor
mal
ized
Out
put P
ower
Simulated 25Gb/s Optical Eye Diagram
-15
0
Falling Edge dneff > 0
Pow
er (d
B)
-15
0
Rising Edge dneff < 0
Pow
er (d
B)
0V - 4V 0V - 4V
Carrier-Depletion Ring Modulator Modeling
• Previous Related Modeling Work• Ring resonator model (only optical dynamics)• Carrier-depletion ring modulator model (lack electrical dynamics)
• Carrier-Depletion Ring Modulator Model• Capture nonlinear electrical and optical dynamics
43
[Smy JOSA B 2011]
[Zhang JSTQE 2010, Buckwalter JSSC 2012, Ban OIC 2015]
[Ashkan Roashan-Zamir, Binhao Wang, Shashank Telaprolu, Kunzhi Yu, Cheng Li, M. Ashkan Seyedi, Marco Fiorentino, Raymond Beausoleil, and Samuel Palermo, “A 40Gb/s PAM4 Silicon Microring Resonator Modulator Transmitter in 65nm CMOS,” accepted in IEEE Optical Interconnects Conference (OIC), 2016]
[Hao Li, Zhe Xuan, Cheng Li, Alex Titriku, Kunzhi Yu, Binhao Wang, Nan Qi, Ayman Shafik, Marco Fiorentino, Michael Hochberg, Samuel Palermo, and Patrick Yin Chiang, “A 25Gb/s, 4.4V Swing, AC-Coupled Ring Modulator-Based WDM Transmitter with Wavelength Stabilization in 65nm CMOS,” IEEE Journal of Solid-State Circuits (JSSC), 50, 3145-3159, 2015]
[Hao Li, Zhe Xuan, Cheng Li, Alex Titriku, Kunzhi Yu, Binhao Wang, Nan Qi, Ayman Shafik, Marco Fiorentino, Michael Hochberg, Samuel Palermo, and Patrick Yin Chiang, “A 25Gb/s, 4.4V Swing, AC-Coupled, Si-Photonic Microring Transmitter with 2-Tap Asymmetric FFE and Dynamic Thermal Tuning in 65nm CMOS,” IEEE International Solid-State Circuits Conference (ISSCC), 2015]
44
Carrier-Depletion Ring Modulator Model
n+ p+n++ p++
Contact Contact
BOX
500nm
950nm 950nm130nm
90nm
Cathode
Anode
∆n = F2(VPN)∆α = F3(VPN)
RN
RP
CPN = F1(VPN)
CSUB
CSUB
RSUB
RSUB
VN
VP
Ring Resonator Dynamics
Input Output
• Lumerical
• Polynomial Curve Fitting
45
Electrical Modeling - Parameter Extraction
2 3 40 1 2 3 4f V a aV a V a V a V
Parameter Unit a0 a1 a2 a3 a4
∆neff - -4.3107 7.3105 8.0106 1.1106 5.2108
∆α dB/cm 0.01 1.5 0.17 -2.3102 1.0103
C fF/μm 0.71 -0.14 5.5102 -1.2102 1.0103
46
Optical Modeling - Optical Dynamics
0
1 1 12 iA cj A j St
o iS S j A
1 1 1c l where
input outputiS oS
t
t
2 2 2 2g cv R
0 02 n n R m
02 0.751 Rl gv e
• Capture non-linear optical dynamics• Require less memory and computation time
[Little JLT 1997]
47
Optical Modeling - Optical Dynamics
Parameter Unit Description Value
τ ps Amplitude decay time 9.07
κ - Coupling ratio 0.188
m - Mode number 28
r µm Ring radius 7.5
• Parameter Extraction
• Extinction ratio (ER) ~10dB• Modulation efficiency ~25pm/V
Co-Simulation with CMOS Driver
• Optical transmitter prototype assembly• Optical transmitter co-simulation schematic
48
500pH
500pH
40fF 40fF
40fF 40fF
PN RingModulator
Cathode
Anode
Wire Bonding
Differential High Swing Transmitter
2-Tap FFE Driver
2-Tap FFE Driver
Measured and Co-Simulated 25Gb/s Eye Diagrams
• Asymmetrical ISI is due to the device nonlinearity• It is compensated by an optimized nonlinear equalizer
49
Proposed AC-Coupled Differential Driver
50
• CC = 3pF → 4.4V differential swing
• ZS < 30Ω to minimize low-pass attenuation
• High-pass cut-off: < 10MHz
Simulated AC-Response of Output Passive Network
106 107 108 109 1010 10110
1
2
3
4
5
6
7
Frequency (Hz)O
utpu
t Sw
ing
(V)
ZS = 15Ω, CS = 150fFZS = 30Ω, CS = 75fF ZS = 60Ω, CS = 30fF
Parameters ValueCCRB
CPAD1LWIRECPAD2
RNRPCPN
3pF40kΩ 70fF
500pH30fF20Ω30Ω25fF
ZS
2×VDD
ZS
2×VDD
Lwire
Lwire
Cathode DC-Bias
Cathode
AnodeDC-Bias
CC
CC
RB
RB Rdamping
Rdamping
CPAD1
CPAD1
CPAD2
CPAD2
Rsub
Rsub
Csub
Rn
Rp
Cpn
~4×VDD Swing
Anode
RingModulator
CS
CS
[H. Li ISSCC 2015]
Conventional Segmented Output Driver
• Cascode transistors suffer VDS overstress
• Large parasitic capacitance due to segmented design
0 1 2 3 4-1
-0.5
0
0.5
1
1.5
2
V DS
(V)
Time (ns)
Simulated VDS
1.7V VDS Overstress
Main Cursor
2.4V
0V
2.4V
VDS
VDS
Segmented Output Stage×4
Post Cursor
Main Cursor
Post Cursor
Main Cursor
Post Cursor
Main Cursor
Post Cursor
SEL
SEL
SELB
SELB 0V
Main Cursor
PostCursor
‘1’ LevelFFE
Control
PostCursor
MainCursor
× 1× 2
× 4× 8
Merged Output Stage
2.4V
0V
1.2V
1.2V
‘0’ LevelFFE
Control
‘0’ LevelImpedance
Control
‘1’ LevelImpedance
Control
Proposed 2-Tap FFE Output Driver
52
• Merged cascode transistors
• No VDS overstress
• Reduced parasitics and area
• Independent ‘1’-Level and ‘0’-Level FFE coefficients
Transmitter Architecture
2.4V2-Tap FFEDriver
27-1PRBSGen.
4:1
2.4V2-Tap FFEDriver
Cathode DC-Bias
Anode DC-Bias
TX<1:5>
12.5GHzClockInput
OpenDrain
4:1
4:1
4:1
Tap0
Tap0b
Tap1
Tap1b
8:4
CML-to-CMOS &Clock Buffers
Tap0HTap0LTap1HTap1L
Tap0bHTap0bLTap1bHTap1bLFixed
Pattern
Clk<1> Clk<2> Clk<3> Clk<4> Clk<5>
1.2V DVDD 2.4V/1.2V DVDD
1.2VCVDD
Level Shifter & Pre Driver
8
4
4
4
4
88
8
FFE Control
8 Sel.
1.2VCVDD
25Gb/s 8:1 CMOS Serializer
• Quadrature quarter-rate architecture eliminates high-speed retiming before final 2:1 MUX
÷2
FF
FF
FF
FF
6.25GHz Quad. CLKs
DEVEN
DODD
DOUT
12.5GHz
2:1
÷22:1
÷22:1
÷22:1
÷2
0º
180º
90º
270º
D0D4
D1D5
D2D6
D3D7
FF
FF
25Gb/s 8:1 CMOS Serializer
• Tri-state inverter-based 2:1 MUX for fast edge rate
1X 2X
2X1X
2X
CLKB
CLK
DEVENDODD
-80 -60 -40 -20 0 20 40 60 80
0
0.5
1
Time (ps)A
mpl
itude
(V)
Pass-Gate MUX
Tri-State MUX
1.5Simulated 25Gb/s DOUT
DOUT
Heterogeneous Integration
56
• Hybrid CMOS-Photonic packaging (<0.5mm bond-wires)
• Stable optical coupling using vertically-attached fibers
25Gb/s Optical Measurement
57
MRM-1, FFE Off, 4.4V Swing QF=4.4, ER=5.3dB
MRM-1, FFE On, 4.4V SwingQF=8.4, ER=6.8dB
500µW 16ps500µW
16ps
Test Channel 1 w/o FFE
Test Channel 1 w/ Asymmetric FFE
Challenge III: Wavelength Stability
• Modulation efficiency depends strongly on wavelength• ER degradation due to temperature fluctuation• Closed-loop control is necessary for robust operation
Average Power Thermal Stabilization
Thermal Tuning Algorithm
Thermal Tuning Algorithm
Thermal Tuning Test
Thermal Tuning Test
Dynamic Thermal Tracking Test
Dynamic Thermal Tracking Test