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DAC-Less PAM-4 Slow-Light Silicon Photonic Modulator Assisted by Coupled Bragg Grating Resonators
Omid Jafari, Sasan Zhalehpour*, Wei Shi, Sophie LaRochelle
Centre d'optique, photonique et laser (COPL), Université Laval, Québec, QC, Canada
*Huawei Technologies Canada, Québec, QC, Canada
2021-06-08
Typical all-silicon modulators
2
- 1Tb/s raw| 833 Gb/s net(32 QAM 100 Gbaud), stable performance
- Larger power consumption, larger footprint
Zhalehpour et al., OFC 2019
Mach-Zehnder modulators (MZM): Micro-ring modulator (MRM):
R. Dube et al., Optica 2016
- 80 Gb/s (PAM4 40 Gbaud), Eb= 6.5 fJ/bit
- Instability in operation, Non-linear response, chirpy pulse
Tradeoff between stability and efficiency
Slow-light modulators
3
• Group index (ng) vs. optical bandwidth (Δλ)
Group index (ng) ~10
Optical bandwidth (Δλ) = C-band
Phase shifter length (L) = 200 μm
Y. Terada et al, Opt. Lett., vol. 42, no. 24, 2017
ng ~ 8×3.8
Δλ = 4 nm
L = 110 μm
Larger Δλ → Larger operating temperature range
ng ~ 8
Δλ = 1.3 nm
L = 500 μm
S. Romero-García et al. Opt. Lett., vol. 42, no. 1, 2017
Brimont, et al. Optics letters vol. 37, no. 17, 2012
MZM assisted by coupled BGRs
4
• MZM assisted by coupled Bragg grating resonators (BGRs)
1480 1520 16001560
-30
-20
-10
0
-80
-40
Ph
ase
resp
on
se (
π )
Tra
nsm
issi
on (
dB
)
-120
NOR=6NOR=2 NOR=4
NOR=8 NOR=10 Transmission
3dB Optical BW= 3.15 nm0
NOP=15
Wavelength (nm)
NOR: number of resonators NOP: number of periods
O. Jafari, et al, “High-Efficiency Silicon Photonic Modulator Using Coupled Bragg Grating Resonators” JLT, 37 (9), 2019.
Wavelength (nm)
1545 1547 1549
60
20
40
Gro
up
in
dex
NOP=15
NOR=6
0
Tra
nsm
issi
on
(d
B)
-5
-10
Operating wavelength range
Slowing down optical waves up to ng = 20 over a nm-range optical bandwidth
MZM assisted by coupled BGRs
5
Ideal grating After Litho. effects
Designed considering Litho. effects
fabricated in a standard 193 nm lithography process
Operating wavelength range
o Length phase shifter: 162 μm
o Operating wavelength range: 3.5 nm @ 30 Gb/s
o Phase modulation enhancement : 7
o Energy consumption: 84 fJ/bit
o Optical bandwidth: 2.9 nm
Design Simulated by FDTD&TMM Experiment
Single-drive push-pull driving scheme
• Experimental characterization of the modulator
O. Jafari et al., “Mach-Zehnder silicon photonic modulator assisted by phase-shifted Bragg gratings”, IEEE PTL, 32(8), 2020
o Six resonators in each arm
DAC-less PAM4 slow-light all-silicon MZM
6
Fabricated in a standard 193 nm lithography process
• Segmented slow-light MZM for enabling high-speed PAM operation
o Optical DAC → Driven by two-binary RF signals
Si Substrate BOXSiO2 P++P+PNN+N++
M2
5.2 0.81 ΔXnΔXp0.83 10.4
10.13
0.09
M1
P+ P N N+ N++
M2
M1
M2
M1
0.25
GSG
S
GB2H6H5 H4B1 H1H3H2
DC probe
Fiber
array
RF probe
GSSGRF probe
GS
SL S-MZM-
BGR
a)
c)
d) e)
f)
Heater
Heater
BGR
Input Output1
Output2
b)
VRF1
VBias1
VRF2
VBias2
342μm 228μm22.8μm
VRF-GroundVDC-BiasVRF-Signal
Via1
Via2
DAC-less PAM4 slow-light all-silicon MZM
7
LSB segment MSB segment
Top arm Bottom arm Both armsSchematic of test structure for transmission
Schematic of test structure for refection
• Experimental characterization of PAM4 slow light modulator
Examine each arm response separately:
DAC-less PAM4 slow-light all-silicon MZM
8
1 2
2 3 4 5
10 9 8 7 6 12
11 12 13 14 15 16
21 20 19 18 17 24
22 23 24 25 26 30
30 29 28 27
31 36
• Experimental characterization of PAM4 slow light modulator
Design reliability test: Wafer-uniformity testing → Examine response of different dies
λBragg
ΔλBW
Each die: 315.8 mm2
DAC-less PAM4 slow-light all-silicon MZM
9
Optical Spectrum:
- 3.2 nm optical bandwidth- 5.5 dB insertion loss
"0 0
"0 1
"1 0
"1 1
VMSB_T VLSB_T VMSB_B VLSB_B
0 0 -3.5 -3
0 -3 -3.5 0
-3.5 0 0 -3
-3.5 -3 0 0
DC characterization:
- Large signal Vπ×L of 0.51 Vcm
- Enhancement factor of γ = 6 in phase modulation
• Experimental characterization of PAM4 slow light modulator
DAC-less PAM4 slow-light all-silicon MZM
10
1 5 10 20 40 60
Frequency (GHz)
-60
-50
-40
-30
-20
-10
0
EE
re
sp
on
se
(dB
)
S11 (LSB segment)
S11 (MSB segment)
Crosstalk
a) b)
1560 1560.5 1561 1561.5 1562 1562.5 1563 1563.5
Wavelength (nm)
30
35
40
45
3-d
B E
O b
an
dw
idth
MSB segment
LSB segment
b)c)
- RF crosstalk less than -40 dB
Electro-optic S-parameter measurement:RF characterization:
• Experimental characterization of PAM4 slow light modulator
- Both segments: S21> 40 GHz EO bandwidth- 2 nm optical bandwidth- Both segments:S11< -15 dB
Small signal measurements:
BPG
SHF
12103 A
PC
Clock
OBF
SHF 41211 C
EDFA
~~~
Laser
PM fiber
Electrical path
Optical path
DC
source
LPFResampling
Downsampling
DD-MMSE
Demapping+Sync.
BER
RTO
Receiver side DSP
OBFEDFA
~~~
In line
attenuator
PA
DC block
DC blockPA PS
PS
DAC-less PAM4 slow-light all-silicon MZM
11
80 GSa/s 63 GHz
32 GHz
PA: RF amplifier (30 GHz and 40 GHz)
• Experimental characterization of PAM4 slow light modulator
Large signal response:
7% FEC
20% FEC
DAC-less PAM4 slow-light all-silicon MZM
12
• Experimental characterization of PAM4 slow light modulator
Large signal response:
o Operating wavelength range: ~2 nm @ 90 Gb/s
o Operating temperature range:
ΔT = 50 C considering Δλ/ΔT
= 40 pm/C
o Energy consumption: 73 fJ/bit
( )
22 1
202
log ( ) 1
Mpp
b
i
CV iE M i
M M M
−
=
= −
−
Eye diagrams are retrieved from date captured by RTO
CONCLUSION
13
Demonstrate DAC-less PAM-4 slow-light silicon photonic modulator providing
high efficiency and stability
o Length phase shifter: 570 μm
o Operating wavelength range: 2 nm @ 90 Gb/s → ΔT = 50 C assuming Δλ/ΔT = 40 pm/C
o Phase modulation enhancement: 6
o Estimated energy consumption: 73 fJ/bit
