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Ming Wu - 2
Large-Scale Silicon Photonic Switches
Ming C. Wu
University of California, Berkeley
Electrical Engineering & Computer Sciences Dept.
& Berkeley Sensor and Actuator Center (BSAC)
Ming Wu - 3
Outline
• Optical switches for data centers
– Why do we need it?
– What’s available now?
– What’s needed in the future?
• Silicon photonic switch
– Is it a game changer?
– How does it scale?
• Silicon photonic MEMS switches
– 64x64 switch on 1-cm2 chip
– Technology scaling
• Discussion about packaging needs
• Summary
Ming Wu - 4
Challenges in Datacenter Networks
• Link rate continues to increase (40G, 100G, 400G, ..)
• Cannot rely on continual scaling of CMOS
– Switch bandwidth-portcount limited by thermal issue, die size,
pin count
• Optical switching can facilitate scaling out datacenters
– Reduce number of hops, transceivers, power consumption
– Critical issues: switching time, arbitration, cost, power
consumption, scalability
Bergman, Rumley,
OECC 2016
Ming Wu - 5
Hybrid Data Center Networks
Example (1): REACToR (UCSD)
• Implementation:
– Using Nistica MEMS WSS
– 30 us reconfiguration time
• Performance
– 10G EPS + 100G OCS ≈100G EPS
Liu, et al. USENIX NSDI 2014
H. Liu, F. Lu, A. Forencich, R. Kapoor, M. Tewari, G. M. Voelker,
G. Papen, A. C. Snoeren, and G. Porter, “Circuit Switching
Under the Radar with REACToR,” USENIX Conference on
Networked Systems Design and Implementation, 2014
• Optical circuit network
– High bandwidth,
– Bufferless TDMA network
– Tx when circuit connects
• Electrical packet switch networks:
– Low bandwidth, small packets
– Buffered all the way
– Tx all the time
Ming Wu - 6
Hybrid Data Center Networks
Example (2): Elastic WDM Switches (UCSB)
• Adding an layer of optical switches
between spine and leaf greatly expand
the scale of network (number of servers)
• Can be space switch or wavelength
switch
• Wavelength routing also investigated by
many other groups (Columbia, UCD,
Nagoya U, NTT, ..)A.A.M. Saleh, A.S.P. Khope, J.E. Bowers, R.C.
Alferness, “Elastic WDM Switching for Scalable Data
Center and HPC Interconnect Networks”, OECC 2016
Ming Wu - 7
Commercially Available Switches
+ High port count
+ Low loss: < 3 dB
− Slow (10 to 25 ms)
− High cost ( $100’s /port)
Calient: 320x320
− Limited port count
− Higher loss: 5 dB
− Slow (3 ms)
+ Low cost (~ $10’s /port ?)
+ Fully integrated
Polatis: 384x384
3D (Free-Space) Switch
NTTElectronics: 16x16 (commercial)
32x32 (publication)
2D (Integrated) Switch
Ming Wu - 8
Scaling of Optical Switches: 2D vs 3D
• 3D switch uses 2N analog
mirrors
– Complex control
• 2D switch uses ~ N2 digital
switching elements
– but simple control
– Monolithic integration
• Si photonics can be a game
changer
– High integration density
– Tight bending radius
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1 10 100 1,000 10,000
No
. S
wit
ch
ing
Ele
me
nts
Port Count
3D Switch
No. = 2N
2D Switch
No. = N2
Largest (commercially available) Switch:
• 3D MEMS: 320x320 (< 3dB)
• 2D PLC: 32x32 (6.6 dB)
NTT
32x32
Calient
320x320
Si Photonics
Switches
Ming Wu - 9
32x32 Non-Blocking PILOSS Switch (AIST, Japan)
• 32x32 switch on 11x25 mm2
• Path-independent insertion loss
(PILOSS) architecture
• Multi-stage topology:
– NxN switch has N stages
• On-chip loss: 15.8 dB
8x8 PILOSS Switch
32x32 PILOSS Switch
K. Tanizawa, et al, “Ultra-compact 32 × 32 strictly-non-blocking Si-wire optical
switch with fan-out LGA interposer,” Optics Express, 2015.
Ming Wu - 10
A Different Approach for High Radix Switch
Traditional Approach: Active Crossbar + EO (or Thermo) Switching
• Many cascaded 2x2 elements
• Lossy in both Bar and Cross states
• High cumulative loss
(~ 0.5∙N dB for NxN)
• Largest switch demonstrated: 32x32
with 16dB on-chip loss
(PILOSS Switch from AIST, Japan)
Berkeley Approach: Passive Crossbar + MEMS Switching
• Single stage switching
• Nearly zero loss in BAR state
• No cumulative loss
• Largest switch demonstrated:
64x64 with < 4dB on-chip loss
• Sub-microsecond switching time
Low Loss
Crossing
(< 0.01 dB)
MEMS
Actuator
(60 dB ER)
Ming Wu - 11
MEMS-Actuated
Vertical Adiabatic Coupler Switch
• Nearly zero loss at OFF state
• Extremely high ON/OFF ratio (60 dB)
• Broadband operation: > 300nm
– Covers S, C, L bandsCalculated Transfer Curve
Ming Wu - 12
Scalability of 2D Si Photonic Switches
NxN Switch Number of Switches Number of Crossing
PILOSS N N-1
Switch-Select 2 log2N (N-1)2
Passive Matrix 1 2N-1
Assumption:
0.25 dB/switch
0.005 dB/crossing
0
10
20
30
40
50
0 20 40 60 80 100
On-ChipLoss(dB)
PortCount
Switch-Select
PILOSS
Passive Matrix
• On-chip loss < 1.25 dB possible for
100x100 Si Photonic 2D MEMS Switch
Ming Wu - 13
MEMS Crossbar Switch: Experimental Implementation
13
64x64
switch
1x1 cm2 die
(a)
In
Drop
Through Vertical Gap
OFF State
(c)
In
Adiabatic Coupler
ON State
(d)
(b)
Drop Ports
Through P
orts
In P
orts
Bus Waveguide
MEMS-Actuated
Adiabatic Coupler
Low-Loss Crossing
Ming Wu - 14
Switch Performance
• Extremely energy efficient
– No dc power consumption, like CMOS
– Low switching energy:
10 pJ per switching
y = – 0.026x – 0.47
Loss per Cell Switching Loss
0.28 µs 0.91 µs
Resonance Frequency : 0.71 MHz
(a) (b) (c)
(d) (e)
< 1us Switching Time
T.J. Seok, N. Quack, S. Han, R.S. Muller, M.C. Wu,
“Large-scale broadband digital silicon photonic
switches with vertical adiabatic couplers,” Optica, 2016
Bell Lab, 8x8
AIST, 8x8
AIST, 32x32
UCB, 50x50UCB, 50x50
1000x
Improvement
UCB64x64
3.7dB
y = – 0.026x – 0.47
Loss per Cell Switching Loss
0.28 µs 0.91 µs
Resonance Frequency : 0.71 MHz
(a) (b) (c)
(d) (e)
Digital Switching On-Chip Loss
Ming Wu - 15
Broadband Operation
• Measured bandwidth > 120 nm (limited by range of tunable laser)
• Simulated bandwidth > 300 nm
Ming Wu - 16
Switch Packaging
Si Photonic
MEMS Switch
(Flip-chip bonded)
AlN Interposer
Silicon Photonic MEMS Switch
PCB Test Board
Interposer
64 Channel Fiber Array
Wire bond
Packaging performed at Tyndall Institute
Ming Wu - 17
System-Level Testing of Packaged
Si Photonic MEMS Switch
Analog 10 Gb/s data out of Switch
10 Gb/s data out of SFP+ Data ModuleTrigger
Time Scale
200 ns/div
Switch Time
10 Gb/s data
10 Gb/s data
Test performed at UCSD (George Papen Group)
Ming Wu - 18
Summary of Si Photonic Switches
18
• Highly scalable matrix switch
• 64x64 switch integrated on 1 cm2 die
• Largest integrated switch in any technology
• Lowest on-chip loss (3.7 dB, or 0.05 dB/port)
• Sub-microsecond switching time
• Scalable to 100’s of ports
• T. J. Seok, N. Quack, S. Han, R. S. Muller, and M. C. Wu, Optica, p. 64, Jan. 2016.
• S. Han, T. J. Seok, N. Quack, B.-W. Yoo, and M. C. Wu, Optica, p. 370, Apr. 2015.
Bronze Medal, 2015 Collegiate
Inventors Competition
64x64
switch
1x1 cm2 die
Bench-Top Probing
Tyndall Packaging
Ming Wu - 19
Acknowledgment
• Berkeley Switch Team
– Dr. Tae Joon Seok, Sangyoon Han, Dr. Niels Quack
– Prof. Richard Muller
• Tyndall Institute at Ireland
– Drs. Peter O’brien, H.Y. Hwang, J.S. Lee, L. Carroll
• UCSD
– Prof. George Papen
• Funding support
– National Science Foundation (NSF) Center for Integrated Access Network
(CIAN) #EEC-0812072
– Defense Advanced Research Project Agency (DARPA) Electronic-Photonic
Heterogeneous Integration (E-PHI) program
Ming Wu - 21
Current and Emerging
Optical Space Switches
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1 10 100 1,000 10,000
SwitchingTim
e(second)
Radix(PortCount)
0
10
20
30
1 10 100 1,000 10,000
Loss(PowerBudget)(dB)
Radix(PortCount)
0
100
200
300
400
1 10 100 1,000 10,000CostPerPort($)
Radix(PortCount)
3D MEMS
UCB
SiPh MEMS
64x64
switch
1x1 cm2 die Silicon
Photonics
3D MEMS
320x320 3D MEMS
(Calient)
64x64 SiPh Switch
(UC Berkeley)
Electro-Optic
SiPh
SJTU
Calient
ms
us
ns
s
AIST, Huawei
Thermo-Optic
SiPh
Ming Wu - 22
Wavelength-Domain Switches/Routers
A. Biberman, B. G. Lee, N. Sherwood-Droz, M.
Lipson, and K. Bergman, IEEE Photonics
Technology Letters. 2010.
A. S. P. Khope, A. A. M. Saleh, J. E. Bowers, and
R. C. Alferness, 2016 IEEE Optical Interconnects
Conference (OI)
R. Yu, S. Cheung, Y. Li, K. Okamoto, R. Proietti, Y.
Yin, and S. J. B. Yoo, Optics Express, 2013.
• Array waveguide grating router
(AWGR) with tunable lasers
• Microrings with fixed or tunable
lasers
• (subjects of many papers in this
conference)