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 - 20

Additional Slides

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)