High-Speed Board-Level Polymer Optical Sub-Systems

High-Speed Board-Level Polymer Optical SubSystems I. H. White, N. Bamiedakis, J. Chen, and R. V. Penty Department of Engineering, University of Cambridge, UK

Motivation

3

Motivation - exponential growth in amount of information exchanged globally - data servers, storage systems, supercomputers Increasing size for a “large” data centre: - 1999: - 2004: - 2009: - 2014:

~ 5 000 ft2 ~ 50 000 ft2 ~ 500 000 ft2 ~ 5 000 000 ft2

 Optical interconnects within high-performance electronic systems 4

Why use Optics in Data Server Units

from OFC 2011 IBM, A. Taunblatt 5

Opto-electronic PCBs use:  optics for high-speed links  electronics for low-speed/control signals and power increase:  interconnect density ( x18 at 10 Gb/s)  reduce board area ( ~60 %) R.C.A Pitw on et al. OI w orkshop, 2014

Intensive research in industry-academia  various: optical material and fabrication methods/ OE board design /OE packaging and assembly

IBM Dellmann L. et al, ECTC, 1288-1293, 2007 Nakagaw a S. et al. ECTC, 256-260, 2008

Xyratex Papakonstantinou I. et al, ECTC, 1769-1775,2008

Mitsui Teck Guan Lim et al, IEEE TAP, pp. 509-516, 2009.

6

Translation for optical interconnects Therefore requirements for optical interconnects:

from J. Kash, Photonics Society Ann. Meeting 2010

 imposes big challenge for next generation short-reach optical links -- cost & power : < $1 Gb/s , < 25 pJ/bit

7

Board-Level Optical Interconnects Various approaches proposed:  free space interconnects  fibres embedded in substrates  waveguide-based technologies

our work

Basic waveguide & component studies

Interconnection architectures

Jarczynski J. et al., Appl. Opt, 2006

Tyco FlexPlane

Board-level OE integration

PCB-integrated Optical units

8

Siloxane Polymer Materials Siloxane materials engineered to exhibit suitable mechanical, thermal and optical properties: • are flexible • exhibit high processability coating, adhesion to substrates, dicing • exhibit high thermal and environmental stability withstands ~ 350 °C (solder reflow) • low intrinsic loss at datacommunications wavelengths: 0.03-0.05 dB/cm @ 850 nm • low birefringence • offer refractive index tunability  suitable for integration on PCBs  offer high manufacturability  are cost effective 9

9

Multimode Waveguides Cost-efficiency targets  relaxed alignment tolerances multimode waveguides - typical cross section used: 50×50 µm 2  1 dB alignment tolerances > ± 10 µm  assembly possible with pick-and-place machines - pitch of 250 µm to match ribbon fibre and VCSEL/PD arrays - facets exposed with dicing saw (low-cost process)

top cladding n ~ 1.5 core n ~ 1.52

20um

50um

bottom cladding n ~ 1.5 substrate

propagation losses: 0.04-0.06 dB/cm @ 850 nm crosstalk (up to 150 µm spacing) < -25 dB  large number of parallel on-board waveguides

10

10

40 Gb/s NRZ data transmission - 1 m long spiral waveguide

A

Pattern generator

Voltage source

Bias Tee

16x

850 nm VCSEL

B

Cleaved 50 μm MMF

50 μm MMF patchcord

40 GHz RF amplifier Oscilloscope

16x

1 m spiral waveguide

MM VOA

30 GHz PD 50 µm 32 µm

 record error-free (BER 40 GHz×m •

Two 1 m long spiral samples tested with different refractive index profiles • “graded”-index (GI) • “step”-index (SI)  different profiles generated by adjusting fabrication parameters  potential for dispersion engineering SI GI

80

input pulse Autocorrelation Trace Amplitude

Autocorrelation Trace Amplitude

Data Gauss fit Sech fit Lore fit

1 0.5

R2 Gaus = 0.999 R2 Sech = 1.000 R2 Loren = 0.988

∆tin

0 -0.5 0

1.2

Data FWHM = 0.25 ps Gaus FWHM = 0.18 ps Sech FWHM = 0.16 ps Loren FWHM = 0.12 ps

1.5

0.25

0.5

0.75 Time (ps)

1

output pulse

Sp2 SI WG#3 In:x10, Out: x16- x= +0.0 m

B2B - x= +0.0 m

2

1 0.8 0.6 0.4 0.2

∆tout

0 -0.2 0

1.25

10

20

30 40 Time (ps)

50

60

Data FWHM = 19.99 ps Gaus FWHM = 14.00 ps Sech FWHM = 12.68 ps Loren FWHM = 10.31 Data ps Gauss fit Sech fit Lore fit R2 Gaus = 0.998 R2 Sech = 0.995 R2 Loren = 0.982

Bandwidth (GHz)

60 50 40

no mode mixer

20 -25 -20 -15 -10 -5 0 5 Offset (m)

10 15 20 25

70

SI

60

32 µm

50 40 30

with mode mixer

20 -25 -20 -15 -10 -5 0 5 Offset (m)

10 15 20 25

35 µm

70

SI GI

80

35 µm

Bandwidth (GHz)

time domain measurements

90

90

30

output

input

GI 32 µm

estimated bandwidth: SI: 30 – 60 GHz GI: 50 – 90 GHz

 potential to achieve 100 Gb/s over a single multimode polymer waveguide J. Chen, et al., in ECOC, paper Mo.3.2.3, pp. 1-3, 2015 J. Chen et al., IEEE JLT, pre-publication available online, 2016

12

Optical board Optical board

Multimode Waveguide Components

OEM

S-bend S-bend OEM

OEM

crossing 90° 90° crossing

90° bend

OEM

90° bend

Use of passive multimode waveguide components:  on-board routing flexibility & advanced topologies

However, limited power budget (e.g. 10 GbE has 8 dB power budget) for high-speed on-board links  low-loss components required

Components designed and fabricated: - Waveguide crossings - Bent waveguides: 90o bends and S-bends) - Y-splitters/combiners - Waveguide couplers - Waveguide Tapers

90° crossings

S-bends

90° bends

Performance characterisation under varying launch conditions and input offsets  restricted launches (SMF, lens) and  partially overfilled launches (MMF)

Y-splitters

FR4 board 13

13

Optical coupling schemes Optical coupling achieved either by: - out-of-plane coupling using beam-turning elements + simplifies assembly and electrical connection of active devices - requires additional fabrication steps  typically, 45° mirrors in optical layer & micro-lenses - end-fired coupling + eliminates the need for additional optical structures - requires embedding the OE devices in the board and efficiently routing the electrical signal from the board surface to the devices

14

14

Optical Coupling Examples End fire

IBM

integrated 45 o mirror optical rod

Neyer A. et al, ECTC, 2005 Takagi Y. et al, IEEE JLT, vol 28 (20), 2010

Dellmann L. et al, ECTC, 1288-1293, 2007

fibre-based 90o connections

S. H. Hwang et al, IEEE PTL, vol 19 (6), 2007

microlens assisted coupling

Ishii Y. et al, IEEE TAP, vol. 26 (2), 2002

15

Cambridge Approach to Optical Coupling Using simple tools, low-cost materials and minimal technical know-how . . .

FR4 substrate

PCB prototyping machine, LPKF Protomat C60

Clad Core layer layer

Mask aligner, EVG 620

VCSEL

Solder reflow machine, PACE Thermoflo 2000

TIA Photodiode

SMA

SMA

FR4 Via

Tx

Rx

Via Solder Copper mask 16

PCB-integrated 10 Gb/s optical units Proof-of-principle demonstrators

10 Gb/s

 integrating optics and high-speed electronics - 10 Gb/s optical transceiver built on low-cost FR4

transmit

- 10 Gb/s chip-to-chip on-board communication link

10 Gb/s receive

Data SMA outputs

Data SMA inputs power input

Tx module LD

OE PCB FR4

Rx module PD

Y-splitter embedded in optical layer

PD

LD

OE PCB waveguide facet

polymer layers waveguide facet

N. Bamiedakis et al., IEEE TCPMT , vol. 3, pp. 592-600, 2013 A .Hahim et al., IET Optoelectronics, vol. 6, pp. 140-146, 2012

17

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On-board interconnection architectures • Blade servers are a popular method of increasing packing density in IT environments. • Network connectivity is currently provided by an electrical backplane capable of providing several Gb/s total throughput.

• Blade servers typically have 14 blades and another 2 external network connections, making a total of 16 backplane connections. • There is a need for a low cost backplane which will enable one blade to talk to any other in the chassis at >1 Gb/s.

Optical Backplanes: Widespread Industry Interest

Numerous demonstrations of simple point-to-point on-board polymer links

Intel optical chip-to-chip link Mohammed et al, Intel Tech. J. 8 (2004)

Asperation Perlos Co/Vtt Electronics Immonen et al, IEEE Trans. Elect. Pack. Manuf. 28 (2005)

Fujitsu Labs optical backplane Glebov et al, Opt. Eng. 46 (2007)

Fraunhofer/Siemens et al Schroder et al, Opt Int. Circ. VIII, Proc.SPIE 6124 (2006)

IBM Terabus Optocard Schares et al, IEEE J. Sel. Top. Q. Elect. 12 (2007)

Advanced on-board interconnection architectures Backplanes are next level of integration of optics into highperformance electronic systems, e.g. blade servers  cost-effective systems with reduced power consumption

Ways to passively optically interconnect different electrical cards/modules Shuffle router  one dedicated waveguide

Optical bus  one common communication channel

for each on-board link on-board waveguide links

Tx1

Rx1

Tx2

Rx2

Tx3

Rx3

Tx4

Rx4 optical backplane

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Shuffle router design details Backplane design

 non-blocking architecture

- exploits all four substrate edges

 scalable waveguide design

- uses low loss waveguide components crossings and 90°bends:

90°

 simultaneous fabrication of all waveguides in single plane  crossing loss ~0.01 dB/crossing with MMF  bend loss ~ 1 dB for RoC > 8 mm

ribbon fibre connection to Rx ribbon fibre connection from Tx

- opposite edges populated with like-connection types (Tx or Rx) & spatial offset  minimises crosstalk reaching I/O connections  requires only one 90° bend per waveguide

- scalable with increasing card number  max # crossings per wg link = N2-N

21

21

10-Card Optical Backplane 4

Rx

x 10

Card interfaces (10 waveguides each) Rx Rx Rx Rx

10’’ FR4 :

10

Tx

100 90°-bends

Tx 8

~ 1800 90°-crossings

Tx Tx

6

Tx

2.25 U (10 cm)

Tx Tx

4

Tx Tx

2

Tx 0 -2

0

Rx

2

Rx

4

Rx

6

Rx

8

Rx

10

Schematic of 10-card backplane layout

• 100 waveguides • single 90° bend per waveguide • 90 crossings or less per waveguide

4

x 10

Input Type

Insertion Loss

Worst-case Crosstalk

50 μm MMF

2 to 8 dB

< -35 dB

SMF

1 to 4 dB

< -45 dB

Terabit capacity enabled by 100 waveguides, each @ 10 Gb/s in multicast mode J. Beals, et al., Applied Physics A, vol. 95, pp. 983-988, 2009,

22

22

Regenerative optical bus architecture Proposed optical bus architecture: - polymeric optical bus modules and multiple optical channel - two optical transmission directions to allow full card connectivity - signal “drop” and signal “add” functions at each card interface M optic

al bus segme n

1

2

3R

ts

optica l signa l directi on

1 2

3R

N

N card

s

M

polym eric w bus str aveguide ucture s

3R

N+1

N+2 2×N

regene

rator u nits M×N

- number of cards per segment limited by available optical power budget - 3R regenerator units to allow bus extension with multiple segments  arbitrary number of cards can be connected onto the bus  implementation costs that linearly scale up with the number of cards 23

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Optical Bus Architecture – Waveguide Design

Schematic of a single bus segment: - two optical transmission directions - signal “drop” at each Rx port and signal “add” at each Tx port

Tx Rx

Rx

Rx

signal “drop”

1 2 N

3R

Tx Rx

Tx Rx

signal “add”

transmission direction

Rx

Tx Rx

Tx Rx

1 2 Rx Tx N

Tx 3R Card 1 Regenerator

3R

Rx

Tx Card 2

Tx Card M

Tx

next bus segment

next bus segment

N optical channels

3R Regenerator

bus repeating unit

24

3R

3R Rx Card

Proof-of-principle bus module

Tx

Tx Rx Card Rx Tx

Tx Rx Card Rx Tx

Rx Tx Card Rx Tx

Tx Rx Card Rx Tx

3R

Tx Rx Card Rx Tx

Tx Rx Card Rx Tx

3R Repeating unit

Power budget studies: using realistic component losses and a 15 dB power budget for 10 Gb/s links  3 cards possible before regeneration required Design of a proof-of-principle 4-channel 3-card bus module  compatible with 1x4 VCSEL/PD arrays and transceivers and 3R chips  size: 90x50 mm2, fits 4’’ wafer Rx1 Rx2 Rx3

abcd

efgh

ijkl

50 µm

50 mm 50 mm

3Rout

90 mm

bus repeating unit

Tx1

Tx2

Tx3

3Rin

1' 2' 3' 4'

5 6 7 8 9 10 11 12 13 14 15 16

50 µm WGs

50 µm WGs 50 µm

50 µm 1 mm

1 2 3 4

50 µm WGs 40 µm

25 µm

100 µm

60 µm

3.35 mm 50 µm, w=0 µm

100 µm

60 µm

3.35 mm

50 µm, w=10 µm

50 µm, w=40 µm

main bus geometry

Optical layer: Y-splitters/combiners, 90° bends, 90°crossings, raised-cosine S-bends, tapers

N. Bamiedakis et al., in Opt. Expr., vol. 20, iss. 11, pp. 11625-11636, 2012

25

25

4-channel 3-card Bus Module

Sample polymeric bus modules fabricated on low-cost FR4 substrates from siloxane materials using standard photolithography  size: 90 x 50 mm2 - facets exposed with dicing saw (no polishing steps) signal drop optical signal

I III

I

II

Fabrication Details

Rx1 Rx2 Rx3

Rx1 Rx2 Rx3

50 mm

b

50 mm

1' 2' 3' 4' 90 mm 5678 910111213141516

Tx1 Tx2 Tx3

Rx3R

Tx3R

abcdefghijkl

1 2 3 4

III

II

signal add

ef

ij

bus outputs

Rx3R

Tx3R

1' 2'

2 bus inputs

optical bus module 5 Tx1

26

Tx1- Rx4 ch2

Rx4

Rx3 3R signal regeneration

Rx1

Tx1- Rx6 ch2

OBUS2_S6

OBUS1_S5

Tx1

Tx3

Rx6

50 mm

Data Transmission

3R Regenerator

Tx4

Tx6

10 Gb/s data transmission experiments for all channels through 3R regeneration e.g. error-free (BER1 mW) can be formed in array configurations  high bandwidth per pixel  larger total output power

µLED array

J.J.D. McKendry et al., JLT, vol. 30, pp. 61-67, 2012.

30 µm wide WGs 62.5 µm

- µLED-based PWG links: - small area ( 1 Tb/s/mm2 using ultra low-cost optical components N. Bamiedakis et al., in ICTON, pp. 1-4, 2015 N. Bamiedakis et al., to be presented in ICTON, 2016

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Other Routes to Higher Bandwidth: Wavelength Division Multiplexing RX

TX

Single Wavelength

TX

1

1

TX

2

2

n

n

TX

RX RX

RX

Multiple Wavelengths

c.f. Australian Photonics Animation

32

Integrated De/multiplexers for Guided-wave WDM Links

Grating collimated field

diffracted field λ1

Parabolic mirror

Component 1 : Slab waveguide – one dimensional confinement (vertical only)

Waveguide connection with slab

λ2 λ 0

Triangular elements for transmission diffraction grating

expanded field

input field

1

input waveguides

output waveguides

Component 2 : Deep etched waveguides two dimensional confinement

Component 3 : Reflecting surfaces

Parabolic collimating mirror

top view Demultiplexer design

33

Integrated De/multiplexers for Guided-wave WDM Links

Grating collimated field

diffracted field λ1

Waveguide connection with slab

λ2 λ 0

Parabolic mirror

Component 1 : Slab waveguide – one dimensional confinement (vertical only) Triangular elements for transmission diffraction grating

expanded field

input field

1

input waveguides

output waveguides

Component 2 : Deep etched waveguides two dimensional confinement

Component 3 : Reflecting surfaces

Parabolic collimating mirror

top view Demultiplexer design

34

Integrated De/multiplexers for Guided-wave WDM Links

Grating collimated field

diffracted field λ1

Parabolic mirror

2

λ2 λ 0

expanded field

input field

1

input waveguides

output waveguides

top view

35

Integrated De/multiplexers for Guided-wave WDM Links

3 Grating collimated field

diffracted field

Component 1 : λ Slab waveguide – one λ0 , λ1 , λ2 dimensional confinement , (vertical only) λ1

Waveguide connection mirror Parabolic 2 with slab

λ2

0

expanded field

input field

1

input waveguides

output waveguides

top view

Component 2 : Deep etched waveguides -

Triangular elements for transmission diffraction grating

λ1 λ0 λ2

Component 3 : Reflecting surfaces 36

Integrated De/multiplexers for Guided-wave WDM Links

3 Grating collimated field

diffracted field λ1

Parabolic mirror

2

λ2 λ 0

4

expanded field

input field

1

5

input waveguides

output waveguides

top view

37

Integrated De/multiplexers for Guided-wave WDM Links Predicted Integrated Multiplexer/Demultiplexer Performance - Gaussian mode power distribution at input  restricted launch condition

- uniform mode power distribution at input  worst-case scenario

0=0.45 m, =10 nm, r0=5.0, w0=8.0m,Sep =125 m,TM Exp (0.25,0.25)

0

0=0.45 m, =10 nm, r0=5.0, w0=8.0m,Sep =125 m,TM Uni Power

0 -5

-10 -15 -20

WG- 1 WG- 2 WG- 3 WG- 4

-35 dB

-25 -30 -35 -40 420

Spectral Response (dB)

Spectral Response (dB)

-5

-10

-10 dB

-15

WG- 1 WG- 2 WG- 3 WG- 4

-20 -25 -30 -35

430

440 450 460 Wavelength (m)

470

480

-40 420

430

440 450 460 Wavelength (m)

470

480

 on-going fabrication work N. Bamiedakis et al., to be presented in ICTON, 2016

38

Conclusions Multimode polymer waveguides:  a cost-effective optical technology for board-level optical interconnects  low loss, low-crosstalk on-board optical links  direct integration onto PCBs, low-cost assembly  various interconnection architectures for passive backplanes  potential to achieve even higher data rates > 100 Gb/s !

Siloxane waveguides

Basic waveguide components

Interconnection architectures

Board-level OE integration

PCB-integrated optical units

39

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References [1] J. Chen, “Polymer Waveguide Based Optical Interconnects For High-speed On-board Communications ”, Ph.D. Thesis, University of Cambridge, June 2016. [2] J. Chen, N. Bamiedakis, P. Vasil’ev, R. V Penty, and I. H. White, “Low-Loss and High-Bandwidth Multimode Polymer Waveguide Components Using Refractive Index Engineering,” in Conference on Lasers and Electro-Optics (CLEO), p. SM2G.7, San Jose, USA, June 2016. [3] J. Chen, N. Bamiedakis, P. Vasil’ev, R. V Penty, and I. H. White, “Bandwidth Enhancement in Multimode Polymer Waveguides Using Waveguide Layout for Optical Printed Circuit Boards,” in Optical Fiber Communication Conference and Exposition (OFC), p. W1E.3, Anaheim, USA, March 2016. [4] N. Bamiedakis, J. Chen, R. V. Penty, and I. H. White, “High-Bandwidth and Low-Loss Multimode Polymer Waveguides and Waveguide Components for High-Speed Board-Level Optical Interconnects,” in Photonics West conference, Proceeding of SPIE, vol. 9753, pp. 975304–1–9, San Francisco, USA, February 2016. [Invited paper] [5] J. Chen, N. Bamiedakis, P. Vasil’ev, T. Edwards, C. Brown, R. Penty, and I. White, “High-Bandwidth and Large Coupling Tolerance Graded-Index Multimode Polymer Waveguides for Onboard High-Speed Optical Interconnects,” Journal of Lightwave Technology, vol. 34, no. 12, pp. 2934–2940, November 2015. [Invited paper] [6] J. Chen, N. Bamiedakis, P. Vasil’ev, R. V Penty, and I. H. White, “Restricted Launch Polymer Multimode Waveguides for Board-level Optical Interconnects with > 100 GHz × m Bandwidth and Large Alignment Tolerance,” in Asia Communications and Photonics Conference (ACP), p. AM3A. 5, Hong Kong, China, November 2015. [7] J. Chen, N. Bamiedakis, P. Vasil’ev, T. J. Edwards, C. T. A. Brown, R. V. Penty, and I. H. White, “Graded-Index Polymer Multimode Waveguides for 100 Gb/s Board-Level Data Transmission,” in European Conference on Optical Communication (ECOC), Valencia, Spain, September 2015. [8] N. Bamiedakis, J. Wei, J. Chen, P. Westbergh, A. Larsson, R. Penty, and I. White, “56 Gb/s PAM-4 Data Transmission Over a 1 m Long Multimode Polymer Interconnect,” in Conference on Lasers and Electro-optics (CLEO), p. STu4F.5, San Jose, USA, May 2015. [9] J. Chen, N. Bamiedakis, T. J. Edwards, C. T. A. Brown, R. V Penty, and I. H. White, “Dispersion Studies on Multimode Polymer Spiral Waveguides for Board-Level Optical Interconnects,” in Optical Interconnects Conference (OIC), p. MD2, San Diego, USA, April 2015. [10] R. V. Penty, N. Bamiedakis, J. Chen, and I. H. White, “Bandwidth Studies on Multimode Polymer Waveguides for High-Speed Board-Level Optical Interconnects,” in Photonics West conference, Proceeding of SPIE, pp. 9368–2, San Francisco, USA, February 2015. [Invited paper] [11] N. Bamiedakis, J. Chen, P. Westbergh, J. S. Gustavsson, A. Larsson, R. V. Penty, and I. H. White, “40 Gb/s Data Transmission Over a 1 m Long Multimode Polymer Spiral Waveguide for Board-Level Optical Interconnects,” Journal of Lightwave Technology, vol. 33, no. 4, pp. 882–888, November 2014. [12] N. Bamiedakis, J. Chen, R. V. Penty, I. H. White, P. Westbergh, and A. Larsson, “40 Gb/s Data Transmission over a 1 m Long Multimode Polymer Spiral Waveguide,” in European Conference on Optical Communication (ECOC), p. P.4.7, Cannes, France, September 2014. [13] N. Bamiedakis, J. Chen, R. V Penty, and I. H. White, “Bandwidth Studies on Multimode Polymer Waveguides for ≥25 Gb/s Optical Interconnects,” IEEE Photonics Technology Letters, vol. 26, no. 20, pp. 2004–2007, July 2014. [14] J. Chen, N. Bamiedakis, R. V. Penty, I. H. White, P. Westbergh, and A. Larsson, “Bandwidth and Offset Launch Investigations on a 1.4 m Multimode Polymer Spiral Waveguide,” in European Conference on Integrated Optics (ECIO), p. P027, Nice, France, June 2014. [15] J. Chen, N. Bamiedakis, R. V. Penty, I. H. White, P. Westbergh, and A. Larsson, “Bandwidth Studies on a 1.4 m Long Multimode Polymer Spiral Waveguide,” in Semiconductor and Integrated OptoElectronics Conference (SIOE), Cardiff, UK, April 2014.