High-Speed Board-Level Polymer Optical Sub-

Systems

I. H. White, N. Bamiedakis, J. Chen, and R. V. Penty

Department of Engineering, University of Cambridge, UK

Motivation

3

Motivation

Optical interconnects within high-performance electronic systems

- exponential growth in amount of information exchanged globally

- data servers, storage systems, supercomputers

Increasing size for a “large” data centre:

- 1999: ~ 5 000 ft2

- 2004: ~ 50 000 ft2

- 2009: ~ 500 000 ft2

- 2014: ~ 5 000 000 ft2

4

5

Why use Optics in Data Server Units

from OFC 2011 IBM, A. Taunblatt

6

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 %)

Dellmann L. et al, ECTC, 1288-1293, 2007 Nakagaw a S. et al. ECTC, 256-260, 2008 Papakonstantinou I. et al, ECTC, 1769-1775,2008 Teck Guan Lim et al, IEEE TAP, pp. 509-516, 2009.

IBM Xyratex Mitsui

Intensive research in industry-academia various:

optical material and fabrication methods/ OE board design /OE packaging and assembly

R.C.A Pitw on et al. OI w orkshop, 2014

7

Translation for optical interconnects

from J. Kash, Photonics Society

Ann. Meeting 2010Therefore requirements for optical interconnects:

imposes big challenge for next generation short-reach optical links

-- cost & power : < $1 Gb/s , < 25 pJ/bit

8

Board-Level Optical Interconnects

Various approaches proposed:

free space interconnects

fibres embedded in substrates

waveguide-based technologies

Tyco FlexPlane

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

our work

Interconnection

architectures

Board-level OE

integration

PCB-integrated

Optical units

Basic waveguide &

component studies

9999

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

Siloxane Polymer Materials

suitable for integration on PCBs

offer high manufacturability

are cost effective

10101010

Multimode Waveguides

substrate

bottom cladding n ~ 1.5

top cladding n ~ 1.5

core

n ~ 1.52

20um

50um

Cost-efficiency targets relaxed alignment tolerances

multimode waveguides

- typical cross section used: 50×50 µm2

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)

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

40 Gb/s NRZ data transmission

11

record error-free (BER<10-12) 40 Gb/s data transmission

1 m spiral

waveguide

Cleaved

50 μm MMF

50 μm MMF

patchcord

16x16x

850 nm VCSEL 30 GHz PD

Pattern

generator

Oscillo-

scope

40 GHz RF

amplifier

MM VOA

A B

Voltage

sourceBias

Tee

N. Bamiedakis et al., IEEE PTL, vol. 26, pp. 2004-2007, 2014

N. Bamiedakis, et al., IEEE JLT, vol. 33, pp. 1-7, 2015

- 1 m long spiral waveguide

32 µm

50

µm

12

Bandwidth studies

Demonstration of

waveguide bandwidth-length product of at > 40 GHz×m

0 0.25 0.5 0.75 1 1.25-0.5

0

0.5

1

1.5

2

Time (ps)

Auto

corr

ela

tion T

race A

mplit

ude

B2B - x= +0.0 m

Data

Gauss fit

Sech fit

Lore fit

Data FWHM = 0.25 psGaus FWHM = 0.18 psSech FWHM = 0.16 psLoren FWHM = 0.12 ps

R2 Gaus = 0.999

R2 Sech = 1.000

R2 Loren =

0.988

0 10 20 30 40 50 60-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Time (ps)

Auto

corr

ela

tion T

race A

mplit

ude

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

Data

Gauss fit

Sech fit

Lore fit

Data FWHM = 19.99psGaus FWHM = 14.00psSech FWHM = 12.68psLoren FWHM = 10.31ps

R2 Gaus =

0.998

R2 Sech =

0.995

R2 Loren =

0.982

input pulse output pulse

∆tin ∆tout

time domain measurements

32 µm

35

µm

32 µm

35

µm

-25 -20 -15 -10 -5 0 5 10 15 20 2520

30

40

50

60

70

80

90

Offset (m)

Bandw

idth

(G

Hz)

SI

GI

-25 -20 -15 -10 -5 0 5 10 15 20 2520

30

40

50

60

70

80

90

Offset (m)

Bandw

idth

(G

Hz)

SI

GI

estimated bandwidth:

SI: 30 – 60 GHz

GI: 50 – 90 GHz

SI GI

• 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

no mode mixerwith mode mixer

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

input output

1313

Multimode Waveguide Components

S-bends 90° bends Y-splitters90° crossings FR4 board

Use of passive multimode waveguide components:

on-board routing flexibility & advanced topologies

Components designed and fabricated:

- Waveguide crossings

- Bent waveguides: 90o bends and S-bends)

- Y-splitters/combiners

- Waveguide couplers

- Waveguide Tapers

Optical board

OEM

OEM OEM

OEM90° bend

S-bend

90° crossing

Optical boardOEM

OEM OEM

OEM

90° bend

S-bend

90° crossing

Performance characterisation under

varying launch conditions and input

offsets

restricted launches (SMF, lens) and

partially overfilled launches (MMF)

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

low-loss components required

141414

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

Optical coupling schemes

15

Optical Coupling Examples

optical rod

Takagi Y. et al, IEEE JLT, vol 28 (20), 2010

integrated 45o mirror

fibre-based 90o connections

Neyer A. et al, ECTC, 2005

microlens assisted coupling

Ishii Y. et al, IEEE TAP, vol. 26 (2), 2002S. H. Hwang et al, IEEE PTL, vol 19 (6), 2007

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

IBM

End fire

Clad

layerCore

layer

FR4

Solder

maskCopper

Cambridge Approach to Optical Coupling

SMASMA

TxVia

VCSEL

RxVia

PhotodiodeTIA

Using simple tools, low-cost materials and minimal technical know-how . . .

Solder reflow machine,

PACE Thermoflo 2000

FR4 substrate Mask aligner, EVG 620PCB prototyping machine,

LPKF Protomat C60

16

1717

PCB-integrated 10 Gb/s optical units

power input

Data SMA inputs

Rx moduleTx module

OE PCB

FR4

polymer layers

Y-splitter

embedded

in optical layer

Data SMA

outputs

waveguide facet

PDLD

waveguide

facet

OE PCB

Proof-of-principle demonstrators

integrating optics and high-speed electronics

- 10 Gb/s optical transceiver built on low-cost FR4

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

10 Gb/s

transmit

receive

PD LD

N. Bamiedakis et al., IEEE TCPMT , vol. 3, pp. 592-600, 2013

A .Hahim et al., IET Optoelectronics, vol. 6, pp. 140-146, 2012

• 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.

On-board interconnection architectures

Optical Backplanes: Widespread Industry Interest

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

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

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

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

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

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

20

Advanced on-board interconnection architectures

Backplanes are next level of integration of optics into high-

performance 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

for each on-board link

Optical bus

one common communication channel

Tx1

Tx2

Tx3

Tx4

Rx1

Rx2

Rx3

Rx4

on-board

waveguide links

optical backplane

21

Shuffle router design details

21

Backplane design

- exploits all four substrate edges

- uses low loss waveguide components 90°

crossings and 90°bends:

simultaneous fabrication of all waveguides in

single plane

crossing loss ~0.01 dB/crossing with MMF

bend loss ~ 1 dB for RoC > 8 mm

- 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

ribbon fibre

connection

to Rx

ribbon fibre

connection

from Tx

non-blocking architecture

scalable waveguide design

222222

10-Card Optical Backplane

Rx Rx Rx Rx Rx

Tx

Tx

Tx

Tx

Tx

Rx Rx Rx Rx Rx

-2 0 2 4 6 8 10

x 104

0

2

4

6

8

10

x 104

Tx

Tx

Tx

Tx

Tx

2.25 U

(10 cm)

Card interfaces (10 waveguides

each)

Schematic of 10-card backplane layout

• 100 waveguides

• single 90° bend per waveguide

• 90 crossings or less per waveguide

Terabit capacity enabled by 100 waveguides, each @ 10 Gb/s in multicast mode

Input TypeInsertion

Loss

Worst-case

Crosstalk

50 μm MMF 2 to 8 dB < -35 dB

SMF 1 to 4 dB < -45 dB

10’’ FR4 :

100 90°-bends

~ 1800 90°-crossings

J. Beals, et al., Applied Physics A, vol. 95, pp. 983-988, 2009,

2323

Regenerative optical bus architecture

23

3R

3R

M optical bus segments

N cards

12

N

N+1N+2

2×N

optical signal direction

3R

1

2

M

M×N

regenerator units

polymeric waveguide bus structures

arbitrary number of cards can be connected onto the bus

implementation costs that linearly scale up with the number of cards

- 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

Proposed optical bus architecture:

- number of cards per segment limited by available optical power budget

- 3R regenerator units to allow bus extension with multiple segments

Optical Bus Architecture – Waveguide Design

12N

Tx

signal

“add”

signal

“drop”

Card 1

Tx Rx

TxRx

3R

Tx Rx

TxRx

3R

Rx

RxTx

TxCard 2

Rx

RxTx

TxCard M

Rx

RxTx

next

bus s

egm

ent n

ext b

us s

egm

ent

3R

Regenerator3R

Regenerator

transmission

direction

N optical

channels

12N

bus repeating

unit

Schematic of a single bus segment:

- two optical transmission directions

- signal “drop” at each Rx port and signal “add” at each Tx port

24

252525

Optical layer:

Y-splitters/combiners, 90° bends, 90°crossings,

raised-cosine S-bends, tapers

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

Power budget studies: using realistic component losses and a 15 dB power

budget for 10 Gb/s links 3 cards possible before regeneration required

3R

Card Card Card Card Card

Tx Rx TxTxTx

Card

Tx

TxTx Tx TxRxRx

Rx

Rx Rx

Rx

Rx

Rx Rx

Tx

Card

Tx

Tx Rx

Rx

Card

Tx

Tx Rx

Rx

Card

Tx

Tx Rx

Rx

Repeating unit

3R 3R

3R

main bus geometry

Proof-of-principle bus module

50 m

m

3R

ou

t

Tx1

Rx1 Rx2 Rx3

3R

in

Tx2 Tx3

1

2

3

4

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

a b c d e f g h i j k l

1'

2'

3'

4'

90 mm

50

mm

bus repeating

unit

50 µm WGs 50 µm WGs 50 µm WGs

50 µm

1 mm

25 µm

50 µm,

w=0 µm

50 µm 100 µm

3.35 mm

60 µm

3.35 mm

100 µm

50 µm,w=10 µm

50 µm,

w=40 µm

50 µm

60 µm

40 µm

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

4-channel 3-card Bus Module

I

II

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)

50

mm

Tx

3R

Tx1

Rx1 Rx2 Rx3

Rx

3R

Tx2 Tx3

1

2

3

4

56789101112 13141516

a b c d e f g h i j k l

1'

2'

3'

4'90 mm5

0 m

m

Tx3R

Rx3R

Tx1

Rx1 Rx2 Rx3

bus inputs

2 2'

optical bus module

5

1'

b e f i jbus

outputs

III

Fabrication Details

optical

signal

signal

drop

signal

add

I III

II

26

27

Data Transmission Tx1- Rx4 ch2

Tx1

Rx1 Rx3

Tx3

OBUS1_S5

50 m

m

3R

sig

na

l re

ge

nera

tio

n

3R Regenerator

3R

sig

na

l re

ge

nera

tio

n3

R s

igna

l re

ge

nera

tio

n

3R Regenerator

Rx4 Rx6

OBUS2_S6

Tx4 Tx6

Tx1- Rx6 ch2

10 Gb/s data transmission experiments for all channels through 3R regeneration

e.g. error-free (BER<10-12)10 Gb/s transmission from Tx1 to Rx4 and Rx6 (channel 2)

2828

Insertion loss characterisation

Insertion losses of all 40 optical paths measured with

butt-coupled 50 µm MMF and a 1×4 VCSEL array

target: loss < 15 dB

- ALL within the 15 dB target

7.0

9.0

10.910.69.7

7.0

8.9

10.4

7.8

10.7

12.1

8.2

10.39.8

8.9

12.1

8.57.6

9.79.0

7.5

10.2

8.1

9.78.8

10.6

7.68.5

7.6

8.8

7.0 7.3

5.4

6.9

8.1

6.1

8.1

6.1

9.7

8.6 8.7

6.5

9.5

8.59.2

7.2

9.6

10.8

7.5

6.2

9.2

7.26.7

8.7

7 6.9

8.37.9

7

9.1

6.57.4

6.1

7.1

5.7

76.4

7.4

6.1

4.8

3.8

5.2

10.0

8.2

9.6

10.610.3

7.6

9.5

10.4

0

2

4

6

8

10

12

14

16

1a

1e

1i

11'

2b

2f

2j

22'

3c

3g

3k

33'

4d

4h

4l

44'

5e

5i

51'

6f

6j

62'

7g

7k

73'

8h

8l

84'

9i

91'

10j

102'

11k

113'

12l

124'

131'

142'

153'

164'

Path

Pa

th L

oss (

dB

)

1x4 VCSEL input

50 µm MMF input

15 dB power budget

3R

ou

t

Tx1

Rx1 Rx2 Rx3

3R

in

Tx2 Tx3

1

2

3

4

5678 9101112 13141516

a b c d e f g h i j k l

1'

2'

3'

4'

90 mm50

mm

N. Bamiedakis et. al, Opt. Expr. vol. 20, 2012

N. Bamiedakis et. al, JLT, vol. 32, pp. 1526-1537, 2014

29

Data Transmission

Tx1

Rx1 Rx3

Tx3

OBUS1_S5

50 m

m

3R

sig

nal re

genera

tion

3R Regenerator

3R

sig

nal re

genera

tion

3R

sig

nal re

genera

tion

3R Regenerator

Rx4 Rx6

OBUS2_S6

Tx4 Tx6

All possible data transmission paths for all

channels when:

- only specific channel ON

- all channels ON (DC biased - not data)

OUT

IN

Rx3 Rx4 Rx6

ONLY ALL ON ONLY ALL ON ONLY ALL ON

Tx1

Ch. 1 EF EF EF EF EF EF

Ch. 2 EF EF EF EF EF EF

Ch. 3 EF EF EF EF EF EF

Ch. 4 EF EF EF EF EF EF

Tx3

Ch. 1 EF EF EF EF

Ch. 2 EF EF EF EF

Ch. 3 EF EF EF EF

Ch. 4 EF EF EF EF

Tx4

Ch. 1 EF EF

Ch. 2 EF EF

Ch. 3 EF EF

Ch. 4 EF EF

Error-free (EF: BER<10-12)

transmission achieved for all

on-board links

ALL ON

ONLY

Rx6

ALL ON

ONLY

Rx4

ALL ON

ONLY

Rx3

Ch. 4Ch. 3Ch. 2Ch. 1Ch. 4Ch. 3Ch. 2Ch. 1Ch. 4Ch. 3Ch. 2Ch. 1

Tx4Tx3Tx1INPUT

OUTPUT

ALL ON

ONLY

Rx6

ALL ON

ONLY

Rx4

ALL ON

ONLY

Rx3

Ch. 4Ch. 3Ch. 2Ch. 1Ch. 4Ch. 3Ch. 2Ch. 1Ch. 4Ch. 3Ch. 2Ch. 1

Tx4Tx3Tx1INPUT

OUTPUT

N. Bamiedakis et. al, JLT, vol. 32, pp. 1526-1537 2014

Toward high-density low-cost interconnects

30

- polymer waveguide arrays

- large arrays feasible high aggregate capacity

- suitable connectors under development

- flexible substrates

- bending radius < 5 mm

- twisted waveguides

850 nm

VCSEL

cleaved

input fibre

fibre

patchcord

mode

mixer

broad area

detector

R

16x

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12 14 16

18

0 d

eg

exce

ss lo

ss (d

B)

Radius (mm)

SMF

50 µm MMF

100 µm MMF

30 µm WGs

R. Dangel,et al., JLT, vol. 31, pp. 3915-3926, 2013

twisted

bent

High-density ultra low-cost interconnects

31

aggregate data transmission > 1 Tb/s/mm2 using ultra low-cost optical components

- interface polymer waveguide arrays with micro-pixelated LED arrays

potential to achieve relatively larger aggregate capacity

with ultra-low cost optical interconnects

- µLED-based PWG links:

- small area (<100 µm) matches waveguide size

- 5 Gb/s demonstrated using a single µLED and a multimode WG

- potential for coarse WDM multiplexing : e.g. 4-λ CDWM

µLED array

62.5 µm

30 µm wide WGs

- micro-pixelated LEDs (µLEDs):

small active area (20 -100 µm)

large bandwidth ( > 100 MHz)

relatively large output power ( >1 mW)

can be formed in array configurations

high bandwidth per pixel

larger total output power

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

N. Bamiedakis et al., in ICTON, pp. 1-4, 2015

N. Bamiedakis et al., to be presented in ICTON, 2016

Other Routes to Higher Bandwidth: Wavelength

Division Multiplexing

TX RXSingle Wavelength

1 1

2

TX

TX

TX

RX

RX

RX

Multiple Wavelengths

n n

2

c.f. Australian Photonics Animation

32

input

waveguidesoutput

waveguides

Grating

Parabolic mirror

input field

expanded field

collimated field diffracted field

λ0λ2

λ1

top view

Triangular elements for

transmission diffraction

grating

Waveguide connection

with slab

Component 2 :

Deep etched waveguides -

two dimensional confinement

Parabolic collimating mirror

Component 1 :

Slab waveguide – one

dimensional confinement

(vertical only)

Component 3 :

Reflecting surfaces

Demultiplexer design

1

Integrated De/multiplexers for Guided-wave WDM Links

33

Triangular elements for

transmission diffraction

grating

Waveguide connection

with slab

Component 2 :

Deep etched waveguides -

two dimensional confinement

Parabolic collimating mirror

Component 1 :

Slab waveguide – one

dimensional confinement

(vertical only)

Component 3 :

Reflecting surfaces

Demultiplexer design

Integrated De/multiplexers for Guided-wave WDM Links

34

input

waveguidesoutput

waveguides

Grating

Parabolic mirror

input field

expanded field

collimated field diffracted field

λ0λ2

λ1

1

top view

35

input

waveguidesoutput

waveguides

Grating

Parabolic mirror

input field

expanded field

collimated field diffracted field

λ0λ2

λ1

1

2

top view

Integrated De/multiplexers for Guided-waveWDM Links

input

waveguidesoutput

waveguides

Grating

Parabolic mirror

input field

expanded field

collimated field diffracted field

λ0λ2

λ1

1

2

3

Triangular elements for

transmission diffraction

grating

Waveguide connection

with slab

Component 2 :

Deep etched waveguides -

two dimensional confinement

Parabolic collimating mirror

Component 1 :

Slab waveguide – one

dimensional confinement

(vertical only)

Component 3 :

Reflecting surfaces

Demultiplexer design

λ2

λ1

λ0

λ0 , λ1 , λ2

,

top view

Integrated De/multiplexers for Guided-waveWDM Links

36

input

waveguidesoutput

waveguides

Grating

Parabolic mirror

input field

expanded field

collimated field diffracted field

λ0λ2

λ1

1

2

3

5

4

top view

Integrated De/multiplexers for Guided-waveWDM Links

37

Predicted Integrated Multiplexer/Demultiplexer Performance

38

420 430 440 450 460 470 480-40

-35

-30

-25

-20

-15

-10

-5

0

Wavelength (m)

Sp

ectr

al R

esp

on

se

(d

B)

0=0.45 m, =10 nm, r

0=5.0, w

0=8.0m,Sep =125 m,TM Exp (0.25,0.25)

WG- 1

WG- 2

WG- 3

WG- 4

-35 dB

420 430 440 450 460 470 480-40

-35

-30

-25

-20

-15

-10

-5

0

Wavelength (m)

Sp

ectr

al R

esp

on

se

(d

B)

0=0.45 m, =10 nm, r

0=5.0, w

0=8.0m,Sep =125 m,TM Uni Power

WG- 1

WG- 2

WG- 3

WG- 4

-10 dB

- uniform mode power distribution at input

worst-case scenario

- Gaussian mode power distribution at input

restricted launch condition

N. Bamiedakis et al., to be presented in ICTON, 2016

on-going fabrication work

Integrated De/multiplexers for Guided-waveWDM Links

3939393939

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

waveguidesInterconnection

architectures

Board-level OE

integration PCB-integrated

optical units

Basic waveguide

components