Polymer Interconnects for Datacom and Sensing

Richard Penty, Ian White, Nikos Bamiedakis, Ying Hao, Fendi HashimRichard Penty, Ian White, Nikos Bamiedakis, Ying Hao, Fendi Hashim

Department of Engineering, University of Cambridge

Outline

• Introduction and Motivation

• Material and Fabrication Process

• Multimode Polymer Waveguide Components- Fundamental transmission studies- Waveguide bends and crossings- Y-splitters/combiners

2

- Y-splitters/combiners

• Integrated Polymer Waveguides / Optoelectronics- OE PCB Fabrication- Transceiver performance

• Non Communications Applications- Prototype gas sensor

• Conclusion

Optical Interconnects

J. Bautista, Optoelectronic Integrated Circuits Vii, pp. 1-8, 2005.

3

Optical interconnects offer significant advantages over their electrical counterparts:- large link bandwidth, reduced power consumption, EMI, thermal management

issues

Successful integration of photonics onto PCBs requires:`- suitable materials- cost-effective fabrication, assembly and packaging schemes compatible with existing manufacturing processes of standard PCBs

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)

Siloxane Polymer Material

4

withstands ~ 350 °C (solder reflow)• low intrinsic loss at datacommunicationswavelengths: 0.03-0.05 dB/cm @ 850 nm

• low birefringence• offer refractive index tunability

� can be integrated with PCBs � offer high manufacturability

(photolithography or embossing techniques)

�are cost effective

Multimode Waveguide Components

• Allow relaxed alignment tolerances – compatible with machine assembly

• But need to have high optical performance (loss, lifetime, bandwidth etc)

• Fabricated by conventional photolithographic techniques on various substrates: silicon, glass , FR4

• Cross section of 50×20 µm2 or 50×50 µm2

• Index step difference ∆n ≈ 0.02

substrate

bottom cladding n ~ 1.5

top cladding n ~ 1.5

coren ~ 1.52

20um

50um

5

• Index step difference ∆n ≈ 0.02• Typical pitch of 250 µm to match ribbon fibre and

VCSEL and photodiode array spacing

Components designed and fabricated:- straight waveguides up to 125 mm- 1.4 m long spiral waveguides- crossing guides- bent waveguides (90o bends, S-bends)- Y-splitters/combiners- couplers

Fundamental Transmission Properties - 1

Transmission properties investigated under varying launches: SMF and MMF inputs

• Propagation loss 0.04-0.06 dB/cm @ 850 nm, 0.4 dB/cm @ 1310 nm

• Coupling loss ~ 0.5 dB for SMF inputs, ~1.5 – 2 dB for MMF input

• Relaxed alignment tolerances± 20 µm for -1 dB and ± 25 µm for -3 dB for SMF launch± 13 µm for -1 dB and ± 20 µm for -3 dB for 50 µm MMF

6

± 13 µm for -1 dB and ± 20 µm for -3 dB for 50 µm MMF

0

0.5

1

1.5

2

2.5

3

3.5

4

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6Wavelength (µm)

Pro

paga

tion

loss

coe

ffici

ent (

dB/c

m)

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35

Input offset (µm)

No

rma

lise

d R

ece

ive

d P

ow

er

(dB

)....

SMFSMF, Simulation50 um MMF50 um MMF, Simulation

N. Bamiedakis, et al., IEEE Journal of Quantum Electronics, vol.45, pp. 415-424, 2009.

Fundamental Transmission Properties - 2

Mode mixing in straight waveguides assessed by far field measurements and near field images under restricted launch (SMF input)

� very small effect for lengths up to 100 mm

Crosstalk performance assessed with arrays of parallel guides with varying pitch and under SMF and MMF launches

Very low crosstalk observed for both input types even for the longest parallel guides (125 mm) and closely spaced (100 µm):

< - 40 dB for SMF and < - 25 dB for a 50 µm MMF

7

< - 40 dB for SMF and < - 25 dB for a 50 µm MMF

-50

-40

-30

-20

-10

0

-300 -250 -200 -150 -100 -50 0X-axis offset (µm)

Nor

mal

ise

d re

ceiv

ed p

ower

(dB

)..

SMF50 µm MMF

WG

WG

0

0.2

0.4

0.6

0.8

1

-9 -6 -3 0 3 6 9

Angle (deg)

No

rma

lised

inte

nsi

ty a

t fa

r fie

ld…..

55 mm71 mm88 mm99 mm

Waveguide Crossings

• offer high routing flexibility• maximise usable on-board area• increase achievable interconnection density

Lowest reported loss: 0.006 dB/crossing for SMF, ~0.01 dB for MMFExcellent crosstalk performance: < - 25 dB even for 100 crossings

8

0

1

2

3

4

5

6

10 20 30 40 50 60 70 80 90 100Number of Crossings

Inse

rtion

Los

s (d

B)

SMF 50 µm MMF 62.5 µm MMF

slope: 0.006 dB/crossing

slope: 0.01 dB/crossing

slope: 0.012 dB/crossing

-50

-45

-40

-35

-30

-25

0 10 20 30 40 50 60 70 80 90 100Number of Crossings

Cro

ssta

lk in

adj

acen

t pa

ralle

l wa

vegu

ide

(dB

)..

SMF 50 µm MMF

Excellent crosstalk performance: < - 25 dB even for 100 crossings

Polymer Backplane: Design Strategy

passive routingRequirements: scalable architecture

low loss & low crosstalk

Backplane

Line cards

Exploit existing technology� ribbon fiber and connectors� VCSEL arrays @ 850 nm

9

� VCSEL arrays @ 850 nm� photo-diode arrays

Mount Tx/Rx arrays on line cards� incremental costs as cards added� dedicated link from each VCSEL in

transmit array to every other card� address appropriate Tx in array on-card

Backplane architecture� passive shuffle scheme� dedicated point-to-point links� strict non-blocking� card connections at board edge

���� no mid-board or out-of-plane

connectors

10 Card Optical Backplane

Tx

Tx

Tx

Tx

Rx Rx Rx Rx Rx

Tx

Tx

Tx

2.25 U

(10 cm)

Card interfaces (10 waveguides each) J. Beals, et al., Applied Physics A, vol. 95, pp. 983-988, 2009.

10

Rx Rx Rx Rx Rx

Tx

Tx

Tx

Tx

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

LossWorst-case Crosstalk

50 µm MMF 2 to 8 dB < -35 dB

SMF 1 to 4 dB < -45 dB

Data Transmission

Link 1 Back to Back 1 Link 2 Back to Back 2

Bit

Err

or

Rat

e

10-3

10-6

0.2 dB penalty for a bit-error-rate of 10-9

Real Gigabit Ethernet Traffic Across Backplane

10 Gb/s link transmission

• links with highest loss and greatest crosstalk• full line-rate data transmission • no dropped packets

11

-16 -15 -14 -13 -12 -11

Received Power (dBm)

Bit

Err

or

Rat

e

10-9

10-12

(back to back) (received)20 ps/div 20 ps/div

Dell PowerEdge 2850 servers for GbE tests

Optical Backplanes: Widespread Industry Interest

• numerous demonstrations of simple point-to-point on-board polymer links• appealing commercial application space

12

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

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

Daimler ChryslerMoisel et al, Opt. Eng. 39 (2000)

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)

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

� Cost and fabrication issues arise

- end-fired coupling

Optical Coupling Schemes

13

- 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

typically, pin-based assembly (MT-ferrules) used for alignment- not space-efficient unless employed at board-edge - not compatible with pick-and-place assembly

and/or flexible PCBs to route electrical signals - minimum bending radius - increased number of electrical interfaces

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

Integration Concept

�simplify optical layer / eliminate the need for beam-turning elements or micro-lenses � end-fired optical coupling schemes

�minimise the number of different types of electrical substrates (flex PCBs/FR4) and electrical interfaces � use one substrate

�allow compatibility with pick-and-place assembly�remove space restrictions: allow electro-optic interface anywhere on the

Motivation: Produce a low complexity/low cost OE PCB

14

�remove space restrictions: allow electro-optic interface anywhere on the board

waveguide

FR4

metal tracks and electrical components

electrical via

mounted connector with VCSEL/PD

connector slot

electrical layer

optical layer

light output/input

waveguide

FR4

electrical via

connector with VCSEL/PD

connector slot

mounted active device (VCSEL or PD)

Waveguide PCB Integration

electronic componentsthrough-board

I/O signal SMA pin connectors

Board design based on low-cost single-layered double-sided FR4 substrate �

Top side: electronic components and power planeBottom side: ground plane and optical waveguides

Through-board slots produced to allow endfire optical coupling

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FR4

electrical layer

optical layer

through-board connector slot

power plane

ground plane

ground vias

I/O signal SMA connectors

waveguide facets

power plane

polymer layers

pin connectors

N. Bamiedakis, et al., Photonics West, San Francisco, 2010.

upper solder mask

OE PCB Fabrication

(i) produce electrical layout on FR4 (plated vias and uniform bottom solder mask)

(ii) fabricate waveguides on the bottom board surface

(iii) attach the electronic components using solder reflow process,

(iv) mill through-board slots to expose waveguide facets.

electronic componentelectronic component

16

FR4

bottom solder mask

ground trackplated-through viaground plane

signal track

FR4

upper solder mask

bottom cladding waveguide core top cladding

bottom cladding waveguide core top cladding

electronic componentelectronic component

through-board trenchthrough-board trench

electrical layer

optical layer

• Electro-optic connectors to:• accommodate active OE devices • interface electrical with optical layer

• L-Connector shape and size• allows pick-and-place assembly (no pins)• can be positioned anywhere on the board• allows electrical connection to the back of the connector

Electro-Optic L-Connectors

• allows electrical connection to the back of the connector• L – shape facilitates vertical and angular alignment: inside surface reference

planes

17

signal vias

active components

1.6 mm

0.4 mm

5 mm copper tracks

7 mmFR4

waveguide core top cladding

electronic component

plated-through via

optoelectronic component

through-board connector

FR4ground plane

light I/O

bottom solder maskbottom cladding

upper solder mask

Optical Transceiver

• Proof-of-principle demonstrator

• Integrates Tx and Rx electronic modules with polymer Y-splitter on a 1-mm single-layered FR4 board

I/O data SMAVoltage

regulators

front view

18

Y-splitterOE PCB

PDLD

Rx

module

Tx

module

planar view

Tolerancing

Y-splitter insertion losses ~ 6.6 and 6.5 dB for VCSEL and PD arms � Input/output coupling losses ~ 3.5 dB and 2 dB

Main loss component: facet quality (Milled – not polished) � optimisation of milling process (tool type, spindle speed, feed rate)

Alignment tolerances:

VCSELOE PCB

x,y and z offset

x10

Broad area detector

Y-splitter

connector slot

Y-splitter

OE PCB

PD 50 µm MMFVCSEL

x,y and z offset

connector slot

19

-4

-3

-2

-1

0

-40 -30 -20 -10 0 10 20 30 40XY-axis offset (µm)

Nor

mal

ised

rec

eive

d po

wer

(dB

)….

X-axis

Y-axis

-4

-3

-2

-1

0

-40 -30 -20 -10 0 10 20 30 40XY-axis offset (µm)

Nor

mal

ised

rec

eive

d p

ower

(dB

) ..

X-axis

Y-axis

-4

-3

-2

-1

0

0 50 100 150 200Z-axis offset (µm)

Nor

mal

ised

Rec

eive

d P

ower

(dB

)

PD arm

LD arm

Alignment tolerances:VCSEL arm: ∆x= ± 8 µm, ∆y= ± 15 µm, ∆z= 70 µmPD arm : ∆x, ∆y= ± 25 µm, ∆z= 120 µm - 1 dB points

VCSEL PD ∆z

connector slot

Optical Transceiver Performance

High-speed receiver

Pattern generator

VCSEL

Y-splitter

OE PCB

BER Tester

RF amplifier

10x10x10x10x

VOAaPD

Transmit mode

Receive mode10 Gb/s

10 Gb/s

20

10x10x 10x10x

VOAVCSEL

Pattern generator

BER Tester

VCSEL

Y-splitter

OE PCB

PD

aReceive mode

Error-free operation (BER < 10-12) achieved for both directions at 10 Gb/s

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

Received optical power at point a (dBm)

Tx module

Rx module

Bit

Err

or R

ate

10-9

10-6

10-3

10-12

Parallel Optical Interconnects

• Integration of 1x4 VCSEL and PD arrays with on-board optical waveguides

Tx e

lect

roni

cs

Rx electronics

OE PCB

- improve RF performance of L-connectors for device arrays

1x4 VCSEL Array

5.6

mm

-60-50-40-30-20-10

010

Nor

mal

ised

Rec

eive

d P

owe

r (d

Be)

S21 Comparison

Exp - CH1 Exp - CH2Model - CH1Model - CH2

CH1

CH2

-60-50-40-30-20-10

010

Nor

mal

ised

Rec

eive

d P

owe

r (d

Be)

S21 Comparison

Exp - CH1 Exp - CH2Model - CH1Model - CH2

CH1

CH2

Tx e

lect

roni

cs

Tx driver Rx driver -14 -12 -10 -8 -6

Received Power (dBm)

10 Gb/s Transmission

PRBS7 TxPRBS7 Rx

Bit

Err

or

Rat

e

10-3

10-12

10-9

10-6

10 Gb/s

Tx

Rx

10.6 mm0 2 4 6 8 10 12 14N

orm

alis

ed R

ecei

ved

Pow

er

Frequency (GHz)

0 2 4 6 8 10 12 14Nor

mal

ised

Rec

eive

d P

owe

r

Frequency (GHz)

Design and test of Tx and Rx electronic circuits for 1x4 parallel links� intial layouts on low-cost FR4 substrates with L-connectors

Conclusions

• Polymer siloxane materials satisfy necessary requirements for low-cost and large-scale integration onto PCBs

• A wide range of useful multimode waveguide components demonstrated with excellent transmission properties

• Automatic assembly compatible integration technique for multi-layer PCB board developed

22

PCB board developed

• Prototype transceiver and on-board links successfully developed for 10Gb/s operation

• Applications in gas and bio sensing being developed

• Initial early studies towards printed waveguides

Multimode Siloxane Waveguides : � a promising technology for use in high-speed short-reach optical

interconnection applications