Polymer Interconnects for Datacom and Sensing. Department of Engineering, University of Cambridge

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1 Polymer Interconnects for Datacom and Sensing Richard Penty, Ian White, Nikos Bamiedakis, Ying Hao, Fendi Hashim Department of Engineering, University of Cambridge

2 Outline Introduction and Motivation Material and Fabrication Process Multimode Polymer Waveguide Components - Fundamental transmission studies - Waveguide bends and crossings - Y-splitters/combiners Integrated Polymer Waveguides / Optoelectronics - OE PCB Fabrication - Transceiver performance Non Communications Applications - Prototype gas sensor Conclusion 2

3 Optical Interconnects J. Bautista, Optoelectronic Integrated Circuits Vii, pp. 1-8, 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 3

$05 db/cm @ 850 nm low birefringence offer refractive index tunability can be integrated with PCBs offer high manufacturability$

4 Siloxane Polymer Material 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: nm low birefringence offer refractive index tunability can be integrated with PCBs offer high manufacturability (photolithography or embossing techniques) are cost effective 4

Multimode Waveguide Components Allow relaxed alignment tolerances compatible with machine assembly But need to have high optical performance (loss, lifetime, bandwidth etc)

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.

5 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 µm 2 or µm 2 Index step difference n 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 m long spiral waveguides - crossing guides - bent waveguides (90 o bends, S-bends) - Y-splitters/combiners - couplers 20um top cladding n ~ 1.5 core n ~ 1.52 bottom cladding n ~ 1.5 substrate 50um 5

6 Fundamental Transmission Properties - 1 Transmission properties investigated under varying launches: SMF and MMF inputs Propagation loss nm, 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 Propagation loss coefficient (db/cm) Wavelength (µm) Normalised Received Power (db) SMF SMF, Simulation 50 um MMF 50 um MMF, Simulation Input offset (µm) N. Bamiedakis, et al., IEEE Journal of Quantum Electronics, vol.45, pp ,

7 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 Normalised intensity at far field Angle (deg) 55 mm 71 mm 88 mm 99 mm Normalised received power (db) WG SMF 50 µm MMF WG X-axis offset (µm) 7

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.

8 Waveguide Crossings offer high routing flexibility maximise usable on-board area increase achievable interconnection density Lowest reported loss: db/crossing for SMF, ~0.01 db for MMF Excellent crosstalk performance: < - 25 db even for 100 crossings Insertion Loss (db) SMF 50 µm MMF 62.5 µm MMF slope: db/crossing slope: 0.01 db/crossing Crosstalk in adjacent parallel waveguide (db) SMF 50 µm MMF 0 slope: db/crossing Number of Crossings Number of Crossings 8

9 Polymer Backplane: Design Strategy Backplane Requirements: passive routing scalable architecture low loss & low crosstalk Exploit existing technology ribbon fiber and connectors VCSEL 850 nm photo-diode arrays Mount /Rx arrays on line cards incremental costs as cards added dedicated link from each VCSEL in transmit array to every other card address appropriate in array oncard Line cards 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 9

10 10 Card Optical Backplane Card interfaces (10 waveguides each) J. Beals, et al., Applied Physics A, vol. 95, pp , Rx Rx Rx Rx Rx 2.25 U (10 cm) Rx Rx Rx Rx Rx Schematic of 10-card backplane layout 100 waveguides single 90 bend per waveguide 90 crossings or less per waveguide 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, 10 Gb/s in multicast mode 10

Data Transmission Bit Er rror Rate 10-3 10-6 10 Gb/s link transmission Link 1 Back to Back 1 Link 2 Back to Back

loss and greatest crosstalk full line-rate data transmission no dropped packets 10-9 10-12 -16-15 -14-13 -12-11

11 Data Transmission Bit Er rror Rate Gb/s link transmission Link 1 Back to Back 1 Link 2 Back to Back db penalty for a bit-error-rate of 10-9 Real Gigabit Ethernet Traffic Across Backplane links with highest loss and greatest crosstalk full line-rate data transmission no dropped packets Received Pow er (dbm) (back to back) (received) 20 ps/div 20 ps/div Dell PowerEdge 2850 servers for GbE tests 11

Optical Backplanes: Widespread Industry Interest numerous demonstrations of simple point-to-point on-board polymer links appealing commercial application space Intel optical chip-to-chip link

12 Optical Backplanes: Widespread Industry Interest numerous demonstrations of simple point-to-point on-board polymer links appealing commercial application space Intel optical chip-to-chip link Mohammed et al, Intel Tech. J. 8 (2004) Fujitsu Labs optical backplane Glebov et al, Opt. Eng. 46 (2007) Asperation Perlos Co/Vtt Electronics Immonen et al, IEEE Trans. Elect. Pack. Manuf. 28 (2005) Daimler Chrysler Moisel et al, Opt. Eng. 39 (2000) 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) 12

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

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 Cost and fabrication issues arise - 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, ,

14 Integration Concept Motivation: Produce a low complexity/low cost OE PCB 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 board connector with VCSEL/PD mounted active device (VCSEL or PD) mounted connector with VCSEL/PD waveguide light output/input waveguide optical layer electrical layer FR4 FR4 metal tracks and electrical components electrical via connector slot electrical via connector slot 14

15 Waveguide PCB Integration Board design based on low-cost single-layered double-sided FR4 substrate Top side: electronic components and power plane Bottom side: ground plane and optical waveguides Through-board slots produced to allow endfire optical coupling electronic components ground vias I/O signal SMA connectors through-board connector slot pin connectors power plane electrical layer power plane FR4 ground plane optical layer polymer layers waveguide facets N. Bamiedakis, et al., Photonics West, San Francisco,

16 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 component upper solder mask plated-through via ground track signal track ground plane electrical layer FR4 FR4 optical layer bottom cladding waveguide core top cladding bottom solder mask through-board trench 16

17 Electro-Optic L-Connectors 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 L shape facilitates vertical and angular alignment: inside surface reference planes electronic component 7 mm 1.6 mm active components 5 mm copper tracks plated-through via through-board connector FR4 FR4 bottom solder mask bottom cladding waveguide core top cladding upper solder mask ground plane light I/O 0.4 mm signal vias optoelectronic component 17

18 Optical Transceiver Proof-of-principle demonstrator Integrates and Rx electronic modules with polymer Y-splitter on a 1-mm single-layered FR4 board front view Voltage regulators I/O data SMA Rx module module LD OE PCB PD Y-splitter planar view 18

19 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: VCSEL arm: x= ± 8 µm, y= ± 15 µm, z= 70 µm PD arm : x, y= ± 25 µm, z= 120 µm - 1 db points connector slot x,y and z offset VCSEL x,y and z offset PD Y-splitter OE PCB Y-splitter OE PCB connector slot x10 Broad area detector 50 µm MMF VCSEL Normalised received power (db) X-axis Y-axis XY-axis offset (µm) Normalised received power (db).. 0 VCSEL PD z X-axis Y-axis XY-axis offset (µm) Normalised Received Power (db) PD arm LD arm Z-axis offset (µm) 19

Tester 10 Gb/s OE PCB VCSEL 10-3 module Rx module Error-free operation (BER < 10-12 ) achieved for both directions

20 Optical Transceiver Performance Transmit mode 10 Gb/s PD Y-splitter VOA a RF amplifier Pattern generator 10x 10x BER Tester Receive mode Pattern generator VCSEL VCSEL 10x VOA 10x OE PCB Y-splitter High-speed receiver a PD BER Tester 10 Gb/s OE PCB VCSEL 10-3 module Rx module Error-free operation (BER < ) achieved for both directions at 10 Gb/s Bit Error Rate Received optical power at point a (dbm) 20

21 Parallel Optical Interconnects Integration of 1x4 VCSEL and PD arrays with on-board optical waveguides - improve RF performance of L-connectors for device arrays electronics OE PCB Rx electronics 1x4 VCSEL Array 10.6 mm Design and test of and Rx electronic circuits for 1x4 parallel links intial layouts on low-cost FR4 substrates with L-connectors 5.6 mm 10 Gb/s Normalised Received Power (dbe) S21 Comparison Exp - CH1-30 Exp - CH2-40 Model - CH1-50 Model - CH CH1 CH Frequency (GHz) 10 Gb/s Transmission driver Rx driver Rx Bit Error Rate PRBS7 PRBS7 Rx Received Power (dbm)

22 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 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 22

High-Speed Board-Level Polymer Optical Sub- Systems

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 - exponential growth