High speed optical and electronic Printed Circuit Boards

High speed optical and electronic Printed Circuit Boards to overcome the bandwidth bottleneck Dr David R. Selviah Department of Electronic and Electrical Engineering, University College London, UCL, UK, d. selviah@ucl. ac. uk Tel: 020 7679 3056 Photonex Tutorial, UCL, Haldane Room, 11. 30 am – 12. 00 noon, 9 th April 2013 © UCL 2013 1

Backplane Motherboards Copyright © 2013 UCL 2

Copper Tracks versus Optical Waveguides for High Bit Rate Interconnects n Copper Track q q q n EMI Crosstalk Loss Impedance control to minimize back reflections, additional equalisation, costly board material Optical Waveguides q q q q Low loss Low cost Low power consumption Low crosstalk Low clock skew WDM gives higher aggregate bit rate Cannot transmit electrical power 3

Integration of Optics and Electronics n n n Backplanes q Butt connection of “plug-in” daughter cards q In-plane interconnection Focus of OPCB project Out-of-plane connection q 45° mirrors q Chip to chip connection possible 4

Direct Laser-writing Setup: Schematic • Slotted baseplate mounted vertically over translation, rotation & vertical stages; components held in place with magnets • By using two opposing 45º beams we minimise the amount of substrate rotation needed 5

Laser written polymer structures SEM images of polymer structures written using imaged 50 µm square aperture (chrome on glass) • Writing speed: ~75 µm / s • Optical power: ~100 µW • Flat-top intensity profile • Oil immersion • Single pass Optical microscope image showing end on view of the 45º surfaces 6

Current Results Laser-writing Parameters: - Intensity profile: Gaussian - Optical power: ~8 m. W - Cores written in oil Polymer: - Custom multifunctional acrylate photo-polymer - Fastest “effective” writing speed to date: 50 mm/s (Substrate: FR 4 with polymer undercladding) 7

Laser direct written backplane • HWU Direct laser written waveguide cores and cladding backplane layout designed by UCL fabricated on FR 4 8

Laser Ablation for Waveguide Fabrication § Ablation to leave waveguides § Excimer laser – Loughborough § Nd: YAG – Stevenage Circuits UV LASER Core Cladding FR 4 PCB Deposit cladding and core layers on substrate Laser ablate polymer SIDE VIEW FR 4 PCB Deposit cladding layer 9

Nd: YAG Ablation upper clad core Lower clad FR-4 layer § Nd: YAG laser based at Stevenage Circuits § Grooves machined in optical polymer and ablation depth characterised for machining parameters § Initial waveguide structures prepared 10

CO 2 Laser Ablation of Polyacrylate Waveguides A cross-section through an array of waveguides fabricated in polyacrylate using CO 2 laser ablation 11

Excimer Laser Ablation of Polyacrylate Waveguides Cross-section through a waveguide (approx. 50 μm x 35 μm) formed in polyacrylate by excimer laser machining. 12 A plan view image of two 45 degree in-plane mirror structures formed in an optical waveguide by excimer laser ablation in polyacrylate.

Inkjetting as a Route to Waveguide Deposition § Print polymer then UV cure § Advantages: § § Deposit Lower Cladding 13 controlled, selective deposition of core and clad less wastage: picolitre volumes large area printing low cost Deposit Core Deposit Upper Cladding

Changing Surface Wettability Contact Angles Core material on cladding Large wetting - broad inkjetted lines Core material on modified glass surface (hydrophobic) Reduced wetting – discrete droplets Identical inkjetting conditions - spreading inhibited on modified surface 14

Final Ink Jet Printed Waveguides of OE 4140 optical polymer inkjet printed onto OE 4141 cladding using multiple print and cure passes. 15 A cross-section through an inkjet printed waveguide of OE 4140 core on cladding prepared using multiple print and cure cycles.

Photolithographic Fabrication of Waveguides Copyright © 2013 UCL 16

Polymer waveguides formed by Photolithography in Truemode® polymer

Optical Power Loss in 90° Waveguide Bends I Input A Rf = Rs + NΔR w Rs+ΔR lin Rs B lout Output Schematic diagram of one set of curved waveguides. O Light through a bent waveguide of R = 5. 5 mm – 34. 5 mm • Radius R, varied between 5. 5 mm < R < 35 mm, ΔR = 1 mm • Light lost due to scattering, transition loss, bend loss, reflection and backscattering • Illuminated by a MM fiber with a red-laser. Copyright © 2013 UCL 18

BPM, beam propagation method modeling of optical field in bend segments w = 50 μm, R = 13 mm (left picture) in the first segment (first 10°). (right picture) in the 30° to 40° degree segment. Copyright © 2009 UCL 19

Surface roughness • RMS side wall roughness: 9 nm to 74 nm • RMS polished end surface roughness: 26 nm to 192 nm. Copyright © 2013 UCL 20

Crosstalk in Chirped Width Waveguide Array 100 µm 110 µm 120 µm 130 µm 140 µm 150 µm • Light launched from VCSEL imaged via a GRIN lens into 50 µm x 150 µm waveguide • Photolithographically fabricated chirped with waveguide array • Photomosaic with increased camera gain towards left Copyright © 2013 UCL 21

ELECTRO-OPTICAL BACKPLANE Hybrid Electro-Optical Printed Circuit Board q Standard Compact PCI backplane architecture q 12 electrical layers for power and C-PCI signal bus and peripheral Optical connector site q. Electrical C-PCI connector slots connections for SBC and line cards q 1 polymeric optical layer for high speed 10 Gb. E traffic q 4 optical connector sites q Dedicated point-to-point optical waveguide architecture Compact PCI slot for single board computer Compact PCI slots for line cards 22

ELECTRO-OPTICAL BACKPLANE Hybrid Electro-Optical Printed Circuit Board Polymer optical waveguides on optical layer q Standard Compact PCI backplane architecture q 12 electrical layers for power and C-PCI signal bus and peripheral Optical connector site q. Electrical C-PCI connector slots connections for SBC and line cards q 1 polymeric optical layer for high speed 10 Gb. E traffic q 4 optical connector sites q Dedicated point-to-point optical waveguide architecture Compact PCI slot for single board computer Compact PCI slots for line cards 23

System Demonstrator Fully connected waveguide layout using design rules Copyright © 2013 UCL 24

The Shortest Waveguide Illuminated by Red Laser Copyright © 2013 UCL 25

Waveguide with 2 Crossings Connected 1 st to 3 rd Linecard Interconnect Copyright © 2013 UCL 26

Output Facet of the Waveguide Interconnection Copyright © 2013 UCL 27

OPTICAL BACKPLANE CONNECTION ARCHITECTURE Backplane and Line Cards Orthogonal Connector housing Parallel optical transceiver Copper layers FR 4 layers Lens Interface Research and Development Overview | Richard Pitwon Optical layer Backplane 28

VCSEL Array for Crosstalk Measurement PIN Array Source: Microsemi Corporation VCSEL Array MT compatible Source: ULM Photonics Gmb. H interface GRIN Lens Array Source: GRINTech Gmb. H Copyright © 2013 UCL 29

Parallel optical transceiver q Mechanically flexible optical platform q MT compatible optical interface q Geometric microlens array q Quad VCSEL driver and TIA/LA q VCSEL / PIN arrays on pre-aligned frame MT pins Microlens array plate Optical platform Drivers Optical Printed Circuit Board and Connector Technology Copyright © Xyratex Technology Limited 2010

PARALLEL OPTICAL PCB CONNECTOR MODULE Parallel optical transceiver circuit Backplane connector module q Small form factor quad parallel optical q Samtec / Xyratex collaborate to develop optical PCB transceiver connector q Microcontroller supporting I 2 C interface q 1 stage insertion engagement mechanism developed q Samtec “SEARAY™” open pin field array q. Xyratex transceiver integrated into connector module connector q Spring loaded platform for optical engagement mechanism q Custom heatsink for photonic drivers Spring loaded platform Samtec field array connector Microcontroller 31

HIGH SPEED SWITCHING LINE CARD Array connector for pluggable active optical connector Compact PCI bus connector PCI Bridge FPGA SMP connector sites 8 x 8 Crosspoint switch XFP ports Transceiver programming port XFP ports Research and Development Overview | Richard Pitwon 32

Demonstrator with Optical Interconnects Copyright © 2013 UCL 33

High speed data transmission measurements 1 st test card q 10 Gb. E LAN test data q Injected into front end Electro-optical midplane q Pluggable connectors q Polymer waveguides Target test card q Retrieved through front end q Signal integrity measured Optical Printed Circuit Board and Connector Technology Copyright © Xyratex Technology Limited 2010

High speed data transmission measurements Test data captured on 8 waveguides q Data rate: 10. 3 Gb/s q Typical Pk to Pk jitter: 26 ps BERT on waveguides q Measured by UCL and Xyratex on all waveguides q BER less than 10 -12 measured Optical Printed Circuit Board and Connector Technology Copyright © Xyratex Technology Limited 2010

Acknowledgments • University College London, UK – Kai Wang, Hadi Baghsiahi, F. Aníbal Fernández, Ioannis Papakonstantinou • Loughborough University, UK – David A. Hutt, Paul P. Conway, John Chappell, Shefiu S. Zakariyah • Heriot Watt University – Andy C. Walker, Aongus Mc. Carthy, Himanshu Suyal • BAE Systems, UK – Henry White • Stevenage Circuits Ltd. (SCL), UK – Dougal Stewart, Jonathan Calver, Jeremy Rygate, Steve Payne • Xyratex Technology Ltd. , UK – Dave Milward, Richard Pitwon, Ken Hopkins • Exxelis Ltd – Navin Suyal and Habib Rehman • Cadence – Gary Hinde • EPSRC and all partner companies for funding © UCL 2013 36