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, 9th April 2013 © UCL 2013
1
Backplane Motherboards
Optical Connector
Optical and Electronic
Interconnects
Backplane
Mezzanine Board (Daughter
Board, Line Card)
Copyright © 2013 UCL
2
Copper Tracks versus Optical Waveguides for
High Bit Rate Interconnects

Copper Track




EMI Crosstalk
Loss
Impedance control to minimize back reflections, additional equalisation, costly
board material
Optical Waveguides







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
Core
Cladding
Multilayer organic substrate
Detector
VCSEL
Core
Cladding
Daughter card
Detector
VCSEL
Daughter card



Backplanes
 Butt connection of
“plug-in” daughter
cards
 In-plane
interconnection
Focus of OPCB project
Out-of-plane
connection
 45° mirrors
 Chip to chip
connection possible
Multilayer organic substrate
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 mW
- Cores written in oil
Polymer:
- Custom multifunctional
acrylate photo-polymer
- Fastest “effective” writing speed
to date: 50 mm/s
(Substrate: FR4 with
polymer undercladding)
7
Laser direct written backplane
• HWU Direct laser written waveguide cores and cladding
backplane layout designed by UCL fabricated on FR4
8
Laser Ablation for Waveguide Fabrication
 Ablation to leave waveguides
 Excimer laser – Loughborough
 Nd:YAG – Stevenage Circuits
UV LASER
Core
Cladding
FR4 PCB
FR4 PCB
Deposit cladding and
core layers on substrate
Laser ablate polymer
SIDE VIEW
FR4 PCB
Deposit cladding layer
9
Nd:YAG Ablation
upper clad
core
Lower clad
FR-4 layer
FR4
 Nd:YAG laser based at Stevenage Circuits
 Grooves machined in optical polymer and ablation depth
characterised for machining parameters
 Initial waveguide structures prepared
10
CO2 Laser Ablation of Polyacrylate Waveguides
FR4
A cross-section through an array of
waveguides fabricated in
polyacrylate using CO2 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
500 m
Waveguides of OE4140
optical polymer inkjet printed
onto OE4141 cladding using
multiple print and cure
passes.
15
A cross-section through an
inkjet printed waveguide of
OE4140 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
 Standard Compact PCI
backplane architecture
 12 electrical layers for power
and C-PCI signal bus and
Optical
connector site
Electrical C-PCI connector slots
peripheral connections
for SBC and line cards
 1 polymeric optical layer for
high speed 10 GbE traffic
 4 optical connector sites
 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
 Standard Compact PCI
backplane architecture
 12 electrical layers for power
and C-PCI signal bus and
Optical
connector site
Electrical C-PCI connector slots
peripheral connections
for SBC and line cards
 1 polymeric optical layer for
high speed 10 GbE traffic
 4 optical connector sites
 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
24
The Shortest Waveguide Illuminated by Red
Laser
Copyright © 2013 UCL
25
Waveguide with 2 Crossings Connected 1st to 3rd
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
FR4 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 GmbH
interface
GRIN Lens Array
Source: GRINTech GmbH
Copyright © 2013 UCL
29
Parallel optical transceiver
 Mechanically flexible optical platform
 MT compatible optical interface
 Geometric microlens array
 Quad VCSEL driver and TIA/LA
 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
 Small form factor quad parallel optical
 Samtec / Xyratex collaborate to develop optical PCB
transceiver
connector
 Microcontroller supporting I2C interface
 1 stage insertion engagement mechanism developed
 Samtec “SEARAY™” open pin field array
Xyratex transceiver integrated into connector module
connector
 Spring loaded platform for optical
engagement mechanism
 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
8x8
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
1st test card
 10 GbE LAN test data
 Injected into front end
Electro-optical midplane
 Pluggable connectors
 Polymer waveguides
Target test card
 Retrieved through front end
 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
 Data rate:
10.3 Gb/s
 Typical Pk to Pk jitter: 26 ps
BERT on waveguides
 Measured by UCL and Xyratex on all
waveguides
 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 McCarthy, 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