Low-loss and high-bandwidth (>47 GHz×m) multimode polymer waveguide crossings (<0.02 /><1dB) are demonstrated. The performance of passive optical backplanes comprising such components is also optimised using refractive-index engineering and launch conditioning.

1. Low-Loss and High-Bandwidth
Multimode Polymer Waveguide Components
Using Refractive Index Engineering
Jian Chen, Nikos Bamiedakis, Peter Vasil'ev, Richard V. Penty, and Ian H. White
Electrical Engineering Division, University of Cambridge, UK
E-mail: jc791@cam.ac.uk
Conference on Lasers and Electro-Optics (CLEO 2016)
June 6th, 2016

2. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Multimode Polymer Waveguides
• Waveguide Components
o 90° Bends
o 90° Crossings
• Conclusions
1

3. Why Optical Interconnects?
Growing demand for data communications link capacity in:
- data centres
- supercomputers
 need for high-capacity short-reach interconnects operating at > 25 Gb/s
Optics better than copper at high data rates (bandwidth, power, EMI, density)
E.Varvarigos, Summer School on Optical Interconnects, 2014.K. Hiramoto, ECOC 2013.
2

4. Board-level Optical Interconnects
• Various approaches proposed:
 free space interconnects
 fibres embedded in substrates
 waveguide-based technologies
M. Schneider, et al., ECTC 2009.
Jarczynski J. et al., Appl. Opt, 2006.
R. Dangel, et al., JLT 2013.
Siloxane
waveguides
Interconnection
architectures
Board-level OE
integration
PCB-integrated
optical units
Basic waveguide
components
Our work:
 Polymer waveguides
3

5. Multimode Polymer Waveguides
- Siloxane Polymer Materials
• low intrinsic attenuation (0.03–0.05 dB/cm at 850 nm);
• good thermal and mechanical properties (up to 350 °C);
• low birefringence;
• fabricated on FR4, glass or silicon using standard techniques
• offer refractive index tunability
- Multimode Waveguide
• Cost-efficiency: relaxed alignment tolerances
 assembly possible with pick-and-place machines
50 μm
core
top cladding
bottom cladding
Substrate
 suitable for integration on PCBs
 offer high manufacturability
 are cost effective
- typical cross section used: 50×50 μm2
- 1 dB alignment tolerances: > ±10 μm
4

6. Technology Development
 increase data rate over each channel
N. Bamiedakis et al., ECOC, P.4.7, 2014.
waveguide link
Finisar, Xyratex
24 channels x 25 Gb/s
K. Shmidtke et al., IEEE JLT, vol.
31, pp. 3970-3975, 2013.
4 channels x40 Gb/s
M. Sugawara et al., OFC, Th3C.5,
2014.
Fujitsu Laboratories Ltd.
1 channel x40 Gb/s
Cambridge University
- numerous waveguide technology demonstrators:
- continuous bandwidth improvement of VCSELs:
- 850 nm VCSELs:
57 Gb/s (2013)
64 Gb/s (OFC 2014, Chalmers - IBM)
71 Gb/s (PTL 2015, Chalmers - IBM)
 high-bandwidth components required !
D. M. Kuchta, et al., IEEE JLT, 2015.
5

7. Tx 1
Tx 2
Tx 3
Tx 4
Rx 1
Rx 2
Rx 3
Rx 4 90 bends
90 crossing
Multimode Waveguide Components
Tx 1
Tx 2
Tx 3
Tx 4
Rx 1
Rx 2
Rx 3
Rx 4 90 bends
90 crossing
Passive multimode waveguide components enable on-board routing flexibility and
advanced topologies:
90° crossing 90° bend S bend Y splitter
 Elementary waveguide
components in complex
interconnection architectures
Components designed and fabricated:
- Waveguide crossings
- Bent waveguides: 90° bends, S bends
- Y-splitters/combiners
N. Bamiedakis et al., IEEE JQE, vol. 45, pp. 415-424, 2009.
J. Beals IV et al., Appl Phys A, vol. 95, pp. 983–988, 2009. 6

8. Interconnection Architectures
Waveguide crossings and bends  elementary components in complex architectures
- meshed waveguide architecture: 1 Tb/s capacity optical backplane  100×10 Gb/s
links
- regenerative optical bus architecture  4 x 10 Gb/s links
10 × 10 cm2 FR4:
100 90° bends
~1800 90° crossings
5 × 9 cm2 FR4:
24 90° bends
36 90° crossings
8 S-bends
 low-loss components required !
J. Beals, et al., Appl. Phys. A, vol. 95, pp. 983-988, 2009.
N. Bamiedakis et. al, IEEE JLT, vol. 32, pp. 1526-1537, 2014. 7

9. Waveguide Components
Radius: 5, 6, 8, 11, 15 and 20 mm
Number of crossings: 1, 5, 10, 20, 40 and 80
A B
A B
Length: ~137 mm
Length: ~137 mm
in
B
Length:~137 mm
A B
WG length: 16.25 cm
WG01 WG02 WG03
x(m)
y(m)
-30 -20 -10 0 10 20 30
-30
-20
-10
0
10
20
30
1.515
1.52
1.525
1.53
x(m)
y(m)
-30 -20 -10 0 10 20 30
-30
-20
-10
0
10
20
30
1.512
1.514
1.516
1.518
1.52
1.522
x(m)
y(m) -30 -20 -10 0 10 20 30
-30
-20
-10
0
10
20
30
1.515
1.52
1.525
1.53
WG01 WG02 WG03
nmax 1.532 1.522 1.531
∆n 0.02 0.01 0.019
Height(μm) 37 53 48
Width(μm) 32 50 29
Radius:5, 6, 8, 11, 15
and 20 mm
A B
Length:~137 mm
A
Length:~137 mm
B
Number of crossings: 1, 5, 10, 20, 40 and 80
B
Length:~137 mm
A B
Parameter WG01 WG02 WG03
max Δn 0.020 0.010 0.019
Size (µm2) 35 × 4055 × 5632 × 53
reference WGs 90° bends 90° crossings
- Components with different RI profiles and dimensions are fabricated and tested:
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- Bends and crossings exhibit differing behaviours with respect to index step Δn:
 bends benefit from larger Δn values (better light confinement)
 crossings exhibit lower loss for smaller Δn values
design trade-off

10. ∆tin
∆tout
Input pulse Output pulse
1. Short pulse generation system
 Femtosecond erbium-doped fibre laser at ~1574 nm
and a frequency-doubling crystal to generate pulses
at wavelength of ~787 nm
2. Matching autocorrelator to record output pulse
3. Convert autocorrelation traces back to pulse traces
 curve fitting is needed to determine the shapes
of the original pulses, i.e. Gaussian, sech2 or Lorentzian.
4. Bandwidth calculation
 waveguide frequency response and bandwidth estimated by comparing Fourier
transforms of input and output pulses
Bandwidth Estimation
0 0.5 1 1.5 2
x 10
12
-20
-17
-14
-11
-8
-5
-2
0
Frequency (Hz)
Intensity(dB)
Output pulse
Input pulse
3 dB
9

11. Reference Waveguides
- Insertion loss and bandwidth of reference waveguide measured under:
 a restricted launch: 9/125 μm SMF input (loss) or 10× lens input (BW)
 a 50/125 μm MMF input (likely encountered in a real-world system)
- restricted launch: similar insertion loss values (~1 dB) and large BLP (> 100 GHz×m)
- 50 μm MMF input: WG02 largest IL (~ 3 dB) but larger BLP (122 GHz×m) due to
smaller Δn value (0.01 vs 0.02)
B
Length:~137 mm
A B
WG length: 16.25 cm
10

12. Bends and Crossings
- Insertion loss the waveguide components measured under:
 a restricted launch: 9/125 μm SMF input (loss)
 a 50/125 μm MMF input (likely encountered in a real-world system)
- crossing loss (XL) and bending loss (BL) obtained by normalising with respect to the
insertion loss of the reference waveguides
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80
Crossingloss(dB)
Numberof crossings
WG_A:9 μm SMF
WG_Α:50 μm MMF
WG_Β:9 μm SMF
WG_Β:50 μm MMF
WG_C:9 μm SMF
WG_C:50 μm MMF
0
1
2
3
4
5
6
7
8
5 8 11 14 17 20Bendingloss(dB)
Radius (mm)
WG01: 9 μm SMF
WG01: 50 μm MMF
WG02: 9 μm SMF
WG02: 50 μm MMF
WG03: 9 μm SMF
WG03: 50 μm MMF
- WG02 largest BL : R > 10 mm for 1 dB BL
 but smallest XL: 0.02 dB/crossing for a 50 μm MMF input, < 0.01 dB/crossing for a
SMF input
- similar BL for WG01 and WG03: R > 6 mm for 1 dB BL, XL of WG01 worse
11

13. Result Summary
- Optimisation of the total loss performance
depends on particular waveguide layout,
launch conditions and BW requirements !!
- example on passive optical backplane:
- worst-case optical path (in red) :
1 bend and 90 crossings
Performance metric WG01 WG02 WG03
SMFinput
IL ref. WGs 1.1 1.5 1.0
XL (dB/crossing) 0.093 0.007 0.033
Radius for BL<1 dB (mm) > 6 > 10 > 6
BLP ref. WGs (GHz×m) 107 154 125
50µmMMFinput
IL ref. WGs 1.6 3.2 1.7
XL (dB/crossing) 0.099 0.019 0.046
Radius for BL<1 dB (mm) > 6 > 11 > 6
BLP ref. WGs (GHz×m) 47 122 48
For a 50 μm MMF input:
- assuming enough area for R = 12 mm
 WG 02 , total loss ~ 6 dB
- assuming R = 8 mm
 WG 03 , total loss ~ 6.2 dB
12

14. Conclusions
• Multimode polymer waveguides constitute an attractive technology for
use in board-level optical interconnects
• Waveguide bends and crossing are essential components in passive
interconnection architectures:
- optimisation of loss and BW performance is based on RI profile (index
step Δn and dimensions)
- depends on particular layout, BW requirements and launch conditions
• Low-loss and high-bandwidth (>47 GHz×m) multimode polymer
waveguide crossings (<0.02 dB/crossing) and bends (<1dB) are
demonstrated using refractive index engineering.
Acknowledgements:
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15. References
[1]. N. Bamiedakis, J. Chen, R. V. Penty, and I. H. White, “High-Bandwidth and Low-Loss Multimode Polymer Waveguides
and Waveguide Components for High-Speed Board-Level Optical Interconnects,” in Photonics West conference, Proceeding
of SPIE, vol. 9753, pp. 975304–1–9 (2016).
[2]. N. Bamiedakis, A. Hashim, J. Beals IV, R. V. Penty, and I. H. White, "Low-Cost PCB-Integrated 10-Gb/s Optical
Transceiver Built With a Novel Integration Method," in IEEE Transactions on Components, Packaging and Manufacturing
Technology, Vol. 3, pp. 592-600 (2013).
[3]. J. Chen, N. Bamiedakis, P. Vasil’ev, T. Edwards, C. Brown, R. Penty, and I. White, “High-Bandwidth and Large Coupling
Tolerance Graded-Index Multimode Polymer Waveguides for On-board High-Speed Optical Interconnects,” in Journal of
Lightwave Technology, vol. 34, no. 12, pp. 2934–2940, (2015).
[4]. J. Beals, N. Bamiedakis, A. Wonfor, R. V. Penty, I. H. White, J. V. DeGrootJr., K. Hueston, T. V. Clapp, M. Glick, "A
terabit capacity passive polymer optical backplane based on a novel meshed waveguide architecture," in Applied Physics A:
Materials Science & Processing, Vol. 95, pp. 983-988 (2009).
[5]. N. Bamiedakis et al., "A 40 Gb/s Optical Bus for Optical Backplane Interconnections," in J. of Lightw. Techn., Vol. 32,
pp.1526-1537 (2014).
[6]. J. Chen, N. Bamiedakis, P. Vasil’ev, T. J. Edwards, C. T. A. Brown, R. V. Penty, and I. H. White, “Graded-Index Polymer
Multimode Waveguides for 100 Gb/s Board-Level Data Transmission,” in European Conference on Optical Communication,
no. 0613 (2015).
[7] J. Chen, N. Bamiedakis, T.J. Edwards, C. Brown, R.V. Penty, and I.H. White, “Dispersion Studies on Multimode Polymer
Spiral Waveguides for Board-Level Optical Interconnects,” in Proceedings of IEEE Optical Interconnects Conference, MD2,
San Diego (2015).
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16. Thank you !
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