We report enhanced bandwidth performance of >100 GHz×m over an offset range of ±10 µm in multimode polymer waveguides under restricted launch, demonstrating the capability to support on-board data rates of >100 Gb/s.

1. Restricted Launch Polymer Multimode Waveguides for
Board-level Optical Interconnects with >100 GHz×m
Bandwidth and Large Alignment Tolerance
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
Asia Communications and Photonics Conference (ACP 2015)
23rd November 2015

2. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Experimental Results
 Simulation Results
• Conclusions

3. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Experimental Results
 Simulation Results
• Conclusions

4. 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.

5. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Experimental Results
 Simulation Results
• Conclusions

6. 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

7. Polymer Multimode 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

8. 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)
- un-cooled operation up to 90°C
- VCSEL arrays with very good uniformity and high bandwidth
D. M. Kuchta, et al., IEEE JLT, 2015.

9. Demand for Higher Bandwidth
 their highly-multimoded nature raises important concerns about their bandwidth
limitations and their potential to support very high on-board data rates (e.g. >100 Gb/s)?
23 GHz (BLP1: 57.5 GHz×m) for a 2.55 m long waveguide2
150 GHz (BLP1: 75 GHz×m) for a 51 cm long waveguide3
1.03 GHz (BLP1: 90 GHz×m) for a 90 m long waveguide4
SI:
GI:
Examples:
Restricted centre
launch
 Effects of launch conditions & input offsets?
2F. Doany, et al., LEOS Summer Topical Meetings, 2004.
3X. Wang, et al., Optics letters, vol. 32, no. 6, pp. 677–679, 2007.
4T. Kosugi , et al., Optics express, vol. 17, no. 18, pp. 15959–15968, 2009.
BLP1: Bandwidth-length product.
- step-index (SI) vs. graded-index (GI) waveguides
 achieve higher bandwidth: renewed interest on ultimate dispersion limits
T. Ishigure, Summer
School on Optical
Interconnects, 2014.

10. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Experimental Results
 Simulation Results
• Conclusions

11. Time Domain Measurements (50 μm MMF)
1 m long spiral waveguide
J. Chen, et al., IEEE Optical Interconnects Conference (OIC 2015), 2015.
J. Chen, et al., accepted to JLT, 2015.
-25 -20 -15 -10 -5 0 5 10 15 20 25
-25
-20
-15
-10
-5
0
5
10
15
20
25
x (m)
y(m)
1.515
1.517
1.519
1.521
1.523
1.525
1.527
1.529
1.531
1.532
-25 -20 -15 -10 -5 0 5 10 15 20 25
-25
-20
-15
-10
-5
0
5
10
15
20
25
x (m)
y(m)
1.515
1.516
1.517
1.518
1.519
1.520
1.521
1.522
1.523
1.524
1.525
1.526
WG 1 WG 2(b) (c)
(a)
- 1 m long spiral multimode waveguide
- cross section ~35×35 µm2
- sample fabricated on 8’’ inch Si substrate
- input/output facets exposed with dicing saw
 this particular features are due to fabrication
process and the mechanism is under study.
50 μm MMF:
mode mixer (MM)
no MM
50 μm MMF 50 μm MMF+MM
Near field images

12. ∆tin
∆tout
Input pulse Output pulse
1. Short pulse generation system
(a) Ti:Sapphire laser emitting at 850 nm
(b) 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

13. Time Domain Measurements (50 μm MMF)
WG 1 WG 2
50 μm MMF: no MM 50 μm MMF: with MM 50 μm MMF: no MM 50 μm MMF: with MM
Bandwidth-length product (BLP): 30 – 60 GHz×m Bandwidth-length product (BLP): 50 – 90 GHz×m
1 m long spiral waveguide
50 μm MMF launch:
J. Chen, et al., IEEE Optical Interconnects Conference (OIC 2015), 2015.
J. Chen, et al., accepted to JLT, 2015.

14. Time Domain Measurements (10× lens)
• Different input positions:
different mode power distributions inside the waveguide  different amount of induced
multimode dispersion.
Restricted launch
(across both horizontal and vertical offsets):
 experimental results
 matching simulation results
TOPTICA laser
Polarization
controller
X10 X10
Nonlinear
crystal
λ ~ 1574 nm
λ ~ 787 nm
Autocorrelator
x10 x16
enhanced bandwidth for a large range of
input offsets (> 10 × 10 μm2)
19.2 cm long waveguide
10× lens
Near field image

15. Experimental Results
100 Gb/s data transmission over a single waveguide channel !
 appropriate launch conditioning scheme
 relatively large alignment tolerances
-16 -12 -8 -4 0 4 8 12 16
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
WG 1 WG 2
-16 -12 -8 -4 0 4 8 12 16
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
BLP >100 GHz×m
Lower bottom part: ~32 × 8 μm2
BLP >100 GHz×m
Upper bottom part: ~20 × 22 μm2

16. Simulation Results (WG 1)
BLP >100 GHz×m
Lower bottom part: ~32 × 8 μm2
WG 1
-16 -12 -8 -4 0 4 8 12 16
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
WG 1
-16 -12 -8 -4 0 4 8 12 16
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
BLP >100 GHz×m
Lower bottom part: ~32 × 6 μm2
- no mode mixing assumed inside the waveguide
 simulation and experimental results exhibit similar trends of bandwidth variation.
Experiment Simulation

17. Simulation Results (WG 2)
- no mode mixing assumed inside the waveguide
BLP >100 GHz×m
Upper bottom part: ~28 × 22 μm2
-16 -12 -8 -4 0 4 8 12 16
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
WG 2 WG 2
-16 -12 -8 -4 0 4 8 12 16
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
BLP >100 GHz×m
Upper bottom part: ~20 × 22 μm2
 simulation and experimental results exhibit similar trends of bandwidth variation.
Experiment Simulation

18. Bandwidth Discussion
18
- Why such a good bandwidth performance ?
some explanations:
1. launch conditioning
 restricted launch excites lower order modes
2. refractive index profiles
 GI waveguides result in reduced multimode dispersion
3. waveguide layout
- bend structure suppresses higher order modes
 reduced multimode dispersion
 ongoing studies to quantify these effects in particular polymer waveguide technology
dispersion engineering
using layout
bandwidth enhancement
using refractive index
engineering
reduced dispersion
using launch conditioning

19. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Experimental Results
 Simulation Results
• Conclusions

20. Conclusions
• Multimode polymer waveguides constitute an attractive technology for
use in board-level optical interconnects
• Bandwidth performance of multimode WGs can be enhanced using
 launch conditions, refractive index engineering, waveguide layout, etc.
• Time domain measurements on 19.2 cm long waveguides
 >100 GHz×m for “step-index” WG1 (~32 × 8 μm2)
 >100 GHz×m for “graded-index” WG2 (~20 × 22 μm2)
• Simulation modelling agrees well with the experimental results.
 potential for 100 Gb/s data transmission over a single waveguide channel !
- Dow Corning
- EPSRC UK
Acknowledgements:
for a restricted launch

21. References
[1] A. F. Benner, M. Ignatowski, J. A. Kash, D. M. Kuchta, and M. B. Ritter, “Exploitation of optical interconnects in future server
architectures,” in IBM Journal of Research and Development, Vol. 49, pp. 755–775 (2005).
[2] D. Miller, “Device requirements for optical interconnects to silicon chips,” in Proceedings of the IEEE, Vol. 97, no. 7, pp. 1166–1185
(2009).
[3] N. Bamiedakis, J. Chen, P. Westbergh, J. Gustavsson, A. Larsson, R. Penty, and I. White, “40 Gb/s Data Transmission Over a 1 m
Long Multimode Polymer Spiral Waveguide for Board-Level Optical Interconnects,” in Journal of Lightwave Technology, Vol. 33, no. 4,
pp. 882 - 888 (2014).
[4] N. Bamiedakis, J. Chen, R. V Penty, and I. H. White, “Bandwidth Studies on Multimode Polymer Waveguides for ≥25 Gb/s Optical
Interconnects,” in IEEE Photonics Technology Letters, Vol. 26, no. 20, pp. 2004–2007 (2014).
[5] J. Chen, N. Bamiedakis, R. V. Penty, I. H. White, P. Westbergh, and A. Larsson, “Bandwidth and Offset Launch Investigations on a
1.4 m Multimode Polymer Spiral Waveguide,” in European Conference on Integrated Optics, p. P027 (2014).
[6] W. Xiaolong et al., “Hard-molded 51 cm long waveguide array with a 150 GHz bandwidth for board-level optical interconnects,” in
Optics Express, Vol. 32, pp. 677–679 (2007).
[7] T. Kosugi et al., “Polymer parallel optical waveguide with graded-index rectangular cores and its dispersion analysis,” in Optics
Express, Vol.17, pp.15959-15968 (2009).
[8] 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).
[9] B.W. Swatowski et al., “Graded index silicone waveguides for high performance computing,” in Proceedings of IEEE Optical
Interconnects Conference, WD2, San Diego (2014).
[10] 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).

22. Thank you !