Dispersion studies demonstrate that waveguide layout can be used to enhance the bandwidth performance of multimode polymer waveguides for use in board-level optical interconnects, providing >40 GHz.m without the need for any launch conditioning.

1. Bandwidth Enhancement in
Multimode Polymer Waveguides Using
Waveguide Layout for Optical Printed Circuit Boards
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
Optical Fiber Communications Conference and Exposition (OFC 2016)
23rd March 2016

2. Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Refractive Index Engineering
 Launch Conditioning
 Waveguide Layout
• Conclusions

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.

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

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

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)
 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)?
D. M. Kuchta, et al., IEEE JLT, 2015.

7. Overfilled
Restricted
Input pulse Output pulse
Input pulse Output pulse
Mode propagation in waveguide
Bandwidth Studies
 Bandwidth (BW) limitation due to modal dispersion
1. Refractive index (RI) engineering
2. Launch conditioning
3. Waveguide layout and waveguide components
T. Ishigure, Summer
School on Optical
Interconnects, 2014.
Overfilled
Restricted
Input pulse Output pulse
Input pulse Output pulse
Mode propagation in waveguide Mode propagation in waveguideMode propagation in waveguide
Input pulse Output pulse Input pulse Output pulse
90° crossing 90° bend S bend Y splitter
N. Bamiedakis et al., IEEE JQE, vol. 45, pp. 415-424, 2009.
Elementary waveguide
components in complex
interconnection architectures

8. Time Domain Measurements
Short
pulse
laser
Autocorrelator10x 16x
Cleaved
50 μm MMF
Short
pulse
laser
Autocorrelator10x 16x
(a)
(b)
MM
10× lens 50 μm MMF 50 μm MMF+MM
1 m long spiral waveguide -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)
- 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.
near field images
- Experimental setup
- Waveguide samples with different RI profiles

9. ∆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

10. WG 1 WG 2
10× lens
BLP >100 GHz×m BLP >100 GHz×m
WG 1 WG 2
50 μm MMF
BLP: 30 – 60 GHz×m BLP: 50 – 90 GHz×m
RI engineering & launch conditioning
J. Chen, et al., IEEE Optical Interconnects Conference (OIC), 2015.
J. Chen, et al., Asia Communications and Photonics Conference (ACP), 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)
RI engineering Launch conditioning
-16 -12 -8 -4 0 4 8 12 1
-16
-12
-8
-4
0
4
8
12
16
Horizontal offset (m)
Verticaloffset(m)
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
-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
WG 1 WG 2
1 m long spiral waveguide 19.2 cm long waveguide

11. Waveguide Layout
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
- Mode filtering schemes: used in multimode fibre systems such as mode-selective ring
resonators and couplers.
 Multimoded on-board optical interconnects using waveguide bends / crossings
- Two waveguide samples with slightly different RI profiles under a SMF (loss) and
50 μm MMF launch (loss, BW)
50 μm MMF
B
Length:~137 mm
A B
WG length: 16.25 cm

12. Experimental Results
-Insertion loss of the crossing and bends measured under:
- 9 μm SMF (restricted launch)
- 50 μm MMF (likely encountered in real-world systems)
- Obtained by normalising with respect to the insertion loss of reference waveguides.
Input
Loss (dB/crossing)
WG A WG B
SMF 0.093 0.033
50 μm MMF 0.098 0.046
- WG A has worse crossing loss
- WG A and B have similar bending loss < 1 dB for radius R > 6 mm.

13. Experimental Results
0 10 20 30 40 50 60 70 80
35
40
45
50
55
60
65
Bandwidth-lengthproduct(GHzm)
Number of crossings
WG A
WG B
0 10 20 30 40 50 60
0
2
4
6
8
10
12
Insertionloss(dB)
Number of crossings
WG A
WG B
6 8 10 12 14 16 18 20
35
40
45
50
55
60
65
Bandwidth-lengthproduct(GHzm)
Radius (mm)
WG A
WG B
6 8 10 12 14 16 18 20
0
1
2
3
4
Insertionloss(dB)
Radius (mm)
WG A
WG B
0 10 20 30 40 50 60 70 80
35
40
45
50
55
60
65
Bandwidth-lengthproduct(GHzm)
Number of crossings
WG A
WG B
0 10 20 30 40 50 60 70 80
0
2
4
6
8
10
12
Insertionloss(dB)
Number of crossings
WG A
WG B
1.55× 1.25× ~1.9 dB
~0.7 dB
1.25×
~1.6 dB
BW Loss
BW Loss
90° Bends vs. Straight WG
90° Crossings vs. Straight WG
R = 5 mm R = 11 mm
BLP
improvement
> 60 GHz×m
(1.55×)
> 50 GHz×m
(1.25×)
Additional
loss
~1.9 dB ~0.7 dB
No. crossings = 10
BLP
improvement
~50 GHz×m
(1.25×)
Additional
loss
~1.6 dB
90° Bends
90° Crossings
BW increases but loss degrades
 design trade-off

14. Conclusions
• Multimode polymer waveguides constitute an attractive technology for
use in board-level optical interconnects
• Bandwidth performance of multimode WGs can be enhanced using
 refractive index engineering, launch conditions, waveguide layout, etc.
• Time domain measurements on waveguide bends and crossings
 potential to get BW improvement via intelligent waveguide layout
o 1.5× BW enhancement, addition loss ~1.9 dB (R = 6 mm)  BLP~60 GHz×m
o 1.25× BW enhancement, addition loss ~1.6 dB (crossing# = 10)  BLP~50 GHz×m
 optimisation of BW and loss performance based on waveguide layout (e.g. optimised
radius), RI profile and launch conditions.
- Dow Corning
- EPSRC UK
Acknowledgements:
50 μm
MMF
 ensure BLP >40 GHz×m to support high on-board data rates while maintaining low
loss performance and without the need for any launch conditioning.

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16. Thank you !