Graded-index Polymer Multimode Waveguides for 100 Gb/s Board-level Data Transmission

Graded-index Polymer Multimode Waveguides for
100 Gb/s Board-level Data Transmission
Jian Chen1, Nikos Bamiedakis1, Peter Vasil'ev1, Tom J. Edwards2, Tom Brown2,
Richard V. Penty1, Ian H. White1
1Electrical Engineering Division, University of Cambridge, UK
e-mail: jc791@cam.ac.uk
2SUPA, School of Physics & Astronomy, University of St Andrews, UK
European Conference on Optical Communication (ECOC 2015)
28th September 2015

Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Experimental Results
 Waveguide Modelling
• Conclusions

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.

Outline
• Introduction to Optical Interconnects
• Board-level Optical Interconnects
• Bandwidth Studies
 Waveguide Modelling
 Experimental Results
• Conclusions

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

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

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
P. Westbergh, et al., IEEE PTL, 2015.

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.

x (m)
y(m)
-25-20-15-10 -5 0 5 10 15 20 25
-25
-20
-15
-10
-5
0
5
10
15
20
25
1.516
1.518
1.52
1.522
1.524
1 m Graded-index (GI) Spiral Multimode Waveguide
- It is described as “GI” here, although it does not have the parabolic GI profile as typically
encountered in MMFs.
 this particular feature is due to fabrication process and the mechanism is under study.
(a) the 1 m long spiral waveguide
illuminated with red light
(a)
Brandon W. Swatowski, et al., IEEE Optical Interconnects Conference
(OIC 2014), WD2, 2014.
- 1 m long multimode spiral waveguide
- cross section 32×36 µm2, ∆n ~ 0.01
- sample fabricated on 8’’ inch Si substrate
- input/output facets exposed with dicing saw
- no polishing steps undertaken
(b) Measured RI profile of the
waveguide at 678 nm
(b)

Frequency Response Measurements
 -3 dB frequency response >35 GHz for all inputs and input positions
 suitable for high-speed transmission of ≥ 40 Gb/s data transmission
N. Bamiedakis, et al., IEEE JLT, vol. 33, pp. 1-7, 2015.
- 1 m long multimode spiral waveguide
- cross section 32×50 µm2, ∆n ~ 0.02
- sample fabricated on 8’’ inch Si substrate
- input/output facets exposed with dicing saw
- no polishing steps undertaken
32 µm
50µm
- frequency response investigated under different launch conditions:
~ 4 µm
50 µm 50 µm
100 µm
exciting increasing number of waveguide modes at waveguide input
4/125 µm SMF
restricted launch
typical (no mode mixer)
50/125 µm MMF
quasi-overfilled (mode mixer)
50/125 µm MMF
100/140 µm MMF
overfilled launch

Time Domain Measurements
• Different launch conditions (10× lens, 50 μm MMF with/without mode mixer):
different mode power distributions at the waveguide input  different levels of multimode
dispersion.
• Different input positions:
different mode power distributions inside the waveguide  different amount of induced
multimode dispersion.
So, what are the bandwidth limits of these particular waveguides ?
 time domain measurements
Short
pulse
laser
Autocorrelator10x 16x
Cleaved
50 μm MMF
Short
pulse
laser
Autocorrelator10x 16x
(a)
(b)
MM

∆tin
∆tout
Input pulse Output pulse
1. Two short pulse generation systems
(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
 allow more detailed study with a range of launch conditions
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

Experimental Bandwidth Results
Estimated bandwidth:
(a) 10× lens: > 200 GHz×m (≤ ±5 μm)
(b) 50 MMF (no MM): > 70 GHz×m (≤ ±10 μm)
(c) 50 MMF (with MM): > 60 GHz×m (≤ ±10 μm)
Mode mixer:
 lower bandwidth
 smaller variation across offsets
Launch conditioning:
 100 Gb/s data transmission !
-20 -15 -10 -5 0 5 10 15 20
0
50
100
150
200
250
300
350
400
Horizontal offset (m)
Bandwidth-lengthproduct(GHzm)
10x lens
50 m MMF
50 m MMF + MM
50
75
100
125
150
32 μm
36μm
Bandwidth-length products
10× lens 50 μm MMF 50 μm MMF+MM
- near field images of different input launch conditions:
exciting increasing number of waveguide modes at waveguide input

Basic Waveguide Modelling
1. Calculate waveguide modes for different waveguide geometries and index step
Δn (FIMMWAVE Mode Solver);
e.g. cross section used: 20x20 µm2 or 60×60 µm2; index step difference Δn  0.005 to 0.03
at 850 nm.
2. Calculate effective and group refractive indices for all waveguide modes;
3. Calculate mode power coefficient for a specific launch condition;
4. Find normalised transfer function from impulse response.
,

Simulation Results
10× lens launch 50 μm MMF launch
- no mode mixing assumed inside the waveguide
-20 -15 -10 -5 0 5 10 15 20
0
50
100
150
200
250
300
350
400
Experiment
Simulation
10x lens
-20 -15 -10 -5 0 5 10 15 20
0
25
50
75
100
125
150
Experiment
Simulation
50/125 m MMF
 >200 GHz×m for a restricted launch
 >70 GHz×m for a MMF launch
 ~60 GHz×m for an overfilled launch
 simulation and experimental results exhibit similar trends of bandwidth variation.

Bandwidth Discussion
18
- Why such a good bandwidth performance ?
some explanations:
1. refractive index profiles
 GI waveguides result in reduced multimode dispersion
2. waveguide layout
- long bends in spiral structure suppress higher order modes
 reduced multimode dispersion
3. mode mixing
 power redistribution inside the waveguides
 BW independent of launch conditions if mode mixing is strong
 ongoing studies to quantify these effects in particular polymer waveguide technology
dispersion engineering
using layout
bandwidth enhancement
using refractive index
engineering
effect important in MMFs
J. Chen, et al., IEEE Optical Interconnects Conference (OIC 2015),
pp. 26–27, 2015.

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 1 m long spiral waveguides
 >200 GHz×m for a restricted launch (≤ ±5 μm)
 >70 GHz×m for a MMF launch (≤ ±10 μm)
• Simulation modelling agrees well with the experimental results.
 potential for 100 Gb/s data transmission over a single waveguide channel !
- Dow Corning
- EPSRC UK
- IET Travel Award
Acknowledgements:

References
[1] N. Bamiedakis et al., “Bandwidth studies on multimode polymer waveguides for ≥ 25 Gb/s
optical interconnects,” IEEE Photon. Technol. Lett., Vol. 26, no. 20, pp. 2004–2007 (2014).
[2] D. Kuchta et al., “64 Gb/s Transmission over 57m MMF using an NRZ Modulated 850nm
VCSEL,” Proc. OFC, Th3C.2, San Francisco (2014).
[3] F. E. Doany et al., “Measurement of optical dispersion in multimode polymer waveguides,”
in IEEE/LEOS Summer Topical Meetings Tech. Dig., MB4.4, San Diego (2004).
[4] W. Xiaolong et al., “Hard-molded 51 cm long waveguide array with a 150 GHz bandwidth
for board-level optical interconnects,” Opt. Lett., Vol. 32, pp. 677–679 (2007).
[5] T. Kosugi et al., "Polymer parallel optical waveguide with graded-index rectangular cores
and its dispersion analysis," Opt. Express, Vol.17, pp.15959-15968 (2009).
[6] N. Bamiedakis et al., “40 Gb/s Data Transmission Over a 1 m Long Multimode Polymer
Spiral Waveguide for Board-Level Optical Interconnects,” J. Lightwave Technol., Vol. 33,
no.4, pp. 1-7 (2015).
[7] J. Chen et al., “Dispersion Studies on Multimode Polymer Spiral Waveguides for Board-
Level Optical Interconnects,” in Proc. IEEE Opt. Interconnects Conf., p. MD2, San Diego,
USA (2015).
[8] P. Pepeljugoski et al., “Modeling and simulation of next-generation multimode fiber links,”
J. Lightwave Technol., Vol. 21, pp. 1242–1255 (2003).

Graded-index Polymer Multimode Waveguides for 100 Gb/s Board-level Data Transmission

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