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

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Experimental Results

Simulation Results

• Conclusions

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Experimental Results

Simulation Results

• 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

Experimental Results

Simulation 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

waveguidesInterconnection

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/sM. 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.

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.

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Experimental Results

Simulation Results

• Conclusions

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)

1.5151.5161.5171.5181.5191.5201.5211.5221.5231.5241.5251.526

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

∆tin∆tout

Input pulse Output pulse1. 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 1012

-20

-17

-14

-11

-8

-5

-2

0

Frequency (Hz)

Inte

nsity (

dB

)

Output pulse

Input pulse

3 dB

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.

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 nmλ ~ 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

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)

Vert

ical off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

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)

Vert

ical off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

BLP >100 GHz×m

Lower bottom part: ~32 × 8 μm2

BLP >100 GHz×m

Upper bottom part: ~20 × 22 μm2

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)

Vert

ical off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

WG 1

-16 -12 -8 -4 0 4 8 12 16

-16

-12

-8

-4

0

4

8

12

16

Horizontal offset (m)

Vert

ical off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

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

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)

Vert

ical off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

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)V

ert

ical off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

BLP >100 GHz×m

Upper bottom part: ~20 × 22 μm2

simulation and experimental results exhibit similar trends of bandwidth variation.

Experiment Simulation

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

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Experimental Results

Simulation Results

• Conclusions

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

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).

Thank you !