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

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Refractive Index Engineering

Launch Conditioning

Waveguide Layout

• 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.

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

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

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)

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.

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 waveguideInput 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

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)

1.5151.5161.5171.5181.5191.5201.5211.5221.5231.5241.5251.526

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

∆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

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)

1.5151.5161.5171.5181.5191.5201.5211.5221.5231.5241.5251.526

WG 1 WG 2(b) (c)

RI engineering Launch conditioning

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

-16

-12

-8

-4

0

4

8

12

16

Horizontal offset (m)

Vert

ica

l 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

ica

l off

set

(m

)

40.060.080.0100.0120.0140.0160.0180.0200.0

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

-16

-12

-8

-4

0

4

8

12

16

Horizontal offset (m)

Ve

rtic

al o

ffse

t (

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)

Ve

rtic

al o

ffse

t (

m)

40.060.080.0100.0120.0140.0160.0180.0200.0

WG 1 WG 2

1 m long spiral waveguide 19.2 cm long waveguide

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

output

input

input

output

- 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 mmA B

WG length: 16.25 cm

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.

InputLoss (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.

Experimental Results

0 10 20 30 40 50 60 70 8035

40

45

50

55

60

65

Ban

dw

idth

-len

gth

pro

du

ct

(GH

zm

)

Number of crossings

WG A

WG B

0 10 20 30 40 50 60 70 800

2

4

6

8

10

12

Insert

ion lo

ss (

dB

)

Number of crossings

WG A

WG B

6 8 10 12 14 16 18 2035

40

45

50

55

60

65

Ban

dw

idth

-len

gth

pro

du

ct

(GH

zm

)

Radius (mm)

WG A

WG B

6 8 10 12 14 16 18 200

1

2

3

4

Insert

ion lo

ss (

dB

)

Radius (mm)

WG A

WG B

0 10 20 30 40 50 60 70 8035

40

45

50

55

60

65

Bandw

idth

-length

pro

duct

(GH

zm

)

Number of crossings

WG A

WG B

0 10 20 30 40 50 60 70 800

2

4

6

8

10

12

Insert

ion loss (

dB

)

Number of crossings

WG A

WG B

6 8 10 12 14 16 18 2035

40

45

50

55

60

65

Bandw

idth

-length

pro

duct

(GH

zm

)

Radius (mm)

WG A

WG B

6 8 10 12 14 16 18 200

1

2

3

4

Insert

ion loss (

dB

)

Radius (mm)

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

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.

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] 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,” Journal of Lightwave

Technology, vol. 34, no. 12, pp. 2934–2940 (2015).

[3] 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).

[4] N. Bamiedakis, J. Chen, P. Westbergh, J. S. Gustavsson, A. Larsson, R. V. Penty, and I. H. White, “40 Gb/s Data Transmission

Over a 1 m Long Multimode Polymer Spiral Waveguide for Board-Level Optical Interconnects,” Journal of Lightwave Technology, vol.

33, no. 4, pp. 882–888 (2014).

[5] J. Chen, N. Bamiedakis, T. J. Edwards, C. T. A. Brown, R. V Penty, and I. H. White, “Dispersion Studies on Multimode Polymer

Spiral Waveguides for Board-Level Optical Interconnects,” in Optical Interconnects Conference (OIC), p. MD2 (2015).

[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 (ECOC),

Mo.3.2.3 (2015).

[7] Z. Haas and M.A. Santoro, “A mode-filtering scheme for improvement of the bandwidth-distance product in multimode fiber

systems,” in Journal of Lightwave Technology, Vol. 11, pp. 1125–1131 (1993).

[8] B. A. Dorin and W.N. Ye, “Two-mode division multiplexing in a silicon-on-insulator ring resonator,” in Optics Express, Vol. 22, pp.

4547–4558 (2014).

[9] J. D. Love and N. Riesen, “Mode-selective couplers for few-mode optical fiber networks,” in Optics Letters, Vol. 37, no. 19, pp.

3990–3992 (2012).

[10] B.W. Swatowski, C.M. Amb, M.G. Hyer, R.S. John, W. Ken Weidner, “Graded index silicone waveguides for high performance

computing,” in OIC, WD2, San Diego (2014).

[11] A. Hashim, N. Bamiedakis, R.V. Penty and I.H. White, “Multimode Polymer Waveguide Components for Complex On-Board

Optical Topologies”, in Journal of Lightwave Technology, Vol. 31, no. 24, pp. 3962–3969 (2013).

Thank you !