Low-Loss and High-Bandwidth

Multimode Polymer Waveguide Components

Using Refractive Index Engineering

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

Conference on Lasers and Electro-Optics (CLEO 2016)

June 6th, 2016

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Multimode Polymer Waveguides

• Waveguide Components

o 90° Bends

o 90° Crossings

• Conclusions

1

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.

2

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

3

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

4

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)

high-bandwidth components required !

D. M. Kuchta, et al., IEEE JLT, 2015.

5

Tx 1

Tx 2

Tx 3

Tx 4

Rx 1

Rx 2

Rx 3

Rx 4 90 bends

90 crossing

Multimode Waveguide Components

Tx 1

Tx 2

Tx 3

Tx 4

Rx 1

Rx 2

Rx 3

Rx 4 90 bends

90 crossing

Passive multimode waveguide components enable on-board routing flexibility and

advanced topologies:

90° crossing 90° bend S bend Y splitter

Elementary waveguide

components in complex

interconnection architectures

Components designed and fabricated:

- Waveguide crossings

- Bent waveguides: 90° bends, S bends

- Y-splitters/combiners

N. Bamiedakis et al., IEEE JQE, vol. 45, pp. 415-424, 2009.

J. Beals IV et al., Appl Phys A, vol. 95, pp. 983–988, 2009. 6

Interconnection Architectures

Waveguide crossings and bends elementary components in complex architectures

- meshed waveguide architecture: 1 Tb/s capacity optical backplane 100×10 Gb/s

links

- regenerative optical bus architecture 4 x 10 Gb/s links

10 × 10 cm2 FR4:

100 90° bends

~1800 90° crossings

5 × 9 cm2 FR4:

24 90° bends

36 90° crossings

8 S-bends

low-loss components required !

J. Beals, et al., Appl. Phys. A, vol. 95, pp. 983-988, 2009.

N. Bamiedakis et. al, IEEE JLT, vol. 32, pp. 1526-1537, 2014. 7

Waveguide Components

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

B

Length: ~137 mmA B

WG length: 16.25 cm

WG01 WG02 WG03

x(m)

y(

m)

-30 -20 -10 0 10 20 30-30

-20

-10

0

10

20

30

1.515

1.52

1.525

1.53

x(m)

y(

m)

-30 -20 -10 0 10 20 30-30

-20

-10

0

10

20

30

1.512

1.514

1.516

1.518

1.52

1.522

x(m)

y(

m)

-30 -20 -10 0 10 20 30-30

-20

-10

0

10

20

30

1.515

1.52

1.525

1.53

WG01 WG02 WG03

nmax 1.532 1.522 1.531

∆n 0.02 0.01 0.019

Height (μm) 37 53 48

Width (μm) 32 50 29

Radius: 5, 6, 8, 11, 15 and 20 mm

A BLength: ~137 mm

ALength: ~137 mm

BNumber of crossings: 1, 5, 10, 20, 40 and 80

B

Length: ~137 mmA B

Parameter WG01 WG02 WG03

max Δn 0.020 0.010 0.019

Size (µm2) 35 × 4055 × 5632 × 53

reference WGs 90° bends 90° crossings

- Components with different RI profiles and dimensions are fabricated and tested:

8

- Bends and crossings exhibit differing behaviours with respect to index step Δn:

bends benefit from larger Δn values (better light confinement)

crossings exhibit lower loss for smaller Δn values design trade-off

∆tin∆tout

Input pulse Output pulse1. Short pulse generation system

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

9

Reference Waveguides

- Insertion loss and bandwidth of reference waveguide measured under:

a restricted launch: 9/125 μm SMF input (loss) or 10× lens input (BW)

a 50/125 μm MMF input (likely encountered in a real-world system)

- restricted launch: similar insertion loss values (~1 dB) and large BLP (> 100 GHz×m)

- 50 μm MMF input: WG02 largest IL (~ 3 dB) but larger BLP (122 GHz×m) due to

smaller Δn value (0.01 vs 0.02)

B

Length: ~137 mmA B

WG length: 16.25 cm

10

Bends and Crossings

- Insertion loss the waveguide components measured under:

a restricted launch: 9/125 μm SMF input (loss)

a 50/125 μm MMF input (likely encountered in a real-world system)

- crossing loss (XL) and bending loss (BL) obtained by normalising with respect to the

insertion loss of the reference waveguides

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80

Cro

ssin

g lo

ss (d

B)

Number of crossings

WG_A: 9 μm SMF

WG_Α: 50 μm MMF

WG_Β: 9 μm SMF

WG_Β: 50 μm MMF

WG_C: 9 μm SMF

WG_C: 50 μm MMF0

1

2

3

4

5

6

7

8

5 8 11 14 17 20B

en

din

g lo

ss (d

B)

Radius (mm)

WG01: 9 μm SMF

WG01: 50 μm MMF

WG02: 9 μm SMF

WG02: 50 μm MMF

WG03: 9 μm SMF

WG03: 50 μm MMF

- WG02 largest BL : R > 10 mm for 1 dB BL

but smallest XL: 0.02 dB/crossing for a 50 μm MMF input, < 0.01 dB/crossing for a

SMF input

- similar BL for WG01 and WG03: R > 6 mm for 1 dB BL, XL of WG01 worse

11

Result Summary

- Optimisation of the total loss performance

depends on particular waveguide layout,

launch conditions and BW requirements !!

- example on passive optical backplane:

- worst-case optical path (in red) :

1 bend and 90 crossings

Performance metric WG01 WG02 WG03

SM

F in

pu

t

IL ref. WGs 1.1 1.5 1.0

XL (dB/crossing) 0.093 0.007 0.033

Radius for BL<1 dB (mm) > 6 > 10 > 6

BLP ref. WGs (GHz×m) 107 154 125

50

µm

MM

F in

pu

t

IL ref. WGs 1.6 3.2 1.7

XL (dB/crossing) 0.099 0.019 0.046

Radius for BL<1 dB (mm) > 6 > 11 > 6

BLP ref. WGs (GHz×m) 47 122 48

For a 50 μm MMF input:

- assuming enough area for R = 12 mm

WG 02 , total loss ~ 6 dB

- assuming R = 8 mm

WG 03 , total loss ~ 6.2 dB

12

Conclusions

• Multimode polymer waveguides constitute an attractive technology for

use in board-level optical interconnects

• Waveguide bends and crossing are essential components in passive

interconnection architectures:

- optimisation of loss and BW performance is based on RI profile (index

step Δn and dimensions)

- depends on particular layout, BW requirements and launch conditions

• Low-loss and high-bandwidth (>47 GHz×m) multimode polymer

waveguide crossings (<0.02 dB/crossing) and bends (<1dB) are

demonstrated using refractive index engineering.

Acknowledgements:

13

References

[1]. 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).

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Transceiver Built With a Novel Integration Method," in IEEE Transactions on Components, Packaging and Manufacturing

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[3]. 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,” in Journal of

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

[4]. J. Beals, N. Bamiedakis, A. Wonfor, R. V. Penty, I. H. White, J. V. DeGrootJr., K. Hueston, T. V. Clapp, M. Glick, "A

terabit capacity passive polymer optical backplane based on a novel meshed waveguide architecture," in Applied Physics A:

Materials Science & Processing, Vol. 95, pp. 983-988 (2009).

[5]. N. Bamiedakis et al., "A 40 Gb/s Optical Bus for Optical Backplane Interconnections," in J. of Lightw. Techn., Vol. 32,

pp.1526-1537 (2014).

[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,

no. 0613 (2015).

[7] 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).

14

Thank you !

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