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

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

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 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.

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Experimental Results

Waveguide Modelling

• Conclusions

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 undertaken32 µm

50

µm

- 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 pulse1. 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 1012

-20

-17

-14

-11

-8

-5

-2

0

Frequency (Hz)

Inte

nsity (

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)

Ba

nd

wid

th-l

en

gth

pro

du

ct

(GH

zm

)

10x lens

50 m MMF

50 m MMF + MM

50

75

100

125

150

Ba

nd

wid

th-l

en

gth

pro

du

ct

(GH

zm

)

32 μm

36μ

m

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 15 200

50

100

150

200

250

300

350

400

Ba

nd

wid

th-l

en

gth

pro

du

ct

(GH

zm

)

Horizontal offset (m)

Experiment

Simulation

10x lens

-20 -15 -10 -5 0 5 10 15 200

25

50

75

100

125

150

Ba

nd

wid

th-l

en

gth

pro

du

ct

(GH

zm

)Horizontal offset (m)

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.

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

Experimental Results

Waveguide Modelling

• 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

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).

Thank you !