dispersion studies on multimode polymer spiral waveguides for board-level optical interconnects

Dispersion Studies on Multimode Polymer Spiral

Waveguides for Board-Level Optical Interconnects

Jian Chen, Nikos Bamiedakis, Richard V. Penty, Ian H. White

Electrical Engineering Division, University of Cambridge, UK

e-mail: [email protected]

Tom J. Edwards, Christian T.A. Brown

School of Physics & Astronomy, University of St Andrews, UK

Acknowledgement:

The authors would like to acknowledge Dow Corning for providing the waveguide samples and EPSRC for supporting the work.

OPTICAL INTERCONNECTS CONFERENCE 2015

20 April 2015

Outline

• Introduction to Optical Interconnects

• Board-level Optical Interconnects

• Bandwidth Studies

• Conclusions

Why Optical Interconnects?

Electrical Interconnects:

• Limited bandwidths;

• Increasing losses;

• Higher crosstalk;

Optical Interconnects:

• Lower losses at high data rates;

• Lower electromagnetic interference;

• Higher power efficiency;

• Density advantages.

Growing demand for data communications link capacity in:

- data centres

- supercomputers

need for high-capacity short-reach interconnects operating at > 10 Gb/s

Evolution of Optical Interconnects

• Optical interconnects will be employed in shorter and shorter links to

meet the bandwidth and power efficiency requirements.

Board Level

[1] A.F. Benner et al, Exploitation of optical interconnects in future server architectures, IBM Journal of Research and

Development, vol 49, Issue 4.5, 2005.

[1]1980’s 1990’s 2000’s > 2012

Outline




• Conclusions

Board-level Optical interconnects

Optics is gradually working in conjunction with electronics for future

communication technologies, however it needs to meet the key

technological requirements at board-level interconnects:

• Cost effectiveness;

• Ability to be integrated into existing architectures;

• Compatibility with existing manufacturing processes of conventional electronic

circuitry.

Polymer Multimode Waveguides

1. Polymer Materials

• Sufficiently low-cost;

• Very low intrinsic attenuation (0.03–0.05 dB/cm at 850 nm);

• Good thermal and mechanical properties (up to 350 °C);

• Fabricated on FR4, glass or silicon using standard techniques such as photolithography

and embossing.

2. Multimode Waveguide

• Cost-efficiency: relaxed alignment tolerances (> ± 10 µm).

Outline




• Conclusions

VCSEL Performance

Continuous improvement in bandwidth performance of VCSELs:

850 nm VCSELs:

44 Gb/s (2012), 57 Gb/s (2013) and 64 Gb/s (OFC 2014, Chalmers - IBM)

performance in longer wavelengths follows same trend

- un-cooled operation up to 90°C: (50 Gb/s Chalmers-IBM, 2014)

- VCSEL arrays with very good uniformity and similarly high bandwidth

[2] P. Westbergh, et al., IEEE PTL, vol. 27, pp. 296-299, 2015

Why do we study the bandwidth of multimode polymer waveguides?

their highly-multimoded nature raises important concerns about their bandwidth

limitations and their potential to support very high on-board data rates.

Frequency Response Measurements

quasi-overfilled 50/125 µm MMF input “overfilled” 100/140 µm MMF input

-3 dB frequency response >35 GHz for all inputs and input positions

suitable for high-speed transmission of ≥ 40 Gb/s data

- results from more overfilled launches into the 1 m long spiral waveguide

50 µm

100 µm

BW > 35 GHz x m

[3] N. Bamiedakis, et al., IEEE JLT, vol. 33, pp. 1-7, 2015

So, what are the bandwidth limits of these particular waveguides ?

time domain measurements

Time Domain Measurements

Back-to-back link

Link with the waveguide

• Different launch conditions (50 μm MMF with and 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.

Short pulse laser

Autocorrelatorx10 x16Cleaved 50 μm MMF

MM

Mode mixer

Short pulse laser

Autocorrelatorx10 x16Cleaved 50 μm MMF

MM

Mode mixer

Bandwidth Estimation

1. Ti:Sapphire laser emitting at 850 nm

input pulse width ~ 250 fs (autocorrelation trace)

2. 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, sech^2 or Lorentzian.

4. Bandwidth calculation:

(a) Calculate the frequency response of the waveguide

Frequency response|WG (dB) = frequency response|system (dB) – frequency response|b2b (dB)

(b) Find the 3dB bandwidth of the WG frequency response.

13

1 m Long Spiral Multimode Waveguide

Our studies show that multimode large waveguides can operate at

higher speed than what people conventionally thought.

(a) the 1 m long spiral waveguide illuminated with red light and facet of the

(b) SI and (c) GI waveguide illuminated with 850 nm light.

32 μm

32μ

m

32 μm

35μ

m

(a) (b)

Experimental Bandwidth Results

-25-20-15-10 -5 0 5 10 15 20 2530

40

50

60

70

80

90

100

Horizontal offset (m)Bandw

idth

-length

pro

duct

(GH

zm

)

-7

-6

-5

-4

-3

-2

-1

0

Norm

alis

ed r

eceiv

ed p

ow

er

(dB

)

-25-20-15-10 -5 0 5 10 15 20 2530

40

50

60

70

80

90

100


idth

-length

pro

duct

(GH

zm

)

-7

-6

-5

-4

-3

-2

-1

0

Norm

alis

ed

rece

ive

d p

ow

er

(dB

)

-25-20-15-10 -5 0 5 10 15 20 2530

40

50

60

70

80

90

100


idth

-length

pro

duct

(GH

zm

)

-7

-6

-5

-4

-3

-2

-1

0

Norm

alis

ed

rece

ive

d p

ow

er

(dB

)

-25-20-15-10 -5 0 5 10 15 20 2530

40

50

60

70

80

90

100


idth

-length

pro

duct

(GH

zm

)

-7

-6

-5

-4

-3

-2

-1

0

Norm

alis

ed

rece

ive

d p

ow

er

(dB

)

SI WG

GI WG

50 μm MMF: no MM 50 μm MMF: with MM

Estimated bandwidth:

SI: 30 – 60 GHz

GI: 50 – 90 GHz

mode mixer:

lower bandwidth

smaller variation

across offsets

Bandwidth Discussion

16

- Why such a good bandwidth performance ?

some explanations (more quantitative details to be reported soon)

1. fabrication effects:

- “SI”-index waveguides might not be strictly-speaking “SI”

some variation in index profile across waveguide cross section

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

dispersion engineering

using fabrication

effect important in MMFs

Outline




• Conclusions

Conclusions

• Multimode polymer waveguides constitute an attractive technology for

use in board-level optical interconnects

• Bandwidth estimation of multimode WGs can be challenging

depends on launch conditions, WG parameters, fabrication and layout

• Time domain measurements on 1 m long spiral waveguides

worst-case BW > 30 GHz for “SI” waveguides (± 10 μm)

worst-case BW > 50 GHz for “GI” waveguides (± 10 μm)

suitable for very high-speed transmission !

References

[1] N. Bamiedakis, J. Chen, R. Penty, and I. 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.

[2] 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.

[3] D. Kuchta, et al., "64 Gb/s Transmission over 57m MMF using an NRZ Modulated 850nm

VCSEL," in Optical Fiber Communication Conference (OFC), pp. 1-3, 2014.

[4] 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.

[5] B. W. Swatowski, C. M. Amb, M. G. Hyer, R. S. John, and W. K. Weidner, "Graded Index

Silicone Waveguides for High Performance Computing," in IEEE Optical Interconnects

Conference (OIC), pp. 1-3, 2014.

Thank you !