Silicon Optical Modulators

1
Silicon Optical Modulators

• Recent developments in fabrication of High Speed
  Modulators
• J
• ee290f

2
Outline

• Motivation
• Png, Reed, et al. work from Surrey.
• Shows basic principle and gives one of the two
  major design types
• Intel device
• History making device, designed and fabricated in
  alternate major design type.
• Conclusions

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Motivation

• Very clear Si modulators means CMOS integration
  and using all our experience in silicon
  micromachining.
• Unfortunately, prior to 2003 the fastest Si
  optical modulator was 20MHz (Lithium Niobate
  modulators are 10GHz).

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p-i-n Si Optical Modulators 1/4

• Follows the work presented by Png, Reed, et al
  from Surrey University (UK).

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p-i-n Si Optical Modulators 2/4
VD is applied to Anode vs. Cathode. ? forward
biased p-i-n junction. ? e and holes injected
into guiding region ? changes refractive index
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p-i-n Si Optical Modulators 3/4
?n ?ne ?nh -8.8 x 10-22(?Ne) 8.5 x
10-18(?Nh)0.8 ?? ??e ??h 8.5 x 10-18 (?Ne)
6.0 x 10-18(?Nh) From R.A. Soref B. R.
Bennett Electrooptical Effects in Silicon Jour.
Of Quan. Elec. 1987.
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p-i-n Si Optical Modulators 4/4

• Simulation results show modulation can be
  optimized to 1.3GHz and a 180? at .7 mA of
  current.
• However, performance is very dependant on doping
  profile and a critical dimensions are not very
  tolerant.
• As of late 2003, fabrication is underway.

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MOS Si Optical Modulators 1/4

• Still uses the plasma dispersion effect, but
  implements a MOS capacitor to induce change in
  free carrier density instead of a
• p-i-n device.

• Again designed for single mode 1.55?m.

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MOS Si Optical Modulators 2/4

• Apply VD to poly. Charge accumulation on both
  sides of gate oxide.
• ?Ne ?Nh ?/etoxt(VD VFB)
• ?ne -8.8 x 10-22?Ne
• ?nh -8.5 x 10-18(?Nh)0.8
• ?? (2?/?)?neffL

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MOS Si Optical Modulators 3/4

• Implemented phase shifter in both arms of a MZI.
• For VD 7.7V ? 16dB total switch.
• Loss is the big key 15.3dB insertion loss
  (4.3dB from coupling 6.7dB from poly guides).

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MOS Si Optical Modulators 4/4
Switching test on psuedo-random time signal.
See 3dB roll off at gt1GHz from MOS cap.
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Conclusion

• MOS modulator has poor loss figures and still an
  order of magnitude slower than commercial
  modulators. Intel argues these can both be
  theoretically fixed by decreasing the size of the
  device and using Si in the guide region instead
  of poly (still integrable?).
• P-i-n modulator is still being fabricated and
  depends on its optimal design for the high values
  achieved, so potentially success or failure from
  fabrication runs.