Silicon Optical ModulatorsSilicon Optical ModulatorsRecent developments in fabrication of Recent developments in fabrication of

High Speed ModulatorsHigh Speed Modulators

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OutlineOutline

MotivationMotivation Png, Reed, et al. work from Surrey.Png, Reed, et al. work from Surrey.

Shows basic principle and gives one of Shows basic principle and gives one of the two major design typesthe two major design types

Intel deviceIntel device History making device, designed and History making device, designed and

fabricated in alternate major design fabricated in alternate major design type.type.

ConclusionsConclusions

MotivationMotivation

Very clear: Si modulators means Very clear: Si modulators means CMOS integration and using all our CMOS integration and using all our experience in silicon experience in silicon micromachining.micromachining.

Unfortunately, prior to 2003 the Unfortunately, prior to 2003 the fastest Si optical modulator was fastest Si optical modulator was ~20MHz (Lithium Niobate modulators ~20MHz (Lithium Niobate modulators are ~10GHz).are ~10GHz).

p-i-n Si Optical Modulators 1/4p-i-n Si Optical Modulators 1/4

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

p-i-n Si Optical Modulators 2/4p-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

p-i-n Si Optical Modulators 3/4p-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.

p-i-n Si Optical Modulators 4/4p-i-n Si Optical Modulators 4/4

Simulation results show modulation Simulation results show modulation can be optimized to 1.3GHz and a can be optimized to 1.3GHz and a 180180 at .7 mA of current. at .7 mA of current.

However, performance is very However, performance is very dependant on doping profile and a dependant on doping profile and a critical dimensions are not very critical dimensions are not very tolerant.tolerant.

As of late 2003, fabrication is As of late 2003, fabrication is underway.underway.

MOS Si Optical Modulators 1/4 MOS Si Optical Modulators 1/4

Still uses the plasma Still uses the plasma dispersion effect, but dispersion effect, but implements a MOS implements a MOS capacitor to induce capacitor to induce change in free carrier change in free carrier density instead of a density instead of a

p-i-n device.p-i-n device.

Again designed for Again designed for single mode single mode 1.551.55m.m.

MOS Si Optical Modulators 2/4MOS Si Optical Modulators 2/4Apply VApply VDD to poly. Charge accumulation on both sides of gate oxide. to poly. Charge accumulation on both sides of gate oxide.

NNee = = NNhh = [ = [/et/etoxoxt]*(Vt]*(VDD – V – VFBFB))

nnee = -8.8 x 10 = -8.8 x 10-22-22NNee

nnhh = -8.5 x 10 = -8.5 x 10-18-18((NNhh))0.80.8

= (2= (2//))nneffeffLL

MOS Si Optical Modulators 3/4MOS Si Optical Modulators 3/4 Implemented phase Implemented phase

shifter in both arms of a shifter in both arms of a MZI.MZI.

For VFor VDD = 7.7V = 7.7V → 16dB → 16dB total switch.total switch.

Loss is the big key: Loss is the big key: ~15.3dB insertion loss ~15.3dB insertion loss (~4.3dB from coupling & (~4.3dB from coupling & ~6.7dB from poly guides).~6.7dB from poly guides).

MOS Si Optical Modulators 4/4MOS Si Optical Modulators 4/4

See 3dB roll off at >1GHz from MOS cap.

Switching test on psuedo-random time signal.

ConclusionConclusion

MOS modulator has poor loss figures and MOS modulator has poor loss figures and still an order of magnitude slower than still an order of magnitude slower than commercial modulators. Intel argues commercial modulators. Intel argues these can both be theoretically fixed by these can both be theoretically fixed by decreasing the size of the device and decreasing the size of the device and using Si in the guide region instead of poly using Si in the guide region instead of poly (still integrable?).(still integrable?).

P-i-n modulator is still being fabricated and P-i-n modulator is still being fabricated and depends on its optimal design for the high depends on its optimal design for the high values achieved, so potentially success or values achieved, so potentially success or failure from fabrication runs.failure from fabrication runs.