MICRO‐RING MODULATOR

Dae-hyun Kwon

High-speed circuits and Systems Laboratory

Paper preview

Title of the paper– Low Vpp, ultralow-energy, compact, high-speed silicon electro-optic

modulator Publication Journal

– Optics express

Contents

Theory– Plasma dispersion effect

Type of modulators– Mach-Zehnder Interferometer(MZI) modulator– Micro-ring modulator

Structure

Design consideration

Experimental results

Conclusion

Theory

Plasma dispersion effect– Change the concentration of free charges change the refractive index of the material

– Drude-Lorenz equation relating the concentration of electrons and holes to the absorption (m*ce=0.26m0, m*ch=0.39m0)

Types of Modulator

Mach-Zehnder interferometer(MZI) modulator– Two waveguide– Phase of signal converted by controlling the bias voltage

Micro-ring modulator– Bus waveguide + ring waveguide– Specific wavelength locked up in ring waveguide by bias voltage

Characteristic of MZI modulator

Changing phase on one or both arms At the output combiner, interferences occur:

– In phase: constructive interferences– Out of phase: destructive interferences

By converting bias voltage depletion width changed free carrier changed effective index changed(plasma dispersion effect) phase shift is done

Power consumption↑, Footprint↑

Characteristic of Micro‐ring modulator

Bus waveguide + ring waveguide Part of the incident beam is coupled into the ring and then specific wavelength

light is locked up in ring waveguide Ring waveguide PN junction

Bus waveguide

Ring waveguide

V

Bias voltage

Micro‐ring modulator

Condition: Light traveling the ring at its at its resonant wavelength must travel an integer number of wavelength m in one round trip Ltot:

Controlling reverse bias voltage Depletion width Tuning effective index modifying the resonant wavelength

Small Vpp for changing the resonant wavelength

Small footprint radius is related to wavelength

Ultra narrow operation BW sensitive to surroundings

mLkL tottot

20

0

MZI vs. Micro‐ring modulator

MZI modulator (a)Micro-ring

modulator (b)

Peak to peak voltage[v]

High (6.5) Low (2)

Power consumption[fJ/bit]

High (3x104) Low (50)

Footprint[㎛2]

Large (1x104) Small (1x103)

Reliability High Low

(a): “High-speed optical modulation based on carrier depletion in a silicon waveguide”, A.Liu(b): ” Low Vpp, ultralow-energy, compact, high-speed silicon electro-optic modulator”, Po Dong

Structure reverse biased micro‐ring modulator

Ring resonator coupled to a neighboring bus waveguide• WG: 500 nm (W), 250 nm (H), 50 nm (slab H) tight waveguide bend radii, down

to a few micros

Design consideration

Asymmetric pn junction optimizing the overlap between the optical mode and the depletion region (p doping: 5x1017 cm-3 , n doping: 1x1018 cm-3)

50 nm offset (pn junction position) hole concentration changes induce a larger index change

than electron concentration changes

Metal to silicon interface: Ohmic contact p++, n++ (1x1020 cm-3)

To overcome sensitivity to surroundings, added the on-chip heater High thermal tuning efficiency achievable

Experimental results

Extinction ratio = =15 dB

Quality factor ~ 14,500 Resonance shift: 18 pm/V Modulation efficiency (V·Lπ) ~ 1.5 V·cm At 1551.84 nm modulation depth = 6.5 dB, insertion loss = 2 dB

Experimental results

3dB modulation bandwidth:

RC: RC time constant

τ : cavity photon lifetime

Measured junction cap ~ 50fF, R of contact ~ 55 ohm 2πRC ~ 50GHz

Mainly limited by cavity photon lifetime: τ ~ 75 ps

Predicted BW ~ 13 GHz

22

2

3

)2()2(1

RCf dB

Experimental results

3 dB BW = 11 GHz PRBS: 223-1, Vpp = 2 V, DC bias = -1 V

Conclusion

The modulator in this paper is consist of ring resonator as an optical platform and realizes index modulation by using a reversed-biased lateral pn diode embedded in the ring

The sensitivity to surroundings can be solved by adding on-chip heater

The low insertion loss is achieved by no carrier-induced losses in the bus waveguide

A compact(1000㎛2), low loss(2 dB) and high-speed(11GHz) silicon electro-optic modulator with a very low Vpp(2V) and ultralow energy consumption (50fJ/bit) has been demonstrated

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