MICRO RING MODULATOR. Dae-hyun Kwon. High-speed circuits and Systems Laboratory

Transcription

1 MICRO RING MODULATOR Dae-hyun Kwon High-speed circuits and Systems Laboratory

2 Paper preview Title of the paper Low Vpp, ultralow-energy, compact, high-speed silicon electro-optic modulator Publication Journal Optics express

3 Contents Theory Plasma dispersion effect Type of modulators Mach-Zehnder Interferometer(MZI) modulator Micro-ring modulator Structure Design consideration Experimental results Conclusion

4 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)

5 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

6 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

7 Characteristic of Micro ring modulator Bus waveguide Ring waveguide V Bias voltage 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

8 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 L tot : Ltot k 0 L 2 0 m Controlling reverse bias voltage Depletion width Tuning effective index modifying the resonant wavelength tot Small Vpp for changing the resonant wavelength Small footprint radius is related to wavelength Ultra narrow operation BW sensitive to surroundings

9 MZI vs. Micro ring modulator MZI modulator (a) Micro-ring modulator (b) Peak to peak voltage [v] Power consumption [fj/bit] Footprint [ μm 2 ] High (6.5) Low (2) High (3x10 4 ) Low (50) Large (1x10 4 ) Small (1x10 3 ) 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

10 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

11 Design consideration Asymmetric pn junction optimizing the overlap between the optical mode and the depletion region (p doping: 5x10 17 cm -3, n doping : 1x10 18 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++ (1x10 20 cm -3 ) To overcome sensitivity to surroundings, added the on-chip heater High thermal tuning efficiency achievable

12 Experimental results Extinction ratio = =15 db Quality factor ~ 14,500 Resonance shift: 18 pm/v Modulation efficiency (V Lπ) ~ 1.5 V cm At nm modulation depth = 6.5 db, insertion loss = 2 db

13 Experimental results 3dB modulation bandwidth: RC: RC time constant (2 ) (2 RC ) 2 f3 db 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

14 Experimental results 3 db BW = 11 GHz PRBS: , V pp = 2 V, DC bias = -1 V

15 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 μm 2 ), low loss(2 db) and high-speed(11ghz) silicon electrooptic modulator with a very low Vpp(2V) and ultralow energy consumption (50fJ/bit) has been demonstrated

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