Post on 29-Jan-2022
transcript
YONSEI UNIVERSITY
YONSEI UNIVERSITY
25Gb/s 1V-driving CMOS ring modulator with
integrated thermal tuning
Kim, Sungjin
YONSEI UNIVERSITY
Contents
2
Introduction
Device design and fabrication
Device DC performance
Small-signal RF tests
High-speed data modulation
Conclusion
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Introduction
3
Carrier-depletion Si ring modulators with thermal tuning
Advantages
- High speed modulation
- consuming low energy for modulation
Disadvantages
- Additional power consumptions occur in thermal tuning
โ Advanced manufacturing techniques and higher modulation speed
can reduce this penalty
Interconnect power and density issues are the
main obstacles for supercomputing
Silicon photonics is candidates incoming
systems
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Waveguide structure
rib waveguide structure for applying electrical modulation
Tradeoff in modulator design
Minimize resistance and capacitance by thicker Si slab and smaller waveguide height
โ Increase optical loss and reduce optical confinement
Minimize optical bending loss with wider waveguide width
โ Difficult to maintain single-mode operation
Device design and fabrication
4
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Ring waveguide dimensions
- 7.6ฮผm ring radius to achieve 12.8nm of free spectral range for 8-wavelength-channel WDM link
- 380nm wide ring waveguide with 220nm etch depth and 80nm thick Si slab
- 300nm wide bus waveguide
PN junction doping
- symmetric lateral PN junction
- vertically uniform doping
- laterally increasing density from junction
- to achieve small capacitance, resistance, optical loss
- caution with impurities diffusion occurred during implantations
and multiple CMOS thermal cycles
Device design and fabrication
5
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Doping profile
The impurity densities decrease linearly from ๐๐ to zero at the center of the junction
PN junction doping determines the depletion width
โ impact to the modulation depth and optical loss
โ determines the quality factor and modulation bandwidth
Impact of the junction doping to the modulation depth
โ๐๐ ๐ ๐ ๐ ๐ โ๐๐น๐น๐น๐น
โ ๐๐1 3โ
๐ผ๐๐๐+๐ผ๐๐ ๐ ๐๐ , ๐ค๐ค๐ค๐ค๐ค ๐ผ๐๐๐ โ ๐๐
โ๐๐ ๐ ๐ ๐ ๐ โถ ๐ก๐ค๐ค ๐ค๐ค๐๐๐๐๐๐๐ค ๐ค๐๐ค๐ค๐ค๐ค๐๐ค๐ก๐ค ๐๐ค๐ ๐ ๐ก ๐ข๐๐ข๐ค๐ค ๐ค๐๐ค๐ก๐๐ค๐ค ๐๐ค๐ ๐๐ค
ฮ๐๐น๐น๐น๐น โถ ๐ ๐ข๐ค๐ค ๐ค๐ ๐ข๐ก๐ค ๐๐ก ๐ค๐๐ค๐ ๐๐๐๐ ๐๐ข๐ ๐๐ ๐ก๐ค๐ค ๐ค๐ค๐๐๐๐๐๐๐ค ๐ค๐ ๐ข๐ก๐ค
๐ผ๐๐๐ โถ ๐ค๐๐๐ ๐๐๐ค๐ ๐ ๐ ๐๐ ๐ค๐๐ก ๐ข๐ข๐ค ๐ก๐ ๐๐ข๐๐๐ก๐ ๐๐ ๐ข๐๐๐ ๐๐ค,๐ผ๐๐ ๐ ๐๐ โถ ๐ค๐๐๐ ๐๐๐ค๐ ๐ ๐ ๐๐ ๐ค๐๐ก ๐ข๐ข๐ค ๐ก๐ ๐๐ก๐ค๐ค๐ค ๐๐ค๐๐ค๐๐๐ ๐๐๐
- to maximize modulation depth, ๐ผ๐๐๐ = 0.5๐ผ๐๐ ๐ ๐๐
- typical ๐ผ๐๐ ๐ ๐๐ is 6dB/cm, which requires junction doping of 3 ร 1017๐๐โ3
Device design and fabrication
6
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Modulation bandwidth
Bandwidth is proportional to the waveguide loss
๐ ๐ = ๐๐ผ ๐๐๐โ
- maximized modulation depth results in small bandwidth of 5GHz
- to achieve higher modulation bandwidth, increase the junction doping density
and sacrifice the modulation depth
- to have ๐ ๐ > 25๐บ๐บ๐บ
๐ < 8000
๐๐ ๐ ๐ ๐ ๐ > 40๐๐
๐๐๐ข๐ข๐ค๐๐ก๐๐ค ๐๐ค๐๐๐ค๐ก๐ > 9๐ข๐
Device design and fabrication
7
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Device layout
67% of the ring : PN diode for high-speed modulation with junction doping of 3 ร 1018๐๐โ3
Upper-right 25% : N-type doped as a Si resistor for thermal tuning
Between the PN diode and the thermal resistor section : undoped waveguide
2๐๐ wide isolation gaps
Device design and fabrication
8
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DC performance
The resonance wavelength shift โ ๐๐ โ ๐ 2 3โ
Positive bias beyond 0.5V will make the diode close to turn-on condition
26pm resonance shift is achieved with voltage swing from -0.5V to 0.5V
Device DC performance
9
< Resonant spectrum with different voltages applied >
< Resonance wavelength shift at different bias voltages >
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DC performance
750ฮฉ of Si resistor
Tuning efficiency of 0.19nm/mW
66mW tuning power is needed to tune the 12.6nm FSR
Device DC performance
10
< Resonant spectrum with different tuning power applied >
< Resonance wavelength versus tuning power >
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Circuit model
High-speed behavior of the ring modulator can be approximated by circuit model
๐ถ๐ : capacitance between the electrodes through top dielectrics
๐ ๐ and ๐ถ๐ฝ : model the current path through the reverse-biased PN junction
๐ ๐๐ and ๐ถ๐๐ : model the current path through the BOX and the Si handle
Based on the circuit model, modulation energy is 7fJ/bit with 25Gb/s pseudo-random data
Small-signal RF tests
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< fitted circuit model > < curve-fitting using the circuit model>
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Experimental results
High-speed data modulation
12
Eye-diagram with 25Gb/s modulation 1Vpp voltage swing
>5dB extinction ratio
Laser wavelength is tuned over the
5.3nm
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Demonstrated an error-free 25Gb/s tunable carrier-depletion ring
modulator with 7.5ฮผm radius
Small junction capacitance leads to 7fJ/bit modulation energy with 1V
driving at 25Gb/s
The carrier-depletion Si ring modulators can enable ultrahigh-speed
and ultralow-power WDM photonic links with compact footprint
Conclusion
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