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High-Speed Circuits & Systems Lab.Dept. of Electrical and Electronic Engineering
Yonsei University
Silicon Optical Modulator
Silicon Optical Photonics Nature Photonics Published online: 30 July 2010
Byung-Min Yu
24 April 2014
2/20 pageYonsei University
Content
1. Introduction
2. Modulation method
3. Performance metrics
4. State-of-art device
5. Conclusion
3/20 pageYonsei University
Introduction
From (www.bretswanson.com)
- Large loss in high frequency
- Cross talks
- Bulky size
Electrical interconnect
- Small loss in high frequency
- EMI insensitive
- Small size
Optical interconnect
4/20 pageYonsei University
Introduction
From (http://citrix.cleanrooms.com/index/packaging/packaging-blogs/ap-blog-display._archives.201104.blogs.ap-blog.html)
Increasing demands for interconnect speed in short-range
communication Optical interconnect !
5/20 pageYonsei University
Introduction
(From http://www.cs.utk.edu/~dongarra/ccgsc2006/Slides)
Silicon photonics: integrate optical devices and electronic circuits on a single chip with SOI (Silicon-On-Insulator) process
6/20 pageYonsei University
Kinds of optical modulator
- Low sensitivity
- Large hard to integrate
(From http://www.cs.utk.edu/~dongarra/ccgsc2006/Slides)
~60 m
MZI Modulator
- High sensitivity
- Small easy to integrate
Ring Modulator
7/20 pageYonsei University
Modulation method
Modulation method
Electro-refractive or electro-absorptive modulation
Electric field: real part (∆n) and imaginary part (∆ ) change in material
1. Pockels effect, Kerr effect (∆n): Refractive index change depending on electric field in semi-conductor
2. Franz-Keldysh effect (∆ ) : Optical absorption change depending on electric field in semi-conductor
3. Thermo-optic effect (∆ ) : Refractive index change depending on temperature
4. Free carrier plasma dispersion effect (∆n): Refractive index change depending on free carriers in material
8/20 pageYonsei University
Modulation method
Free carrier plasma dispersion effect
∆ = −8.8 × 10 × ∆ − 8.5 × 10 × ∆ . = 1550∆ = 8.5 × 10 × ∆ + 6.0 × 10 × ∆ = 1550
9/20 pageYonsei University
Modulation method
Electrical manipulation of the charge density
Carrier accumulation Carrier injection Carrier depletion
1. Carrier accumulation: capacitor structure
2. Carrier injection: p-i-n structure, forward bias
3. Carrier depletion: p-n structure, reverse bias (width of depletion region change)
10/20 pageYonsei University
Performance metrics
Kinds of performance metrics
1. Modulation speed or bandwidth
2. Modulation depth (extinction ratio, on/off ratio)
3. Insertion loss
4. Power consumption
5. Footprint (device size)
6. Optical bandwidth
7. Temperature
11/20 pageYonsei University
Performance metrics
Modulation speed & electro-optic bandwidth
- Electro-optic bandwidth: defined by 3dB bandwidth
- Modulation speed: eye opening
- High modulation speed is imperative for optical interconnect
3GHz optical modulator 10Gb/s eye diagram
12/20 pageYonsei University
Performance metrics
Modulation depth & Insertion loss
- Modulation depth: >7dB for interconnect application, 4~5dB for overall system
- Insertion loss: passive loss because of reflection, absorption and mode coupling
Nor
mal
ized
Tra
nsm
itted
Pow
er
Wavelength [ m]
= 0 V
= -2 V
on/off ratio
Insertion loss
13/20 pageYonsei University
Power consumption & Footprint
- Power consumption: commonly small compare to electrical circuits! optical interconnect target: electrical interconnect(~1 pJ/bit) modulator target: ~10 fJ/bit
- MZI modulator: ~5 pJ/bit @10 Gb/s, 200-μm-long device
- Footprint
- MZI modulator: about hundreds or thousands of μm
- Ring modulator: about dozens of μm
Performance metrics
14/20 pageYonsei University
Optical bandwidth & Temperature
- Optical bandwidth: operational wavelength range of a device
- MZI modulator: relatively large bandwidth (~20 nm)
- Ring modulator: relatively small bandwidth (~100 pm)
- Temperature: device characteristics change due to temperature
- Ring modulator is relatively large temperature sensitive
1. Thermo-electric controller (±1℃) must be needed (power consumption)
2. Temperature robust device (multiple rings, ring coupled to and MZI)
3. Electro absorption device (low speed)
Performance metrics
15/20 pageYonsei University
Trade off in performance metrics
Reducing footprint
- Reducing power consumption, drive signal loss
- Phase shift reduce in MZM, reducing modulation depth
Multiple ring modulator
- Reduce optical bandwidth and thermal sensitivity
- Complexity increased, large footprint and power consumption
Speed increased (MRM)
- Modulation depth decreased
16/20 pageYonsei University
State-of-art devices• Research start of silicon optical modulator: mid-1980s
• Carrier injection
• p-i-n structure with carrier injection device is usually used (very slow)
• mid-2000s: GHz optical modulator (optimizing and reducing structure)
• Carrier accumulation
• 2004: >1GHz carrier accumulation device is developed (Intel)
• Optimized to ~10GHz and 3.8dB E.R. (2005) & ~10GHz and 9dB E.R. (2009)
• ! Resistance and capacitance is major speed limit rather than minority carrier life time
• Carrier depletion
• Theoretically ~50GHz 3dB bandwidth
• 2007: 30Gb/s optical modulator is developed (Intel)
Improved to 40Gb/s data transmission
17/20 pageYonsei University
State-of-art devices
• 2005: High-speed ring modulator was first introducedCarrier injection based ring modulator (~1.5 Gb/s) ~10 GHz using pre-emphasis driving signal
• Carrier depletion based ring modulator (>35 GHz)
18/20 pageYonsei University
State-of-art devices
19/20 pageYonsei University
Conclusion
1. Modulator type
1) MZI based modulator
2) Ring resonator based modulator
2. Modulation method
1) Pockels effect, Kurr effect
2) Franz-Keldysh effect
3) Thermo-optic effect
4) Free carrier plasma dispersion effect
3. Electrical carrier change
1) Carrier accumulation
2) Carrier injection
3) Carrier depletion
4. Performance metrics
- Modulation speed, Modulation depth, Insertion lossPower consumption, Foot print, Optical bandwidth, Temperature
20/20 pageYonsei University
Paper review
Silicon Optical Modulator, Nature photonics (2010)
Byung-Min Yu
dbqudals1989@gmail.com
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