Silicon Optical Modulator Silicon Optical Photonics Nature Photonics Published online: 30 July 2010 Byung-Min Yu 24 April 2014 High-Speed Circuits & Systems Lab. Dept. of Electrical and Electronic Engineering
Content 1. Introduction 2. Modulation method 3. Performance metrics 4. State-of-art device 5. Conclusion 2/20 page
Introduction From (www.bretswanson.com) Electrical interconnect - Large loss in high frequency - Cross talks - Bulky size Optical interconnect - Small loss in high frequency - EMI insensitive - Small size 3/20 page
Introduction From (http://citrix.cleanrooms.com/index/packaging/packaging-blogs/ap-blogdisplay._archives.201104.blogs.ap-blog.html) Increasing demands for interconnect speed in short-range communication Optical interconnect! 4/20 page
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 5/20 page
Kinds of optical modulator ~60 m (From http://www.cs.utk.edu/~dongarra/ccgsc2006/slides) MZI Modulator Ring Modulator - Low sensitivity - Large hard to integrate - High sensitivity - Small easy to integrate 6/20 page
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 7/20 page
Modulation method Free carrier plasma dispersion effect = 8.8 10 8.5 10. = 1550 = 8.5 10 +6.0 10 = 1550 8/20 page
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) 9/20 page
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 10/20 page
Performance metrics Modulation speed & electro-optic bandwidth 3GHz optical modulator - Electro-optic bandwidth: defined by 3dB bandwidth - Modulation speed: eye opening - High modulation speed is imperative for optical interconnect 10Gb/s eye diagram 11/20 page
Performance metrics Modulation depth & Insertion loss Normalized Transmitted Power = 0 V = -2 V Insertion loss on/off ratio Wavelength [ m] - Modulation depth: >7dB for interconnect application, 4~5dB for overall system - Insertion loss: passive loss because of reflection, absorption and mode coupling 12/20 page
Performance metrics 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 13/20 page
Performance metrics 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) 14/20 page
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 15/20 page
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 16/20 page
State-of-art devices 2005: High-speed ring modulator was first introduced Carrier injection based ring modulator (~1.5 Gb/s) ~10 GHz using pre-emphasis driving signal Carrier depletion based ring modulator (>35 GHz) 17/20 page
State-of-art devices 18/20 page
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 loss Power consumption, Foot print, Optical bandwidth, Temperature 19/20 page
Paper review Silicon Optical Modulator, Nature photonics (2010) Byung-Min Yu dbqudals1989@gmail.com 20/20 page