Namkyoo Park Nanoscale Energy Conversion and Information Processing Devices

1. Next Generation Optical Amplifiers requirements, bottlenecks, possible resolutions(Approach concerned on Cost, Footprint , Functionalityrather than Efficiency, utilizing nano-photonics) Namkyoo Park Nanoscale Energy Conversion and Information Processing Devices September 24 th, 2006 Photonic Systems Laboratory School of EE, Seoul National University http://stargate.snu.ac.kr nkpark@snu.ac.kr Photonic Systems Lab School of EECS, S.N.U.

2. Research Topics in PSL : Past / Present • Photon Generation • Raman Amplifier • Erbium Amplifier • Thulium Amplifier • nano-Photonics : Er / Raman based Si Amplifier / Laser) This presentation • Photon Transport • Transient control & amplified transmission line design • Polarization Mode Dispersion tolerant transmission format • Multi-level Optical Transmission • Photon Control – Coding, Detection, Logic • Optical Coding (CDMA, Noise reduction) • Super-resolution Techniques (2D / 3D Imaging) • Surveillance system for FTTH network • Distributed / Multi-port Temperature sensor • Semiconductor Amplifier & SOA based Logic Gates • Integration with / Applications to IT, BT & NT • Tunable Optical devices (including Photonic Crystals, MEMS) • Application to Medical-Photonics (3-D Tomography)

3. Long Haul More of the same (higher speed, more wavelength, longer reach…) Metro/Access will shape the next wave of innovative components Tunable, intelligent, distributed amplification The “siliconization” of photonics Drive scalable manufacturing and cost efficiency Push optics further into the network and ensure sustainable growth of the industry Network Evolution – Market Calls for METRO and Below Backbone Continent to Continent Coast to Coast all over Fiber at 10 Gbps & up Metro City to City-Town to Town all over Fiber at 1Gbps 10 Gbps Access To the optically Fibered World “First Mile / Last Mile” 56kbps 1 Gbps The First Mile LAN Desktop to Desktop – Floor to Floor 10 Mbps 1 Gbps Jonathan Thatcher, OFC2002, Tutorial Sessions(2002)

4. Network Evolution – Challenges in Technology OITDA2000

5. Network Evolution – Challenges : Met for METRO network Now in the Market ! (2002)

6. Network Evolution – Beyond Metro : How far ? You will need more photons for Your desktop PC / Processors Electronics-photonics must converge !

7. WDM Two pillars of information revolution: Si IC & photonics “A chip that can transfer data using laser light”, NYT, 20060918 • By Intel and UC-Santa Barbara “$6 million project to develop silicon-based laser”, 20060804 • By MIT, the Microphotonics Center from US DoD • Using nanocrystalline silicon as an sensitizer for Er “Electronics-photonics must converge”, MIT, 20050520 • By MIT from 3-year study vs

8. Optical Sub-Assembly Electronics : Vacuum tube Transistor IC VLSI Common User $’s per 0.1M 100’s M Gb Memory TO OPEN THE PHOTONIC AGE, compatible to that of semiconductor industry, Need Cost reduction Need Smaller Footprint Need Integrated functionality Need optical power lines (amplification function) Electronics Sub-Assembly Zolo Technologies Status of Photonics – Compared to Electronics • 25% of the material cost is in the package • Another 25% is in the assembly • Beyond automation and off-shore assembly • Packaging really hasn’t advanced much until recently • Costly 70’s-era technology • Poor signal integrity • Poor Thermal properties • Need : New materials, Athermal designs, and Packaging standards

9. Challenges and Promises • Challenges in achieving Photonic Age -- if it comes ^^; • Cost 10’s of $ • Footprint Size of PCMCIA • Functionality More than Serial integration • Promises made to meet above challenges, with some technologies • OEIC mostly for active devices Compound semiconductor • PLC mostly for passive devices Silica, Polymer • MEMS mostly for switching devices Silicon, Glass, etc.. • EDWA mostly for amplification devices Silica • Hybrids • Si-Photonics • Plasmonics • Ph-Xtals • ….. • …. • How far do we need to go ? • Is it really possible to meet these Promises ? • Let’s sit back and look at the status of Silica based technologies (EDWA / PLC )

10. Si Photonics – Optical Communication in the Chip • Leveraging the astronomical Si processing technology for photonics • Photonics based on Si-based materials and Si-compatible processes • Passives, Modulators, Detectors but still missing Photon Generators

11. Cost Not allowed by physics E D A High Power DFB PLC + EDA ?????? Gain E D F A PLC E D W A Integration / Functionality ?????? Discrete Integration Discrete components PLC Loss What are we missing ? - Functionality / Size • For Photonics, there is lack of optical power lines which is compatible to that of electrical PCB • For Increased data rates, we need more photon (per bit) : Photon generator (amplifier, laser) • For SOA, with its strong electron - electron interaction and high noise figure • For EDWA, true integration is impossible after certain level (e.g. Splitting OA) : Only serial

12. Challenges in Amplifier Technology OITDA2000 Gbps/user  need more photons/bit, but not with $1K/unit nor at current size !!

13. Amplifier evolution before millennium

14. Relative Cost (A.U.) 4.0 Tap Tap 3.0 Nortel Networks,1999 2.0 1.0 Pump laser diode Photodiode Gain media (whatever) 0 Photodiode 0 10 20 30 40 50 60 70 80 90 100 110 120 Bandwidth (nm) DGFF Isolator Pump MUX Isolator Amplifier – Bandwidth and Cost

15. Amplifiers – Any challenges left ? • Optical Amplifier now 40 + year old, mature technology • Researchers have touched most issues on amplifiers • Gain flattening • Transient • Temperature • Power Conversion efficiency • Noise, Scattering, Fiber structure, Host materials, Co-Dopants • Various types of OAs have been commercialized, by numerous vendors • EDFA • TDFA • Raman • Hybrid • EDWA • SOA (bulk, QW, QD) • Not much issues left for OAs, especially for LH, trunk line applications (personal opinion) • Let’s sit back and look at the Technology / Bottlenecks of OA for Metro and beyond

16. Er-doped Waveguide Gain Block Array Isolators Pump LD For N=8 : 80  1 part • Hybrid integrated actives • Photodiode • Pump laser die • Yields ? Delivery ? x N PD Status of Amplifier – for Metro and beyond, to FTTH Photonics : Different material / structure for each function, with high losses 1st Generation of Integration : Parallel (Laser, Detector, VOA arrays) 2nd Generation of Integration : Serial (ILM, Router, Receiver..) Amplifier : 10-20 components with intensive package SERVING JUST ONE FUNCTION NOT have been integrated with any other functional devices Size reduction achieved to reasonable level to Metro, but not yet enough Cost reduction achieved to reasonable level to Metro, but not yet enough Is it possible to achieve above requirements with EDWA ?

17. The current status of EDWA • Exactly the same schematic as that of an EDFA • Efforts on the amplifying section only : > 10 M$ to make the cheapest part cheaper • Larger than the smallest available conventional EDFA • Need every component in one plane : severely restricts further reduction in size • Integration no more than an addition : Amplifying splitter = AMP + splitter

18. 4F9/2 Pump laser diode Er energy level diagram $ 4I9/2 4I11/2 4I13/2 1.53m 0.98m 0.80m 0.66m 4I15/2 Cost-centered view of an EDA Energy level determined by QM Story behind the Limitation - Cost • For EDFA & EDWA, optical excitation occurs through direct photon absorption • Very small absorption cross section (210-21 cm2)  Requires a long interaction length btw pump and signal  Efficiency, Size • Narrow absorption band – requires finely tuned lasers  Requires an expensive pump LD with wavelength (temperature) control  Cost

19. Reflecting mirror Half MUX/ DMUX Amplifying Section Splitting Section Story behind the Limitation - Structure 2-D Structure : Point Access Impossible Serial Integration : Lower Yield • PLC and EDWA shares the same platform but the real integration is Difficult • Limited to serial integration  Marginal reduction in the footprint with lower chip yield • Point amplification impossible  Series of resistors and filters adds in system noise • Photonic Lightwave circuit without optical power line, EDWA as a mimic of EDFA

20. Story behind the Limitation - Material • PLC is a stabilized, patterned fiber arrays using the same material • Mode size limitation dictates the minimum device size (much bigger than memory chip) • Wafer uniformity affects the yield of the chip  higher index for smaller device size • To keep the Er numbers same within smaller volume, concentration have to be much higher • Increased Er concentration  much lower ( ~ x 2 ) PCE from the quenching process

21. Faults of Integrated Amplifiers proposed so far • EDFA on a substrate • Similar properties under similar conditions  competes against established products with only an incremental advantage • Can never be integrated with anything else  can never truly “siliconize” photonics • Still requires an expensive pump LD • Transfers the control over the final price of the device to LD suppliers • The better you are, the worse this problem gets! • The smaller you get, you lose more pump power from Er quenching • Dictates the smallest possible size of EDWA • Not different at all when compared to EDFA again • Current OA technology not enough to support for metro – access network • Cost Too high Photon Price, dictated by Electrical – Optical – Optical pumping • Footprint Limited by Erbium on Silica wafer • Functionality Limited by 2-D structure • Any solutions… ?

22. Contemplations on Photon Generators • Compound Semiconductor • Bandgap - electrical : fast, strong interaction  modulation, switching • Strong interaction  Smaller device size • Energy source (electrical pump) independent from signal plane • Feedback structure : LED  FP, DFB but at much increased cost • Bandgap engineering  Wider, adjustable bandgap • Difficulties in pigtailing  Cost • Differences in refractive index with fiber  AR coating for SOA • Silica base Rare Earth • Atomic level - optical : slow, weak interaction  amplification without crosstalk • Weak interaction  Larger device size, Low efficiency • Energy source (optical pump) requires waveguide • Feedback structure : Fiber laser but no modulation capability • Bandgap engineering  None • Compatibility in Pigtailing • Next generation Optical Amplifier – photon wavelength converter • Eliminate an expensive LD source : just need to provide inversion  COST • Require dimensional separation of Pump and Signal plane  FUNCTIONALITY • Need stronger interaction mechanism for the excitation  FOOTPRINTS

23. Si-Photonic Optical Amplifier ?

24. Pump photons Si nanoclusters Interacting medium Er ions Signal photons 20 nm Signal photons Conversion mechanism SiO 2 (host matrix) Nanocrystal-Si sensitized EDWA Amplifier is Wavelength Converter in its nature Why do we use expensive coherent photons ?

25. Material properties • Continuous excitation spectra from IR to UV: anything bluer than green works! • No need of Frequency control (or cooling) • >100 times Er3+ luminescence intensity even with half the photon flux • nc-Si completely dominates excitation (100 x larger with 477nm than 980nm)

26. SRSO / Er layer deposition • Deposition process • ECR – PECVD (electron cyclotron resonant plasma enhanced chemical deposition) • Silicon rich silicon oxide to construct silicon nanocluster • Silicon contents control : Ar , SiH4, O2 (automated MFC) • Evaporation / sputtering  Er

27. Silicon nanocrystal control • Silicon nanocrystal • Controlled by RTA annealing condition / silicon contents • Best energy coupling condition to Er ions • Size control : Quantum energy state of nanocrystal • State control : Crystal or amorphous

28. Bulk performance measurement • PL measurement • PL intensity and lifetime for various pump wavelength (980nm for Er, 477nm for NC-Er) • Activity of silicon nanocluster and Er, coupling efficiency • RBS measurement • Atomic composition estimation for layer depth

29. Waveguide characterization (amplification) H 2 mm LED array W 1,000 mm x L 5 mm Ridge Waveguide W 8 mm x L 11 mm • Butt-coupled tapered fibers for signal input and output • 15mm linear array of commercial, 470 nm LEDs • Need to clear the fibers and cover glass: 2mm separation between LED and waveguide, pump only center 5mm portion of the waveguide

30. Wavelength-dependence of signal change • Signal change: Itrans(P)/Itrans(0): typical inversion curves for Er3+ • With LED pump: low pump density due to unoptimized alignment  lower inversion, optical gain at 1545 nm • Simulation of high LED pump power with 477 nm laser:  full inversion with 3 dB/cm optical gain at 1533 nm

31. Numerical Assessment / Design • Parameters from experimental results • Much larger effective excitation cross-section and signal absorption cross-section • Emission cross-section from PL measurement • Absorption cross-section from McCumber relation • Simulation scheme (top pumped NC-Si EDWA) • 2-D propagation equations (with 10x10x400 segments) • 1500 ~ 1610 nm with 1nm resolution

32. Inversion of NC-Si EDWA Inversion of conventional EDFA Signal intensity (dBm/cm2) Signal intensity (dBm/cm2) Pump intensity (dBm/cm2) Pump intensity (dBm/cm2) Population inversion characteristics of NC-Si Er • Much larger pump absorption cross-section than signal emission cross-section • Over 50% inversion with small # of pump photons (Left-shift of red region in below figures) • Top pumping scheme • Large doping area than conventional EDF • Doping area confined to the center of the fiber core for high inversion (conventional EDF) • Large doping areaEnhancement of overlap factor with signal  high gain per length Population inversion

33. Device Structure & Feasibility • Performance comparison • 4 cm EDWA without coupling loss • NC-Si EDWA with type A core (7x7 μm2) • Type A core EDWA with bottom mirror (100% reflection) • Large gain by reusing of wasted pump power • Adiabatic designed large core (type B, 100x7 μm2) • Saturation gain enhancement by increasing pump collection area • Small signal gain enhancement by overlap factor enhancement Type A w/ mirror

34. Optimization : Pump LEDs • High intensity visible (blue) pump LED • Chip LED for illumination application (Cree) • Max 25W/cm2 (hard contact) • Easy to align (waveguide width 50um < LED 250um) • 7mW(at 20mA) x 64 • Chip size : 300um x 300 um • Array size : 0.03(cm) x 3(cm) • Total Power : 5W/cm2 LED Die-Bonding pattern Emission of LED Array(0.3x3 cm)

35. Optimization : Material Composition Estimated result Evaporation Sputtering Gain: 2.4dB/cm PL : 93000 LT : 6.3 ms x 17 x 12 Experimental result Gain: 0.2dB/cm PL : 8000 LT : 9.3 ms

36. Amplifying Section Amplifying Section Splitting Section Pump WDMs Pump & Signal LED Pump array SRSO wafer Amplifying Splitter Pump light VCPAC Splitter (Splitting section = Amplifying section) Schematic of a SRSO based VCPAC (pump WDM removed) Examples & Implications in the applications Totally NEW concept, Reduced Complexity & Higher Chip Yield ! True integration for Active PLC Ultra-compact, low-cost

37. Much things you can do with NANO Si !! That’s a good news for Photonics Engineers Summary # of Amplifier Worldwide # of nano-particle Worldwide # of Amplifier Engineers