Recent developments in high bandwidth optical interconnects Brian Corbett
Outline Introduction to photonics for interconnections Polymeric waveguides and the Firefly project Silicon on insulator (SOI) platform for photonics Transfer Print (TP) technology Laser on silicon using TP Conclusions
Tyndall National Institute Dedicated Research Institute Initiated in 1985 460 people (120 PhD students) 32M pa turnover 140M Irish government investment Unique combination of III-V, Si, MEMS Photonics: Epitaxy Process technology Device characterisation Systems
Motivation Why optics for communications? Frequency of 1550nm light is 200 THz 40 GHz << baseband Optical fibre waveguides No skin effect 10 mm core for optical isolation Loss ~ 0.2 db / km Multiplexing Weak interaction of light with itself (especially at low intensity) Dense wavelength division multiplexing to fill the spectrum Factor of 2 increase in bandwidth by adding polarization The result Long haul / metro communications all based on light Fibre migrating (slowly) to the home
Continuing growth in bandwidth Bandwidth is increasing at every level Data centres and data management Supercomputing All following exponential curves (which may saturate) Log(Traffic) Services Time Requires increased bandwidth over internet, on board and at chip level
Long Haul developments Increase bits per symbol ~1 b/s/hz 60nm gain bandwidth of Fibre Amplifiers 7.5 Tb/s per fibre Coherent techniques Decrease loss (to allow higher threshold for nonlinear effects) and allow higher Signal/Noise Increase number of spatial modes
Reach of photonics http://mphotonics.mit.edu/ Petar Pepeljugoski Towards Exaflop Servers and Supercomputers: The Roadmap for Lower Power and Higher Density Optical Interconnects
Optics for data centres < 1km Active optical cable for bidirectional connection between peripherals From 5 to 100 Gbps 850nm surface emitting laser arrays and 1550nm edge emitting arrays Example from Molex: 0.78W for 10 Gbps (78mW/Gb/s) Luxtera: 4x10 Gb/s at 0.78W Targetting 4 x 25 Gb/s cables for 100 GbE Source: IBM
Supercomputing needs Exascale (10 18 ) computing Factor of 10 increase every 4 years: x 100 by 2020 IBM Roadrunner system (1 PetaFlop): 48,000 fibres to the boards IBM Power P775 system: Optical fibres at the board level Need bandwidth to scale at all levels (inter-rack, backplane, card chip) Low energy per bit <1mW/Gbps Scaling manufacturing Including the wires http://www.leti-annualreview.com/presentations/session_e/3_offrein.pdf
Optical circuit Light source Edge/Surface emitting 850, 1300, 1550nm Detector Si, InGaAs, InP Interconnect medium Fibres (single/ multimode), polymer, silicon, glass like dielectrics,. Drive and receive circuits Need decisions where integration is essential Eg wavelength, waveguide, light source,..
Optics for backplanes Firefly project: Multilayer Photonic Circuits made by Nano-Imprinting of Waveguides and Photonic Crystals http://www.fp7-firefly.eu/pages/about.htm 5 10 cm Photonic crystals Fibre output Single mode polymer waveguides
Firefly strategy Choice of wavelength: 1300 / 1550nm compatible with internet communications and with on chip Silicon On Insulator (SOI) platform Choice of source: Surface emitting laser Choice of waveguide: Single mode Imprintable, photo-processable
Firefly strategy Add photonics crystal structures for more compact functions Wavelength controlled scattering from high index TiO2 spheres
Optics using Silicon on insulator Silicon on insulator (SOI) Silicon 0.22 mm thick; Oxide > 1mm thick Transparent waveguide for wavelengths > 1100nm Single mode waveguides with width of 0.2 mm High index (3.5) permitting bends with radius of curvature <2mm Precision fabrication and manufacturing scale of silicon Carrier depletion effects permit modulation at >25 GHz Integrate detectors by depositing and crystallising Ge Optical circuits eg Luxtera using 0.13 µm SOI process Foundry model for development now established (epixfab, OpSiS)
Photonic Integrated Circuits Complex photonic integrated circuits possible using SOI platform Layout using PICDraw: F. Peters et al (Tyndall/UCC)
Packaging results (epixnet) Connecting to SOI circuits Packaged SOI ring resonator Ring Resonator performance Peter O Brien, Brad Snyder (Tyndall)
The light source Silicon Optoelectronics Technology Work Group To provide a truly compelling solution, silicon microphotonics will likely need to achieve a high degree of monolithic integration with at most a small degree of hybrid integration, (e.g. laser sources) in order to offer low cost and increased functionality.
Integration of lasers Sparse array of lasers are required in optical circuits Laser patch desired Mismatch between dimensions of III-V and of silicon wafers Coupling to external lasers challenging Options Hybridisation of pre-fabricated devices Wafer bonding (of patch of material) Transfer print and post processing
Transfer print concept A A Transfer printing stamp B P-Cladding Guide Active Guide N-Cladding Release layer Substrate C GaAs Laser Wafer with pre-etched coupons C B D D Pick-up of GaAs coupons Silicon Wafer Silicon Wafer Populated with GaAs coupons A 100mm epi wafer will have > 130,000 printable 400x100mm 2 coupons
Transfer print: examples Laser epitaxial material 840 mm 350 mm Silicon substrate Allows co-processing of all devices with lithographic alignment John Justice, et al, Nat. Photon, 6, pp. 612 616 (2012)
Laser fabrication P-metal P-metal Contact Contact Laser Laser Ridge Ridge N-Metal N-Metal Contact Contact N-GaAs N-GaAs P-AlGaAs P-AlGaAs Active Active Region Region N-AlGaAs N-AlGaAs Silicon Silicon Substrate Substrate 100 mm John Justice, et al, Nat. Photon, 6, pp. 612 616 (2012)
Silicon III-V interface P-GaAs P-AlGaAs N-AlGaAs N-GaAs 2 x Quantum wells Silicon substrate 5 mm John Justice, et al, Nat. Photon, 6, pp. 612 616 (2012)
Laser performance Power per facet (mw) 30 25 20 15 10 5 20 0 C 30 0 C 40 0 C 50 0 C 60 0 C 70 0 C 80 0 C 90 0 C 100 0 C 20 0 C 100 0 C 0 0 20 40 60 80 100 Current (ma) Power (db) -50-60 -70 820 822 824 826 Wavelength (nm) John Justice, et al, Nat. Photon, 6, pp. 612 616 (2012)
Coupling to waveguides Laser Diode Laser Dielectric waveguide Dielectric Waveguide Fabry-Perot cavity Below threshold Retro-reflector cavity Above threshold
Plasmonics Can optics meet the dimensions of electronics? SOI waveguides 200 nm x 200 nm Plasmonics Collective oscillations of electrons at optical frequencies on a metal surface Fundamental dimensions in the 10s of nm
Conclusions Photonics is making a difference to the upcoming generations of computing systems Optical interconnect now competitive at distances of 10 m Next is the 10 cm 1 m range Future: IC sitting on a silicon photonics chip Signals to optical circuit boards Then to the internet Enormous new opportunities Ambient intelligence platform Environmental sensing (spectrometer on chip + sensing) Information signalling in buildings
Acknowledgements Tyndall researchers