High Speed VCSELs and Optical Interconnects Petter Westbergh Photonics Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg SE-41296, Sweden. E-mail: [email protected] Optics & Photonics in Sweden 2014 11 - 12 November 2014 Chalmers University of Technology, Göteborg Outline • • • • • • Introduction VCSEL speed limitations VCSEL design for high speed Photon lifetime Performance Summary VCSELs probed on wafer VCSELs arrays Introduction Vertical-Cavity Surface-Emitting Laser – VCSEL Surface emitting semiconductor laser with optical feed-back provided by highly reflective mirrors (DBRs) • Small volume (10x10x10 µm3) → low drive current, low power consumption • Wafer scale production and testing (100 000 lasers on single 3” wafer possible) → low fabrication cost • GaAs-based materials → emission at 850 nm wavelength Bias current + data signal 10110010011101 • DBR (p-type) oxide aperture gain medium (MQW) VCSELs probed on wafer DBR (n-type) effective refractive index index guiding of optical field SEM image of high speed VCSELs Introduction Optical interconnect = short reach optical link for high capacity data transfer Datacenters High performance computing Optical Thunderbolt Optical USB 3.0 DataCom ComputerCom Consumer links rack - rack Multimode optical fiber shelve - shelve VCSEL based optical interconnects board - board Polymer optical waveguide module - module 10-2 10-1 100 101 Distance (m) 102 103 Introduction • Datacom is today the most rapidly growing segment of the optical communications market • GaAs-based 850 nm wavelength VCSELs are key components The first of Facebook’s set of three 28,000 m2 data centers in Luleå, Sweden Existing and emerging data transmission standards Warehouse-scale datacenters and supercomputers: ~106 optical interconnects (2012) → ~109 optical interconnects (2020) Requirements for datacom VCSELs: • High speed • High operating temperature • Low power consumption VCSEL speed limitations Transmission at high data rates require a large modulation bandwidth Intrinsic response (damping limited) I2 I1 I3 Intrinsic response + electrical parasitics Intrinsic response + parasitics + thermal I1 I1 I2 Gen 2 11um 55 nm etch K=0.14 ns, γ0=16.8ns-1, D=4.44current GHz/sqrt(mA), increased fp=17GHz, fr,max = 15 GHz I3 I1< I2 < I3 typically > 50 GHz • The modulation bandwidth is limited by a combination of damping, electrical parasitics, and thermal saturation • These parameters can be tuned in the VCSEL design to optimize the modulation bandwidth VCSEL design for high speed • Thick polymer for reduced pad capacitance • Multiple oxide layers for reduced capacitance SEM • Graded interfaces and modulation doping in DBRs • AlAs in bottom DBR for improved thermal conductivity • Active region optimized for rapid bandwidth increase with current Chalmers 3rd generation high speed 850 nm VCSEL design 4 µm aperture 7 µm aperture 28 GHz P. Westbergh et al., Electron. Lett. 48, 517 (2012); P. Westbergh et al., IEEE Photon. Techn. Lett. 25, 768 (2013); P. Westbergh et al., SPIE Proc. 8639, 86390X (2013) Photon lifetime • The intrinsic high speed properties of the VCSEL depend strongly on the photon lifetime* • The large refractive index step at the semiconductor/air interface has a strong impact on DBR reflectivity and photon lifetime • Nanometer thinning of top layer → reduced mirror reflectivity and photon lifetime Photon lifetime 0 nm 25 nm 40 nm 55 nm *Photon lifetime = the lifetime of a photon in the laser resonator before it escapes P. Westbergh et al., IEEE J. Sel. Top. Quantum Electron. 17, 1603 (2011) Photon lifetime τp≈6.4 ps 0 nm τp≈5.3 ps 25 nm τp≈3.3 ps τp≈1.3 ps 40 nm 55 nm etch depth 0 nm 25 nm 40 nm 55 nm Modulation bandwidth (GHz) 15.0 17.0 23.0 22.0 • There exists an optimum photon lifetime for maximum modulation bandwidth • > 50% increase in modulation bandwidth achieved by optimizing the reflectivity of the top DBR P. Westbergh et al., IEEE J. Sel. Top. Quantum Electron. 17, 1603 (2011) VCSEL-based optical interconnect - limiting receiver • Gen 3, 7 µm aperture diameter, 26 GHz BW • 40 Gbit/s @ 25°C Photon lifetime optimized for maximum modulation BW • VI-Systems R40-850 photoreceiver (~30 GHz BW, limiting TIA) • Error-free transmission: bit-error-rate (BER) < 10-12 Back-to-back 40 Gbit/s @ 85°C 40G 25°C 40G 85°C 47G 25°C 47 Gbit/s @ 25°C P. Westberg et al., IEEE Photon. Techn. Lett. 25, 768 (2013) VCSEL-based optical interconnect - linear receiver • Gen 3, 8 µm aperture diameter, 24 GHz BW • 50 Gbit/s @ 25°C VCSEL has slightly higher damping (longer photon lifetime) → flatter response → reduced ringing and timing jitter* • New Focus 1484-A-50 photoreceiver (22 GHz 3 dB BW, linear TIA) Back-to-back 55 Gbit/s @ 25°C 50G 25°C 55G 25°C 57G 25°C 57 Gbit/s @ 25°C *For more information, see Emanuel Haglund’s Poster ”Optimum Damping Level for High-Speed Large Signal VCSEL Modulation” P. Westbergh et al. Electron. Lett. 49, 1021 (2013) VCSEL-based optical interconnect - equalization • 26 GHz VCSEL (Gen 3 Chalmers), > 30 GHz photodiode (Sumitomo) • Driver and receiver circuits with two tap feed forward equalization (FFE) • IBM SiGe BiCMOS 8HP (130 nm) • Error-free transmission over MMF up to 71 Gb/s at 25°C and 50 Gb/s at 90°C Electrical eyes Optical eyes Driver VCSEL 25 - 71 Gb/s BER vs. Datarate 25-71Gb/s, 7m OM3 25G 32G 40G 50G 60G 64G 68G 71G -3 40Gb/s -4 50Gb/s 60Gb/s log10(BER) -5 -6 -7 -8 71Gb/s -9 -10 -11 -12 -13 -12 -10 -8 OMA (dBm) D.M. Kuchta et al., Proc. OFC, paper Th3C.2 (2014); D.M. Kuchta et al., IEEE Photon. Technol. Lett. (2014) -6 -4 -2 VCSEL-based optical interconnect - multi-level modulation • Increased capacity through improved spectral efficiency • Pulse amplitude modulation (PAM) – low complexity, low power consumption, good receiver sensitivity • PAM-4 modulation (4 levels, 2 bits/level) • 60 Gb/s (30 Gbaud) transmission over a 20 GHz link 25 Gbaud (50 Gbps) VCSEL 30 Gbaud (60 Gbps) PAM signal generator Back-to-back K. Szczerba et al., Electron. Lett. 49, 953 (2013) MMF 20 GHz VOA optical receiver 50/100 m OM4 fiber error analyzer VCSEL-based optical interconnect -onboard polymer waveguide interconnect • Polymer waveguide embedded in backplanes and circuit boards to interconnect boards and modules • 40 Gb/s NRZ and 56 Gb/s PAM-4 transmission over 1 m polymer waveguide • Record speed-distance products (40 and 56 Gbps∙m) 25 Gb/s 36 Gb/s 40 Gb/s 56 Gb/s PAM-4 Back-to-back Waveguide N. Bamiedakis et al., IEEE J. Lightw. Technol. (2014); N. Bamiedakis et al., Proc. ECOC, paper ? (2014); N. Bamiedakis et al., Proc. OFC (2015) VCSEL array for MCF links Multi-core optical fiber (MCF) for increased capacity and bandwidth density • 240 Gb/s (6×40 Gb/s ) single fiber capacity 10 10 Ch5 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 8 Ch4 Power (mW) Ch6 Ch1 6 25 °C 4 25 °C Ch3 Øox≈8 μm 0 0 2 4 6 8 10 12 Current (mA) Ch 1 25 Gb/s 40 Gb/s P. Westbergh et al., IEEE Photon. Technol. Lett. (2014) 2 85 °C Ch2 40 Gb/s -3 85 °C 2 25 Gb/s 8 6 4 -2 14 16 18 Ch 2 0 20 85 °C -4 log(BER) • 6-channel VCSEL array matched to MCF geometry Voltage (V) • MCF Single core MMF 85 °C -5 -6 25 °C 25 °C -7 -8 -9 -10 -11 Øox≈8 μm -12 -13 -15-14-13-12-11-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 OMA (dBm) Ch 3 Ch 4 Ch 5 Ch 6 Summary • VCSEL based links for datacom is the fastest growing segment of the optical communications market • • Approaching 30 GHz bandwidth for directly modulated 850 nm VCSELs Binary modulation: • 57 Gb/s at 25°C and 40 Gb/s at 85°C unequalized • 71 Gb/s at 25°C and 50 Gb/s at 90°C equalized Multi-level modulation • 60 Gb/s at 25°C VCSEL arrays • 6×40 Gb/s = 240 Gb/s over a single MCF • • Summary • VCSEL based links for datacom is the fastest growing segment of the optical communications market • • Approaching 30 GHz bandwidth for directly modulated 850 nm VCSELs Binary modulation: • 57 Gb/s at 25°C and 40 Gb/s at 85°C unequalized • 71 Gb/s at 25°C and 50 Gb/s at 90°C equalized Multi-level modulation • 60 Gb/s at 25°C VCSEL arrays • 6×40 Gb/s = 240 Gb/s over a single MCF • • Thank you! Acknowledgement The team at Chalmers IQE Europe (UK) Anders Larsson Johan Gustavsson Jörgen Bengtsson Åsa Haglund Benjamin Kögel Rashid Safaisini Erik Haglund Emanuel Haglund Tyndall Institute (Ireland) Krzysztof Szczerba Magnus Karlsson Peter Andrekson Technical University of Berlin (Germany) Cambridge University (UK) CNR-IEIIT (Italy) IBM (USA) HP Labs (USA) Ghent University (Belgium) ULM Photonics (Germany) Johnny Karout (S2) Erik Agrell (S2) IHP (Germany) OFS (Denmark) VTT (Finland) Financing European Union (FP7 projects VISIT and MERLIN) Swedish Foundation for Strategic Research (projects LASTECH, MuTOI)
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