Hybrid Photonic Integration: Enabling Technology for Terabit/s Communications

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1 Hybrid Photonic Integration: Enabling Technology for Terabit/s Communications Robert Palmer, Christian Koos Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany Institute of Photonics and Quantum Electronics and Institute of Microstructure Technology KIT University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association Robert Palmer

2 Outline Silicon photonics: Promises and challenges Photonic wire bonding and multi-chip integration SOI fiber-chip-coupling concepts The photonic wire bonding concept Towards photonic multi-chip systems Silicon-organic hybrid (SOH) modulator How to build a silicon modulator SOH MZM SOH IQ-modulator Future application prospects SOH lasers SOH frequency comb generator Integrated terabit/s interconnects Karlsruhe Institute of Technology (KIT)

3 Silicon photonics has many advantages like High integration density Use of CMOS foundries for photonic devices Potentially low cost devices Multi-project-wafer (MPW) shuttle runs, e.g., epixfab ( or OpSIS ( Process design kits (PDK) and open libraries of standard devices ( mm) but comes along with several challenges: Light source on chip Fiber-to-chip coupling Chip-to-chip coupling Hybrid photonic integration Electro-optic modulation Karlsruhe Institute of Technology (KIT)

4 Outline Silicon photonics: Promises and challenges Photonic wire bonding and multi-chip integration SOI fiber-chip-coupling concepts The photonic wire bonding concept Towards photonic multi-chip systems Silicon-organic hybrid (SOH) modulator How to build a silicon modulator SOH MZM SOH IQ-modulator Future application prospects SOH lasers SOH frequency comb generator Integrated terabit/s interconnects Karlsruhe Institute of Technology (KIT)

5 Challenge: Fiber-chip coupling and packaging Important: Low-loss Large bandwidth Alignment tolerance Coupling of both polarizations Single-mode fiber core n 10-3 core diameter 10 μm degenerate polarization states Si SiO 2 n = 1.44 SOI strip waveguide n 2 width 400 nm height 200 nm 1mm Karlsruhe Institute of Technology (KIT)

6 Inverse Tapers for Horizontal Fiber-Chip Coupling Fiber-chip coupling loss ~ 1 db +/- 0.5 µm misalignment tolerance (1 db coupling penalty) 450 nm Tapered fiber MFD ~ 3 µm 220 nm 80 nm Broadband operation Requires cleaved facet Karlsruhe Institute of Technology (KIT)

7 Grating couplers for Out-of-plane Fiber-Chip Coupling 10 SMF SMF TE Picture courtesy of Wim Bogaerts, UGENT/IMEC Wafer-scale testing 0.6 db coupling loss demonstrated* 1dB-bandwidth: 40 nm But: TE only *W. S. Zaoui, et al., CMOS-compatible nonuniform grating coupler with 86% coupling efficiency, ECOC Karlsruhe Institute of Technology (KIT)

8 Grating couplers for Out-of-plane Fiber-Chip Coupling 10 SMF 10 SMF TE TE TE Picture courtesy of Wim Bogaerts, UGENT/IMEC Picture courtesy of Wim Bogaerts, UGENT/IMEC Wafer-scale testing Polarization diversity 0.6 db coupling loss demonstrated* 1dB-bandwidth: 40 nm But: TE only *W. S. Zaoui, et al., CMOS-compatible nonuniform grating coupler with 86% coupling efficiency, ECOC Karlsruhe Institute of Technology (KIT)

9 Challenge: Photonic Chip-to-Chip Interconnects Progress in Integrated Photonics High integration density in Silicon and III-V-PICs But: Scalable packaging concepts are still missing Fiber-Chip-Coupling Slow and expensive fabrication Low interconnect density Chip-to-Chip-Interconnects High mechanical alignment accuracy required No flexibility 1 mm IBM photonic integrated circuit Needed: Flexible and scalable technology for high-density chip-to-chip interconnects. Off-chip connection by fiber-chip coupling Karlsruhe Institute of Technology (KIT)

10 Heterogeneous Integration and Chip-Chip Coupling Goal: Assemble photonic systems from discrete chips combine strengths of different material systems Example: WDM transmitter??????? Use best available platform for each subsystem. Optimize individual fabrication processes for performance, not for compatibility! Enable diversified value chain: Dedicated supplier for each chip! Challenge: Single-mode connections between photonic chips, featuring Low loss High integration density Photonic wire bonding Potential for mass production Karlsruhe Institute of Technology (KIT)

11 Wire Bonding in Electronics and Photonics! State of the Art: Electronic wire bonding Stacked-die package Automated fabrication Small pitch ± 2 µm bond placement accuracy Tight control of the loop trajectory Novel Concept: Photonic Wire Bonding 3D freeform polymer waveguide for chip-to-chip connections No active chip-alignment required High interconnect density Fast fabrication Single mode connection Picture source: Kulicke & Soffa, Karlsruhe Institute of Technology (KIT)

12 Concept: Photonic Wire Bonding Mounting of components on submount SOI Chip Submount Karlsruhe Institute of Technology (KIT)

13 Concept: Photonic Wire Bonding Mounting of components on submount SOI Chip III-V Chip Submount Karlsruhe Institute of Technology (KIT)

14 Concept: Photonic Wire Bonding Mounting of components on submount SOI Chip Fiber III-V Chip Submount Karlsruhe Institute of Technology (KIT)

15 Concept: Photonic Wire Bonding Pattern-recognition and calculation of optimized interconnect waveguide SOI Chip Fiber III-V Chip Photoresist Karlsruhe Institute of Technology (KIT)

16 Concept: Photonic Wire Bonding Embedding in photoresist SOI Chip Fiber III-V Chip Submount Photoresist Karlsruhe Institute of Technology (KIT)

17 Concept: Photonic Wire Bonding 3D structuring by two-photon polymerization SOI Chip III-V Chip nm Karlsruhe Institute of Technology (KIT)

18 Concept: Photonic Wire Bonding SOI Chip Fiber III-V Chip Submount Photoresist Karlsruhe Institute of Technology (KIT)

19 Concept: Photonic Wire Bonding SOI Chip Fiber III-V Chip Submount Photoresist Karlsruhe Institute of Technology (KIT)

20 Concept: Photonic Wire Bonding Photonic Wire Bond SOI Chip Fiber III-V Chip Submount Photoresist Karlsruhe Institute of Technology (KIT)

21 Concept: Photonic Wire Bonding Development Free standing Photonic Wire Bond Photonic Wire Bond SOI Chip Fiber III-V Chip Submount Karlsruhe Institute of Technology (KIT)

22 Karlsruhe Institute of Technology (KIT)

23 Coupling to Nanophotonic Waveguides Inverse Simulation Si-taper Results: combined with 3D polymer taper 3D taper as part of the Photonic Wire Bond Transmission [db] w tip = 20 nm 100 nm Polymer w tip Si SiO 2 Transmission [db] Wavelength [nm] Karlsruhe Institute of Technology (KIT)

24 On-Chip PWB Pitch 25 µm Karlsruhe Institute of Technology (KIT)

25 Karlsruhe Institute of Technology (KIT) Chip-to-Chip PWB

26 Transmission measurements Insertion loss: (1.6 ± 0.3) db within C-band (1535 nm 1565 nm) (2.5 ± 1.1) db between 1240 nm and 1580 nm Lindenmann et al., Opt. Express 20, (2012) Karlsruhe Institute of Technology (KIT)

27 Towards a Universal Integration Platform: Reproducibility Fabrication of nominally identical wire bonds and investigation of insertion losses Low insertion losses can be routinely reproduced! Insertion loss: (1.1±0.8) db SOI reference waveguides Bonds + SOI waveguides Length of inverse SOI taper: 70 µm Insertion loss: (0.7±0.8) db SOI reference waveguides Bonds + SOI waveguides Length of inverse SOI taper: 80 µm Karlsruhe Institute of Technology (KIT)

28 KIT people involved in PWB activities Nicole Lindenmann Prof. Christian Koos Muhammad Rodlin Billah Tobias Hoose Prof. Wolfgang Freude Dr. Sebastian Koeber Karlsruhe Institute of Technology (KIT) Prof. Max Mustermann - Title

29 Outline Silicon photonics: Promises and challenges Photonic wire bonding and multi-chip integration SOI fiber-chip-coupling concepts The photonic wire bonding concept Towards photonic multi-chip systems Silicon-organic hybrid (SOH) modulator How to build a silicon modulator SOH MZM SOH IQ-modulator Future application prospects SOH lasers SOH frequency comb generator Integrated terabit/s interconnects Karlsruhe Institute of Technology (KIT)

30 Electro-Optic Modulation I want to build a high-speed silicon modulator, but how? High-performance electro-optic modulators in other material systems (LiNbO 3, GaAs, InP) rely on the Pockels-effect. Pockels effect? Ah, yes, now I remember... (1) D 0E P 0E 0 E D r (1) (2) E E r n n 2 2 n 2n n (2) 0 EE higher order effects 1 ( 2) n E n re r = 4 2n n 2 n that s when the refractive index of a material changes linearly with an external electric field E! but in silicon (2) =0, due to crystal symmetry. E: Electric field D: Displacement field density P: Electric polarization (n) : Suszeptibility of n-th order (2) Karlsruhe Institute of Technology (KIT)

31 Electro-Optic Modulation but in silicon (2) =0 What now? Option 1 Option 2 Use plasma dispersion instead! pn/pin-junctions Find a way to add (2) -functionality! A) Break crystal symmetry Or B) Hybridization with a (2) material Green et al., Opt. Expr. 15(25), 2007, Xu et al., Nature 435, 2005, Karlsruhe Institute of Technology (KIT)

32 Option 1: Free-carrier dispersion Carrier Injection Carrier Depletion Plasma effect: Phase and amplitude modulation linked Performance of Mach-Zehnder modulators: V p L 0.4 Vmm f 3dB 1 GHz W bit 1 pj/bit V p L Vmm f 3dB 30 GHz W bit 200 fj/bit Images: Reed et al., Silicon optical modulators, Nature Photon., vol. 4, pp , Aug Karlsruhe Institute of Technology (KIT)

33 Option 2A: Breaking the Symmetry of the Crystal Lattice Break symmetry by inducing strain: Regular Si WG Strained Si WG Compressively strained Si 3 N 4 layer z x y (2) = 0 pm/v Strained Si : (2) xxy 122 pm/v, r 1.6 pm/v 12 LiNbO : 3 (2) zzz 360 pm/v, r 30 pm/v 33 R. S. Jacobsen et al., Nature 441, (2006) Chmielak et al., Opt. Express 19, (2011) Strain-induced electro-optic coefficients not (yet) sufficient for practical devices Karlsruhe Institute of Technology (KIT)

34 Option 2B: Silicon-Organic Hybrid Approach SOI waveguides High integration density Mature CMOS processing EO Organic materials Tailored optical properties EO coefficients up to r 33 =450 pm/v Slot waveguide Si SiO nm Karlsruhe Institute of Technology (KIT)

35 Phase modulator with > 100 GHz bandwidth Increase bandwidth by highly conductive charge accumulation layer: Conductive strips Electron accumulation U gate SOI substrate used as gate electrode Flat frequency response: 3 db bandwidth > 100 GHz (All-silicon devices: 30 GHz) Pure phase modulation (All-silicon devices: Strong amplitude-phase coupling) Alloatti et al., Opt. Express 19 (12), (2011) Alloatti et al., accepted for publication in Light: Science and Applications (2013) Karlsruhe Institute of Technology (KIT)

36 SOH Mach-Zehnder Modulator G + S U pol - G EO Organic Material SiO 2 Si U pol / ( 2w ) slot U drive U pol / ( 2w ) slot SiO 2 n + -Si 2 phase modulators driven by 1 CPW (GSG) Slot WG confines light to 160 nm slot, filled with EO organic material Rails of slot WG connected to CPW by n-doped Si slabs (Ar, cm - ³) High modulation field in slot, strong overlap with optical mode Poling: Field-induced acentric orientation of the nonlinear molecules Push-pull operation: Poling antisymmetric wrt modulation field Karlsruhe Institute of Technology (KIT)

37 SOH MZM - Improving the Nonlinear Material Material r 33 in SOH r 33 (reference) Publication YLD124(25%)/APC 32 pm/v 80 pm/v YLD156(25%)/PMMA 19 pm/v 50 pm/v AJLZ53(15%)/PMMA 19 pm/v 60 pm/v YLD124(25%)/APC 30 pm/v 80 pm/v YLD124(25%)/PSLD41 56 pm/v 285 pm/v Takayesu et al., J. Lightwave Technol. Vol. 27(4), 2009 Takayesu et al., J. Lightwave Technol. Vol. 27(4), 2009 Gould et al., Optics Express, Vol. 19(5), 2011 Baehr-Jones et al., Appl. Phys. Lett. Vol. 92(16), 2008 Takayesu et al., J. Lightwave Technol. Vol. 27(4), 2009 EO coefficients in SOH devices are a factor 2-5 smaller than in respective parallel-plate poled references Karlsruhe Institute of Technology (KIT)

38 SOH MZM - Improving the Nonlinear Material YLD124/PMMA Maximum E poling EO coefficient x r 33 = 29 pm/v y Electrical breakdown at high poling f fields 1 zzz : First hyperpolarizability N : Chromophore density cos 3 3 How to increase r 33? r 2 N cos g n 33 zzz r33 [pm/v] : Average acentric order parameter g : Local field factor n: Refractive index z: dipole axis [Takayesu 2009] [1] [Baehr-Jones [9] [4] 2008] [7] [2] [Gould [3] 2011] [8] E poling [V/µm] YLD124/PMMA 0.23 nm 2 /V 2 We increase r 33 by increasing chromophore density and molecular orientation Karlsruhe Institute of Technology (KIT)

39 SOH MZM - Improving the Nonlinear Material Thermal randomisation Dipole-dipole electrostatic interaction Dipole - poling field interaction E poling z z z z z z z z z z z z z z z z z z z z z z z z z z z z z Isotropic (r 33 =0) Isotropic z z Centric Order (r 33 =0) Centric Ordering Acentric Ordering (r 33 >0) Karlsruhe Institute of Technology (KIT)

40 SOH MZM - Improving the Nonlinear Material Novel material: DLD164 Monolithic material, i.e. no polymer host matrix is used. High chromphore density Coumarin Stabilizing side groups Donor + EO-core Acceptor Novel Concept: Coumarin side groups: Stabilization of glassy matrix (reducing dipole-dipole interaction) N Enhancement of acentric order by reduction of rotational degrees of 3 freedom (coumarin-coumarin interaction) cos _ Karlsruhe Institute of Technology (KIT)

41 SOH MZM - Improving the Nonlinear Material Well 1 known: YLD124 1-d Novel: DLD164 Typical poling energy <cos 3 > d 3-d zzz : Microscopic hyperpolarizability The same Average acentric order N : Chromophore density nearly doubles 2.3 times higher 1 3 cos : Average acentric order parameter 2 times higher Poling µe p Energy /kt a.u. 1 : (assuming equal density) Koeber et al., to be published. Figure: Dalton et al., Optical Materials 32, 658 (2010) Karlsruhe Institute of Technology (KIT)

42 Transmission [db] SOH MZM- Improving the Nonlinear Material DLD164 Light out 0 1 mm-long MZM Light out V U π,mzm 0.5V U L p 0.5 Vmm Bias Voltage [V] Karlsruhe Institute of Technology (KIT) Palmer, R. et al., ECOC'13, London, Paper We.3.B.3 [invited]

43 Transmission [db] Extracting the Electro-Optic Coefficient V Bias Voltage [V] w slot = 160nm: Slot width = 1540nm: Wavelength L = 1mm: Device length n slot = 1.82: Cladding refract. index : Field interaction factor Karlsruhe Institute of Technology (KIT) 1 (2) 1 3 n 333 E n r33e n 2 E U U U w slot slot π,mzm 3 2Lnsl ot r33 π,mzm w 0.5V x 1 r 33 n ( x, y) (, )da clad x x y 2 Z 4 r 33 : EO coefficient r33 180pm/V Acentric order enhanced? slot

44 SOH MZM - Improving the Nonlinear Material Acentric order enhanced? DLD164 Maximum EO coefficient r 33 = 190 pm/v In-device r 33 greater than reference r 33 (137 pm/v) r 2 N cos g n 33 zzz r Karlsruhe Institute of Technology (KIT) 3 4 cos 33,DLD164 zzz,dld164 DLD164 DLD164 DLD ,YLD124 zzz,yld124 NYLD124 cos g n YLD124 YLD124 r N Chromophore orientation improved by a factor 2.1! 3 g n Palmer, R. et al., ECOC'13, London, Paper We.3.B.3 [invited]

45 Photonic Integrated Circuit 1 mm long MZM 5 db loss Grating Coupler 4µm Fiber-to-fiber: -16 db On-chip: -6 db 0.1 db loss MMI 2µm 0.01 db loss -60 db reflection 0.02 db loss Karlsruhe Institute of Technology (KIT) Palmer, R., IEEE Photonics J.; Vol. 5; issue 1; Article # ; Jan. 2012

46 Transmission and Reception of OOK Data 12.5 Gbit/s 25 Gbit/s 40 Gbit/s - No RF amplifier - U drive = 950 mv pp Palmer, R. et al., ECOC'13, London, Paper We.3.B.3 [invited] Karlsruhe Institute of Technology (KIT)

47 Transmission and Reception of OOK Data 40 Gbit/s using shorter modulators L = 1mm L = 500µm L = 250µm Terminated Terminated Not terminated U drive = 0.95 V pp U drive = 2 V pp U drive 4 V pp (U Source = 2 V pp ) Karlsruhe Institute of Technology (KIT)

48 Low Drive Voltage Experiments 10 fj/bit 100 U drive [mv pp ] 1.6 fj/bit 0.6 fj/bit Palmer, R. et al., ECOC'13, London, Paper We.3.B.3 [invited] Karlsruhe Institute of Technology (KIT)

49 Energy Efficient Silicon Modulators pn-mzm 1 (U-Wash) pn-µ-disk 2 (MIT) SOH-MZM (KIT) U p L Vmm n.a. 0.5 Vmm Device length L / diameter D L = 5 mm D = 3.5 µm L = 1 mm Drive voltage 630 mv mv 130 mv Energy consumption Insensitive to wavelength and temperature drifts 200 fj/bit 3 fj/bit 1.6 fj/bit Yes No Yes 1 T. Baehr-Jones, Opt. Express, vol. 20(11), May M.R. Watts, Opt. Express, vol. 19(22), Oct This Work Karlsruhe Institute of Technology (KIT)

50 Multilevel Generation using an SOH MZM Push-Pull Modulator: Δ V Drive E out E in sin Opt. Amplitude Opt. Amplitude 4-ASK I V Drive t Q IQ-Modulator 16-QAM I V Drive Q t Karlsruhe Institute of Technology (KIT)

51 Multilevel Generation using an SOH MZM FPGA DAC M-ASK Tx 50W OMA V Bias ECL 1559nm G S G DUT EDFA 0.6 nm VOA EDFA 3 nm Signal LO fj/bit pj/bit 1.55 pj/bit 800 fj/bit 1 bit/symbol 28 Gbit/s 2 bits/symbol 56 Gbit/s 3 bits/symbol 84 Gbit/s R. Palmer et al., IEEE Photonics Journal 5, (25% overhead soft-fec (2013) 67.2 Gbit/s) Karlsruhe Institute of Technology (KIT)

52 Multilevel Generation using an SOH MZM fj/bit pj/bit 1.55 pj/bit 800 fj/bit 1 bit/symbol 28 Gbit/s 2 bits/symbol 56 Gbit/s 3 bits/symbol 84 Gbit/s R. Palmer et al., IEEE Photonics Journal 5, (25% overhead soft-fec (2013) 67.2 Gbit/s) Karlsruhe Institute of Technology (KIT)

53 SOH IQ modulators: QPSK and 16 QAM I -p/2 OUT Q QPSK at 28 GBd: 16 QAM at 28 GBd: 28 GBd, 112 Gbit/s BER = EVM = 10.3 %, I Q D. Korn et al., Opt. Express 21; (2013) Karlsruhe Institute of Technology (KIT)

54 KIT people involved in SOH activities Dietmar Korn Prof. Christian Koos Robert Palmer Matthias Lauermann Prof. Wolfgang Freude Dr. Sebastian Koeber Karlsruhe Institute of Technology (KIT)

55 and external partners Gent University IMEC, Gent, Belgium: Wim Bogaerts, Hui Yu AMO GmbH, Aachen, Germany: Thorsten Wahlbrink, Jens Bolten, Michael Waldow University of Washington, Seattle, WA, USA: Larry Dalton, Delwin Elder GigOptix, Bothell, WA, USA: Raluca Dinu Karlsruhe Institute of Technology (KIT)

56 Outline Silicon photonics: Promises and challenges Photonic wire bonding and multi-chip integration SOI fiber-chip-coupling concepts The photonic wire bonding concept Towards photonic multi-chip systems Silicon-organic hybrid (SOH) modulator How to build a silicon modulator SOH MZM SOH IQ-modulator Future application prospects SOH lasers SOH frequency comb generator Integrated terabit/s interconnects Karlsruhe Institute of Technology (KIT)

57 SOH Lasers Strip waveguide Polymer + Dye Si SiO 2 Polymer + Dye Si Si SiO 2 Slot waveguide Pulsed pump 1064nm λ/2 WP Alignment HeNe PBS L1 Beam dump L2 CL Fiber 1 SOH Chip Fiber 2 Slot waveguide: 850 mw on-chip pulse peak power D. Korn et al., CLEO 2012, paper CTu2I.1 M. Lauermann et al., IPR 2013, paper IM3A Karlsruhe Institute of Technology (KIT)

58 Relative Optical Power 10 db SOH Comb Generator RBW 1.44pm 40 GHz, modulation depth 1.5π No termination used 7 comb lines within 2 db spectral flatness Previous work: T. Sakamoto et al, El. Lett., vol. 43, no. 19, pp , 2007: Commercial LiNbO 3 MZM, 10 GHz line spacing 11 comb lines within 1.1 db Karlsruhe Institute of Technology (KIT) Weimann, C., ECOC'13, London, UK, Paper Th.2.B.1; Sept 22 26, 2013

59 SOH Comb Generator 4x Carrier: x pol. EVM: 31.1% 33.3% 23.7% 22.0% 18.5% 19.5% 19.7% 19.8% 24.6% 26.0% 19.0% 18.4% 16.5% 17.3% 18.7% 18.0% 33.6% 34.6% y pol. BER: 1.6e-3 8.2e-6 <1.0e-6 <1.0e-6 7.8e-5 4x 75 km transmission span 28 Gbd NRZ Aggregate data rate Tbit/s Karlsruhe Institute of Technology (KIT) <1.0e-6 <1.0e-6 <1.0e-6 3.4e-3 Weimann, C., ECOC'13, London, UK, Paper Th.2.B.1; Sept 22 26, 2013

60 The Vision: Terabit/s Transmitter Terahertz waveform out t Frequency comb source Digital data in Tbits/s Karlsruhe Institute of Technology (KIT)

61 PWB - Summary Photonic wire bonding offers a flexible way for single-mode interconnects between photonic chips. Polymer w tip Si SiO 2 3D 0.0 routing algorithm and double taper w tip = 20 nm approach -0.2 allow for optimized photonic wire bond design. 100 nm -0.4 Transmission [db] Wavelength [nm] Low-loss interconnects covering the entire wavelength range of optical communications offering flawless data transmission. Lindenmann et al., Opt. Express 20, (2012) Karlsruhe Institute of Technology (KIT)

62 SOH Modulators - Summary Fast MZI Modulators (MZM) 40 Gbit/s OOK with 1mm MZM 84 Gbit/s 8-ASK with 1mm MZM R. Palmer et al., PJ, vol. 5(2), April 2013 I Complex Modulation Formats 112 Gbit/s 16-QAM with 1.5mm IQ-Modulator Q Energy Efficient Modulation D. Korn et al., Opt. Express, vol. 5(2), June 2013 Down to 0.6 fj/bit at 12.5 Gbit/s 1 mm long MZM Electro-optic coefficient of r 33 = 180 pm/v R. Palmer et al., ECOC2013, paper We.3.B Karlsruhe Institute of Technology (KIT)