Submicron SOI waveguides Dries Van Thourhout Trento 05

Submicron SOI waveguides Dries Van Thourhout Trento 05 http://photonics.intec.ugent.be

Acknowledgements The European Union IST-PICCO and IST-PICMOS project The European Space Agency The Belgian IAP-PHOTON network The Flemish Institute for the industrial advancement of Scientific and Technological Research (IWT) The Photonic Research Group at Ghent University IMEC Pieter Dumon, Wim Bogaerts, Dries Van Thourhout, Dirk Taillaert, Bert Luyssaert, Peter Bienstman, Joris Van Campenhout, Gunther Roelkens, Ilse Christiaens The Silicon Process division at IMEC Vincent Wiaux, Stephan Beckx, Johan Wouters, Diziana Vangoidsenhoven, Rudi De Ruyter, Johan Mees http://photonics.intec.ugent.be 2

Outline Submicron SOI-wires Introduction: why do we need them Basic properties: design, loss, wavelength, polarization Fabrication Devices: couplers, crossings, filters III-V on Silicon Introduction Coupling of light Fabrication PICMOS (Photonic Interconnect on CMOS) http://photonics.intec.ugent.be 3

Scale difference Electronics interconnects gate transistor width flip-flop Active opto-electronics Wavelength-scale photonics LED VCSEL detector stripe laser 2R regenerator Passive photonics Wavelength-scale photonics fibre core linewidth in current PIC Bend radius taper spot-size convertor AWG in Silica on Silicon 100nm 1µm 10µm 100µm 1mm 1cm http://photonics.intec.ugent.be 4

PICs: today and future Today (InP, Silica-on-Silicon...): Size of components on a chip (both functional components and interconnect components): 10 3-10 6 µm 2 Number of components on a chip: 1-10 3 Future (10-20 years from now): Size of components on a chip (both functional components and interconnect components): 1-10 3 µm 2 Number of components on a chip: 10 3-10 6 http://photonics.intec.ugent.be 5

Silica-on-silicon NTT (e.g. Miya e.a., IEEE STQE 00 pp.38) http://photonics.intec.ugent.be 6

Reduce PIC-size / increase density WE NEED: Ultra-compact waveguiding with Sharp bends (Bend radius < 10µm) Compact splitters and combiners Short mode-conversion distances Compact wavelength selective functions Highly dispersive element Small, high-q resonators Compact non-linear functions Increase power density by using tight confinement http://photonics.intec.ugent.be 7

High refractive index contrast (>2:1) High refractive index contrast allows for: Very tight bends Compact resonators with low loss Wide angle mirrors Very compact mode size strong field strength strong non-linear effects small volume to be pumped in active devices faster and/or lower power Photonic band gap effects air semiconductor dielectric High refractive index contrast is key for ultra-compact photonic circuits http://photonics.intec.ugent.be 8

Index Contrast Conventional PICs High Nanophotonics Out-of-plane index contrast Low Low In-plane (effective) index contrast High http://photonics.intec.ugent.be 9

Materials for nanophotonic waveguides In-plane index Out-of-plane contrast index contrast Si/SiO 2 (SOI) 3.5 to 1 3.5 to 1.5 Si/air 3.5 to 1 3.5 to 1 (membrane) GaAs/AlOx 3.5 to 1 3.5 to 1.5 InP/SiO 2 3.3 to 1 3.3 to 1.5 SiON/SiO 2 2 to 1.5/1 2 to 1.5 GaAs/AlGaAs 3.5 to 1 3.5 to 3.2 InGaAsP/InP 3.3 to 1 3.3 to 3.17 http://photonics.intec.ugent.be 10

SOI nanophotonics Width + Height Waveguide Definition Start: BOX SOI-Wafer thickness Thin Silicon layer Thick SiO 2 buffer http://photonics.intec.ugent.be 11

SOI-wires Group Date h [nm] w [nm] loss [db/cm] BOX [um] top clad Fab. IMEC Apr. '04 220 500 2.4 1 no DUV IBM Apr. '04 220 445 3.6 2 no EBeam Cornell Aug. '03 270 470 5.0 3 no EBeam NTT Dec. '02 300 300 6.0 3 no EBeam Yokohama Dec. '02 320 400 105.0 1 no EBeam MIT Dec. '01 200 500 32.0 1 yes G-line 50 200 0.8 +oxidation LETI / LPM Apr. '05 300 300 15.0 1 yes DUV 200 500 5.0 Columbia Oct. 03 260 600 110.0 1 yes EBeam (Table partly from Vlasov, McNab, Opt. Expr. 04, pp1630) http://photonics.intec.ugent.be 12

Outline Submicron SOI-wires Introduction: why do we need them Basic properties: theory, design, loss, wavelength, polarization Fabrication Devices: couplers, crossings, filters III-V on Silicon Introduction Coupling of light Fabrication PICMOS (Photonic Interconnect on CMOS) http://photonics.intec.ugent.be 13

Back to basics: cavities and waveguides How does light propagate in waveguides and cavities? n 2 n 2 n 1 n 1 Propagation in waveguide n 2 Emission within waveguide/cavity n 2 n 2 n 2 n 1 n 1 Propagation through cavity n 2 Propagation through waveguide discontinuity n 2 What is the role of n 2 /n 1? http://photonics.intec.ugent.be 14

Back to basics: cavities and waveguides Radiation modes d n 1 n 2 k z n 2 dispersion: continuity: k k x ω k = nk = n = k x + k c k = x, 1 x,2 2 2 0 y k k z n 2 k 0 n 1 k 0 Light line Total Internal Reflection k x For slab waveguide: 2 n 2 /n 0.99 1 0.9 0.5 n2 Fraction of (2D) k-space confined by TIR = 1 14% 44% 87% n 1 For channel waveguide: 2 n2 Fraction of (3D) k-space confined by TIR = 1 2% 19% 75% n http://photonics.intec.ugent.be 15 1

Back to basics: cavities and waveguides n 2 k z d n 1 k z k k x n 2 k z π = m Resonance if d d large many resonances d small few resonances π/d k x Two types of resonances: guided modes confined by TIR resonantly enhanced radiation modes Only one guided mode if 2n d λ 1 ( ) n 2 2 n 1 http://photonics.intec.ugent.be 16 1

Back to basics: cavities and waveguides Waveguide/cavity types: conventional waveguide: n 2 /n 1 0.9-1 single mode even for d substantially larger than λ/n 1 lots of radiation modes only small fraction of k-space well controlled by TIR hence bends, couplers need to be based on slow adiabatic transitions long interference with long coupling length between guided modes long http://photonics.intec.ugent.be 17

Back to basics: cavities and waveguides Waveguide/cavity types: high contrast waveguide: n 2 /n 1 << 1 if d is of the order of λ/n 1 : waveguide with few (0, 1, 2 ) guided modes and few (0, 1, 2 ) resonantly enhanced radiation modes large fraction of k-space well controlled in few modes hence bends, couplers can be based on fast adiabatic transitions short interference with short coupling length between modes short applications: ultra-compact waveguide circuits resonant cavity light emitting diodes (RCLEDs) http://photonics.intec.ugent.be 18

Back to basics: periodic stacks k z Λ n 1 n 2 n 1 n 2 n 1 d 2 d 1 2π/Λ n 2 k 0 n 1 k 0 k x Strong reflection and little transmission if: (for normal incidence) d1 d2 k1 + k2 Λ Λ (Bragg condition) π = m Λ λ Λ = m 2n with : n av = n av 1 d1 Λ + n d2 Λ http://photonics.intec.ugent.be 19 2

Back to basics: periodic stacks Cases: low contrast stack: n 2 /n 1 1 many periods needed for strong reflection strong reflection only for narrow angular and spectral range high contrast stack: n 2 /n 1 << 1 few periods needed for strong reflection strong reflection for broad angular and spectral range : photonic bandgap http://photonics.intec.ugent.be 20

Spectral accuracy and geometrical accuracy High index contrast components: - interference based filters, λ λ - cavity resonance wavelength λ λ - photonic crystal λ d λ d d d d d with d the waveguide width ( λ) with d the cavity length (a few λ) with d the hole diameter ( λ) if tolerable wavelength error : 1 nm tolerable length scale error : (of the order of) 1 nm http://photonics.intec.ugent.be 21

Basic Properties Effective Index h=220nm λ=1550nm 2D calc 2.75 2.5 TE0 Effective Index 2.25 2 1.75 1.5 TM0 TE1 TM0 Cladding (1.44) 1.25 TE1 1 300 400 500 600 700 800 Waveguide Width [nm] Single-Mode Width http://photonics.intec.ugent.be 22

Basic Properties Effective Index h=220nm λ=1550nm 2D calc 2.75 Effective Index 2.5 2.25 2 1.75 1.5 TE0 TM0 TE1 TE1 TM0 Cladding (1.44) 1.25 1 300 400 500 600 700 800 Waveguide Width [nm] Single-Mode Width http://photonics.intec.ugent.be 23

Basic Properties n eff -n group 5 4.5 4 3.5 3 2.5 2 1.5 1 Group Index h=220nm λ=1550nm 2D calc n g 1500 1525 1550 1575 1600 Wavelength [nm] dn = n λ dλ w=400-600 w=400-600 dn = n + ν dν 5 4.5 4 3.5 3 2.5 Determines filter properties 2 n eff n group 1.5 400 450 500 550 600 Waveguide Width [nm] http://photonics.intec.ugent.be 24

Substate Leakage Substrate Leakage Loss [db/cm] w=500nm w=300nm 1dB/cm h=220nm λ=1550nm BOX Buffer Thickness [um] http://photonics.intec.ugent.be 25

Losses Group IMEC Date Apr. '04 h [nm] 220 w [nm] 500 loss [db/cm] 2.4 BOX [um] 1 top clad no Fab. DUV σ roughness [nm] < 5 IBM Apr. '04 220 445 3.6 2 no EBeam 2.5 Cornell Aug. '03 270 470 5.0 3 no EBeam NTT Dec. '02 300 300 6.0 3 no EBeam Yokohama Dec. '02 320 400 105.0 1 no EBeam 11 MIT Dec. '01 200 500 32.0 1 yes G-line 50 200 0.8 +oxidati on LETI/ LPM Apr. '05 300 300 15.0 1 yes DUV 200 500 5.0 Columbia Oct. 03 260 600 110.0 1 yes EBeam http://photonics.intec.ugent.be 26

Loss (IMEC) Surface Roughness Field at interface α s σ 2 2 Es 2 E dx n 2 Refractive index contrast 40 35 Loss [db/cm] Losses (db/cm) 30 25 20 15 10 5 IBM 0 300 350 400 450 500 550 Width Wire width [nm] (nm) w 400nm 440nm 450nm 500nm Propagation losses 33.8 ± 1.7 db/cm 9.4 ± 1.8 db/cm 7.4 ± 0.9 db/cm 2.4 ± 1.6 db/cm http://photonics.intec.ugent.be 27

Loss (IBM) α s σ 2 2 Es 2 E dx n 2 (Vlasov, McNab, Optics Express, 04) TM TE 3.5dB/cm Loss [db/cm] w=450nm h=220nm Wavelength [nm] http://photonics.intec.ugent.be 28

Loss - other Oxidation Difference TE/TM TM higher substrate leakage TM higher scattering at vertical roughness TE higher field intensity Roughness Correlation length Grillot e.a., PTL 04, pp. 1661 (Grillot) (MIT) http://photonics.intec.ugent.be 29

Polarisation Group IMEC Date Apr. '04 h [nm] 220 w [nm] 500 loss [db/cm] 2.4 BOX [um] 1 top clad no Fab. DUV σ roughness [nm] < 5 IBM Apr. '04 220 445 3.6 2 no EBeam 2.5 Cornell Aug. '03 270 470 5.0 3 no EBeam NTT Dec. '02 300 300 6.0 3 no EBeam Yokohama Dec. '02 320 400 105.0 1 no EBeam 11 MIT Dec. '01 200 500 32.0 1 yes G-line 50 200 0.8 +oxidati on LETI/LPM Apr. '05 300 300 15.0 1 yes DUV 200 500 5.0 Columbia Oct. 03 260 600 110.0 1 yes EBeam http://photonics.intec.ugent.be 30

Polarisation Issues: Rectangular cross-section: very different n eff Square cross-section: n eff,te = n eff,tm But: polarisation conversion + higher losses Polarisation conversion in bends studied by Sakai FDTD: Conversion < 25dB (R>1um) Experiment: -13dB to -10dB Reason: side wall angle (85 o ) Polarisation insensitivity: hopeless?? Use polarisation diversity (Sakai, Fukazawa, Baba, JLT 04, pp. 520) http://photonics.intec.ugent.be 31

Temperature dependence dλ Classical Filters: c dn = dλ dt dn Si 4 1 dλ = 1.79 10 K c 140pm/K dt dt = (λ c : central wavelength) http://photonics.intec.ugent.be 32

Temperature Dependence Use Temperature Dependence for TO-switch Espinola e.a. (PTL 03, pp. 1366) L h =650um Switching time = 3.5us Switching power = 50mW http://photonics.intec.ugent.be 33

Waveguide Density Min. Center-to-center [µm] Single mode Crosstalk < 20dB/cm Waveguide Width [µm] Photonic Crystal Guides have smaller mode diameter but require several rows of holes!!! Further scaling: increase height, increase index (?) Surface Plasmon waveguides? http://photonics.intec.ugent.be 34

Bends Group IBM IMEC NTT Yokohama MIT LETI/LPM h [nm] 220 220 300 320 200 220 w [nm] 445 500 300 400 500 500 Radius [um] 1.0 2.0 5.0 1.0 2.0 5.0 2.0 3.0 1.0 1.0 resonant 2.0 5.0 Loss [db/90] 0.086 0.013 0?? 0 0.46 0.17 3 0.5 0.3 0.15 0.05 Note 20 bends 24 bends 12 bends poly-si 40 40 Columbia 340 400 resonant 1.3 2 bends (Table partly from Vlasov, McNab, Opt. Expr. 04, pp1630) http://photonics.intec.ugent.be 35

Bends (Vlasov, Mc Nab) Wide λ-range No need for resonant bends? http://photonics.intec.ugent.be 36

Fabrication: Review Group loss [db/c m] BOX [um] top clad Fab. Mask Etch Method IMEC 2.4 1 no DUV Resist Cl2/He/Hbr/O2 IBM 3.6 2 no EBeam SiO 2 CF4/CHF3/Ar (Oxide) + HBr (Silicon) Cornell 5.0 3 no EBeam ICP NTT 6.0 3 no EBeam SF6/CF4 etch, ECR-etch Yokohama 105.0 1 no EBeam metal ICP (CF4 + Xe) MIT 32.0 0.8 yes G-line SiO 2 oxidation RIE (SF6) LETI 15.0 1 yes DUV SiO 2 HBr etching 5.0 mixed Columbia 110.0 1 yes EBeam Aluminium RIE (CF4:Ar) http://photonics.intec.ugent.be 38

Fabrication EBeam Best suited for research, Features < 50nm Slow, not compatible with mass-fabrication (?) Standard Litho (G-line, I-Line) OK for 500nm lines Problem for smaller features (gaps in direction coupler, PhC, taper tips DUV (248nm, 193nm) Resolution OK (but characterisation needed!) Compatible with Mass-Fabrication Expensive mask http://photonics.intec.ugent.be 39

Original fabrication process AR-coating Si SiO 2 Si-substrate Photoresist Photoresist Bare wafer Photoresist Soft bake AR coating Illumination (UV3) (248nm deep UV) Post bake Development Silicon etch Oxide etch Resist strip http://photonics.intec.ugent.be 40

Line width with exposure dose 900 Line width (nm) 800 700 600 Designed Line Width 200 300 400 500 400 500 600 700 300 200 10 15 20 25 30 35 40 Exposure dose (mj) http://photonics.intec.ugent.be 41

Hole size with exposure dose 550 500 450 400 350 300 Hole size (nm) Marginally sufficient Sufficient Process window Designed Pitch/diameter 400/240 400/320 450/270 450/360 500/300 500/400 550/220 550/330 550/440 600/240 250 600/360 600/480 200 150 10 15 20 25 30 35 40 Exposure dose (mj) http://photonics.intec.ugent.be 42

Process Window Design Pitch/Size: 500nm /300nm Target size: 300nm Focus [µm] +0.4 +0.2 Best focus -0.2 5% deviation ellipse λ = 248nm NA = 0.63 resist = UV3 λ = 248nm NA = 0.7 resist = UV3 λ = 248nm NA = 0.7 resist = TIS -0.4-15% -10% -5% Best +5% +10% +15% energy Exposure Energy [ E/E 0 ] λ = 193nm NA = 0.63 resist = TIS http://photonics.intec.ugent.be 43

Process Window Design Pitch/Size: 400nm /240nm Target size: 200nm Focus [µm] +0.2 +0.1 Best focus -0.1 5% deviation ellipse λ = 248nm NA = 0.63 resist = UV3 λ = 248nm NA = 0.7 resist = UV3 λ = 248nm NA = 0.7 resist = TIS -0.2-15% -10% -5% Best +5% +10% +15% energy Exposure Energy [ E/E 0 ] λ = 193nm NA = 0.63 resist = TIS http://photonics.intec.ugent.be 44

Optical proximity effects example: triangular lattice resist pitch = 530nm Diameter = 420nm r/a = 0.4 Bulk hole = 420nm Border hole = 380nm Corner hole = 350nm 1um http://photonics.intec.ugent.be 45

Optical proximity effects W1 waveguide pitch = 500nm hole Ø in bulk lattice = 300nm a/λ 0.32 0.31 Ø border =300nm Light cone Ø border =310nm Ø border =320nm λ(nm) Light cone Light cone 1560 1610 0.30 Odd lattice modes MSB Odd lattice modes MSB Odd lattice modes MSB 1665 0.29 1725 0.28 1785 0.27 1850 Even lattice modes Even lattice modes Even lattice modes 0.26 0.40 0.42 0.44 0.46 0.48 k 0.50 0.40 1925 0.42 0.44 0.46 0.48 0.50 0.40 0.42 0.44 0.46 0.48 0.50 k http://photonics.intec.ugent.be k 46

Optical Proximity Correction Problem: Optical Proximity Effects Holes at lattice boundary are different than holes in bulk due to interference effects Correction on mask required (also on PICCO_03) Original Mask layout Resist on wafer Mask layout with OPC http://photonics.intec.ugent.be 47

Optical proximity corrections Determine empirically from PICCO_01 Example: 500nm pitch, 300 320nm holes Hole size deviation (nm) Border hole Bulk Corner hole Corner hole bias (nm) Border hole bias (nm) http://photonics.intec.ugent.be 48

Deep Etch Roughness Example: Ring resonator z Straight wire=400nm z Ring wire = 500nm http://photonics.intec.ugent.be 49

Roughness reduction Oxidation Thermal oxidation of top Silicon layer. Lithography Si+SiO 2 Etch 20-60nm thermal oxide optional Oxide removal with HF dip 10nm oxide 30nm oxide 50nm oxide Roughness on air-oxide interface http://photonics.intec.ugent.be 50

Roughness reduction Shallow etching Shallow etching less roughness But: Scattering at bottom of hole Re-fill hole with oxide to reduce asymmetry optional Lithography Si Etch 10nm oxide Oxide deposition 200nm hole http://photonics.intec.ugent.be 51

Silicon-only etch Deep etching Si-only etching Less roughness http://photonics.intec.ugent.be 52

Etch bias with Silicon-only etch Thick resist layer: 800nm UV3 needed for deep etching 200-300nm hole Ø: high aspect ratio causes litho-etch bias Litho Etch Result 800nm 300nm hole Ø shadow of thick resist 230-250nm hole Ø http://photonics.intec.ugent.be 53

Etch bias with Silicon-only etch Optimal Solution: new litho + etch development: no time. Short-term Solution: Resist-hardening/Resist Trimming plasma treatment Litho RH Etch Result 800nm 300nm hole Ø smaller shadow 300nm hole Ø Still slightly sloped sidewalls http://photonics.intec.ugent.be 54

Updated Fabrication Process AR-coating Si SiO 2 Si-substrate Photoresist Photoresist Bare wafer Photoresist Soft bake AR coating Illumination (UV3) (248nm deep UV) Post bake Development Resist Hardening Silicon etch Resist strip http://photonics.intec.ugent.be 55

2-step processing Two types of structures Waveguides: requires deep etch (al least through Silicon) Fibre couplers: require 50nm etch Two-step processing Fibre couplers first: 50nm etch gives little topography Wafer-scale alignment: Alignment markers on the wafer and reticle periphery, not between the structures http://photonics.intec.ugent.be 56

2-step processing shallow fibre coupler deep trench http://photonics.intec.ugent.be 57

CMOS-compatible? Well, it is Silicon It is processed in a CMOS line But CMOS = layered We: lines, holes, gaps, tips all in same layer CMOS = vias but rather low density Phot Crystals = superdense lattices Line-edge roughness: no issue in CMOS (till now) Roughness kills everything http://photonics.intec.ugent.be 58

Outline Submicron SOI-wires Introduction: why do we need them Basic properties: design, loss, wavelength, polarization Fabrication Devices III-V on Silicon Couplers Introduction Crossings Coupling of light Ring Resonators AWG Fabrication Cascaded MachZehnder PICMOS (Photonic Fibre-chip Interconnect couplers on CMOS) http://photonics.intec.ugent.be 59

Couplers - Splitters Directional Couplers Used in ring resonators, cascaded MZI Easy to choose splitting ratio Sensitive to fabrication issues (optical proximity, deviations in widths) Multi-mode interference couplers (MMI) Fabrication tolerant Standard Y-juncion Symmetric, narrow gap Advanced Couplers Sakai, Fukazawa, Baba, IEICE Trans 02, 1033 http://photonics.intec.ugent.be 60

Couplers Yokohama Nat. Univ Simulation: <0.1dB excess loss Experiment: 0.3dB excess loss Some imbalance due to opt. Prox. Other LETI-LPM: 1x8 MMI-coupler (imbalance 0.5dB) (Sakai, intec Fukazawa, 2004 Baba, IEICE Trans 02, 1033) http://photonics.intec.ugent.be 61

Crossings Crosstalk free crossings in optics? Standard crossing Large diffraction Large crosstalk (-9dB) Large loss (1.4dB) Enhanced versions Better performance Larger NOT acceptable for large density circuits Use multiple waveguide layers?? http://photonics.intec.ugent.be 62

Fibre coupling Mode mismatch between waveguide and fibre µm SOI PhC wg InP ridge wg SM-fibre core http://photonics.intec.ugent.be 63

Coupling to fiber Important: Large bandwidth Low loss Fabrication Limited extra processing Tolerant to fabrication deviations Coupling tolerance If coupling to SMF: same for all types of taper Coupling to high-na fiber: lower http://photonics.intec.ugent.be 64

Fiber-chip coupling Regular taper Difficult to fabricate Multi-mode Facet coating required http://photonics.intec.ugent.be 65

Coupling to fiber Inverse taper 0.4µm 0.2µm Broad wavelength range Single mode 500 µm 80nm polished facet Easy to fabricate (if you can do the tips) Low facet reflections http://photonics.intec.ugent.be 66

Coupling to fiber Group h [nm] w [nm] L [um] tip width [nm] Cladding Material Clad ding Size Loss IMEC 220 500 Polymer tbd IBM 220 445 150.0 75.0 Polymer 2x2 < 1dB Cornell 270 470 40.0 100.0 SiO2 2x00 < 4dB NTT 300 300 200.0 60.0 Polymer 3x3 0.8 0.2µm 0.4µm 500 µm 80nm polishe d facet http://photonics.intec.ugent.be 67

Coupling to fibre Tip fabrication EBeam Modified DUV (resist trimming) <100nm 220nm http://photonics.intec.ugent.be 68

Coupling to fiber The vertical fiber coupler use a grating to couple light from/to a fiber perpendicular to the PIC use a spot-size convertor in plane wafer scale, no need to cleave/polish the devices good alignment tolerances relatively broadband works for TE only spot size convertor Single mode fiber core (artist s impression) http://photonics.intec.ugent.be 69

Out-of-plane coupler Grating couplers : Second order grating (Λ=λ /n eff ) First order diffraction couples light out of the waveguide producing a surface normal propagating field What is new? Other grating couplers long (>100µm) very narrow bandwidth couple in and out high efficiency (>50%) Our grating coupler short (10µm) bandwidth > 50nm possible couple in and out high efficiency? http://photonics.intec.ugent.be 70

Fabricated Devices Alternative: Grating couplers Waferscale testing Waferscale packaging High alignment tolerance -5 Wavelength [nm] 1500 1520 1540 1560 1580 1600 From Fibre Single mode fiber core Transmission [db] -10-15 -20-25 -30 shallow fibre coupler deep trench λ 1dB = 35nm Towards optical circuit -35-40 http://photonics.intec.ugent.be 71

bcb cladding, ring resonator with bend coupling, R=8µm 1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600-15 -20-25 -30 T [db] -35-40 pass drop -45-50 -55-60 wavelength [nm] http://photonics.intec.ugent.be 72

Fiber Couplers Coupling light from waveguide to optical fiber on top http://photonics.intec.ugent.be 73

Experimental results 0.40 fiber coupling efficiency 0.30 0.20 0.10 620nm period 630nm period 620nm theory 630nm theory 0.00 1520 1560 1600 1640 wavelength (nm) 33% efficiency (4.8dB coupling loss) 35-40nm 1dB bandwidth http://photonics.intec.ugent.be 74

2D grating fiber coupler Fiber to waveguide interface for polarisation independent photonic integrated circuit 2D grating couples each fiber polarisation in its own waveguide in the waveguides the polarisation is the same (TE) Allows for polarisation diversity approach patent Single mode fiber core http://photonics.intec.ugent.be 75

Experimental results Fabrication SOI: 220nm Si / 1000nm SiO 2 Etch depth: 90nm Square lattice of holes: 580nm period http://photonics.intec.ugent.be 76

Optical Ring Resonators Optical Ring Resonators Relevant Characteristics: Free Spectral Range (Period) Quality Factor Determined by: Coupling ratio Round-trip loss Length (=radius) For a given loss: trade-off High Q Low coupling But Low coupling Low drop efficiency!!! http://photonics.intec.ugent.be 77

Optical Ring Resonator 1um 5um http://photonics.intec.ugent.be 78

Optical Ring Resonators -10.00-20.00-30.00-40.00 Ring resonator demux 4 rings in series Linearly increasing radius λ c does not increase linearly as expected!! -50.00-60.00 1520 1540 1560 1580 Fabrication problem: mask discretisation Solution: vary parameter which is less sensitive to fabrication Other: Peak splitting due to reflections http://photonics.intec.ugent.be 79

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Increasing Index Contrast 5 cm Low Contrast - Fiber Matched (silica or polymer based) Bend Radius ~ 5 mm Size ~ several cm^2 5 mm 200 µm Ulra-high Contrast (SOI based) Bend Radius < 50µm Medium Contrast (InP-InGaAsP) Bend Radius ~ 500µm http://photonics.intec.ugent.be 81

AWG Design Various devices were designed : R 200µm To grating couplers ν = 400GHz FSR = 8 x 400GHz w g w i w g w w = 0.5µm # arms = 18 or 24 R = 75µm ~ 150µm w i = 0.6µm ~ 1.0µm w g = 0.6µm ~ 1.0µm g = 0.2µm g w i All designs fabricated with different exposure doses (during litho) different actual waveguide widths http://photonics.intec.ugent.be 82

AWG Results 200µm AWG, 400GHz spacing, 8 channels ν = 340GHz 360GHz (different exposure times) 8dB on-chip loss -5 Wavelength [nm] 1500 1520 1540 1560 1580 1600 Transmission [db] -10-15 -20-25 http://photonics.intec.ugent.be 83

AWG Results 200µm 5 x 8 AWG, 400GHz spacing, 8 Channels 300µm x 300µm area 8dB on-chip loss 6-10 db crosstalk -5 O2 Wavelength [nm] 1500 1520 1540 1560 1580 1600 Transmission [db] -10-15 -20 7dB -25 http://photonics.intec.ugent.be 84

AWG Yokohama Nat. University (Fukazawa, Ohno, Baba, Jap. J of Appl. Physics, 04) http://photonics.intec.ugent.be 85

AWG Crosstalk Origin Possible reasons for crosstalk Overspill in star-coupler Reflections in star-coupler Phase errors in grating arms http://photonics.intec.ugent.be 86

AWG Crosstalk Origin Phase errors in Waveguide arms? Assume standard deviation for phase-error given by : 1 σ φ = π L i i f c Calculated Crosstalk vs. f c f c Crosstalk Level [db] f c =100 Roughness [nm] 10 5 f c =100 f c 0.01 0.1 1 10 100 10 3 10 4 Correlation length [µm] http://photonics.intec.ugent.be 87

Cascaded MZ Filter Example: 5 stage CMZ 3.2nm bandwidth 17nm FSR coupling efficiency ~80% -10 db crosstalk gap width = 220nm waveguide width = 535nm waveguide width = 565nm normalized output [db] 0.00-5.00-10.00-15.00-20.00 pass drop -25.00 1520.00 1530.00 1540.00 1550.00 1560.00 1570.0 wavelength [nm] L = 32.8µm 20µm 14µm 20µm 20µm 14µm 20µm http://photonics.intec.ugent.be 88

PICCO04: Cascaded MZ Filter Example: 5 stage CMZ 2.6nm bandwidth 17nm FSR coupling efficiency ~100% gap -10 width db = 220nm crosstalk waveguide width = 535nm waveguide width = 565nm normalized output [db] 0.00-5.00-10.00-15.00-20.00-25.00 pass drop 1520.00 1530.00 1540.00 1550.00 1560.00 1570.0 wavelength [nm] L = 32.8µm 26µm 14µm 20µm 20µm 14µm 26µm http://photonics.intec.ugent.be 89

Optical Proximity Effects Optical Lithography Images of neighbouring structures interfere Effect can be additive of subtractive = optical proximity effects Example: isolated line width: 565 gap width: 220nm w c w g line width in coupling section: 535nm W. Bogaerts et al. to be published in JLT (Oct 2004) w i http://photonics.intec.ugent.be 90

Conclusions Sub-micron SOI-waveguides: Powerful platform for high-density Photonic circuits We have all basic building blocs (and no need for these complicated Photonic Crystals) Fabrication issues to be solved Optical proximity (narrow lines, fine gaps) Phase-Errors, control central wavelength of devices Further reduction losses needed? Next step: active functionality? http://photonics.intec.ugent.be 91