OTuP1.pdf 2009 OSA/OFC/NFOEC 2009 Holographic Bragg Reflectors: Designs and Applications T. W. Mossberg, C. Greiner, D. Iazikov LightSmyth Technologies OFC 2009
Review - Volume Holograms (mode-selective diffraction) Holographic Interference Holographic PatternInterference Pattern Output keyed to (1) Input wavelength (2) Input Output Beam Wrong wavefront input wavefront = suppressed output Spatial Wavefront Selectivity Input Beam
Holographic Bragg Reflectors in Planar Waveguides Uses Spectral filtering (like integrated thin-film filter) Compact multiplexers with very flexible passband control Integrated high-q reflective resonators Photonic interconnect fabrics Holograms in planar waveguides: Why Now? Flexible fabrication finally available (if you can design it you can build it!) Computer Design simulated input/output beams Laser writer (large spatially coherent writing area) reticle at 0.3 micron resolution DUV stepper-based photo-reduction Lithography X4 demagnification of reticle onto wafer (Si or SiO 2 ) Volume production with low cost
Layout of a Simple Holographic Bragg Reflector Upper cladding not shown Out In Access Channels Diffractive Hologram Contours
Photolithographically Scribed Planar Holograms (SiO 2 on Si)
Photolithographic Nanofab pathway Computer Design and Tape-Out Render Computed Design (Laserwriter) 200-1000 nm pixel full spatial coherence Device Fab (Silicon, Glass) 4x Reduction DUV Photolithography 50-250 nm Master Fab 4x Reduction DUV Photolithography 50-250 nm Device Fab nanoimprinting Device Fab molding/stamping Final Devices: cm-scale pattern, Full spatial coherence, 50-250 nm feature size, Low-cost mass production via stepper/nanoimprint/injection molding
Laser Mask Writer Arbitrary Pattern, Submicron Pixels Micronic Laser Systems AB Write time (6 mask) 1 h 45 min Minimum main feature 220 nm Address grid 1.25 nm CD uniformity (global, 3 σ) 7 nm Registration (global, 3 σ) 15 nm
Modern DUV Reducing Stepper/Scanner Reduction Factor 4x (from mask) Resolution 65 nm Field Size 26 X 33 mm Throughput 122 wph 300 mm wafers 125 exposures
Slab-Waveguide Holographic Filter (Silica-on-Si) Core Thickness 2-4 microns Cladding Thickness 15 microns Materials Doped SiO 2 Substrate Silicon Wafer Device Length 0-2 cm Lines/cm 20,000 (1.5 µm) Line Spacing Λ λ/2n 7 top view out HBR in Cladding Core 7 Cladding cross sectional view (of slab waveguide)
Powerful Filter Passband Control ε(z) = reflection amplitude z = depth into device Throughput T(ν) 2 HBR T(ν ) ε ( z) exp{ 2π iν z/c} dz frequency (ν) out in
Spatial Coherence of Stepper-Written Holographic Filters Measured reflection bandwidth 0.08 nm Calculated weak signal width 0.05 nm ( 6 GHz) Spatial Coherence Length 1.5 cm Relative Power Measured Spectra of a 19-mm uniform grating transmission 1527.2 nm reflection 0.1 nm Resolution: ν c 15GHz 2nL nl( cm) Wavelength 10 GHz for 1-cm Silica Device
Binary Etch Compatible Apodization Approaches (Tailor Passband) Constant amplitude (a) Partial line writing (b) Interferometric contour placement (c) 500 nm line spacing
Partial-Fill GrayScale 2D Holographic Filter 1D Channel Grating Λ 100% integrated holographic grating 33% top view Partially Scribe Contours Percentage Scribed Controls Net Reflective Amplitude Λ out in w
Interferometric GrayScale Incident Field d o Reflected Field z What is net reflected field? δ i d o /2- δ i d o δ i E R E R ( δi, εi) exp(2ikoβi )cos(2koδi ) Phase Amplitude, β i = (z i -z 0 i ) z 0 i z i
Interferometric GrayScale - Visual Full Grating Delete every 3 rd line (resolution control) Displace for amplitude control 500 nm
Example of Apodization flat top CWDM filter 0 Reflected Power (db) -10-20 -30 unapodized apodized Linearly chirped holographic reflector with and without apodization -40 1480 1490 1500 Wavelength (nm)
Coupling Strength Issues Core Cladding Best Coupling (diffractive elements span mode) Diffractive elements of limited depth/aspect ratio Difficult to get strong coupling
Strong Coupling via Dual Core Construction Grating Layer (higher index) Shape edge for adiabatic transition Focus mode on grating For 1/e penetration depth, d, grating length L νtot 10GHz L( cm) d 2 ( cm ) d 0.027 cm fabricated
Broad Bandwidth Reflector via Strong Coupling 0 69 nm Reflected Power (db) -10-20 -30 linear chirp linear chirp with apodization -40 1480 1500 1520 1540 1560 1580 Wavelength (nm)
Losses Out-of-plane scattering (loss) In-plane scattering (guided) λ Cladding Core Cladding ρ 1 n n clad core Λ/2 No Losses! λ = Λ ( ) 1 ρ 2 Λ
Multiplexers
Coarse WDM Mux (only integrated solution) Τ (ν) 8 Τ 6 Τ 4 Τ 2 Τ 1 Τ 2 Τ 3 Τ 4 Τ 5 Τ 6 Τ 7 Τ 8 1 2 3 4 5 6 7 8 IN Τ 1 Τ 3 Τ 5 Τ 7 r 18.8 mm
Polarization Properties
Close-up of Integrated HBR CWDM Mux
Integrated vs Discrete CWDM Mux Integrated CWDM mux Discrete TFF CWDM Mux
Photo of integrated holographic multiplexers Filter Sections Input Waveguides
Stacked Integrated Holographic Devices for Multiplexing Τ8 (ν) Τ6 Τ4 Τ2 Τ6 Τ7 Τ8 Τ Τ 5 Τ1 Τ2 Τ3 4 Stack upper cladding core IN lower cladding Τ1 Τ3 Τ5 Τ7 r Overlay Τ4(ν) Τ2 Interleave IN Τ1 Τ3 Slab Waveguide Λ d
8-channel, 100-GHz, HBR-based, Gaussian MUX 0 (a) Simulation 0-10 -20 (a) Simulation (constant index) -10-30 Relative Insertion Loss (db) -20-30 0-10 -20-30 λ 1 λ 2 λ 3 λ 4 λ 5 λ 6 λ 7 λ 8-40 (b) Measurement Relative Insertion Loss (db) 0-10 -20-30 -40 0-10 -20-30 -40 (b) (c) Measurement (1-channel) Simulation (apodization index coupling included) 1529 1530 1531 1532 1533 1534 1535 Wavelength (nm) 1529 1530 1531 1532 Wavelength (nm)
4-channel, 50-GHz, Gaussian, HBR MUX 0 (a) Simulation -5 Relative Insertion Loss (db) -10-15 0-5 -10 (b) Measurement λ 1 λ 2 λ 3 λ 4 1.6 db Insertion Loss -15 1529.0 1529.5 1530.0 1530.5 Wavelength (nm)
Integrated Reflective Resonators
Integrated Reflective High-Q Resonator Input 4 mm Reflection Reflection (TM) HBR R = 73% Transmission (TM) HBR R = 73% Transmission Power (arb. units) Finesse: ~10 Losses < 1.2 % per pass Cavity Q almost 10 5 (reflectivity-limited) Surface figure λ/20 1527 1529 1531 1533 1535 Wavelength (nm) 1531.0 1531.2 1531.4 1531.6 Wavelength (nm)
Photo of integrated holographic concentric cavity
Nearly Million-Q Integrated Reflective Resonator Reflected Power (arb. units) 0.4 0.3 0.2 0.1 0.0 1524 1525 1526 1527 1528 Wavelength (nm) Relative Reflected Power 1.0 0.9 0.8 0.7 0.6 0.5 0.4 1525.30 1525.31 Wavelength (nm) 2.6 pm Q 10 6 Finesse 80 FSR = 220 pm FWHM = 2.6 pm Flatness Figure = 8 nm
Forward Scattering Holograms
Integrated Forward coupling Holographic lens 1.0 0.8 Power 0.6 0.4 0.2 0.0-30 -20-10 0 10 20 30 Position (µm)
Photonic Crystals vs Holograms (Strong vs Weak Scattering) Photonic Crystals Space filling = lattice + basis = highly constrained structure. Control all optical modes = very short scattering length = high refractive index contrast. high losses to out-of-plane scattering Good for suppression of spontaneous radiative decay Volume Holograms Not space filling, translationally invarient, flexible structure Control specific signal modes, leave others alone = long scattering length = low refractive index contrast low out-of-plane scattering Good for optical signal filtering and routing in photonic circuits
Integrated Holographic Optical Interconnects
In-plane/Out-of-plane Coupling via Nanoprinted Diffractive Structures Input from Surface Mount Transmitter Output to Detector Diffractive Coupler Diffractive Coupler 2D Waveguide
Interconnection via Couplers and Photonic Transport Plane 1-D Solution: Fiber 2-D Solution: Channels waveguides wire analogs the optimal solution? What s the alternative? Photonic transport plane 1. slab waveguide 2. wavelength+wavefront selective I/O coupling along slab normal 3. contentionless links, point-to-point, point-to-multipoint 4. photonic localization only at source and sink 5. compatible with standard semiconductor fab 6. low effective index contrast for mode selectivity and low loss (anti-photonic bandgap) A C B A B D C D C cladding cladding Guiding diffractive structure core layer(s)
Low Cost Fabrication for Ubiquitous Photonics
Integrated Holographics, Replication, Plastics Litho Scribed Stamp/Mold UV phase mask write Upper Cladding (poured) UV writing beam Cladding Core Cladding Direct Grating Write Upper Cladding (poured) Cladding
Summary Fabrication advances open the door to planar volume holographics devices Functional equivalent of integrated thin film filters Multiplexers (small with flexible passband) Integrated reflective resonators (high finesse, high Q) Truly photonic integrated circuits Overlapping, delocalized signal paths Signal selection via mode-specific holographic structures Consistent with low cost volume manufacture DUV photolithography Nanoimprinting Many basic questions remaining to possible function, loss management, mode selectivity, non-linear effects, etc., remain.
LightSmyth Publications (www.lightsmyth.com) 1. T. W. Mossberg, Planar holographic optical processing devices, Opt. Lett. 26, 414 416 (2001). 2. C. Greiner, D. Iazikov and T. W. Mossberg, Fourier-transform-limited performance of a lithographically-scribed planar holographic Bragg reflector, Photon. Technol. Lett. 16, 840 842 (2004). 3. C. Greiner, D. Iazikov, and T. W. Mossberg, Lithgraphically-fabricated planar holographic Bragg reflectors, J. Lightwave Tech. 22, 136 145 (2004). 4. D. Iazikov, C. Greiner, and T. W. Mossberg, Effective grayscale in lithographically scribed planar holographic Bragg reflectors, Appl. Opt. 43, 1149 1155 (2004). 5. C. Greiner, T. W. Mossberg, and D. Iazikov, Bandpass engineering of lithographically-scribed channel-waveguide Bragg gratings, Opt. Lett. 29, 806 808 (2004). 6. D. Iazikov, C. Greiner, and T. W. Mossberg, Apodizable integrated filters for coarse WDM and FTTH-type applications, J. Lightwave Tech. 22, 1402 1407 (2004). 7. T. W. Mossberg, D. Iazikov, and C. Greiner, Planar-waveguide integrated spectral comparator, submitted to J. Opt. Soc. America A. 21, 1088 1092 (2004). 8. C. Greiner, D. Iazikov and T. W. Mossberg, Wavelength-division multiplexing based on apodized planar holographic Bragg reflectors, Appl. Opt. 43, 4575 4583 (2004). 9. C. Greiner, D. Iazikov and T. W. Mossberg, Low-loss silica-on-silicon two-dimensional Fabry Perot cavity based on holographic Bragg reflectors, Opt. Lett. 30, 38 40 (2005). 10. T. W. Mossberg, C. Greiner, and D. Iazikov, Interferometric amplitude apodization of integrated gratings, Opt. Exp. 13, 2419 2416 (2005). 11. Yue-Kai Haung, Varghese Baby, Paul R. Prucnal, Christoph M. Greiner, Dmitri Iazikov and Thomas W. Mossberg, Integrated holographic encoder for wavelength-hopping/time-spreading optical CDMA, Photon. Technol. Lett. 17, 825 827 (2005). 12. Jose M. Castro, David F. Geraghty, Seppo Honkanen, Christoph M. Greiner, Dmitri Iazikov and Thomas W. Mossberg, Demonstration of mode conversion using anti-symmetric waveguide Bragg gratings, Optics Express 13, 4180 4184 (2005). 13. T.W. Mossberg and M.G. Raymer, Optical code-division multiplexing: the intelligent optical solution, Opt.and Phot. News 12, 50 54 (2001). 14. T. W. Mossberg, Lithographic holography in planar waveguides, SPIE Holography Newsletter 12, (2001). 15. T. W. Mossberg, C. Greiner, and D. Iazikov, Holographic Bragg reflectors, photonic bandgaps, and photonic integrated circuits, Optics and Photonics News 15, 25 33 (2004). 16. T. Mossberg, C. Greiner and D. Iazikov, Submicron photolithography opens new areas of holographic design, Laser Focus World 40, 73 76 (2004). 17. T.W. Mossberg, D. Iazikov, C.M. Greiner, Impact of High-resolution photolithography on integrated photonics, Microlithography World 14, 8 11 (2005).