Holographic Bragg Reflectors: Designs and Applications

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1 OTuP1.pdf 2009 OSA/OFC/NFOEC 2009 Holographic Bragg Reflectors: Designs and Applications T. W. Mossberg, C. Greiner, D. Iazikov LightSmyth Technologies OFC 2009

2 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

3 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

$not shown Out In Access Channels Diffractive Hologram$

4 Layout of a Simple Holographic Bragg Reflector Upper cladding not shown Out In Access Channels Diffractive Hologram Contours

5 Photolithographically Scribed Planar Holograms (SiO 2 on Si)

6 Photolithographic Nanofab pathway Computer Design and Tape-Out Render Computed Design (Laserwriter) nm pixel full spatial coherence Device Fab (Silicon, Glass) 4x Reduction DUV Photolithography nm Master Fab 4x Reduction DUV Photolithography nm Device Fab nanoimprinting Device Fab molding/stamping Final Devices: cm-scale pattern, Full spatial coherence, nm feature size, Low-cost mass production via stepper/nanoimprint/injection molding

7 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

8 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

9 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)

10 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

11 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 nm reflection 0.1 nm Resolution: ν c 15GHz 2nL nl( cm) Wavelength 10 GHz for 1-cm Silica Device

12 Binary Etch Compatible Apodization Approaches (Tailor Passband) Constant amplitude (a) Partial line writing (b) Interferometric contour placement (c) 500 nm line spacing

13 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

14 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

15 Interferometric GrayScale - Visual Full Grating Delete every 3 rd line (resolution control) Displace for amplitude control 500 nm

16 Example of Apodization flat top CWDM filter 0 Reflected Power (db) unapodized apodized Linearly chirped holographic reflector with and without apodization Wavelength (nm)

17 Coupling Strength Issues Core Cladding Best Coupling (diffractive elements span mode) Diffractive elements of limited depth/aspect ratio Difficult to get strong coupling

18 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 cm fabricated

19 Broad Bandwidth Reflector via Strong Coupling 0 69 nm Reflected Power (db) linear chirp linear chirp with apodization Wavelength (nm)

20 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

21 Multiplexers

22 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 IN Τ 1 Τ 3 Τ 5 Τ 7 r 18.8 mm

23 Polarization Properties

24 Close-up of Integrated HBR CWDM Mux

25 Integrated vs Discrete CWDM Mux Integrated CWDM mux Discrete TFF CWDM Mux

26 Photo of integrated holographic multiplexers Filter Sections Input Waveguides

27 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

28 8-channel, 100-GHz, HBR-based, Gaussian MUX 0 (a) Simulation (a) Simulation (constant index) Relative Insertion Loss (db) λ 1 λ 2 λ 3 λ 4 λ 5 λ 6 λ 7 λ 8-40 (b) Measurement Relative Insertion Loss (db) (b) (c) Measurement (1-channel) Simulation (apodization index coupling included) Wavelength (nm) Wavelength (nm)

29 4-channel, 50-GHz, Gaussian, HBR MUX 0 (a) Simulation -5 Relative Insertion Loss (db) (b) Measurement λ 1 λ 2 λ 3 λ db Insertion Loss Wavelength (nm)

30 Integrated Reflective Resonators

31 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/ Wavelength (nm) Wavelength (nm)

32 Photo of integrated holographic concentric cavity

33 Nearly Million-Q Integrated Reflective Resonator Reflected Power (arb. units) Wavelength (nm) Relative Reflected Power Wavelength (nm) 2.6 pm Q 10 6 Finesse 80 FSR = 220 pm FWHM = 2.6 pm Flatness Figure = 8 nm

34 Forward Scattering Holograms

35 Integrated Forward coupling Holographic lens Power Position (µm)

36 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

37 Integrated Holographic Optical Interconnects

38 In-plane/Out-of-plane Coupling via Nanoprinted Diffractive Structures Input from Surface Mount Transmitter Output to Detector Diffractive Coupler Diffractive Coupler 2D Waveguide

39 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)

40 Low Cost Fabrication for Ubiquitous Photonics

41 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

42 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.

43 LightSmyth Publications ( 1. T. W. Mossberg, Planar holographic optical processing devices, Opt. Lett. 26, (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, (2004). 3. C. Greiner, D. Iazikov, and T. W. Mossberg, Lithgraphically-fabricated planar holographic Bragg reflectors, J. Lightwave Tech. 22, (2004). 4. D. Iazikov, C. Greiner, and T. W. Mossberg, Effective grayscale in lithographically scribed planar holographic Bragg reflectors, Appl. Opt. 43, (2004). 5. C. Greiner, T. W. Mossberg, and D. Iazikov, Bandpass engineering of lithographically-scribed channel-waveguide Bragg gratings, Opt. Lett. 29, (2004). 6. D. Iazikov, C. Greiner, and T. W. Mossberg, Apodizable integrated filters for coarse WDM and FTTH-type applications, J. Lightwave Tech. 22, (2004). 7. T. W. Mossberg, D. Iazikov, and C. Greiner, Planar-waveguide integrated spectral comparator, submitted to J. Opt. Soc. America A. 21, (2004). 8. C. Greiner, D. Iazikov and T. W. Mossberg, Wavelength-division multiplexing based on apodized planar holographic Bragg reflectors, Appl. Opt. 43, (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, (2005). 10. T. W. Mossberg, C. Greiner, and D. Iazikov, Interferometric amplitude apodization of integrated gratings, Opt. Exp. 13, (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, (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, (2005). 13. T.W. Mossberg and M.G. Raymer, Optical code-division multiplexing: the intelligent optical solution, Opt.and Phot. News 12, (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, (2004). 16. T. Mossberg, C. Greiner and D. Iazikov, Submicron photolithography opens new areas of holographic design, Laser Focus World 40, (2004). 17. T.W. Mossberg, D. Iazikov, C.M. Greiner, Impact of High-resolution photolithography on integrated photonics, Microlithography World 14, 8 11 (2005).

Integrated Photonics based on Planar Holographic Bragg Reflectors

Integrated Photonics based on Planar Holographic Bragg Reflectors C. Greiner *, D. Iazikov and T. W. Mossberg LightSmyth Technologies, Inc., 86 W. Park St., Ste 25, Eugene, OR 9741 ABSTRACT Integrated