Planar Waveguide Illuminator with Variable Directionality and Divergence William Maxwell Mellette, Glenn M. Schuster, Ilya P. Agurok, Joseph E. Ford Electrical & Computer Engineering Department University of California, San Diego 11/05/13 Photonics Systems Integration Lab Presented at 2013 OSA Optics & Photonics Conference: Renewable Energy and the Environment, Solid State & Organic Lighting (SOLED)
Motivation for New Illumination System Context: Conventional LED Illumination Systems • Directional, collimated “spot” illumination. • Diffuse “flood” illumination. • Cannot switch due to fixed optical path. Directional Illumination Diffuse Illumination Maxik et al. Bergmann et al. Walezak et al. Yuen Bolta et al. US pat. D528,673 S US pat. D587,832 S US pat. 7,744,259 US pat. D553,267 S US pat. 7,234,844 • Based on old technologies: Diffuse illumination = Directional illumination = Localized source Localized source + diffuser + reflector (lens) C. H. Muckenhirn M. C. Meigs. US pat. 1,288,124 (1918) US pat. 209,178 (1878) Goal: System with variable directionality and divergence for efficient use of light energy 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Waveguide Based Illumination System 1) LED Sources Lens Array • High luminance, high efficacy. LEDs 2) Coupling • Tradeoff between spatial power density and divergence. 3) Guiding and Extraction • Confinement by total internal Couplers reflection. • Periodic extraction features scatter light toward lens array. Extraction Waveguide Features 4) Beam Steering & Divergence • Lenses image extraction features to an infinite conjugate. • Translations between lenslet and extraction arrays steer total beam by steering individual beams in the same direction. • Rotations between arrays steer individual beams in different directions, altering divergence of the total beam. Aligned Translated Rotated Continuous control over directionality and divergence through small mechanical actuation 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Planar Micro-Optic Solar Concentrator Research UCSD Photonics Basic Concept: Low-cost planar concentrator optics Micro-Tracking Waveguide Concentrator 1.0 mm 0.87 mm Wide Angle Doublet PMMA Microlens Design Lenlet Array 0.36 Polycarbon mm ate Waveguide Coupling facets Concentrated mm 1.0 & Uniform BK7 Waveguide Output Higher-Efficiency Orthogonal Concentrators - - - 0 1 3 3 ° 35 30 15 5 0 5 ° ° ° ° ° ° 70 ° field of view (2.5mm path) Worm drive motors Self-contained micro-tracking mechancial system prototype Fresnel End Mirror & Angled Injection Normalized Optical Efficiency vs. Time Facets 100 Output 90 Uniformity Trackin 80 g Normalized Optical Efficiency 70 “on” 60 50 40 30 20 Trackin Cloud 10 g “off” cover 0 0 10 20 30 40 50 60 J. H. Karp et al, “Orthogonal and secondary concentration in “Lateral translation micro-tracking of planar micro-optic solar concentrator,” Time (minutes) planar micro-optic solar collectors,” Optics Express, May 2011. SPIE Conference on Solar Energy & Technology , Paper 7769-03 August 2010. PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Design Considerations LEDs Cree Xlamp XM-L2 Active area: 2.5x2.5mm • From conservation of radiance: brightness of Emittance: 116.5 lm/mm 2 output determined by brightness of source. Power: 6.2 W Efficacy: 159.13 lm/W Want large package high luminance LEDs. 2.5 mm Lenses • Determine max. steering angle, crosstalk, and min. divergence angle. Want low F/#, low divergence source, small source. Light Guiding & Extraction 1) Constant Mode Volume 2) Stepped Mode Volume • Faceted extraction features: divergence maintaining, broadband, axially symmetric. Input • Waveguide confines light by TIR. • Two configurations: Output Want thin waveguide. Input Couplers • Efficient coupler must conserve radiance. Coupler • Impose above constraints, design becomes etendue matching problem. Needed: efficient coupling structure. Large Source Thin Waveguide 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Coupler Design Approach h 2 , θ 2 h 2 x h 2 , θ 2 Mh 2 x t wg , θ 2 h 1 , θ 1 Aperture Collimation Source Transformation 1) Collimation [ ] ( ) ( ) ( ) • Compound parabolic concentrators (CPCs) = − + − + ∈ 2 2 B t 1 t P 2 1 t tP t P , t 0 , 1 0 1 2 provide nearly etendue limited concentration, Parameterized by variable t. Points P 0 , P 1 , P 2 ϵ R 2 . and likewise, collimation. 4 • When used as a collimator, the conventional CPC has poor spatial uniformity at the output. 3 • Quadratic Bezier curve allows tradeoff between 2 Conventional CPC spatial uniformity and divergence. Modified Bezier Curve 1 “Method to improve spatial uniformity with lightpipes”, 0 0 2 4 6 8 10 12 14 16 18 Fournier, Cassarly, Rolland, Optics Letters, Vol. 33, No. 11, June 1, 2008. CAD models Angular Output Spatial Output • Optimized in Nonsequential Zemax. – Merit function: CPC: • Minimize standard deviation (RMS from mean) of all nonzero intensity values. • Minimize radial RMS from 0º (on axis) Bezier: in polar space. – Variables: • Control point (P 1 ) and axial length. 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Coupler Design 2) Space Variant Aperture Transformation • Define structures which segment and rearrange a square aperture into a rectangular aperture. • Designed for perfectly collimated input, modeled in Zemax for varying degrees of divergence. i. Faceted Structure Approaches square input aperture as aspect ratio increases. ii. Curved Structure TIR limit planar guide R outer θ t output input − t θ = − cos 1 1 2 R outer “Analysis of Curved Optical Waveguides by Conformal Transformation”, Heiblum, Harris, IEEE, Vol. QE-11, No. 2, Feb 1975 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Analytic Model & Optimization Motivation Analytic Approach • Optimization difficult in standard raytracing software. • Use equations from imaging and nonimaging optics. • Create analytic optimization procedure. • Find optimal designs in a constrained space. • Show that optimal designs have useful performance. • Verify predicted performance using Zemax. Etendue: Geometrical Optics: (Lens F/# = f/D) ψ max π β 2 ( ) ( ) ( ) ∫ ∫∫ = θ θ θ ϕ = β ( ) 2 2 2 2 1 G n cos sin d d dS n h sin D ψ = − − − θ 1 1 sin n sin tan tan ( ) max 2 2 F /# S 0 0 ( ) ( ) θ = θ h sin h sin h 1 1 1 2 2 (Half divergence angle (Waveguide index) φ within waveguide) θ 1 (Spatial extent) (Facet width) θ 2 (Half div. angle) f w Radiometry: Geometry: − − ϕ = facet h 2 1 1 ( ) sin n sin tan η = sin θ 2 = ⋅ 2 f h M t beam 1 2 wg (# of segments) (Lens focal length) t wg 2 options: = (Waveguide index) N # of facets 1) Constant Mode 2) Stepped Mode Volume Waveguide Volume Waveguide γ = 45º D γ = 45º θ w facet w facet w facet ( ( ) ( ) ) w facet D θ = − ϕ 1 cos 2 N F /# tan t ( ) 2 − θ = 2 wg w ( ) N cos w ( ) θ = = < facet γ tan facet A w 2 t ( ) ( ) ( ) tan − + θ θ facet facet wg N N 1 cos sin 2 11/05/2013
Optimized Design Performance 2 feet 2 feet Physical realization of optimal Input stepped mode volume design: F/0.5 Desired: 1.5x10 4 lx • Cree Xlamp XML2 Modeled: 1.3x10 4 lx • Curved coupling structure Input • F/0.5 reflective spherical lenses Aspect Ratio = 14:1 • 40 x 40 lenses in full system. N = 20 No translation Translation Optical Efficiency Analytic Model Zemax Model Luminous intensity (linear scale) Good agreement between models No rotation Optical Efficiency Rotation Zemax Model Luminous Intensity (log scale) 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
Simulated System Application Far field intensity modeled in Zemax, exported as .ies file. Room illumination modeled in Dialux software using radiosity method. • FEM approach to global illumination. Applies to Lambertian surfaces. Iterates through subsequent scattering steps until convergence. θ θ ( ) ( ) ( ) ( ) ( ) cos cos ∫ = + ρ Iterative solution to radiosity method. x x ' B x dA E x dA x dA B x ' Vis x , x ' dA ' π 2 r S Conventional 2x2’ system (53W, 4000lm) UCSD SMV2a optimized design, 2x2’ aperture (54.82W, 5700lm) Y X Translation: ( Δ x, Δ y) = (-3, 3) mm Translation: ( Δ x, Δ y) = (5, 0) mm Rotation = 1º *Nonlinear Scale 11/05/2013 PHOTONIC SYSTEMS INTEGRATION LABORATORY – UCSD JACOBS SCHOOL OF ENGINEERING
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