Planar Waveguide Illuminator with Variable Directionality and Divergence

Transcription

1 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 013 OSA Optics & Photonics Conference: Renewable Energy and the Environment, Solid State & Organic Lighting (SOLED)

2 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. US pat. D58,673 S Bergmann et al. US pat. D587,83 S Walezak et al. US pat. 7,744,59 Yuen US pat. D553,67 S Bolta et al. US pat. 7,34,844 Based on old technologies: Directional illumination = Localized source + reflector (lens) Diffuse illumination = Localized source + diffuser M. C. Meigs. US pat. 09,178 (1878) C. H. Muckenhirn US pat. 1,88,14 (1918) Goal: System with variable directionality and divergence for efficient use of light energy

3 Waveguide Based Illumination System LEDs Lens Array 1) LED Sources High luminance, high efficacy. ) Coupling Tradeoff between spatial power density and divergence. Couplers 3) Guiding and Extraction Confinement by total internal reflection. Periodic extraction features scatter light toward lens array. Aligned Translated Extraction Features Rotated Waveguide 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. Continuous control over directionality and divergence through small mechanical actuation

4 Planar Micro-Optic Solar Concentrator Research UCSD Photonics Basic Concept: Low-cost planar concentrator optics Micro-Tracking Waveguide Concentrator 1.0 mm Lenlet Array Waveguide Coupling facets Concentrated & Uniform Output Higher-Efficiency Orthogonal Concentrators Wide Angle Doublet Microlens Design 0.87 mm 0.36 mm 1.0 mm PMMA Polycarbon ate BK7 Waveguide field of view (.5mm path) Worm drive motors Fresnel End Mirror & Angled Injection Facets Output Uniformity Self-contained micro-tracking mechancial system prototype Normalized Optical Efficiency Normalized Optical Efficiency vs. Time Trackin g on Time (minutes) J. H. Karp et al, Orthogonal and secondary concentration in Lateral translation micro-tracking of planar micro-optic solar concentrator, planar micro-optic solar collectors, Optics Express, May 011. SPIE Conference on Solar Energy & Technology, Paper August Trackin g off Cloud cover

5 Design Considerations LEDs From conservation of radiance: brightness of output determined by brightness of source. Want large package high luminance LEDs..5 mm Cree Xlamp XM-L Active area:.5x.5mm Emittance: lm/mm Power: 6. W Efficacy: lm/w Lenses Determine max. steering angle, crosstalk, and min. divergence angle. Want low F/#, low divergence source, small source. Light Guiding & Extraction Faceted extraction features: divergence maintaining, broadband, axially symmetric. Waveguide confines light by TIR. Two configurations: Want thin waveguide. Input Output Input 1) Constant Mode Volume ) Stepped Mode Volume Couplers Efficient coupler must conserve radiance. Impose above constraints, design becomes etendue matching problem. Needed: efficient coupling structure. Coupler Large Source Thin Waveguide

6 Coupler Design Approach h, θ h x h, θ Mh x t wg, θ Source h 1, θ 1 Collimation Aperture Transformation 1) Collimation Compound parabolic concentrators (CPCs) provide nearly etendue limited concentration, and likewise, collimation. When used as a collimator, the conventional CPC has poor spatial uniformity at the output. Quadratic Bezier curve allows tradeoff between spatial uniformity and divergence. Method to improve spatial uniformity with lightpipes, Fournier, Cassarly, Rolland, Optics Letters, Vol. 33, No. 11, June 1, 008. Optimized in Nonsequential Zemax. Merit function: Minimize standard deviation (RMS from mean) of all nonzero intensity values. Minimize radial RMS from 0º (on axis) in polar space. Variables: Control point (P 1 ) and axial length B ( t) = ( t) P + ( 1 t) tp + t P, t [ 0,1] Parameterized by variable t. Points P 0, P 1, P ϵ R. Conventional CPC Modified Bezier Curve CPC: Bezier: CAD models Angular Output Spatial Output

7 Coupler Design ) 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 t R outer output θ input θ cos 1 1 = t R outer Analysis of Curved Optical Waveguides by Conformal Transformation, Heiblum, Harris, IEEE, Vol. QE-11, No., Feb 1975

8 Analytic Model & Optimization Motivation Optimization difficult in standard raytracing software. Create analytic optimization procedure. Show that optimal designs have useful performance. Analytic Approach Use equations from imaging and nonimaging optics. Find optimal designs in a constrained space. Verify predicted performance using Zemax. Etendue: π β = n cos S sin 1 sin G ( θ ) h ( θ ) h = (Spatial extent) (Half div. angle) Radiometry: ( ) η beam = sin θ1 ( θ ) sin( θ ) dθdϕds = n h sin ( β ) Geometry: h = M h 1 θ 1 θ t wg (# of segments) N = # of facets h f options: D ψ max φ ψ t wg Geometrical Optics: max = sin 1 nsin tan ϕ = sin 1 1 (Waveguide index) nsin tan (Waveguide index) (Lens F/# = f/d) 1 ( F /#) 1 w f tan (Facet width) facet ( θ ) (Half divergence angle within waveguide) (Lens focal length) 1) Constant Mode Volume Waveguide ) Stepped Mode Volume Waveguide γ = 45º θ D γ = 45º D w facet A facet = w facet < twg w facet w facet θ = cos tan( θ ) = N 1( N( F /#) tan( ϕ) ) N cos ( θ ) ( N 1) + cos( θ ) sin( θ ) t w facet = tan wg ( γ ) w facet w facet

9 Optimized Design Performance Physical realization of optimal stepped mode volume design: Cree Xlamp XML Curved coupling structure F/0.5 reflective spherical lenses 40 x 40 lenses in full system. Aspect Ratio = 14:1 N = 0 F/0.5 Input feet Desired: 1.5x10 4 lx Modeled: 1.3x10 4 lx feet Input No translation Optical Efficiency Analytic Model Zemax Model Luminous intensity (linear scale) Translation No rotation Good agreement between models Optical Efficiency Zemax Model Luminous Intensity (log scale) Rotation

10 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. B x x' ( x) da = E( x) da + ρ( x) da B( x ) Vis( x, x' ) S cosθ cosθ πr ' da' Iterative solution to radiosity method. Conventional x system (53W, 4000lm) UCSD SMVa optimized design, x aperture (54.8W, 5700lm) Y X Rotation = 1º Translation: (Δx, Δy) = (-3, 3) mm Translation: (Δx, Δy) = (5, 0) mm *Nonlinear Scale

11 Optimized System Components Prototype Components SMVa Optimized Design Stepped mode volume waveguide Acrylic Sheet 0.1 x 4 x 4 inches. Machined with 1mm diameter hemispherical holes. Upper bound on absorption measured: α = 1 m -1 Flat faceted extraction features Cree Xlamp XML Curved coupling structure 1mm dia. ball bearings PCB designed to provide adequate heat sinking using thin FR4 substrate and silver adhesive. Custom spherical reflective lenses Edge Coupler LEDs registered with high precision using custom fixture during reflow soldering. Detail SunLED right angle SMD LED Designed CPC, Sputtered Silver Refractive Fresnel lens array Two F/1.04 lenses oriented grooves out = F/0.7 lens in PMMA. Array is 4 x 4 lenses = 3 x 3 inches.

12 Prototype Model and Measurement Unit Cell System : Lab measurement to determine performance. Far field intensity pattern: Far Field Pattern Cross Section, Luminous Intensity Analytic Model Zemax Model Measurement (a.u.) Measurement Model Superposition of 3 patterns from 3 LEDs. Fresnel Lens Good agreement between analytic model, Zemax model, and measurement. Non-ideal off-axis Fresnel lens performance eliminates crosstalk lobes. Paraxial lens model for Zemax and analytic models

13 Experimental System CAD Design Magnetic eccentric cams as actuation mechanism. 8 x 8 lens array. 304 LED sources coupled to edges of waveguide. x foot aperture System Fabrication and Test Ceiling 9 feet Prototype

14 Summary of Results Design of new illumination system Continuously variable directionality and divergence allows efficient use of light energy. Optimized designs can achieve performance metrics matching those of conventional illumination systems, while simultaneously providing new functionality. Prototype demonstration Measurements of unit cell prototype validate the accuracy of Zemax and analytic models used in design process. Full x experimental system provides proof of principle in a large aperture system. Thank you wmellett@ucsd.edu psilab.ucsd.edu