A Thesis Submitted to the Faculty of Engineering of the University Of
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(V7 UNG ASPHERIC GEODESIC LENSES FOR AN INTEGRATED OPTICAL SPECTRUM ANALYSER A Thesis Submitted to the Faculty of Engineering of the University of Glasgow for the degree of Doctor of Philosophy by Gordon Frank Doughty,, B. A. A November 1983 This work is dedicated to the author's wifer Patricia. PROLOGUE every one who has succeeded in this line has done so,, not by following written or communicated instructions, but by striking out a new line for himself; and I think I am correct in saying that there is hardly to be found any case of a person attaining notable success in the art of figuring optical surfaces by rigidly following directions or instructions given or bequeathed by others. " Sir Howard Grubb, 1886. TABLE OF CONTENTS ACKNOWLEDGMENTS & DECLARATION viii SUMMARY xiii KEY TO ABBREVIATIONS AND SYMBOLS xv CHAPTER 1 INTRODUCTION 1.1 Integrated Optical Signal-Processing 1 1.2 The integrated-optical spectrum analyser 2 1.2.1 Spectrum analysis by acousto-optic Bragg-deflection 2 1.2.2 The integrated-optical spectrum analyser (IOSA) 7 1.2.3 User requirements 10 1.3 Choice of materials and components for an IOSA 10 1.4 Titanium-diffused waveguides in lithium niobate 13 1.5 Focusing in integrated optics 15 1.5.1 Gratings and zone plates 15 1.5.2 Mode-index lenses 19 1.5.3 Configurational elements 19 1.6 Choice of geodesic lenses for the lithium niobate IOSA 21 1.7 Main findings of this work 24 CHAPTER 2 THEORY AND DESIGN OF GEODESIC LENSES 26 2.1 Specification of lenses for an IOSA 26 2.1.1 Choice of practical IOSA and lens dimensions 26 2.1.2 Propagation of truncated Gaubsian-beam 28 2.1.3 Field curvature 29 2.1.4 Lens dimensions and image size 32 , 2.2 Geometrical optics of geodesic lenses 33 2.2.1 Introduction 33 2.2.2 Fermat's principle 33 2.2.3 Geodesics on surfaces of revolution 35 2.2-. 4 Ray-tracing expressions 40 2.3 Methods of designing geodesic lenses 43 2.. 3, 'l Spherical lenses and their limitations 43 i 2.3.2 Edge-rounding of spherical lenses 48 2.3.3 Numerical optimisation: aspheric lens-profiles 49 2.3.4 Transformation of Luneberg-lens designs 50 2.3.5 Exact solution for lenses with perfect ray-traces 51 2.4 Modifi cations to the exact-solution profile of Sot tini, Russo and Righini 57 2.4.1 Problems with small radii of curvature 57 2.4.2 Bridging over locally highly-curved regi. ons 58 2.4.3 Profile tolerances related to spectrum analyser performance 64 2.5 Conclu sions 67 CHAPTER 3 REVIEW OF LITHIUM NIOBATE COMPONENT FORMATION- PROCESSES AND METHODS USED FOR PLANE-SUBSTRATE PRODUCTION 3.1 Introduction 68 3.2 Aspherical surfaces 68 3.3 Cutting lithium niobate crystals 69 3.4 Machines for generating and polishing components 70 3.5 Grinding and lapping 77 3.6 Polishing 81 3.6.1 Surface finish 81 3.6.2 Laps and surface flatness 85 3.6.3 Rate of material removal in polishing 86 3.7 Preparation of plane parallel substrates for the present work 89 3.8 Conclusions 91 CHAPTER 4 ULTRASONIC GENERATION OF GEODESIC LENSES 4.1 Introduction 92 4.2 Modification of an ultrasonic drill 94 4.3 Methods of preparing ultrasonic tpols 96 4.4 Tool wear 102 4.5 Lens generation 103 4.6 Conclusions 108 ii CHAPTER 5 GENERATION OF GEODESIC LENSES BY COMPUTER- CONTROLLED MACHINES 5.1 Review of computer-controlled aspheric generators 110 5.2 Lens generation on a jig grinder 112 5.2.1 Introduction 112 5.2.2 Plane grinding and polishing 117 5.2.3 Lens generating and polishing 121 5.3 Single-point diamond-turning of plane lithium niobate surfaces 125 5.4 Diamond-turning of geodesic lenses 133 5.4.1 Diamond-turning on a contact-lens lathe 133 5.4.2 Diamond-turning on a high-precision machine 135 5.4.3 Additions and setting adjustments to aspheriser 138 5.4.4 Turning of geodesic lenses 146 5.5 Conclusions 154 CHAPTER 6 POLISHING OF GENERATED LENSES 6.1 Large-area flexible-polisher 155 6.2 Small-area microcomputer-controlled polishing machine (MCPM) 159 6.3 Development*of small-area polishing head 160 6.4 Construction of MCPM - hardware 165 6.4.1 Accuracy and design of mechanical components 165 6.4.2 Lens mounting and rotation spindle 166 6.4.3 Polishing-head positioning-unit 168 6.4.4 Positioning-unit electronicsý,, 170 6.4.5 Auxiliary equipment 172 6.5 Construction of MCP14 - software 173 6.6 Use and evaluation of MCPM 180 6.7 Conclusions 183 iii CHAPTER 7 EVALUATION OF GEODESIC LENSES 7.1 Measurement of lens profile 184 7.1.1 Interferometric methods 184 7.1.2 Scanning depth-probe methods 188 7.1.3 Sira surface-form gauge 191 7.1.4 Development of geodesic . lens me&suring-machine 196 7.2 Formation of waveguides and coupling 200 7.2.1 Formation of titanium-diffused waveguid es on plane and concave surfaces 200 7.2.2 Methods of coupling light into waveguides 201 7.3 Assessment of surface roughness 206 7.3.1 Roughness of plane lithium niobate surf aces before and after waveguide formation 206 7.3.2 Measurements of roughness of lens surfaces 209 7.4 Assessment of lens imaging-performance 211 7.4.1 Measurement of lens focal-length 211, 7.4.2 Measurement of image profile and width 212 7.5 Comparison of measured imaging-performance wit h that calculated from profile measurements 222 7.6 Conclusions 225 CHAPTER 8 ECONOMIC ASPECTS OF GEODESIC LENS FABRICATION 8.1 Introduction 228 8.2 Cost of machines and labour 228 8.2.1 Introduction 229 8.2.2 Capital equipment 229 8.2.3 Labour requirement 231 8.2.4 Number of machines required 232 8.3 Fabrication cost per lens 233 8.3.1 Introduction 233 8.3.2 'Small number of lenses 234 8.3.3 Number of lenses which justifies MCPD1purchase 235 lenses 235 8.3 .4 Large number of 8.4 Conclusions 238 iv CHAPTER 9 CONCLUSIONS 9.1 Introduction 239 9.2 Overall IOSA design: substrate and lenses 239 9.3 Geodesic lens design 240 9.4 Lens generation by ultrasonic impact-grinding 243 9.5 Lens generation by CNC grinding 244 9.6 lens generation by diamond-turning 245 9.7 Lens polishing 246 9.8 Lens evaluation 247 9.9 Choice of a geodesic-lens fabrication method 248 I ihi's -, 9.10 summary of-origi nal work in theiiis'. 249 9.11 6iicýt_ions_____ 249 REFERENCES 250 APPENDIX I Derivation of expressions for the slope of exact ray-tracing lens (Sottini, Russo and Righini), and for the second derivatives. I-1 APPENDIX II Derivation of approximations used for small arcs, tool setting and profile assessment errors. V LIST OF TABLES AVID ILLUSTRATIONS TABLES 1. Aberration of a spherical geodesic-lens. 47 2. Circular arcs fitted to a Sottini, Russo and Righini lens profile without bridging over highly--rcurved regions. 63 3. Circular arcs fitted to a Sottinif Russo and Righi ni lens profile bridging over highly-curved regions. 63 4. Sottini, Russo and Righini rule-of-thumb approximation for tolerable lens-depth errors. 65 5. Measurements of ultrasonic tool and lens profiles. 98 6. Format and details of data required to control a Moore G18 jig grinder with GE 1050 computer for producing a 21-arc lens. 116 7. Variation of polishing feed-rate with radial-zone position. 120 8. Machines which could possibly diamond-turn geodesi c lenses. - 126 9. Diamond turning of plane lithium-niobate substrates on Bryant Symons facing-lathe. 129 10. Linear-interpolation points used for diamond- turning on a Moore aspberising machine. 140 11. Typical series of cuts and tests during diamond- turning of a geodesic lens. 149 12. Sequence of energising stepper-motor coils. 170 13. 8 bit coding of polishing-machine steps. 176 14. All possible step-values. 177 15. Memory map of loaded system. 179 16. Sign rules for the four loops through the polishing data-file. 179 17. Measured attenuation of narrow light-beams through lens. 215 18. Five combinations of lens-fabrication techniques. 229 19. Labour time per lens required by each combination of fabrication methods. 232 vi FIGURES 1.1 Spectrum analysis by acousto-optic Bragg deflection 3 1.2 Geodesic-lens integrated-optical microwave-spectrum analyser. 8 1.3 Focusing elements for surface waveguides. 16 2.1 Fitting one or two IOSA onto a Glmm diameter boule section. 27 2.2 Effect of field curvature on defocusing at detector array. 30 2.3 RAtio of worst-case defocused spot-width to detect or- element size as a function of lens focal-length. 31 2.4 Layout and dimensions of IOSA with two identical lenses. 34 2.5 Relationship between infinitesimals on a geodesic surface. 36 2.6 Plane projection of ray paths on and beyond an axisymmetrical geodesic lens. 36 2.7 Geodesic path on a spherical surface. 44 2.8 Geometrical trace of rays from infinity through a spherical-section geodesic lens, illustrating spherical aberration. 46 2.9 Definition of terms used in SRR derivation of perfectly-focubing geodesic-lens profile. 53 2.10 A family of lens profiles with F/12.7 relative aperture. 55 2.11 A geodesic lens profile derived by the SRR methody with diameter 10mm, and edge-rounding beyond 3.7mm, for conjugates infinity and 18.5mm.