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Optical Systems for Laser Scanners1 to Provide Yet Another Perspective

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Optical Systems for Laser Scanners1 to Provide Yet Another Perspective 2 OpticalSystemsforLaserScanners Stephen F. Sagan NeoOptics Lexington, Massachusetts, USA CONTENTS 2.1 Introduction .......................................................................................................................... 70 2.2 Laser Scanner Configurations ............................................................................................71 2.2.1 Objective Scanning..................................................................................................71 2.2.2 Post-objective Scanning ..........................................................................................72 2.2.3 Pre-objective Scanning............................................................................................72 2.3 Optical Design and Optimization: Overview .................................................................72 2.4 Optical Invariants ................................................................................................................ 74 2.4.1 The Diffraction Limit .............................................................................................. 76 2.4.2 Real Gaussian Beams .............................................................................................. 76 2.4.3 Truncation Ratio ....................................................................................................... 77 2.5 Performance Issues .............................................................................................................. 79 2.5.1 Image Irradiance ......................................................................................................79 2.5.2 Image Quality ........................................................................................................... 79 2.5.3 Resolution and Number of Pixels.......................................................................... 81 2.5.4 Depth of Focus Considerations.............................................................................. 81 2.5.5 The F–Θ Condition ..................................................................................................83 2.6 First- and Third-Order Considerations ............................................................................84 2.6.1 Correction of First-Order Chromatic Aberrations ..............................................87 2.6.2 Properties of Third-Order Aberrations ................................................................88 2.6.2.1 Spherical Aberration ................................................................................. 89 2.6.2.2 Coma ...........................................................................................................89 2.6.2.3 Astigmatism ...............................................................................................89 2.6.2.4 Distortion ...................................................................................................90 2.6.3 Third-Order Rules of Thumb ................................................................................. 91 2.6.4 Importance of the Petzval Radius .........................................................................92 2.7 Special Design Requirements ............................................................................................ 93 2.7.1 Galvanometer Scanners .......................................................................................... 93 2.7.2 Polygon Scanning ....................................................................................................94 2.7.2.1 Bow .............................................................................................................. 94 2.7.2.2 Beam Displacement .................................................................................. 94 2.7.2.3 Cross-Scan Errors...................................................................................... 94 2.7.2.4 Summary ....................................................................................................97 2.7.3 Polygon Scan Efficiency .......................................................................................... 98 2.7.4 Internal Drum Systems ...........................................................................................99 2.7.5 Holographic Scanning Systems ........................................................................... 100 69 70 Handbook of Optical and Laser Scanning 2.8 Lens Design Models .......................................................................................................... 100 2.8.1 Anatomy of a Simple Scan Lens Design............................................................. 101 2.8.2 Multiconfiguration Using Tilted Surfaces.......................................................... 106 2.8.3 Multiconfiguration Reflective Polygon Model ................................................... 108 2.8.4 Example Single-Pass Polygon Setup ................................................................... 109 2.8.4.1 Multiconfiguration Code V Lens Prescription ................................... 110 2.8.4.2 Lens Prescription Model ........................................................................ 111 2.8.5 Dual-Axis Scanning .............................................................................................. 112 2.9 Selected Laser Scan Lens Designs ................................................................................... 112 2.9.1 A 300 DPI Office Printer Lens λ( = 633 nm) ....................................................... 113 2.9.2 Wide-Angle Scan Lens (λ = 633 nm) ................................................................... 114 2.9.3 Semiwide Angle Scan Lens (λ = 633 nm) ........................................................... 114 2.9.4 Moderate Field Angle Lens with Long Scan Line (λ = 633 nm) ...................... 115 2.9.5 Scan Lens for Light-Emitting Diode (λ = 800 nm) ............................................. 116 2.9.6 High-Precision Scan Lens Corrected for Two Wavelengths (λ = 1064 and 950 nm) ............................................................................................ 116 2.9.7 High-Resolution Telecentric Scan Lens (λ = 408 nm) ....................................... 117 2.10 Scan Lens Manufacturing, Quality Control, and Final Testing .................................. 118 2.11 Holographic Laser Scanning Systems ............................................................................ 118 2.11.1 Scanning with a Plane Linear Grating ............................................................... 119 2.11.2 Line Bow and Scan Linearity ............................................................................... 120 2.11.3 Effect of Scan Disc Wobble ................................................................................... 121 2.12 Noncontact Dimensional Measurement System Using Holographic Scanning.......122 2.12.1 Speed, Accuracy, and Reliability Issues ............................................................. 124 2.12.2 Optical System Configuration .............................................................................. 125 2.12.3 Optical Performance .............................................................................................. 127 2.13 Holographic Laser Printing Systems .............................................................................. 129 2.14 Closing Comments ............................................................................................................ 131 Acknowledgments ...................................................................................................................... 131 References ..................................................................................................................................... 132 2.1 INTRODUCTION This chapter builds on the original work of Robert E. Hopkins and David Stephenson on optical systems for laser scanners1 to provide yet another perspective. The goal of this chapter is to provide the background knowledge that will help develop an insight and intuition for optical designs in general and scanning systems in particular. Combined with a familiarity with optical design tools, these insights will help lead to optical designs with higher performance and fewer components. Design issues and considerations for holographic scanning systems are discussed in detail. The interactions between optical requirements and constraints imposed on optical sys­ tems for laser scanners are explored. The optical components that many applications of laser scanning depend on to direct and focus the laser beam are discussed, including lenses, mirrors, and prisms. OpticalSystemsforLaser Scanners 71 The optical invariant, first-order issues, and third-order lens design theory as they relate to scanning systems are presented as a foundation to the layout and design of the optical systems. Representative optical systems, with their characteristics, are listed along with drawings showing the lenses and ray trajectories. Some of the optical systems used for scanning that require special methods for testing and quality control are reviewed. 2.2 LASER SCANNER CONFIGURATIONS Optical system configurations for laser scanners can vary in complexity from a simple col­ limated laser source and scanner to one including beam conditioning optical components, modulators, cylinders, anamorphic optical relays, laser beam expanders, multiple scan­ ners, and anamorphic optical components for projecting the scanned beam. The scanned laser beam can be converging, diverging,
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