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Tutorial: Design, Fabrication, and Testing of Aspheric Surfaces. Chia-Ling Li College of Optical Sciences, University of Arizona Dec. 12. 2013. Outline. Introduction Design Mathematical representation of aspherical surfaces Aspheric shape design guide

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tutorial design fabrication and testing of aspheric surfaces

Tutorial:Design, Fabrication, and Testing of Aspheric Surfaces

Chia-Ling Li

College of Optical Sciences, University of Arizona

Dec. 12. 2013

outline
Outline
  • Introduction
  • Design
    • Mathematical representation of aspherical surfaces
    • Aspheric shape design guide
    • Tolerances for aspherical optical elements
  • Fabrication
  • Testing
    • Profilometry
    • Interferometry in reflection
    • Interferometry in transmission
  • Summary
what is an aspherical surface
What is an aspherical surface?
  • The aspheric surface means not spherical.
  • It can be thought as comprising a base sphere and an aspheric cap.

Aspherical surface

Aspherical cap

Spherical base surface

why is it important
Why is it important?
  • It can correct aperture dependent aberrations, like spherical aberration.
  • It can correct field dependent aberrations, like distortion and field curvature.
  • It can reduce lens weight, make optical systems more compact, and in some cases reduce cost.
  • Fewer elements are needed in a system with aspherical surfaces: making systems smaller, lighter and shorter.
mathematical representation of aspherical surfaces
Mathematical representation of aspherical surfaces

Q-Type Asphere:

Even Asphere:

Polynomial:

Zernike Standard Sag

aspheric shape design guide
Aspheric shape design guide
  • When designing an aspheric surface, some surface shapes should be avoided because they could increase the manufacture difficulty and the cost.
  • The slope of the aspheric departure often has a larger impact on manufacturing difficulty than the amplitude of the asphere.

Kreischer Optics, Ltd., “Aspheric Design Guide”

tolerances for aspherical optical elements 1
Tolerances for aspherical optical elements (1)

http://www.optimaxsi.com/capabilities/aspheres/

tolerances for aspherical optical elements 2
Tolerances for aspherical optical elements (2)

ISO 10110

  • 3/4(0.8/0.4) :a sag error of 4 fringes (@ λ = 546 nm), a total irregularity of 0.8 fringes, and a rotational symmetric irregularity of 0.4 fringes
  • 4/ : tolerance for the tilt angle
  • B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
different process technologies
Different process technologies
  • http://www.optimaxsi.com/capabilities/aspheres/
  • B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
the manufacturing cost of different materials
The manufacturing cost of different materials
  • Crystals: CNC machining or diamond turning
  • Glasses:CNC machining or precision molding
  • Polymers: injection-molding
  • B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
classical optics fabrication
Classical optics fabrication
  • The actual production sequence is iterative; several steps must be taken between surface shaping and measurement before the required accuracy level is achieved.
  • B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
the characteristic features of each process step
The characteristic features of each process step
  • B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
moore nanotech 350fg ultra precision freeform generator
Moore Nanotech® 350FGUltra-Precision Freeform® Generator
  • Five-axis CNC machining
  • Used for on-axis turning of aspheric and toroidal surfaces; slow-slide-servo machining (rotary ruling) of freeform surfaces; and raster flycutting of freeforms, linear diffractives, and prismatic optical structures
  • Workpiece Capacity: 500mm diameter x 300mm long
  • Programming Resolution: 0.01 nm linear / 0.0000001º rotary
  • Functional Performance: Form Accuracy (P-V) ≤ 0.15µm / 75mm dia, 250mm convex aluminum sphere.

http://www.nanotechsys.com/

profilometer 2d map
Profilometer - 2D map
  • It is less accurate than an interferometer.
  • It can measure almost any surface.
  • Multiple profilometer traces can map the surface more accurately.
  • Measurement certainty is ~0.1 µm at best.
  • Limit: slope<40°, sag<25mm
  • http://www.optimaxsi.com/capabilities/aspheres/
stitching interferometry 3d map
Stitching interferometry-3D map
  • Measure overlapping smaller patches
  • Use phase shifting interferometry for individual measurements
  • Calculate the final surface height map by stitching all the patches

Annular ring stitching

Sub-aperture stitching

  • Part is moved in Z to focus on different annular zones.
  • Limit: surface departure from a sphere <800μm
  • Part is moved in Z, tip, and tilt to focus on different patches.
  • Limit: surface departure from a sphere <650μm
  • http://www.optimaxsi.com/capabilities/aspheres/
null testing in reflection
Null testing in reflection

Computer generated hologram, CGH

Spherical null lens

Spherical

wavefront

Aspherical

wavefront

  • Part specific
  • Takes time and money
  • Limit: surface departure from a sphere <100μm
  • Part specific
  • Takes time and money
  • Surface departure from a sphere can be high.
  • http://www.optimaxsi.com/capabilities/aspheres/
null testing in transmission
Null testing in transmission
  • Field is less than ±5°.
  • Limit: surface departure from a sphere <100μm
  • http://www.optimaxsi.com/capabilities/aspheres/
flexible measurement technique
Flexible measurement technique
  • Many wavefronts simultaneously impinge onto the surface under test.
  • It’s rapid, flexible and precise.
  • Wide dynamic range in the asphericities is allowed.
  • Special calibration is needed.

MA=microlens array;

PA=point source array;

M=source selection mask

  • C. Pruss, E. Garbusi and W. Osten, “Testing Aspheres”, Optics & Photonics News, pp. 25-29, Apr. 2008.
summary
Summary
  • Aspheres, which are designed to null out a unique set of aberrations, are specified using the aspheric equation.
  • A suitable manufacturing method is chosen according to the lens materials and the required accuracy.
  • There are many metrology options, with selection driven by surface departure, form error and cost objectives.