chia-ling li college of optical sciences, university of arizona dec. 12. 2013
TRANSCRIPT
Tutorial:Design, Fabrication, and Testing of Aspheric Surfaces
Chia-Ling LiCollege of Optical Sciences, University of Arizona
Dec. 12. 2013
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
Outline
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Introduction
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The aspheric surface means not spherical.
It can be thought as comprising a base sphere and an aspheric cap.
What is an aspherical surface?
Spherical base surface
Aspherical surfaceAspherical
cap
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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.
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Design
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Mathematical representation of aspherical surfaces
Even Asphere:
Polynomial:
Q-Type Asphere:
Zernike Standard Sag 6
Aspheric shape design guide
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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)
8http://www.optimaxsi.com/capabilities/aspheres/
Tolerances for aspherical optical elements (2)
9B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
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
ISO 10110
Fabrication
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Different process technologies
11B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.http://www.optimaxsi.com/capabilities/aspheres/
The manufacturing cost of different materials
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Crystals: CNC machining or diamond turning Glasses: CNC machining or precision
molding Polymers: injection-moldingB. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
The actual production sequence is iterative; several steps must be taken between surface shaping and measurement before the required accuracy level is achieved.
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Classical optics fabrication
B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
The characteristic features of each process step
14B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
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.
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Moore Nanotech® 350FGUltra-Precision Freeform® Generator
http://www.nanotechsys.com/
Testing
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Profilometer - 2D map
17http://www.optimaxsi.com/capabilities/aspheres/
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
Stitching interferometry-3D map
18http://www.optimaxsi.com/capabilities/aspheres/
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
Spherical null lens
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Null testing in reflectionComputer generated hologram, CGH
http://www.optimaxsi.com/capabilities/aspheres/
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.
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Null testing in transmission
http://www.optimaxsi.com/capabilities/aspheres/
Field is less than ±5°. Limit: surface departure from a sphere <100μm
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.
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Flexible measurement technique
C. Pruss, E. Garbusi and W. Osten, “Testing Aspheres”, Optics & Photonics News, pp. 25-29, Apr. 2008.
MA=microlens array; PA=point source array; M=source selection mask
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.
Summary
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Thank you!
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