Tutorial: Design, Fabrication, and Testing of Aspheric Surfaces

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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

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
• Tolerances for aspherical optical elements
• Fabrication
• Testing
• Profilometry
• Interferometry in reflection
• Interferometry in transmission
• Summary
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?
• 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

Q-Type Asphere:

Even Asphere:

Polynomial:

Zernike Standard Sag

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)

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

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
• http://www.optimaxsi.com/capabilities/aspheres/
• B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
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
• 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
• B. Braunecker, etc., “Advanced Optics Using Aspherical Elements”, SPIE ebook, 2008.
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
• 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
• 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

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
• Field is less than ±5°.
• Limit: surface departure from a sphere <100μm
• http://www.optimaxsi.com/capabilities/aspheres/
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;