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Freeform Optics: The Next Generation of Precision Optical
Components
By: Edward Fess OptiPro Systems
Laser Focus World Webinar · Sept. 2012 Revised: August 2013
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• Introduction – How we got to where we are today
• Define freeform optics – Discuss potential applications
• Manufacturing challenges/techniques – General
• Datums, file formats – Specific
• Single point diamond turning • Grinding • Polishing • Metrology
• Bi-aspheric arch process example • Summary
Outline
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• Advancements in technology have led to the ability to create more complex optical shapes.
• Like aspheres 20 years ago we are on the cusp of a new era of optical components.
• Freeform optical components have the capability to revolutionize the optics industry!!
• There still lie ahead many challenges in manufacturing freeform optical components.
• Brief history of how we got to this point is required…
Introduction
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University Industry Department of Defense
An effective partner and focal point for development, demonstration, and implementation of manufacturing technology that meets industry and DoD needs.
Center for Optics Manufacturing
U.S. Army Center for Excellence
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Harvey M. Pollicove 1944 – 2004
• COM Co-founder with Duncan Moore • COM Director (’90-’04) • APOMA* Executive Board (*American Precision Optics Manufacturers
Association) • Eastman Kodak (’64-’91)
– Loaned executive (manufacturing/engineering)
• OSA honorary member – Rochester • Optical & Manufacturing Societies
SPIE/ASPE/ASME/OEOCS/ISO • Photonics Spectra’s “Distinction in
Photonics” Award – posthumously awarded January 26, 2004
The “Dauntless Visionary” – Rochester Business Journal (6/17/94)
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COM’s Virtual Network
and Technical Advisory
Board 2001
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Deterministic process • Flexible CNC consistency • Transferable technology
· Deterministic Microgrinding · Magnetorheological Finishing
COM has developed the next generation of optics
manufacturing technology
Historical Industry Methods Deterministic Manufacturing
Emphasis >>>> Art to Science
…an industry paradigm shift
Non-deterministic process • Labor and skill intensive • Specialized tooling • Artisan resident technology
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• Twenty plus years ago — you couldn't buy CNC machines
for manufacturing optics • Now –
you can't be in business without them
Commercialization:
…changing the factory floor OptiPro’s Opticam SX, the
world’s first affordable CNC Optical Grinder
OptiPro introduces the e Series line of CNC Optical
Grinders
OptiPro introduces the PRO Series line of CNC Optical
Production Machines
1991 2001 2011
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• Introduction • Define freeform optics
– Discuss potential applications • Manufacturing challenges / Techniques
– General • Datums, file formats
– Specific • Single point diamond turning • Grinding • Polishing • Metrology
• Bi-aspheric arch process example • Summary
Outline
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We have spheres, aspheres, and now there are “FreeForm optics”.
What are Freeform Optics?
What constitutes a freeform optic?
Might I suggest that from a manufacturers perspective, freeform optics are shapes that are not manufactured by standard spherical or aspheric manufacturing techniques. They can include a wide range of geometries and can usually be broken down into the following sub-classes;
-Off-axis sections of rotationally symmetric parts -Rotationally symmetric non standard shapes -Conformal optics (Optics that conform to the platform that they reside in) -Other FreeForm Geometries
Potential Uses:
Energy Research
Medical Devices
Automotive lighting
Mobile Displays
Infrared and Military Optics
Semiconductor Industry
Optical Transforms
Remote sensing
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Off-Axis Sections Off-axis Segment
Parent Mirror
This is probably the most common type of freeform optic. They can be spherical or aspheric. They are commonly used as
reflectors in target detection systems, collimators, beam expanders, space
optics, and many more...
If possible, you would manufacture the parent mirror, and then core out the
segment(s) from the parent.
In many cases this is not possible due to the size of the parent mirror.
If the part is too far off axis to make using “standard” techniques, then the part must be processes using freeform manufacturing techniques.
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Non- Standard Symmetric Shapes These shapes may include ogives, cones, or other non-standard rotationally symmetric surface definitions. They may commonly be used in laser systems, imaging systems, or for aerodynamic
purposes. Typically they can be manufactured by rotating the part. However,
the tool path required for manufacturing is custom generated, depending on the surface definition. Typically , like aspheres, these surfaces require sub-aperture polishing techniques to
finish.
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Conformal Optics These are optical components that conform to the shape of the platform that they reside in.
They may or may not have any rotational symmetry to them. Some examples may
include of uses in the automotive optics, and optics that might conform to the leading edge
of an airplane wing. In both of these cases, the goal would be to reduce the effects of
drag for better performance, but still maintain the “correct” optical path.
Corvette headlight
Due to the complexity that these shapes might have, they might be extremely difficult to define, manufacture, and
measure!!
Aircraft wing optics
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Other FreeForm Geometries
Phase mapping for enhanced viewing
Wavefront Coding - Phase Mapped Optical Surface
…a 25% increase in near depth of Field
Phase Mapped Optics …a better view
…Basically everything else!
There are a whole host of computational methods for defining freeform surfaces; Phi-Polynomials (i.e. Zernike) Radial Basis Functions (RBFs)
Control Point Surfaces (i.e. NURBS) and many, many more!!!
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OSA Incubator Meeting: OPN June & Sept. 2012
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The new NSF Center for Freeform Optics
• The 1st-generation of freeform surfaces, ϕ-polynomial surfaces, provide a dramatic new degree-of-freedom
• Historically, coma, a field linear aberration has immediately limited the ultimate capabilities of any off-axis optical design
• The 1st generation surfaces allow, for the first time, direct, independent correction of coma, the system dominant aberration
Impact of Freeform Surfaces on Optical System Design
• All three mirrors are freeform (Zernike) • This system presents the largest circular
aperture in the smallest volume • These mirrors have been fabricated and the
system is in assembly
The World’s 1st Truly rotationally nonsymmetric imaging optical system
Primary
Secondary
Tertiary
8°
6°
Diffraction Limit @ 10µm =
Fuerschbach, K., J.P. Rolland, and K.P. Thompson, “A new family of optical systems employing φ-polynomial surfaces”, Optics Express 19(22), 21919-21928 (2011).
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• Introduction • Define freeform optics
– Discuss potential applications • Manufacturing challenges / techniques
– General • Datums, file formats
– Specific • Grinding • Polishing • Metrology
• Bi-aspheric arch process example • Summary
Outline
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General Considerations No matter what type of Freeform optic you are going to manufacture, you will most likely have to deal with some or all of the following;
1. Surface definition and format Is it a simple equation that can be regenerated, some complex
mathematical equation, a CAD file, or a cloud of points? Each has it’s own advantages and limitations.
2. Tool path generation for your manufacturing process Will you use a CAD/CAM system to generate tool path, excel, or MatLab?
Due to the wide range of surface definition possibilities, it is much more complex than spheres or aspheres to have a simple all encompassing solution.
3. Alignment surface and reference datum Many types of freeform shapes may not have reference surfaces to align
or check the surface from (i.e. – no center thickness or diameter). Surfaces should have alignment features that can be easily picked up by a machine operator.
4. Metrology Many of these complex shapes cannot be measured by any standard
interferometric techniques.
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Grinding Contour Deterministic Microgrinding (CDMG)
The complex shapes of these optics dictate that they be ground using raster grinding techniques. These techniques may require that there are 3 -5 axis of simultaneous motion to
maintain tool and part tangency during the grinding cycle. As shown below, typically you would use a “spherically shaped” diamond grinding wheel like you might use for asphere grinding.
Tool Rotation
Tool Path
OptiPro eSX grinding machine
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Polishing Freeforms require sub-aperture polishing
Raster UFF
Most freeform geometries will require a 3-5 axis tool path to move the polishing tool across the surface while maintaining tangency. These complex tool paths also need to be dwell based to correct for form errors on the parts surface.
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Metrology If you can’t measure it…
Some freeform shapes lend themselves to be tested using custom interferometric techniques. However, many do not. This necessitates the need for another metrology solution. The two main
commercially available options are a coordinate measuring machine (CMM) or UltraSurf.
Most CMM’s use touch probe technology. The UltraSurf uses
a variety of non-contact optical pens. For all of these
instruments, having the correct data file format as a reference is critical in being
able to properly measure and analyze data for the part
under test. Zeiss Micura OptiPro UltraSurf
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• Introduction • Define freeform optics
– Discuss potential applications • Manufacturing challenges / Techniques
– General • Datums, file formats
– Specific • Single point diamond turning • Grinding • Polishing • Metrology
• Bi-aspheric arch process example • Summary
Outline
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Bi-Aspheric Arch Anamorphic Asphere / 5-axis solution required
•Glass bi-aspheric arch with dimensions of 100 mm x 78 mm x 38.1 mm. CT=3.5 mm
•The arch has no rotational symmetry to it, but has bi-lateral symmetry.
100 mm
78.125 mm
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5-Axis Process Flow Import tool path and error map
Set parameters (tool and removal)
Regenerate the part coordinates
Fit the part coordinates
Determine dwell times
Output tool path with dwell 26
External data used as input
This section is what OptiPro is developing
internally.
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CAD/CAM Tool Path Generation • Use MasterCam to generate the 5-axis tool path. • Have integrated UFF into MasterCam’s machine simulation package to evaluate 5-axis tool paths before execution of them.
OptiPro UFF machine simulation in MasterCam
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Obtain Metrology Data Glass corrector arch– Initial Grind
• Obtain the metrology Data for the initial surface error.
• This map will be used to generate the dwell map that will modify the G-code program.
3D UltraSurf map of arch surface (left) and top down map (right). Lateral units are in mm, and color scale is in microns.
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Glass Corrector Arch Glass corrector arch – Initial Polish (uniform removal)
• Several passes were made using a 5-axis uniform removal polishing routine on UFF that was generated in MasterCam. • The program was ~45 minutes long, and was run until the surface was no longer grey. • The surface was then measured on UltraSurf.
Polishing condition
Value Unit
Wheel Diameter
40 mm
Wheel Hardness
60 Shore A
Belt Type CeO2 Bound
n/a
Compression 0.150 mm
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Glass Corrector Arch Glass corrector arch – Initial Uniform Removal Polish Result
pv : 25.5 microns rms: 3.97 microns
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Glass Corrector Arch First UFF figure correction
pv : 9.86 microns rms: 1.75 microns
Polishing Notes This was the first 5-axis correction attempt ever at OptiPro. We attempted to run a 50% correction routine, and ended up with ~57% correction!
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Glass Corrector Arch 2nd UFF correction
pv : 11.59 microns rms: 1.82 microns
Polishing Notes We learned that there was a data mirror problem in the long direction of the arch.
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Glass Corrector Arch After final UFF figure correction
pv : 3.94 microns rms: 0.53 microns
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Glass Corrector Arch 5-axis polishing Summary
0.0
1.0
2.0
3.0
4.0
5.0
0 2 4 6 8 10rms f
orm
err
or (m
icro
ns)
Polishing Iteration
rms form error progression
0
5
10
15
20
25
30
0 2 4 6 8 10Pv fo
rm e
rror
(mic
rons
)
Polishing iteration
Pv form error progression
Initial Surface
Final Surface
We continue to explore limit of accuracy.
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Summary • Freeform optics have the potential to revolutionize the precision optics industry, and advancements in manufacturing technology have allowed us to begin to create optical shapes that were never before thought possible.
• As we move forward, much collaboration between optical design and manufacturing will be required to facilitate a successful implementation of freeform optical systems.
• OptiPro is continuing to explore developing new technologies and refining existing ones to further manufacturing capabilities.
• OptiPro hopes that strong collaboration with the Center for Freeform Optics will enable a bright and focused future for freeform optics.
