Who’s afraid of freeform optics?*

Webinar – 17 January 2018

Outline• About the presenters • Spectrum Scientific, Inc. (SSI)• Challenges of using freeform optics• How to use freeform optics• Low cost replicas from complex components• Advantages of the replication process• Introduction to freeform optics• Benefits of utilizing freeform optics • Freeform surface design in OpticStudio• Manufacturability• Metrology • Transitioning into manufacturing• Assembly considerations

PresentersIntroductionSpectrum Scientific

Daphnie ChakranPresident & CEOSpectrum Scientific

PresenterDavid CookGeneral ManagerSpectrum Scientific

Q&ADave EricksonCTOSpectrum Scientific

PresenterZemax

Erin ElliottOptical Research & Prototyping EngineerZemax

Spectrum Scientific, Inc.

• Space qualified • Clean room facility• ISO 9001:2015 certified• In-house capabilities

o Manufacturingo Designo Coatingo System Assemblyo Engineeringo Precision Metrology

Spectrum Scientific, Inc. (SSI) was established in 2004

Spectrum Scientific, Inc.

Spherical mirrorsAspheric mirrorsFreeform mirrorsOff-axis parabolic mirrorsEllipsoidal mirrorsHolographic diffraction gratingsHollow retroreflectorsUV spectrometers

Capabilities

Challenges of using freeform optics

Understanding & Visualization

Designing & Specifying for Manufacturability

Cost

Assembling your Freeform

Measurement

How to use freeform optics

Add

Replace

Redo

Low cost replicas from complex components

Precision Machining Expensive Polishing Fixtures

Expensive Polishing & Metrology Processes

Expensive Original

Advantages of the replication processReplication is the most cost effective method for high volume applications

Spectrum Scientific’s replication process delivers:• High fidelity reflective aspheric mirrors• λ/10 reflective optics• High volume production of freeform mirrors at

a reasonable cost and with high repeatability

Advantages of the replication process• Exceptional quality surfaces are faithfully

replicated to very high fidelityo Component figure: macroscopic scaleo Scratches and digs; microscopic scaleo surface roughness: sub nanometer scale.

• Mounting features can be easily incorporated into the component

• Greater design flexibility• UV, VIS, IR coatings• Lower OEM cost than conventional polishing

and/or diamond machining

Master optic

Release agent

Thin film coating

Polymer layer

Replicated mirror

Introduction to freeform opticsDefinition of freeform: A curved optic without axial symmetry

1900s 2000s1970s 2010s1950s

Flat & Spherical optics1

Initial freeform designs2

Significant commercial freeform manufacturing:

Polaroid SX-703, 4, 5

Deterministic freeform manufacturing methods

(DT & MRF)1

Mainstream systems integrate freeforms into

production (Beam Shaping, HUDs, VR/AR)6

Introduction of commercially available

aspheric optics3

High precision, state of the art freeform manufacturing

Benefits of utilizing freeform optics• Single optical surface allowing for compound compensation• Improved system performance • Redistribution of tolerances in the system design • Reduces the number of optics• Additional aberration correction

The SCUBA-2 telescope 3, 5, 7

System design - freeform benefit example

Overall volume decreased from 530 cm3 to 100 cm3

Offner baselineall-spherical design

Offner compact freeform design8

Reimers, Jacob, et al. "Increased Compactness of an Imaging Spectrometer Enabled by Freeform Surfaces." International Optical Design Conference. Optical Society of America, 2017.

Freeform tools in OpticStudio• Surfaces in sequential mode

• Standard surfaces – allow a ROC and conic constant. • Biconic surfaces – allow separate ROCs and conics in X and Y.• Toroidal, superconic• Even Asphere, Extended Asphere, Odd Asphere, Extended Odd Asphere, Q-type Aspheres, Alternate Even/Odd Asphere• Zernike Standard Phase/Sag, Zernike Fringe Phase/Sag, Biconic Zernike• Polynomial, Extended Polynomial, Chebyshev Polynomial• Grid Phase/Sag

• Analysis tools• ISO Element drawings• Surface and curvature tables and cross-sections• Power as a function of pupil coordinate, field angle• Universal plots

• Optimization tools• FTLT/FTGT – full thickness less than and greater than• POWF – Power at a given field point• POWP – Power at a given point in the pupil• BFSD – Best fit sphere data• SCUR – Surface curvature at a given X and Y coordinate• SDRV – Derivative of the sag at a given X and Y coordinate

Optimizing with freeforms in OpticStudio• Three Mirror Anastigmat example• Traditionally done with decentered conics.• With freeforms, can mimic those shapes with on-axis parts.• But, freeforms open up the design space beyond the

traditional conics.• Design with freeforms requires caution!

• Optimization has many more parameters available. Typically need to reduce the # of parameters in an intelligent way.

• Never use terms that violate the symmetry of the system.• TMA has symmetry about the YZ plane, so terms that are odd

in X are disallowed.• Start with lowest-order term – power – by connecting X and Y

radii.• Then, release to allow astigmatism, then coma, etc.• Always include at least a very small field angle. Easy to create

systems that are perfect at one field point.• Never turn all the terms on to see what happens!• Never keep a term that doesn’t improve the performance!

Optical Testing of Freeforms• OpticStudio can be used to model the optical test setup for each

freeform.• The test method sets the maximum slope limitation for the

freeform part.• Typical optical testing methods:

• Interferometric testing (unassisted):• For freeforms without large departure from spherical surfaces.• Max. slope of the freeform part is limited by the resolution of the interferometer.

• Interferometric testing with a null lens:• Removes part of the wavefront departure so that the interferometer can resolve

the fringes.• Null lens must be designed for precision manufacture, so can’t be too complex.• Max. slope of the freeform part is limited by the precision required for the null

lens.• Interferometric testing with a CGH:

• Also removes part of the wavefront departure.• Max. slope of the freeform part is limited by the resolution of the CGH.

ManufacturabilityManufacturing Methods• Originals by direct manufacturing• Copies from masters (injection molding,

pressed glass, nano-imprint lithography)• Large volume, repeatable process using

the SSI replication process

Superior stray light performance from replicated mirror compared to diamond machined mirror

Metrology• Measurements down to a nm-scale• Wide range of contact & non-contact measurement methods

o Contact - common, slow µm-scaleo Non-contact - higher precision9

Transitioning Into manufacturing• Communication with the manufacturer is critical• Specifying the optical surface

o Surface equationo Sag table

• Additional parameters to considero Surface profile (figure error)o Micro-surface roughness (RMS)o Mid-spatial frequency (slope errors)11 Surface Equation10

Assemble – your instrument Alignment considerations for optic manufacturing and system integration

Summary

Nobody should be afraid of freeform optics*They can offer significant benefits and cost savings in modern optical systems

Thank YouThis is an exciting time in our industry

Freeforms have the potential revolutionize a lot of different markets

*with apologies to Edward Albee

References1. Thompson, Kevin P., and Jannick P. Rolland. "Freeform optical surfaces: a revolution in imaging optical design.” Optics and Photonics

News 23.6 (2012): 30-352. Kanolt, Clarence W. "Multifocal ophthalmic lenses.” U.S. Patent No. 2,878,721. 24 Mar. 1959.3. Fang, F. Z., et al. "Manufacturing and measurement of freeform optics.“ CIRP Annals-Manufacturing Technology 62.2 (2013): 823-846.4. Plummer, William T. "Free-form optical components in some early commercial products." Proc. SPIE. Vol. 5865. 2005.5. Henselmans, Rens. "Non-contact measurement machine for freeform optics." Macromolecules 35 (2009): 607-24.6. Cakmakci, Ozan, and Jannick Rolland. "Head-worn displays: a review." Journal of display technology 2.3 (2006): 199-216.7. Hoogstrate, André M., et al. "Manufacturing of high precision aspherical and freeform optics." Proc. SPIE. Vol. 8450. 2012Interferometric

Measurement of Mid-Spatial Frequency Wavefront Errors, Smythe, Aikens, IODC Denver June 2017.8. Reimers, Jacob, et al. "Increased Compactness of an Imaging Spectrometer Enabled by Freeform Surfaces." International Optical

Design Conference. Optical Society of America, 2017.9. Non-contact measurement of optical freeforms – current solutions and advantages, F. Reischer, S. Mühlig, E. Grüner, J. Siepmann, S.

Mika, A. Beutler, M. Lotz, A. Wiegmann, Mahr GmbH. Laser World of Photonics, June 2017.10. Zemax, LLC. “OpticStudio 16 SP2 Help Files." OpticStudio 16 SP2. (2016): 476. Print.11. ISO11010-8

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