This paper briefly tells the story of the importance of metrology in optics fabrication at Optimax, and highlights Wyko and Jim Wyant’s contribution to early Optimax success.

1. EARLY OPTIMAX

Optimax¹ was founded in 1991 on the premise of utilizing Computer-Numerically-Controlled (CNC) machining for precision optics. This innovation enabled delivery times for custom optical components to be reduced from 16 weeks to 1 week. Founded with the commitment to use advanced manufacturing technology, the third OptiCam² machine OptiPro Systems³ (CNC Systems in 1991) was installed at Optimax. At the time, Optimax relied on test plates for power and surface figure testing. This lack of metrology regularly limited what was required for verification in those early days.

Wyko let Optimax borrow their 6” interferometer – a memory from Mike Mandina

2. LEAN MANUFACTURING

In 1994 Optimax began to market “Prototype Optics in One Week”. A combination of manufacturing and testing capabilities created opportunities for Optimax to support prominent customers, such as the lithography market, NASA, the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). As Optimax continued to grow, they transitioned to lean manufacturing[4] which required everyone to learn to manufacture and test their own surfaces. Instead of “throwing the optic over the wall” to the next department (ex. grinding to polishing), each optician learned onepiece-flow and was able to take the raw material all the way through the process to the finished product. This process is illustrated in Figure 1.

Figure 1  Diagrams illustrating the difference between traditional manufacturing (left) and lean manufacturing (right).lance between MSF error correction and resulting loss of form, requiring an iterative loop (Figure 4).

3. ASPHERE METROLOGY

By early 2000 industry demand for aspheres required more metrology options and capabilities.[5-7] In asphere manufacturing, historically the gating item is the metrology, as metrology enables deterministic correction. This yields the adage and the title of this presentation “if you can’t measure it, you can’t make it”. During this time of growth Optimax was quickly growing its metrology capabilities in order to keep up industry demand. With additional capabilities and capacity Optimax developed an asphere decision tree to help promote better communication with customers about asphere metrology. This decision tree is shown as Figure 2.

Figure 2  Optimax’s Asphere Decision Tree, highlighting the importance of metrology in asphere manufacturing. The inset on the left defines the different asphere shapes.

4. FREEFORM METROLOGY AND TOTAL ERROR

Freeform optics manufacturing is similar to manufacturing high departure and complex aspheres.[8-14] The freeform shape is typically initiated in generation and measurement is also a gating item. In addition to surface irregularity form error, total error is important for freeforms.[15] As depicted in Figure 3, total error is the combination of surface irregularity, surface texture and positioning errors.

Figure 3  Total Error = (Surface Irregularity + Surface Texture + Positioning) errors

Positioning or errors in locating a freeform in space lead to measurement errors. Alignment errors in measurement can manifest as surface shape errors. For spherical surfaces the alignment error would be tip or tilt, aspheric surfaces the error is coma, but for freeform errors it could be almost anything. The freeform errors depend on the shape of the freeform surface and the type of misalignment. An example of a biconic surface with 0.5o rotational offset is showing in Figure 4 where the rotational alignment error causes astigmatism and higher order terms.

Figure 4  Simulation on a biconic 100 mm circular aperture. A rotational offset of 0.5^o causes 19 µm of PV astigmatism.

The best way to control positioning error in freeform measurements is to measure the position relative to fiducials or a global coordinate system.[16] This shows a major advantage of including a coordinate measuring machine (CMM) into the optical manufacturing and testing process for freeforms. Multiple instruments are required for full spatial frequency measurement coverage for freeforms. Optimax is continuously searching for additional metrology tools capable of measuring total error to sub-wave precision on freeforms.

5. SUMMARY

Measurements are extremely important in the optics manufacturing process. Increasing design complexity demands increasingly better metrology. Thank you to Jim Wyant, and all of the metrology innovators out there. You can only make an optic as good as you can test it!

References

[1] Optimax Systems Inc., 6367 Dean Parkway, Ontario, NY, Phone (585) 265-1020, Website: www.optimaxsi.com.
[2] Novak, R., et al. Lens Blocking Method for Opticam. in Optical Fabrication and Testing Workshop. 1992. Boston, MA: Optical Society of America.
[3] OptiPro Systems, 6368 Dean Parkway, Ontario, NY 14519, Phone:(585) 265-0160, www.optipro.com.
[4] Liker, J.K., The Toyota way : 14 management principles from the world's greatest manufacturer. 2004, New York: McGraw-Hill.
[5] Dumas, P., et al. Complete sub-aperture pre-polishing and finishing solution to improve speed and determism in asphere manufacture. in Optical Manufacturing and Testing VII. 2007. San Diego, CA: SPIE.
[6] Forbes, G.W., Robust, efficient computational methods for axially symmetric optical aspheres. Optics Express, 2010. 18(19): p. 19700-19712.
[7] Greivenkamp, J.E., Testing aspheric surfaces. OPN, 1990(June): p. 25-27.
[8] Brooks, D., et al., Manufacturing of a large, extreme freeform, conformal window with robotic polishing. SPIE Optical Engineering + Applications. Vol. 10742. 2018: SPIE.
[9] Kim, D.W. and J.H. Burge, Rigid conformal polishing tool using
non-linear visco-elastic effect. Optics Express, 2010. 18(3): p. 2242-2257.
[10] Kordonski, W.I., A.B. Shorey, and M. Tricard, Magnetorheological jet (MR Jet (TM)) finishing technology. Journal of Fluids Engineering-Transactions of the Asme, 2006. 128(1): p. 20-26.
[11] Nelson, J.D., The evolution of freeform fabrication and testing: lessons learned and the roadmap to higher precision, E.P.I.C. (EPIC), Editor. 2019: EPIC Meeting on Precision and Freeform Optics at WZW-OPTIC.
[12] Powers, S., M. Brunelle, and M. Novak. Design and Manufacturing Considerations for Freeform Optical
Surfaces. in Optical Design and Fabrication 2019 (Freeform, OFT). 2019. Washington, DC: Optical Society of
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[13] Walker, D.D., et al. New results extending the Precessions process to smoothing ground aspheres and producing freeform parts. 2005. San Diego: SPIE.
[14] Wolfs, F., et al., Freeform grinding and polishing with PROSurf. SPIE Optifab. Vol. 9633. 2015: SPIE.
[15] DeGroote Nelson, J., et al., Using total error measurements of freeforms during manufacturing to aid in alignment. SPIE Astronomical Telescopes + Instrumentation. Vol. 11451. 2020: SPIE.
[16] Brunelle, M., et al., Importance of fiducials on freeform optics. SPIE Optifab. Vol. 9633. 2015: SPIE.

SPIE Optical Engineering + Applications, 2021, San Diego, California, United States
Proc. of SPIE Vol. 11813, 118130H | © 2021 SPIE · CCC code: 0277-786X/21/$21 · doi: 10.1117/12.25712

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