Computer-Generated Hologram Metrology Alignment: Challenges and Solutions

Computer Generated Hologram (CGH) metrology has several benefits, such as 1/100th wave accuracy and snapshot measurement time. However, alignment of aspheric and freeform parts in CGH setups can be challenging. Unlike spherical or cylindrical optics, for example, more complex shapes allow perfect compensation of tilt with decenter. Blindly aligning to reach minimum surface figure error (SFE) is a valid approach, but only if these degrees of freedom can also be aligned in the final application. Further, misalignment signatures can be mistaken as surface figure error during manufacturing, imprinting more errors onto an optic.

How can one capture true surface figure error relative to mechanical features that are relevant in manufacturing or final assembly? In this talk, Shelby Ament presents several alignment features that can be written onto the CGH itself to provide feedback that will speed up alignment and ensure SFE is measured relative to relevant datums. The following topics are addressed:

• Examples of commonly used reference datums and their intended purpose.
• Types of CGH patterns to consider as part of an overall alignment plan, including:
• Projected spots for coarse visual alignment;
• Converging wavefronts that reflect off of tooling balls or sphere-mounted retroreflectors (SMRs);
• Catseye rings or spots that come to focus on a test optic; and
• Line focus reference patterns used in concert with corner cubes or SMRs.
• Test optic mounting and alignment solutions.

About the presenter
Shelby Ament, Ph.D., is a senior optical engineer at Arizona Optical Metrology (AOM). She holds a doctorate from the University of Arizona’s Wyant College of Optical Sciences, with specialization in diffractive optics. She has five years of industry experience designing, fabricating, and using CGHs to test a variety of optics: from catalog cylinders and aspheres to meter-class telescope mirrors. At AOM, she designs and certifies CGHs for customers and develops next-generation metrology products.