Absolute testing of rotationally symmetric surfaces with computer-generated holograms

Area of Science:

• Optical metrology
• Nanotechnology
• Holography

Background:

• High accuracy is critical for optics operating at short wavelengths.
• Interferometric testing of aspheres and freeforms relies on null optics like computer-generated holograms (CGHs).
• Measurement uncertainty is often limited by the accuracy of the test wavefront, influenced by CGH and interferometer optics.

Purpose of the Study:

• To present a method for absolute testing of rotationally symmetric surfaces using CGH null optics.
• To achieve measurement accuracy significantly higher than the test wavefront accuracy through error separation.
• To validate the proposed method experimentally.

Main Methods:

• Absolute testing combining the N-position method and the shift-rotation method.
• Assumption of negligible rotationally symmetric error in raster-scanned off-axis holograms.
• Theoretical modeling and experimental validation using white light interferometry to characterize fabrication errors.

Main Results:

• Demonstration of a method for separating wavefront errors from CGH and transmission flats from the absolute surface shape.
• Experimental validation achieving sub-nanometer uncertainty.
• Confirmation of the fabrication error assumption through hologram structure characterization.

Conclusions:

• The proposed absolute testing method effectively separates errors associated with CGH null optics.
• Sub-nanometer uncertainty is achievable for rotationally symmetric surfaces using this technique.
• The method provides a pathway for highly accurate optical surface metrology.