Stitching interferometry for long X-ray mirrors with lateral multi-shift-based absolute calibration.

Area of Science:

• Optical metrology
• Surface characterization
• X-ray optics

Background:

• Fizeau interferometers are crucial for measuring X-ray mirrors, but suffer from reference mirror (REF) errors.
• Accurate measurement of high aspect ratio X-ray mirrors is challenging due to these inherent errors.
• Existing methods struggle to provide absolute 2D surface maps without REF calibration.

Purpose of the Study:

• To present a novel reference calibration method for Fizeau interferometry.
• To eliminate reference mirror errors in X-ray mirror surface measurements.
• To enable absolute 2D surface mapping of X-ray mirrors.

Main Methods:

• Introduced a lateral multi-shift measurement technique at the hundreds-micrometer pixel level.
• Extended calibration by using differences between multiple shifted measurements of the surface under test (SUT).
• Constructed an augmented multi-matrix to extract the absolute 2D surface, applicable to arbitrary measurement regions.
• Stitched reference-subtracted sub-apertures obtained by millimeter-scale shifts for full 2D map reconstruction.

Main Results:

• Achieved sub-nanometer repeatability for all 2D discrepancies in experimental results.
• Demonstrated simultaneous measurement and calibration of benchmark sub-apertures and the REF in a single process.
• Validated the method through comparison with the long trace profiler (LTP).

Conclusions:

• The proposed lateral multi-shift method effectively eliminates reference mirror errors in Fizeau interferometry.
• The technique provides accurate absolute 2D surface mapping for X-ray mirrors.
• The method meets the stringent requirements for X-ray mirror metrology.