Phase-Shifting Interferometry Vibration Compensation
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Solution Overview
Problem
Phase-shifting interferometry methods face errors in surface profile measurement due to non-uniform scanning motions and vibrations, which can cause incorrect sign determination of phase maps, leading to incorrect surface topography calculations.
Innovation Solution
The method involves collecting PSI data using an initial sequence of gradual mechanical phase shifts followed by rapid mechanical phase shifts to establish the sign of the phase map, with the data from gradual phase shifts refining the accuracy after sign determination. This approach reduces the impact of vibrations by using the rapid phase shifts to determine the correct sign and the gradual phase shifts to refine the phase map.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If rapid mechanical phase shifts are used to determine the sign of the phase map, then the robustness against vibration-induced sign changes is improved, but additional vibration and noise are introduced
Solution Approach 1:
The phase shift sequence is divided into two distinct segments: an initial segment with gradual phase shifts for sign determination, and a subsequent segment with rapid phase shifts for refined measurement. This segmentation allows each segment to be optimized for its specific purpose, minimizing the overall impact of vibrations while ensuring correct sign determination.
Solution Approach 2:
The initial sequence of gradual phase shifts is performed before the rapid phase shifts to establish the correct sign of the phase map. This preliminary action ensures that subsequent rapid phase shifts, which are more sensitive to vibrations, are interpreted with the correct sign information, preventing sign errors in the final measurement.
2Object-affected harmful factors
If gradual mechanical phase shifts are used for the entire sequence, then vibration sensitivity is reduced, but the sign determination accuracy deteriorates under large vibrations
Solution Approach 1:
The phase shifting mechanism dynamically adjusts its speed based on the requirements of each measurement phase. The system transitions from gradual phase shifts during the initial sign-determination phase to rapid phase shifts during the refined measurement phase, optimizing both vibration resistance and measurement precision at different stages.
3Device complexity
If conventional PSI algorithms assuming constant scanning motion are used, then the system complexity is reduced, but measurement precision deteriorates due to non-uniform scanning and vibrations
Solution Approach 1:
The algorithm dynamically determines the actual phase shift increments by analyzing the sinusoidal dependence of intensity values across multiple interferograms, rather than assuming constant phase shift increments. This parameter adaptation allows the system to compensate for non-uniform scanning motions and vibrations, significantly improving measurement precision without requiring complex additional hardware.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method provides a robust phase map with the correct sign, even in the presence of large vibrations, ensuring accurate surface topography measurements by minimizing the effect of vibration-induced errors in phase shift increments.
Implementation Method 1
one can use an interferometer to combine a measurement wavefront reflected from the measurement surface with a reference wavefront reflected from a reference surface to form an optical interference pattern
Data Source
Figure 1~2
Figure 3A~3C
Figure 3D~5
AI summary
A phase-shifting interferometry (PSI) method and corresponding system including: (i) recording an interferogram for each phase in a sequence of phases between test light reflected from a test surface and reference light reflected from a reference surface, the interferograms defining an interferometry signal for each of different transverse locations of a cavity defined by the test and reference surfaces; (ii) calculating an initial phase map for the cavity based on at least some of the recorded interferograms; (iii) calculating an estimate for each of at least some of the phase shift increments based on the initial phase map and at least some of the recorded interferograms; and (iv) calculating an improved phase map based on the calculated estimates for the phase shift increments and at least some of the recorded interferograms.