Crosstalk Cancellation in Phase-Shifting Interferometer
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Solution Overview
Problem
Phase-shifting interferometers suffer from fringe print-through errors, which are not adequately reduced by existing methods, especially in simultaneous phase shifting measurements, due to issues like polarizer leakage and vibration-induced errors.
Innovation Solution
A method and algorithm that determine multiple transfer functions, compute crosstalk terms, and display a phase-difference map to minimize variance in the modulation function, using techniques like MATLAB code for crosstalk cancellation, effectively reducing fringe print-through errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If simultaneous phase shifting interferometry is used to achieve high-speed measurement, then measurement speed is improved, but fringe print-through errors increase due to polarizer leakage and crosstalk
Solution Approach 1:
The patent extracts and separates the crosstalk components from the interferometric signals by determining transfer functions for each pixel that characterize the crosstalk between phase-shifted channels. By identifying and isolating these harmful crosstalk terms, the system can then remove them through computational correction, thereby maintaining high-speed simultaneous phase shifting while eliminating the associated measurement errors
Solution Approach 2:
The patent implements a feedback mechanism where the measured interferograms are processed to determine transfer functions that characterize system crosstalk. These transfer functions are then used to compute correction terms that are applied back to the phase calculation process, creating a closed-loop system that continuously compensates for polarizer leakage and crosstalk effects to maintain measurement accuracy
2Device complexity
If polarizer leakage is present in the optical system, then device complexity is reduced, but crosstalk between channels increases causing fringe print-through
Solution Approach 1:
The patent introduces transfer functions as intermediary mathematical models that characterize the crosstalk behavior of each pixel in the system. These transfer functions act as mediators between the physical polarizer leakage and the measured interferometric signals, allowing the system to quantify and compensate for leakage effects without requiring complex optical modifications. The transfer functions enable computational correction that accounts for polarizer imperfections while maintaining the simplicity of the optical design
Solution Approach 2:
The patent changes the approach from modifying optical parameters to modifying computational parameters. Instead of attempting to reduce polarizer leakage through optical design changes, the system determines transfer function parameters that characterize the leakage and uses these parameters in computational algorithms to correct the measured signals. This parameter-based correction approach maintains optical simplicity while achieving high measurement accuracy
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
The proposed solution significantly reduces fringe print-through errors by up to a factor of 2-3 in surface amplitude measurements and spectral content, improving the accuracy of phase difference maps in phase-shifting interferometers.
Implementation Method 1
a pixilated phase-mask (PPM) 14 that introduces an effective phase-delay between the reference and test wavefronts at each pixel and, subsequently, interferes the transmitted light
Data Source
AI summary
A method of minimizing fringe print-through in a phase-shifting interferometer, includes the steps of: (a) determining multiple transfer functions of pixels in the phase-shifting interferometer; (b) computing a crosstalk term for each transfer function; and (c) displaying, to a user, a phase-difference map using the crosstalk terms computed in step (b). Determining a transfer function in step (a) includes measuring intensities of a reference beam and a test beam at the pixels, and measuring an optical path difference between the reference beam and the test beam at the pixels. Computing crosstalk terms in step (b) includes computing an N-dimensional vector, where N corresponds to the number of transfer functions, and the N-dimensional vector is obtained by minimizing a variance of a modulation function in phase shifted images.