Grating Shearing Interferometer Wavefront Measurement
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Solution Overview
Problem
Current grating shearing interferometers face challenges in achieving high-precision wavefront measurements due to interference from high-order diffraction beams, which introduce system errors and reduce accuracy, especially as the numerical aperture increases, with existing methods being complex and inconvenient to operate.
Innovation Solution
A method using a grating shearing interferometer system with a one-dimensional and two-dimensional diffraction grating plate, where the two-dimensional diffraction grating plate is moved to eliminate high-order diffraction beam interference by determining the required phase-shifting steps based on the shear ratio and numerical aperture, allowing only ±1st-order beams to interfere, and using Fourier transforms to obtain gradient phases for wavefront reconstruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-order diffraction beams are allowed to participate in interference, then the measurement range and sensitivity are improved, but phase retrieval precision deteriorates due to system errors from multiple diffraction orders
Solution Approach 1:
The patent extracts and eliminates high-order diffraction beams from the interference field, keeping only the ±1st-order beams and 0th-order beam. This is achieved through algorithmic processing that identifies and removes contributions from higher diffraction orders, thereby simplifying the interference field and improving phase retrieval precision without requiring physical filtering components.
Solution Approach 2:
The patent changes the parameters of the interference field by selectively modulating the coherence of different diffraction orders. Through mathematical processing of the interferograms, the system transforms the multi-order interference pattern into an effective two-beam interference pattern, changing the operational parameters to achieve high-precision phase retrieval.
2Measurement precision
If a double-window mask is used to filter high-order diffraction beams, then phase retrieval precision is improved, but device complexity and operational convenience deteriorate due to mask replacement requirements
Solution Approach 1:
The patent replaces the mechanical double-window mask filtering system with an algorithmic approach. Instead of physically blocking high-order beams with masks that require manual replacement, the system uses computational methods to identify and eliminate high-order diffraction contributions from the interferograms, thereby maintaining precision while dramatically improving operational convenience.
Solution Approach 2:
The patent introduces an algorithmic intermediary processing step between light source and detector that mathematically separates and eliminates high-order diffraction effects. This computational mediator performs the filtering function that would otherwise require physical masks, enabling precise phase retrieval without mechanical intervention.
3Adaptability or versatility
If the numerical aperture of the optical system is increased, then the measurement capability and sensitivity are improved, but phase retrieval accuracy deteriorates due to increased high-order diffraction beams
Solution Approach 1:
The patent converts the harmful effect of high-order diffraction beams (which increase with numerical aperture) into a beneficial diagnostic tool. By algorithmically analyzing the interference patterns, the system identifies and quantifies high-order beam contributions, then eliminates them computationally. This approach not only maintains accuracy but actually enables higher numerical aperture measurements by managing the increased diffraction complexity.
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 approach achieves precise phase retrieval by eliminating the influence of high-order diffraction beams, improving wavefront measurement accuracy and allowing for a larger measurable range of numerical apertures with a simpler and more convenient operation.
Implementation Method 1
due to the diffraction of the grating at the object plane, there are multistage high-order diffraction beams
Implementation Method 2
interference can occur between these high-order diffraction beams
Data Source
AI summary
A method for wavefront measurement of optical imaging system based on grating shearing interferometry, the grating shearing interferometer comprising: a light source and illumination system, an optical imaging system to be tested, a one-dimensional diffraction grating plate, a two-dimensional diffraction grating plate, a two-dimensional photoelectric sensor and a computing unit. The one-dimensional diffraction grating plate and the two-dimensional diffraction grating plate are respectively placed on the object side and the image side of the optical imaging system to be tested. By collecting N sets of interferograms with a2πNphase-shifting interval (where,N=2(fix(ceil(1/s)2)+1),s is the shear ratio of the grating shearing interferometer), combined with a certain phase retrieval algorithm, the influence of all high-order diffraction beams on the phase retrieval accuracy is eliminated, and finally the wavefront measurement accuracy for the optical imaging system is improved.