Calibrating Diffractive Optical Modules for Interferometric Shape Measurement
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Solution Overview
Problem
Existing interferometric methods for determining the shape of optical surfaces with non-rotationally symmetric forms suffer from insufficient accuracy due to inadequate calibration of diffractive optical elements, particularly when measuring nano free-form surfaces that deviate significantly from rotationally symmetric reference surfaces.
Innovation Solution
A method involving a diffractive optical calibration module to generate a calibration wave and determine a position assignment distribution between the module plane and detector plane, allowing for precise evaluation of interferograms and improved accuracy in shape measurement by accounting for the difference between the target shape and the test wave's wavefront.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a test wave with a wavefront merely approximated to the target shape is used for measurement, then the measurement process becomes simpler and more adaptable to various surface types, but the measurement accuracy deteriorates due to insufficient calibration
Solution Approach 1:
The patent applies preliminary action by performing calibration before measurement using a calibration module with a known calibration wavefront. This calibration step establishes a position assignment distribution that accounts for optical path deviations, enabling subsequent measurements with approximated test waves to achieve high accuracy. The calibration is done in advance using a calibration interferogram, so the actual measurement can use simpler approximated wavefronts without sacrificing precision.
2Measurement precision
If a test wave with a wavefront adapted to the target shape is used, then measurement accuracy is improved, but the device complexity increases due to the need for precise calibration modules and position assignment determination
Solution Approach 1:
The patent uses an intermediary approach by introducing a calibration module that generates a calibration wave with a known wavefront (such as a spherical or planar wave). This calibration wave serves as an intermediary to determine the position assignment distribution, which then enables accurate measurement using test waves with approximated wavefronts. The intermediary calibration step simplifies the overall system compared to requiring direct complex wavefront adaptation.
3Productivity
If traditional interferometric methods are used without position assignment distribution determination, then the measurement process is faster and simpler, but the accuracy deteriorates for non-rotationally symmetric surfaces
Solution Approach 1:
The patent applies preliminary action by performing a calibration step before measurement to determine the position assignment distribution. This preliminary calibration using a calibration interferogram establishes the mapping between the module plane and detector plane, enabling subsequent measurements to achieve high accuracy for non-rotationally symmetric surfaces. The calibration is done once in advance, so it does not significantly impact the speed of actual measurements.
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 enhances the accuracy of shape determination for optical surfaces with non-rotationally symmetric forms by enabling precise position assignment and distortion correction, resulting in more accurate measurement results compared to traditional methods.
Implementation Method 1
arranging a diffractive optical calibration module in the module plane for the purposes of generating a calibration wave
Implementation Method 2
superimposing a reference wave with the reflected test wave
Data Source
AI summary
A method for calibrating a measuring device (10) for interferometrically determining a shape of an optical surface (12) of an object under test (14). The measuring device includes a module plane (32) for arranging a diffractive optical test module (30) which is configured to generate a test wave (34) that is directed at the optical surface and that has a wavefront at least approximately adapted to a target shape (60) of the optical surface. The method includes: arranging a diffractive optical calibration module (44) in the module plane for generating a calibration wave (80), acquiring a calibration interferogram (88) generated using the calibration wave in a detector plane (43) of the measuring device, and determining a position assignment distribution (46) of points (52) in the module plane to corresponding points (54) in the detector plane from the acquired calibration interferogram.