Wavefront Measurement Using Sinusoidal Gratings to Suppress Diffraction Noise
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Solution Overview
Problem
Conventional wavefront measuring devices based on shearing interference systems suffer from higher order diffracted lights interfering with the interference fringe, leading to reduced accuracy in wavefront aberration measurement.
Innovation Solution
A wavefront measuring device using a diffraction grating with a sinusoidal transmittance distribution is employed to minimize the effect of higher order diffracted lights, ensuring accurate wavefront information measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a diffraction grating with rectangular-shaped openings is used, then the interference fringe can be generated, but higher order diffracted lights with relatively high light intensity exit from the grating, causing noise to mix with the interference fringe and lowering measurement accuracy
Solution Approach 1:
The patent changes the shape parameter of the diffraction grating openings from rectangular to circular. This parameter change modifies the diffraction characteristics, causing higher order diffracted lights to have lower light intensity while maintaining sufficient intensity for the zeroth and first order lights needed for interference fringe generation. This resolves the contradiction by reducing noise from higher order lights without compromising the measurement capability.
2Reliability
If pinholes are arranged in rows on the object plane, then light beams can be generated and transmitted through the optical system, but the higher order diffracted lights from the grating still interfere with the measurement accuracy
Solution Approach 1:
The patent modifies the geometric parameter of the diffraction grating by changing the opening shape from rectangular to circular. This change affects the diffraction efficiency distribution across different orders, suppressing higher order diffracted lights while preserving the necessary zeroth and first order lights for reliable wavefront measurement through shearing interference.
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 device achieves high-accuracy wavefront aberration measurement by reducing the impact of higher order interference lights, enhancing the precision of optical system assessment.
Implementation Method 1
a diffraction grating having a pattern composed of a large number of rectangular-shaped openings periodically arranged in two mutually orthogonal directions; and an interference fringe, produced by interference between laterally-shifted or deviated wavefronts of a plurality of diffracted lights generated (exiting, outgoing) from the diffraction grating
Implementation Method 2
an interference fringe, produced by interference between laterally-shifted or deviated wavefronts of a plurality of diffracted lights
Data Source
Figure 1
Figure 2(A)~2(E)
Figure 3(A)~3(C)
AI summary
There is provided a wavefront measuring method for obtaining wavefront information of a projection optical system, the method including: irradiating a light beam exited from a pinhole array of a measurement reticle onto the projection optical system; allowing the light beam passed via the projection optical system to come into a diffraction grating having periodicities in the X and X directions and of which transmittance distributions in the X and Y directions have a sinusoidal shape; and obtaining the wavefront information of the projection optical system (PO) based on an interference fringe formed by a plurality of light beams generated from the diffraction grating. The wavefront information of an optical system to be inspected can be measured with high accuracy, based on the interference fringe obtained using the diffraction grating.