Dual-Grating Littrow Interferometry for Alignment-Resilient Precision
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Solution Overview
Problem
Conventional Littrow incident optical paths in grating interferometers face challenges in achieving multiple diffractions and are sensitive to precise alignment and angular changes, affecting measurement precision.
Innovation Solution
A Littrow grating interferometry device employing two diffraction gratings positioned opposite to each other, with a specific optical path configuration allowing secondary diffraction and using quarter-wave plates and reflective mirrors to maintain consistent polarization and path stability, enabling optical subdivision and reducing angular sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional Littrow incident optical path is used, then measurement resolution can be improved by using higher reticle density grating, but multiple diffractions cannot be achieved and the system is very sensitive to precise alignment and angular changes
Solution Approach 1:
The patent divides the single grating system into two separate gratings (first diffraction grating and second diffraction grating) positioned at different locations in the optical path. This segmentation allows each grating to perform a specific diffraction function, achieving multiple diffractions while reducing the sensitivity to alignment errors that would affect a single high-density grating system.
Solution Approach 2:
The patent introduces a beam splitter as an intermediary component between the light source and the gratings, and between the gratings and the detector. This intermediary allows the optical path to be divided and recombined in a controlled manner, enabling multiple diffractions to occur while maintaining stable interference patterns that are less sensitive to angular changes.
2Measurement precision
If higher reticle density grating is used in Littrow incidence, then measurement resolution is improved, but the system becomes more sensitive to angular changes and posture deviation
Solution Approach 1:
By segmenting the measurement function across two gratings rather than relying on a single high-density grating, the system achieves the desired measurement resolution while distributing the angular sensitivity across multiple components. This reduces the impact of any single grating's angular deviation on the overall measurement accuracy.
Solution Approach 2:
The patent transitions from a single-plane diffraction system to a multi-plane system where diffraction occurs at different locations and orientations. This dimensional change in the optical path allows the system to achieve high resolution through the combined effect of multiple diffractions rather than relying on a single high-density grating, thereby reducing angular sensitivity.
3Device complexity
If conventional Littrow optical path is used, then device structure can be simplified, but secondary diffraction cannot be achieved and optical subdivision at higher multiple is limited
Solution Approach 1:
The optical path is segmented into distinct sections with the first grating performing one diffraction and the second grating performing a second diffraction. This segmentation enables optical subdivision at higher multiples while keeping each individual grating and its associated optical components relatively simple, balancing device complexity with measurement precision.
Solution Approach 2:
The patent adds a second diffraction stage to the optical path, transitioning from single-plane to multi-plane diffraction. This dimensional expansion in the optical path enables higher optical subdivision multiples without requiring excessive complexity in any single component, as the complexity is distributed across multiple simpler stages.
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 precision displacement measurements with improved optical subdivision and maintains consistent interference contrast, reducing measurement errors due to angular changes, suitable for applications requiring high integration and precision.
Implementation Method 1
the output light of a two-frequency orthogonal polarization light source achieves secondary diffraction under Littrow incidence between the two diffraction gratings
Implementation Method 2
a first quarter-wave plate is disposed between the reflection assembly and the polarizing beam splitting prism; a second quarter-wave plate is disposed between the first diffraction grating and the polarizing beam splitting prism; a third quarter-wave plate is disposed between the second diffraction grating and the polarizing beam splitting prism
Implementation Method 3
the polarizing beam splitting prism is used to separate the first measuring light and the second measuring light, such that the light in a horizontal polarization state is transmitted and the light in a vertical polarization state is reflected
Implementation Method 4
both the first diffraction grating and the second diffraction grating reflect diffracted light along an original incident optical path
Data Source
AI summary
The invention relates to the technical field of grating interferometry, in particular to a Littrow grating interferometry device and a use thereof. The device comprises: a two-frequency orthogonal polarization light source, a polarizing beam splitting prism, a reflection assembly, a detector, and a first diffraction grating and a second diffraction grating having identical parameters. The output light of the two-frequency orthogonal polarization light source is split by the polarizing beam splitting prism into horizontally polarizing measuring light which is transmitted and vertically polarizing measuring light which is reflected. The two beams of measuring light undergo diffraction twice between the first diffraction grating and the second diffraction grating, and finally the two beams of measuring light interfere with each other and are incident on the detector. Grating displacement is calculated according to the interference pattern. The invention achieves secondary diffraction under Littrow incidence, and improves the precision of displacement measurement.