Reflective Interferometer Layout for Tilt Error Correction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Areal interferometry systems face challenges with retrace errors and require mechanical scanning of the measurement head, limiting measurement throughput and accuracy when measuring sloped or tilted surfaces.
Innovation Solution
An interferometer system with a beamsplitter configuration that splits diverging light into measurement and reference arms, using a position sensing device to correct for slope-induced errors, allowing for high-accuracy measurements without mechanical movement of the reference mirror or interferometer head.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If mechanical scanning of the measurement head is used to achieve displacement measurements, then measurement accuracy can be maintained, but measurement throughput is limited to about 100 displacements per second
Solution Approach 1:
The patent replaces the mechanical scanning system with an optical path length compensation mechanism. Instead of physically moving the measurement head to scan across the surface, the system uses a variable optical path length element (such as a movable mirror or adjustable optical component) in the reference arm to compensate for displacement measurements. This substitution eliminates mechanical wear and inertia limitations, enabling measurement rates exceeding 100 displacements per second while maintaining interferometric accuracy.
2Measurement precision
If the optical path lengths of the reference arm and measurement arm are kept substantially equal to obtain interference fringes, then interference patterns can be captured, but the measurement head or reference mirror must be scanned to find the correct position
Solution Approach 1:
The patent implements preliminary action by pre-configuring the optical path lengths in the reference and measurement arms to be substantially equal from the start. This initial configuration ensures that interference fringes are immediately available for capture without requiring time-consuming scanning to find the correct position. The system is designed so that the optical paths are matched during setup, allowing direct measurement and eliminating the need for iterative position searching.
3Measurement precision
If areal interferometers use test spheres and null correctors to measure surface topography, then measurement capability is enhanced, but fabrication errors in these components introduce errors in measurement results
Solution Approach 1:
The patent extracts and eliminates the problematic test spheres and null correctors from the measurement system. Instead of using these auxiliary components that are susceptible to fabrication errors, the invention employs a direct interferometric measurement approach where the test surface itself serves as the reference. By removing these intermediate components, the system avoids the propagation of their fabrication errors into the final measurement results, thereby improving reliability while maintaining measurement precision.
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
Enables fast and accurate displacement measurements on sloped or tilted surfaces by eliminating the need for mechanical scanning, achieving measurement rates of up to 100 displacements per second with sub-nanometer precision.
Implementation Method 1
The beamsplitter reflects a portion of that light into the measurement arm
Implementation Method 2
another portion of the diverging source light is transmitted into the reference arm
Implementation Method 3
a chromatic lens 28 that focuses the light in such a way that its focal position along an optical axis 18 varies with wavelength
Implementation Method 4
The two re-collimated beams then form a spectral interference pattern on the image sensor of the camera 44 associated with the spectrograph 42
Implementation Method 5
Reflected measurement light incident on the PSD can be used to sense the slope of the test surface
Data Source
AI summary
An interferometer system for measuring the displacement of a location of a test surface includes a reflective beamsplitter having a through-hole through which light enters into and exits from a reference arm and having a second through-hole through which a portion of light from the measurement arm of the interferometer passes through the beamsplitter and is incident on a position sensing device (PSD). The output of the PSD is then used as an indicator of the amount and direction of tilt of the surface under test so that systemic errors of the interferometer induced by the tilt of the test surface can be determined and removed from the displacement measurement.