Chromatic Confocal Spectral Interferometry Depth Resolution
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Solution Overview
Problem
Chromatic confocal spectral interferometry faces a trade-off between spectral range and depth measurement resolution, leading to reduced accuracy in determining the depth position of objects with non-continuous surfaces, due to limited spectral bandwidth and increased nonlinearities in the interferometric optical path.
Innovation Solution
A method and assembly that generate multiple spectral wavelets with distinct wavenumbers, increasing the spectral bandwidth and reducing nonlinearities, allowing for higher depth measurement resolution and accuracy by using a multispectral source, a chromatic object path, and an achromatic reference path with a multifocal optical component that produces confocal foci coinciding in a single point, enabling simultaneous detection of multiple foci with different colors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If spectral analysis is performed at the output of an interferometer using conventional methods, then depth measurement can be achieved, but high lateral resolution cannot be achieved in case of objects with large depth extension
Solution Approach 1:
The patent divides the spectral detection into multiple discrete spectral channels or bands. Instead of using a continuous spectrum, the interference signal is segmented into multiple spectral wavelets at different wavenumbers. This segmentation allows each spectral component to be detected independently, improving depth measurement resolution while managing the spectral bandwidth limitation.
Solution Approach 2:
The patent transitions from spatial resolution to spectral resolution by encoding depth information in the spectral domain. Instead of relying on lateral spatial separation, the system uses spectral frequency encoding where different depth positions correspond to different spectral frequencies. This dimensional transformation enables high depth resolution without requiring large spectral bandwidth.
2Measurement precision
If chromatic depth splitting is performed using a diffractive lens with a tunable laser, then focus position and wavenumber relationship is established, but nonlinearities in the detected spectral wavelet are introduced
Solution Approach 1:
The patent changes the optical parameters by using a Schwarzschild objective with specific refractive powers designed for different wavenumbers. This parameter optimization ensures that the chromatic depth splitting produces linear spectral wavelets, eliminating the nonlinearities that would complicate signal evaluation. The system is designed so that the focal length variation with wavenumber follows a predictable linear relationship.
3Reliability
If a Pellicle beam splitter is used for depth scanning, then interferometric measurement is possible, but vibrations and multiple reflections occur, and free working distance is reduced
Solution Approach 1:
The patent extracts the beam splitting function from a Pellicle beam splitter and implements it through wavelength-dependent focal length variation in the Schwarzschild objective. By removing the physical beam splitter component, the system eliminates the associated vibrations, multiple reflections, and working distance limitations. The beam splitting is achieved optically through the chromatic aberration of the objective lens itself.
4Measurement precision
If refractive materials are used in the optical path for chromatic depth splitting, then focus position and wavenumber relationship is established, but dispersions lead to nonlinearities in the detected spectral wavelet
Solution Approach 1:
The patent uses a composite optical system combining a Schwarzschild objective with specific refractive materials chosen for their dispersion characteristics. The refractive materials are selected and arranged to compensate for chromatic aberrations and ensure linear spectral wavelet detection. This composite material approach optimizes the refractive power variation with wavenumber to maintain linearity in the spectral domain.
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 significantly enhances the depth measurement resolution and reduces measurement uncertainty, enabling precise determination of depth positions even on non-continuous surfaces, while minimizing the influence of dispersions and vibrations, making it suitable for industrial applications.
Implementation Method 1
a multifocal optical component configured for producing chromatic depth splitting of foci in a two-beam interferometer
Implementation Method 2
spectral interferometry
Data Source
AI summary
The present invention relates to a method and an assembly for chromatic confocal spectral interferometery, in particular also for spectral domain OCT (SD-OCT) using multi-spectral light. A multiple (e.g. two, three, four, etc.) axial splitting of foci in the interferometric object arm is performed using a multifocal (e.g. bifocal, trifocal, quattro-focal, etc.) optical component, forming thereby at least two, three or even several groups of chromatically split foci in the depth direction. The multifocal optical component is made of a diffractive optical element (712) and a Schwarzschild objective (5). At least two, three, four or even more differently colored foci of different groups of foci coincide in at least one confocal point in the object space of the setup. Thus, at least two, three or even more spectral wavelets are formed in the case of optical scanning of an object measurement point and spectral detection in the wavenumber domain, which wavelets are at least slightly spectrally separated from each other. This results in a significant increase in the optical primary data in the wavenumber domain and reduces the trade-off of the chromatic confocal spectral interferometry between axial measurement range and depth resolution. From the detected data, it is possible to calculate tan (alpha) as the quotient of the absolute phase shift delta_phi and the associated wavenumber difference delta_k, the Fourier transform over the spectral data, in order to respectively determine the optical path difference.