Spectrally Controllable Light Sources for Coherence Noise Reduction
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Solution Overview
Problem
Interferometric methods face limitations in achieving high-accuracy shape measurements due to the drawbacks of both high-coherence laser interferometers, which are susceptible to coherence noise and require clean environments, and low-coherence white-light interferometers, which need complex mechanical designs for scanning.
Innovation Solution
The application of Fourier Transforms to manipulate the spectral distribution of the light source to produce localized interference fringes at selected locations, allowing for the use of Fizeau interferometers to perform white-light measurements without coherence noise and eliminating the need for mechanical scanning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high-coherence laser sources are used in interferometry, then measurement precision is improved, but coherence noise increases and the system requires clean environments
Solution Approach 1:
The patent modulates the spectral distribution of the light source to change its coherence properties dynamically. By adjusting the spectral width and shape, the system achieves high coherence when needed for precise measurements while reducing coherence noise when not required, thus resolving the contradiction between measurement precision and coherence noise
Solution Approach 2:
The interferometer system dynamically adjusts the coherence properties of the light source during operation. The spectral distribution is modulated in real-time to match the measurement requirements, allowing the system to transition between high-coherence and low-coherence states as needed, thereby eliminating the need for clean environments while maintaining measurement accuracy
2Object-generated harmful factors
If low-coherence white-light sources are used in interferometry, then coherence noise is reduced, but device complexity increases due to required mechanical scanning
Solution Approach 1:
The patent replaces the mechanical scanning mechanism with spectral modulation of the light source. Instead of physically moving components to achieve interference patterns, the system modulates the spectral distribution to create localized fringes, thereby eliminating complex mechanical designs while maintaining the ability to perform precise measurements
Solution Approach 2:
The system changes the spectral parameters of the light source to achieve the desired interference patterns without mechanical movement. By modulating the spectral width and shape, the system creates localized fringes that eliminate the need for scanning mechanisms, thus reducing device complexity while maintaining measurement capability
3Reliability
If white-light interferometers are used to eliminate coherence noise, then reliability is improved, but ease of operation deteriorates due to scanning requirements
Solution Approach 1:
The patent eliminates the mechanical scanning system by using spectral modulation to create localized interference fringes. This substitution maintains the reliability benefits of white-light interferometry while dramatically improving ease of operation, as the system requires no moving parts or complex scanning procedures
Solution Approach 2:
The light source itself performs the function previously requiring mechanical scanning. By modulating its spectral distribution, the light source automatically creates localized fringes at the desired measurement locations, making the system self-sufficient and eliminating the need for external scanning mechanisms
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 enables high-accuracy shape measurements with reduced coherence noise and simpler construction, achieving the benefits of both laser and white-light interferometry while avoiding interference from spurious reflections and eliminating the need for mechanical parts.
Implementation Method 1
interferometric methods rely on the interference of two beams of light to produce interference patterns
Implementation Method 2
the application of Fourier Transforms to manipulate the spectral distribution of the light source
Data Source
AI summary
The time delay (and therefore the OPD) between object and reference beams in an interferometer is manipulated by changing the spectral properties of the source. The spectral distribution is tuned to produce a modulation peak at a value of OPD equal to the optical distance between the object and reference arms of a Fizeau interferometer, thereby enabling the use of its common-axis configuration to carry out white-light measurements free of coherence noise. Unwanted interferences from other reflections in the optical path are also removed by illuminating the object with appropriate spectral characteristics. OPD scanning is implemented without mechanical means by altering the source spectrum over time so as to shift the peak location by a predetermined scanning step between acquisition frames. Finally, the spectrum is controlled on a pixel-by-pixel basis to create a virtual surface that matches the profile of a particular sample surface.