Interferometric Optical System Aperture Blocking Unwanted Reflections
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Solution Overview
Problem
Optical interferometers face challenges in separating reflections from multiple conformal surfaces of test objects due to unwanted reflections, particularly when using spatially extended light sources, which reduce fringe contrast and signal quality.
Innovation Solution
An interferometric optical system employing a partially reflective reference surface, spatially extended light sources, and compensation elements such as phase plates or variable transmission plates to adjust the optical path lengths and phase delays, ensuring high fringe contrast by blocking unwanted reflections and compensating for defocus effects caused by the spatial extension of the light source.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a spatially extended light source is used, then the interferometer can accommodate larger source sizes and potentially improve brightness, but the fringe contrast deteriorates due to defocus effects and reduced spatial coherence
Solution Approach 1:
A phase compensation plate is introduced as an intermediary element between the light source and the interferometer components. This plate contains phase-delaying structures that compensate for the path length differences caused by the spatial extension of the light source, thereby restoring fringe contrast while allowing the use of extended sources for improved brightness
Solution Approach 2:
The patent modifies the optical path length parameters by introducing variable transmission plates and phase compensation elements. These elements adjust the phase relationships between different ray paths originating from different parts of the extended light source, changing the interference conditions to maintain high fringe contrast despite the spatial extension
2Adaptability or versatility
If multiple conformal surfaces are measured, then the measurement capability is improved, but unwanted reflections from different surfaces interfere with each other, reducing signal quality
Solution Approach 1:
The patent uses variable transmission plates and aperture stops to selectively block unwanted reflected beams while allowing desired interference patterns to pass. This extracts the harmful reflections from the measurement path, enabling clear measurement of multiple surfaces without cross-interference
Solution Approach 2:
Variable transmission plates are employed that can dynamically adjust their transmission properties to selectively pass or block different beam paths. This dynamic control allows the system to adapt to different measurement configurations and eliminate unwanted reflections from specific surfaces while maintaining measurement capability
3Object-generated harmful factors
If the mutual coherence length is reduced below twice the optical distance between reference and test surfaces, then unwanted reflections are suppressed, but the interference signal becomes weaker
Solution Approach 1:
Phase compensation plates serve as intermediaries that extend the effective coherence by compensating for path length differences. This allows the use of shorter coherence length light sources to suppress unwanted reflections while maintaining strong interference signals through phase correction
Solution Approach 2:
The patent changes the phase parameters of the light waves using compensation elements, effectively extending the coherence properties without changing the physical coherence length of the source. This parameter transformation allows suppression of unwanted reflections while preserving signal strength
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 system achieves high fringe contrast and signal-to-noise ratio by selectively blocking unwanted reflections and compensating for defocus effects, enabling accurate topography measurement of multiple surfaces with improved interference patterns.
Implementation Method 1
the two input beams have a mutual coherence length smaller than twice an optical distance between the reference surface and any of the test object surfaces
Implementation Method 2
an aperture positioned to selectively block light from reaching the detector, wherein the angle between the first and second input beams is selected to cause the aperture to block light from the first input beam reflected by the reference surface
Implementation Method 3
a reference object comprising a partially reflective reference surface
Implementation Method 4
Optical interferometers are optical systems that combine optical wave fronts to generate a measurable interference signal (e.g., a modulation in optical intensity) indicative of phase information of one wave front relative to the other wave front
Data Source
AI summary
An interferometric optical system for measuring a test object, including: i) a reference object comprising a partially reflective reference surface; ii) a light source module configured to direct first and second input beams through the reference surface to the test object at an angle to one another; iii) a detector positioned to detect light reflected from the reference surface and one or more surfaces of the test object; and iv) an aperture positioned to selectively block light from reaching the detector, wherein the angle between the first and second input beams causes the aperture to block light from the first input beam reflected by the reference surface and pass light from second input beam reflected by the reference surface, wherein the two input beams have a mutual coherence length smaller than twice an optical distance between the reference surface and the test object.