Equal-path interferometer with tilted optics for transparent surface profiling
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Solution Overview
Problem
Existing equal-path interferometers face challenges in accurately measuring surfaces of partially transparent objects while being insensitive to other surfaces, particularly due to issues with unwanted reflections and maintaining equal path lengths for low-coherence light sources.
Innovation Solution
The implementation of an interferometer design featuring partially reflective surfaces oriented at non-normal angles, an aperture stop for filtering reflections, and a dispersion compensator to ensure equal path lengths and minimize unwanted reflections, allowing for the use of low-coherence light sources and precise surface profiling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If equal-path interferometer configuration is used with low-coherence light sources, then measurement of partially transparent object surfaces is enabled, but unwanted reflections from multiple surfaces interfere with measurement precision
Solution Approach 1:
The interferometer separates the measurement beam and reference beam into distinct spatial paths using beam splitters and mirrors, allowing independent control and optimization of each path to minimize unwanted reflections while maintaining equal optical path lengths
Solution Approach 2:
Anti-reflective coatings are applied to optical surfaces to reduce unwanted reflections, and aperture stops are introduced as intermediary elements to block spurious reflections from entering the detection path
2Adaptability or versatility
If unequal-path laser Fizeau interferometer is adapted to equal-path geometry, then low-coherence light source compatibility is achieved, but device complexity increases
Solution Approach 1:
The interferometer is designed with adjustable optical path length mechanisms that allow it to operate with different types of light sources (laser and low-coherence sources) by modifying the path lengths to achieve equality, making the device universally applicable to multiple light source types
Solution Approach 2:
The optical path lengths are made dynamically adjustable through movable mirrors and adjustable beam paths, allowing the system to adapt from unequal-path configuration for laser sources to equal-path configuration for low-coherence sources
3Measurement precision
If aperture stop is introduced to block spurious reflections, then measurement precision improves, but light intensity is reduced
Solution Approach 1:
The aperture stop is strategically positioned and sized to block only the specific angular range of spurious reflections while allowing the main measurement and reference beams to pass through with minimal attenuation, achieving selective filtering
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 design enables accurate profiling of selected surfaces of partially transparent objects by effectively blocking spurious reflections and maintaining equal path lengths for measurement and reference beams, resulting in a robust and sensitive interference pattern analysis.
Implementation Method 1
A portion of the incoming source light passes through a partially-reflective surface of the reference element to the interferometer beamsplitter. The beamsplitter divides the portion of the source light into reference and measurement beams.
Implementation Method 2
The measurement beam returns to the interferometer beamsplitter and combines approximately coextensively and coaxially with the reference beam, resulting in an interference pattern at the camera.
Implementation Method 3
The reference element and the beamsplitter are tilted in such a way that spurious reflections from the various surfaces of the interferometer components are blocked by the aperture stop or its equivalent, resulting in a two-beam interference pattern at the imaging device.
Data Source
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AI summary
An optical assembly for use in an interferometer is provided. The optical assembly includes first and second partially reflective surfaces positioned along an optical axis and oriented at different non-normal angles to the optical axis. The second partially reflective surface is configured to receive light transmitted through the first partially reflective surface along the optical path, transmit a portion of the received light to a test object to define measurement light for the interferometer and reflect another portion of the received light back towards the first partially reflective surface to define reference light for the interferometer. The reference light makes at least one round trip path between the second and first partially reflective surfaces.