Interferometer with Tunable Coherent Length for Shape Measurement
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Solution Overview
Problem
Interferometers used for measuring the shape of objects face interference noises due to rear-surface reflections, especially when measuring surfaces with large waviness, and require precise setup adjustments, making it difficult to achieve accurate measurements.
Innovation Solution
An interferometer with a wavelength-variable light source that emits light with varying wavelengths, allowing for the separation and analysis of interference fringe images to compute the shape of the object, while controlling the coherent length to minimize rear-surface interference and facilitate easy adjustment of the measuring object.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a low-coherence light source is used to prevent rear-surface reflection interference, then interference noises are reduced, but it becomes difficult to measure target surfaces with large waviness and requires strict setup adjustment
Solution Approach 1:
The patent employs a tunable coherent length that can be dynamically adjusted based on measurement requirements. By controlling the coherence properties of the light source, the system can adapt between short coherent length (for suppressing rear-surface interference) and long coherent length (for measuring surfaces with large waviness), eliminating the need for strict fixed setup adjustments
Solution Approach 2:
The invention changes the coherence parameter of the light source to resolve the contradiction. By adjusting the coherent length parameter, the system can suppress interference noises from rear-surface reflections when needed, while also accommodating measurements of surfaces with large waviness, thus improving ease of operation without sacrificing measurement capability
2Object-affected harmful factors
If a short coherent length is used to suppress rear-surface interference, then interference noises are reduced, but measurement of target surfaces with large waviness becomes difficult
Solution Approach 1:
The system dynamically adjusts the coherent length parameter according to the specific measurement task. When measuring surfaces with large waviness, the coherent length is extended to accommodate the optical path differences, while still maintaining suppression of rear-surface interference through controlled coherence management
Solution Approach 2:
By changing the coherence parameter of the light source, the invention enables the system to measure target surfaces with large waviness while simultaneously suppressing interference noises from rear-surface reflections, thus resolving the contradiction between noise suppression and measurement capability
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 effectively reduces interference noises and simplifies the adjustment of the measuring object, enabling accurate shape measurement by varying the coherent length and analyzing interference fringe images integrated over a specific wavelength range.
Implementation Method 1
an interferometer for use in measurement of the shape of a measuring object based on interference fringes
Implementation Method 2
a light reflected from a front surface to be measured (a target surface) and a light reflected from an undesired, rear surface may interfere with each other
Data Source
AI summary
A wavelength-variable light source is configured to emit a light with a wavelength (λ), which is variable within a scan width (Δλ). An interferometer has a coherent length (ΔL), which is determinable from (Δλ) and (λ). A controller determines an appropriate magnitude of the scan width (Δλ) while a CCD camera captures a fringe image in an exposure time (Te), which is set longer than a time for wavelength scanning.