Interferometer Real-Time Fringe-Free Imaging via OPD Modulation
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Solution Overview
Problem
Interferometric surface profilers often require operators to interpret interference patterns, which can be challenging in dim or complex samples, leading to washed-out features and confusing images due to interference fringes, necessitating a 'fringe-free' mode without additional hardware.
Innovation Solution
A surface topography interferometer that displays a real-time 'fringe-free' image by modulating the optical path difference at a rate faster than the detector's integration time, reducing fringe contrast through periodic or sinusoidal modulation, and synchronizing with illumination intensity modulation, allowing for adjustable fringe reduction and maintaining focus on the sample surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If mechanical shutters are used to block reference light, then fringe-free imaging is achieved, but device complexity increases
Solution Approach 1:
The patent replaces mechanical shutters with temporal modulation of the optical path difference. By modulating the OPD at a frequency higher than the detector integration time, the reference light contribution is averaged out, eliminating fringes without mechanical moving parts. This substitutes a mechanical system with an optical control system.
Solution Approach 2:
The patent employs periodic modulation of the optical path difference between test and reference light. The OPD is modulated sinusoidally or with other periodic waveforms at frequencies above the detector integration rate, causing the reference light to contribute uniformly across all phases, thereby averaging out the interference pattern and eliminating visible fringes.
2Measurement precision
If reference light intensity is increased to improve signal, then measurement sensitivity improves, but fringe contrast increases making images harder to interpret
Solution Approach 1:
The patent modulates the optical path difference periodically at frequencies higher than the detector integration time. This causes the reference light to be distributed uniformly across all phase positions during integration, converting the interference pattern into a uniform background that does not obscure sample features, thus maintaining high signal intensity while improving image interpretability.
Solution Approach 2:
The patent introduces dynamic modulation of the optical path difference during the detector integration period. By continuously varying the OPD throughout the integration time, the system transforms static high-contrast fringes into a dynamic average that appears as uniform background illumination, allowing the reference light to contribute strongly without creating obscuring patterns.
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
Enables improved contrast and visibility of sample features by substantially reducing or eliminating interference fringes, facilitating easier operation and metrology data acquisition while avoiding detector saturation, and allowing instantaneous switching between fringe-free and interferogram displays.
Implementation Method 1
Interferometers generally include an actuator for varying the optical path in an interferometric cavity... The interferometric information is useful in many ways... the presence of the interference fringes is undesirable
Implementation Method 2
an actuator for varying the optical path in an interferometric cavity, for example a PZT-mounted mirror or beamsplitter
Data Source
AI summary
Methods include: directing test light and reference light along different optical paths, where a test object is in a path of the test light; forming an image of the test object on a multi-element detector by directing test light from the test object to the detector; overlapping the reference light with the test light on the detector; detecting an intensity of the overlapped test and reference light with the detector, the intensity being detected at a frame rate; and modulating an optical path difference (OPD) between the test and reference light at the detector while detecting the light intensity. The OPD is modulated at a rate and amplitude sufficient to reduce a contrast of fringes in a spatial interference pattern formed by the light at the detector over a frame of the detector. Accordingly, fringe-free images may be acquired real-time.