Scan Error Correction in Low Coherence Interferometry
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Solution Overview
Problem
Low coherence interferometry measurements are prone to errors due to perturbations in the optical path length difference, particularly from vibrations, which are not effectively captured by conventional phase-shifting algorithms and result in increased system noise and reduced accuracy.
Innovation Solution
The implementation of a system that monitors the true scan history using a laser displacement measuring interferometer or other sensors to correct for scan errors by generating monitor interference signals with a coherence length longer than the OPD scan range, allowing for the determination of high-frequency vibration effects and subsequent correction of interferometric data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional phase-shifting algorithms are used to process low coherence interferometry data, then the measurement process is simple and fast, but the accuracy is reduced due to uncorrected scan errors from vibrations
Solution Approach 1:
A laser displacement measuring interferometer is introduced as an intermediary device to monitor the actual scan history and provide correction data. This mediator captures vibration-induced scan errors without interfering with the primary low coherence interferometry measurements, enabling accurate error compensation while maintaining measurement simplicity.
Solution Approach 2:
The system implements feedback by continuously monitoring the actual optical path difference scan history using the laser displacement interferometer and using this information to correct the interferometry data. The feedback loop captures vibration effects and applies real-time corrections, significantly improving measurement accuracy despite increased system complexity.
2Measurement precision
If the optical path length difference is scanned over a large range to capture all interference fringes, then complete surface profile information is obtained, but high-frequency vibration effects are not effectively captured by conventional algorithms
Solution Approach 1:
The laser displacement measuring interferometer serves as an intermediary that specifically monitors high-frequency vibration effects during the OPD scan. This mediator captures the dynamic scan history including high-frequency perturbations, enabling reliable measurement even when the OPD is scanned over large ranges where conventional algorithms would fail to capture vibration effects.
3Measurement precision
If a laser displacement measuring interferometer is added to monitor scan history, then scan errors can be corrected and measurement accuracy improves, but device complexity and cost increase
Solution Approach 1:
The laser displacement measuring interferometer is positioned as a separate intermediary monitoring system rather than an integrated component. This approach allows scan error correction capability to be added without redesigning the core low coherence interferometry system, thereby improving measurement precision while minimizing the increase in overall system complexity.
Solution Approach 2:
The laser displacement measuring interferometer performs multiple functions: it monitors the actual optical path difference scan history, captures high-frequency vibration effects, and provides correction data for the primary measurements. This multi-functionality justifies the added device complexity by delivering comprehensive error correction capabilities.
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 significantly reduces noise and enhances the accuracy of low coherence interferometry measurements by accounting for vibrations and other mechanical disturbances, improving the resolution and reliability of surface profiling and metrology.
Implementation Method 1
an interferometer combines measurement light reflected from the surface of interest with reference light reflected from a reference surface to produce an interferogram
Implementation Method 2
a laser displacement measuring interferometer or other sensors to correct for scan errors by generating monitor interference signals
Data Source
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AI summary
An apparatus includes a broadband scanning interferometry system including optics for combining test light from a test object with reference light to form an interference pattern on a detector. The apparatus includes a stage configured to scan an optical path difference (OPD) between the test and reference light from a common source to the detector and a detector system including the detector for recording the interference pattern for each of a series of OPD increments, the frequency of each OPD increment defining a frame rate. The optics are configured to produce at least two monitor interferometry signals indicative of changes in the OPD as it's scanned, the detector system being configured to record the monitor interferometry signals. The apparatus includes a processor configured to determine information about the OPD increments with sensitivity to perturbations to the OPD increments at frequencies greater than the frame rate.