Laser Penetration Depth Measurement via Optical Coherence Tomography
Solution Overview
Problem
Current laser processing devices lack a reliable method to precisely measure the penetration depth of a laser beam into a workpiece, leading to inaccuracies in machining results due to fluctuations in the vapor capillary's shape and brightness, which affects welding, cutting, and machining quality.
Innovation Solution
A method utilizing an optical coherence tomograph to generate two measuring beams, one directed at the base of the vapor capillary and the other at the workpiece surface, allowing for precise measurement of penetration depth by calculating the difference between these distances, with adjustments for correction factors and geometric data to ensure accurate readings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If indirect measurement methods (brightness observation) are used to estimate penetration depth, then the measurement can be performed under challenging vapor capillary conditions, but the measurement precision is insufficient leading to inaccurate control
Solution Approach 1:
The patent introduces an intermediary reference surface (the top surface of the workpiece) that can be easily measured by optical methods. By measuring the distance from this reference surface to the vapor capillary base and subtracting the known capillary length, the penetration depth is calculated indirectly but accurately. This mediator approach avoids direct measurement challenges within the vapor capillary while maintaining high precision.
Solution Approach 2:
The patent replaces indirect optical estimation methods (brightness observation) with a more precise optical measurement system (OCT - Optical Coherence Tomography). This substitution enables accurate distance measurement through the vapor capillary medium by using light interference patterns rather than relying on brightness intensity, thereby achieving high measurement precision under challenging conditions.
2Measurement precision
If a single scanning measuring beam is used to measure the vapor capillary, then the measurement system remains simple, but the measurement accuracy is insufficient due to the small size and constant shape changes of the vapor capillary
Solution Approach 1:
The patent segments the measurement task into two distinct parts: (1) measuring the distance from the reference surface to the vapor capillary base using the OCT system, and (2) obtaining the vapor capillary length from process data or separate measurement. This segmentation allows each measurement to be optimized independently, with the OCT system focused on accurate distance measurement while the capillary length is obtained from reliable process parameters.
Solution Approach 2:
The patent uses the workpiece surface as an intermediary reference point that is easily accessible and measurable. By establishing this stable reference surface and measuring distances relative to it, the system achieves accurate penetration depth measurement without requiring complex tracking of the moving vapor capillary shape, thereby maintaining relatively simple device architecture.
3Measurement precision
If the measuring beam is directed into the vapor capillary, then the penetration depth can be measured directly, but the measurement is affected by the bright thermal radiation and changing capillary shape
Solution Approach 1:
The patent uses the workpiece surface as an intermediary reference that is not affected by the harsh conditions inside the vapor capillary. By measuring the distance to this stable reference surface and combining it with capillary length data, the system obtains accurate penetration depth measurements while avoiding the harmful effects of thermal radiation and shape changes within the capillary.
Solution Approach 2:
The patent replaces direct visual observation methods that are susceptible to thermal radiation interference with OCT technology, which uses low-coherence light interference. This substitution enables accurate distance measurement through the bright thermal radiation environment by detecting optical path differences rather than relying on visible light intensity, thereby eliminating the harmful effects of thermal radiation on measurement accuracy.
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 enhances measurement accuracy, enabling precise control of the laser processing parameters, such as power and focal spot position, to achieve high-quality machining results by directly influencing the penetration depth and maintaining it within target values.
Implementation Method 1
optical coherence tomography (OCT) enables highly accurate and non-contact optical distance measurement
Implementation Method 2
a laser radiation source, which can be, for example, a fiber laser or a disk laser
Implementation Method 3
the focal spot creates a vapor capillary in the workpiece
Data Source
Figure 1~3b
Figure 2
Figure 4~5
AI summary
The invention relates to a method for measuring the depth of penetration of a laser beam (19) into a workpiece (24, 26), wherein the laser beam is focused in a focal spot (22) by means of a focusing optical unit (14) arranged in a machining head. The focal spot produces a vapor capillary (88) in the workpiece. An optical coherence tomograph (40) produces a first measurement beam (70a) and a second measurement beam (70b). The first measurement beam (70a) is directed at a first measurement point (MPa) at the base of the vapor capillary (88) in order to thereby measure a first distance (a1) between a reference point and the first measurement point (MPa). At the same time, the second measurement beam (70b) is directed at a second measurement point (MPb) on a surface (92) of the workpiece (24) which faces the machining head (14) and which is outside of the vapor capillary (88) in order to thereby measure a second distance (a2) between the reference point and the second measurement point (MPb). The depth of penetration (d) of the laser beam then results as the difference between the second distance (a2) and the first distance (a1).