Optical Delay Path for Extended Interferometric Range
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Solution Overview
Problem
Interferometric distance measuring methods with frequency-modulated laser beams are limited by coherence length, restricting the measurable distance range and requiring extensive control efforts to scan and measure objects completely in a short time, especially in applications with varying surface structures and larger geometries.
Innovation Solution
The introduction of an optical delay path, implemented using a Mach-Zehnder interferometer, allows for a delayed radiation field that shifts the operating point towards the target, effectively increasing the measurable distance range beyond the coherence length limitations, enabling faster measurement times and larger travel speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a frequency-modulated laser beam is used for interferometric measurement, then measurement precision is improved, but the measurable distance range is limited by coherence length
Solution Approach 1:
The patent applies preliminary action by introducing an optical delay path that pre-delays the reference beam before it reaches the detector. This delay is calculated to compensate for the expected round-trip time of the measuring beam to distant targets, allowing the reference and measuring beams to remain coherent even over distances exceeding the laser's natural coherence length. The delay path length is set to L_delay = c × Δt, where c is the speed of light and Δt is the time delay matching the round-trip time to the target.
2Adaptability or versatility
If the measuring range is increased beyond coherence length, then adaptability is improved, but control complexity increases due to extensive scanning requirements
Solution Approach 1:
The patent uses an optical delay path as an intermediary element between the laser source and the detector. This delay path acts as a mediator that extends the effective coherence length by introducing a controlled time delay to the reference beam, allowing it to remain synchronized with the measuring beam even when the latter travels to distant targets. This intermediary component enables extended measuring range without requiring complex control systems or multiple lasers.
3Measurement precision
If white light interferometry with spectrally resolved detection is used, then measurement accuracy is improved, but measurement time increases due to slow scanning
Solution Approach 1:
The patent replaces the mechanical scanning approach of white light interferometry with a frequency-modulated laser-based interferometric system. Instead of physically moving components to achieve spectral resolution, the system uses frequency modulation of the laser combined with an optical delay path to achieve both extended range and accurate measurements. This substitution of mechanical scanning with optical frequency modulation dramatically reduces measurement time while maintaining or improving 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 enhances the measurable distance range, reduces measurement times, and allows for more flexible and efficient scanning of objects with varying surface structures by shifting the operating point of the interferometric arrangement, thus overcoming the constraints imposed by coherence length.
Implementation Method 1
a laser source (1) with a coherence length of more than 1 millimeter, preferably more than 60 millimeters
Implementation Method 2
One of the two radiation components is guided without delay over the distance to be measured to the target and back to the radiation detector, while the other component passes through at least one optical delay element or delay path (3)
Implementation Method 3
A delay component with two optical couplers (2), one of which is designed as a beam splitter for the measuring radiation of the frequency-modulated laser source (1), splitting it into two radiation components
Implementation Method 4
a radiation detector (11) to receive the measuring radiation MS backscattered from the surface (13)
Implementation Method 5
interferometric distance measuring methods with frequency-modulated laser beams
Data Source
Figure 1~3
Figure 4~6
Figure 7~9b
AI summary
The invention relates to a distance measuring method for measuring surfaces (13). A laser beam, the wavelength of which can be tuned in a wavelength range by modulating the frequency of a laser source (1), is generated with a coherence length in order to provide a measuring beam (MS) and is emitted at the surface (13), which is located within a specified distance range, as a measuring beam (MS). The measuring beam (MS), which is back-scattered by the surface (13), is received again and used to interferometrically measure the distance from a reference point to the surface (13), a measurement and reference interferometric arm being used. The specified distance range lies at least partly outside of the coherence length, and the measuring beam is separated into two beam portions. One of the beam portions is temporally delayed with respect to the other beam portion such that the one optical path difference caused by the delay matches the optical path difference that corresponds to a distance in the specified distance range plus or minus the coherence length of the laser.