Overview
FrançaisABSTRACT
Laser interferometry is a technique allowing for contactless dimensional measurements of high precision with a dynamic range of several meters in air and nanometric resolution. It is used for calibrating translational displacements on machine tools or for characterizing motorized or piezoelectric micrometric mechanical actuators.
The principles of homodyne, heterodyne, or frequency-scanning interferometry as well as various laser sources used for dimensional metrology are presented. The performances of different types of commercially available devices are detailed. The main industrial applications of laser interferometry used for dimensional measurement are also discussed.
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Read the articleAUTHORS
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Hervé GILLES: Professor at ENSICAEN, PhD in physics from the University of Caen Laboratoire CIMAP, UMR n° 6252, ENSICAEN Caen, France
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Sylvain GIRARD: Professor at ENSICAEN, PhD in physics from the University of Caen Laboratoire CIMAP, UMR n° 6252, ENSICAEN Caen, France.
INTRODUCTION
Laser interferometry techniques are widely used for metrological dimensional control. They combine a laser source with a perfectly controlled optical frequency and an interferometric assembly to measure relative or absolute distance.
Compared with other approaches, the laser interferometry technique is the only high-precision dimensional measurement method with a dynamic range typically from 0 to 10 m and resolution on the nanometer scale (0.1 to 1 nm). Distance measurement can be carried out continuously on a fixed cooperative target (mirror, cube corner, reflecting sphere) or in translational motion with a maximum displacement speed of the order of 0 to 1 m/s.
It is therefore widely used for the periodic calibration of translational displacements on machine tools or the characterization of mechanical actuators (displacement range, absolute accuracy and resolution), such as motorized or piezoelectric micrometric translation tables.
The homodyne measurement technique uses a light wave with a single optical frequency. It involves interferometric detection of the optical phase difference between a reference light signal and the signal reflected by the mechanical part whose position is being checked.
The heterodyne measurement technique is based on the detection of a beat to determine the difference between two optical frequencies. One optical frequency is the reference, while the other is reflected by the mechanical part under test. These homodyne and heterodyne laser interferometry techniques can only measure a variation in distance, and are therefore relative dimensional measurement techniques. In addition, they require the measurement to be carried out without discontinuity during the entire movement of the mechanical part, which is a major constraint in real-life conditions.
To obtain an absolute distance measurement, the laser source undergoes a continuous optical frequency sweep. This frequency sweep approach has several advantages:
the distance is measured absolutely;
the measurement can be interrupted during travel without the absolute distance information being lost.
On the other hand, it requires a seamless sweep of the optical frequency, which places a significant constraint on the laser technology used.
For all these techniques, the absolute accuracy and resolution of distance measurement are directly linked to the quality of control exercised over the optical frequency (wavelength) of the laser source, and to the corrections made to take account of variations in the refractive index of the air through which the laser beam passes during measurement...
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KEYWORDS
laser | optical sensors | interferometry | metrology | dimensional measurements
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Dimensional measurements using laser interferometry
Bibliography
Standards and norms
- Laser safety – Part 1: Equipment classification and requirements - NF EN 60825-1 - Janvier 2008
- Length measuring instruments – Fringe-counting and laser-source interferometers. - E11-016 - Avril 1987
Directory
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API Metrology
https://apimetrology.com/xd-laser/
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https://www.attocube.com/en/products/laser-displacement-sensor
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