Dimensional measurements by laser interferometry

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...