Speaker
Description
In IWAA 2024, we presented an absolute distance meter (ADM) for long-range measurements, achieving uncertainties below 1 mm (k=1) over distances of up to 12 km. This instrument uses two laser wavelengths, one in the visible and the other in the infrared, to measure the difference in optical path length between them. This additional measured quantity corresponds to the accumulated effect of the atmospheric dispersion along the propagation path and eliminates the need to determine the air refractive index. The instrument inherently compensates for the contributions of temperature and pressure to the air refractive index. However, achieving an uncertainty better than 300 µm with this ADM for distances below 100 m remains a challenge, whereas some applications for accelerator alignment require an uncertainty of a few µm.
To address this limitation, we propose a new instrument inspired by the above principle. It combines an ADM operating at 1550 nm with a dual-wavelength homodyne interferometer (IFM) operating at 532 nm and 1064 nm. The first measures an absolute optical path length with an uncertainty of 2 µm, while the second determines the difference in optical path length between the two interferometric wavelengths, i.e. the dispersion effect, with an uncertainty of about 20 nm, which enables a fine compensation of the atmospheric effects. The operating principle and the first experimental results will be presented, along with the issues to be resolved. Currently, the ADM can measure absolute distances up to at least 66 m, while the IFM can track subsequent distance variations caused by changes in the air refractive index. A comparison was performed between the combined ADM-IFM system and a 3-m-long interferometric bench, which demonstrated a standard deviation of less than 3 µm between the two systems. Measurements were also carried out indoors over several days at distances of 12 m and 66 m, demonstrating resolution of 2–3 µm.
