Optical distance measurements at two wavelengths with air refractive index compensation

We present our first results in the realization of an absolute distance meter that will bring well-defined and metrologically traceable measurements for distances of several kilometers. The purpose is to achieve an accuracy of 1 mm up to 5 km and beyond. To this end, the developed prototype operates at two different wavelengths, 780 nm and 1560 nm. For each of these wavelengths, the measured distance is determined from the phase accumulated by a radio-frequency carrier propagated in air by a laser beam modulated in intensity. Thus, using the dispersion relation between these two measurements, a compensation of the refractive index of air can be applied, i.e. no measurement of temperature, pressure and CO2 content of the air is required. In this paper, the principle of the two-wavelength absolute distance meter is explained, and preliminary results are presented, for each wavelength and with air refractive index compensation.