Experiment with simultaneous measurements at two optical wavelengths in the FG5X absolute gravimeter

The FG5X gravimeters are the most accurate commercially available absolute gravimeters at present. They traditionally use one optical wavelength in their interferometer to measure the gravity acceleration of the freely falling test mass in a vacuum. In this paper, for the first time, it is demonstrated the possibility to track the test mass simultaneously with two optical wavelengths of 633 nm and 771 nm and to evaluate the gravity acceleration from both these measurements. We show the technical solution, mathematical methods and error sources that have to be taken into account for the realization of simultaneous interferometric measurements. The achieved results show agreement of the gravity accelerations at the level of 2-4 microgals and increased low-frequency noise in residuals at 771 nm due to optical optimisation of a gravimeter to the wavelength of 633 nm. We evaluated the sensitivity of gravity measurements to the used wavelength as a new contribution in the uncertainty budget that for the FG5X gravimeter reached 0.25 mu Gal and 2 mu Gal at wavelengths of 633 nm and 771 nm, respectively. Further, we discuss that the optimisation of a gravimeter to a certain wavelength is related to the applied antireflective coating on the optical elements of the gravimeter among them the glass retroreflector plays the key role since its movements being dominant.