Design of New Launch and Interferometer Systems for the IMGC-02 Absolute Gravimeter

For the measurement of the acceleration due to gravity, INRiM developed a transportable ballistic rise-and-fall absolute gravimeter, the IMGC-02. It uses laser interferometry to measure the symmetrical free rising and falling motion of a test mass in the gravity field. The launch system is composed of a moveable carriage fixed to two pairs of springs loaded by an electric stepper motor, which vertically throw up a corner cube retroreflector in vacuum. The interferometer system is a modified Mach-Zehnder interferometer where the launched corner cube acts as the reflector in one of the optical arms of the interferometer and the other retroreflector acts as inertial reference during the measurement. However, both systems entail some practical problems and uncertainty contributions that need to be reduced. In particular, the current launch system might cause beam shear and rotational effects due to unavoidable small different loadings of the springs, while the current interferometer system poses problems in the alignment of the mirrors, which is a highly time-consuming procedure and has to be performed before and, sometimes, during the measurement session. For this reason, a new launch system consisting of an electric linear motor which produces a linear force along its length, and a modified Jamin interferometer system entailing a simpler alignment and a better stability in time, have been designed. This works deals with the description of these new systems.