Novel concept of interferometric control of the moving coil in watt balance

One of the most promising projects for the new definition of the kilogram is the watt balance. It uses a two-mode procedure: static mode and dynamic mode. One of the key points is to control the velocity, the yaw and the pitch of a moving coil during the dynamic mode. We have developed a specific method based on the use of a homemade double pass heterodyne interferometer, a two-level translation stage, and a home-made high frequency phase shifting electronic circuit. However, the commercial heterodyne sensor is limited in power and has polarization limitations. In this paper, we propose a novel and specific heterodyne interferometer able to insure all previous demands to control the displacements of the moving coil in order to reduce the velocity noise. The laser source is powerful enough to ensure beams of measurements, the laser is frequency stabilized at 532 iodine atomic transition to ensure traceability, and all the polarization defaults of the classical heterodyne interferometer have been minimized in order to minimize non-linearity effects. The most original point is that the reference beam is lock-in on an external electronic reference, which allows to be less sensitive to vibration and polarization defects of optical components.