The present work describes the process of revision and improvement of the micro-Newton thrust balance used by Airbus Defence & Space in Friedrichshafen for electric propulsion testing; in particular, the replacement of the interferometer device used to evaluate the differential translation between the two hanging pendulums of the thrust stand setup. The new device is assembled and the characterization is performed; the necessary hardware modifications are designed and implemented for installation inside the vacuum chamber. The interferometer demonstrated the capacity for differential pathlength measurement, with a noise level under 1 nm/√Hz between 31 mHz and 1 Hz, and the capacity for Differential Wavefront Sensing (DWS) is enhanced from the previous setup.