Progress on the optical interferometer for the new calculable cross-capacitor at NMIA

This paper details the recent progress made on the optical interferometer, developed to perform precision length measurements for the new calculable cross-capacitor at the National Measurement Institute, Australia (NMIA). We have confirmed both the alignment of the optical cavity to the capacitor's axis using an external cavity set-up, and the proper functioning of the optical cavity after installation of the upper guard electrode. Development of the dual-frequency Pound-Drever-Hall locking system is ongoing, as is the iodine-locking for SI traceability. Current results are on track for achieving the targeted measurement uncertainty of 1 in 10⁹ .