Fabry-Perot displacement interferometry for next-generation calculable capacitor

We describe a Fabry-Perot interferometer system designed to measure the variable spacing between two electrodes in the new NIST calculable capacitor. Using narrow-linewidth telecom lasers, we span a frequency interval of 18 GHz, and achieve a fractional uncertainty of δv/Δ ~1.7 × 10
^-8
without an optical frequency standard. In the near future we will introduce a rubidium optical standard and frequency comb to make the displacement uncertainty a negligible component of the total target uncertainty of 0.1 zF for capacitance measurements.