A new type vertical calculable capacitor has been built at National Institute of Metrology (NIM) cooperated with National Measurement Institute of Australia (NMIA). The calculable capacitor is the highest accuracy equipment apparatus except the quantum voltage and the quantum resistance in the electromagnetic metrological field. In order to measure the capacitance precisely, the accurate displacement measurement among the two guard electrodes in the calculable capacitor is a pivotal part. This paper describes a method of measuring the displacement of a Fabry-Perot interferometer, and this interferometer is component of two mirrors in two guard electrodes of the calculable capacitor at NIM. One concave reflective mirror, with 5 m radius and 70% reflectivity, is on the top of the bottom fixed guards electrodes. The other planar mirror is placed at the end of the moveable guard electrodes. This Fabry-Perot interferometer employs a home-made lamb-dip stabilization He-Ne laser at 633 nm to measure the displacement of the movable guard electrode. The internal modulation, which is used for laser stabilization, is also employed for locking the Fabry-Perot interferometer. The displacement of the movable guard electrode could be measured, when the Fabry-Perot interferometer is locked to the stabilization laser at two positions respectively. An iodine stabilization He-Ne laser at 633 nm is employed to simultaneously calibrate the wavelength of lamb-dip working laser. A reproducibility of 1.43×10-8(k=3) for the range of 205 mm can be obtained at present, and that is estimated from the experimental results of calculable capacitor.
Instead of traditional heating method, the cavity length of an internal-mirror He-Ne laser is controlled by air cooling which is generated by a mini cooling fan. A PID servo controlling system is designed to drive the cooling fan tuning the frequency of the laser. The frequency is stabilized by balancing the power of two operating longitudinal modes. Beating with an iodine stabilized He-Ne laser, a relative uncertainty(Δf / ̅f ) of 4.3×10-9 in 5 months, a frequency fluctuation of less than 2.6 MHz and an Allan deviation of 3×10-11 (τ=100 s) in 75 h are obtained.
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