Development of a refractive index measurement system for vacuum pressure measurement

Many studies have recently been conducted to replace the mercury manometer that has been used as a national standard because of the prohibition of mercury. In particular, ‘the new realization of the Pascal’ by using photon technology shows some comparable results in terms of performance compared to the existing primary pressure standard based on the mercury manometer. In this study, we describe the development of a refractive index measuring system for precise vacuum pressure measurements to develop a mercury-free standard. If the refractive index of a medium is to be measured using a laser interferometer, designing a cavity as a pressure medium and a laser as a light source is important. We designed a Fabry–Perot (FP) cavity structure having two channels to calculate an arbitrary pressure by comparing the laser resonance frequency in a vacuum state, which served as a reference pressure, and that in an arbitrary pressure state. Zerodur was used as the material for the cavity. The cavity was designed to be a rectangular parallelepiped with a width of 50 mm and a length of 150 mm considering the wavelength (633 nm) of the He–Ne laser used as a light source, the pressure range to be implemented, and the implementation range of a single mode. We independently fabricated a single-mode (SM) He–Ne laser instead of a commercially available laser to modulate the frequency freely and to achieve a single polarization regardless of the quantum number. With our system, the beat frequency was measured to the internal pressure of a cavity. A 1 MHz beat frequency was found to be approximately equal to a pressure of 1 Pa.