Performance of a dual Fabry–Perot cavity refractometer

We have built and characterized a refractometer that utilizes two Fabry–Perot cavities formed on a dimensionally stable spacer. In the typical mode of operation, one cavity is held at vacuum, and the other cavity is filled with nitrogen gas. The differential change in length between the cavities is measured as the difference in frequency between two helium-neon lasers, one locked to the resonance of each cavity. This differential change in optical length is a measure of the gas refractivity. Using the known values for the molar refractivity and virial coefficients of nitrogen, and accounting for cavity length distortions, the device can be used as a high-resolution, multi-decade pressure sensor. We define a reference value for nitrogen refractivity as $n-1=(26485.28\pm 0.3)\times {10}^{-8}$ at $p=100.0000\mathrm{kPa}$, $T=302.9190\mathrm{K}$, and ${\lambda }_{\mathrm{vac}}=632.9908\mathrm{nm}$. We compare pressure determinations via the refractometer and the reference value to a mercury manometer.