Pascal Realization by Comb-Spectral-Interferometry Based Refractometer

To break through the limitations of existing pressure standards, which rely on the gravity and toxic mercury, the national metrological institutes prefer a quantum-based pressure standard. Combining the ideal gas law with helium refractivity measurement, we demonstrate a scheme for the realization of the pressure unit. The refractometer is based on a spectral interferometry with an optical frequency comb and a double-spaced vacuum cell. Through fast Fourier transform of the spectral interferograms of the two beams propagating inside and outside the vacuum cell, the helium refractivity can be obtained with a combined standard uncertainty u(n) of 2.9\times 10^-9. Moreover, the final u(p) is \sim8.7\times 10^-6 in a measurement range of several megapascals (MPa). Our apparatus is compact, fast (15 ms for one single measurement) and easy to handle. Furthermore, the measurement uncertainty will be improved to \sim1\times 10^-9 or lower if a VIPA-based spectrometer is used. The value of u(p) will thus increase to 3\times 10^-6 or better in several MPa.