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） of2.9 × 10-9. Moreover, the final u（n） is -8.7 × 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-1 × 10-9 or lower if a VIPA-based spectrometer is used. The value of u（p） will thus increase to 3 × 10-6 or better in several MPa.