Uncertainty reduction in the volume measurement of Si spheres by an optical interferometer to determine the Avogadro constant

In the determination of the Avogadro constant using the x-ray crystal density method, volume measurements of Si spheres by optical interferometry play a crucial role. For the accurate volume determination, geometrical shape of the optical components of the interferometer has been evaluated by using ray-tracing method. To reduce the noise in the interference fringes, a new etalon has been developed. An optical frequency comb has been also introduced in the interferometer as the optical frequency standard for more reliable and precise volume measurement. Details of the improvements are described.