Non-contact absolute internal distance measurement between two mirrors by using a low-coherence tandem interferometer (2st Report)

Evaluation of the length compensation value due to difference of group refractive index

Abstract

A novel absolute measurement method for precisely measuring the internal distance between two plane mirrors is proposed by a tandem low-coherence interferometer. In this method, a beam splitter is positioned in between two plane mirrors such that the internal distances between two plane mirrors can be measured. The difference of group refractive index between air and beam splitter causes a length difference between mechanical and optical lengths. Hence, a new technique for measuring the length compensation method due to the difference group refractive index between air and beam splitter is proposed. The technique uses a tandem configuration of low-coherence interferometers. The length compensation for the group refractive index of the beam splitter can be measured without prior knowledge of its geometrical thickness.