Low-coherence interferometer system for the simultaneous measurement of refractive index and thickness

Abstract

We have developed a low-coherence interferometer system used for the simultaneous measurement of refractive index n and thickness t of transparent plates. Both the phase index np and group index ng can be determined automatically in a wide thickness range of from 10 µm to a few millimeters. Two unique techniques are presented to measure np, ng, and t simultaneously. One allows us to determine np, ng, and t accurately by using a special sample holder, in which the measurement accuracy is 0.3% for the thickness t above 0.1 mm. In the other technique the chromatic dispersion δn of index is approximately expressed as a function of (np - 1) on the basis of measured values of np and ng for a variety of materials, and then the simultaneous measurement is performed with a normal sample holder. In addition, a measurement accuracy of less than 1% is achieved even when the sample is as thin as 20 µm. The measurement time is also 3 min or more.