<title>Simultaneous, remote, and interferometric measurement of thickness and group refractive index</title>

Abstract

A low-coherence optical fiber interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive materials is described. The technique, immune to the dispersion-induced asymmetry of the low-coherence interferograms obtained, overcomes some of the drawbacks associated with recently reported low-coherence approaches to this simultaneous measurement. The technique uses the experimental configuration of a tandem interferometer with the samples to be characterized placed in an air-gap in one arm of the measurement interferometer. Dispersion- insensitive measurements of the group delay imbalances in the measurement interferometer are made using dispersive Fourier transform spectrometry (DFTS). Sample thickness and group refractive index are calculated from these group delays which are unambiguously determined from the optical frequency dependence of the measured phases of the interferograms. Thickness measurements accurate to within 1 micrometer and group index measurements accurate to within one part per thousand have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 micrometers.