Dispersion-insensitive measurement of thickness and group refractive indexby low-coherence interferometry

We describe a low-coherence interferometric technique for simultaneous meas urement of geometric thickness and group refractive index of highly dispers ive samples. The technique is immune to the dispersion-induced asymmetry of the interferograms, thus overcoming Limitations associated with some other low-coherence approaches to this simultaneous measurement. We use the expe rimental configuration of a tandem interferometer, with the samples to be c haracterized placed in an air gap in one arm of the measurement interferome ter. Unambiguous, dispersion-insensitive measurements of critical group-del ay imbalances in the measurement interferometer are determined from the opt ical frequency dependence of interferogram phases, by means of dispersive F ourier transform spectrometry. Sample thickness and group refractive index are calculated from these group delays. A thickness measurement precision o f 0.2 mu m and group index measurement accuracy of 5 parts in 10(5) across a wavelength range of 150 nm have been achieved for BK7 and fused-silica gl ass samples in the thickness range 2000 to 6000 mu m. (C) 2000 Optical Soci ety of America OCIS codes: 060.2370, 260.2030.