SIMULTANEOUS MEASUREMENT OF REFRACTIVE-INDEX AND THICKNESS OF TRANSPARENT PLATES BY LOW-COHERENCE INTERFEROMETRY

We demonstrate a novel low coherence Michelson interferometer which ca n provide simultaneous measurement of the refractive index and thickne ss of transparent plates used as a measured object. Unlike the existin g low coherence interferometers reported so fat, either an object or a focusing lens aligned on the signal arm is scanned repeatedly by a pr ecise translation stage in synchronization with movement of a reflecti on mirror on the reference arm. The so-called object or lens scanning method gives us two measured quantities a movement distance of the sta ge between two light focusing states on the front and rear planes of a n object and the corresponding optical path difference. These two meas ured quantities, result in desirable values of the index and thickness of the object with a short calculation. The measurement accuracy of l ess than or equal to 0.1% is expected for a thickness of more than 1 m m. In the experiment using the object scanning method, the accuracy of 0.3% or less was successfully attained for nearly l-mm thick plates o f fused quartz, sapphire, LiTaO3 and slide glass.