The role of diffraction gratings in a heterodyne detector for low-coherence interferometry

Abstract

The use of low-coherence interferometry (LCI) in industrial applications is increasing as it is a nondestructive technique with high resolution for material characterization. However, the measurement range of around 3 mm of the conventional spectrometer based Fourier Domain LCI systems limits its field of application. The employment of a heterodyne detection system for FD-LCI signals could increase the measurable optical depths to 5 cm at least. In this work, the study of the effect of the diffraction gratings used in this type of devices is presented. The characteristics of the gratings as well as its configuration played a determinant role in the detector bandwidth and dynamic range. Diffraction gratings with fewer lines per millimeter are shown to be more suitable for large axial range measurement without losing resolution.