Chromatic confocal spectral interferometry (CCSI)

Abstract

In this paper, we report on the recent development of a novel low coherence interferometry technique for the purpose of 3D-topography measurements. It combines the well established techniques of spectral-interferometry (SI) and chromatic-confocal microscopy (CCM). Measuring the optical interference in the spectral-domain allows for the detection of a reflecting or scattering object's depth position, without the necessity of a mechanical axial-scan. Focusing the white-light detection field with a microscope objective combined with a diffractive optical element leads to an expansion of the axial-range of the sensor beyond the limited depth-of-focus, imposed by the numerical aperture (NA) of the focusing objective. Focusing with a high NA objective and confocally filtering the detection light field causes the reduction of the lateral dimension of the area sampled upon the object. By this, the lateral resolution of the sensor is enhanced and due to the high NA, a high light collection-efficiency is achieved as well. The attained interferometric signals consist of high-contrast wavelets, measured in the optical-frequency domain. The depth position of an investigated point of the object is given by the modulation-period of the wavelets. Therefore, unlike in CCM, positionwavelength referencing is not necessary.