A white light interferometer uses a broadband light source (i.e. with low temporal coherence), so that interference fringes are observed only in a narrow range around the point of zero arm length difference. Likewise, the above mentioned ambiguity is effectively removed. In figure 1 is shown a typical image of a flat and smooth mirror acquired with the white light interferometer (Zygo) available in the Optical Metrology Lab. A wavelength tunable laser can be used to record the detector signal for different optical frequencies. From such signals, the arm length difference can be unambiguously retrieved. If one of the mirrors is intentionally tilted, an interference fringe pattern is obtained. Any change in arm length difference will then move the fringe pattern. This method makes possible to measure phase changes sensitively and also to measure position-dependent phase changes in some optical element. The spatial wavelength range where this kind of instrument can operate start from microns up to few millimetres with a few angstroms height accuracy.


 Example of Phase Shift Interferometric measurement results during the inspection of a good quality optics. The measured rms roughness has been estimated to be about 2.6 A ̊.




Last Updated on Wednesday, 20 January 2021 08:45