Several micron-range measurements with sub-nanometric resolution by the use of dual-wavelength digital holography and vertical scanning
Reflection digital holographic microscopy (DHM) is a very powerful technique allowing measuring topography with a sub-nanometer axial resolution from a single hologram acquisition. But as most of interferometer methods, the vertical range is limited to half the wavelength if numerical unwrapping procedure could not be applied (very high aspect ratio specimen). Nevertheless, it was already demonstrated that the use of dual-wavelength DHM allows increasing the vertical range up to several microns by saving the single wavelength resolution if conditions about phase noise are fulfilled (the higher the synthetic wavelength, the smaller the phase noise has to be). In this paper, we will demonstrate that the choice of a synthetic wavelength of about 17 microns allows measuring precisely a 4.463μm certified step. Furthermore, we will show the feasibility of a sub-nanometer resolution on a range higher than the synthetic wavelength by being able to map the dual- wavelength measurement on data acquired from a vertical scanning process, which precision is about 1 μm.