Tomography using multiple wavelengths in digital holographic microscopy (2006)

This thesis presents a new tomographic imaging method using multiple wavelengths in digital holographic microscopy. It is based on the addition of several reconstructed wavefronts measured at different wavelengths. The resulting diffraction tomographic visibility is then enhanced and the position of the interfaces is determined with ultrahigh precision. Digital holographic microscopy is a method enabling the recording of complex wavefronts and its numerical reconstruction. In our case, it is based on the recording, in off-axis geometry, of the interference between a reference wave and an object wave reflected by a microscopic specimen and magnified by a microscope objective. A couple charged device (CCD) camera records consecutively the resulting holograms at several wavelengths equally separated in the frequency domain. An adapted reconstruction algorithm has been developed to perform an achromatic reconstruction of the different wavefronts. Tomography is then performed by adding the reconstructed wavefronts. Each wavefront phase is individually adjusted to be equal in a given plane of interest. The result of this operation is a constructive addition of complex waves in the selected plane and destructive addition in the others. The amplitude image is attenuated according to a function, called filter function. For perfectly constructive phases no attenuation is present. Varying the plane of interest enables the scan of the object in depth. The obtained diffraction tomographic resolution is better than the one obtained in OCT, with an equivalent spectral width, in low-coherence or short-pulse lasers in optical coherence tomography. Nevertheless, no axial scanning is needed and the shape of the spectrum can be post acquisition tailored. A method is then proposed to enhance the visibility and to precisely determine the position of the interfaces. An axial scan allows the extraction of depth profiles. The maxima detection and a least square fit algorithm are used to perform this enhancement. Moreover, weights are introduced in the addition of the wavefronts, in order to tailor the filter function. Different weights distributions are discussed in terms of separation limit and precision on the interface position. It is shown that position resolution increases for higher separation limit. Finally, the tomographic method has been simplified for cases of a total reflective surface to perform topography. This allows measurements on specimen of several microns high without phase ambiguities. The results are directly measured in optical pathlengths. Experimental measurements have been performed on a specifically designed and homemade target, as well as on a multilayer specimen. Twenty wavelengths in the range of 480-700 nm have been used, resulting in tomographic sections of 725 nm. It is shown that the experimental results perfectly correspond the simulations. Enhanced tomographic results show a separation limit of 725 nm with a position resolution under 130 nm. Measures performed using the Kaiser weight distribution allow a position resolution of a few nanometers, for an increasing separation limit to 3 µm.