Multi-Wavelength Digital Holographic Microscopy for High Resolution Inspection of Surfaces and Imaging of Phase Specimen

Main drawbacks of using laser light in digital holographic microscopy (DHM) are coherent noise and parasitic reflections in the experimental setup as these disturbances affect the reconstructed images and restrict the measurement accuracy. Partially coherent light reduces such effects. On the other hand, the application of light sources with a low coherence length requires a precise alignment of the experimental equipment. Thus, it was investigated, if coherence properties of spectral broadened light sources can be generated synthetically with laser light. Therefore, amplitude and phase distributions are superposed that result from the reconstruction of digital holograms which are recorded separately at slightly different wavelengths. In this way, the robust alignment of a laser-based experimental setup due to long coherence lengths is combined with the noise reduction advantage of partial coherent light. By using a single fiber coupled tuneable laser the multi-wavelength approach can be used with already existing DHM setups, e. g., in combination with commercial microscopes. The performance of the method for the observation of phase objects is illustrated by results obtained from the topography analysis of reflective surfaces and from the application for quantitative phase contrast imaging of thin living tumor cells.