Multi-wavelength off-axis digital holographic microscopy with broadly tunable low-coherent sources: theory, performance and limitations

Abstract

Multi-wavelength digital holographic microscopy (MDHM) is widely used in biological and industrial applications because of increased unambiguous height measurement range and the ability to measure concentration from the spectral dependence of phase delay. Acousto-optic tunable filters (AOTFs) provide the simultaneous selection of several bands with tunable central wavelengths to create a multiplexed hologram, but may limit the field of view (FOV) in off-axis holography because of the short coherence length of the filtered light. We analyzed the performance of the AOTF-based off-axis MDHM setup with a diffraction grating or a prism in the reference arm necessary to increase the efficiency of angular multiplexing. This allows varying the number of spectral channels selected simultaneously without setup realignment. Mathematical description relates the spectral bandwidth of the AOTF, tilt of the coherence plane induced by the angular dispersion of a prism or a grating, width of the FOV determined by interference pattern visibility, spatial resolution, and optimal intermediate wavelengths. We theoretically and experimentally demonstrated that the FOV may be expanded by changing the angle of light incidence on the AOTF and that the prism changes the wavelength dependence of the FOV. We validated this technique by single-shot acquisition of the height maps of the transparent test chart at four wavelengths with an error similar to that of four sequentially captured single-wavelength holograms. The results may be helpful for multiple applications of MDHM using spectrally tunable light sources.