This paper by Koner et al. uses low coherence digital holography for simultaneous optical sectioning at multiple planes separated by a fixed interval. For a low coherence source, the interference (or modulated) signal is observed only when the path difference between the object section and reference beam is within the short coherence length. The unmodulated signal corresponding to the background may then be separated by digital processing. Multiple sections may then be imaged by scanning the object along the depth dimension. The present authors use a femtosecond frequency comb laser as the low coherence source. The comb laser sources have now matured as frequency standards. The important characteristic of this source for the present application is that its temporal coherence function shows periodic behaviour. This enables the authors to implement optical sectioning of multiple planes simultaneously – a novel feature of their setup. The positions of these planes is periodic in depth direction with the period corresponding to half the periodicity of the temporal coherence function in a reflection based configuration for the object beam. Numerical reconstruction methods used in digital holography are then used for obtaining focused images of the optical sections. With the progress in technology of frequency combs as projected by the authors, it may be feasible to obtain optical sections separated by ~ 100 microns using this approach in coming years.
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