Short temporal coherence digital holography with a femtosecond frequency comb laser for multi-level optical sectioning
Spotlight summary: Optical sectioning refers to imaging of slices of a 3D sample along its depth dimension by optical means as opposed to cutting the sample into thin slices and imaging the individual slices using some imaging system (e.g. a microscope). Being able to image a thin slice of a sample at a time is challenging for a conventional microscope. While the microscope objective is focused on a particular slice along the depth direction, the back-scattered light from other planes creates a diffuse background noise which obscures the image of interest. Several methods/ techniques have been developed in the past for addressing the optical sectioning problem, e.g. Optical Coherence Tomography, confocal microscopy, differential interference contrast, multiphoton imaging, computational deconvolution methods etc. They all aim at separating the diffuse background from the image of interest.
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.
Technical Division: Optical Design and Instrumentation
ToC Category: Holography
|OCIS Codes:||(090.0090) Holography : Holography|
|(120.3940) Instrumentation, measurement, and metrology : Metrology|
|(090.1995) Holography : Digital holography|
|(140.3538) Lasers and laser optics : Lasers, pulsed|
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