Common-path intrinsically achromatic optical diffraction tomography

Quantitative phase microscopy (QPM) [1-4] is gaining a lot of interest, as it provides measurable information about phase distribution of the tested semi-transparent object, without the need of staining the sample. It showed its powerful imaging capabilities in plethora of applications, e.g., biomedicine [5], neuroscience [6], blood and flow cytometry [7,8] or cell pathophysiology [9]. The label-free imaging is achieved thanks to the intrinsic contrast of transparent (phase) samples - each component of the sample introduces different optical delay to the light. However, QPM imaging only delivers an averaged phase delay (along the Z axis), losing the information about internal structure of the imaged object. Additionally, phase delay (optical thickness) can change due to the local variation of both refractive index and physical thickness of the sample. Therefore, In this work we propose an open-top like common-path intrinsically achromatic optical diffraction tomography system. It operates as a total-shear interferometer and employs Ronchi-type amplitude diffraction grating, positioned in between the camera and the tube lens without an additional 4f system, generating three-beam interferograms with achromatic second harmonic. Such configuration makes the proposed system low cost, compact and immune to vibrations. We present the results of the measurements of 3D-printed cell phantom using laser diode (coherent) and superluminescent diode (partially coherent) light sources. Broadband light sources can be naturally employed without the need for any cumbersome compensation because of the intrinsic achromaticity of the interferometric recording (holograms generated by -1st and +1st conjugated diffraction orders are not affected by the illumination wavelength). The results show that the decreased coherence offers much reduced coherent noise and higher fidelity tomographic reconstruction especially when applied nonnegativity constraint regularization procedure.