Quantitative phase imaging has many applications for label-free studies of the nanoscale structure and dynamics of cells and tissues. It has been demonstrated that optical coherence phase microscopy (OCPM) can provide quantitative phase information with very high sensitivity. The excellent phase stability of OCPM is obtained by use of a reflection from the microscope cover glass as a local reference field. For detailed intracellular studies a large numerical aperture (N.A.) objective is needed in order to obtain the required resolution. Unfortunately, this also means that the depth of field becomes too small to obtain sufficient power from the cover glass when the beam is focused into the sample. To address this issue, we designed a setup with a dual-beam sample arm. One beam with a large diameter (filling the 1.2 N.A. water immersion objective) enabled high-resolution imaging. A second beam with a small diameter (underfilling the same objective) had a larger depth of field and could detect the cover glass used as a local phase reference. The phase stability of the setup was quantified by monitoring the front and back of a cover glass. The standard deviation of the phase difference was 0.021 rad, corresponding to an optical path displacement of 0.9 nm. The lateral and axial dimensions of the confocal point spread function were 0.42 and 0.84 μm, respectively. This makes our dual-beam setup ideal for three-dimensional intracellular phase imaging.
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