Harmonically-matched Gratings-based Quadrature Phase Imaging

Observing unstained transparent biological samples with high resolution imaging system is very important for a wide range of biomedical studies. Traditional phase-contrast imaging techniques such as Zernike phase and Nomarski differential interference contrast (DIC) render excellent phase contrast images; however, the phase information is only qualitative in nature. We are developing a new method for obtaining non-trivial phase difference between the output ports of a Mach-Zehnder interferometer through the use of diffraction gratings as the beam splitter/combiner. Phase imaging based on this technique involves simultaneously acquiring images from two or more of the output ports; the imaging speed of this technique is limited only by the cameras' speed. So we can use this technique to observe dynamics of biological samples. Fig. 1 shows the interferometer schemes using simple beamsplitter, a single shallow diffraction grating and our harmonically-matched G1G2 grating. Fig. 2 shows our experimental setup. Fig. 3 and 4 show some imaging results.

Fig. 1. (a) Simple beamsplitter based interferometer: 180^o phase shift between output ports; (b) Single shallow grating based interferometer: still 180^o phase shift between output ports; (c) G1G2 grating based interferometer: non-trivial phase shift can be obtained.

Fig. 2. Experimental setup for phase imaging. BS: beam splitter; O1 and O2: objective lenses 1 and 2; P: pinhole; L1-4: lens 1-4; S: sample; G1G2: the harmonically matched grating pair (G1G2 grating) on a holographic plate.

 
Fig. 3. Images of "CIT" logo by our iamging system. (a) Intensity image; (b) Phase image; (c) 3D reconstruction of the phase image; (d) Step-height measurement.

Fig. 4. Images of onion skin cells. (a) Intensity image; (b) Phase image; (c) 3D reconstruction of the phase image.

REFERENCE:

Z. Yaqoob, J. Wu, X. Cui, X. Heng, and C. Yang, "Harmonically-related diffraction gratings-based interferometer for quadrature phase measurement," Optics Express, 14, 2817 (2006).

J. Wu, Z. Yaqoob, X. Heng, L. M. Lee, X. Cui, and C. Yang, "Full field phase imaging using a harmonically matched diffraction grating pair based homodyne quadrature interferometer," Applied Physics Letters, 90, 151123 (2007).

J. Wu, Z. Yaqoob, X. Heng, X. Cui, and C. Yang, "Harmonically matched grating-based full-field quantitative high-resolution phase microscope for observing dynamics of transparent biological samples," Optics Express, 15, 18141 (2007).

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