Digital holography is an emerging field of new paradigm in general imaging applications. We present a review of a subset of the research and development activities in digital holography, with emphasis on microscopy techniques and applications. First, the basic results from the general theory of holography, based on the scalar diffraction theory, are summarized, and a general description of the digital holographic microscopy process is given, including quantitative phase microscopy. Several numerical diffraction methods are described and compared, and a number of representative configurations used in digital holography are described, including off-axis Fresnel, Fourier, image plane, in-line, Gabor, and phase-shifting digital holographies. Then we survey numerical techniques that give rise to unique capabilities of digital holography, including suppression of dc and twin image terms, pixel resolution control, optical phase unwrapping, aberration compensation, and others. A survey is also given of representative application areas, including biomedical microscopy, particle field holography, micrometrology, and holographic tomography, as well as some of the special techniques, such as holography of total internal reflection, optical scanning holography, digital interference holography, and heterodyne holography. The review is intended for students and new researchers interested in developing new techniques and exploring new applications of digital holography.

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Principles and techniques of digital holographic microscopy

Digital holographic microscopy enables a quantitative phase contrast metrology that is suitable for the investigation of reflective surfaces as well as for the marker-free analysis of living cells. The digital holographic feature of (subsequent) numerical focus adjustment makes possible applications for multifocus imaging. An overview of digital holographic microscopy methods is described. Applications of digital holographic microscopy are demonstrated by results obtained from livings cells and engineered surfaces.

Digital holographic microscopy for live cell applications and technical inspection

Techniques of digital holography are improved in order to obtain high-resolution, high-fidelity images of quantitative phase-contrast microscopy. In particular, the angular spectrum method of calculating holographic optical field is seen to have significant advantages including tight control of spurious noise components. Holographic phase images are obtained with 0.5 μm diffraction-limited lateral resolution and largely immune from the coherent noise common in other holographic techniques. The phase profile is accurate to about 30 nm of optical thickness. Images of SKOV-3 ovarian cancer cells display intracellular and intranuclear organelles with clarity and quantitative accuracy.

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High-resolution quantitative phase-contrast microscopy by digital holography

An approach is proposed for removing the wave front curvature introduced by the microscope imaging objective in digital holography, which otherwise hinders the phase contrast imaging at reconstruction planes. The unwanted curvature is compensated by evaluating a correcting wave front at the hologram plane with no need for knowledge of the optical parameters, focal length of the imaging lens, or distances in the setup. Most importantly it is shown that a correction effect can be obtained at all reconstruction planes. Three different methods have been applied to evaluate the correction wave front and the methods are discussed in detail. The proposed approach is demonstrated by applying digital holography as a method of coherent microscopy for imaging amplitude and phase contrast of microstructures.

Compensation of the inherent wave front curvature in digital holographic coherent microscopy for quantitative phase-contrast imaging

We present a procedure that compensates for phase aberrations in digital holographic microscopy by computing a polynomial phase mask directly from the hologram. The phase-mask parameters are computed automatically without knowledge of physical values such as wave vectors, focal lengths, or distances. This method enables one to reconstruct correct and accurate phase distributions, even in the presence of strong and high-order aberrations. Examples of applications are shown for microlens imaging and for compensating for the deformations associated with a tilted thick plate. Finally we show that this method allows compensation for the curvature of the specimen, revealing its surface defects and roughness. Examples of applications are shown for microlenses and metallic sphere imaging.

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Automatic procedure for aberration compensation in digital holographic microscopy and applications to specimen shape compensation.

In this paper we study silicon MEMS (Microelectromechanical systems) structures subjected to thermal loading. Digital holography has been investigated as inspection tool to evaluate the deformation induced by the thermal loading. Application of DH on structures with several different geometries and shapes, like cantilever beams, bridges and membranes is reported and result will be discussed. Dimensions of the inspected microstructures, varies in the range 1÷50μm. The experimental results shown that a "bimorph-effect" induces a deformation in MEMS structures. The difficulties encountered in performing the deformation analysis by digital holography in real-time will be afforded and discussed. A method with automatic focus tracking in Digital Holography is proposed allowing inspection of MEMS, under thermal loading, in real-time.

Carlo Magro

Papers

Investigation of silion MEMS structures subjected to thermal loading by digital holography