Chau Jern Cheng

Optical sectioning with a low-coherence phase-shifting digital holographic microscope

The properties of a low-coherence phase-shifting digital holographic microscope are first studied and analyzed. We then demonstrate en face imaging with transverse resolution of 3 μm and axial resolution of 10 μm through a thickness of 300 μm onion membrane. In addition, with the above said resolutions, optical sectioning of the eye and spine of a live zebra fish has been demonstrated. To the best of our knowledge, this is the first time that a short coherence phase-shifting holographic microscope has been applied to the internal structure visualization of a biological specimen under an in vivo environment.

Determining the refractive index profile of micro-optical elements using transflective digital holographic microscopy

This study proposes a novel method for directly measuring the spatial distribution of the refractive index of transparent or semi-transparent micro-optical elements using digital holographic microscopy, which involves transmission and reflection architectures. The proposed transflective digital holographic microscope system simultaneously records both the transmitted and the reflected waves from the micro-optical element and then numerically reconstructs the phase information and refractive index profile. No prior knowledge of the thickness and curvature of the element to be measured is required. In this study, the spatial refractive index profiles of a microlens array and a gradient index lens are demonstrated experimentally. Unlike the conventional scanning refractive ray method, the proposed method supports the rapid full-field nondestructive measurement and direct observation of the spatial index variation and structure of micro-optical elements.

A Fragile Watermarking Algorithm for Hologram Authentication

A fragile watermarking algorithm for hologram authentication is presented in this paper. In the proposed algorithm, the watermark is embedded in the discrete cosine transform (DCT) domain of a hologram. The watermarked hologram is stored in spatial domain with finite precision level. By enhancing the precision for storing the watermarked hologram pixels, the distortion produced by the proposed watermarking scheme can be lowered. While providing high perceptual transparency, the proposed algorithm also attains high performance detection to delivery errors and malicious tampering. Experimental results reveal that the proposed algorithm can be used as an effective filter for blocking polluted or tampered holograms from 3D magnitude and/or phase reconstruction.

Sectional imaging of spatially refractive index distribution using coaxial rotation digital holographic microtomography

We propose and demonstrate a novel optical section imaging technique to measure the spatially refractive index (RI) distribution inside a sample using coaxial rotation digital holographic microtomography (DHμT). In this method, the illuminated laser source and image sensor are lined up and simultaneously rotated around the sample. The transmitted waves collected from the sample are used to generate a symmetric frequency, coverage as in a sample-rotation approach was discussed and simulated theoretically. We experimentally performed the sectional imaging of the RI inside a silicon micro-bead immersed in a liquid surrounding medium, to confirm the coaxial rotation DHμT as having the phase stability for tomographic recording and the reconstruction process. This sectional imaging technique is applied to measure the spatially RI distribution of a fusion structure between a single mode fiber and a polarization-maintaining fiber.

Phase measurement accuracy in digital holographic microscopy using a wavelength-stabilized laser diode

This work describes the influence of coherent illumination on phase measurement accuracy in digital holographic microscopy (DHM). To improve net phase accuracy in a DHM system, the phase referencing and temporal averaging techniques are applied simultaneously to suppress the phase noises caused by the laser source and image sensor. A comparison between a laser diode operated in single- and multi-modes is given to demonstrate the coherence effect on the fringe visibility and the reconstructed phase accuracy of digital holograms. Axial sub-nanometer accuracy in DHM with the fewest successive hologram recordings is performed using a wavelength-stabilized laser diode in single-mode operation.

Resolution enhancement of spectrum normalization in synthetic aperture digital holographic microscopy

This study describes the overlapping of spatial frequency bands for synthetic aperture in digital holography using spectrum normalization to effectively enhance the spatial resolutions of image reconstruction. Synthesized spectrum swelling induced by excessive frequency overlaps can be normalized through the inverse apodization of an adjustable window function, which is similar to the effects of suppressing low-frequency expansion and strengthening high-frequency components of the reconstructed images. The results indicated that using the normalized spectrum synthesis that requires only a few frequency bands effectively enhances the spatial resolution and phase sensitivity of reconstructed images in digital holographic microscopy.

Digital Hologram Authentication Using a Hadamard-Based Reversible Fragile Watermarking Algorithm

This paper presents a novel reversible fragile watermarking algorithm for hologram authentication. In the algorithm, the watermark is embedded in the transform domain. The marked hologram is then stored in the spatial domain with finite resolution level. The resolution level is allowed to be pre-specified for attaining different degrees of transparency. The algorithm is based on Hadamard transform for both watermark embedding and extraction. The Hadamard transform and its inverse can be operated by simple addition, subtraction and shift operations. Due to the simplicity of the transform, a sufficient condition on the resolution level of marked holograms is derived for guaranteeing the reversibility of watermarking. In addition, from the condition, it is also observed that the hiding capacity for reversible watermarking increases with the resolution level. The proposed algorithm therefore provides high flexibility for reversible watermarking allowing transparency and hiding capacity to be varied by selecting different resolution levels. Moreover, a novel scheme is presented for supporting self-containedness for 3D object reconstruction without requiring the delivery of additional files consisting of parameters for diffraction computation. Because of its low degradation in transparency, the algorithm is also able to operate in conjunction with the existing spatial domain based reversible algorithms to further enhance the hiding capacity while maintaining high visual transparency.

Optically driven full-angle sample rotation for tomographic imaging in digital holographic microscopy

This study presents a novel tomographic imaging technique for living biomedical samples using an optically driven full-angle rotation scheme based on digital holographic microscopy, in which the three-dimensional refractive index distribution inside the sample can be measured and analyzed. To accomplish the full-angle sample rotation, two optical traps are driven by highly focused spots on the top and bottom of the sample. The rim image of the sample outside the focal depth at the different rotation angles and propagation distances can be corrected and compensated, respectively, via numerical focusing; therefore, tomographic imaging of the sample can be conducted. The proposed approach shows that an entire symmetric spectrum can be acquired for tomographic reconstruction without the missing apple core problem as in traditional sample-rotation schemes. The three-dimensional refractive index of living yeast in a fluid medium is measured and verified.

Hologram authentication based on a secure watermarking algorithm using cellular automata

A secure watermarking algorithm for hologram authentication is presented in this paper. The algorithm exploits the noise-like feature of holograms to randomly embed a watermark in the domain of the discrete cosine transform with marginal degradation in transparency. The pseudo random number (PRN) generators based on a cellular automata algorithm with asymmetrical and nonlocal connections are used for the random hiding. Each client has its own unique PRN generators for enhancing the watermark security. In the proposed algorithm, watermarks are also randomly generated to eliminate the requirements of prestoring watermarks in the clients and servers. An authentication scheme is then proposed for the algorithm with random watermark generation and hiding.

Complex Modulation Characterization of Liquid Crystal Spatial Light Modulators by Digital Holographic Microscopy

We propose and demonstrate the characterization of complex modulation of liquid crystal devices by phase-shifting digital holographic microscopy with lensless imaging configuration. The complex-modulated wavefronts of the liquid crystal spatial light modulator are measured directly from the reconstructed field of the phase-shifted digital holograms. The characteristics of various operation modes of the commercial twisted nematic liquid crystal devices are specified as functions of input gray levels.