Han Yen Tu

Optical image encryption based on polarization encoding by liquid crystal spatial light modulators

This work describes a polarization-encoding scheme with cascaded liquid crystal spatial light modulators for two-dimensional exclusive OR logic encryption. The transfer characteristic of the encryption system is analysed to determine suitable polarization modulation for optical realization. Performance is compared across systems that use various displays, including twisted nematic, supertwisted nematic and homogeneously parallel aligned nematic liquid crystal devices. Experiments show that binary image encryption/decryption is accomplished with fair image quality by choosing the orientation of the polarization and modulation conditions. Both simulated and experimental results are presented and discussed.

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.

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.

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.

In situ mapping of light-induced refractive index gratings by digital holographic microscopy

This work presents a technique for in situ measurement of light-induced refractive index gratings in epoxy resin using digital holographic microscopy (DHM). The reconstructed phase image derived from a digital hologram can exhibit the grating structure and refractive index profile of an epoxy resin hologram. Reconstruction properties of finite aperture effect in the DHM system are considered and analyzed theoretically. Due to the high spatial frequency components collected by the objective lens, the DHM system can measure fine grating structures and numerically determine the reconstructed images in increased detail. Grating formation and dynamic behavior during the light-induced holographic process can be demonstrated experimentally and characterized using the proposed scheme. Compared with an optical holographic readout, the proposed technique facilitates direct observation and substantial understanding of the holographic recording mechanism in a microscopic view.

Full phase encoding for digital holographic encryption using liquid crystal spatial light modulators

This work describes a full phase encoding technique for digital holographic encryption based on liquid crystal spatial light modulators, which are operated in the phase modulation mode to perform the phase-encoding object and the double random phase key masks for optical Fresnel encryption. The architecture of four-step phase-shifting digital holography is used to generate the double key holograms for implementing the encryption and decryption. Experimental results show the feasibility of the full phase encoding encryption with double keys for high-data-security properties. The proposed encryption system with electrically addressed spatial light modulators provides the flexibility of the key mask design by on-line processing.

Image formation of holographic three-dimensional display based on spatial light modulator in paraxial optical systems

Abstract. This work describes the image formation and properties of holographic three-dimensional (3-D) display based on spatial light modulators (SLMs) combined with optical imaging systems. Existing pixelated SLMs with periodic mesh structures affect the holographic reconstruction and display properties. According to a holographic 3-D display architecture based on SLM in paraxial optical systems, this study applied the ray matrix optics and scalar diffraction theory to regard the light wave emitting from the holographic plane to the image plane as an optical system composed of four matrix elements. The image quality and depth of field (DOF) of the holographic 3-D display system are investigated, and the relationship between the impulse response and the matrix elements of the holographic imaging system is derived. In addition, the imaging properties and DOF are explored and verified through optical experimentation.

Coded aperture structured illumination digital holographic microscopy for superresolution imaging

This paper proposes a coded aperture structured illumination (CASI) technique in digital holographic microscopy (DHM). A CASI wave is generated using two binary phase codes (0° and 120°) for spatial phase shifting. The generated CASI wave then interferes with a reference wave to form a coded Fresnel hologram at a single exposure with compressive sensing (CS) to avoid the temporal phase-shifting process of the structured illumination (SI). The CS algorithm is applied to retrieve the missing data of decoded phase-shifted SI-modulated waves, which are used to separate overlapped spatial frequencies for obtaining a larger spatial frequency coverage to provide superresolution imaging. Two phase-only spatial light modulators are applied to generate a directional SI pattern for obtaining a coded aperture with a suitable size to perform one-shot acquisition in the DHM system.

Superresolution imaging in synthetic aperture digital holographic microscopy

This study presents a superresolution imaging technique based on synthetic aperture digital holographic microscopy (SA-DHM). The synthetic aperture for superresolution imaging in digital holographic microscopy is theoretically derived and analyzed, which indicates that the frequency coverage and spatial resolution increase as the effective aperture size. We experimentally performed the scanning synthetic aperture for superresolution and tomographic imaging in the DHM system. The experimental results show that the spatial resolution of the DHM system can increase up to two times than that in using conventional numerical-aperture microscopy.