Xuechang Ren

Four-image encryption scheme based on quaternion Fresnel transform, chaos and computer generated hologram

A novel four-image encryption scheme based on the quaternion Fresnel transforms (QFST), computer generated hologram and the two-dimensional (2D) Logistic-adjusted-Sine map (LASM) is presented. To treat the four images in a holistic manner, two types of the quaternion Fresnel transform (QFST) are defined and the corresponding calculation method for a quaternion matrix is derived. In the proposed method, the four original images, which are represented by quaternion algebra, are processed holistically in a vector manner by using QFST first. Then the input complex amplitude, which is constructed by the components of the QFST-transformed plaintext images, is encoded by Fresnel transform with two virtual independent random phase masks (RPM). In order to avoid sending entire RPMs to the receiver side for decryption, the RPMs are generated by utilizing 2D–LASM, which results that the amount of the key data is reduced dramatically. Subsequently, by using Burch’s method and the phase-shifting interferometry, the encrypted computer generated hologram is fabricated. To improve the security and weaken the correlation, the encrypted hologram is scrambled base on 2D–LASM. Experiments demonstrate the validity of the proposed image encryption technique.

Color image watermarking scheme based on quaternion Hadamard transform and Schur decomposition

Based on quaternion Hadamard transform (QHT) and Schur decomposition, a novel color image watermarking scheme is presented. To consider the correlation between different color channels and the significant color information, a new color image processing tool termed as the quaternion Hadamard transform is proposed. Then an efficient method is designed to calculate the QHT of a color image which is represented by quaternion algebra, and the QHT is analyzed for color image watermarking subsequently. With QHT, the host color image is processed in a holistic manner. By use of Schur decomposition, the watermark is embedded into the host color image by modifying the Q matrix. To make the watermarking scheme resistant to geometric attacks, a geometric distortion detection method based upon quaternion Zernike moment is introduced. Thus, all the watermark embedding, the watermark extraction and the geometric distortion parameter estimation employ the color image holistically in the proposed watermarking scheme. By using the detection method, the watermark can be extracted from the geometric distorted color images. Experimental results show that the proposed color image watermarking is not only invisible but also robust against a wide variety of attacks, especially for color attacks and geometric distortions.

Fabrication of off-axis holographic Fresnel lens used as multiplexer/demultiplexer in optical communications

Off-axis holographic Fresnel lenses were designed and fabricated, with the aim of using them as multiplexer/demultiplexer in optical networks. The reduction of third-order aberrations was the main consideration. The theoretical analysis indicates that third-order aberration can be effectively reduced if the condition tanαC/tanαR = -λ2/λ1 is applied in geometry design and long object distance is selected. The experimental results verify the theoretical analysis.

A QDCT- and SVD-based color image watermarking scheme using an optimized encrypted binary computer-generated hologram

Based on quaternion discrete cosine transform (QDCT) and singular value decomposition (SVD), a novel color image watermarking scheme using an optimized encrypted binary computer-generated hologram (CGH) as the watermark is presented. A Fibonacci transform-based binary CGH technique that uses particle swarm optimization (PSO) algorithm to improve the reconstructed image quality has been proposed to generate a hologram of a watermark. By use of QDCT, the host color image is treated holistically as a vector field. The component of the host color image after QDCT, which is suitable for embedding watermark, is analyzed. With SVD, the mark CGH is inserted into the host color image by modifying the U matrix. To embed the binary CGH, multiple embedding strength factors are used in this study. To withstand the geometric attacks, based on Zernike moment algorithm and invariant centroid, an improved geometric distortion detection algorithm is proposed. Considering the significant color information, this algorithm is performed on the color invariance model. To obtain better imperceptibility and robustness, PSO is used for finding the proper multiple embedding strength factors. Compared with traditional geometric estimation method, the improved algorithm can estimate the transform parameters of the geometric distorted color image more accurately. Experimental results show that the proposed color image watermarking is not only secure and invisible, but also robust against common image processing operations and geometrical distortions.

An Adaptive Secure Watermarking Scheme for Images in Spatial Domain Using Fresnel Transform

A new adaptive blind watermarking scheme with the watermarks encrypted by Fresnel transform in spatial domain is presented. By utilizing fuzzy c-means clustering (FCM) technique and human visual system (HVS), the watermark can be adaptively embedded according to block classification. To keep imperceptibility and robustness, a novel iterative embedding algorithm is adopted to change the to-be-embedded pixel-values. The experimental results demonstrate that the proposed scheme provides good robustness to withstand different kinds of common attacks.

Adaptive watermarking scheme using a gray-level computer generated hologram

We present a new adaptive blind watermarking scheme using a gray-level computer generated hologram in the discrete wavelet transform domain. By utilizing an improved fuzzy clustering technique and human visual system , the watermark can be adaptively embedded according to block classification. To keep imperceptibility and robustness, a novel iterative embedding algorithm is adopted to change the to-be-embedded coefficients. Compared with the standard Fuzzy c-means (FCM) clustering, the suggested improved FCM (IFCM) converges more quickly and can avoid local optimum effectively. The experimental results demonstrate that the proposed scheme provides good robustness to withstand different kinds of common attack. Compared with other methods, the proposed method has the distinct advantage of better robustness to a JPEG compression attack.

Fabrication of holographic Fresnel lens used as a solar concentrator

One of the more intriguing of solar collection techniques, holographic concentration, has been addressed in many ways, but it has so far met with only limited success. The major attraction of holography is that unlike conventional concentrators, the holographic concentrator can track the sun across the sky while remaining completely stationary. A simple system using holographic Fresnel lenses (HFLs) was proposed in this paper. We present the theoretical analysis and the technique for fabricating HFLs used for solar concentrator. Several HFLs fabricated using corresponding recording geometries, resulted in a combined device that collects light at any angle, with each lens responsible for gathering sunlight for a certain portion of the day. Three HFLs with different fringe pattern were achieved, which operates in the morning, at noon and in the afternoon, respectively. The primary experimental results of the concentrator were presented, which effectively concentrates the visible spectrum over a 60 degrees daily angular variation. Several advantages of the holographic concentrator we obtained include longer lifetime, low cost, and the small size of a flat piece of film compared to a mirror or lens. The dispersive characteristics applied to photovoltaic power generation would offer a novel type of generator with high conversion efficiencies.

One-in-hundreds-out laser splitting generated by diffraction of single holographic optical element

A technique using a holographic optical element to split one incident laser beam into hundreds is proposed. The holographic optical element is fabricated with hexagonal packed lattice structure using 4-beam interference method. When the element is illuminated by a single laser beam with normal incidence, hundreds of beams are generated by diffraction. The element has the potential to be used as the device for interconnection and clock distribution in optical and electronic systems.

Fabrication of 2D photonic crystals with micron to sub-micron hexagonal lattices using single-exposure holographic technique

A single-exposure holographic technique for fabricating 2-dimensional photonic crystals (PCs) with hexagonal lattices is presented. A specially made holographic optical element (HOE), which consists of three holographic gratings, is used to generate three interference beams simultaneously under the illumination of a single laser beam. Theoretical analysis indicates that by selecting appropriate grating period and illuminating wavelength, the hexagonal lattice patterns formed by the interference of the three beams can have the lattice constants in the range from micron to sub-micron. Moreover, to obtain uniform lattice pattern with high contrast, the gratings in the HOE must have proper grating shape and depth. In the experiment, hexagonal lattices with lattice constants from 5 μm to 620 nm were fabricated in photoresist using the method. The patterned photoresist layers can be used as the masks to generate PCs in semiconductor materials through dry or wet etchings.

Influence of fabrication parameters on groove profile of blazed gratings made with two-step holographic technique

The two-step holographic technique of fabricating blazed gratings has been successfully applied to common positive photoresist. Influence of three fabrication parameters in the second step on groove profile of blazed gratings was studied experimentally. The three fabrication parameters are exposure, incident angle of laser beam and development time. The exposure distribution on the cross-section of one groove stripe of grating was modeled. Results of experiment and modeling with various exposures reveal that the technique is effective only when the symmetrical contour lines of exposure in the first step fade out with increasing second exposure. With other parameters fixed, experiments were also carried out with the beam incident angle varying from 10° to 80° and with the development time varying from 1 s to 9 s. Blazed gratings with quasi-triangular are obtained when incident angle reaches 40° and when development time reaches 5 s. Scanning electron micrographs of the results are presented. Blazed gratings with observed blaze angles of 20° to 50° were obtained by changing the incident angle. Efficiencies of different diffraction orders for gratings with various incident angles are also given.