Embedding of computer-generated hologram in a dithered image

We have developed an encryption method using a computer-generated hologram (CGH) embedded in a dithered image. First, confidential information is converted into a CGH. Next, the CGH data undergo two separate dithering processes in parallel: one corresponding to CGH white pixels and one corresponding to CGH black pixels. The results from both processes are used to form a dither matrix for creating the final dithered and encoded image. In this way, confidential information can be embedded into the image. The confidential information can be extracted using a technique similar to CGH optical reconstruction.     

Demonstration of a computed-tomography imaging spectrometer using a computer-generated hologram disperser

We have constructed a computed-tomography imaging spectrometer that uses a phase-only computer-generated hologram (CGH) array illuminator as the disperser. This imaging spectrometer collects multiplexed spatial and spectral data simultaneously and can be used for flash spectral imaging. The CGH disperser has been designed to maintain nearly equal spectral diffraction efficiency among a 5 x 5 array of diffraction orders and to minimize diffraction efficiency into higher orders. Reconstruction of the (x, y, lambda) image cube from the raw, two-dimensional data is achieved by computed-tomography techniques. The reconstructed image and spectral-signature data compare favorably with measurements by other spectrometric methods.     

Absolute interferometric test of aspheres by use of twin computer-generated holograms

A complete absolute interferometric test of axially symmetric aspheres is presented. The method is based on a specially designed computer-generated hologram (CGH) that reconstructs an aspherical wave as well as a spherical auxiliary wave. Since both phase functions have the same symmetry and their pattern is simultaneously encoded, we call this type of multiplex hologram a Twin-CGH. The spherical wave is used for calibration. The aberrations of the spherical auxiliary wave are measured absolutely with either a spherical mirror or an absolute test for Fresnel zone plates. Thus the two types of aberration inherent in the CGH can be identified and separated from each other. The errors of the spherical wave can be transferred to those of the aspherical wave. Two different methods thatuse Twin-CGHs for absolute testing of aspheric surfaces are described. Test procedures are explained, equations are derived, and experimental results are presented. A mutual comparison of the two results and a comparison with the established N-position rotation test are given.     

Fast computation for generating CGH of a 3D object by employing connections between layers

An approximate method for generating computer-generated holograms (CGH) of a 3D object with six times faster speed than the conventional algorithm is presented. In the conventional algorithm, a 3D object is sliced into many layers and treated as a collection of self-illuminated point light source. The propagation process of a light ray from every point of an object to all the points on the hologram plane is simulated and interfered with by the reference beam to form a CGH. In our proposed method, under the assumption that the depth of a 3D object is much smaller than the recording distance, we just need to calculate the oblique distance between the first layer and the hologram plane, and then the oblique distances from the other layers to the hologram plane can be obtained from a simple relation, thus the computational time is much reduced. The CGH is optically reconstructed and the quality of the reconstructed image agrees well with that from the conventional algorithm.     

Reflection volume holographic scanners with field-curvature corrections

A new type of polygonal holographic scanner that combines a reflection volume hologram with a computer-generated hologram (CGH) is described. The scanner is free from the aberration of field curvature. Such a scanning system can allow for a compact folded version of the scanner and Bragg diffraction into only a single order. The equations expressing the spatial-variable image distance are derived and are fit to the phase function designated by polynomials incorporated into a CGH in terms of the least-squares method. A reflection scanner with field-curvature correction is made by interfering a diffracted wave front from this CGH with a spherical wave front having scanning focal power through a second plane hologram. Experiments demonstrating the feasibility of this scanner are presented. Raster-scan patterns using a multifaceted scanner are shown. Helpful data on the diffraction efficiency and the spectrally diffracted intensity of reflection holograms are also presented.     

Properties of DMDs for holographic displays

Digital micromirror device’s (DMD) properties as being a display device for holographic displays are investigated. High speed, a large separation between reconstructed image and reconstruction beam, two symmetric diffraction patterns, and low intensity (0,0)th-order beam at a blazed grating condition are the desired properties for the displays. The blazed grating condition of a DMD can reconstruct images with higher diffraction efficiency than the line grating condition. DMD’s high speed enables to present colors and gray levels to the reconstructed image. However, reconstructed images from a gray-level computer-generated hologram (CGH) and its binary form hologram reveal no noticeable difference between them, except the background noise in the image from the CGH.     

Diffractive optical Hough transform implemented with phase singularities

The conventional Hough transform is implemented with a computer-generated hologram. The transmission function of the computer-generated hologram is computed with an extension of Bryngdahl's technique, which incorporates branch-point phase singularities. With this implementation it is shown that the branch-point technique can be used to implement point transforms successfully with an implicit transformation equation such as the Hough transform. This is done first by expression of the implicit transform in terms of several explicit transforms. After computation of the complex-valued transmission functions of the explicit transform, the functions are added together to form the transmission function for the implicit transform. Results are obtained by computation of Fresnel diffraction patterns of the Hough-transform computer-generated hologram, illuminated by different input images.     

Effective reduction of the novel look-up table memory size based on a relationship between the pixel pitch and reconstruction distance of a computer-generated hologram

In this paper, we propose an approach, new to our knowledge, to effectively generate and reconstruct the resolution-enhanced computer-generated hologram (CGH) of three-dimensional (3-D) objects with a significantly reduced in memory size novel look-up table (N-LUT) by taking into account a relationship between the pixel pitch and reconstruction distance of the hologram pattern. In the proposed method, a CGH pattern composed of shifted versions of the principal fringe patterns (PFPs) with a short pixel pitch can be reconstructed just by using the CGH generated with a much longer pixel pitch by controlling the hologram reconstruction distance. Accordingly, the corresponding N-LUT memory size required for resolution-enhanced hologram patterns can be significantly reduced in the proposed method. To confirm the feasibility of the proposed method, experiments are carried out and the results are discussed.     

Computer-generated holograms of a real three-dimensional object based on stereoscopic video images

A novel method of using stereoscopic video images to synthesize the computer-generated hologram (CGH) patterns of a real 3D object is proposed. Stereoscopic video images of a real 3D object are captured by a 3D camera system. Disparity maps between the captured stereo image pairs are estimated and from these estimated maps the depth data for each pixel of the object can be extracted on a frame basis. By using these depth data and original color images, hologram patterns of a real object can be computationally generated. In experiments, stereoscopic video images of a real 3D object, a wooden rhinoceros doll, are captured by using the Wasol 3D adapter system and its depth data are extracted from them. Then, CGH patterns of 1280 pixels x 1024 pixels are generated with these depth-annotated images of the wooden rhinoceros doll, and the CGH patterns are experimentally displayed via a holographic display system.     

Optical pattern recognition using a unidirectional photorefractive oscillator coupled with an angular multiplexing volume hologram

Abstract

We propose and demonstrate a unidirectional photorefractive ring oscillator that couples with an external angular multiplexing volume hologram for pattern recognition. By configuring the hologram externally, a computer-generated hologram (CGH) and a spatial light modulator are utilized to generate reference beams for the hologram. Two-dimensional images are stored in the form of resonating beams in the system, whereby the nonlinear interaction between the beams allows the image that most resembles an input object to be recognized. Five images were used to illustrate the pattern recognition ability of the oscillator. It was found that the input object was successfully recognized within 10s. The fact that CGHs can generate a large number of beamlets enhances significantly the storage capacity in this system.     

Computer-generated holograms on a metal ion-doped polymer system: contact copying

Dichromated poly(acrylic acid) films with dimethyl formamide have been used as a real-time recording medium for volume transmission holograms. In this study a simple computer-generated holographic grating with a sinusoidal amplitude profile is copied onto this recording material by the contact-copying technique. The experimental setup for the contact copying of a computer-generated hologram (CGH) is explained, and the phase modulation is observed for a different exposure time. Theoretical and experimental diffraction efficiency values for the CGH copy are also evaluated. Photographs of the original and copy of the CGH on the photopolymer film are presented.     

Null test for a highly paraboloidal mirror

A circular null computer-generated hologram (CGH) was used to test a highly paraboloidal mirror (diameter, 90 mm; f number, 0.76). To verify the null CGH test a classic autocollimation test with a flat mirror was performed. Comparing the results, we show that the results of the null CGH test show good agreement with results of the autocollimation test.     

Four-channel self-focus computer-generated hologram

A new type of computer-generated hologram (CGH) is described in this research. Upon the base of a two-channel CGH, it can generate four independent images in four different directions with the addition of positive or negative quadratic phase factors on the object spectrum; it has the character of self-focus. Results of the experiment are provided.     

Decomposition storage of information based on computer-generated hologram interference and its application in optical image encryption

An information-encryption method based on computer-generated hologram (CGH) interference is presented. In this method the original information is decomposed into two parts, and then each part is encoded on a separate CGH. When these two encoded CGHs are aligned and illuminated, a combined interference pattern is formed. The original information is obtained from this pattern. It is impossible to decrypt the original information from one CGH alone; two matched CGHs must be put together to make it available.     

Holographic femtosecond laser processing using optimal-rotation-angle method with compensation of spatial frequency response of liquid crystal spatial light modulator

Holographic femtosecond laser processing performs high-speed parallel processing using a computer-generated hologram (CGH) displayed on a liquid crystal spatial light modulator. A critical issue is to precisely control the intensities of the diffraction peaks of the CGH. We propose a method of compensating for the spatial frequency response in the design of CGH using the optimal-rotation-angle method. By applying the proposed method, the uniformity of the diffraction peaks was improved. We demonstrate holographic femtosecond laser processing with two-dimensional and three-dimensional parallelism.     

Calculation method for computer-generated holograms with cylindrical basic object light by using a graphics processing unit

It takes an enormous amount of time to calculate a computer-generated hologram (CGH). A fast calculation method for a CGH using precalculated object light has been proposed in which the light waves of an arbitrary object are calculated using transform calculations of the precalculated object light. However, this method requires a huge amount of memory. This paper proposes the use of a method that uses a cylindrical basic object light to reduce the memory requirement. Furthermore, it is accelerated by using a graphics processing unit (GPU). Experimental results show that the calculation speed on a GPU is about 65 times faster than that on a CPU.     

Multiplexed computer-generated hologram with polygonal apertures

A novel type of multiplexed computer-generated hologram (CGH) is designed with more than one billion of pixels per period. It consists of elementary cells divided into arbitrary-shaped polygonal apertures, the division being identical in all cells. The cells are further digitized into pixel arrays to exploit the huge space-bandwidth product of electron-beam lithography. The polygonal apertures in the same location inside the cells constitute a subhologram. With the Abbe transform that has never, to our knowledge, been used in other CGH designs, the subhologram images (subimages) are obtained with fast Fourier transforms. It is therefore possible to design a multiplexed CGH that has a size thousands of times larger than the manageable size of a conventional CGH designed with the iterative Fourier transform algorithm (IFTA). A much larger object window than that of the conventional CGH can also be achieved with the multiplexed polygonal-aperture CGH, owing to its extremely large dimensions. The multiplexed polygonal-aperture CGH is designed with the novel iterative subhologram design algorithm, which considers the coherent summation of the subimages and applies constraints on the total image, subimages, and subholograms. As a result, the noise appearing in the preceding multiplexed-CGH designs is avoided. The multiplexed polygonal-aperture CGH has a much higher diffraction efficiency than that resulting from either the preceding multiplexed-CGH designs or the conventional CGH designed by the IFTA.     

基于数字全息和离散余弦变换的数字水印技术

提出了一种基于数字菲涅耳全息和离散余弦变换的数字水印技术.利用全息技术得到原始水印图像的菲涅耳全息图,之后将其嵌入宿主图像离散余弦变换域的中频系数,将水印添加位置和光学系统的几何结构参数作为恢复水印信息的密钥,并且算法属"盲数字水印技术",从而该算法具有较好的安全性和实用性.对算法的测试结果也表明,该算法对噪声,滤波、剪切,旋转、有损压缩等常见的图像处理操作具有较高的稳健性.因此,该算法具有一定的实用价值.     

Computer-generated holograms of three-dimensional realistic objects recorded without wave interference

We propose a method of synthesizing computer-generated holograms of real-life three-dimensional (3-D) objects. An ordinary digital camera illuminated by incoherent white light records several projections of the 3-D object from different points of view. The recorded data are numerically processed to yield a two-dimensional complex function, which is then encoded as a computer-generated hologram. When this hologram is illuminated by a plane wave, a 3-D real image of the object is reconstructed.     

Analysis of the disturbing diffraction orders of computer-generated holograms used for testing optical aspherics

Aspheric surfaces are increasingly used in the design of high-quality optical imaging systems. Therefore accurate testing methods for aspherics are also necessary. One possibility is to use a computer-generated hologram (CGH) as a part of a null lens in an interferometric testing device. However, CGHs normally have more than one diffraction order, thus causing disturbing areas in the interferogram. Here a simple approximative analytical expression is given for the spatial frequencies of the disturbing light in the interferogram coming from the different diffraction orders of the CGH. This expression also enables one to calculate the size and the shape of the disturbing areas in the interferogram. Some design examples for CGHs are given in an application of the expression.