Dual Fresnel mirror system for 360 degree viewable three-dimensional displays.

360-degree viewable three-dimensional (3D) display systems have gained considerable attention for the unique manner in which they display objects. Most of the optical display devices in these systems employ two parabolic mirrors facing each other separated by a distance equal to the focal length of the mirrors. However, the current configuration is limited to unity magnification and provides a small image relative to the volume of the system. This paper presents a novel 3D display system based on two Fresnel mirrors with different focal lengths facing each other. The distance between the mirrors can be adjusted to alter the magnification of the resulting 3D image. Wave-optics analysis of an optical dual-lens system equivalent to the proposed dual-mirror system was used to simulate the image distance, the transverse and longitudinal magnification, and the minimum length of the proposed system. This paper also addresses issues pertaining to the design and manufacture of Fresnel mirrors. An experiment system using Fresnel mirrors with focal lengths of 60mm and 90mm clearly demonstrated the efficacy of the proposed scheme in terms of 3D image magnification.

Iterative pixelwise approach applied to computer-generated holograms and diffractive optical elements.

This paper presents a novel approach to optimizing the design of phase-only computer-generated holograms (CGH) for the creation of binary images in an optical Fourier transform system. Optimization begins by selecting an image pixel with a temporal change in amplitude. The modulated image function undergoes an inverse Fourier transform followed by the imposition of a CGH constraint and the Fourier transform to yield an image function associated with the change in amplitude of the selected pixel. In iterations where the quality of the image is improved, that image function is adopted as the input for the next iteration. In cases where the image quality is not improved, the image function before the pixel changed is used as the input. Thus, the proposed approach is referred to as the pixelwise hybrid input-output (PHIO) algorithm. The PHIO algorithm was shown to achieve image quality far exceeding that of the Gerchberg-Saxton (GS) algorithm. The benefits were particularly evident when the PHIO algorithm was equipped with a dynamic range of image intensities equivalent to the amplitude freedom of the image signal. The signal variation of images reconstructed from the GS algorithm was 1.0223, but only 0.2537 when using PHIO, i.e., a 75% improvement. Nonetheless, the proposed scheme resulted in a 10% degradation in diffraction efficiency and signal-to-noise ratio.

Compact Holographic Projection Display Using Liquid-Crystal-on-Silicon Spatial Light Modulator.

This paper presents a holographic projection display in which a phase-only spatial light modulator (SLM) performs three functions: beam shaping, image display, and speckle reduction. The functions of beam shaping and image display are performed by dividing the SLM window into four sub-windows loaded with different diffractive phase elements (DPEs). The DPEs are calculated using a modified iterative Fourier transform algorithm (IFTA). The function of speckle reduction is performed using temporal integration of display images containing speckles. The speckle contrast ratio of the display image is 0.39 due to the integration of eight speckled images. The system can be extended to display full-color images also by using temporal addition of elementary color images. Because the system configuration needs only an SLM, a Fourier transform lens, and two mirrors, the system volume is very small, becoming a potential candidate for micro projectors.

Laser speckle reduction by phase range limited computer generated hologram in laser projection display system.

The speckle phenomenon is an annoyance in laser projection display systems. We propose a novel speckle suppression method that utilizes the interference concept on a pixel point, which reduces the speckle contrast (SC) of the project image by limiting the phase distribution range in the optical field. The SC formula is derived in the uniform interval phase range for partially developed speckle conditions, showing that the SC can be lowered by lessening the phase range limitation. In the ideal simulation model, the SC can be reduced from 98.77% to 0% as the phase range limitation varies from 2π to 0. The phase range limitation model is a novel method using a computer generated hologram to provide beam shaping and phase limitation. In a more realistic simulation model, the SC is reduced from 99.18% to 16.68%.

Full-frame projection displays using a liquid-crystal-on-silicon spatial light modulator for beam shaping and speckle suppression.

This paper presents a full-frame laser projection display system in which a spatial light modulator (SLM) is used for beam shaping and speckle suppression. Phase-only computer-generated holograms (CGHs) are used to transform a cross section of the incident laser beam into a square nearly the same size as that of the display device. Under different initial conditions, the diffraction patterns generated by the CGHs possess identical intensity distributions but differ with regard to random phase distribution. Image speckles can be suppressed via the temporal addition of diffraction patterns from the CGHs when displayed by the SLM. The addition of 16 speckled images resulted in speckle suppression ratios of 0.290 in simulations and 0.345 under experimental conditions. Not only were the speckles suppressed, but the quality of the overall image was also improved considerably. The proposed approach presents a simple design with low power consumption and stable display architecture for application in pico-projectors.

Optimizing the parameters for measuring laser speckle and speckle contrast.

Evaluating the contrast in speckle patterns produced by laser projection displays can facilitate the development of methods to suppress such imperfection. Computer simulations were first conducted to characterize the contrast (C(gs)) of fully developed speckle patterns with spatial factor k and power factor r. Results showed that 0.1≤r≤2.0 and k≥4.0 were required to obtain a C(gs) with less than 5% error. Experimental results, however, revealed that a power factor within the range 0.5≤r≤2.0 was needed, meaning that the speckle dimension was at least four times the pixel pitch and the largest speckle intensity was of the order of magnitude of the saturation level of the camera. The method proposed here is that the spatial factor be determined by adjusting the distance between the object and the camera, and the power factor be determined by monitoring the real-time histogram representing the speckle pattern.

Orthogonal incidence method for efficient sunlight collection from asymmetric light couplers in tree-structured light guiding systems.

Directly transporting sunlight for use in indoor lighting applications is an efficient way to utilize solar energy. This study proposes a tree-structured light guiding system (TLGS) to collect sunlight and transport it for indoor illumination. The use of asymmetric light couplers in a TLGS increases the amount of accumulated sunlight. An analytic ray tracing model of the asymmetric coupler is proposed to present the angle and height distributions of the propagated rays. The cutoff angles were derived, and this cutoff condition was used to determine which rays are able to travel through the coupling region. In simulations, the couplers with coupling angles (θ(coup)) of 30° and 50° were conducted, and the large θ(coup) coupler provided high coupling efficiency (0.450). The orthogonal incidence method was adopted to increase coupling efficiency (0.646), and subsequently the amount of accumulated sunlight. The amount of accumulated sunlight in a TLGS was increased by 44%.

Optical effects of shadow masks on short circuit current of organic photovoltaic devices.

In this paper, we have employed different shadow masks attached on top of organic photovoltaic (OPV) devices to study the optical effects of the former on the short circuit current (J(SC)). To rule out possible lateral electrical conduction and simplify the optical effects inside the device, a small-molecular heterojunction OPV device with a clear donor/acceptor interface was employed with a hole extraction layer exhibiting high resistance intentionally. Careful calibration with a shadow mask was employed. By attaching two layers of opaque masks in combination with a suitable holder design to shield the light from the edges and backside, the value of J(SC) approached that of the dark current, even under 1-sun radiation. With different illumination areas, we found that the photons illuminating the non-active region of the device contributed to 40% of the J(SC) by optical effect within the width of about 1 mm around the active region. When illuminating the non-active area with 12 mm to the active area, a 5.6 times improvement in the J(SC) was observed when the incident angle was 75°. With the introduction of a microstructured film onto the OPV device and an increase in the reflection from the non-active region, a 15% enhancement of the J(SC) compared to the control device was achieved.

Speckle suppression in holographic projection displays using temporal integration of speckle images from diffractive optical elements.

Speckles on images in holographic projection displays (HPDs) were efficiently suppressed by the temporal sum of two diffractive images generated from diffractive optical elements (DOEs). Using a modified iterative Fourier transform algorithm, we obtained pairs of phase-only DOEs that generated the diffractive images with high negative correlation coefficients of -0.827 and -0.490 in the one-dimensional and the two-dimensional simulations, respectively. The suppression ratios of the speckles in the two simulations were 0.301 and 0.457, which were 61% and 35% lower, respectively, than the sum of the two uncorrelated images. We have successfully demonstrated that the sum of two negatively correlated images from DOEs can effectively reduce the image speckles and improve the image quality in HPD systems.

Implementation of phase-shift patterns using a holographic projection system with phase-only diffractive optical elements.

We proposed a method to implement spatial phase-shift patterns with subdiffraction limited features through a holographic projection system. The input device of the system displayed phase-only diffractive optical elements that were calculated using the iterative Fourier-transform algorithm with the dummy-area method. By carefully designing the target patterns to the algorithm, the diffractive optical elements generated the Fourier-transformed images containing the phase-shift patterns in which the widths of dark lines were smaller than the diffraction limit. With these demonstrations, we have successfully shown that the near-field phase-shift lithographic technique can be realized through an inexpensive maskless lithographic system and can still achieve subdiffraction limited images.

Optimal quantization method for uneven-phase diffractive optical elements by use of a modified iterative Fourier-transform algorithm.

A method based on an iterative Fourier-transform algorithm (IFTA) with phase optimization for phase-only multilevel diffractive optical elements (DOEs) is presented. Phase optimization by minimizing the mean-squared error composed of an amplitude-weighted probability-density function is performed in the IFTA iterations to ensure that the wavefront difference is minimized and thus to improve the DOEs' performance. Using the proposed method, we obtained a small standard deviation of diffraction efficiency of 20 uneven-phase DOEs, showing that DOEs with high diffraction efficiency did not vary significantly with the initial conditions. The simulation results of even- and uneven-phase four-level DOEs designed by use of direct and stepwise quantization IFTA methods are compared.

Backward iterative quantization methods for designs of multilevel diffractive optical elements.

Four types of backward iterative quantization (BIQ) methods were proposed to design multilevel diffractive optical elements (DOEs). In these methods, the phase values first quantized in the early quantization steps are those distant from the quantization levels, instead of the neighboring ones that the conventional iterative method began with. Compared with the conventional forward iterative quantization (FIQ), the Type 4 BIQ achieved higher efficiencies and signal-to-noise ratios for 4- level unequal-phase DOEs. For equal-phase DOEs, the Type 4 BIQ performed better when the range increment of each quantization step was large (> 15 degrees ), while the FIQ performed better when the range increment was small (< 15 degrees ).

Optimal quantization by use of an amplitude-weighted probability-density function for diffractive optical elements.

We present a method for determining the optimal quantitized phase levels for phase-only diffractive optical elements by taking into account a modified probability-density function (PDF). This modified PDF was obtained from the PDF of the phase distribution weighted by the incident amplitude distribution. We applied the proposed method to construct a multilevel Fresnel zone plate with a focal length of 8 m and a diameter of 6.2 mm upon which was incident a Gaussian beam with a 1/e width of 1.4 mm. The efficiency of a binary design was improved by 26.4% and the signal-to-noise ratio by was improved by 18.2% compared with those obtained by the uniform quantization method.