The optical combiner is an important part of the optical see-through augmented reality display system. Waveguide is an appropriate solution due to its advantages such as light weight and compact structure. Because grating has replicability, it is a promising solution to the waveguide’s coupler for mass-production. In this paper, a grating coupler for waveguide is designed by using the rigorous coupled wave analysis (RCWA) to increase the accuracy of the simulation due to the critical dimension is similar to the wavelength. The uniformity of the diffraction efficiency is considered as an important parameter for a better displaying performance. The downhill algorithm is used to optimize the parameters of the grating. In order to obtain a large field of view, the thickness of the grating should be controlled carefully. Finally, two gratings are designed for the waveguide which can extend pupil horizontally. The displaying performance of the waveguide is simulated, and the grating couplers are fabricated by the nanoimprint lithography method. The characteristics of the gratings are tested such as transmittance and diffraction efficiency. The results show the proposed gratings can be utilized for waveguide’s coupler. It is believed that our results will give a better alternative for the augmented reality display system.
Metasurface optical elements such as metalenses have drawn great attentions for their capabilities of manipulating wavefront versatilely and miniaturizing traditional optical devices into ultrathin counterparts, and multi-functional metasurfaces such as bifocal metalenses have attracted tremendous interests due to their potential in system integration. In this paper, an approach to design polarization-dependent bifocal metalenses which are able to independently generate longitudinally or transversely bifocal spots under the incidence of circularly polarized light with arbitrary ellipticity is proposed and demonstrated by full-wave simulations. When the designed devices are illuminated with elliptically polarized lights at wavelength of 532 nm, both of the helicity-multiplexed bifocal spots appear simultaneously, and the relative intensity of both focal spots can be tuned in terms of the ellipticity of the polarization state. In addition, a polarization-independent metalens based on geometric phase modulation is illustrated and the focusing efficiency of it maintains stable ignoring the polarization state of the incident waves, which could be of vital importance in real applications. This design is of enormous potential of being applied in real compact optical systems such as imaging, display, microscopy, tomography, optical data storage and so on.
A simple yet effective method to realize holographic three-dimensional (3D) display by shifted Fraunhofer diffraction has been presented in this paper. After a 3D object is divided into a set of layers in axial direction, these layers are calculated into corresponding sub-holograms by Fraunhofer diffraction. The hologram uploaded on SLM consists of sub-holograms in a tiling approach. Both simulations and experiments are carried out to verify the feasibility of shifted Fraunhofer diffraction. Detailed analysis of computational cost has also been carried out, and the comparison between shifted Fresnel diffraction and shifted Fraunhofer diffraction in the proposed method has been analyzed. The experimental results demonstrate that our method can reconstruct multi-plane 3D object with continuous depth map and the process of 3D modeling is simple, that is the computational complexity is accordingly reduced.
In compressive spectral imaging, three-dimensional spatio-spectral data cubes are recovered from two-dimensional projections. The quality of the compressive-sensing-based reconstruction is dependent on the coherence of the sensing matrix, which is determined by the system projection and the sparse prior. Studies on the optimization of the system projection, which mainly deals with the coded aperture, successfully decreases the coherence of the sensing matrix and improves the reconstruction quality. However, the optimization of the sparse prior considering the relationship between the system projection and the sparse prior remains a challenge. In this paper, we propose a gradient-descent-based sparse prior optimization algorithm for the coherence minimization of the sensing matrix in compressive spectral imaging. The Frobenius norm coherence is introduced as the cost function for the optimization, and the overcomplete dictionary is chosen as the sparse prior to solve the optimal sparse representation in the reconstruction as it provides higher degree of freedom for optimization compared to common orthogonal bases. The optimized dictionary effectively decreases the coherence of the sensing matrix from 0.880 to 0.604 and significantly improves the quantitative image quality metrics of the reconstructed hyperspectral images with the corresponding peak signal-to-noise ratio (PSNR) increased by 9 dB, the structural similarity (SSIM) above 0.98, and the spectrum angular mapper (SAM) below 0.1. Furthermore, the requirement of the sampling snapshots is reduced, which is shown by similar image quality metrics between the reconstructed hyperspectral images of only 1 snapshot with the optimized dictionary and of more than 5 snapshots with the non-optimized dictionary.
Metasurfaces are expected to realize the miniaturization of conventional refractive optics into planar structures; however, they suffer from large chromatic aberration due to the high phase dispersion of their subwavelength building blocks, limiting their real applications in imaging and displaying systems. In this paper, a high-efficient broadband achromatic metasurface (HBAM) is designed and numerically demonstrated to suppress the chromatic aberration in the continuous visible spectrum. The HBAM consists of TiO2 nanofins as the metasurface building blocks (MBBs) on a layer of glass as the substrate, providing a broadband response and high polarization conversion efficiency for circularly polarized incidences in the desired bandwidth. The phase profile of the metasurface can be separated into two parts: the wavelength -independent basic phase distribution represented by the Pancharatnam-Berry (PB) phase, depending only on the orientations of the MBBs, and the wavelength-dependent phase dispersion part. The HBAM applies resonance tuning for compensating the phase dispersion, and further eliminates the chromatic aberration by integrating the phase compensation into the PB phase manipulation. The parameters of the HBAM structures are optimized in finite difference time domain (FDTD) simulation for enhancing the efficiency and achromatic focusing performance. Using this approach, this HBAM is capable of focusing light of wavelengths covering the entire visible spectrum (from 400 nm to 700 nm) at the same focal plane with the spot sizes close to the diffraction limit. The minimum polarization conversion efficiency of most designed MBBS in such spectrum is above 20%. This design could be viable for various practical applications such as cameras and wearable optics.
Augmented reality (AR) technology has been applied in various areas, such as large-scale manufacturing, national defense, healthcare, movie and mass media and so on. An important way to realize AR display is using computer-generated hologram (CGH), which is accompanied by low image quality and heavy computing defects. Meanwhile, the diffraction of Liquid Crystal on Silicon (LCoS) has a negative effect on image quality. In this paper, a modified algorithm based on traditional Gerchberg-Saxton (GS) algorithm was proposed to improve the image quality, and new method to establish experimental system was used to broaden field of view (FOV). In the experiment, undesired zero-order diffracted light was eliminated and high definition 2D image was acquired with FOV broadened to 36.1 degree. We have also done some pilot research in 3D reconstruction with tomography algorithm based on Fresnel diffraction. With the same experimental system, experimental results demonstrate the feasibility of 3D reconstruction. These modifications are effective and efficient, and may provide a better solution in AR realization.
Optical tweezers is an increasingly important technique for controlling and probing particles since computer-generated holography (CGH) make steering of multiple traps individually possible. In addition, the dark focus of orbital angular momentum (OAM) beams is increasingly widely used in trapping reflecting, absorbing or low-dielectric-constant objects. In this paper, we present a method to create arbitrary three-dimensional configurations of orbital angular momentum modes to achieve manipulation of micro-particles. Compared with conventional optical tweezers, this method can steer mixed arrays of traps individually and randomly by producing three-dimensional structure of optical vortices. These optical traps we used was formed by a CGH generated complex phase mask, which has three components: 1) a helical phase mask to change the transverse phase structure, 2) a blazed grating phase mask to vary the propagation direction of the incident beams, and 3) a modulated grating phase mask to divert the focal plane from the planar configurations. The latter one ensure that we can form threedimensional trapping patterns. The trap patterns can also be generated dynamically by holographic display system based on liquid crystal on silicon (LCoS). The experimental results show that the refresh frequency of reconfiguring achieves 24fps. Our method is effective and promise an exciting new opportunity to be used as a valuable non-contact manipulation tool in various applications.
A color transparent screen was designed in this paper, utilizing a planar glass combined with lens array holographic optical elements (HOEs). The lens array HOEs were exposed using two coherent beams, one of which was the reference wave directly illuminating on the holographic material and the other was modulated by the micro lens array. The lens array HOEs can display the images with see-through abilities. Unlike the conventional ones that used the lens array HOEs as the screen solely, planar glass was adopted here as the waveguide. The projecting light was totally internalreflected in the planar glass to eliminate undesired zero-order diffracted light and realize high system compactness. Colorful display can be realized in our system as the holographic materials were capable for multi-wavelength display. To verify the effectiveness of this method, a color transparent screen incorporating the lens array HOEs and waveguide was designed. Results showed the circular display area with 20mm in diameter and the pixel resolution of 100μm were achieved in the system. This simple and effective method could be an alternative in the augment reality (AR) applications, such as transparent phone and television.
In this paper, an overlapped sub-block gray-level average method for contrast enhancement is presented. The digital
image correction of uneven illumination under microscope transmittance is a problem in image processing, also sometimes
the image in the dark place need to correct the uneven problem. A new correction method was proposed based on the mask
method and sub-blocks gray-level average method because Traditional mask method and background fitting method are
restricted due to application scenarios, and the corrected image brightness is low by using background fitting method, so
it has some limitations of the application.
In this paper, we introduce a new method called AOSCE for image contrast enhancement. The image is divided into
many sub-blocks which are overlapped, calculate the average gray-level of the whole image as M and the calculate the
average gray-level of each one as mi, next for each block it can get d = mi - m, each block minus d to get a new image,
and then get the minimum gray-level of each block into a matrix DD to get the background, and use bilinearity to get the
same scale of the image. over fitting the image in matlab in order to get smoother image, then minus the background to
get the contrast enhancement image.
Ultrashort laser pulse has been widely used in various applications. Its parameters, such as the pulse duration and the
spectral bandwidth, should be controlled accurately in order to achieve high spatial and temporal resolution, as well as
high local field intensity. In this paper, we have proposed a method to trace the propagation of ultrashort pulses through
optical systems, especially the complex optics. The approach, in which both the material’s dispersion and optical
aberrations are taken into consideration, is developed based on the geometrical ray-tracing combined with wave theories.
This method is validated by simulating the propagation of a femtosecond pulse through a specific practical imaging system.
As the numerical result shows that the spatial-temporal performances of pulses are influenced greatly by optical elements,
the calibration arrangement is employed to compensate for those undesired distortions. The negative dispersion of the
optical grisms (the combination of gratings and prisms) is utilized in the calibration process to offset the positive dispersion
introduced by lenses. The final result shows effectiveness of the correction.