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.
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.
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.
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.
Fourier ptychographic microscopy (FPM) is a recently developed computational imaging technology, which achieves high-resolution imaging with a wide filed-of-view by overcoming the limitation of the optical spatial-bandwidth-product (SBP). In the traditional FPM system, the aberration of the optical system is ignored, which may significantly degrade the reconstruction results. In this paper, we propose a novel FPM reconstruction method based on the forward neural network models with aberration correction, termed FNN-AC. Zernike polynomials are used to indicate the wavefront aberration in our method.Both the spectrum of the sample and coefficients of different Zernike modes are treated as the learnable weights in the trainable layers.By minimizing the loss function in the training process, the coefficients of different Zernike modes can be trained, which can be used to correct the aberration of the optical system. Simulation has been performed to verify the effectiveness of the FNN-AC.
To obtain surreal and richer visual experience, augmented reality (AR) technology has been widely used in various areas. As a popular solution of AR, display using computer generated hologram (CGH) is often accompanied by blurring which is caused by uncontrolled interference. In this paper, a modified algorithm based on double-phase hologram (DPH) algorithm is proposed to reduce speckle noise in holographic reconstruction. The macro-pixels in the original hologram are separated into multiple sub-holograms, and these sub-holograms are displayed alternately in high frequency, which reduces the speckle noise generated from the interference between adjacent macro-pixels. Meanwhile, the method is less time-consuming than the traditional Gerchberg-Saxton algorithm because no iteration is needed. The simulation and the optical experiment based on liquid crystal on silicon (LCoS) have been conducted, and the results confirm the feasibility of the proposed method to improve the image quality.
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.
Light exerts non-visual effects on a wide range of biological functions and behavior apart from the visual effect. Light can regulate human circadian rhythms, like the secretion of melatonin and cortisol. Light also has influence on body’s physiological parameters, such as blood pressure, heart rate and body temperature. However, human cognitive performance, alertness and mood under different lighting conditions have not been considered thoroughly especially for the complicated visual task like surgical operating procedure. In this paper, an experiment was conducted to investigate the cognition, alertness and mood of healthy participants in a simulated operating room (OR) in the hospital. A LED surgical lamp was used as the light source, which is mixed by three color LEDs (amber, green and blue). The surgical lamp is flexible on both spectrum and intensity. Exposed to different light settings, which are varied from color temperature and luminance, participants were asked to take psychomotor vigilance task (PVT) for alertness measurement, alphabet test for cognitive performance measurement, positive and negative affect schedule (PANAS) for mood measurement. The result showed the participants’ cognitive performance, alertness and mood are related to the color temperature and luminance of the LED light. This research will have a guidance for the surgical lighting environment, which can not only enhance doctors’ efficiency during the operations, but also create a positive and peaceful surgical lighting environment.
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.