Ping Su

Optical filter based on contra-directional waveguide coupling in a 2D photonic crystal with square lattice of dielectric rods.

A coupler-type optical filter in 2D photonic crystal (PhC) with square lattice of dielectric rods in air is presented. The reduced-index and increased-index waveguides of filter have dispersion curves with opposite slopes to realize contra-directional coupling, and the point of anti-crossing is designed below the light line to avoid vertical radiation. The filter has a broad operable bandwidth due to the absence of mini stop bands. The transmission properties are analyzed using coupled modes theory (CMT) and simulated using the finite-difference time-domain (FDTD) method. The results show that a filtering bandwidth of 4 nm can be achieved in the range of 1500~1600 nm, and over 83% drop coefficient is obtained.

Improving accuracy of multichannel volume holographic correlators by using a two-dimensional interleaving method.

We introduce a two-dimensional interleaving method to improve the output accuracy of volume holographic correlators. The method redistributes the pattern of every input and stored image without changing their inner products, so it can eliminate the impact of correlation pattern difference on calculated inner product values. Experimental tests show that this method can achieve more accurate output intensity for every channel, which agrees much better with the theoretical value than without this method.

Fast Computer-Generated Hologram Generation Method for Three-Dimensional Point Cloud Model

A fast computer-generated hologram (CGH) generation method for three-dimensional point cloud models is proposed. The points of the model are classified into parallel two-dimensional layers after a layer spacing δz is set. Then, the CGH can be calculated by a layer-based method. In addition, a simple approach for occlusion culling is introduced. The computation time is reduced by thousands of times compared to point-based method. Discussion results show that the proposed method is more effective for larger models, and for models with huge number of points, the computation efficiency can be further enhanced by choosing larger layer spacing and optimized classification method. The proposed method will be an ideal CGH generation method for point cloud models, extremely suitable for real-time holographic reconstruction of the real scene.

Demonstration of an Autostereoscopic Three-Dimensional Light-Emitting Diode Display Using Diffractive Optical Elements Sheet

A prototype autostereoscopic three-dimensional (3D) light-emitting diode (LED) display using a cylindrical diffractive optical elements (C-DOEs) sheet as the optical steering element is proposed. The operation of the system and the calculation method of the system parameters are described in detail. The DOEs sheet is placed from a distance from the LED display panel which is five times smaller of the existing technology, and the column spacings of the pixels of the LED display panel are nonequal in order to equalize the distance between the viewing zones. The prototype has a 1.33 m2 display panel, 384 ×144 resolution and a horizontal field of view of 60°. The experimental result shows that the proposed method is a potential autostereoscopic technology for the large area LED displays.

Computer generated hologram null test of a freeform optical surface with rectangular aperture

In null computed generated hologram (CGH) test of optical elements, fitting method is needed in null CGH design to generate continuous phase function from the ray-traced discrete phase data. The null CGH for freeform testing usually has a deformed aperture and a high order phase function, because of the aberrations introduced by freeform wavefront propagation. With traditional Zernike polynomial fitting method, selection of an orthogonal basis set and choosing number of terms are needed before fitting. Zernike polynomial fitting method is not suitable in null CGH design for freeform testing; a novel CGH design method with cubic B-spline interpolation is developed. For a freeform surface with 18×18  mm2 rectangular aperture and 630 μm peak-to-valley undulation, the null CGH with a curved rectangular aperture is designed by using the method proposed. Simulation and experimental results proved the feasibility of the novel CGH design method.

Micro-Displacement Sensor With Large Dynamic Range Based on Photonic Crystal Co-Directional Coupler

A novel micro-displacement sensor based on a photonic crystal (PhC) co-directional coupler and its sensing technique are presented. The coupler consists of a fixed and a movable PhC segments. Due to the translational symmetry of the PhC structure, this sensing technique is valid for a large displacement on the order of na (n is an integer, and a is the lattice constant of the PhC). After optimization, the outputs of the sensor are approximately sine functions of the displacement, thus it has the potential to achieve high resolution using a subdivision approach. These two characteristics ensure the large dynamic range of the micro-sensor. The properties of the micro-displacement sensor are analyzed using the coupled-mode theory and simulated using the finite-difference time-domain method

High accurate volume holographic correlator with 4000 parallel correlation channels

Volume holographic correlator allows simultaneously calculate the two-dimensional inner product between the input image and each stored image. We have recently experimentally implemented in VHC 4000 parallel correlation channels with better than 98% output accuracy in a single location in a crystal. The speckle modulation is used to suppress the sidelobes of the correlation patterns, allowing more correlation spots to be contained in the output plane. A modified exposure schedule is designed to ensure the hologram in each channel with unity diffraction efficiency. In this schedule, a restricted coefficient was introduced into the original exposure schedule to solve the problem that the sensitivity and time constant of the crystal will change as a time function when in high-capacity storage. An interleaving method is proposed to improve the output accuracy. By unifying the distribution of the input and stored image patterns without changing the inner products between them, this method could eliminate the impact of correlation pattern variety on calculated inner product values. Moreover, by using this method, the maximum correlation spot size is reduced, which decreases the required minimum safe clearance between neighboring spots in the output plane, allowing more spots to be parallely detected without crosstalk. The experimental results are given and analyzed.

Magnification of optical image in holography projection using lensless Fresnel holography

Jianshe MaPing SuFeipeng XiaZhenbo RenTong LiuTsinghua UniversityGraduate School at ShenzhenShenzhen, Guangdong 518055, ChinaE-mail: su.ping@mail.sz.tsinghua.edu.cnAbstract. A digital micro-mirror device (DMD) acting as a real-time holo-gram is an emerging technology in dynamic holographic projection. Thispaper presents a lensless image magnification method in DMD hologra-phy by using a Fresnel hologram. By analyzing the diffraction order dis-tribution in the image plane of a hologram produced by DMD, we find thefactors that limit the size of the magnified image. We perform a lenslessmagnification experiment that shows good magnified images in accor-dance with the numerical results. Finally, we discuss methods to eliminatelongitudinal error and chromatic aberration in three-dimensional (3-D) andcolor projection, respectively, and present a 3-D image reconstructionresult that shows lensless magnification of a 3-D image without distortion.Itisbelievedthatthistechniquecanbeusedinfuturereal-timeholographicprojection based on digital light processing technology.

An Integral Model of Two-Order and Three-Order Scattering for Non-Line-of-Sight Ultraviolet Communication in a Narrow Beam Case

The study on multiple scattering propagation of non-line-of-sight ultraviolet communication is based on the stochastic Monte Carlo method in the previous studies. In this letter, an integral model of two-order and three-order scattering is deduced in a narrow beam case. A prolate-spheroidal coordinates system and an isotropic scattering hypothesis are used to simplify the three-order scattering model. Compared with the Monte Carlo method, the computational efficiency and the stability of proposed model are much better and do not decrease with the increase of the communication range. This letter provides a new way to obtain the path loss of multiple scattering in long communication ranges.

Estimation and optimization of computer-generated hologram in null test of freeform surface

Freeform surfaces are increasingly used in the design of compact optical systems. Interferometric null test with computer generated hologram (CGH), which has been successfully used in highly accurate test of aspheric surfaces, is adopted to test the freeform surfaces. The best fitting sphere of the freeform surface under the test is firstly calculated to quickly estimate the possibility of null test. To decrease the maximum spatial frequency of the null CGH, the position of the CGH and the direction of optical axis are optimized. The estimated maximum spatial frequency of the CGH is 7.8% apart from the optimized one, which shows the validity of the best fitting sphere.