He Qing-Sheng

Diarylethene Materials for Rewritable Volume Holographic Data Storage

The photochromic diarylethene, 1,2-bis(2-methyl-5-(4-formylphenyl)-thien-3-yl)perfluorocyclopentene (1a) is studied and its applicable potential in rewritable volume holographic data storage is verified. Holographic recording films of 10-µm thickness have been fabricated. The refractive index modulation (Δn = 1.15×10-3) between the open- and close-ring forms is detected to be large enough so that the films are suitable for the production of volume holographic storage. The experiments of angle multiplexing and rewriting holograms show that the materials are fit for volume holographic data storage.

Holographic grating formation in dry photopolymer film with shrinkage

An important issue in developing applications for photopolymers in holography is the effect of shrinkage on recording properties. In this paper, we introduce a model to describe real-time formation of a single grating in photopolymers at any geometrical angle, under the assumption that the shrinkage is in proportion to the polymerization. This model combines polymerization kinetics with the coupled-wave theory, explaining the shrinkage effect on the diffraction efficiency. The model is validated by comparing its predictions with the experimental results for a film of 99μm thickness.

Holographic Grating Formation in Cationic Photopolymers with Dark Reaction

We propose a new formula to describe the dynamics of holographic grating formation under low intensity pulse exposures in cationic photopolymers, in which the dark reaction contributes dominantly to the grating strength. The formula is based on the living polymerization mechanism and the diffusion-free approximation. The analytical solution indicates that the grating formation time depends on the termination rate constant, while the final grating strength depends linearly on the total exposure energy. These theoretical predictions are verified experimentally using the Aprilis HMC-400 μm photopolymer. The results can provide guidelines for the control and optimization of the holographic recording conditions in practical applications.

Holographic Grating Formation in Photochromic Diarylethene-Doped Polymeric Thin Films

We introduce a model to describe real-time grating formation in holographic photochromic diarylethene-doped polymeric thin films. This model, which combines photochromic chemical reaction with the coupled-wave theory, indicates that the grating recording time depends on the molecular absorption coefficient of molecules and the quantum yield of the photochromic reaction at a certain holographic recording intensity. The model is validated by comparing its predictions with the experimental results in which photochromic molecule 1,2-bis(2-methyl-5-(4-formylphenyl)-3-thienyl) perfluorocyclopentene doped PMMA films were used.

Nonlinear Blind Equalization for Volume Holographic Data Storage

We investigate the nonlinear blind equalization for volume holographic data storage channel. Base on the recurrent neural network channel model, we describe a novel blind equalizer for the volume holographic data storage system to improve the bit error rate and hence to make the storage densities achievable. The experimental results also indicate that a significant improvement in the bit error rate to 2.55×10−3 is possible with the nonlinear blind equalization.

Multiplexing Nonvolatile Holograms in LiNbO3:Cu:Ce Crystals

A practical experimental system for two-centre recording by angular multiplexing and 90° geometry is designed with red and blue light. The erasure mechanism of lithium niobate crystals doubly doped with copper and cerium for two-centre recording is discussed and a modified decremental recording schedule for angular multiplexing is proposed subsequently. Finally, 50 nonvolatile holograms with almost the same diffraction efficiency are recorded.

Epoxy Resin-Photopolymer Composite with None-Shrinkage for Volume Holography

A novel photopolymer storage medium for holographic storage is presented. In particular, the refractive index of the monomer methyl methacrylate is much lower than that of the binder epoxy resin solidified by m-xylylenediamine, and the material can be developed into samples with dimension stability and high optical quality and thickness of several millimetres or more. Some none-polymeric macromolecule impurity is uniformly mixed into the medium to aid the monomer diffusing and to enhance the diffraction efficiency. A holographic recording medium of 0.8 mm thickness has been fabricated, and relative theoretical analysis and experiments are presented.

Numerical Analysis of Three-Dimensional Object Recognition with Digital Holography

We describe an intrinsic correlation peak of three-dimensional (3D) pattern recognition, which is higher than that of the two-dimensional (2D) pattern recognition and whose difference can reach up to five orders of magnitude in our numerical simulation. The relationship between the 3D objects and their recognition result is formalized and the comparison between the 3D and 2D pattern recognitions as well as the formers independency of the reconstruction distance is presented. Finally, the augmentation of the 3D pattern recognition sensitivity with the increasing surface complexity of the 3D object is also demonstrated.

Insensitivity of Speckle Multiplexing to Multi-longitudinal Modes of Laser in Volume Holographic Storage

If a diode pumped solid state laser is used in a holographic storage system, its multi longitudinal modes may damage the angular selectivity of the hologram and introduce more cross talk in the system. By theoretical analysis, we found that with adopting the speckle multiplexing scheme, holographic systems are no longer sensitive to the multi longitudinal modes of the laser source, and consequently the damage described above could be well suppressed. Moreover, the following high density storage experimental results also express strong advocacy of this conclusion. This result may greatly prompt the miniaturization of a holographic storage system.

Uniform Diffraction Efficiency for Three-Dimensional Dynamic-Static Speckle Multiplexing Holographic Storage

We analyse and optimize the exposure-schedule of three-dimensional dynamic-static speckle multiplexing (DSSM) schemes. The uniform diffraction efficiency for the DSSM volume holographic storage system is realized in an ion and indium doped LiNbO3 crystal. An overlap-factor matrix is introduced into the system to compensate for the complex erasure effects caused by the static speckle multiplexing. We obtain 100 holograms demonstrating the exposure-schedule theory. The uniformity of the diffraction efficiency is improved by about 8%.