Sujuan Huang

Hologram-based watermarking capable of surviving print-scan process

We propose a watermarking scheme for hardcopy pictures based on computer-generated holography. A hologram of the watermark is produced using a conjugate-symmetric extension technique, and its spectrum is inserted into the discrete cosine transform domain of the image. Adjusting the watermark placement in a data array, a trade-off between transparency and robustness is achieved. Anticropping and the interference-resisting capability of holograms make the watermark robust against manipulations commonly performed on digital images during postprocessing, including contrast enhancement, moderate smoothing and sharpening, and, in particular, geometric transformation. Most importantly, the proposed hologram-based watermarking can withstand the printing-scanning attack and, therefore, is useful in protecting copyright of digital photographs both as electronic and hardcopy versions.

Research of optical vortex’s energy efficiency and diffraction angle based on spatial light modulator

We systematically study the effects of computer-generated hologram characteristics and spatial light modulator (SLM) performance on the quality of the optical vortex (OV). To improve the energy efficiency of the OV, the blazed fork gratings are introduced to transfer most of the energy to +1 diffraction order of SLM, and the relationship among three kinds of holograms is discussed. A mathematical model, based on the digital blazed grating theory, is proposed to analyze the diffraction angle and energy efficiency of +1 order. The theoretical analysis is further validated by experimental results. The research provides useful guidelines for OV-related applications.

Dimensional properties of Laguerre–Gaussian vortex beams

We derive and analyze the dimensional properties of Laguerre-Gaussian vortex beams theoretically and numerically. The analytical expressions of inner and outer radii are given out and proved to be proportional to the transverse beam size, when the topological charge remains. The ratio of the outer and inner radius only depends on the topological charge, having nothing to do with the waist radius and the propagation distance, and the ratio decreases as the topological charge increases. Using a spatial light modulator loaded with computer-generated holograms, we generate vortex beams. The experimental results are in good agreement with our numerical simulations. This research may provide useful insights into the study of the optical vortex beam and its further applications.