The cavity supports the orthogonal reference beam families as its eigenmodes while enhancing the reference beam power. Such orthogonal eigenmodes are used as additional degree of freedom to multiplex data pages, consequently increase storage densities for volume Holographic Data Storage Systems (HDSS) when the maximum number of multiplexed data page is limited by geometrical factor. Image bearing holograms are multiplexed by orthogonal phase code multiplexing via Hermite-Gaussian eigenmodes in a Fe:LiNbO3 medium with a 532 nm laser at multiple Bragg angles by using Liquid Crystal on Silicon (LCOS) spatial light modulators (SLMs) in reference arms. Total of nine holograms are recorded with three angular and three eigenmode.
Phase retrieval algorithm is gradually applied to the terahertz in-line digital holography, as its effectiveness in the removal of the zero-order diffraction light and twin image. Based on the experiments, the reconstruction results of the double-exposed phase retrieval algorithm are obtained which demonstrate that the method can improve the quality of reconstructed images, compared to the angular spectrum method. Furthermore, the influence of the superimposed frame number of the holograms in double-exposed phase retrieval algorithm is discussed. The result shows that the recording time can be reduced by reasonably decreasing the frames of the holograms and it contributes to real-time imaging and the algorithm practicality is also further improved.
Terahertz (THz) holography is a frontier technology in terahertz imaging field. However, reconstructed images of holograms are inherently affected by speckle noise, on account of the coherent nature of light scattering. Stationary wavelet transform (SWT) is an effective tool in speckle noise removal. In this paper, two algorithms for despeckling SAR images are implemented to THz images based on SWT, which are threshold estimation and smoothing operation respectively. Denoised images are then quantitatively assessed by speckle index. Experimental results show that the stationary wavelet transform has superior denoising performance and image detail preservation to discrete wavelet transform. In terms of the threshold estimation, high levels of decomposing are needed for better denoising result. The smoothing operation combined with stationary wavelet transform manifests the optimal denoising effect at single decomposition level, with 5×5 average filtering.
Proc. SPIE. 9522, Selected Papers from Conferences of the Photoelectronic Technology Committee of the Chinese Society of Astronautics 2014, Part II
KEYWORDS: Image filtering, Electronic filtering, Linear filtering, 3D image reconstruction, Digital holography, Digital filtering, Optical filters, Speckle, Filtering (signal processing), Digital imaging
Terahertz digital holography combines the terahertz technology and digital holography technology at present, fully exploits the advantages in both of them. Unfortunately, the quality of terahertz digital holography reconstruction images is gravely harmed by speckle noise which hinders the popularization of this technology. In this paper, the maximum a posterior estimation (MAP) filter is harnessed for the restoration of the digital reconstruction images. The filtering results are compared with images filtered by Wiener Filter and conventional frequency-domain filters from both subjective and objective perspectives. As for objective assessment, we adopted speckle index (SPKI) and edge preserving index (EPI) to quantitate the quality of images. In this paper, Canny edge detector is also used to outline the target in original and reconstruction images, which then act as an important role in the evaluation of filter performance. All the analysis indicate that maximum a posterior estimation filtering algorithm performs superiorly compared with the other two competitors in this paper and has enhanced the terahertz digital holography reconstruction images to a certain degree, allowing for a more accurate boundary identification.
The phase-only liquid crystal spatial light modulator (SLM) is a real-time electro-optic device capable of modulating the phase of an optical wavefront in space. SLMs have been harnessed for beam shaping. In this paper, an accelerated GS phase retrieval and iteration algorithm is used for designing the SLM phases which transformed a single-mode He-Ne laser into a ring-shaped pattern. These generated ring beams are investigated experimentally and the phenomena of the added prism phases are also observed. The experimental results showed that a given single-mode laser beam could be converted into a ring-shaped intensity distribution which corresponds to our designation.