Single-pixel three-dimensional imaging of the terahertz-wave by complex-field synthesis.

We propose a novel three-dimensional (3D) imaging technique by terahertz (THz) waves. Specifically, we modulate the THz wave using diffusers to produce three different speckle-like illumination patterns. The object is raster scanned by the three illumination patterns to generate three raw images via the single-pixel detection method.

Off-axis optical scanning holography [Invited].

Optical scanning holography (OSH) involves the principles of optical scanning and heterodyning. The use of heterodyning leads to phase-preserving, which is the basic idea of holography. While heterodyning has numerous advantages, it requires complicated and expensive electronic processing.

Down-sampling slim camera using a micro-lens array.

The thickness of a camera is proportional to the image distance, although the lens can be replaced by a flat optics, such as a meta lens. However, there is no suitable method to reduce this thickness for low-cost applications. Here we proposed a novel down-sampling slim camera based on a micro-lens array (MLA) and an array sensor.

Unconventional Imaging: introduction to the feature issue.

This feature issue of Applied Optics is dedicated to the international meeting of Information Photonics 2020 (IP'20), which was held September 11-12, 2020, in Taipei, Taiwan. IP'20 covered a broad range of topics, including advanced display techniques, optical computing, and optical storage. This feature issue, however, limits topics to unconventional imaging techniques, such as digital holography, artificial-intelligence associated imaging, compressive imaging, and single-pixel imaging.

Optical scanning holography with a polarization directed flat lens.

Recently, an optical scanning holographic system with a polarization directed flat lens was proposed to realize coaxial scanning holography (CSH). The advantage of CSH is its small form factor and the stability. However, the diffraction efficiency of the polarization directed flat lens cannot be 100%, and thus there is always zeroth order light in the scanning beam.

Fast calculation of high-definition depth-added computer-generated holographic stereogram by spectrum-domain look-up table [Invited].

High-definition depth-added computer-generated holographic stereogram (DA-CGHS) is superior in its high quality, easy realization, and auto-shading effect. However, its computing cost is extremely high because numerous scenes together with depth information must be calculated. Here, we proposed a fast calculation scheme of DA-CGHS by the spectrum-domain look-up table (SDLUT) method.

3D display by binary computer-generated holograms with localized random down-sampling and adaptive intensity accumulation.

In this paper, we proposed a new technique to realize a high-quality three-dimensional (3D) display by using binary holograms. First, we applied a localized random down-sampling (LRDS) mask to down-sample the object function and generated a binary CGH by direct sign-thresholding. Subsequently, we devised the display by adaptive intensity accumulation (AIA).

Recording of a curved digital hologram for orthoscopic real image reconstruction.

Based on the concept of optical scanning holography, a holographic system for recording a curved digital hologram was proposed and demonstrated. In the system, an interference beam without the object information is first generated and then used to two-dimensionally scan a three-dimensional object along a cylindrical path. As a result, a complex curved hologram of a real object is digitally holographically recorded for the first time, to the best of our knowledge.

Error diffusion method with optimized weighting coefficients for binary hologram generation.

The error diffusion method can effectively reduce quality degradation by propagating thresholding errors to neighboring pixels in the conversion of a gray-scale hologram to a binary hologram. In previous works, the four weighting coefficients in error diffusion are mostly set as the Floyd-Steinberg coefficient, which was determined empirically and originally proposed for photograph processing. In this work, we point out that the Floyd-Steinberg coefficients can be suboptimal for hologram error diffusion binarization.

Enhanced direct binary search algorithm for binary computer-generated Fresnel holograms.

The direct binary search (DBS) algorithm was originally invented for the synthesis of a binary Fourier hologram, and was applied for the generation of a binary Fresnel hologram recently. DBS performs quality evaluation on every pixel. Therefore, both the quality and diffraction efficiency of the generated binary hologram are better among various algorithms of the binary hologram.

Fast occlusion processing for a polygon-based computer-generated hologram using the slice-by-slice silhouette method.

In a polygon-based computer-generated hologram (CGH), the three-dimensional (3D) model is represented as a polygon, which consists of numerous small facets. Lighting effect, material texture, and surface property can be included in the polygonal model, which enables polygon-based CGH to realize high-fidelity 3D display. On the other hand, the occlusion effect is an important depth cue for 3D display.

Nonlinearity compensation and complex-to-phase conversion of complex incoherent digital holograms for optical reconstruction.

Incoherent digital holography (IDH) can be realized by optical scanning holography or self-interference incoherent holography. Although IDH can exhibit high quality reconstruction due to its inherently speckle-free property, direct display of an incoherent hologram is a challenge because of its amplitude nonlinearity and the demand of complex modulation. In this paper we propose to compensate the amplitude nonlinearity at the object plane, and use bidirectional error-diffusion method to convert the complex-type incoherent Fresnel hologram to a phase-only Fresnel hologram for display.

Coherence experiments in single-pixel digital holography.

In optical scanning holography (OSH), the coherence properties of the acquired holograms depend on the single-pixel size, i.e., the active area of the photodetector.

High-axial-resolution, full-field optical coherence microscopy using tungsten halogen lamp and liquid-crystal-based achromatic phase shifter.

A high-axial-resolution, full-field optical coherence microscope (FFOCM) for topography and tomography applications is presented. The FFOCM is based on a polarization Linnik interference microscope equipped with a tungsten halogen lamp. The phase difference between the reference and test beams in the microscope is precisely and quickly shifted by using an achromatic liquid-crystal phase shifter (LCPS).

Feature issue of digital holography and 3D imaging (DH): introduction.

The OSA Topical Meeting "Digital Holography and 3D Imaging (DH)" was held in Seattle, Washington, 13-17 July 2014. Feature issues based on the DH meeting series have been released by Applied Optics (AO) since 2007. In this year (2014), Optics Express (OE) and AO jointly decided to have one such feature issue in each journal.

Spatial coherence analysis for optical scanning holography.

In optical scanning holography (OSH), the system can be operated in coherent mode by using a pinhole detector, or in incoherent mode by using a spatially integrating detector. In the coherent mode, the three-dimensional (3D) amplitude transparency of an object is recorded and thus the phase of the object can be retrieved. On the other hand, it is the 3D intensity transparency of the object recorded in the incoherent mode and thus the speckle can be suppressed.

Vignetting-free computer-generated Fresnel holograms by mask shifting.

In computer-generated Fresnel holography, direct sampling (DS) and simple shading (SS) are two common ways to generate sampled Fresnel zone plates (FZPs) on the hologram plane. Nevertheless, either aliasing or vignetting, or both, will occur in the reconstructed image when the DS method or the SS method is applied. To avoid vignetting together with aliasing in the two sampling methods, either the object size or the object distance must be restricted in generating the holograms.

Controlling the aliasing by zero-padding in the digital calculation of the scalar diffraction.

The well-sampling conditions for the digital calculations of the scalar diffraction, including the Huygens convolution method (HCM), the angular spectrum method (ASM), and the Fresnel diffraction integral (FDI), were discussed. We found the aliasing always occurs unless proper zero-padding--that is, to pad zero-value pixels around the initial field--is applied prior to the simulation of the diffraction. From the aspect of well-sampling, the ASM is applicable to a short propagation distance, while the HCM is applicable to a long propagation distance.

Vertical-bandwidth-limited digital holography.

Optical scanning holography (OSH) is a promising technique to acquire a big-size digital hologram. However, the acquisition speed is limited by the mechanical scanner. In this Letter we apply the OSH in conjunction with an anisotropic low-pass filtering pupil to acquire vertical-bandwidth-limited (VBL) holograms.

Complex Fresnel hologram display using a single SLM.

We propose a novel optical method to display a complex Fresnel hologram using a single spatial light modulator (SLM). The method consists of a standard coherent image processing system with a sinusoidal grating at the Fourier plane. Two or three position-shifted amplitude holograms displayed at the input plane of the processing system can be coupled via the grating and will be precisely overlapped at the system's output plane.

Comparison of two-, three-, and four-exposure quadrature phase-shifting holography.

In standard (four-exposure) quadrature phase-shifting holography (QPSH), two holograms and two intensity maps are acquired for zero-order-free and twin-image-free reconstruction. The measurement of the intensity map of the object light can be omitted in three-exposure QPSH. Furthermore, the measurements of the two intensity maps can be omitted in two-exposure QPSH, and the acquisition time of the overall holographic recording process is reduced.

Projected fringe profilometry using a liquid-crystal spatial light modulator to extend the depth measuring range.

An approach using a liquid-crystal spatial light modulator (LC-SLM) to enlarge the depth measuring range of the projected fringe profilometry is presented. This approach is especially applicable to detect dynamic objects with micro-scale sizes. Compared with a typical 2D image system, the LC-SLM provides a better performance for a 3D shape sensing system.

Three-dimensional complex image coding using a circular Dammann grating.

Recently, optical image coding using a circular Dammann grating (CDG) has been proposed and investigated. However, the proposed technique is intensity based and could not be used for three-dimensional (3D) image coding. In this paper, we investigate an optical image coding technique that is complex-amplitude based.

Angle-multiplexed holographic data storage with minimum cross talk noise.

The cross talk noise-to-signal ratio (NSR) of an angle-multiplexed holographic data storage system is studied, and we propose a method to determine the optimized multiplexing spacing with which the cross talk noise can be less than the conventional method. In our method, the optimization location at the image plane can be chosen arbitrarily, so the multiplexing of asymmetrical image patterns can be optimized. In particular, we investigate the 90° scheme and the transmission scheme angle multiplexing.

Fast generation of Fresnel holograms based on multirate filtering.

One of the major problems in computer-generated holography is the high computation cost involved for the calculation of fringe patterns. Recently, the problem has been addressed by imposing a horizontal parallax only constraint whereby the process can be simplified to the computation of one-dimensional sublines, each representing a scan plane of the object scene. Subsequently the sublines can be expanded to a two-dimensional hologram through multiplication with a reference signal.