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. Subsequently, we synthesize a complex field using the three raw images. Finally, the retrieved image is calculated using the phase correlation of the complex point spread function. The proposed imaging system is simple and highly cost-effective. Therefore, it is a promising technique that can be adopted for industrial inspection and security screening.

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. We investigate an off-axis approach to OSH, thereby eliminating the use of heterodyning for phase retrieval. We develop optical scanning theory for holographic imaging and show that by properly designing the scanning beam, we can perform coherent and incoherent holographic recording. Simulation results are provided to verify the proposed idea.

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. By down-sampling the multiple micro images with a suitable array sensor, an enlarged image directly appears. Since the imaging module only consists of a low-resolution array sensor and an MLA, the thickness of the camera can be reduced to sub-millimeter. The proposed low-cost slim camera is suitable for imaging and sensing of internet-of-things (IoT) in particular. It also has a great application potential in the imaging of non-visible light.

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. In SDLUT, diffraction fields on the hogel plane of selected reference points in the object space are calculated. Subsequently, the fields are Fourier transformed to the spectrum domain. Because the signal energy always concentrates in a small spectrum region, these regions are cropped as the elemental tables. In the computing of the hogels, the field superposition is conducted in the spectrum domain by using the elemental tables. In our demonstration, the table size of SDLUT is only 0.44% that of the look-up table (LUT). Because the table size is very small, the computing time of SDLUT method can be nearly 80 times faster than that of conventional LUTs in the spatial domain, while the image quality is comparable.

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. The imperfect diffraction property of the polarization directed flat lens results in an incomplete scanning Fresnel zone plate. Consequently, the reconstructed image is blurred and noisy. In this paper, we compared different methods, including the back propagation, the phase correlation, and inverse filtering, for the hologram reconstruction. It is demonstrated that inverse filtering is the only method that can retrieve the high-frequency component of the hologram. However, additional noise also arises with the use of inverse filtering. Therefore, the imaging performance of CSH by using a polarization directed flat lens is inherently worse than that of conventional OSH.

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.

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). In AIA, multiple CGHs of the same object are generated. However, selective sampling points of the same scene are removed according to the reconstructed image of previous binary CGHs as the second and more binary CGHs are generated. Finally, these holograms are sequentially displayed on a fast spatial light modulator, a digital micromirror device (DMD). Thus, a high-quality 3D image is reconstructed without artifacts and speckle noise.

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. The method of digital reconstruction and the properties of the curved digital hologram are then discussed.

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. Furthermore, the weighting coefficients are optimized for each different hologram adaptively. Compared with conventional coefficients, our optimized coefficients can better preserve the fidelity of a reconstructed image after a hologram is binarized.

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. However, DBS is a time-consuming algorithm and thus is impractical for the generation of high-definition computer-generated holograms. In this paper, we proposed an enhanced DBS (E-DBS) method to speed up the hologram computation. E-DBS is based on the same pixelwise evaluation strategy of DBS, but the diffraction field of a single pixel is precomputed as a lookup table. In evaluating any pixel value, only a small area in the region of interest affected by the diffraction field of single pixel is calculated. In addition, it is also found that qualified results can be obtained by using only 4% of the area of the diffraction field. As a result, the computing complexity of E-DBS can be reduced by at least 2 orders of magnitude in contrast to conventional DBS.

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. In polygon-based CGH, however, occlusion processing is difficult and time-consuming work. In this paper, we proposed a simple and fast occlusion processing method, the slice-by-slice silhouette (S3) method, for generating the occlusion effect in polygon-based CGH. In the S3 method, the polygonal model is sliced into multiple thin segments. For each segment, a silhouette mask is generated and located at the backside of the segment. The incident light is first shaded by the mask and superimposes on the light emitted from the facets of the evaluated segment. In this way, every segment can be processed sequentially to get the resulting object light. Our experimental result demonstrates that the S3 method can generate a high-definition hologram with qualified occlusion effect. The computing complexity of the S3 method is lower than that of previous methods. In addition, the S3 method can be parallelized easily, and thus can be further speeded up by applying a parallel computing framework, such as multi-core CPU or GPU.

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. A spatial light modulator is used to reconstruct the phase-only hologram optically to demonstrate the validity of our proposed method.

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. For the first time, to the best of our knowledge, we have demonstrated coherent, partial coherent, and incoherent three-dimensional (3D) imaging by experiment in such a single-pixel digital holographic recording system. We have found, for the incoherent mode of OSH, in which the detector of the largest active area is applied, the 3D location of a diffusely reflecting object can be successfully retrieved without speckle noise. For the partial coherent mode employing a smaller pixel size of the detector, significant speckles and randomly distributed bright spots appear among the reconstructed images. For the coherent mode of OSH when the size of the pixel is vanishingly small, the bright spots disappear. However, the speckle remains and the signal-to-noise ratio is low.

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. OSH in both coherence modes has been well investigated. However, there is no discussion on the case between the coherent mode and incoherent mode, namely, the partial-coherent mode. In this paper, we derived for the first time, to the best of our knowledge, the formula of OSH in various modes of coherence. We found the detector in OSH plays the role of a kind of filter for the field. The retrieved amplitude transparency of the object is thus nonlinearly processed by the mask function of the detector. Consequently, the reconstructed image cannot benefit from the implementation of the partial-coherent mode. On the contrary, significant artifacts usually appear among the reconstructed image and thus the image quality degrades.

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. The feature issue includes 27 papers and covers a large range of topics, reflecting the rapidly expanding techniques and applications of digital holography and 3D imaging. The DH meeting will continue in the future, as expected, and the next meeting is scheduled to be held on 24-28 May 2015, at Shanghai Institute of Optics and Fine Mechanics, Shanghai, China.

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). The cross-sectional amplitude and phase maps of an interferogram are retrieved by using a three-step phase-shifting technique. The LCPS consists of three identical nematic liquid-crystal (NLC) cells sandwiched between two quarter-wave plates so that it functions as a typical quarter-half-quarter phase shifter. Instead of using high-cost NLC cells with precise thickness of half-wave retardation, a method is proposed to operate thicker NLC cells without scarifying the axial resolution. Experimental results reveal that the FFOCM is able to perform three-dimensional micrometer-resolution imaging.

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. In this paper we propose a mask-shifting (MS) method to generate the sampled FZPs. The main concept of the MS method is that the center of the FZP can be shifted relative to the center of the mask against the FZP when the FZP is at the margin of the hologram. The shifting of the mask will result in only a phase shift and will not change the intensity distribution of the reconstructed point. Thus, by using the MS method, aliasing and vignetting are simultaneously alleviated in any combination of object size and object 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. The size and the acquisition time of the VBL hologram can be reduced by one order of magnitude while the horizontal resolution remains the same as the conventional scanning hologram. The VBL hologram can be coded as an off-axis hologram without any postfiltering. Meanwhile, the full horizontal bandwidth of the displaying device can be capitalized.

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. However, we found that the free-space point spread function in the HCM is low-pass filtered when the propagation distance is long. As a result, it is recommended to always use the ASM in conjunction with sufficient zero-padding for the digital calculation of the diffraction field. The FDI can be directly applied to a long-distance propagation without the necessity of the zero-padding, provided only the intensity is of interest. If the phase of the diffraction field is important, the zero-padding is necessary and the propagation distance is severely restricted.

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. The main advantages include (1) a much higher allowance to increase in the depth measuring range, (2) easiness to compensate perspective distortion and geometric distortion, (3) very high accuracy (in the micron-range) and (4) only one phase measurement needed for operation.