Showing papers in "Applied Optics in 2019"
TL;DR: The past, present, and future industry prospects of virtual reality (VR) and augmented reality (AR) are presented and the future of VR/AR technology based on holographic display is predicted by analogy with theVR/AR based on binocular vision display and light field display.
Abstract: The past, present, and future industry prospects of virtual reality (VR) and augmented reality (AR) are presented. The future of VR/AR technology based on holographic display is predicted by analogy with the VR/AR based on binocular vision display and light field display. The investigations on holographic display that can be used in VR/AR are reviewed. The breakthroughs of holographic display are promising in VR/AR with high resolution. The challenges faced by VR/AR based on holographic display are analyzed.
105 citations
TL;DR: An architecture that utilizes a single electrical to optical conversion by designing a free-space optical frontend unit that implements the linear operations of the first layer with the subsequent layers realized electronically.
Abstract: The parallelism of optics and the miniaturization of optical components using nanophotonic structures, such as metasurfaces, present a compelling alternative to electronic implementations of convolutional neural networks. The lack of a low-power optical nonlinearity, however, requires slow and energy-inefficient conversions between the electronic and optical domains. Here, we design an architecture that utilizes a single electrical to optical conversion by designing a free-space optical frontend unit that implements the linear operations of the first layer with the subsequent layers realized electronically. Speed and power analysis of the architecture indicates that the hybrid photonic-electronic architecture outperforms a fully electronic architecture for large image sizes and kernels. Benchmarking of the photonic-electronic architecture on a modified version of AlexNet achieves high classification accuracies on images from the Kaggle's Cats and Dogs challenge and MNIST databases.
81 citations
TL;DR: The proposed microchannel incorporated photonic crystal fiber-based surface plasmon resonance (SPR) sensor for detection of low refractive index (RI) at near-infrared wavelength would be an excellent candidate for a wide range of RI detection with higher accuracy for applications such as pharmaceutical inspection and leakage monitoring, bio-sensing, and other low RI analytes.
Abstract: A microchannel incorporated photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor for detection of low refractive index (RI) at near-infrared wavelength is presented in this paper. To attain a simple and practically feasible mechanism, plasmonic material gold (Au) and sensing medium are placed outside the fiber. A thin layer of TiO2 is employed as an adhesive layer to strongly attach the Au with the silica glass. In the sensing range of 1.22 to 1.37, maximum sensitivities of 51,000 nm/RIU (RI unit) and 1872 RIU−1 are obtained with resolutions of 1.96×10−6 and 9.09×10−6 RIUs using wavelength and amplitude interrogation methods, respectively. To the best of the authors’ knowledge, the obtained maximum wavelength sensitivity and resolution are the highest among reported PCF-based SPR sensors to date. The sensor also exhibits a maximum figure of merit of 566. Therefore, the proposed sensor would be an excellent candidate for a wide range of RI detection with higher accuracy for applications such as pharmaceutical inspection and leakage monitoring, bio-sensing, and other low RI analytes.
79 citations
TL;DR: This paper addresses the challenging problem of determining the in-focus reconstruction depth of Madin-Darby canine kidney cell clusters encoded in digital holograms by addressing the challenging issue of deep convolutional neural network learning.
Abstract: Deep artificial neural network learning is an emerging tool in image analysis. We demonstrate its potential in the field of digital holographic microscopy by addressing the challenging problem of determining the in-focus reconstruction depth of Madin-Darby canine kidney cell clusters encoded in digital holograms. A deep convolutional neural network learns the in-focus depths from half a million hologram amplitude images. The trained network correctly determines the in-focus depth of new holograms with high probability, without performing numerical propagation. This paper reports on extensions to preliminary work published earlier as one of the first applications of deep learning in the field of digital holographic microscopy.
68 citations
TL;DR: A highly sensitive temperature sensor is demonstrated experimentally, which is fabricated based on a Fabry-Perot interferometer filled with polydimethylsiloxane (PDMS) and exhibits a high temperature sensitivity of 17.758 nm/°C.
Abstract: A highly sensitive temperature sensor is demonstrated experimentally, which is fabricated based on a Fabry–Perot interferometer (FPI) filled with polydimethylsiloxane (PDMS). The sensor’s sensitivity is −0.653 nm/°C by utilizing the thermal expansion effect of PDMS, which has been greatly improved compared to that of the traditional FPI temperature sensor. Moreover, in order to further improve the sensitivity, a scheme where two parallel FPI structures are used to form the Vernier effect is proposed, which are a sensing FPI and reference FPI, respectively. Such a temperature sensor based on the FPI filled with PDMS and the Vernier effect exhibits a high temperature sensitivity of 17.758 nm/°C. Meanwhile, the proposed sensors show the advantages of high sensitivity, simplicity, and low cost.
60 citations
TL;DR: This paper investigates the influence of lateral misalignment, i.e., the distance between the beam axis of a probe light and the rotationaxis of a spinning object, on the rotational Doppler effect and deduces the generalized formula of rotational doppler shift based on a local scattering model.
Abstract: The discovery of the optical rotational Doppler effect associated with orbital angular momentum of light paves a new way to detect the rotational speed of spinning objects. In this paper, we investigate the influence of lateral misalignment, i.e., the distance between the beam axis of a probe light and the rotation axis of a spinning object, on the rotational Doppler effect. First, we analyze the mechanism of the rotational Doppler effect of optical vortices based on the linear Doppler effect. Specifically, we consider the general case where the center of the optical vortex does not coincide with the rotation axis, and deduce the generalized formula of rotational Doppler shift based on a local scattering model. It is found that the bandwidth of the rotational Doppler signal depends proportionally on the amount of lateral misalignment, whereas the value of rotational Doppler shift remains constant. A proof-of-concept experiment is performed, and the measured results agree well with theoretical predictions. These findings may be useful for practical application of the optical rotational Doppler effect in remote sensing and metrology.
53 citations
TL;DR: A fusion algorithm based on multi-level Gaussian curvature filtering (MLGCF) image decomposition is proposed that performs effectively in most cases by subjective and objective evaluations and even exceeds many high-level fusion algorithms.
Abstract: The aim of infrared and visible image fusion is to obtain an integrated image that contains obvious object information and high spatial resolution background information. The integrated image is more conductive for a human or a machine to understand and mine the information contained in the image. To attain this purpose, a fusion algorithm based on multi-level Gaussian curvature filtering (MLGCF) image decomposition is proposed. First, a MLGCF is presented and employed to decompose the input source images into three different layers: small-scale, large-scale, and base layers. Then, three fusion strategies—max-value, integrated, and energy-based—are applied to combine the three types of layers, which are based on the different properties of the three types of layers. Finally, the fusion image is reconstructed by summing the three types of fused layers. Six groups of experiments demonstrate that the proposed algorithm performs effectively in most cases by subjective and objective evaluations and even exceeds many high-level fusion algorithms.
53 citations
TL;DR: A two-dimensional photonic-crystal-based structure was used for designing a novel structure for realizing an all-optical half-subtractor that works completely in the linear region, and therefore it does not require a high amount of optical power.
Abstract: A two-dimensional photonic-crystal-based structure was used for designing a novel structure for realizing an all-optical half-subtractor. The proposed structure was designed by combining the phase shift keying technique with the optical beam interference mechanism. The proposed structure works completely in the linear region, and therefore it does not require a high amount of optical power. The delay time for the proposed structure is about 2 ps.
52 citations
TL;DR: This work uses a generalized approach for generating long-range self-healing beams with a phase-only element encoded on a spatial light modulator and imbue them with OAM to present an alternative orbital angular momentum (OAM) solution for free-space optical communications in the form of shape-invariant Bessel beams.
Abstract: We present an alternative orbital angular momentum (OAM) solution for free-space optical communications in the form of shape-invariant Bessel beams. We use a generalized approach for generating these long-range self-healing beams with a phase-only element encoded on a spatial light modulator and imbue them with OAM. We study the performance of helical OAM beams as well as these long-range Bessel-like OAM beams over a real-world outdoor optical link of 150 m and show comparable performance. In the process, we characterize the link and its impact on modal cross-talk.
50 citations
TL;DR: A photonic crystal (PhC)-based structure was proposed for designing a 1-bit all-optical comparator using nonlinear ring resonators inside PhCs and the maximum rise time obtained was about 4 ps.
Abstract: Optical comparators can play crucial roles in the all-optical digital circuits required for optical calculation and optical processing. A typical digital comparator is a logic circuit that can compare two binary codes and show the results by activating one of its three output ports. In this paper, a photonic crystal (PhC)-based structure was proposed for designing a 1-bit all-optical comparator. The proposed structure was designed using nonlinear ring resonators inside PhCs. Optical intensity of ${1}\,\,{\rm W/}\unicode{x00B5} {{\rm m}^2}$1W/µm2 at the input ports is required for the proper operation of the proposed structure. For the final structure, the maximum rise time obtained was about 4 ps.
49 citations
TL;DR: This research aims to reduce the size of the HUD footprint while offering a wide FOV projected in the far field with an expanded pupil, by coupling the image-bearing light into a waveguide under total internal reflection conditions, redirecting that light in the orthogonal direction, and outcoupling the light toward the pilot.
Abstract: Head-up displays (HUDs) are used in aircraft to overlay relevant flight information on the vehicle’s externals for pilots to view with continued focus on the far field. In these systems, the field of view (FOV) is traditionally limited by the size of the projection optics. Though classical HUD systems take a significant amount of space in the flight deck, they have become a necessity in avionic transportation. Our research aims to reduce the size of the HUD footprint while offering a wide FOV projected in the far field with an expanded pupil. This has been accomplished by coupling the image-bearing light into a waveguide under total internal reflection conditions, redirecting that light in the orthogonal direction, and then outcoupling the light toward the pilot. Each step was achieved using holographic optical elements. The injection hologram has optical power to obtain longitudinal magnification, whereas the redirection hologram expands the pupil in one dimension and the extraction hologram expands the pupil in a second dimension. Varying diffraction efficiency along the direction of the light propagation ensures even image intensity throughout the expanded pupil. We used ray tracing optical simulations to optimize the design of the system and present a fully operational demonstrator of the HUD. This HUD produces an image with a FOV of 24°×12.6° at a viewing distance of 4.5 in. (114 mm) from the waveguide, with infinite longitudinal magnification and 1.9× by 1.6× horizontal and vertical pupil expansion, respectively.
TL;DR: Experimental results on real THz images show that the proposed method significantly improves the quality ofTHz images with increased resolution and decreased noise, which proves the practicability and exactitude of the proposedmethod.
Abstract: We propose an effective and robust method for terahertz (THz) image super-resolution based on a deep convolutional neural network (CNN). A deep CNN model is designed. It learns an end-to-end mapping between the low- and high-resolution images. Blur kernels with multiple width and noise with multiple levels are taken into the training set so that the network can handle THz images very well. Quantitative comparison of the proposed method and other super-resolution methods on the synthetic THz images indicates that the proposed method performs better than other methods in accuracy and visual improvements. Experimental results on real THz images show that the proposed method significantly improves the quality of THz images with increased resolution and decreased noise, which proves the practicability and exactitude of the proposed method.
TL;DR: Experimental results show remarkable performance improvement of the proposed method over conventional linear equalizers, and significant enhancement at high launch power compared with Volterra filtering.
Abstract: This paper proposes a nonlinear equalization technique enabled by long short-term memory (LSTM) recurrent neural networks. The proposed technique is implemented at the end of offline digital signal processing. And two approaches utilizing the LSTM network are experimentally tested and demonstrated in transmission of a 50-Gb/s four-level pulse amplitude modulation intensity modulation direct detection link over 100-km standard single-mode fiber. The first approach uses the LSTM network-based equalizer to directly categorize the received signal into four amplitude levels, and the second approach uses the LSTM network to estimate signal noise for compensating the received signal. The experimental results show remarkable performance improvement of the proposed method over conventional linear equalizers, and significant enhancement at high launch power compared with Volterra filtering. Also, the proposed method reveals better short-time universality.
TL;DR: In this method, the system FOV is expanded stepwise in the design process, and the surface optical freeform polynomial terms are extended based on the judgment of image quality and some constraint conditions, and to obtain a prospective ultrawide FOV system.
Abstract: Unobscured reflective optical systems with a wide field of view (FOV) have significant application values. However, the aberration increases with the increase of the system FOV, so a wide FOV system is difficult to design. In this paper, a design method that is effective in achieving off-axis three-mirror systems with ultrawide FOV is proposed. In this method, the system FOV is expanded stepwise in the design process, and the surface optical freeform polynomial terms are extended based on the judgment of image quality and some constraint conditions, and to obtain a prospective ultrawide FOV system. A freeform off-axis three-mirror imaging system with a focal length of 1000 mm, an F-number of 10, and an ultrawide FOV of 80°×4° is designed as an example. This design result shows that the system has a high imaging quality of RMS wavefront error value of 0.040λ(λ=0.633 μm), and it demonstrates that the method is effective in achieving off-axis three-mirror systems with an ultrawide FOV.
TL;DR: The technique was implemented for measuring and analyzing combined beams with OAM anomalies and it was found that the dips and bursts in the OAM spectrum are caused by the vortex avalanche induced by weak perturbations of the holographic grating responsible for shaping the beam.
Abstract: A new technique for measuring the degenerate spectra of optical vortices and orbital angular momentum (OAM) of singular beams is theoretically studied and experimentally verified. The technique is based on measuring the intensity moments of higher orders of a beam containing vortices with both positive and negative topological charges. The appropriate choice of the vortex mode amplitudes of the combined beam forms anomalous regions in the form of resonant dips and bursts in the OAM spectrum. Since the intensity moments for vortices with positive and negative topological charges are the same (degenerate) for an axially symmetric beam, it was necessary to break the symmetry of the beam, so measurements were taken at the plane of the double focus of a cylindrical lens. The calibration measurements showed that the experimental error is not higher than 3.5%. The technique was implemented for measuring and analyzing combined beams with OAM anomalies. It was found that the dips and bursts in the OAM spectrum are caused by the vortex avalanche induced by weak perturbations of the holographic grating responsible for shaping the beam. The OAM dips or bursts are controlled by the ratio between the energy fluxes of the vortex avalanche with positive or negative topological charges.
TL;DR: The authors' simulations reveal that the Fano resonance at longer wavelengths can be used for increasing the sensitivity and FOM and this absorber is an appropriate component for the design of highly sensitive optical sensors.
Abstract: In this paper, we present various optical metamaterial nanoabsorbers with the aim of improving the refractive index sensitivity using the Fano response. The proposed absorbers consist of various parasitic elements such as single cross, broken cross, Jerusalem cross, and also single L and double L models. We numerically study their absorption and reflection using the three-dimensional finite-difference time-domain method and calculate the sensitivity and figure of merit (FOM) in every absorption peak (reflection dip). Our simulations reveal that the Fano resonance at longer wavelengths can be used for increasing the sensitivity and FOM. The proposed absorbers have been coated with an external material with a maximum thickness of 100 nm and refractive indices in the range of 1.05–1.2. We also study and compare the FOM for these structures. They are modified for 900 and 1200–1300 nm. The maximum FOM is achieved around 2400 RIU−1 for the double L nanoabsorber coated with 1.05 indexed material, while its sensitivity is 473 nm/RIU. This absorber is an appropriate component for the design of highly sensitive optical sensors.
TL;DR: This work presents a simple design for the detection and sensing of protein using one-dimensional defective photonic crystals and uses the characteristic matrix method and curve fitting to detect the concentration of a protein solution using an efficient, accurate, and simple method.
Abstract: The sensing of protein is of great importance because of its prominent role in building and repairing tissues. In this work, we present a simple design for the detection and sensing of protein using one-dimensional defective photonic crystals. The main idea of our work is included in the theoretical investigation of the transmittance properties of the resonant mode produced inside the photonic band gap. Our study uses the characteristic matrix method and curve fitting. The main reason for our study is to detect the concentration of a protein solution using an efficient, accurate, and simple method. Here, the defect layer is filled with a protein solution. Our idea depends on two hypotheses, and the first one is based on the appearance of a resonant peak on the photonic band gap. The second one depends on a change in the position of this resonant peak with the concentration of the protein solution. The effect of many parameters on the performance of our sensor, such as the thickness of the defect layer and the sensitivity, is demonstrated. The numerical results could present a simple way to design an accurate, stable, efficient, and low-cost protein sensor compared to other current methods and techniques.
TL;DR: This work develops a phase retrieval algorithm that utilizes two DNNs together with the model-based HIO method that not only achieves state-of-the-art reconstruction performance but also is more robust to different initialization and noise levels.
Abstract: The classical phase retrieval problem is the recovery of a constrained image from the magnitude of its Fourier transform. Although there are several well-known phase retrieval algorithms, including the hybrid input-output (HIO) method, the reconstruction performance is generally sensitive to initialization and measurement noise. Recently, deep neural networks (DNNs) have been shown to provide state-of-the-art performance in solving several inverse problems such as denoising, deconvolution, and superresolution. In this work, we develop a phase retrieval algorithm that utilizes two DNNs together with the model-based HIO method. First, a DNN is trained to remove the HIO artifacts, and is used iteratively with the HIO method to improve the reconstructions. After this iterative phase, a second DNN is trained to remove the remaining artifacts. Numerical results demonstrate the effectiveness of our approach, which has little additional computational cost compared to the HIO method. Our approach not only achieves state-of-the-art reconstruction performance but also is more robust to different initialization and noise levels.
TL;DR: Collinear phase-shifting holography is demonstrated for measuring complex optical modes of twisted light beams with orbital angular momentum (OAM) generated by passing a laser through a spatial light modulator (SLM).
Abstract: We demonstrate collinear phase-shifting holography for measuring complex optical modes of twisted light beams with orbital angular momentum (OAM) generated by passing a laser through a spatial light modulator (SLM). This technique measures the mode along the direction of propagation from the SLM and requires no additional optics, so it can be used to aid alignment of the SLM, to efficiently check for the effects of beam wander, and to fully characterize generated beams before use in other experiments. Optimized error analysis and careful SLM alignment allow us to generate and measure OAM with purity as high as 99.9%.
TL;DR: A compact polarization beam splitter based on a gold-filled photonic crystal fiber with a square lattice is proposed, which may be a promising candidate in communication due to its ultra-short length and ultra-broad bandwidth.
Abstract: A compact polarization beam splitter based on a gold-filled photonic crystal fiber with a square lattice is proposed. The full vector finite element method is used to design and characterize the proposed ultra-compact and ultra-broadband polarization splitter. The plasmonic plays an important role in order to achieve an ultra-short length of 56.33 µm with a high extinction ratio of 132.92 dB at the wavelength of 1.55 µm. It can ensure an ultra-broad bandwidth of 530 nm, from 1225 to 1755 nm, covering all the communication bands with an extinction ratio better than 20 dB. The proposed polarization splitter may be a promising candidate in communication due to its ultra-short length and ultra-broad bandwidth.
TL;DR: The results of computational studies show that the variations in diagnostic volumes due to arterial pulse wave are notably (up to 45%) different in visible and near-infrared spectral ranges in both transmitted and back-scattered probing geometries.
Abstract: Recent advances in the development of ultra-compact semiconductor lasers and technology of printed flexible hybrid electronics have opened broad perspectives for the design of new pulse oximetry and photoplethysmography devices. Conceptual design of optical diagnostic devices requires careful selection of various technical parameters, including spectral range; polarization and intensity of incident light; actual size, geometry, and sensitivity of the detector; and mutual position of the source and detector on the surface of skin. In the current study utilizing a unified Monte Carlo computational tool, we explore the variations in diagnostic volume due to arterial blood pulsation for typical transmitted and back-scattered probing configurations in a human finger. The results of computational studies show that the variations in diagnostic volumes due to arterial pulse wave are notably (up to 45%) different in visible and near-infrared spectral ranges in both transmitted and back-scattered probing geometries. While these variations are acceptable for relative measurements in pulse oximetry and/or photoplethysmography, for absolute measurements, an alignment normalization of diagnostic volume is required and can be done by a computational approach utilized in the framework of the current study.
TL;DR: A class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera are reviewed.
Abstract: Imaging with THz radiation has proved an important tool for both fundamental science and industrial use. Here we review a class of THz imaging implementations, named coherent lensless imaging, that reconstruct the coherent response of arbitrary samples with a minimized experimental setup based only on a coherent source and a camera. After discussing the appropriate sources and detectors to perform them, we detail the fundamental principles and implementations of THz digital holography and phase retrieval. These techniques owe a lot to imaging with different wavelengths, yet innovative concepts are also being developed in the THz range and are ready to be applied in other spectral ranges. This makes our review useful for both the THz and imaging communities, and we hope it will foster their interaction.
TL;DR: The experimental results show that the proposed method can obtain desired results even when the quality of ESPI fringe images is considerably low because of variable density, high noise, and low contrast, and the method can denoise multi-frame fringe patterns simultaneously.
Abstract: The denoising of electronic speckle pattern interferometry (ESPI) fringe patterns is a key step in the application of ESPI. In this paper, we propose a method for batch denoising of ESPI fringe patterns based on a convolution neural network (CNN). In the proposed method, the network is first trained by our training dataset, which consists of the noisy ESPI fringe patterns and the corresponding noise-free images. We propose a new computer-simulated method of ESPI fringe patterns to create our training dataset. After training, the other multi-frame ESPI fringe patterns to be processed are fed to the trained network simultaneously, and the corresponding denoising images can be obtained in batches. We demonstrate the performance of the proposed method via application to 50 computer-simulated ESPI fringe patterns and three groups of experimentally obtained ESPI fringe patterns. The experimental results show that our method can obtain desired results even when the quality of ESPI fringe images is considerably low because of variable density, high noise, and low contrast, and our method can denoise multi-frame fringe patterns simultaneously. Moreover, we use the computer-simulated ESPI fringe patterns to train the network; after training, the trained network can be used to denoise either computer-simulated ESPI fringe patterns or the experimentally obtained ESPI fringe patterns. The proposed method is especially suitable for processing a large number of ESPI fringe patterns.
TL;DR: An overview of spatial resolution criteria in classical optics, digital optics, and holography is presented and it is shown that, for coherent light, the classical Abbe and Rayleigh resolution criteria do not provide accurate estimation of the lateral and axial resolution.
Abstract: This work presents an overview of spatial resolution criteria in classical optics, digital optics, and holography. Although the classical Abbe and Rayleigh resolution criteria have been thoroughly discussed in the literature, there are a few issues that still need to be addressed, e.g., the axial resolution criteria for coherent and incoherent radiation (which is a crucial parameter in 3D imaging), the resolution criteria in the Fresnel regime, and the lateral and the axial resolution criteria in digital optics and holography. This work discusses these issues and provides a simple guide on which resolution criteria should be applied for a particular imaging scheme: coherent/incoherent, far- and near-field, lateral and axial resolution. Different resolution criteria such as two-points resolution and the resolution obtained from the image spectrum (diffraction pattern) are compared and demonstrated with simulated examples. It is shown that, for coherent light, the classical Abbe and Rayleigh resolution criteria do not provide accurate estimation of the lateral and axial resolution. The lateral and axial resolution criteria based on the evaluation of the spectrum of the diffracted wave provide more precise estimation of the resolution for coherent and incoherent light. It is also shown that the resolution criteria derived in approximation of the far-field can be applied for the near-field (Fresnel) imaging regime.
TL;DR: A chip-scale refractive index sensor based on multiple Fano resonances based on a metal-insulator-metal (MIM) structure, which is constructed by two side-coupled semi-ring cavities and a vertical cavity, which may have great applications in highly integrated photonic circuits.
Abstract: A chip-scale refractive index sensor based on multiple Fano resonances is proposed by using a metal-insulator-metal (MIM) structure, which is constructed by two side-coupled semi-ring cavities and a vertical cavity. The finite-difference time-domain method and multimode interference coupled-mode theory are employed to simulate and analyze the transmission spectra of this structure, respectively. First, dual Fano resonances are generated in the MIM structure with a baffle and a semi-ring cavity. By arranging two additional cavities, the mode interferences successfully induce up to six ultra-sharp and asymmetrical Fano peaks. The calculated sensing performances are available with ultra-high sensitivity of 1405 nm/RIU and figure of merit of 3.62×105. This chip-scale refractive index sensor may have great applications in highly integrated photonic circuits.
TL;DR: This paper proposes a novel design configuration of freeform off-axis three-mirror systems for automotive head-up display (HUD) application, where the image source, the flat mirror, and the freeform mirror are allocated on the same horizontal level to guarantee a compact structure.
Abstract: Automotive head-up display (HUD), a typical application in augmented reality (AR), has gained popularity in recent years. In this paper, we propose a novel design configuration of freeform off-axis three-mirror systems for automotive head-up display (HUD) application. In the configuration, the image source, the flat mirror, and the freeform mirror are allocated on the same horizontal level to guarantee a compact structure as well as to make the optical elements easy to be assembled. The whole design philosophy and procedure, including the analytic method to determine initial structure, structure constraints, and optimization strategy, are demonstrated in detail. Superior optical performance is achieved regardless of the pupil being located at any position inside a rectangular eye box.
TL;DR: A novel single-shot 3D shape measuring system that utilizes a coaxial fringe projection system and a 2CCD camera that combines the two-step phase-shifting algorithm, Fourier transform profilometry, and the optimum three-frequency selection method to improve the speed and accuracy.
Abstract: Fringe projection profilometry has been widely used in high-speed three-dimensional (3D) shape measurement. To improve the speed without loss of accuracy, we present a novel single-shot 3D shape measuring system that utilizes a coaxial fringe projection system and a 2CCD camera. The coaxial fringe projection system, comprising a visible light (red, green, and blue) projector and an infrared (IR) light projector, can simultaneously project red, green, blue, and IR fringe patterns. The 2CCD camera, as the name suggests, has two CCD chips that can acquire visible and IR fringe patterns at the same time. Combining the two-step phase-shifting algorithm, Fourier transform profilometry, and the optimum three-frequency selection method, 3D shape measurement of complex surfaces such as large slopes or discontinuous objects can be obtained from single-shot acquisition. A virtual fringe projection measurement system has been established to generate pre-deformed fringe patterns to correct positional deviations of the coaxial fringe projection system. This method has been applied to simulations and experiments on static and dynamic objects with promising results.
TL;DR: A novel terahertz refractive index sensor based on Tamm plasmon-polaritons (TPPs), comprising a Bragg reflector and a graphene layer, is proposed, using a semi-analytical transfer matrix method to study the proposed structure and its sensing performance.
Abstract: A novel terahertz (THz) refractive index sensor based on Tamm plasmon-polaritons (TPPs), comprising a Bragg reflector and a graphene layer, is proposed. A semi-analytical transfer matrix method is used to study the proposed structure and its sensing performance. The sensor demonstrates a sensitivity of 0.744 THz per refractive index unit (THz/RIU), or equivalently, 175.5 μm/RIU, and a figure of merit of 10.33 RIU−1 at the operating frequency of 1.132 THz. The effects of structural parameters on the sensing performance are studied, offering new methods for improving TPP-based sensors. The proposed approach is a simple and practical alternative to traditional, and often more complex, THz sensing approaches, due to the ease of excitation, which lifts the requirement of phase and polarization-matching devices such as polarizers, prisms, and gratings. The proposed structure is studied for gas sensing, and its performance is compared with previous THz refractive index sensing structures.
TL;DR: It is pointed out that the Floyd-Steinberg coefficients can be suboptimal for hologram error diffusion binarization, and the weighting coefficients are optimized for each different hologram adaptively to better preserve the fidelity of a reconstructed image after a hologram is binarized.
Abstract: 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.
TL;DR: A fiber-optic Fabry-Perot high-temperature pressure sensor based on sapphire direct bonding is proposed and experimentally demonstrated, demonstrating the sensing capabilities for pressures from 20 kPa to 700 k Pa up to 800°C.
Abstract: In this study, a fiber-optic Fabry–Perot (FP) high-temperature pressure sensor based on sapphire direct bonding is proposed and experimentally demonstrated. The sensor is fabricated by direct bonding of two-layer sapphire wafers, including a pressure diaphragm wafer and a cavity-etched wafer. The sensor is composed of a sensor head that contains a vacuum-sealed cavity arranged as an FP cavity and a multimode optical fiber. The external pressure can be measured by detecting the change in FP cavity length in the sensor. Experimental results demonstrate the sensing capabilities for pressures from 20 kPa to 700 kPa up to 800°C.