The 50 most-cited articles in Applied Optics, 2020

The complex refractive indices of polymers have important applications in the analysis of their components and the study of radiation endothermic mechanisms. Since these materials have high transmittance in the visible to near-infrared ranges, it is difficult to accurately measure their complex refractive indices. At present, the data for complex refractive indices of polymers are seriously lacking, which greatly limits the applications of these materials in the field of thermal radiation. In this work, spectroscopic ellipsometry (SE) combined with the ray tracing method (RTM) is used to measure the complex refractive indices of five polymers, polydimethylsiloxane, poly(methyl methacrylate) (PMMA), polycarbonate, polystyrene, and polyethylene terephthalate, in the spectral range of 0.4-2 µm. The double optical pathlength transmission method (DOPTM) is used to measure the complex refractive indices of three polymers, PMMA, polyvinyl chloride, and polyetherimide, in the 0.4-2 µm range. The complex refractive index of PMMA measured by the DOPTM almost coincides with the data measured…

In this paper a novel, to the best of our knowledge, deep neural network (DNN), VUR-Net, is proposed to realize direct and accurate phase unwrapping. The VUR-Net employs a relatively large number of filters in each layer and adopts alternately two types of residual blocks throughout the network, distinguishing it from the previously reported ones. The proposed method enables the wrapped phase map to be unwrapped precisely without any preprocessing or postprocessing operations, even though the map has been degraded by various adverse factors, such as noise, undersampling, deforming, and so on. We compared the VUR-Net with another two state-of-the-art phase unwrapping DNNs, and the corresponding results manifest that our proposal markedly outperforms its counterparts in both accuracy and robustness. In addition, we also developed two new indices to evaluate the phase unwrapping. These indices are proved to be effective and powerful as good candidates for estimating the quality of phase unwrapping.

Camera modeling and calibration are essential tasks in modern optics. Conventionally, the pinhole model is adopted with a further extension for lens distortion. However, pinhole and distortion models are mutually dependent; thus, the standard approach induces systematic camera calibration errors. This research presents a unifying distorted pinhole camera model that includes a telecentric, distortion-free pinhole, and radial lens distortion as particular cases. An iterative calibration method based on the derived distorted pinhole model is proposed, and experimental evaluation by calibrating a camera with high radial distortion is performed. The calibration results are compared with the standard and fisheye models using a well-known commercial camera calibrator software. The proposed method outperforms the standard model and achieves accuracy comparable to the fisheye model, and the proposed approach is a versatile and accurate tool for diverse optical metrology applications.

Primary production and photoacclimation models are two important classes of physiological models that find applications in remote sensing of pools and fluxes of carbon associated with phytoplankton in the ocean. They are also key components of ecosystem models designed to study biogeochemical cycles in the ocean. So far, these two classes of models have evolved in parallel, somewhat independently of each other. Here we examine how they are coupled to each other through the intermediary of the photosynthesis–irradiance parameters. We extend the photoacclimation model to accommodate the spectral effects of light penetration in the ocean and the spectral sensitivity of the initial slope of the photosynthesis–irradiance curve, making the photoacclimation model fully compatible with spectrally resolved models of photosynthesis in the ocean. The photoacclimation model contains a parameter θ m , which is the maximum chlorophyll-to-carbon ratio that phytoplankton can attain when available light tends to zero. We explore how size-class-dependent values of θ m could be inferred from field…

In this research, the photonic and phononic response of one-dimensional multilayer phoxonic crystals (PxCs) with normal incident of electromagnetic and acoustic waves is discussed. The presented design can work as a highly sensitive sensor for measuring three binary alcohol/water mixtures (i.e., 1-propanol/water, ethanol/water, and methanol/water) for a wide range of concentrations. The PxC sensor is able to detect small changes in the refractive index and longitudinal sound velocity of the alcohol/water mixture with initially neglecting the acousto-optical interaction. The sensor design is a defective structure as [ ( S i / S i O 2 ) 4 ( m i x t u r e w t . % ) ( S i O 2 / S i ) 4 ]. Also, we studied the effects of changing mixture concentrations from 0 wt. % to 100 wt. % on the physio-chemical parameters and resonant mode frequency. In our results, we have achieved high performance for the three alcohol mixtures in both phononic and photonic sensors especially for low concentrations. For example, in the photonic sensor we obtained sensitivity, Q value, and figure of merit of 873…

This paper contributes a novel design of sensor with a heart-shaped dual-core photonic crystal fiber (PCF) to detect cancerous cells in human cervical, blood, adrenal glands, and breast. Cancer-infected cells and their normal cells are considered in liquid form having their own refractive indices. In the designed PCF, the two heart-shaped cores separated by a large circular air hole serve as two independent waveguides. The large circular air hole is infiltrated by sample cells from different body parts. Detection of cancer-contaminated cells by the proposed PCF is based on the mode-coupling theory. According to the mode-coupling theory, the guided optical light transmits periodically from one core to another, throughout the PCF length. During this transmission, the optical light interacts with the cancerous cell, which is filled in the center air hole of the PCF. Due to this interaction, the dip wavelength of the transmission spectrum is sensitive to the corresponding cancerous cell filled in the center air hole of the PCF. The variation in the PCF transmission spectrum for…

In this paper, we describe a method used to determine the optical properties, namely, the effective scattering and absorption coefficients, employing an optimized three-dimensional-printed single integrating sphere. The paper consists of two parts, and in Part 1, the theoretical investigation of an optimized measurement and the evaluation routine are presented. Using an analytical and a numerical model for the optical characterization of the integrating sphere, errors caused by the application of a non-ideal sphere (the one with ports or baffles) were investigated. Considering this research, a procedure for the precise determination of the optical properties, based on Monte Carlo simulations of the light distribution within the sample, was developed. In Part 2, we present the experimental validation of this procedure.

We devise an inline digital holographic imaging system equipped with a lightweight deep learning network, termed CompNet, and develop the transfer learning for classification and analysis. It has a compression block consisting of a concatenated rectified linear unit (CReLU) activation to reduce the channels, and a class-balanced cross-entropy loss for training. The method is particularly suitable for small and imbalanced datasets, and we apply it to the detection and classification of microplastics. Our results show good improvements both in feature extraction, and generalization and classification accuracy, effectively overcoming the problem of overfitting. This method could be attractive for future in situ microplastic particle detection and classification applications.

Based on theoretical investigations of the light propagation within an integrating sphere, we developed an accurate method to determine the optical properties of scattering media using an integrating sphere-based setup. The method takes into account the exact sphere geometry as well as the different angular distributions of the reflected and transmitted light from the sample and the calibration standard. We tested our novelties successfully in theory with Monte Carlo simulations and in practice using a 3D printed and professionally coated integrating sphere. As a result, we were able to determine precisely the effective scattering coefficient, μ s ′ , and the absorption coefficient, μ a , between 400 nm and 1500 nm in a range of μ a = 1 e − 3 m m − 1 to 10 m m − 1 and μ s ′ = 0.2 m m − 1 to 100 m m − 1 . Usually, the accuracy was around 1% for μ s ′ and around 3% for μ a for turbid phantom media with an optical thickness τ = μ s

We demonstrate a three-dimensional (3D)-printed miniature optical fiber-based polymer Fabry-Perot (FP) interferometric pressure sensor based on direct femtosecond laser writing through two-photon polymerization. An unsealed cylinder column with a suspended polymer diaphragm is directly printed on a single-mode fiber tip to form an FP cavity. Here, two FP cavities with different lengths and the same diaphragm thickness (5 µm) are presented. The fabricated FP interferometer has a fringe contrast larger than 15 dB. The experimental results show that the fabricated device with a 140 µm cavity length has a linear response to the change of pressure with a sensitivity of 3.959 nm/MPa in a range of 0-1100 kPa, and the device with a 90 µm cavity length has a linear pressure sensitivity of 4.097 nm/MPa. The temperature sensitivity is measured to be about 160.2 pm/°C and 156.8 pm/°C, respectively, within the range from 20 to 70°C. The results demonstrate that 3D-printing techniques can be used for directly fabricating FP cavities on optical fiber tips for sensing applications.

In this paper, an electro-optical 4-to-2 encoder based on a photonic crystal is presented. The structure is composed of four silicon waveguides, four photonic crystal structures including the graphene– A l 2 O 3 stacks, and two optical combiners. Two one-dimensional arrays of air holes in the silicon background are designed parallel to the waveguides. Also, a graphene– A l 2 O 3 stack is placed at the center of each array, which provides the desired interferences. This feature is used for controlling the optical wave transmission through the waveguides. Using two optical combiners at the end of two waveguides, the received signals from the waveguides will be guided toward the output ports. The amount of the transmitted signal from input ports to the output of the encoder can be controlled by applying the proper chemical potential to the graphene-based stacks. The simulation results show that the encoding operation can be achieved by using 0.2 eV and 0.8 eV for chemical potentials. In addition, the normalized output power margins for logic 0 and 1 are calculated to be 8.2% and 46.7%,…

Mode-locked laser operation near 2.05 µm based on a mixed sesquioxide T m : L u Y O 3 ceramic is demonstrated. Continuous-wave and wavelength-tunable operation is also investigated. Employing a GaSb-based semiconductor saturable absorber mirror as a saturable absorber, a maximum average output power of 133 mW is obtained for a pulse duration of 59 fs. Pulses as short as 54 fs, i.e., eight optical cycles are generated at a repetition rate of ∼ 78 M H z with an average output power of 51 mW. To the best of our knowledge, this result represents the shortest pulse duration ever achieved from Tm-based solid-state mode-locked lasers.

The process monitoring of the top side and the evaluation of surface quality were applied in adjustable ring mode (ARM) laser welding of 316 stainless steel by using visual monitoring and confocal sensor technology. Experiments were conducted in different power arrangements of an ARM laser. In this study, the process status was evaluated based on the area of keyhole entrance and the width of the pool. The geometric characteristics of the topside weld were measured by a confocal sensor. The shape of the fusion zone was analyzed based on its cross section. The process mechanism of a different mode of ARM laser welding was investigated by analyzing the process status and geometric characteristics of the surface bead. Experimental results showed that dual-mode laser welding could stabilize the keyhole entrance with a uniform weld surface formation in comparison to the pure ring-mode laser weld. The dual-mode laser also generated a wider molten pool with a wider fusion zone than the pure center laser weld.

The performance of an ultra-compact all-optical D flip-flop using photonic crystal waveguides is numerically analyzed and examined by optimized parameters such as refractive index and silicon rod radius. In the field of optical networking and computing, flip-flops are used to reduce the complexity of digital circuits. The phenomenon of optical interference effect is used to implement a D flip-flop at a wavelength of 1550 nm. This structure is designed using T-shaped waveguides without using non-linear material. The proposed design is small, has low insertion losses of 0.087 dB when operated at low power level, and provides high contrast ratio of 25 dB and transmission ratio of more than 96%.

To derive the impact of chip size reduction on optical efficiency in micro-LED array panels, blue InGaN/GaN LEDs, which consist of 21 × 7 arrays (60 ppi display) with different mesa sizes on sapphire substrates, are designed and fabricated in this study. Changing the mesa area of the chip is first proposed to investigate the luminous efficiency (cd/A) of the screen. The current efficiency with a peak wavelength of 450 nm reaches up to 14.29 cd/A for the biggest pixel 50 µ m × 60 µ m and to 12.25 cd/A for the 15 µ m × 25 µ m chip, delivering high-level efficiencies to the current LED research field. The mechanisms of size-dependent efficiency variation trends and efficiency droops of blue LEDs are investigated experimentally, confirming that the current efficiency is more efficient at high injection current density while exhibiting poorer performance at the low current density region for smaller chips. The peak efficiency corresponds to higher current density with a decrease in chip size according to the carrier recombination ABC model. Moreover, the characteristic curve of the…

The rapid development of photonic integrated circuits demands the design of efficient and compact waveguide devices such as waveguide tapers and crossings. Some components in the silicon nitride (SiN) waveguide platform are superior to their counterparts in the silicon waveguide platform. Designing a compact SiN waveguide taper and crossing is crucial to reduce the size of SiN photonic components. In this paper, we utilize the focusing property of the Luneburg lens to design an SiN taper connecting a 10-µm-wide waveguide to a 1-µm-wide waveguide. Three-dimensional full-wave simulations indicate that the designed 13-µm-long taper has an average transmission efficiency of 92% in the wavelength range of 1500–1600 nm. We also present an in-plane SiN waveguide crossing based on the imaging property of the square Maxwell’s fisheye lens designed with quasi-conformal transformation optics. The designed waveguide crossing occupies a compact footprint of 5.65 µ m × 5.65 µ m , while its average insertion loss is 0.46 dB in the bandwidth of 1500–1600 nm. To the best our knowledge, the designed…

New (to the best of our knowledge) photonic crystal optical filters with unique optical characteristics are theoretically introduced in this research. Here, our design is composed of a defect layer inside one-dimensional photonic crystals. The main idea of our study is dependent on the tunability of the permittivity of graphene by means of the electro-optical effect. The transfer matrix method and the electro-optical effect represent the cornerstone of our methodology to investigate the numerical results of this design. The numerical results are investigated for four different configurations of the defective one-dimensional photonic crystals for the electric polarization mode. The graphene as a defect layer is deposited on two different electro-optical materials (lithium niobate and polystyrene) to obtain the four different configurations. The electro-optical properties of graphene represent the main role of our numerical results. In the infrared wavelength range from 0.7 µm to 1.6 µm, the reflectance properties of the composite structures are numerically simulated by varying…

The demand for single-shot and common-path holographic systems has become increasingly important in recent years, as such systems offer various advantages compared to their counterparts. Single-shot holographic systems, for example, reduce computational complexity as only a single hologram with the object information required to process, making them more suitable for the investigation of dynamic events; and common-path holographic systems are less vibration-sensitive, compact, inexpensive, and high in temporal phase stability. We have developed a single-shot common-path off-axis digital holographic setup based on a beam splitter and pinhole. In this paper, we present a concise review of the proposed digital holographic system for several applications, including the quantitative phase imaging to investigate the morphological and quantitative parameters, as a metrological tool for testing of micro-optics, industrial inspection and measurement, and sound field imaging and visualization.

The description and validation of an ImageJ open-source plugin to numerically simulate and reconstruct digital lensless holographic microscopy (DLHM) holograms are presented. Two modules compose the presented plugin: the simulation module implements a discrete version of the Rayleigh-Somerfield diffraction formula, which allows the user to directly build a simulated hologram from a known phase and/or amplitude object by just introducing the geometry parameters of the simulated setup; the plugin's reconstruction module implements a discrete version of the Kirchhoff-Helmholtz diffraction integral, thus allowing the user to reconstruct DLHM holograms by introducing the parameters of the acquisition setup and the desired reconstruction distance. The plugin offers the two said modules within the robust environment provided by a complete set of built-in tools for image processing available in ImageJ. While the simulation module has been validated through the evaluation of the forecasted lateral resolution of a DLHM setup in terms of the numerical aperture, the reconstruction module is…

In this paper, a photonic crystal structure based on nonlinear cavities has been proposed to improve the time response of a 2-to-4 decoder. The structure includes an array of chalcogenide rods with an air gap in which the spatial period of rods is 500 nm. The radius of the fundamental rods is assumed to be 125 nm, which results in a photonic bandgap of 1092–1724 nm at TM mode. Three cavities, including the nonlinear rods with a Kerr coefficient of 10 − 14 m 2 / W , drop the incoming waves concerning the amount of optical intensity. The finite-difference time-domain method was used to calculate the components of the electric and magnetic fields throughout the structure. The time analysis of the device shows the rise time is equal to 200 fs, which is less than one for the previous structures. The area of 110 µ m 2 and the margins of 4% and 88% for logics 0 and 1 are other advantages of the proposed structure. Based on the obtained results, it was proven that the performance of the 2-to-4 photonic crystal-based decoder has been improved by this work.

Reversible logic gates are capable of designing lossless digital systems, which have received a great deal of attention in photonic integrated circuits due to their advantages, such as less heat generation and low power dissipation. In this paper, all-optical reversible Feynman and Toffoli logic gates are designed for optical computing devices and low-power integrated circuits. Proposed designs of all-optical reversible logic gates are implemented with two-dimensional photonic crystal waveguides without using any nonlinear material. The finite-difference time-domain method is used to simulate and verify the proposed design, and it is operated at a wavelength of 1550 nm. The structure of all-optical reversible logic gates requires much less area, and Feynman logic gates offer a contrast ratio (CR) of 12.4 dB, transmittance of 0.96, and less insertion loss of − 0.015 d B , while Toffoli logic gates offer a CR of 32.5 dB, transmittance of 0.9, and less insertion loss of − 0.04 d B .

Coating thermal noise is a fundamental limit for precision experiments based on optical and quantum transducers. In this review, after a brief overview of the techniques for coating thermal noise measurements, we present the latest worldwide research activity on low-noise coatings, with a focus on the results obtained at the Laboratoire des Matériaux Avancés. We report new updated values for the T a 2 O 5 , T a 2 O 5 − T i O 2 , and S i O 2 coatings of the Advanced LIGO, Advanced Virgo, and KAGRA detectors, and new results from sputtered N b 2 O 5 , T i O 2 − N b 2 O 5 , T a 2 O 5 − Z r O 2 , M g F 2 , A l F 3 , and silicon nitride coatings. Amorphous silicon, crystalline coatings, high-temperature deposition, multi-material coatings, and composite layers are also briefly discussed, together with the latest developments in structural analyses and models.

We report on efficient diode-pumped mid-infrared lasers based on T m : L i Y F 4 , T m : Y 3 A l 5 O 12 , and T m : Y A l O 3 crystals. These lasers operate in the continuous-wave (CW) regime and deliver watt-level output power at the wavelengths of 2.2–2.3 µm (the 3 H 4 → 3 H 5 T m 3 + transition). In particular, a 1.8 at. % T m : Y A l O 3 laser pumped at 789 nm generates a maximum CW output power of 1.32 W at 2272–2277 nm with a slope efficiency of 33.1% (with respect to the absorbed pump power), a linear laser polarization ( E ∥ b ), and a fundamental transverse output mode (the measured M x 2 = 1.26 , M y 2 = 1.68 ). In the quasi-CW regime, the output peak power is scaled up to 2.69 W. The pump quantum efficiency and the fractional heat loading are estimated and discussed.

We propose and demonstrate a hybrid fiber-based sensor combining a multimode interference (MMI) structure and a surface plasmon resonance (SPR) structure for simultaneous measurement of temperature and refractive index (RI) of a liquid sample. We configure the MMI structure by connecting a single-mode fiber, a no-core fiber, and a single-mode fiber sequentially. We set up the SPR structure by coating a gold film with a thickness of 50 nm on the surface of the no-core fiber. We measure the sensitivity of RI and the temperature of the MMI and SPR structure, respectively. Then we obtain the coefficient matrix to simultaneously measure the temperature and RI of a liquid sample and obtain the highest RI sensitivity of 2061.6 nm/RIU and temperature sensitivity of 37.9 pm/°C. We verify the feasibility of the sensor in liquid alcohol. The testing results indicate that the proposed sensor and testing method are feasible, accurate, and convenient.

A noncontact method to identify sparsely distributed plastic pellets is proposed by integrating holography and Raman spectroscopy in this study. Polystyrene and poly(methyl methacrylate) resin pellets with a size of 3 mm located in a 20 cm water channel were illuminated using a collimated continuous wave laser beam with a diameter of 4 mm and wavelength of 785 nm. The same laser beam was used to take a holographic image and Raman spectrum of a pellet to identify the shape, size, and composition of material. Using the compact system, the morphological and chemical analysis of pellets in a large volume of water was performed. The reported method demonstrates the potential for noncontact continuous in situ monitoring of microplastics in water without collection and separation.

In this paper, we present a noniterative method for 3D computer-generated holography based on deep learning. A convolutional neural network is adapted for directly generating a hologram to reproduce a 3D intensity pattern in a given class. We experimentally demonstrated the proposed method with optical reproductions of multiple layers based on phase-only Fourier holography. Our method is noniterative, but it achieves a reproduction quality comparable with that of iterative methods for a given class.

An optical fiber having the properties of photonic crystal and offering new diversity and features beyond a conventional optical fiber is the photonic crystal fiber (PCF). In this paper, a simplified version of a highly sensitive plasmonic sensor, called a “slotted PCF based plasmonic biosensor,” is studied numerically with asymmetric air holes using the finite element method. From numerical records through the interrogation method, the maximum obtained wavelength sensitivity and amplitude sensitivity are 22000 nm/RIU and 1782.56 R I U − 1 , respectively, with a maximum wavelength resolution of 4.54 × 10 − 6 R I U − 1 RIU for the y -polarized mode. Finally, optimization of the sensor performance is scrutinized, and the effect of different parameters is studied with proper resonance wavelength curve fitting. The design structure of the fiber is simple, symmetrical, easy to fabricate, and cost effective and has higher sensitivity than other PCF based sensors. Having a symmetric orientation of air holes, classic geometric structure, and higher sensitivity, it has the capability to be…

As the research work in surface plasmon resonance (SPR)-based photonic crystal fiber (PCF) is getting tighter, a perfectly circular-shaped PCF with elliptical air holes is proposed where the performance parameters are improved significantly. The performances among our designed elliptical, circular, and rectangular air holes are compared, and the best result is achieved with the elliptical air holes. The technique used for the investigation is the finite element method, and for the simulation of data COMSOL Multiphysics 5.3a software is used. The method covers a wider range of the optical spectrum from 0.59 to 1.05 µm. The highest confinement loss achieved through our design is 340 dB/cm. The wavelength sensitivity and amplitude sensitivity are 13,000 nm/RIU and 1189.46 R I U − 1 , respectively. The sensor resolution is 7.69 × 10 − 6 for our proposed design. The proposed sensor also achieved a maximum birefringence of 2.8 × 10 − 3 , which is, to our knowledge, the highest birefringence reported so far for a PCF-SPR sensor. This enables the fiber to be operated in a dual-polarized…

In this work, the design, construction, and testing of the most cost-effective digital lensless holographic microscope to date are presented. The architecture of digital lensless holographic microscopy (DLHM) is built by means of a 3D-printed setup and utilizing off-the-shelf materials to produce a DLHM microscope costing U S $ 52.82 . For the processing of the recorded in-line holograms, an open-source software specifically developed to process this type of recordings is utilized. The presented DLHM setup has all the degrees of freedom needed to achieve different fields of view, levels of spatial resolution, and 2D scanning of the sample. The feasibility of the presented platform is tested by imaging non-bio and bio samples; the resolution test targets, a section of the head of a Drosophila melanogaster fly, red blood cells, and cheek cells are imaged on the built microscope.

We present the performance enhancement of the surface plasmon resonance (SPR) sensor for deoxyribonucleic acid (DNA) hybridization employing black phosphorus (BP), silver (Ag), and silicon (Si) configurations and numerical analysis. The combination of Ag and BP demonstrates the sensitivity of 91.54°/RIU with degradation of detection accuracy (DA), quality factor (QF), and figure of merit (FOM). To enhance the DA, QF, and FOM, a novel SPR is proposed with Si, which demonstrates a DA and QF of 69 and 1061.6 R I U − 1 , which are 8.53 and 11.04 times, and an FOM of 554.58, which is 4.47 and 5.77 times higher than both the conventional and graphene-based SPR sensors, respectively.

We introduce an approach to expand the eye-box in a retinal-projection-based near-eye display. The retinal projection display has the advantage of providing clear images in a wide depth range; however, it has difficulty in practical use with a narrow eye-box. Here, we propose a method to enhance the eye-box of the retinal projection display by generating multiple independent viewpoints, maintaining a wide depth of field. The method prevents images projected from multiple viewpoints from overlapping one other in the retina. As a result, our proposed system can provide a continuous image over a wide viewing angle without an eye tracker or image update. We discuss the optical design for the proposed method and verify its feasibility through simulation and experiment.

Fresnel incoherent correlation holography (FINCH) is a technology that can acquire three-dimensional information of incoherent objects such as fluorescence with an in-line optical system. However, it is difficult to apply FINCH to dynamic phenomena, since FINCH has to detect phase-shifted holograms sequentially to eliminate twin and zero-order images. In this paper, a method in which the phase-shifted holograms can be obtained simultaneously with an in-line setup by using an optimized simulated diffraction optical element (sDOE), realized by a phase-only spatial light modulator, is proposed. The optimized sDOE is an optical device with a dual-focus lens, 2D grating, and spatial phase shifter. Therefore, the sDOE is called a dual-focus checkerboard lens. The optical experiment confirms the feasibility of the proposed method.

The key to accurate 3D shape measurement in fringe projection profilometry (FPP) is the proper calibration of the measurement system. Current calibration techniques rely on phase-coordinate mapping (PCM) or back-projection stereo vision (SV) methods. PCM methods are cumbersome to implement as they require precise positioning of the calibration target relative to the FPP system, but they produce highly accurate measurements within the calibration volume. SV methods generally do not achieve the same accuracy level. However, the calibration is more flexible in that the calibration target can be arbitrarily positioned. In this work, we propose a hybrid calibration method that leverages the SV calibration approach using a PCM method to achieve higher accuracy. The method has the flexibility of SV methods, is robust to lens distortions, and has a simple relation between the recovered phase and the metric coordinates. Experimental results show that the proposed hybrid method outperforms the SV method in terms of accuracy and reconstruction time due to its low computational complexity.

Holographic tomography allows the 3D mapping of the refractive index of biological samples thanks to reconstruction methods based on the knowledge of illumination directions or rotation angles of the imaged sample. Recently, phase contrast tomographic flow cytometry by digital holography has been demonstrated to reconstruct the three-dimensional refractive index distribution of single cells while they are flowing along microfluidic channels. In this system, the illumination direction is fixed while the sample's rotation is not deterministically known a priori but induced by hydrodynamic forces. We propose here a technique to retrieve the rolling angles, based on a new phase images similarity metric that is capable of identifying a cell's orientations from its 3D positioning while it is flowing along the microfluidic channel. The method is experimentally tested and also validated through appropriate numerical simulations. We provide demonstration of concept by achieving reconstruction of breast cancer cells tomography.

Sound field imaging techniques have been found very useful for acoustic designs. Building on this idea, innovative techniques are needed and presented in this paper, where we report on developed imaging of the sound field radiated from speakers by parallel phase-shifting digital holography. We adopted an ultrasonic wave radiated from a speaker for an object. The phase distribution of the light wave was modulated by the sound field radiated from the speaker. The modulated phase distribution was recorded in the form of multiplexed phase-shifted holograms at the frame rate of 100,000 fps. A 40,000 Hz sound field radiated from a speaker is used as an observation target. Our proposed method can implement the imaging of the sound field successfully. Also, in order to demonstrate the digital refocusing capability of digital holography, we set two speakers, whose difference in depth positions was 6.6 cm, as a long-depth object. We demonstrated the digital refocusing on the two speakers along with the capability of measuring the positions of the objects. Furthermore, we succeeded in imaging…

We present a performance analysis of compact monolithic optomechanical inertial sensors that describes their key fundamental limits and overall acceleration noise floor. Performance simulations for low-frequency gravity-sensitive inertial sensors show attainable acceleration noise floors on the order of 1 × 10 − 11 m / s 2 H z . Furthermore, from our performance models, we devised an optimization approach for our sensor designs, sensitivity, and bandwidth trade space. We conducted characterization measurements of these compact mechanical resonators, demonstrating mQ -products at levels of 250 kg, which highlight their exquisite acceleration sensitivity.

An adhesive-free encapsulation sapphire Fabry–Perot interferometer (FPI) is proposed and demonstrated for high-temperature pressure measurements. The sapphire FPI sensor is packaged by zirconia ferrules and a zirconia sleeve, which is easy to be configured and low in cost. Owing to this packaging technology, the sapphire FPI sensor presents good stability and high temperature resistance. The pressure and temperature properties of the sapphire FPI sensor are investigated within a temperature range from − 50 ∘ C to 1200°C and a pressure range from 0.4 to 4.0 MPa. Experimental results show the FPI has a temperature sensitivity of 23 pm/°C and still works as the temperature is up to 1200°C. Meanwhile, the wavelength shift of the sapphire FPI versus the applied pressure is linear at each tested temperature. The pressure sensitivity is measured to be 1.20 nm/MPa at 1200°C, and the linear response shows the proposed sensor has good repeatability within 0.4–4.0 MPa. Such a sapphire FPI sensor has potential applications in engineering areas, such as the oil industry and gas boilers.

We propose an asymmetric optical image cryptosystem based on biometric keys and singular value decomposition (SVD) in the Fresnel transform domain. In the proposed cryptosystem, the biometric keys are palmprint phase mask generated by a palmprint, a chaotic phase mask, and an amplitude truncated Fourier transform, which can provide the cryptosystem with more data security due to the uniqueness of the palmprint. Two images are first encoded into a complex function, which then is modulated by the palmprint phase mask. A Fresnel transform and then an SVD operation are performed on the modulated result. The SVD operation is used to generate private secret keys, which makes the encryption secret keys and decryption secret keys different, and thus the encryption process and decryption process are different. In addition, multiple images are encrypted into a real-valued ciphertext, making it convenient to transport and record. Numerical simulation results have demonstrated that our proposed encryption system has robustness against statistical, occlusion, noise, and chosen-plaintext attacks.

Binary hologram generation based on deep learning is proposed. The proposed method can reduce the severe effect of quality degradation from binarizing gray-scaled holograms by optimizing the neural network to output binary amplitude holograms directly. In previous work on binary holograms, the calculation time for generating binary holograms was long. However, in the proposed method, once the neural network is trained enough, the neural network generates binary holograms much faster than previous work with comparable quality. The proposed method is more suitable for opportunities to generate several binary holograms under the same condition. The feasibility of the proposed method was confirmed experimentally.

A hybrid model based on a wavelet threshold de-noising (WTD) and recursive feature elimination with cross-validation (RFECV) method was proposed to improve the measurements in quantitative analysis of coal properties using laser-induced breakdown spectroscopy (LIBS). First, a modified threshold of WTD was proposed based on wavelet coefficient theory. Interference of noise in the LIBS spectrum was reduced by using this modified method. Then, the RFECV method was applied to extract effective features from the de-noised LIBS spectrum. Finally, support vector regression (SVR) models of coal properties were established by the selected features. A validation set was used to verify the effectiveness and robustness of the hybrid model. The improvement of the hybrid model on the quantitative analysis of each index of coal properties (heat value, ash, volatile content) was studied and discussed. By using the proposed model, the determination coefficient ( R 2 ), root mean square error of prediction, average relative error, and relative standard deviation were all significantly improved over…

Deep learning has been actively investigated for various applications such as image classification, computer vision, and regression tasks, and it has shown state-of-the-art performance. In diffuse optical tomography (DOT), the accurate estimation of the bulk optical properties of a medium is paramount because it directly affects the overall image quality. In this work, we exploit deep learning to propose a novel, to the best of our knowledge, convolutional neural network (CNN)-based approach to estimate the bulk optical properties of a highly scattering medium such as biological tissue in DOT. We validated the proposed method by using experimental, as well as, simulated data. For performance assessment, we compared the results of the proposed method with those of existing approaches. The results demonstrate that the proposed CNN-based approach for bulk optical property estimation outperforms existing methods in terms of estimation accuracy, with lower computation time.

We calibrate and test a division-of-focal-plane red-green-blue (RGB) full-Stokes imaging polarimeter in a variety of indoor and outdoor environments. The polarimeter, acting as a polarization camera, utilizes a low dispersion microretarder array on top of a sensor with Bayer filters and wire-grid linear polarizers. We also present the design and fabrication of the microretarder array and the assembly of the camera and validate the performance of the camera by taking multiple RGB full-Stokes images and videos. Our camera has a small form factor due to its single-sensor design and the unique capability to measure the intensity, color, and polarization of an optical field in a single shot.

Using optical beam interference inside photonic-crystal-based waveguides is a promising method for designing and realizing all-optical logic gates and other digital devices. In this paper we design and propose an all-optical 1-bit comparator using optical beam interference. In the proposed structure, the logic states of input ports are determined based on their initial phases. The 180 deg and 0 deg phases are used as logic 0 and 1. However, the logic states of the output ports are determined based on the amplitude of the optical signal at the output ports. For the proposed structure, the maximum rise and fall times are about 0.6 ps and 0.3 ps, respectively.

This paper reports on the influence of the bromine (Br) atoms substitution on the structures and optoelectronic traits of CsPbI 3 , wherein the density functional theory (DFT) simulation was performed, using all electrons full potential linearized augmented plane-wave method. Furthermore, the generalized gradient approximation, local density approximation, and modified Becke–Johnson exchange-correlation potential were used to improve the optimization and band structure calculations. The calculated lattice constants of CsPbI 3 and CsPbBr 3 were consistent with the experimental values. All the studied compounds revealed wide and direct bandgap energies at the R-symmetry point, which varied from 1.74–2.23 eV. The obtained refractive indices of the CsPbI 3 , CsPbBrI 2 , CsPbIBr 2 , and CsPbBr 3 compounds were correspondingly 2.265, 2.245, 2.090, and 2.086. Present findings may contribute towards the development of experimental studies on the proposed compounds with controlled properties useful for the solar cells.

We present a baseline correction method based on improved asymmetrically reweighted penalized least squares (IarPLS) for the Raman spectrum. This method utilizes a new S-type function to reduce the risk of baseline overestimation and speed up the reweighting process. Simulated spectra with different levels of noise and measured spectra with strong fluorescence background from different samples are used to validate the performance of the proposed algorithm. Considering the drawbacks of the weighting rules for the asymmetrically reweighted penalized least squares (arPLS) method, we adapt an inverse square root unit (ISRU) function, which performs well in baseline correction. Compared with previous penalized least squares methods, such as asymmetric least squares, adaptive iteratively reweighted penalized least squares, and arPLS, experiments with the simulated Raman spectra have confirmed that the proposed method yields better outcomes. Experiments with the measured Raman spectra show that the IarPLS method can improve real Raman spectra within 20 ms. The results show that the…

Defects or discontinuities are inevitable during the melting and consolidation process of metal additive manufacturing. Online inspection of microdefects during the processing of layer-by-layer fusion is urgently needed for quality control. In this study, the laser ultrasonic C-scan imaging system is established to detect the surface defects of selective laser melting (SLM) samples that have a different surface roughness. An autosizing method based on the maximum correlation coefficient and lag time is proposed to accurately measure the defect length. The influences of the surface roughness on the laser ultrasound signal-to-noise ratio distribution and defect sizing accuracy are also studied. The results indicate that the proposed system can detect notches with a depth of 50 µm and holes with a diameter of 50 µm, comparable in size to raw powder particles. The average error for the length measurement can reach 1.5% if the notch is larger than 2 mm. Meanwhile, the sizing error of a 1 mm length notch is about 9%. In addition, there is no need to remove the rough surface of the…

A magnetically tunable metasurface comprising meta-atoms with InSb-patched, InAs-patched, and unpatched pixels was simulated using commercial software to maximize the absorption of normally incident radiation in the terahertz spectral regime, with the patches decorating the illuminated face of a gold-backed polyimide substrate. Maximum absorptance of 0.99 and minimum absorptance of 0.95 can be obtained in 0.14–0.23-THz-wide bands in the 2–4-THz spectral regime, with an average tuning rate of and 0.24-THz dynamic range when the controlling magnetostatic field is aligned parallel to the incident electric field. The use of both InSb and InAs patches is much superior to the use of patches of only one of those materials. The design can be adapted for neighboring spectral regimes by exploiting the scale invariance of the Maxwell equations.

We experimentally demonstrate high-speed visible light communication (VLC) and high-quality solid-state lighting (SSL) using polymethyl-methacrylate-doped phosphor film based on cesium lead bromide quantum dot ( CsPbB r 3 -QD) and potassium fluorosilicate K 2 Si F 6 : M n 4 + , which is excited by a blue gallium nitride laser diode. A 1.6 Gbps data rate is achieved by employing a non-return-to-zero on–off keying modulation scheme. The measured bit error rate of 2.7 × 10 − 3 adheres to the standard threshold ( 3.8 × 10 − 3 ) of forward error correction. Moreover, the generated white-light source has a high color rendering index of 93.8 and a correlated color temperature of 4435 K, and it exhibits a Commission Internationale de l’Eclairage (CIE) 1931 chromaticity coordinate at (0.3556, 0.3520), which is close to the ideal CIE value of white light (0.3333, 0.3333). This work opens up exciting possibilities for future high-speed indoor VLC and high-quality SSL.

Breast cancer accounts for the highest number of female deaths worldwide. Early detection of the disease is essential to increase the chances of treatment and cure of patients. Infrared thermography has emerged as a promising technique for diagnosis of the disease due to its low cost and that it does not emit harmful radiation, and it gives good results when applied in young women. This work uses convolutional neural networks in a database of 440 infrared images of 88 patients, classifying them into two classes: normal and pathology. During the training of the networks, we use transfer learning of the following convolutional neural network architectures: AlexNet, GoogLeNet, ResNet-18, VGG-16, and VGG-19. Our results show the great potential of using deep learning techniques combined with infrared images in the aid of breast cancer diagnosis.

The propagation model of orbital angular momentum (OAM) modes carried by the perfect vortex (pv) beam through anisotropic oceanic turbulence links is established and the factors influencing the OAM propagation are discussed. The findings show that the self-focusing property of pv beams is beneficial to the propagation of OAM modes: a smaller topological charge, a smaller initial radius, and an optimized half-ring width can alleviate degrading effects of turbulence on the pv beam. Additionally, the pv beam with a longer wavelength is more resistant to turbulent interference. The oceanic conditions with a higher dissipation rate of kinetic energy per unit mass of seawater, larger values of anisotropy and inner-scale factor, a smaller temperature-salinity contribution ratio, or a lower mean-squared temperature dissipation rate can improve the signal mode detection probability. The results are expected to further optimize the design of OAM-based underwater wireless communication systems.