The 50 most-cited articles in Applied Optics, 2019

Thin films of Ge, ZnS, Y b F 3 , and L a F 3 produced using e-beam evaporation on ZnSe and Ge substrates were characterized in the range of 0.4–12 µm. It was found that the Sellmeier model provides the best fit for refractive indices of ZnSe substrate, ZnS, and L a F 3 films; the Cauchy model provides the best fit for Y b F 3 film. Optical constants of Ge substrate and Ge film as well as extinction coefficients of ZnS, Y b F 3 , L a F 3 , and ZnSe substrate are presented in the frame of a non-parametric model. For the extinction coefficient of ZnS, the exponential model is applicable. Stresses in Ge, ZnS, Y b F 3 , and L a F 3 were estimated equal to ( − 50 ) M P a , ( − 400 ) M P a , 140 MPa, and 380 MPa, respectively. The surface roughness does not exceed 5 nm for all films and substrates.

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…

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.

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.

Believing that the detection of hemoglobin possesses a vital role in the discovery of many diseases, we present in this work a simple method for sensing and detecting hemoglobin based on one-dimensional photonic crystals. Implementing hemoglobin as a defect layer inside the proposed photonic crystal results in a resonant peak evolving within the bandgaps. The strong dependence of these resonant peaks on concentration and the consequent refractive index are the essential bases of the detection process. The role played by these parameters together with the angle of incidence on performance and efficiency of our sensor is demonstrated. In the vicinity of the investigated results, we demonstrate the values of sensitivity, figure of merit (FOM), signal-to-noise ratio (SNR), and resolution to optimize the performance of our sensor. The numerical results show a significant effect of polarization mode on performance of this sensor. For TE polarization with an angle of incidence equal to 45°, we investigated sensitivity of 167nmRIU−1 , SNR of 0.23, FOM of 0.63RIU−1 , and resolution of 257 nm.

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.

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.

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…

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.

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…

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.

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…

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.

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.

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.

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.…

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.

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…

To improve our understanding of the complex role of aerosols in the climate system and on air quality, measurements are needed of optical and microphysical aerosol. From many studies, it has become evident that a satellite-based multiangle, multiwavelength polarimeter will be essential to provide such measurements. Here, high accuracy (∼0.003) on the degree of linear polarization (DoLP) measurements is important to retrieve aerosol properties with an accuracy needed to advance our understanding of the aerosol effect on climate. SPEX airborne, a multiangle hyperspectral polarimeter, has been developed for observing and characterizing aerosols from NASA's high-altitude research aircraft ER-2. It delivers measurements of radiance and DoLP at visual wavelengths with a spectral resolution of 3 and 7-30 nm, respectively, for radiance and polarization, at nine fixed equidistant viewing angles from -56° to +56° oriented along the ground track, and a swath of 7° oriented across-track. SPEX airborne uses spectral polarization modulation to determine the state of linear polarization of…

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.

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…

Quantum key distribution (QKD) has undergone significant development in recent decades, particularly with respect to free-space (air) and optical fiber channels. Here, we report a proof-of-principle experiment for the BB84 protocol QKD over a water channel. First, we demonstrate again the polarization preservation properties of the water channel in optical transmission according to the measured Mueller matrix, which is close to the unit matrix. The reason for the polarization preservation, revealed by Monte Carlo simulation, is that almost all the received photons are unscattered. Then, we performed the first polarization encoding BB84 protocol QKD over a 2.37 m water channel. The results show that QKD can be performed with a low quantum bit error rate, less than 3.5%, with different attenuation coefficients.

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 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.

Sub-wavelength diffractive optics, commonly known as metasurfaces, have recently garnered significant attention for their ability to create ultra-thin flat lenses with a high numerical aperture. Several materials with different refractive indices have been used to create metasurface lenses (metalenses). In this paper, we analyze the role of refractive index on the performance of these metalenses. We employ both forward and inverse design methodologies to perform our analysis. We found that, while high-refractive-index materials allow for extreme reduction of the focal length, for moderate focal lengths and numerical aperture (<0.6), there is no appreciable difference in the focal spot size and focusing efficiency for metalenses made of different materials with refractive indices ranging between 1.2 and 3.43 in forward design, and 1.25 and 3.5 in inverse design.

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…

This work demonstrates a broadband polarization filter based on copper-filled photonic crystal fiber (CFPCF). The proposed fiber is fabricated using the conventional stack-and-draw method. The polarization filter properties of the proposed CFPCF are investigated numerically by considering the cross-sectional scanning electron microscopy image of the fabricated CFPCF. It is observed that the magnitude of cross talk reached up to -206 dB over 0.8 mm length with a broad bandwidth of 282 nm at a central wavelength of 1790 nm. In addition, the polarization characteristics of the CFPCF including cross talk, central wavelength, and bandwidth can be adjusted by varying the diameter of the copper wire. It is shown that the resonance wavelength of the proposed filter can be tuned over the wide range of wavelengths from 1390 to 1890 nm. We have shown that by adjusting the copper wire diameter to 0.32Λ and 0.48Λ μm (Λ is pitch size), the proposed filter can operate at communication bands of 1310 and 1550 nm, respectively. The results suggest high-potential of the proposed fiber for polarization…

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…

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…

A new method is presented for performing the Abel inversion by fitting the line-of-sight projection of a predefined intensity distribution (FLiPPID) to the recorded 2D projections. The aim is to develop a methodology that is less prone to experimental noise when analyzing the projection of axisymmetric objects-in this case, co-flow diffusion flame images for color ratio pyrometry. A regression model is chosen for the light emission intensity distribution of the flame cross section as a function of radial distance from the flame center line. The forward Abel transform of this model function is fitted to the projected light intensity recorded by a color camera. For each of the three color channels, the model function requires three fitting parameters to match the radial intensity profile at each height above the burner. This results in a very smooth Abel inversion with no artifacts such as oscillations or negative values of the light source intensity, as is commonly observed for alternative Abel inversion techniques, such as the basis-set expansion or onion peeling. The advantages of…

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.

In this paper, a planar comb-shaped antenna array for terahertz sensing based on the excitation of spoof surface plasmon modes is proposed. The structure is constructed by an array of three periodic rectangular grooves perforated through metal stripes on top of a silicon substrate. The effective detection of lactose is given as an example to demonstrate the ability of this structure to enhance detection sensitivity. In transmission mode, the sensing signal of lactose using the antenna array was 7.6 times larger than that of using a silicon substrate. In reflection mode, the sensing signal of lactose increased almost 13 times using our proposed antenna array compared to that of using a silicon substrate, exhibiting high sensitivity in terahertz sensing. Further, lactose thickness could be predicted based on the reflectance at the peak using our proposed structure. Our results indicate that the proposed structure has great potentiality in the field of biological and chemical sensing.

Freeform optics offers more degrees of freedom to optical design that can benefit from a compact package size and a large field of view for imaging systems. Motivated by the advances in modern optical fabrication and metrology, freeform optics has been found in many applications. In this paper, we will describe the challenging optical design, fabrication, metrology, and assembly of an all-aluminum unobscured two-mirror freeform imaging telescope. The telescope has a large field of view of 20 ∘ × 15 ∘ . The freeform aluminum mirrors are manufactured by diamond turning based on a feedback modification strategy. The freeform mirrors are measured by a computer-generated hologram-based interferometric null test method. All-aluminum configuration has the advantages of being athermal and cost-effective.

This paper proposes a particle volume reconstruction directly from an in-line hologram using a deep neural network (DNN). Digital holographic volume reconstruction conventionally uses multiple diffraction calculations to obtain sectional reconstructed images from an in-line hologram, followed by detection of the lateral and axial positions, and the sizes of particles by using focus metrics. However, the axial resolution is limited by the numerical aperture of the optical system, and the processes are time consuming. The method proposed here can simultaneously detect the lateral and axial positions, and the particle sizes via a DNN. We numerically investigated the performance of the DNN in terms of the errors in the detected positions and sizes. The calculation time is faster than conventional diffracted-based approaches.

In this paper, a compact optical high-speed 1-bit comparator is proposed based on photonic crystals. In this structure, the nonlinear rods are used at the cross-connecting point of two optical waveguides. The optical transmission and reflection from these rods depend on the amount of the optical intensity. In response to the different states of the input ports, different values of the optical power reach these rods and the interference patterns make the correct function of the output ports. The refractive index and the Kerr coefficient of nonlinear rods are 1.4 m 2 / W and 10 − 14 m 2 / W , respectively. The footprint of the structure is 55 µ m 2 , which is much smaller than the previous works. Besides, the lower delay time is the other advantage of this work compared with the previous works. Based on the simulation results, the proposed structure can be used in integrated optical circuits.

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.

We experimentally introduce a normalized differential method to enhance the time domain signal-to-noise ratio (SNR) of an optical fiber distributed acoustic sensor (DAS). The reported method is calibrated against the typical differential method in noisy DAS systems, including those utilizing a relatively wide linewidth laser or few-mode fiber. In these two systems, the normalized differential method respectively identifies the position information of various vibration events with 1.7 dB and 0.53 dB SNR improvement. We further demonstrate the ability to locate positions along a fiber that are subjected to vibrations of frequencies higher than the theoretical maximum, but without determining these frequencies.

We developed transmission diffraction grating-based spectroscopic single-molecule localization microscopy (sSMLM) to collect the spatial and spectral information of single-molecule blinking events concurrently. We characterized the spectral heterogeneities of multiple far-red emitting dyes in a high-throughput manner using sSMLM. We also investigated the influence of spectral dispersion on the single-molecule identification performance of fluorophores with large spectral overlapping. The careful tuning of spectral dispersion in grating-based sSMLM permitted simultaneous three-color super-resolution imaging in fixed cells with a single objective lens at a relatively low photon budget. Our sSMLM has a compact optical design and can be integrated with conventional localization microscopy to provide add-on spectroscopic analysis capability.

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.

In early 2013, three airborne polarimeters were flown on the high altitude NASA ER-2 aircraft in California for the Polarimeter Definition Experiment (PODEX). PODEX supported the pre-formulation NASA Aerosol-Cloud-Ecosystem (ACE) mission, which calls for an imaging polarimeter in polar orbit (among other instruments) for the remote sensing of aerosols, oceans, and clouds. Several polarimeter concepts exist as airborne prototypes, some of which were deployed during PODEX as a capabilities test. Two of those instruments to date have successfully produced Level 1 (georegistered, calibrated radiance and polarization) data from that campaign: the Airborne Multiangle Spectropolarimetric Imager (AirMSPI) and the Research Scanning Polarimeter (RSP). We compared georegistered observations of a variety of scene types by these instruments to test whether Level 1 products agreed within stated uncertainties. Initial comparisons found radiometric agreement, but polarimetric biases beyond measurement uncertainties. After subsequent updates to calibration, georegistration, and the measurement…

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%.

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…

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.

We study the use of three effective medium theory models, namely Maxwell-Garnett, Bruggeman, and Landau-Lifshitz-Looyenga, for the dielectric response of biological tissue in the terahertz band of the electromagnetic spectrum. In order to accomplish our objectives, we performed measurements on water-dehydrated basil binary mixtures encompassing the entire concentration range, and we further analyze the dielectric function with the models. Our results indicate that the Landau-Lifshitz-Looyenga and Bruggeman models provide marginally better fit to the experimentally measured dielectric function in the terahertz band. We further discuss the biological relevance of the models in the context of our experimental data based on their fundamental assumptions.

A Nomarski polarizing prism has been used in conjunction with a focused laser differential interferometer to measure the phase velocity of a density disturbance at sampling frequencies ≥ 10 M H z . Use of this prism enables the simultaneous measurement of density disturbances at two closely spaced points that can be arbitrarily oriented about the instrument’s optical axis. The orientation is prescribed by rotating the prism about this axis. Since all four beams (one beam pair at each measurement point) propagate parallel to one another within the test volume, any bias imparted by density fluctuations away from the measurement plane on the disturbance phase velocity is minimized. A laboratory measurement of a spark-generated shock wave and a wind tunnel measurement of a second-mode instability wave on a cone model in a Mach 6 flow are presented to demonstrate the performance of the instrument. High-speed schlieren imaging is used in both cases to verify the results obtained with the instrument.

Two or more focal planes are required in augmented reality head-up displays (AR HUDs) to respectively present basic and interactive driving information to car drivers; whereas, solutions using two separated picture generation units (PGUs) with two sets of optics incur increased cost, reduced reliability, and expanded volume. To develop an AR HUD using a single PGU and a single curved mirror, we propose to set two logically separated regions on a single PGU and optically relay one of them to a new position to create two focal planes. A single freeform mirror acquired through careful optical and mechanical design optimization produces high image quality in an eyebox of 120 mm by 60 mm, simultaneously for a far image (9 m, 10° by 3°) and a near image (2.5 m, 6° by 2°). Finally, an AR HUD product with a compact volume of 8.5 L is fabricated to experimentally verify the design.

This study develops a novel automatic all-sky imaging system, namely, an all-sky camera (ASC) system, for cloud cover assessment. The proposed system does not require conventional solar occulting devices and can capture complete hemispheric sky images. Cloud detection is performed innovatively using a convolutional neural network model (i.e., the optimized U-Net model). Experiments demonstrate that the optimized U-Net model can effectively detect clouds from sky images. In terms of cloud cover, the estimation results of the ASC system exhibit a high correlation with those obtained via manual observation, thereby indicating the applicability of the ASC system in ground-based cloud observation and analysis.

In this study, merging two photonic crystal-based structures, a new design for an all-optical 2-to-4 decoder has been proposed. The switching operation is based on the Kerr effect and refractive index modification. The structure consists of one nonlinear ring resonator and three nonlinear cavities that have been modified for entering the slow-light regime in order to enhance coupling through waveguides. The maximum group index of 94 has been obtained for the proposed slow-light waveguides. With this approach, the maximum and minimum normalized output powers for logic 0 and 1 are 4% and 82%, respectively. The data transfer rate of the decoder is 220 GHz, and the size of the structure is 24×9.5 μm2. The maximum insertion loss and cross talk are −7.45 dB and −16.38 dB, respectively. Considering the above characteristics, the proposed decoder can be qualified as a part of optical integrated circuits.

In this paper, we proposed a large-coverage underwater visible light communication (VLC) system utilizing quadrature amplitude modulation-discrete multitone (QAM-DMT) and integrated 2×2 positive-intrinsic-negative (PIN) array reception to achieve equal-gain combining (EGC) receiving. The transmitter consists of a lens group and blue-emitting silicon substrate light-emitting diode peak emission, the wavelength of which is 458 nm. After 1.2 m underwater transmission, a data rate of over 1 Gbit/s is successfully achieved with a spot of 25 cm in diameter and 11 cm alignment tolerances. A data rate of 1.8 Gbit/s is also achieved when using parallel light reception. Both are the highest data rates to our knowledge so far in a large-coverage underwater VLC system, which shows the benefit and feasibility of using an integrated PIN array with QAM-DMT modulation and EGC.

Machine vision techniques, including camera calibration methods, are of great importance for the development of vision-based measurements. However, in multi-camera calibration methods, rapidly constructing accurate geometric relationships among different coordinates is very difficult. Herein, we present a multi-camera calibration method capable of calibrating the intrinsic and extrinsic parameters of four cameras using only a single captured image per camera. Unlike Zhang's method, which relies on multiple captured images to calibrate the cameras, the method uses a multi-plane stereo target containing multiple fixed planes to which coded patterns are attached. This target greatly reduces the time required for calibration and improves calibration robustness. The proposed method was experimentally compared with traditional camera calibration. The problem affecting the calibration accuracy in single calibration of multiple cameras is that the feature points on the captured images produce occlusion or different degrees of blurring; in the calibration of multiple cameras multiple times,…

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.