The 50 most-cited articles in Applied Optics, 2018

Ethanol is widely used in chemical industrial processes as well as in the food and beverage industry. Therefore, methods of detecting alcohol must be accurate, precise, and reliable. In this content, a novel Zeonex-based photonic crystal fiber (PCF) has been modeled and analyzed for ethanol detection in terahertz frequency range. A finite-element-method-based simulation of the PCF sensor shows a high relative sensitivity of 68.87% with negligible confinement loss of 7.79×10−12 cm−1 at 1 THz frequency and x-polarization mode. Moreover, the core power fraction, birefringence, effective material loss, dispersion, and numerical aperture are also determined in the terahertz frequency range. Owing to the simple fiber structure, existing fabrication methods are feasible. With the outstanding waveguiding properties, the proposed sensor can potentially be used in ethanol detection, as well as polarization-preserving applications of terahertz waves.

We present a method for computer-generated holography based on deep learning. The inverse process of light propagation is regressed with a number of computationally generated speckle data sets. This method enables noniterative calculation of computer-generated holograms (CGHs). The proposed method was experimentally verified with a phase-only CGH.

In this work, we proposed a high-resolution D-shaped photonic crystal fiber (PCF) surface plasmon resonance (SPR) sensor based on gold grating. Gold grating is introduced to modulate the resonance wavelength and enhance the refractive index (RI) sensitivity. Structure parameters of PCF and gold grating are analyzed by the finite element method for optimizing the SPR sensor. The simulation results indicate that air hole pitch, air hole diameter, and gold thickness and grating constant have little influence on the sensitivity of the refractive index, which reduces the requirement of precise processing. For improving the resolution of RI sensing, a two-feature (2F) interrogation method, which combines wavelength interrogation and amplitude interrogation, is used. The maximum theoretical resolution of the SPR sensor reaches to 5.98×10−6 RIU in the range of 1.36–1.38, and the wavelength sensitivity reaches to 3340 nm/RIU. The proposed SPR sensor shows potential applications for developing a high-sensitivity, real-time, and fast-response SPR-RI sensor.

The past, present, and future industry prospects of virtual reality (VR) and augmented reality (AR) are presented. The future of VR/AR technology based on holographic display is predicted by analogy with the VR/AR based on binocular vision display and light field display. The investigations on holographic display that can be used in VR/AR are reviewed. The breakthroughs of holographic display are promising in VR/AR with high resolution. The challenges faced by VR/AR based on holographic display are analyzed.

The NASA Langley airborne second-generation High Spectral Resolution Lidar (HSRL-2) uses a density-tuned field-widened Michelson interferometer to implement the HSRL technique at 355 nm. The Michelson interferometer optically separates the received backscattered light between two channels, one of which is dominated by molecular backscattering, while the other contains most of the light backscattered by particles. This interferometer achieves high and stable contrast ratio, defined as the ratio of particulate backscatter signal received by the two channels. We show that a high and stable contrast ratio is critical for precise and accurate backscatter and extinction retrievals. Here, we present retrieval equations that take into account the incomplete separation of particulate and molecular backscatter in the measurement channels. We also show how the accuracy of the contrast ratio assessment propagates to error in the optical properties. For both backscattering and extinction, larger errors are produced by underestimates of the contrast ratio (compared to overestimates), more extreme…

Fixed-pattern noise (FPN), which is caused by the nonuniform opto-electronic responses of microbolometer focal-plane-array (FPA) optoelectronics, imposes a challenging problem in infrared imaging systems. In this paper, we successfully demonstrate that a better single-image-based non-uniformity correction (NUC) operator can be directly learned from a large number of simulated training images instead of being handcrafted as before. Our proposed training scheme, which is based on convolutional neural networks (CNNs) and a column FPN simulation module, gives rise to a powerful technique to reconstruct the noise-free infrared image from its corresponding noisy observation. Specifically, a comprehensive column FPN model is utilized to depict the nonlinear characteristics of column amplifiers in the readout circuit of FPA. A large number of high-fidelity training images are simulated based on this model and the end-to-end residual deep network is capable of learning the intrinsic difference between undesirable FPN and original image details. Therefore, column FPN can be accurately…

In this study, by using an equivalent circuit method, a polarization-insensitive terahertz (THz) absorber based on multilayer graphene-based metasurfaces (MGBMs) is systematically designed, providing an extremely broad absorption bandwidth (BW). The proposed absorber is a compact, three-layer structure, comprising square-, cross-, and circular-shaped graphene metasurfaces embedded between three separator dielectrics. The equivalent-conductivity method serves as a parameter retrieval technique to characterize the graphene metasurfaces as the components of the proposed circuit model. Good agreement is observed between the full-wave simulations and the equivalent-circuit predictions. The optimum MGBM absorber exhibits >90% absorbance in an extremely broad frequency band of 0.55-3.12 THz (BW=140%). The results indicate a significant BW enhancement compared with both the previous metal- and graphene-based THz absorbers, highlighting the capability of the designed MGBM absorber. To clarify the physical mechanism of absorption, the surface current and the electric-field distributions, as…

retrieval using both HSRL and RSP measurements.

A plethora of research in recent years has been reported on biosensing in the surface plasmon resonant systems. However, very little research has reported a tunable and highly sensitive biosensor in a nanoscale platform. In this regard, we propose a nanoscale hyperbolic metamaterial (HMM)-based prism coupled waveguide sensor (PCWS) in the near-infrared range. The HMM layer makes up one of the constituents of the PCWS-comprised of a periodically arranged assembly of silver nanostrips. The structure is numerically simulated by the finite difference time domain method. It is demonstrated that the sensitivity of the reflected light can be tuned through the refractive index (RI) of the solution. Moreover, the effects of alteration of constituents of PCWS on the sensitivity have been analyzed. Results show that the sensitivity of PCWS can be harnessed by altering the thickness, slant angle of HMM layer, volume fraction (f) of metal in the HMM layer, and the incidence angle of light. For this purpose, the structure is numerically simulated by the finite difference time domain method. In…

In this paper, we aim to propose an optical structure for realizing half-adder operation without using the nonlinear Kerr effect. Constructive and destructive interference of optical beams is the main working mechanism of the proposed structure. The phase difference required for the destructive interface will be created by choosing different lengths for the input waveguides of the XOR gate. For the proposed structure, the on–off contrast ratios for SUM and CARRY are 9.77 dB and 6.98 dB, respectively. Also, the delay time and footprint of the proposed structure are about 4 ps and 1056 μm2, respectively.

The technique of digital image correlation (DIC), which has been widely used for noncontact deformation measurements in both the scientific and engineering fields, is greatly affected by the quality of speckle patterns in terms of its performance. This study was concerned with the optimization of the digital speckle pattern (DSP) for DIC in consideration of both the accuracy and efficiency. The root-mean-square error of the inverse compositional Gauss-Newton algorithm and the average number of iterations were used as quality metrics. Moreover, the influence of subset sizes and the noise level of images, which are the basic parameters in the quality assessment formulations, were also considered. The simulated binary speckle patterns were first compared with the Gaussian speckle patterns and captured DSPs. Both the single-radius and multi-radius DSPs were optimized. Experimental tests and analyses were conducted to obtain the optimized and recommended DSP. The vector diagram of the optimized speckle pattern was also uploaded as reference.

Fourier ptychography captures intensity images with varying source patterns (illumination angles) in order to computationally reconstruct large space-bandwidth-product images. Accurate knowledge of the illumination angles is necessary for good image quality; hence, calibration methods are crucial, despite often being impractical or slow. Here, we propose a fast, robust, and accurate self-calibration algorithm that uses only experimentally collected data and general knowledge of the illumination setup. First, our algorithm makes a fast direct estimate of the brightfield illumination angles based on image processing. Then, a more computationally intensive spectral correlation method is used inside the iterative solver to further refine the angle estimates of both brightfield and darkfield images. We demonstrate our method for correcting large and small misalignment artifacts in 2D and 3D Fourier ptychography with different source types: an LED array, a galvo-steered laser, and a high-NA quasi-dome LED illuminator.

In this paper, a plasmonic nanosensor, by using a nanorod array in a square resonator coupled with two slot cavities, with properties for the detection of glucose concentration in water, is proposed and analyzed. We investigated the sensing feature by changing the concentration of the glucose from 0 to 60%. Obtained results show that, by placing different water samples in a square resonator and two cavities, resonance wavelengths can be changed. These resonances demonstrate different dependence on the glucose concentration of water samples. Further, varying the physical parameters of the configuration can also change the resonance wavelength and can be simply tuned. These features recommend flexibility to propose the structure. Simulation results show that the values of sensitivity and figure of merit can be obtained as 892 nm/RIU and 3.5×106 RIU−1, respectively, which can help researchers to discover applications in the plasmonic sensor domain.

We propose a D-shaped photonic crystal fiber (PCF) refractive index sensor with ultrahigh sensitivity and a wide detection range. The gold layer is deposited on the polished surface, avoiding filling or coating inside the air holes of the PCF. The influences of the gold layer thickness and the diameter of the larger air holes are investigated. The sensing characteristics of the proposed sensor are analyzed by the finite element method. The maximum sensitivity can reach 31,000 nm/RIU, and the refractive index detection range is from 1.32 to 1.40. Our proposed PCF has excellent sensing characteristics and is competitive in sensing devices.

We demonstrate a movable, Rydberg atom-based radio frequency (RF) electric (E) field probe. The technique is based on electromagnetically induced transparency and Autler-Townes splitting. Two fibers attached to a 10 mm cubic Cs133 vapor cell are used to couple counter-propagating probe and control lasers through the cell. This all-dielectric fiber-coupled sensor can be moved from the optics table to locations more suitable for RF (gigahertz to sub-terahertz) E-field measurements and calibrations.

Information transfer rates in optical communications may be dramatically increased by making use of spatially non-Gaussian states of light. Here, we demonstrate the ability of deep neural networks to classify numerically generated, noisy Laguerre-Gauss modes of up to 100 quanta of orbital angular momentum with near-unity fidelity. The scheme relies only on the intensity profile of the detected modes, allowing for considerable simplification of current measurement schemes required to sort the states containing increasing degrees of orbital angular momentum. We also present results that show the strength of deep neural networks in the classification of experimental superpositions of Laguerre-Gauss modes when the networks are trained solely using simulated images. It is anticipated that these results will allow for an enhancement of current optical communications technologies.

In this paper, a dual-band perfect metamaterial absorber based on graphene is proposed in the terahertz region. The metamaterial absorber consists of two sizes of graphene disks and a gold film separated by a dielectric spacer in a unit cell. The numerical results demonstrate that the dual-band perfect absorption can be achieved by the superposition of the specific absorption peaks induced by different disks. The resonance frequency can be tuned via controlling the graphene conductivity and the sizes of the disks. The metamaterial absorber can achieve selectively frequency tunability and it can tune each resonance independently. And the dual-band absorption will not be changed when the small disks move along the diagonal within the range of our research. In addition, owing to the symmetry of the structure, the absorber is insensitive to polarization and can keep a high absorptivity with a wide angle. The flexible and simple design makes it possible for our proposed single-layer graphene absorber to be applied in many metamaterial fields, such as sensing, detecting, and cloaking…

In this paper, an all-metal nanostructure is designed with a large frequency ratio (∼6) and a large bandwidth ratio (∼32), and consists of period slit-box cavities and nanodisk clusters. It is a nearly perfect absorber at 1.064 μm to achieve laser stealth, a frequency-selective emitter with low emissivity in wavelength ranges 3-5 and 8-14 μm to achieve infrared stealth, and also an emitter with near unity emissivity at 2.709 μm and 6.107 μm to compensate for the decrease of radiation heat transfer owing to the low emissivity. The absorption/emission peaks are all the Lorentzian shape, and the bandwidths, defined as full width at half-maximum, are 35, 408, and 1124 nm at 1.064, 2.709, and 6.107 μm, respectively. The electric and magnetic field distribution shows that the slit behaves like a capacitor, the box behaves like an inductance, and the nanodisk clusters can excite electric dipole resonance. Considering the solar irradiation, the nanostructure maintains middle-wavelength infrared signal reduction rates greater than 80% from 450 to 1000 K, and long-wavelength infrared signal…

We propose a new type of lensless camera enabling light-field imaging for focusing after image capture and show its feasibilities with some prototyping. The camera basically consists only of an image sensor and Fresnel zone aperture (FZA). Point sources making up the subjects to be captured cast overlapping shadows of the FZA on the sensor, which result in overlapping straight moiré fringes due to multiplication of another virtual FZA in the computer. The fringes generate a captured image by two-dimensional fast Fourier transform. Refocusing is possible by adjusting the size of the virtual FZA. We found this imaging principle is quite analogous to a coherent hologram. Not only the functions of still cameras but also of video cameras are confirmed experimentally by using the prototyped cameras.

In this paper, using a graphene-based metasurface, we demonstrate a unique design to develop a highly efficient, broadband, mid-infrared cross-polarization converter. The proposed graphene-based metasurface structure comprises periodical ϕ-shaped graphene on the top surface of a noble-metal-backed dielectric silicon dioxide (SiO2). The reported structure converts the incident linearly polarized wave into cross-polarized components with a peak polarization conversion ratio of more than 0.9 over a large band. Furthermore, the metasurface structure exhibits the full width at half-maximum bandwidth of 41.98% with respect to its center frequency of 5.98 THz. The physical insights behind electromagnetic polarization conversion are supported by field distributions and retrieved electromagnetic parameters. The structure works as a broadband cross-polarization converter up to 40° incident angle for both TE and TM polarizations. In addition, the structure is found to be as thin as ∼λ/6 with respect to lowermost frequency of the polarization conversion. The period of the unit cell is ∼λ/24 to…

A polymer-coated fiber Bragg grating (PCFBG) is examined for real-time temperature and strain monitoring in composite materials at cryogenic temperatures. The proposed sensor enables the simultaneous measurement of temperature and strain at extremely low temperatures by tracking the changes in the reflected center wavelengths from a pair of PCFBGs embedded in a composite material. The cryogenic temperature sensing was realized by introducing polymer coatings onto bare FBGs, which resulted in high temperature sensitivity under cryogenic conditions. A comparison of wavelength responses of the Bragg grating with and without a polymer coating toward temperatures ranging from 25°C to -180°C was performed. The polymer-coated FBG exhibited a sensitivity of 48 pm/°C, which is 10 times greater than that of the bare FBGs. In addition, the encapsulation of the FBG in a capillary tube made it possible to evaluate the strain accumulated within the composite during operation under cryogenic conditions.

Underwater optical wireless communications (UOWC) performance is affected by turbulence. However, not much research has been carried out to estimate the probability density function (PDF) of the received optical power. In this paper, we investigate the effect of turbulence on the UOWC system using a new experimental setup with a variable link span in a water pool. Different turbulence levels are created by changing the temperature and the rate of an injected water flow in the pool water to obtain the PDF. Results show that lognormal distribution closely matches the measured PDF for a range of link spans. In UOWC systems, the link span is one of the main factors influencing fluctuations of the received optical power, and it has not been thoroughly investigated. In this work, the scintillation index and turbulence-induced power loss are obtained for a range of turbulence strengths and transmission link spans. Finally, we show that there is a good agreement between the experimental and simulated results.

In this work, we present a 3D-printed waveguide that provides effective electromagnetic guidance in the THz regime. The waveguide is printed using low-cost polycarbonate and a conventional fused deposition modeling printer. Light guidance in the hollow core is achieved through antiresonance, and it improves the energy effectively transported to the receiver compared to free space propagation. Our demonstration adds to the field of 3D-printed terahertz components, providing a low-cost way of guiding terahertz radiation.

In this work, issues in phase retrieval in the coherent diffractive imaging (CDI) technique, from discussion on parameters for setting up a CDI experiment to evaluation of the goodness of the final reconstruction, are discussed. The distribution of objects under study by CDI often cannot be cross-validated by another imaging technique. It is therefore important to make sure that the developed CDI procedure delivers an artifact-free object reconstruction. Critical issues that can lead to artifacts are presented and recipes on how to avoid them are provided.

We report on the design, in-depth analysis, and characterization of a novel elliptical array shaped core rectangular shaped cladded photonic crystal fiber (PCF) for multichannel communication and polarization maintaining applications of terahertz waves. The asymmetrical structure of air holes in both core and cladding results in increased birefringence, while a compact geometry and different cladding air hole size makes the dispersion characteristic flat. The modal characteristics of the PCF are calculated using a finite element method. The simulated results show a near-zero dispersion flattened property of ±0.02 ps/THz/cm, high birefringence of 0.063, low effective material loss of 0.06 cm−1, and negligible confinement loss of 5.45×10−13 cm−1 in the terahertz frequency range. Additionally, the core power fraction, effective area, physical attributes, and potential fabrication possibilities of the fiber are discussed.

Conducting quantum key distribution (QKD) through existing optical fibers together with conventional communication signals is a viable way to expand its practical application, but weak quantum signals can be severely disrupted by co-propagating classical signals. In this paper, the suppression of four-wave mixing (FWM) noise and Raman noise is considered simultaneously for the first time, to the best of our knowledge, and the joint optimized channel allocation (JOCA) scheme is proposed. In the JOCA scheme, the quantum channels and classical channels are interleaved with each other to avoid FWM noise and optimal quantum channel positions are chosen in variable conditions according to the Raman scattering spectrum. Experimental measurements of the noise photons show that the JOCA scheme can effectively reduce the impairments on quantum signals compared with the single-target schemes. Additionally, simulation results verify that the JOCA scheme can increase the secure key generation rate and transmission distance, and that it also enables the DWDM-QKD system to tolerate higher-power…

:YLF laser end pumped by blue laser diodes. As the pump source, we used four 5 W single-emitter blue laser diodes and a >20 W fiber-coupled module. In single-emitter-diode pumping, we obtained pump-limited output powers of 6.7 and 3.7 W at 640 and 607 nm, respectively. We successfully suppressed thermal aberration and obtained an output power of 3.4 W at a slope efficiency of 25% at 640 nm with good beam quality in the fiber-coupled module pumping.

This paper presents a polymer-optical-fiber (POF)-based sensor system for simultaneous measurement of angle and temperature. The main contribution is obtaining a sensor with higher temperature sensitivity and lower hysteresis on the angle measurements. The annealing was made on the fibers under the conditions of low relative humidity and under water, and a third set of samples without any heat treatment was applied for comparison with the annealed ones. Results of temperature and angle characterization show that the fibers annealed under water presented higher temperature sensitivity and lower errors when compared with the fibers annealed with low humidity or the fibers without annealing. Furthermore, the fibers annealed under water also presented lower hysteresis on the angle characterization. For these reasons, such fibers were employed for the temperature and angle measurements, which results in a sensor system capable of simultaneously measuring the angle and temperature with root-mean-squared error of 0.82°C for temperature and 2.20° for angle, which is further reduced to 1.20°…

Spectrometers employing acousto-optic tunable filters (AOTFs) rapidly gain popularity in space, and in particular on interplanetary missions. They allow for reducing volume, mass, and complexity of the instrumentation. To date, space operations of 11 AOTF spectrometers are reported in the literature. They were used for analyzing ocean color, greenhouse gases, atmospheres of Mars and Venus, and for lunar mineralogy. More instruments for the Moon, Mars, and asteroid mineralogy are in flight, awaiting launch, or in the state of advanced development. The AOTFs are used in point (pencil-beam) spectrometers for selecting echelle diffraction orders, or in hyper-spectral imagers and microscopes. We review the AOTF-employing devices flown in space or ready to set off. The paper considers basic principles of the AOTF and science applications of the AOTF spectrometers, and describes developed instruments in some detail. We also address some advanced developments for future missions and plans. In addition, we discuss lessons learned during instrument design, build, calibration, and…

Benefiting from the memory effect (ME) for speckle intensity correlations, only one single-shot speckle pattern can be used for the high-quality recovery of objects. However, ME gives a restriction to the field of view (FOV) for imaging with scattering media. Objects beyond the ME region cannot be recovered and produce unwanted speckle patterns, which cause reduction in the speckle contrast and recovery quality. Nevertheless, all the spatial information from a large object is embedded in a single speckle image. Here, we extract the spatial information from these unavoidable speckle patterns and enlarge the FOV of the imaging system with scattering media. Regional point spreading functions, which are fixed and only need to be recorded once for all-time use, are employed to recover corresponding spatial regions of an object by deconvolution. Then, an automatic weighted averaging in an iterative process is performed to obtain the object with significantly enlarged FOV. Our results present an important advancement of imaging techniques with strongly scattering media.

In this paper, a 2-to-4 all-optical decoder based on photonic crystal ring resonators is introduced. The photonic crystal structure has a 2D square chalcogenide rod lattice whose maximum response time is 2 ps. Three ring resonators including nonlinear rods with 9×10−17 m2/W for a Kerr coefficient carry out a switching operation at 1550 nm wavelength. The switching speed of the device is 500 GHz, which is more than that in previously presented works. Also, the small size of the structure is sufficient for optical integrated circuits.

We have developed a three-dimensional (3D) dynamic integral-imaging (InIm)-system-based optical see-through augmented reality display with enhanced depth range of a 3D augmented image. A focus-tunable lens is adopted in the 3D display unit to relay the elemental images with various positions to the micro lens array. Based on resolution priority integral imaging, multiple lenslet image planes are generated to enhance the depth range of the 3D image. The depth range is further increased by utilizing both the real and virtual 3D imaging fields. The 3D reconstructed image and the real-world scene are overlaid using an optical see-through display for augmented reality. The proposed system can significantly enhance the depth range of a 3D reconstructed image with high image quality in the micro InIm unit. This approach provides enhanced functionality for augmented information and adjusts the vergence-accommodation conflict of a traditional augmented reality display.

In this paper, we present a rigorous analytical solution for the bi-aspheric singlet lens design problem. The input of the general formula presented here is the first surface of the singlet lens; this surface must be continuous and such that the rays inside the lens do not cross each other. The output is the correcting second surface of the singlet; the second surface is such that the singlet is free of spherical aberration.

A birefringent single-layer coating photonic crystal fiber biosensor based on surface plasmon resonance is proposed to realize high sensitivity, which is easy to implement, in that only gold is deposited externally. The birefringent nature of the structure provides the sensor with high sensitivity. The results show that the biosensor can obtain the wavelength sensitivity of 15180 nm/refractive index unit (RIU) and high linearity with the analyte RI range of 1.40–1.43, corresponding to the resolution of 5.6818×10−6 RIU. Owing to the high sensitivity and simple structure, the proposed sensor can find important applications in biochemical and biological analyte detection.

We present simulation results quantitatively showing that circularly polarized light persists in transmission through several real-world and model fog environments better than linearly polarized light over broad wavelength ranges from the visible through the infrared. We present results for polydisperse particle distributions from realistic and measured fog environments, comparing the polarization persistence of linear and circular polarization. Using a polarization-tracking Monte Carlo program, we simulate polarized light propagation through four MODTRAN fog models (moderate and heavy radiation fog and moderate and heavy advection fog) and four real-world measured fog particle distributions (Garland measured radiation and advection fogs, Kunkel measured advection fog, and Sandia National Laboratories' Fog Facility's fog). Simulations were performed for each fog environment with wavelengths ranging from 0.4 to 12 µm for increasing optical thicknesses of 5, 10, and 15 (increasing fog density or sensing range). Circular polarization persists superiorly for all optical wavelength bands…

A quantitative study of image matching techniques applied to fiber specklegram sensor analysis is presented. The fiber status is modulated by a microbending transducer, so the output speckle field can be correlated to the input displacements. Once acquired and preprocessed, the specklegrams’ variations were evaluated according to seven approaches. Although the average intensity did not provide reliable information per se, the correlation and sum of differences methods yielded ∼11 mm−1 and ∼14 mm−1 sensitivities, respectively, within a ∼0.06 mm range and low linearity and hysteresis errors, with enhancement possibility by intensity level cancellation. Moreover, the phase-only correlation and the mutual information metrics provided very high sensitivities (22 mm−1 and 120 mm−1, respectively) for a <0.02 mm range, making these techniques suitable for detecting subtle variations in the fiber status due to physical or chemical stimuli.

We have developed a new spectro-gonio radiometer, SHADOWS, to study in the laboratory the bidirectional reflectance distribution function of dark and precious samples. The instrument operates over a wide spectral range from the visible to the near-infrared (350–5000 nm) and is installed in a cold room to operate at a temperature as low as −20°C. The high flux monochromatic beam is focused on the sample, resulting in an illumination spot of about 5.2 mm in diameter. The reflected light is measured by two detectors with high sensitivity (down to 0.005% in reflectance) and absolute accuracy of 1%. The illumination and observations angles, including azimuth, can be varied over wide ranges. This paper presents the scientific and technical constraints of the spectro-gonio radiometer, its design and additional capabilities, as well as the performances and limitations of the instrument.

By applying an underwater hyperspectral imager (UHI) to a selection of archaeological artifacts, we have found spectral signatures that are representative of materials likely to be present at wreck sites. By successfully using the signatures to classify a subset of said artifacts placed on the seabed at 61 m depth, we demonstrate that it is possible to detect archaeological objects of interest in UHI data acquired by a remotely operated vehicle. Correct UHI classification of rust and glass bottles in situ on a historical wreck site further supports the viability of the method for marine archaeological applications.

Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. We have designed a terahertz (THz) semiconductor metamaterial (MM) waveguide system, which exhibits a significant slow-light effect, based on a classical electromagnetically induced transparency phenomenon. The potential of MMs for THz radiation originates from a resonant electromagnetic response that can be tailored for specific applications. By appropriately adjusting the distance between the two radiative and nonradiative modes, a flat band corresponding to a nearly constant group index (of the order of 4924) in the THz regime can be achieved. Finite-difference time-domain simulations show that the incident pulse can be slowed down. The proposed device from a paucity of naturally occurring materials has useful applications in electronic or photonic properties at terahertz frequencies. This proposed compact configuration may find potential applications in plasmonic slow-light systems, optical buffers, and thermal and electromagnetic modulating…

In this paper, we study two adaptive beam control techniques, where the beam divergence angle is adjusted at the transmitter to (i) maximize link availability or (ii) minimize transmitter power while maintaining target link availability. For this purpose, we provide closed-form expressions about the link availability and optimum beam divergence angle under the effect of generalized two-dimensional Gaussian distribution of the alignment error between the transmitter and receiver. These simple and closed-form expressions reduce the computational complexity for performance optimization. Thus, they can be used to (i) reduce the power consumption required for adaptive beam control and (ii) facilitate the fast operation of the control techniques. The results show that the adaptive beam control techniques can improve system performance under various scenarios.

We present a range-gated camera system designed for real-time (10 Hz) 3D estimation underwater. The system uses a fast-shutter CMOS sensor (1280×1024) customized to facilitate gating with 1.67 ns (18.8 cm in water) delay steps relative to the triggering of a solid-state actively Q-switched 532 nm laser. A depth estimation algorithm has been carefully designed to handle the effects of light scattering in water, i.e., forward and backward scattering. The raw range-gated signal is carefully filtered to reduce noise while preserving the signal even in the presence of unwanted backscatter. The resulting signal is proportional to the number of photons that are reflected during a small time unit (range), and objects will show up as peaks in the filtered signal. We present a peak-finding algorithm that is robust to unwanted forward scatter peaks and at the same time can pick out distant peaks that are barely higher than peaks caused by sensor and intensity noise. Super-resolution is achieved by fitting a parabola around the peak, which we show can provide depth precision below 1 cm at high…

We demonstrate a dynamic metasurface aperture as a unique tool for computational ghost imaging at microwave frequencies. The aperture consists of a microstrip waveguide loaded with an array of metamaterial elements, each of which couples energy from the waveguide mode to the radiation field. With a tuning mechanism introduced into each independently addressable metamaterial element, the aperture can produce diverse radiation patterns that vary as a function of tuning state. Here, we show that fields from such an aperture approximately obey speckle statistics in the radiative near field. Inspired by the analogy with optical correlation imaging, we use the dynamic aperture as a means of illuminating a scene with structured microwave radiation, receiving the backscattered intensity with a simple waveguide probe. By correlating the magnitude of the received signal with the structured intensity patterns, we demonstrate high-fidelity, phaseless imaging of sparse targets. The dynamic metasurface aperture as a novel ghost imaging structure can find application in security screening,…

The field of view of traditional heads-up display systems is limited by the size of the projection optics. Our research is focused on overcoming this limitation by coupling image-bearing light into a waveguide using holographic elements, propagating the light through that waveguide, and extracting the light several times with additional holographic optical elements. With this configuration, we demonstrated both longitudinal magnification and pupil expansion of the heads-up display. We created a ray-trace model of the optical system to optimize the component parameters and implemented the solution in a prototype that demonstrates the merit of our approach. Longitudinal magnification is achieved by encoding optical power into the hologram injecting the light into the waveguide, while pupil expansion is obtained by expanding the size of the hologram extracting the light from the waveguide element. To ensure uniform intensity of the image, the diffraction efficiency of the extracting hologram is modulated according to the position. Our design has a 12°×8° field of view at a viewing…

We demonstrate a directional curvature sensor based on tilted few-mode fiber Bragg gratings (FM-FBGs) inscribed by a UV laser. The eigenmodes of LP01 and LP11 mode groups are simulated along with the fiber bending. The directional curvature sensor is based on the LP11 mode resonance in the tilted FM-FBG. For curvature from 4.883 to 7.625 m−1, the curvature sensitivities at direction of 0° and 90° are measured to be −2.67 and 0.128 dB/m−1, respectively. The temperature variation barely affects the resonance depth of LP11 mode. The proposed curvature sensor clearly demonstrates the potential to simultaneous directional curvature and temperature measurement with the resolutions of 9.15×10−4 m−1 and 0.952°C, respectively.

The volumetric calibration of a plenoptic camera is explored to correct for inaccuracies due to real-world lens distortions and thin-lens assumptions in current processing methods. Two methods of volumetric calibration based on a polynomial mapping function that does not require knowledge of specific lens parameters are presented and compared to a calibration based on thin-lens assumptions. The first method, volumetric dewarping, is executed by creation of a volumetric representation of a scene using the thin-lens assumptions, which is then corrected in post-processing using a polynomial mapping function. The second method, direct light-field calibration, uses the polynomial mapping in creation of the initial volumetric representation to relate locations in object space directly to image sensor locations. The accuracy and feasibility of these methods is examined experimentally by capturing images of a known dot card at a variety of depths. Results suggest that use of a 3D polynomial mapping function provides a significant increase in reconstruction accuracy and that the achievable…

Most modern security systems depend on biometrics. Unfortunately, these systems have suffered from hacking trials. If the biometric databases have been hacked and stolen, the biometrics saved in these databases will be lost forever. Thus, there is a desperate need to develop new cancelable biometric systems. The basic concept of cancelable biometrics is to use another version of the original biometric template created through a one-way transform or an encryption scheme to keep the original biometrics safe and away from utilization in the system. In this paper, the optical double random phase encoding (DRPE) algorithm is utilized for cancelable face and iris recognition systems. In the proposed cancelable face recognition scheme, the scale invariant feature transform is used for feature extraction from the face images. The extracted feature map is encrypted with the DRPE algorithm. The proposed cancelable iris recognition system depends on the utilization of two iris images for the same person and features are extracted from both images. The features extracted from one of the iris…

This paper introduces a structured light system with two projectors and one camera for three-dimensional (3D) shape measurement to alleviate problems created by a single projector such as the shadow problem. In particular, we developed (1) a system calibration framework that can accurately calibrate each such camera-projector system; (2) a residual error correction method based on the system error function; and (3) a data fusion method utilizing the angle between the projection direction and surface normal. Experimental results demonstrate that the proposed dual-projector structured light system improves the measurement accuracy besides extending the measurement range of a single projector system.

Fringe projection profilometry has been widely used in many fields for its advantages such as high speed, high accuracy, and robustness to environmental illumination and surface texture. However, it is vulnerable to high dynamic range (HDR) objects. To this end, we propose a technique that can enhance the dynamic range of the fringe projection profilometry system. According to the surface reflectivities of the measured objects, several groups of fringe patterns with optimal light intensities are generated based on the intensity response function of a camera. The HDR fringe images are acquired by fusing these fringe patterns, and a three-step phase-shifting algorithm is used to obtain the unwrapped phase from the fused images. Experimental results demonstrate that the proposed technique can accurately measure objects with an HDR of surface reflectivity variation.

With the fast development of deep learning, its performance in image classification and object recognition has presented dramatic improvements. These promising results could also be applied to better understand speckle patterns in scattering media imaging. In this paper, a multimode fiber is used as the scattering media, and 4000 face and nonface original images are transmitted generating speckle patterns. A SpeckleNet is proposed and trained with these 3600 speckle patterns based on a convolutional neural network, and its output layer is activated for a support vector machine (SVM) classifier. The binary classification accuracy of the proposed CNN-architecture SpeckleNet for face and nonface speckle patterns classification tested on another 400 speckle patterns is about 96%, which has been improved compared with the accuracy of the pure SVM method. The promising results confirm that the combination with deep learning could lead to lower optical and computation costs in optical sensing and contribute to practical applications in optics.

Atmospheric turbulence is a major challenge for practical orbital angular momentum (OAM)-based free-space optical (FSO) communication systems that causes intermodal crosstalk and degrades the performance of the system. Herein, we propose a hybrid input-output algorithm (HIOA)-based adaptive optics (AO) system to compensate for distorted OAM beams. The principle and parameters of the HIOA-based AO system in an OAM-based FSO system are analyzed, and the performance is discussed. The simulation results indicate that the HIOA-based AO system can effectively correct distorted OAM beams and that the HIOA improves the compensation performance and convergence speed compared to the traditional Gerchberg-Saxton algorithm. Moreover, we analyze the compensation performance based on different probe beams. Using an OAM beam with state l=1 as a probe beam can yield better correction effects than a Gaussian beam. This work verifies the feasibility of using an HIOA for adaptive turbulence compensation and provides new insights into OAM communication systems.