Complementary metal-oxide semiconductor image sensors suffer from undesirable geometric image distortion due to the rolling shutter effect. However, conventional algorithms cannot be used for strict real-time use in mobile devices. An extremely low computational method to compensate rolling shutter effect for the mobile devices is proposed. The proposed method efficiently predicts the rotational camera motion in three-dimensional space from the global motion vectors predicted in two-dimensional image space. Hence, the computational complexity is extremely reduced. To further improve the video quality, the video stabilization to reduce the high-frequency jitter of the camera motion without increasing the complexity is introduced. The results of simulation demonstrate that while the computational complexity is extremely low, the proposed algorithm provides acceptable performance.

A theoretical model is proposed to analyze the time-domain spectral phase en/decoding and evaluate the unconditional security of the optical code-based secure optical communication systems with DPSK modulation. The confidentiality of the systems with and without bit-by-bit code scrambling technique is investigated. It mathematically proves that the system without code scrambling lacks confidentiality and the system employing code scrambling can realize unconditionally secure transmission. Furthermore, encoding only within the central band of the spectrum is sufficient for perfect secrecy, and the secrecy rate can be potentially improved. It allows the system to operate in the burst mode for code generation and encoding and thus have an idle time for rearm.

In recent times, both the windowed Fourier ridges (WFR) and sampling moiré (SM) algorithms have been extensively used due to their high effectiveness in the demodulation of carrier fringe patterns. As they are developed independently, they are mostly recognized as completely different techniques, but we theoretically prove that SM is a special WFR with a specific window shape and a preset local frequency. This unifies the two different algorithms and enhances the understanding of their theoretical aspects, which helps to simplify the selection of these algorithms in real applications.

A strip-loaded channel waveguide was fabricated by deposition of Au nanoparticles (AuNPs) in a thin strip via organometallic chemical vapor deposition on top of a polystyrene slab waveguide. The beam confinement is demonstrated experimentally, despite the absence of a classical closed layer for strip loading. The AuNP density conditions for plasmonic-waveguide hybrid transmission devices are discussed.

We propose a four-channel signal-receiver optical subassembly (ROSA) for partial-discharge signal transmission using a coarse-wavelength-division multiplexing (CWDM) thin-film filter (TFF). The proposed four-channel ROSA consists of an ultrasmall-factor-based demultiplexer module, which is fabricated with a parallel optical block, four CWDM TFFs, and four 28-Gbps integrated positive-intrinsic-negative photodiode blocks. The partial discharge on a 22.9-kV XLPE cable joint was successfully measured using the proposed module with phase-resolved partial-discharge analysis and a pulse-diagnosis system. The experimental results demonstrated the feasibility and effectiveness of the proposed ROSA module for the diagnosis of partial discharge, which is important for monitoring electric power facilities.

This guest editorial introduces the Special Section on Advances in Optical Design.

Reflecting anastigmatic optical systems hold several inherent advantages over refracting equivalents, such as compactness, absence of color, high “refractive efficiency,” wide bandwidth, and size scalability to enormous apertures. Such advantages have led to these systems becoming, increasingly since their first deliberate development in 1905, the “go-to” solution for various classes of optical design problems. This paper describes in broad terms the history of the development of this class of optical system, with an emphasis on the early history.

When symmetry is broken, the wave-aberration function generalizes to have not only odd-orders but also even-orders. Geometric interpretation of the lowest-order coefficients (i.e., second) and insights offered by consideration of these terms are presented.

A glass selection optimization algorithm is developed for primary and secondary color correction in thick lens systems. The approach is based on the downhill simplex method and requires manipulation of the surface color equations to obtain a single glass-dependent parameter for each lens element. Linear correlation is used to relate this parameter to all other glass-dependent variables. The algorithm provides a statistical distribution of Abbe numbers for each element in the system. Examples of several lenses, from two- to six-element systems, are performed to verify this approach. The optimization algorithm proposed is capable of finding glass solutions with high color correction without requiring an exhaustive search of the glass catalog.

A systematic method for the design of nonaxially symmetric optical systems is described. Free-form optical surfaces are constructed by superposition of a conic segment and a polynomial, and successfully applied to design relatively fast wide field-of-view optical systems.

Driven by the development of freeform four-mirror solutions, we review and compare analytical methods to generate starting point designs with various states of correction, surface types, symmetry, and obscuration. The advantages and disadvantages of each are examined. We have combined several concepts and techniques from the literature to analytically generate unobscured freeform starting point designs that are corrected through the third-order image degrading aberrations. The surfaces in these starting point designs are described as base off-axis conics that image stigmatically for the central field point, also known as Cartesian reflectors, with an aspheric departure “cap” (quartic with the aperture) added to the Cartesian reflectors. Tilt angles are chosen to cancel field-asymmetric field-linear astigmatism and unobscure the system. Paraxial data from an equivalent on-axis system are used to solve a system of linear equations to determine the magnitude of the aspheric departure “caps” that are placed on top of the base Cartesian reflectors, in order to eliminate the remaining third-order image degrading aberrations. In this approach, each aspheric departure “cap” is centered about the intersection of the optical-axis-ray, also known as the base ray, with the base surface, rather than being centered about the axis of rotational invariance.

Several designs are shown, which are alternatives to Bernhard Schmidt’s largely unknown 1934 three-lens corrector replacement for an aspheric plate in his famous telescope.

Gabor’s 1941 catadioptric design can be modified to give much higher performance versions. Some designs are shown that are 0.999 NA in air over a wide field with diffraction-limited correction on a curved image.

Mounting aspheres is often challenging because of the higher sensitivity to decenter and tilt compared with spherical lenses. This paper first describes aspheric surface decenter and tilt error as per ISO 10110 standard. Then, the most common lens mounting and alignment method for aspheric lenses are discussed in detail. Finally, an innovative mounting method that uses surface contact mounting is presented. This autocentering method uses the optical surfaces as mounting interfaces to provide a high level of centering accuracy for aspheric lenses. Centering measurement results for different aspheric lenses mounted using this method are also presented.

System assessment for design often involves averages, such as the standard deviation of the wavefront error, which are estimated by ray tracing through a sample of points within the entrance pupil. General-purpose sampling and weighting schemes are presented, and it is also shown that optical design can benefit from tailored versions of these schemes. It turns out that the type of Gaussian quadrature that has long been recognized for efficiency in this domain requires about 40% to 50% more ray tracing to attain comparable accuracy to generic versions of the new schemes. Even greater efficiency gains can be won, however, by tailoring such sampling schemes to the optical context where azimuthal variation in the wavefront is generally weaker than the radial variation. These new schemes are special cases of what is known in the mathematical world as cubature. Our initial results also led to the consideration of simpler sampling configurations that approximate the newfound cubature schemes. We report on the practical application of a selection of such schemes and make observations that aid in the discovery of cubature schemes relevant to optical design of systems with circular pupils.

Consumer-grade range cameras are widely used in three-dimensional reconstruction. However, the resolution and stability limit the quality of reconstruction, especially with transparent objects. A method is proposed to reconstruct the transparency while improving the reconstruction quality of the indoor scene with a single RGB-D sensor. We propose the method to localize the transparent regions from zero depth and wrong depth. The lost surface of transparency is recovered by modeling the statistics of zero depth, variance, and residual error of signed distance function (SDF) with depth data fusion. The camera pose is first initialized by the error minimization of depth map on the SDF and k-color-frame constraint. The pose then is optimized by the penalized coefficient function, which lowers the weight of voxels with higher SDF error. The method is proved to be valid in localizing the transparent objects and can achieve a more robust camera pose under a complex background.

Stereoscopic image quality assessment (SIQA) is an important task in many applications. An efficient SIQA model should not only deliver high-quality prediction accuracy but also be computationally efficient. We propose a simple but very efficient quality assessment index to evaluate image quality using the global variation of image gradient magnitude. The quality assessment index is computed by modeling the perceptual gradient attributes of the reference stereoscopic images and the distorted stereoscopic images based on the deviation pooling strategy. Experimental results demonstrate that the proposed algorithm can serve as an efficient predictive image quality feature, which not only delivers highly competitive prediction accuracy, but also low computational complexity.

Midwave infrared (MWIR) measurements of a well-characterized JetCat turbine engine were made using a Telops infrared Fourier transform spectrometer (IFTS) hyperspectral imager (HSI), which utilizes a 320  ×  256 indium-antimonide (InSb) focal plane array, calibrated to give a spectral distribution of radiance for each pixel within the image. Measurements were also made using a Santa Barbara focal plane IR imager (InSb, 320  ×  256 array) that reported integrated radiance values in the MWIR. These measurements were made at varying engine RPM and different nozzle orientations to determine temperature and concentrations of the flow. The different orientations allow for the determination of the degree of asymmetry in the flow in two dimensions. The spectral information from these measurements was used to determine temperature and species concentrations of carbon dioxide and water vapor produced by the combustion of the Jet-A fuel. This was done utilizing the hyperspectral tomography (HT) process, which combines multiple measurement paths and high-resolution spectral data to determine the temperatures and concentrations of CO₂ and H₂O throughout the volume. Corresponding local temperature and pressure readings were taken in order to corroborate the radiometric data. Solution sets on a grid of 0.15-in. spacing based on measurements made at 2  cm^−  1 spectral resolution were generated for six exhaust planes throughout the exhaust flow of a 4  in  .    ×  1  in  .   rectangular exhaust nozzle. The HT process generated core plume temperature values within ±35  K of measured thermocouple data for exhaust planes near the nozzle exit plane. These results show the successful application of the HT algorithms to IFTS HSI data for temperature and species concentration reconstruction.

An automatic video tracker gives tracking error after processing video frames. Since frame rate is limited to 25 or 30 Hz, tracking error update rate is also limited to this order. In addition, there is a processing delay of about one frame in calculating target movement. Due to these factors, track loop bandwidth is limited to 2 to 5 Hz. This restricts tracking performance against fast moving and accelerating targets. A control scheme has been proposed where dynamics between target and tracking system has been modeled as line-of-sight rate, caused by disturbance. This rate has been estimated using a disturbance observer and used to augment conventional tracking loop output. Combined signal has been used to steer line-of-sight of the tracking system and keep the target at aim-point mark. This has resulted in a highly accurate following of the fast moving target. Also, steering rate of camera motion has been controlled dynamically in accordance to its field-of-view and integration time during auto-acquisition of the target to avoid motion blur.

It is always a challenging task to detect an infrared (IR) small target with high-detection rate, low false alarm rate, and high detection speed since the target usually has a small size and dim gray value, and the background is usually complex. Local contrast, which is based on the human visual system, has been proved efficient for IR small target detection, but existing local contrast algorithms are either difference-form or ratio-form and cannot enhance true target and suppress background simultaneously. In addition, most of them are pixel-level algorithms and require a huge amount of calculations. A simple but efficient method named subblock-level ratio-difference joint local contrast measure (SRDLCM) is proposed; it can enhance real small target and suppress complex background simultaneously. In addition, SRDLCM is calculated for each subblock but not each pixel, so its calculation amount can be reduced significantly. The experimental results on seven real IR sequences and one single-frame image dataset show that the proposed algorithm can achieve a good detection performance in detection rate and false alarm rate with a good robustness, and the time consumed for a single frame is only less than 0.04 s.

To improve the timeliness of the three-dimensional (3-D) maximum entropy method, an image segmentation method based on equivalent 3-D entropy and artificial fish swarm optimization algorithm is proposed. An equivalent 3-D entropy method without logarithmic operation is developed, and its equivalence is proved theoretically. The optimal threshold is determined based on the artificial fish swarm optimization algorithm so as to avoid exhaustive search and improve algorithm efficiency. The experimental results demonstrate that the proposed method is more time-efficient than the traditional 3-D entropy method and the equivalent 3-D entropy method without affecting segmentation. Compared with the one-dimensional entropy method and the two-dimensional entropy method, it is obviously superior in noise immunity and detail preservation.

Light sheet fluorescence microscopy is an imaging method in which the sample is illuminated from the side by a wide and relatively thin light sheet. This method minimizes the out-of-focus excitation, provides a good balance between spatial resolution and temporal resolution, and mitigates photobleaching and photodamage. Unfortunately, the axial resolution of light sheet fluorescence microscopy is much lower than lateral resolution, which limits the achievement of isotropic imaging. We propose to combine the subtraction method with light sheet fluorescence microscopy to improve the axial resolution. The experimental results demonstrate that the proposed method can produce higher axial resolution than that of conventional light sheet fluorescence microscopy. In addition, a way to reduce negative values in subtraction method is also discussed.

We propose a fast algorithm to extract the phase in the center of an interferogram with circular fringes obtained in a simple Michelson interferometer setup with one rough surface. The phase in the center is a linear function of the displacement of an object, which is very advantageous. It avoids the uncertainty in displacement associated with intensity measurements near a maximum or minimum. Also the direction of movement is always clear. The method used also overcomes the problems associated with speckle, unavoidable with a rough surface, common in industrial applications, when using laser as a light source. Using a large number of points in the interferogram to obtain one value largely reduces noise and also increases the accuracy. Displacement is measured with an accuracy <10  nm.

It is known that certain geometrical-optics predictions often agree well with optical turbulence field observations even though theoretical constraints for ignoring diffraction may be violated. Geometrical optics assumptions can simplify analyses, and ray optics can significantly reduce simulation computation time. Here, an investigation into angle-of-arrival fluctuations is presented involving wave optics and geometrical (ray) optics computer simulations of a plane wave of visible light propagating through a turbulent refractive-index field. The simulation and Rytov-based theory results for the variances of aperture-filtered angle-of-arrival fluctuations generally agree well for weak scattering (Rytov variance, σR2≲0.2), but for increasing Rytov variance, the simulation results demonstrate a positive slope that can be significantly shallower than that predicted by the theory. For weak-to-moderate scattering regimes ( σR2≲2.67), a comparison of the ray and wave results show they match for aperture diameters greater than about two Fresnel lengths. This result is consistent with a previous theoretical analysis by Cheon and Muschinski. For the strongest scattering case studied ( σR2=26.7), the wave and ray simulations match for aperture diameters greater than about 10 Fresnel lengths. For smaller apertures, we attribute the disparity between the wave and ray simulation results to a Fresnel filtering effect.

Yarn speed and length are two important parameters in the winding process. A noncontact method is proposed to measure the speed and length of the microtension moving yarn automatically in the winding process. A moving yarn could vibrate sharply in the microtension range. With a line laser illuminating the moving yarn, a linear charge-coupled diode camera was used to capture the images. Based on different laser reflection of the yarn texture, we calculated the speed and length of the moving yarn by extracting different features of yarn texture. Yarns with different materials were used to prove the validity of the proposed method under different winding speed. Experiment works have been performed and compared with a direct contact sensor, and the results proved that the proposed method is effective.

Phase measuring deflectometry (PMD) with structured light projection and phase-shifting technique is a highly accurate optical surface measuring method based on the law of reflection. Generally, for surface shape measurement of transparent planar elements, PMD suffers from parasitic reflection. To avoid the unexpected effect of parasitic reflection, a method based on fringe frequency tuning and Fourier-transform is introduced. Numerical simulations and experiments are both conducted to evaluate the performance of the proposed method. Experiment results are shown. Surface figures measured using the proposed method are compared with those measured using traditional phase-shifting algorithm and Fizeau interferometer. Both an optical planar element with a thickness of 24.5 mm and a planar window glass with a thickness of 10 mm are measured. For the optical element, the proposed method is accurate and the measurement error is within 200-nm PV. For window glass, the proposed method yields a better surface figure than the traditional phase-shifting algorithm does.

The measurement of current in electric power transmission lines is crucial for the electrical sector, thus demanding optimal control of the system. Such tasks can be carried out by optical sensors. We present a polarimetric study of an electric current sensor using magneto-optical crystal, as well as the complete development of a mobile optical meter for electric current in low- and medium-voltage lines. Based on our polarimetric study of the optical sensor (transducer), we discuss and compare Faraday rotation levels with the developed mathematical model, presenting a peak-to-peak amplitude variation of 1.8534 in the plane of polarization of light. The development of the transducer is described considering the measurement curves, calibration, linearity analysis, design of the mechanical structure, construction of the electronic structure of data acquisition, and the complete structure of the device. In the end, we present a field test. For the measurements carried out in the laboratory, the sensor presented a linear behavior with sensitivity of 2.4  mV  /  A, as well as a high degree of linearity, in comparison to a device with a linearity of 0.001 A. In addition, we show the development of the optical, mechanical, and electronic structures of the device, as well as field measurements in a distribution line of 13.8 kV. The measurements were performed in an energized line, and presented values compatible with those read in the feeder of the distribution company. At the time, the measured current was 220 A. Therefore, this work is presented as a solution for the realization of safe actions in certain high-voltage lines for companies that operate at high-voltage levels, primarily electric power distribution companies.

A full-view three-dimensional (3-D) measurement method for complex surfaces is proposed, where 3-D data for standard balls with different angles are used to calibrate the rotation axes of a turntable and obtain transformation matrices of 3-D data of adjacent views. It can achieve accurate registration of 3-D data of views with different angles and obtains full-view 3-D data for complex surfaces in conjunction with the method for principal point calibration of cameras and modified triple-frequency six-step phase-shifting phase demodulation methods. Experiments show that the developed system based on the proposed method can achieve automatic registration of 3-D data of views with different angles, and good full-view 3-D measurement precision for complex surfaces.

Scanning beam interference lithography (SBIL) is an advanced large-area grating manufacturing technology. To produce gratings with nanoscale phase accuracy, the phase detector is demanded for a high precision in SBIL. A well-designed homodyne detector with a four-orthogonal detection is proposed to meet the harsh measurement requirements. However, the phase detection could suffer from periodic nonlinearity error, which changes slowly over time. A real-time correction method based on an extended Kalman filter is applied to estimate and compensate the periodic nonlinearity. The simulation results show that this method has great advantages in eliminating the time-varying periodic nonlinearity, compared with the conventional least squares fitting method. An experimental setup is also established to validate the proposed correction method.

Accurate characterization of atmospheric turbulence is useful for performance assessment of optical systems operating in real environments and for designing systems to mitigate turbulence effects. Irradiance-based techniques such as scintillometry, suffer from saturation, and hence commercial scintillometers have limited operational ranges. A method to estimate turbulence parameters, such as path weighted Cn2 and Fried’s coherence diameter r₀ from turbulence-induced random, differential motion of extended features in the time-lapse imagery of a distant target is presented. Since the method is phase-based, it can be applied to longer paths. It has an added advantage of remotely sensing turbulence without the need for deployment of sensors at the target location. The approach uses a derived set of path weighting functions that drop to zero at both ends of the imaging path, the peak location depending on the size of the imaging aperture, and the relative sizes and separations of the features whose motions are being tracked. Using different sized features separated by different amounts, a rich set of weighting functions can be obtained. These weighting functions can be linearly combined to approximate a desired weighting function such as that of a scintillometer or that of r₀ in inhomogeneous turbulence. The time-lapse measurements can thus mimic the measurements of a scintillometer or any other instrument. The method is applied to images captured along two different paths, and the estimates are compared to co-located scintillometer measurements.

A method based on uniform experimental design and the backpropagation (BP) neural network to analyze the sensitivity of measurement errors in the binocular vision system is proposed. Six main error factors that affect the measurement accuracy of the system are summarized according to the binocular vision principle. The measurement experiment was carried out based on uniform design. The mapping relationship between the total measurement error and each error factor was established by BP neural network. Then the sensitivity coefficient of each error factor can be calculated by this mapping. These sensitivity coefficients provide a measure of the degree of influence each factor had on the total measurement error. The results show that the sensitivity coefficients of lens distortion and the image noise are 0.287 and 0.243, respectively, which are significantly higher than the other error factors. This means that these two error factors need to be emphatically suppressed during the measurement process.

We propose an optical method for uroflowmetry, exploiting the laser speckle contrast imaging (LSCI) technique onto an intermediate tubing apparatus having an elastic wall that can sensitively respond to flow-induced shedding vortices. Based on the method, we devised and fabricated an elastic-walled U-shaped tubing apparatus (EWUSTA), using the three-dimensional printing technique. We utilized the spatiotemporal contrast scheme for the LSCI as a fast and reliable computational algorithm. We investigated three different materials of flex-vinyl, ninja-flex, and natural rubber latex for the elastic wall of the EWUSTA in steady flow conditions, and verified that their optimal operational ranges could extend up to 7, 15, and 25 ml/s, respectively. We characterized the natural-rubber-latex-based EWUSTA in dynamic flow conditions in comparison with a commercial reservoir-weight-transducer-based gravimetric flowmeter, and verified its feasibility. We stress that the proposed method can offer precise and accurate information on flow dynamics. In addition, we found that the upper limit of the optimal operational range of the proposed apparatus had strong correlation with the tensile strength of the elastic-wall material. We reckon that the proposed and demonstrated method has great potential not only for uroflowmetry but also for other flow-related medical and industrial applications.

In recent years, the correlation coefficient has been used as a tool for comparison between experimental and synthetic interferograms in algorithms of evaluation, in the areas of optical fabrication and testing. This coefficient has been used with the aim of eliminating the observer criterion during the fabrication process, and in this sense, to make a quantitative test to compare experimental and synthetic interferograms using the correlation as a parameter of evaluation. However, this coefficient is dramatically affected when laboratory conditions are not adequate. Therefore, in this work, we present a detailed analysis of the correlation behavior when interferograms with different values of visibility, Gaussian noise, and background illumination are evaluated/correlated. To analyze the correlation behavior, we simulated different interferograms, where these parameters were varied and examined how they affect the interference patterns. We found that a bad illumination dramatically affects the value of correlation, causing it to decrease to 0.1046, with σ  =  0.1.

An optimization function-based phase-matching method is proposed for the three-dimensional (3-D) fringe projection method. As an essential procedure in 3-D fringe projection metrology, the phase-matching processing has a significant influence on the measurement accuracy. The traditional phase-matching method obtains the corresponding points in different images by comparing the phase-values directly, so its matching precision is easily affected by the phase-measurement error. The proposed method determines the corresponding points automatically and accurately by minimizing the cost function represented by the absolute phase values. The cost function consists of two parts: one for calculating the squared difference of the absolute phase to ensure matching precision, and the other is selected to guarantee optimization convergence. A series of experiments enable us to analyze the precision and effectiveness in practical application.

Precise and stable centroid estimation is crucial for many tasks that require locating a light spot. We present an algorithm for fitting a deformed sigmoid function (two-dimensional sigmoid function) to the edge shape and profiled intensity of a light spot. The information of the spot (such as the center position, the size, and the tilt,) can be obtained by this fitting. The robustness of our proposal against noise, contrast, aspect ratio, and tilt is assessed compared with the adaptive circle–ellipse fitting and the centroid weighted Kalman filter algorithm, and the performance is validated experimentally. We show that our proposed algorithm provides better results than these algorithms, especially with regards to noise immunity. It is an effective and precise algorithm for estimating the centroid location of a light spot and can be used in a wide variety of optical sensing applications.

Accommodation–convergence mismatch is still an unsolved issue within the field of augmented reality, virtual reality, and three-dimensional systems in general. Solutions suggested to correct the focus cue in recent years require additional bandwidth, or compromise the image resolution. Our simple approach to overcome this issue is by using an eye-tracking system and electronic lenses. We propose an electronic hybrid lens system composed of segmented phase profile liquid crystal and Pancharatnam phase lenses. For practical application, eye tracking is necessary for measuring the toe-in of the user’s pupil to calculate the object depth. This information is used to determine the required diopteric power of the hybrid system. The optical performance and imaging quality of the proposed hybrid system are evaluated.

The bidirectional reflectance distribution function (BRDF) describes optical scattering off realistic surfaces. The microfacet BRDF, while computationally simple, lacks accuracy especially for grazing angles. An approximation, which replaces mathematically problematic elements of the microfacet model with the polarization factor from wave optics, has proved useful in accurately modeling the grazing region. We now expand upon this analysis by additionally varying the microfacet distribution function—a fundamental part of microfacet BRDF models. We find that after choosing the best microfacet distribution, 12 of the 18 materials studied show a significant improvement in the BRDF fit at grazing angles using the proposed approximation. Additionally, there was one case for which the approximation produced a model statistically tied for best within the experimental uncertainty of the data. The remaining five materials have significant sources of error outside the grazing region and will be further studied in future work.

We present a general analysis of thickness errors in multilayers using the classical existing methods, and adding a complimentary tool based on admittance perturbed circumferences that allow us to obtain a deeper insight into the spectral performance sensitivity due to each layer composing the system. An analysis of the standard deviation of reflectance based on perturbing the system with statistical thickness errors is considered. The proposed method represents a useful tool to study the sensitivity of any multilayer to determine the manufacturing feasibility in the design stage that can save a number of trial deposition experiments while setting up the manufacturing process of a given system. This fact is demonstrated by considering different classical systems with highly stable and instable spectral regions.

The InGaN-based blue light-emitting diodes (LEDs) with double AlGaN/InGaN superlattice (SL) electron-blocking layers are investigated theoretically. The simulation results indicate that the output power, the radiative recombination rates, and the spontaneous emission rates of the blue LED with double AlGaN/InGaN superlattice layers are evidently improved, meanwhile, the efficiency droop phenomenon is remarkably alleviated, compared with the original structure. The reasons are that the new structure with double AlGaN/InGaN SLs can not only suppress the electron leakage but also enhance the hole injection. Specifically, it can improve the radiative recombination rates, which leads to higher output power than original structure.

Defocusing phase-shifting profilometry is now widely utilized in fast 3-D shape measurements for its high efficiency and accuracy. However, motion-induced ripples and phase unwrapping errors can still be observed while the object measured is moving. An efficient motion-compensated defocusing phase-shifting profilometry, which accurately estimates the motion-induced shifts regarding each pixel, both on the image plane and the phase map, to reduce the artifacts occurring in the dynamic scenes, is proposed. The phase error is analyzed by N-step phase-shifting theory, and with the help of optical flow method and five-step phase-shifting method; this error can be processed according to the motion information obtained. The phase unwrapping errors are also fixed with the proposed motion-compensated algorithm. Compared with the traditional phase-shifting profilometry and state-of-the-art methods, our method demonstrates competitive performance on dynamic objects.

Phase space provides the natural formalism with which to formulate optical imaging problems as a system with constraints. We consider the general formulation of optical imaging problems and look at two examples. The first example is a completely asymmetric freeform prism that has titled surfaces. The second example is a gradient index medium that is exactly solvable. This gives an alternative formalism to standard Seidel aberrations and nodal aberration theory, which can be used in the design process.

A set of high-efficiency optical coupler design methods that can be used to collect multiple light sources for compression and integrate them into one light source outlet is proposed. It adopts parabolic confocal principle and high reflectivity coating and completes the structural design with the formulation concept. In addition to direct sunlight collection, the optical coupler is also coupled to the exit source of the collection subsystem of a natural light illumination system (NLIS). The concept of NLIS is to collect sunlight and direct it into indoor lighting. Due to the numerous light sources and the nonparallel light exiting from the light collection subsystem, it is inconvenient for space utilization and back-end transmission. Therefore, the actual NLIS structure is considered as a coupler to improve the efficiency of this design. The optical software used in this study was Fred. Finally, coupling 900 light sources still maintains 50.28% coupler efficiency. The results show that this optical coupler can provide perfect use in the application, and also solve the export problem, and finally achieve high coupler efficiency.

We present a simple method to assess surface figure error tolerances of optical systems. From a statistical analysis, an empirical equation is obtained for estimating the overall wavefront error contribution from surface figure error tolerance assigned on each surface. The same equation can be used to evaluate the inverse sensitivity for surface figure error specification given an error budget.

This paper aims to present a Lorenz chaotic free-space optical (FSO) communication system (LC-FSO-CS), and to study the impact of FSO channel on a transmitted Lorenz chaotic signal infused by an arbitrary random binary message. It examines and corroborates the LC-FSO-CS by an analytical approach, using autocorrelation and power spectral density in the presence of the channel’s multiplicative noise. To achieve this task, the paper exploits an autocorrelation model for the multiplicative scintillation noise while approximating the transmitted chaotic signal modeled by a quasirandom telegraph signal. The method presented here considers the effect of the additive noise in the FSO channel as negligible, while successfully employing the linear system concepts and logarithmic operation to analyze the influence of scintillation noise on the transmitted chaotic waveform. The emphasis of this work is on last-mile FSO communication with simulation experiments performed and results presented throughout the paper to validate the LC-FSO-CS.

Indoor visible light positioning has become a research hotspot owing to its high accuracy, low cost, and easy implementation. However, many existing indoor positioning systems based on visible light communication (VLC) can only achieve two-dimensional (2-D) positioning and those focusing on three-dimensional (3-D) positioning usually contain various sensors or use complex algorithms. Consequently, a high-precision indoor 3-D positioning system based on VLC using modified artificial fish swarm algorithm is proposed, avoiding the problems mentioned above. To simplify the solution procedure of searching for 3-D coordinates, the 3-D positioning problem is transformed into a global optimization problem with the corresponding fitness function designed according to Lambertian radiation pattern. The simulation executed on 1350 points shows an average 3-D positioning error of 4.05 mm in an indoor space of 4  m  ×  4  m  ×  6  m, and 90% positioning errors are below 5.95 mm. Moreover, when tracking a moving target in the positioning space, the average error is 3.57 mm and the maximum error is 7.25 mm. All in all, the system proposed in this paper stands a good chance of widespread application for its excellent performance in indoor 3-D positioning.

We describe a low-latency optical process-level network (OPLN) scheme based on all-optical edge nodes for sample value (SV) service in substations. The edge nodes we present and demonstrate consist of optical splitters and a wavelength division multiplexer, enabling the proposed scheme to realize optical intrabay physical-layer multicasting and interbay signal gathering. A scalability model for the OPLN and a latency model of the intrabay transmission subsystem are established. Simulation results show that 1  ×  32 optical splitters can be used in the all-optical edge nodes with error-free transmission and that the end-to-end latency for the intrabay transmission of SV flows can be reduced by 46.6% compared with an Ethernet switch under the 100BASE protocol. A prototype of the all-optical edge node is developed and tested, achieving ultralow latency (less than 500 ns) and good compatibility with terminals using an Ethernet interface in a substation. A field-experiment system for SV service, covering eight bays with four all-optical edge nodes, is constructed in a 220-kV substation in Sichuan Province, China. This system achieves low-latency transmission of SV flows without frame loss for more than 10 h of continuous operation.

A visible light positioning (VLP) system based on a single light-emitting diode (LED) using a symmetrical receiver is presented. In the proposed receiver, a horizontal photodetector (PD) is located at the center and surrounded by four tilted PDs, which are arranged symmetrically to increase the accuracy in the actual VLP environments. Based on the received signal strength, the high-accuracy positioning can be easily achieved with only the LED’s location regardless of LED’s and PDs’ characteristics. In addition, the relationship between the positioning error and the PDs’ polar angles (α) is analyzed, which indicates that the polar angles changing from 20 deg to 30 deg are optimum. The experimental results show that when the polar angles are 20 deg, the positioning error changes from 0.79 to 3.97 cm and the average error is 2.32 cm. The proposed VLP system removes the intercell interference caused by multiple LEDs and can be easily applied in the LED-limited scenarios of low costs, low complexity, and high accuracy.

A scheme that can produce a continuous wave (CW) phase-coded signal and phase-coded pulses is proposed. The proposed scheme is based on a dual-polarization Mach–Zehnder modulator (DPol-MZM), a polarization modulator (PolM) and a balanced photodetector (BPD) without the help of an electrical phase shifter or an electrical power divider. In DPol-MZM, a carrier-suppressed double sideband (CS-DSB) signal and an optical carrier are, respectively, produced in the upper and lower arms, which are in two orthogonal polarization states. The output is then sent to a PolM. The polarization multiplexed CS-DSB and optical carrier are phase modulated by driving the RF port of the PolM with a coding signal. After beating between the modulated CS-DSB and optical carrier in a BPD, a stable CW phase-coded signal or phase-coded pulses can be obtained. The proposed scheme is experimentally verified. Phase-coded microwave waveforms at 6 and 11 GHz are generated and phase-coded pulses at 11 GHz are also produced.

A compact triangular waveform generator, consisting of a laser diode, a single dual-output Mach–Zehnder modulator (DOMZM), a variable optical delay line and a balanced photodetector (BPD), is proposed and experimentally demonstrated, using a single-frequency RF signal as the input. The DOMZM is biased at the quadrature point, so that no even-order harmonics of the RF frequency would be generated after the BPD. Then, the modulation index is properly selected to let the generated electrical first- and third-order harmonics approximately equal those of the triangular waveform. The use of the balanced detection not only increases the output power by 6 dB but also removes the DC component and the common-mode noise. No optical filtering is required, which guarantees the simplicity and wide frequency range of the system. A proof-of-concept experiment is carried out. Triangular waveforms with repetition rate tuning from 1 to 13 GHz are successfully generated, with the even-order harmonics suppressed by more than 54 dB.

High-quality Bessel beams with adjustable cone angles are realized using a unimorph deformable mirror (DM) with 61 actuators. The DM not only generates an adjustable diverging or converging conical mirror surface in high precision, but also corrects the optical aberrations in the optical system. Furthermore, the annular beams are generated via the Fourier transform of the generated Bessel beams. The core size of the Bessel beam and the radius of the annular beam are proved to be flexibly controlled by the DM without changing the optical elements and realigning the optical path. Since the unimorph DM has a very high damage threshold, the proposed method paves the way for potential applications involving high-power lasers.

We propose an unequally spaced channel allocation algorithm, i.e., decreasing base units-unequally spaced (DBU-US) algorithm tested for high-capacity dense wavelength division multiplexed (DWDM) transmission systems. The new scheme decides upon the number of base units (BUs) covering the entire bandwidth and the allocation of different number of channels in the BUs. Starting from BU_{i  =  0}, the number of channels in each subsequent BU(BU_{i  =  1  ,  2  ,  3}  )   reduces by a factor of 2, “i” being the index of the BUs. The BU_i  ,  BU_{i  +  1}  ,    …   have even and odd slot frequency allocations, which creates a substantial degree of unequal channel spacing, which virtually proves to reduce the four wave mixing (FWM) products effectively for any number of transmitted channels. When compared with existing unequal channel allocation schemes of unequally spaced repeated unequally spaced (URUS), alternately paired URUS (APRUS), a reduction of 6.25% and 13.7% in bandwidth is observed, respectively. Similarly, a reduction of FWM products by 45.5%, 41.3%, and 31.7% is observed, when compared with URUS, paired URUS, and APRUS, respectively. The FWM intensities generated by DBU-US are reported to be 5 dBm lesser when compared to widely accepted URUS scheme. Therefore, it is concluded that DBU-US is superior and is considered suitable for DWDM transmission systems.

The transmission characteristics of 384  Gbit  /  s free-space laser link with dense wavelength division multiplexing (DWDM) are demonstrated, with channel spacing of 100 GHz. We use a simulated atmosphere channel with a tunable weak turbulence, and bit error rate (BER) curves of back-to-back transmission, which are measured and contrasted, respectively. To our knowledge, it is the first demonstration in the research of DWDM-DP-QPSK high-speed free-space optical (FSO) transmission in an atmospheric channel with tunable turbulence. For 100 GHz channel spacing, receiving sensitivities can be −33.6, −32.7, and −32.1  dBm for 300, 336, and 384  Gbit  /  s DWDM-PM-QPSK signals, respectively, at BER of 3.8  ×  ^{10  −  3} (FEC limit). In comparison with −37.6, −36.1, and −35.1  dBm for B2B case, the power penalties are 4, 3.4, and 3.0 dB, respectively. This study indicates that high-speed coherent DWDM-DP-QPSK FSO transmission is feasible.

We demonstrate a sensing network with 5000 identical ultraweak fiber Bragg gratings (uwFBGs) at equal separations of 100 mm based on optical frequency domain reflectometry. The fiber is 510-m long and includes 5000 uwFBGs with a reflectivity of 0.02% and with identical peak reflection wavelengths of ∼1553.9  nm. With an applied strain from 0 to 600  μϵ on the uwFBGs, this system can unambiguously distinguish the shifts in the wavelengths and the locations of the uwFBGs. Compared with a traditional FBG interrogation method, this technique achieves a high-spatial resolution and a large number of multiplexed uwFBGs. The distinct advantages of the proposed sensor network make it an excellent candidate for distributed strain sensing measurements.

A miniature fiber-optic sensor fabricated by fuse splicing a section of hollow silica tube between two sections of single-mode fiber is proposed and experimentally demonstrated for simultaneous measurement of the liquid refractive index (RI) and temperature. The sensitivities of RI and temperature are measured by linear fitting, respectively, and corresponding detecting resolutions are obtained by the sensitivity matrix. The different response sensitivities of two dips to RI and temperature manifest that the structure proposed can be used for simultaneous measurement. The experimental evidence indicates that the miniature fiber sensor with advantages of easy fabrication, ultracompactness, and robustness is insensitive to axial strain.

Nonlinear phase-drift on 16-QAM optical signals due to semiconductor optical amplifier gain saturation were measured for two devices—a linear and a nonlinear one—considering the input optical power and the net optical gain. The signal when amplified by the nonlinear device is marred even for low P_in (−20  dBm), and for moderate input powers (−15  dBm) signal quality stays always above the forward error correction (FEC) limit (error vector magnitude  =  16  %  ). The nonlinear device as expected induced more degradation, being able to operate fairly just for small gain and input power; for moderate gain (12 dB), the amplified signals stay always above the FEC limit. The linear device showed fair operation even for moderate P_in  =    −  15  dBm, working under FEC limit for OSNR (at Rx) higher than 16.5 dB for 10 Gbaud, and higher than 20 dB for 25 Gbaud, with small penalties (<2  dB) in relation to the back-to-back configuration.

A high-sensitivity and low-cost optical fiber sensor for seawater temperature sensing is designed and fabricated. The large core-offset splicing sensing structure was made of single-mode fiber only. To fabricate the sensing structure precisely with a length deviation <10  μm, the relative fabrication equipment was designed and assembled. Temperature measurement experiment of the sensor was performed in a seawater bath. Also, the experimental results show that redshift of the wavelength occurs with the increase of external temperature. Its temperature response curve conforms to the quadratic curve. The temperature sensitivity can reach up to 2.293 nm/°C, and the fitting degree is up to 0.9998. Moreover, the excellent repeatability of our sensor has been verified. The sensor has great potential to measure the temperature in the ocean.

Plasmonic demultiplexers using improved circular nanodisk resonators (CNRs) and metal-insulator-metal waveguides have been designed. The proposed structures use air and silver as insulator and metal layers, respectively. The relative permittivity of silver has been characterized by Drude, Palik, and Drude–Lorentz models in our finite-difference time-domain simulations. To obtain demultiplexers, first two filters based on improved CNRs are designed. One of the most outstanding features of a CNR is that the resonance wavelength can be tuned by changing its radius. It is shown that increasing the CNRs radii for the single-mode bandpass filter increases the resonance wavelength, linearly. Accordingly, double and triple-wavelength demultiplexers (DeMuxes) have been proposed for the wavelength range of 600 to 2000 nm. Due to their small area, the proposed demultiplexers can be applied in integrated optical circuits for optical communication purposes.

We propose a design for silicon-on-chip integrated eight-channel wavelength division multiplexing (WDM) demultiplexer, which consists of parallel-arrayed one-dimensional (1-D) photonic crystal nanobeam cavities (PCNCs) with high-Q over 10⁵ and large free spectral range of ∼200  nm. To the best of our knowledge, this is for the first time that a 1-D PCNC-based demultiplexer is presented. The performance of the device is investigated theoretically by using three-dimensional finite-difference time-domain method. To enable eight-channel parallel arrayed 1-D PCNCs to be coupled to on-chip optical networks for higher integration and multiplex application, an 1  ×  8 taper-type equal optical power splitter is used to connect all channels simultaneously. The total device footprint is as small as 12  μm  ×  15  μm (width  ×  length), which is decreased by five times compared to that per channel in the recent two-dimensional (2-D) PC-based demultiplexer. Moreover, the average channel spacing smaller than 115 GHz is achieved, which is more than two times smaller than that of 2-D PC nanocavity devices, demonstrating that the arrayed nanocavities have the potential for developing ultracompact 100-GHz spaced filters in a dense WDM system. Thus, we believe that the results demonstrated in this work is promising for the future on-chip photonics integrated circuits and optical communication systems.

A processing technique for enhancing imagery and ranging data collected by an underwater modulated pulse laser system is presented. Laser-based sensors offer high-resolution and high-accuracy for imaging and ranging applications in the underwater environment. However, these capabilities can be degraded in turbid waters due to scattering. We present experimental results demonstrating a technique inspired by image processing which reduces the effects of both backscatter and forward scatter. By combining individual return waveforms of the modulated pulse laser system together and applying the processing approach as described, images can be formed in which an object in the scene can be distinguished from scatter using an edge detector. Results obtained by applying the technique to laboratory experimental data are presented and compared to a baseline approach. Useful three-dimensional imagery was generated out to 6.9 attenuation lengths, a 25% improvement over the baseline. In range finding experiments without a range-gate, a test object was detected at 11.1 attenuation lengths downrange, compared to 5.7 attenuation lengths for the baseline.

We propose a photonic crystal dual-resonant microcavity and waveguide-coupled temperature sensor structure. The resonant characteristics of photonic crystal microcavities are simulated by finite-difference time-domain method. Due to the positive thermo-optic effect of silicon and the negative thermo-optic effect of SU-8 photoresist, a resonant wavelength shift is detected for temperature sensing in the opposite direction, which significantly improves the sensitivity of the sensor. Simulation results highlight that the sensitivity of the temperature sensor is 124.69  pm  /    °  C, the temperature measuring range is ∼150  °  C, and the limitation of the temperature sensing area is reduced. The sensor can be integrated to lab-on-chip and system-on-chip to achieve real-time temperature measurement in different microregions.

An optical fiber ocean bottom vector magnetometer (OOBVM) array based on fiber laser sensing technology is proposed. The fiber laser is modulated at the natural frequency of the sensing structure by the Ampere force in the magnetic field. In this way, the sensitivity is enhanced and the effect of power fluctuation of fiber laser is eliminated. The test result shows that its magnetic field resolution is about 0.86  nT  /    √  Hz@100  mA. The design and assembly process of the OOBVM array are demonstrated. A field test in South China Sea demonstrated that the OOBVM array can detect the movement of ferromagnetic objects in water, which implies the potential for applications in underwater object detection and marine geophysics.

In recent years, the optical resonators of whispering gallery modes (WGMs) have attracted increasing interest in the scientific community. These are the axially symmetric dielectric resonators (spherical, disk, torus, bottle-shaped, etc.) with smooth edges that support the existence of WGMs due to the total internal reflection from the surface of the resonator. The rotation of the WGM resonators with respect to the inertial space causes changes in their radii. This, in turn, leads to the reciprocal (the same for light beams travelling in opposite directions) spectral shift of the WGMs. Thus, the rotation of such resonators causes a spectral shift, by which it is possible to calculate the motion parameters. Earlier the prospects of creation of the angular velocity sensor on the basis of this effect were discussed. Spherical, torus, and disk-shaped resonators were considered as possible sensitive elements. This work is devoted to the analysis of the influence of rotation on the mutual shift of the WGM of bottle-shaped resonators. An important part of the work is the study of cross-sensitivity of bottle resonators to rotation.

Fiber ring laser gyroscopes (FRLGs) are among the most sensitive rotation sensors. Recently, we have proposed a scheme for improving the sensitivity of such gyroscopes significantly by using the fast light effect. The resulting device, called the active fast light fiber-optic sensor (AFLIFOS), makes use of a pair of counter-propagating superluminal Brillouin lasers in a fiber cavity. Compared to a conventional FRLG, the sensitivity of the AFLIFOS is expected to be enhanced by nearly four orders of magnitude. For both conventional FRLG and the AFLIFOS, the overall sensitivity increases with increasing output power. However, when the power of the pump laser used for producing the Brillouin gain exceeds a threshold value, cascaded higher order Brillouin lasing may occur, thus complicating the dynamics of the AFLIFOS, and limiting the maximum achievable sensitivity. We experimentally study the parameters that determine the onset of second- and third-order Brillouin lasing in a fiber cavity. We also analyze how the pump power affects the AFLIFOS operation and show how the measured threshold for second-order Brillouin lasing sets a practical limit for the AFLIFOS sensitivity.

This erratum corrects an error in the authorship of the original paper.