It is predicted that the number of internet-of-things (IoT) devices will be <28 billion in 2020. Due to the shortage of the conventional radio-frequency spectrum, using visible light communication (VLC) for IoT can be promising. IoT networks may only require very low-data rate communication for transmitting sensing or identity information. The implementation of a VLC link on existing computer communication standards and interfaces is important. Among the standards, universal asynchronous receiver/transmitter (UART) is very popular. We propose and demonstrate a VLC-over-UART system. Bit error rate analysis is performed. Different components and modules used in the proposed VLC-over-UART system are discussed. Then, we also demonstrate a real-time simultaneous temperature, humidity, and illuminance monitoring using the proposed VLC link.

Star tracker is an important instrument of measuring a spacecraft’s attitude; it measures a spacecraft’s attitude by matching the stars captured by a camera and those stored in a star database, the directions of which are known. Attitude accuracy of star tracker is mainly determined by star centroiding accuracy, which is guaranteed by complete star segmentation. Current algorithms of star segmentation cannot suppress different interferences in star images and cannot segment stars completely because of these interferences. To solve this problem, a new star target segmentation algorithm is proposed on the basis of mathematical morphology. The proposed algorithm utilizes the margin structuring element to detect small targets and the opening operation to suppress noises, and a modified top-hat transform is defined to extract stars. A combination of three different structuring elements is utilized to define a new star segmentation algorithm, and the influence of three different structural elements on the star segmentation results is analyzed. Experimental results show that the proposed algorithm can suppress different interferences and segment stars completely, thus providing high star centroiding accuracy.

An accurate algorithm for three-dimensional (3-D) pose recognition of a rigid object is presented. The algorithm is based on adaptive template matched filtering and local search optimization. When a scene image is captured, a bank of correlation filters is constructed to find the best correspondence between the current view of the target in the scene and a target image synthesized by means of computer graphics. The synthetic image is created using a known 3-D model of the target and an iterative procedure based on local search. Computer simulation results obtained with the proposed algorithm in synthetic and real-life scenes are presented and discussed in terms of accuracy of pose recognition in the presence of noise, cluttered background, and occlusion. Experimental results show that our proposal presents high accuracy for 3-D pose estimation using monocular images.

This paper presents an effective target binarization method for a linear timed address-event (TAE) vision system. In the preprocessing phase, TAE data are processed by denoising, thinning, and edge connection methods sequentially to obtain the denoised- and clear-event contours. Then, the object region will be confirmed by an event-pair matching method. Finally, the image open and close operations of morphology methods are introduced to remove the artifacts generated by event-pair mismatching. Several degraded images were processed by our method and some traditional binarization methods, and the experimental results are provided. As compared with other methods, the proposed method performs efficiently on extracting the target region and gets satisfactory binarization results from object images with low-contrast and nonuniform illumination.

Conventional two-dimensional (2-D) barcode patterns are printed with black and white squares to encode texts. A few papers have proposed special 2-D barcode patterns with extra encrypted data, but the security of extra encrypted data is not emphasized usually. Therefore, this paper proposes color 2-D barcode patterns composed of black, blue, white, and yellow subsquares to extra encrypt binary data with higher security. Because blue looks like black and yellow looks like white, a color 2-D barcode pattern performs like a conventional 2-D barcode pattern. On the other hand, black, blue, white, and yellow subsquares are used to denote binary data. The security of extra encrypted data depends on an image scrambling algorithm by using the sinusoidal function, and the image scrambling algorithm can make a scrambled image have a high image scrambling degree percentage even after image scrambling is operated only one time.

It is essential for accurate image reconstruction to obtain a set of parameters that describes the x-ray scanning geometry. A geometric estimation method is presented for x-ray digital intraoral tomosynthesis (DIT) in which the detector remains stationary while the x-ray source rotates. The main idea is to estimate the three-dimensional (3-D) coordinates of each shot position using at least two small opaque balls adhering to the detector surface as the positioning markers. From the radiographs containing these balls, the position of each x-ray focal spot can be calculated independently relative to the detector center no matter what kind of scanning trajectory is used. A 3-D phantom which roughly simulates DIT was designed to evaluate the performance of this method both quantitatively and qualitatively in the sense of mean square error and structural similarity. Results are also presented for real data acquired with a DIT experimental system. These results prove the validity of this geometric estimation method.

This paper combines the prism and single camera and puts forward a method of stereo imaging with low cost. First of all, according to the principle of geometrical optics, we can deduce the relationship between the prism single-camera system and dual-camera system, and according to the principle of binocular vision we can deduce the relationship between binoculars and dual camera. Thus we can establish the relationship between the prism single-camera system and binoculars and get the positional relation of prism, camera, and object with the best effect of stereo display. Finally, using the active shutter stereo glasses of NVIDIA Company, we can realize the three-dimensional (3-D) display of the object. The experimental results show that the proposed approach can make use of the prism single-camera system to simulate the various observation manners of eyes. The stereo imaging system, which is designed by the method proposed by this paper, can restore the 3-D shape of the object being photographed factually.

Census transform (CT), a stereo matching algorithm, has a strong advantage in radial distortion and brightness changes. However, CT is noise-sensitive because it compares the brightness of a single central pixel based on the brightness values of neighborhood pixels within a matching window. Star-census transform, which compares the brightness of pixels separated by a certain distance along a symmetrical pattern within the matching window, is presented. The proposed method can select the distance between the pixels for comparison and comparison patterns. The experiment results show that the proposed method yields a better performance than the previous CT methods.

We define the displacement, smear, and jitter components of image motion and derive the two-dimensional statistical image motion optical transfer function (OTF) corresponding to each component. These statistical OTFs are parameterized by means and covariances, which are computed most conveniently from a weighted power spectrum of the line-of-sight motion. Another feature of these results is the realization that all temporal and spatial frequencies contribute to each statistical OTF and that one can determine the frequencies that contribute most significantly to each OTF. Additionally, optical system design is typically based upon the properties of an individual image. In a comprehensive optical system design, the statistical properties of an ensemble of images should also be considered. For individual images subject to a constant but possibly unknown smear length, the OTF is a sinc function. This is called a deterministic smear OTF because it does not describe the smear statistically. The statistical smear OTF describes the average smear OTF for an ensemble of images.

Gaze tracking systems are widely used in human–computer interfaces, interfaces for the disabled, game interfaces, and for controlling home appliances. Most studies on gaze detection have focused on enhancing its accuracy, whereas few have considered the discrimination of intentional gaze fixation (looking at a target to activate or select it) from unintentional fixation while using gaze detection systems. Previous research methods based on the use of a keyboard or mouse button, eye blinking, and the dwell time of gaze position have various limitations. Therefore, we propose a method for discriminating between intentional and unintentional gaze fixation using a multimodal fuzzy logic algorithm applied to a gaze tracking system with a near-infrared camera sensor. Experimental results show that the proposed method outperforms the conventional method for determining gaze fixation.

This paper proposes a method for calculating phase-only computer-generated hologram (CGH) in holographic display with reduced speckle noise. The method works by encoding the desired complex-amplitude field of object into a phase-only CGH by a linear canonical transform algorithm. The complex-amplitude field can then be reconstructed independently from the encoded CGH using a filter at the Fourier plane of a single-lens optical system. The feasibility and effectiveness of the proposed method was verified by a simulation experiment. An optical experiment for holographic display was also conducted with reduced speckle using a single phase-only spatial-light modulator. The object was, in fact, reconstructed with different depth of focus clearly without speckle noise due to the simultaneous modulation of both amplitude and phase, confirming our method’s ability to suppress speckle noise in holographic displays by modulating complex amplitude in three-dimensional space.

Fingerprint identification systems have been widely applied in both civilian and governmental applications due to its satisfying performance. However, the fingerprint identification systems can be easily cheated by the presentation of artificial fingerprints made from common materials. Therefore, it reduces the reliability and misleads the decision of the fingerprint identification systems. In this work, we propose a software-based fingerprint liveness detection method based on multiscale difference co-occurrence matrix (DCM). In doing so, multiscale wavelet transform operation is first conducted on the original image. After the preprocessing of the decomposition of the original image, DCMs are computed by using the Laplacian operator. Horizontal and vertical difference co-occurrence matrices are constructed in our method. In order to reduce the dimensionality of the feature vectors, truncation operation is introduced for DCMs. Then, the elements of processing DCMs are regarded as the texture features of original fingerprint images. Finally, classification accuracy of feature vectors is predicted based on a support vector machine classifier. The experimental results have shown that the performance of our method is very promising and meanwhile achieve better accurate classification compared with the best algorithms of LivDet2013 and LivDet2011, while being able to recognize spoofed fingerprints with better recognition accuracy.

A method based on digital image correlation (DIC) is implemented for measuring the height of the roll waves developed in a non-Newtonian fluid flowing on an inclined channel. A projector and a high-resolution digital camera, placed vertically above the fluid surface, are used to project and record a random speckle pattern located on the free liquid surface, where the pattern is deformed due to the developed roll waves. According to the experimental geometry, the height of the roll waves associated to the out-of-plane deformation of the dots is obtained through a quantitative relationship between the experimental parameters and the in-plane displacement field in the flow direction. In terms of this, the out-of-plane deformation is found using a DIC criterion based on the speckle comparison between a reference image without the deformed pattern and an image with a deformed pattern. The maximum height of the roll waves computed with this technique is compared with the height measured using a lateral camera, with both results differing by <10% over the set of experimental instances.

We propose a stress measurement system based on a projection moiré method and heterodyne interferometry for thin films on a flexible substrate. In the measurement setup, a CMOS camera in which every pixel can receive a series of heterodyne moiré signals by using a continuously relative displacement with a constant velocity is used. Furthermore, the phase of the optimized sinusoidal curve and the surface profile of the flexible substrate are determined using a least-squares sine fitting algorithm. The thin-film stress is obtained by representing the cross-sectional curve of the surface profile by using a polynomial fitting method, estimating the resultant curvature radii of the uncoated and coated substrates, and using these two radii in the corrected Stoney formula. The proposed measurement system has the advantages of high accuracy, high resolution, and high capacity for substrates with high flexibility and a large measurement depth.

Traditional electrical sensor or traditional fiber Bragg grating sensing technology is not applicable to the measurement of nonuniform strain in composite material. Therefore, the distributed nonuniform strain in the lap plate position of composite interlining material is measured using a single fiber with optical frequency domain reflection technology in this study. The experimental results show consistency with the experiment phenomena, and the measurement accuracy could be increased to the submillimeter level.

In part 1 of the study, background-oriented schlieren (BOS) and fringe deflection (FD) were numerically compared when used for the measurement of temperature. The aim of this part is to experimentally corroborate the obtained numerical results. In this regard, we analyze an axisymmetric flame issued by a gas nozzle. Fringe deflection and BOS images are recorded at two different points in time and the corresponding displacement results are compared. Furthermore, we implement a variation of the techniques that allows us to carry out simultaneous displacement measurements by them. In this case, the signals of the techniques are encoded on the RGB channels of a color background image. The results confirm that FD slightly outperforms BOS, in particular for images that contain relatively high temperature gradients or regions with low contrast.

Diamond turning of high-precision molds is a vital process for the roll-to-roll-based ultraviolet resin imprinting process in fabricating subwavelength gratings. The effects of the grating shape and grating period on diffraction efficiencies and diffraction angles were simulated. Experiments were then conducted to examine the effects of shape design, grating period, and cutting speed on machinability of the mold. According to the optical measurement results, the performance of the subwavelength gratings matched the design well at various incident angles. The results confirm that diamond turning of high-precision molds is a feasible approach for ensuring the continual mass production of subwavelength gratings.

Thulium (Tm)-doped fiber lasers offer a broad emission bandwidth in the 2-μm region, providing the perfect basis to develop broadly tunable laser sources, e.g., for spectroscopic applications. Recently, a tuning principle for pulsed fiber lasers has been reported, which is based on a fiber Bragg grating (FBG) array as a discrete spectral filter. This concept uniquely combines an unrivaled spectral freedom for tailored tuning ranges with a monolithic layout preserving the inherent advantages of fiber-integrated systems. In this study, we investigate this discrete tuning method using a Tm-doped fiber laser in the spectral domain around 1950 nm. While the laser emits linearly polarized light based on a polarization-maintaining (PM) resonator, we also examine the possibility of using standard FBG arrays inscribed in non-PM fiber. In order to highlight the prospect for tunable high-power operation, the tunable seed laser is implemented in a master oscillator power amplifier configuration scaling the average power to ∼28  W. With a tuning range of up to 76 nm, the emission characteristics of the system are investigated showing pulse durations down to 11 ns and a very good spectral signal contrast with narrow linewidth.

With their large field of view, anamorphosis, and areas of enhanced magnification, panomorph lenses are an interesting choice for navigation systems for mobile robotics in which knowledge of the surroundings is mandatory. However, panomorph lenses special characteristics can be challenging during the calibration process. This study focuses on the calibration of two panomorph stereoscopic systems with a model and technique developed for narrow-angle lenses, the “Camera Calibration Toolbox for MATLAB.” In order to assess the performance of the systems, the mean reprojection error (MRE) related to the calibration and the reconstruction error of control points of an object of interest at various locations in the field of view are used. The calibrations were successful and exhibit MREs of less than one pixel in all cases. However, some poorly reconstructed control points illustrate that an acceptable MRE guarantees neither the quality of 3-D reconstruction nor its uniformity in the field of view. In addition, the nonuniformity in the 3-D reconstruction quality indicates that panomorph lenses require a more accurate estimation of the principal point (center of distortion) coordinates to improve the calibration and therefore the 3-D reconstruction.

Damage action, such as human disruption, is one of the major threats to pipeline operation. It is essential to monitor perturbation behavior and locate the position in real time. A pipeline security monitoring system is proposed using a line structure Sagnac distributed optic fiber interferometer with a 3×3 coupler that can modulate the optic signal phase without special modulation and demodulation. The optic structure of the system is simplified, signal processing accuracy improved, and the effect of polarization reduced. The working principle of the monitoring in ideal conditions and phase demodulation are analyzed and the location of the possible damage point is formulated. Simulation and validation tests confirm the feasibility of the proposed monitoring system and indicate that the low frequency signals <1  kHz can be detected effectively. A disturbance can be accurately located over long monitoring distances.

Optical modulation of terahertz surface plasmon polaritons (THz SPPs) propagating in an intrinsic indium antimonide surface is demonstrated in this paper. The modulation is mediated by the modification of free carrier density with optical illumination. Simulation and experimental results show that a THz modulator can be realized by tuning the propagation lengths of THz SPPs, which could be controlled to be larger or shorter than the distance of two razor blades used for the coupling of the THz wave and the THz SPPs. In comparison with conventional THz modulation approaches, this method of manufacturing is simpler and the switching bandwidth is wider. The maximum modulation frequency of the modulators is anticipated to be above gigahertz, thus leading to the possibility of communication applications using the THz baseband.

The design of a scanning photothermal accessory is presented, which can be attached to the camera port of commercial microscopes to measure thermal diffusivity maps with micrometer resolution. The device is based on the thermal expansion recovery technique, which measures the defocusing of a probe beam due to the curvature induced by the local heat delivered by a focused pump beam. The beam delivery and collecting optics are built using optical fiber technology, resulting in a robust optical system that provides collinear pump and probe beams without any alignment adjustment necessary. The quasiconfocal configuration for the signal collection using the same optical fiber sets very restrictive conditions on the positioning and alignment of the optical components of the scanning unit, and a detailed discussion of the design equations is presented. The alignment procedure is carefully described, resulting in a system so robust and stable that no further alignment is necessary for the day-to-day use, becoming a tool that can be used for routine quality control, operated by a trained technician.

When directly applying optical transforms, such as fractional Fourier transform (FrFT), to a single image or real image (input image), the resulting image will become complex-valued, which leads to the doubling of data volume. This data expansion problem can be found in many existing single-image optical encryption schemes. We propose a folding technique to offset the data expansion by constructing a complex input image of half size. And we devise an optical single-image encryption scheme based on double FrFTs, in which this technique together with compressed sensing can bring about the possible maximum compression of encrypted images. Moreover, the chaos-based random circular shift for scrambling is introduced to enhance security. The chaotic random signum matrix is also tried as the measurement matrix, and it displays a good performance. Simulation results demonstrate the validity and security of the proposed scheme.

A refractometer system using four modified Wu-type heterodyne interferometers with a variable length vacuum cell is presented. The proposed system has two working modes: (1) a moving mode for measuring the absolute air refractive index at the start of a measurement and (2) a static mode for monitoring the air refractive index fluctuation with the same bandwidth as a traditional displacement interferometer. The system requires no gas filling or pumping during the measurement and can be used for real-time refractive index compensation. Comparison experiments with empirical equations are conducted to investigate the feasibility and performance of the proposed system. The standard deviation of the measurement difference between the proposed system and empirical equation is 2.8 parts in 10⁷, which is close to the uncertainty of our refractive index reference based on the accuracy of the environmental sensors. The relative refractive index tracking is a few parts in 10⁸ with a bandwidth of 10 Hz, but high bandwidths are readily achievable.

Calibration is a crucial step in fringe projection profilometry, which establishes the relationship between unwrapped phase and (FPP) three-dimensional (3-D) shape data (X,Y,h). For an arbitrarily arranged FPP system, a simple geometrical model and mathematical descriptions of the relationships among phase, height distribution, and transverse coordinate are presented. Based on this, a flexible global calibration method is presented to reconstruct 3-D shape by just using a checkerboard with known separation and alternating white and blue. The calibration board is placed at several random positions to determine the relationship between phase and height, and the relationship between pixel position and X, Y coordinates. To get high accuracy, distortion for each pixel is considered. The validity, flexibility, and practicality of this system and calibration technique are verified by experiments.

Soldering using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glass, ceramics, and optical crystals. This is due to a localized and minimized input of thermal energy. Solderjet bumping technology has been used to assemble a lens mount breadboard using specifications and requirements found for the optical beam expander for the European Space Agency EarthCare Mission. The silica lens and a titanium barrel have been designed and assembled with this technology in order to withstand the stringent mission demands of handling high mechanical and thermal loads without losing the optical performance. Finally, a high-precision optomechanical lens mount has been assembled with minimal localized stress (<1  MPa) showing outstanding performance in terms of wave-front error and beam depolarization ratio before and after environmental tests.

We present a large-core single-mode “windmill” single crystal sapphire optical fiber (SCSF) design, which exhibits single-mode operation by stripping off the higher-order modes (HOMs) while maintaining the fundamental mode. The “windmill” SCSF design was analyzed using the finite element analysis method, in which all the HOMs are leaky. The numerical simulation results show single-mode operation in the spectral range from 0.4 to 2  μm in the windmill SCSF, with an effective core diameter as large as 14  μm. Such fiber is expected to improve the performance of many of the current sapphire fiber optic sensor structures.

A fiber Bragg grating (FBG) sensing network with a bus chain typology structure based on time-division multiplexing (TDM) technology has been developed. Each FBG sensor was placed in an isolated branching circuit separated by an optical splitter. By doing this, multiple reflection and spectrum shadow, which are common in a traditional TDM network, were eliminated since incident light reflected by each sensor did not go through the other sensors. Interference among different FBGs was also avoided. The system was experimentally verified by constructing such a network with 17 FBGs involved. Wavelength and position interrogation were successfully realized. Temperature experiment was carried out on four of the FBGs and the sensitivity was 9.87, 9.92, 9.91, and 9.97  pm/°C, respectively. The durability, reliability, and measuring accuracy of the sensing network were effectively improved due to the bus chain typology structure.

We demonstrated a Q-switched Nd:LuVO₄ laser with fundamental mode at 1064 nm using BaB2O4 electro-optic Q-switching. High-efficiency operation of Q-switched laser with dynamic to static ratio of 91.4% was realized. When the absorbed pump power was 6.59 W, the maximum average output power of 2.88 W was achieved with a repetition rate of 50 kHz. The optical conversion efficiency and slope efficiency were 43.7% and 55.5%, respectively. The minimum pulse width of 17.8 ns was achieved. Meanwhile, the pulse energy and peak power were 57.6  μJ and 3.2 kW, respectively. To the best of our knowledge, this study is the first to demonstrate about the electro-optically Q-switched laser in Nd:LuVO₄ crystal.

An extended interference formula has been derived permitting simultaneous determination of geometrical and optical parameters of optical fibers whether the cores or stress-applying parts are nonconcentric to cladding. This formula can be applied for normal and oblique transverse interferometric techniques. In this paper, this interference formula is combined with computer-aided Mach–Zehnder microinterferometer for characterization of these optical fibers. The advantage of the presented interference formula is that it can be applied for all types of optical fibers. Also, it is used to minimize any uncertainty of the thickness measurement. In comparison with the well-known conventional interference formula, the measurement accuracy of this extended formula is discussed.

We propose a protection scheme based on preconfigured k-edge-connected structures (p-kecs) and study the spectrum resource redundancy of flexible bandwidth optical networks under single-link-failure and multiple-link-failure environments. The lower bound of redundancy and the upper bound on efficiency of preconfigured protection structures in flexible bandwidth optical networks are provided. With sufficient capacity, the analysis shows that p-cycle is the optimal structure against single-link failure and p-kecs are the optimal structures to address multiple-link failures. Straddling links reduce the network redundancy while requiring little or no increase in spare spectrum capacity relative to the total spectrum capacity consumed in the protection process. We theoretically prove that the spectrum resource redundancy of the p-kecs has the same lower bound as that of flexible bandwidth optical networks under multiple-link failures. Numerical results show that p-kecs can achieve or approximate the lower bound on redundancy and support protection against multiple-link failures in static and dynamic networks. These results support the theoretical underpinning for the efficiency of p-kecs in providing protection against multiple-link failures in flexible bandwidth optical networks.

A Rayleigh backscattering (RBS) assisted Brillouin erbium fiber laser scheme with multiwavelength narrow linewidth output is proposed and investigated experimentally. The stimulated Brillouin scattering and RBS take place at two conventional single-mode fibers (SMFs), respectively. RBS is used as a mechanism to compress the linewidth of each Stokes component, and it has been realized and maximized in conventional SMF by optimizing injection power of Stokes light through adjusting variable optical attenuator (VOA). By adjusting VOA attenuation, the laser can obtain three wavelengths output with 3 dB linewidth less than 2 KHz for each wavelength, or six wavelengths output with 3 dB linewidth less than 5 KHz.

We propose an efficient time-domain channel estimation method for polarization interleaving orthogonal frequency-division multiplexing passive optical network with direct detection. Compared with the frequency-domain least-square channel estimator and intrasymbol frequency-domain averaging, the time-domain channel estimator with overhead reduction demonstrates better system robustness against transmission impairments, such as optical noise, chromatic dispersion, and polarization mode dispersion.

We established a theoretical model for a single knot-ring resonator and investigated the transmission spectrum by Jones matrix. The numerical results show that two orthogonal polarization modes of knot-ring, which are originally resonated at the same wavelength, will split into two resonant modes with different wavelengths. The mode splitting is due to the coupling between the two orthogonal polarization modes in the knot-ring when the twisted angle of the twist coupler is not exactly equal to 2mπ (m is an integer). It is also found that the separation of the mode splitting is linearly proportional to the deviation angle δθ with a high correlation coefficient of 99.6% and a slope of 3.17  nm/rad. Furthermore, a transparency phenomenon analogous to coupled-resonator-induced transparency was also predicted by the model. These findings may have potential applications in lasers and sensors.

We report more than two octave spanning mid-IR flat-top supercontinuum (SC) generation using all normal A_s2S₅-borosilicate hybrid photonic crystal fiber. Our design is based on a chalcogenide A_s2S₅ photonic crystal fiber (PCF), where the first ring composed of six air holes is made by borosilicate glass. By injecting 50-fs pulses with 1.6 nJ energy at 2.5  μm in the all normal dispersion (ANDi) regime, a flat-top broadband SC extending from 1 to 5  μm with high-spectral flatness of 8 dB is obtained in only 4-mm fiber length. To the best of our knowledge, we present the broadest flat mid-IR spectrum generated in the ANDi regime of an optical fiber. The self-phase modulation and the optical wave breaking are identified as the main broadening mechanisms. The obtained broadband light source can be potentially used in the field of spectroscopy and in high-resolution optical coherent tomography owing to the high-spectral SC flatness generated by our designed fiber.

A two-core optical fiber composed of a single-mode core and a few-mode core is proposed. Index-matched coupling between two fundamental modes in two cores can be achieved by applying a long-period fiber grating in the few-mode core. Mode-field conversion between the small mode-field area and the large mode-field area with low loss can be achieved by this configuration. Numerical simulation shows that the operation bandwidth of the mode-field converter can be as large as 36 nm if the insertion loss of the converter should be lower than 0.5 dB. Results show that this structure has higher conversion efficiency and lower cross talk as compared to the scheme of direct connection between the single-mode fiber and the few-mode fiber.

To compensate for the gain saturation effect in the high-energy laser amplifier, a modified polarization beam combination (PBC) method is introduced to reshape temporal waveform of the injected laser pulse to obtain a controlled high-energy laser pulse shape after amplification. One linearly polarized beam is divided into two orthogonal polarized beams, which spatially recombine together collinearly after propagating different optical paths with relative time delay in PBC structure. The obtained beam with polarization direction being rotated by the following half wave plate is divided and combined again to reform a new beam in another modified polarization beam structure. The reformed beam is injected into three cascaded laser amplifiers. The amplified pulse shape can be controlled by the incident pulse shape and amplifier gain, which is agreeable to the simulation by the Frank–Nodvik equations. Based on the simple method, the various temporal waveform of output pulse with tunable 7 to 20 ns pulse duration can be obtained without interferometric fringes.

We present a fiber-optic wavelength-modulated sensor for pH applications. Fiber Bragg grating (FBG) is functionalized with a stimulus-responsive hydrogel that induces a strain on FBG due to mechanical expansion of the gel in response to ambient pH changes. The gel is synthesized from the blends of poly (vinyl alcohol)/poly (acrylic acid). The induced strain results in a shift of FBG reflected peak that is monitored by an interrogator. The sensor system shows good linearity in the acidic pH range of 3 to 7 with a sensitivity of 12.16  pm/pH. In addition, it shows good repeatability and oscillator behavior, which proves it to be fit for pH sensing applications.

Currently, an acceleration sensor based on fiber Bragg grating (FBG) has been widely used. A cantilever FBG accelerometer is designed. The simulation of this structure was implemented by finite element software (ANSYS) to analyze its sensing performance parameters. And then the optimized structure was produced and calibration experiments were conducted. On the basis of simulation, optical fiber is embedded in the inner tank of the vibrating mass, and Bragg grating is suspended above the cantilever structure, which can effectively avoid the phenomenon of center wavelength chirp or broadening, and greatly improve the sensitivity of the sensor. The experimental results show that the FBG accelerometer exhibits a sensitivity of 75  pm/(m/s²) (100 Hz) and dynamic range of 60 dB. Its linearity error is <2.31% and repeatability error is <2.76%. And the resonant frequency is ∼125  Hz. The simulation results match the experimental results to demonstrate the good performance of FBG accelerometer, which is expected to be used in the actual project.

We propose and analyze an optimized Lambertian order (OLO) of light-emitting diode for both indoor cellular optical wireless communication and positioning systems. We carry out analysis for the system consisting of a Lambertian source and a tilted optical receiver, and develop an expression for OLO for four-, six-, and nine-cell configurations. We investigate the channel characteristics including the optical path loss, impulse response, transmission bandwidth, and positioning accuracy for the proposed systems with and without OLO, showing that there is a significant improvement in the transmission bandwidth as well as the positioning accuracy when employing OLO. For example, for a four-cell configuration with the optimum Lambertian order, 99% of cumulative distribution function of the estimation errors is within the Cramer–Rao bound (CRB) accuracy of 6.7 to 26.7 cm, compared to the CRB accuracy of 12.8 to 29.7 cm for the Lambertian order of m=1.

The performance of absolute added correlative coding (AACC) modulation format with direct detection has been numerically and analytically reported, targeting metro data center interconnects. Hereby, the focus lies on the performance of the bit error rate, noise contributions, spectral efficiency, and chromatic dispersion tolerance. The signal space model of AACC, where the average electrical and optical power expressions are derived for the first time, is also delineated. The proposed modulation format was also compared to other well-known signaling, such as on-off-keying (OOK) and four-level pulse–amplitude modulation, at the same bit rate in a directly modulated vertical-cavity surface-emitting laser-based transmission system. The comparison results show a clear advantage of AACC in achieving longer fiber delivery distance due to the higher dispersion tolerance.

The performance of the dual-parallel polarization modulator based optical single-sideband modulator (PSSBM) or frequency shifter (FS) has been studied theoretically. There are various factors impacting the performance of PSSBM/FS, such as the state of polarization (SOP), imbalanced power ratio, and direct current (dc) bias control, and they all have been validated through the VPI software. Based on our simulation results, the desired high-quality SSB frequency shift can be achieved through the PSSBM/FS by applying the optimized parameters while only one dc bias control is required. The results show that PSSBM/FS has the advantages and potentiality to be a commercial product used in various scenarios.

Recently, indoor visible light localization has become attractive. Unfortunately, its performance is limited by diffuse reflection. The diffuse reflection is estimated by the bilinear interpolation-based method. A received signal strength-based iterative visible light positioning algorithm is proposed to reduce the influence of diffuse reflection by subtracting the estimated diffuse reflection signal from the received signal. Simulations are made to evaluate the proposed iterative positioning algorithm in a typical scenario with different parameters of the field-of-view (FOV) of the receiver and the reflectivity of the wall. Results show that the proposed algorithm can reduce the average positioning error by 12 times in a typical scenario and can reduce the positioning error greatly with various FOV of the receiver and the reflectivity of the wall. The proposed algorithm is effective and robust to reduce the degradation caused by diffuse reflection in a positioning system and will have many potential applications in indoor localization scenarios.

We investigated wavelength conversion for polarization multiplexing signal based on four-wave mixing in a semiconductor optical amplifier. We found that the converted signals endured crosstalk among the pol-muxed channels. We also proposed and demonstrated a wavelength conversion scheme with polarization diversity technique. By utilizing the technique, the converted polarization multiplexing signal can be received without crosstalk. In addition, the performance of the proposed system is numerically analyzed with respect to the bit error rate of the converted signal, different frequency spacing between signal and pump and modulated data rate. The simulation results show that the proposed scheme may be a promising method to realize transparent wavelength conversion for polarization multiplexing signals.

An investigation has been done on the parameters of a hysteretic bistable optical Schmitt trigger device. From a design point of view, it is important to know the regions where this bistability occurs and is fully functional with respect to its subsystem parameters. Otherwise experimentally reaching such behavior will be very time-consuming and frustrating, especially with multiple devices employed in a single photonic circuit. A photonic Schmitt trigger consisting of two feedbacked inverting amplifiers, each characterized by −m (slope), A (y-intercept), and B (constant base) parameters is considered. This system is investigated dynamically with a varying input to find its stable and unstable states both mathematically and with simulation. In addition to a complete mathematical analysis of the system, we also describe how m, A, and B can be properly chosen in order to satisfy certain system conditions that result in bistability. More restrictions are also imposed to these absolute conditions by the system conditions as will be discussed. Finally, all results are verified in a more realistic photonic simulation.

A nanofiber-plane-grating composite slow-light waveguide to achieve wideband slowlight propagation with no distortion is proposed. The waveguide is formed by embedding a tapered nanofiber into a V-groove on a plane-grating surface. By optimizing the waveguide structural parameters, a slow-light effect with bandwidth of about 1453 GHz is obtained. Based on finite-difference time-domain (FDTD) method, we analyze the waveguide’s optical properties and slow-light characteristics. Simulation results show that a picosecond optical pulse propagating in the slow-light waveguide can be delayed for about 980 fs and without distortion. The group velocity of the optical pulse can be reduced to about 0.3c (c is the speed of light in vacuum). This study will provide important theoretical basis and innovative ideas for the development of new-type slow-light elements.

Chaotic sequences can be applied to realize multiple user access and improve the system security for a visible light communication (VLC) system. However, since the map patterns of chaotic sequences are usually well known, eavesdroppers can possibly derive the key parameters of chaotic sequences and subsequently retrieve the information. We design an advanced encryption standard (AES) interleaving aided multiple user access scheme to enhance the security of a chaotic code division multiple access-based visible light communication (C-CDMA-VLC) system. We propose to spread the information with chaotic sequences, and then the spread information is interleaved by an AES algorithm and transmitted over VLC channels. Since the computation complexity of performing inverse operations to deinterleave the information is high, the eavesdroppers in a high speed VLC system cannot retrieve the information in real time; thus, the system security will be enhanced. Moreover, we build a mathematical model for the AES-aided VLC system and derive the theoretical information leakage to analyze the system security. The simulations are performed over VLC channels, and the results demonstrate the effectiveness and high security of our presented AES interleaving aided chaotic CDMA-VLC system.

Anderson localization has been previously demonstrated both theoretically and experimentally for transmission of a Gaussian beam through long distances in an optical fiber consisting of a random array of smaller fibers, each having either a higher or lower refractive index. However, the computational times were extremely long. We show how to simulate these results using a fast Fresnel diffraction algorithm. In each iteration of this approach, the light passes through a phase mask, undergoes Fresnel diffraction over a small distance, and then passes through the same phase mask. We also show results where we use a binary amplitude mask at the input that selectively illuminates either the higher or the lower index fibers. Additionally, we examine imaging of various sized objects through these fibers. In all cases, our results are consistent with other computational methods and experimental results, but with a much reduced computational time.

Low-cost single-mode four-channel optical transmitter and receiver modules using the wavelength-division multiplexing (WDM) method have been developed for long-reach fiber optic applications. The single-mode four-channel WDM optical transmitter and receiver modules consist of two dual-wavelength optical transmitter and receiver submodules, respectively. The integration of two channels in a glass-sealed transistor outline-can package is an effective way to reduce cost and size and to extend the number of channels. The clear eye diagrams with more than about 6 dB of the extinction ratio and the minimum receiver sensitivity of lower than −16  dBm at a bit error rate of 10⁻¹² have been obtained for the transmitter and receiver modules, respectively, at 5  Gbps/channel. The 4K ultrahigh definition contents have been transmitted over a 1-km-long single-mode fiber using a pair of proposed four-channel transmitter optical subassembly and receiver optical subassembly.

Spatial domain multiplexing/space division multiplexing (SDM) can increase the bandwidth of existing and futuristic optical fibers by an order of magnitude or more. In the SDM technique, we launch multiple single-mode pigtail laser sources of the same wavelength into a carrier multimode fiber at different angles. The launching angles decide the output of the carrier fiber by allocating separate spatial locations for each channel. Each channel follows a helical trajectory while traversing the length of the carrier fiber, thereby allowing spatial reuse of optical frequencies. We launch light from five different single-mode pigtail laser sources (of same wavelength) at different angles (with respect to the axis of the carrier fiber) into the carrier fiber. Owing to helical propagation, five distinct concentric donut-shaped rings with negligible crosstalk at the output end of the fiber were obtained. These SDM channels also exhibit orbital angular momentum (OAM), thereby adding an extradegree of photon freedom. We present the experimental data of five spatially multiplexed channels and compare them with simulated results to show that this technique can potentially improve the data capacity of optical fibers by an order of magnitude: A factor of five using SDM and another factor of two using OAM.

A Brillouin optical time-domain analysis (BOTDA) sensor that combines the conventional complementary coding with the pulse prepump technique for high-accuracy and long-range distributed sensing is implemented and analyzed. The employment of the complementary coding provides an enhanced signal-to-noise ratio (SNR) of the sensing system and an extended sensing distance, and the measurement time is also reduced compared with a BOTDA sensor using linear coding. The combination of pulse prepump technique enables the establishment of a preactivated acoustic field in each pump pulse of the complementary codeword, which ensures measurements of high spatial resolution and high frequency accuracy. The feasibility of the prepumped complementary coding is analyzed theoretically and experimentally. The experiments are carried out beyond 50-km single-mode fiber, and experimental results show the capabilities of the proposed scheme to achieve 1-m spatial resolution with temperature and strain resolutions equal to ∼1.6°C and ∼32  μϵ, and 2-m spatial resolution with temperature and strain resolutions equal to ∼0.3°C and ∼6  μϵ, respectively. A longer sensing distance with the same spatial resolution and measurement accuracy can be achieved through increasing the code length of the prepumped complementary code.

Commercial picosecond sources have found widespread applications. Typical system parameters are pulse widths below 20 ps, repetition rates between 0.1 and 2 MHz, and microjoule level pulse energies. Most systems are based on short pulse mode-locked oscillators, regenerative amplifiers, and pockel cells as active beam switches. In contrast, we present a completely passive system, consisting of a passively Q-switched microchip laser, a single-stage amplifier, and a pulse compressor. The Q-switched microchip laser has a 50-μm-long Nd:YVO₄ gain material optically bonded to a 4.6-mm-thick undoped YVO₄ crystal. It delivers pulse widths of 40 ps and repetition rates of 0.2 to 1.4 MHz at a wavelength of 1.064 μm. The pulse energy is a few nanojoule. These 40-ps pulses are spectrally broadened in a standard single-mode fiber and then compressed in a 24-mm-long chirped Bragg grating to as low as 3.3 ps. The repetition rate can be tuned from ∼0.2 to 1.4 MHz by changing the pump power, while the pulse width and the pulse energy from the microchip laser are unchanged. The spectral broadening in the fiber is observed throughout the pulse repetition rate, supporting sub-10-ps pulses. Finally, the pulses are amplified in a single-stage Nd:YVO₄ amplifier up to the microjoule level (up to 4 μJ pulse energy). As a result, the system delivers sub-10-ps pulses at a microjoule level with about 1 MHz repetition rate, and thus fulfills the requirements for ps-micromachining. It does not contain any active switching elements and can be integrated in a very compact setup.

It is shown experimentally that the near-infrared femtosecond laser irradiation and subsequent thermal treatment of silver-containing silicate glasses result in the formation of luminescent silver molecular clusters (MCs) and silver nanoparticles (NPs). In glasses doped also with halides (mostly Br), the nonluminescent silver NPs are formed because of the presence of halogenide shells on their surfaces, whereas, in glasses with no Br ions, the luminescent silver nanoparticles provides an emission in the 600- to 750-nm range. Two possible mechanisms of luminescence of glass with silver NPs are considered: (i) luminescence of silver NPs without halogenide shell and (ii) the luminescence of silver MCs Agm (m=1 to 4) remaining even after the formation of silver NPs.

Cross-talk characterization results of high-fill-factor single-photon avalanche diode (SPAD) arrays in CMOS 150-nm technology are reported and discussed. Three different SPAD structures were designed with two different sizes (15.6 and 25.6  μm pitch) and three guard ring widths (0.6, 1.1, and 1.6  μm). Each SPAD was implemented in an array, composed of 25 (5×5) devices, which can be separately activated. Measurement results show that the average cross-talk probability is well below 1% for the shallow-junction SPAD structure with 15.6  μm pitch and 39.9% fill factor, and 1.45% for the structure with 25.6  μm pitch and 60.6% fill factor. An increase of cross-talk probability with the excess bias voltage is observed.

A metallic packaging technique of fiber Bragg grating (FBG) sensors is developed for measurement of strain and temperature, and it can be simply achieved via one-step ultrasonic welding. The average strain transfer rate of the metal-packaged sensor is theoretically evaluated by a proposed model aiming at surface-bonded metallic packaging FBG. According to analytical results, the metallic packaging shows higher average strain transfer rate compared with traditional adhesive packaging under the same packaging conditions. Strain tests are performed on an elaborate uniform strength beam for both tensile and compressive strains; strain sensitivities of approximately 1.16 and 1.30  pm/μϵ are obtained for the tensile and compressive situations, respectively. Temperature rising and cooling tests are also executed from 50°C to 200°C, and the sensitivity of temperature is 36.59  pm/°C. All the measurements of strain and temperature exhibit good linearity and stability. These results demonstrate that the metal-packaged sensors can be successfully fabricated by one-step welding technique and provide great promise for long-term and high-precision structural health monitoring.

We theoretically and numerically investigate all-optical Mach–Zehnder interferometer switching based on the phase-shift multiplication effect of an all-optical analog on the electromagnetically induced transparency effect. The free-carrier plasma dispersion effect modulation method is applied to improve the tuning rate with a response time of picoseconds. All observed schemes are analyzed rigorously through finite-difference time-domain simulations and coupled-mode formalism. Compared with no phase-shift multiplication effect, the average pump power of all-optical switching required to yield the π-phase shift difference decreases by 55.1%, and the size of the modulation region is reduced by 50.1% when the average pump power reaches 60.8 mW. This work provides a new direction for low-power consumption and miniaturization of microstructure integration light-controlled switching devices in optical communication and quantum information processing.

We report an integrated whispering gallery mode microresonator–based sensor probe for refractive index sensing. The probe was made by sealing a borosilicate glass microsphere into a thin-wall glass capillary pigtailed with a multimode optical fiber. The intensities of the resonant peaks were found decreasing exponentially (linearly in a log scale) with the increasing refractive index of the medium surrounding the capillary. The sensing capability of the integrated probe was tested using sucrose solutions of different concentrations and the resolution was estimated to be about 2.5×10−⁵ in the index range of 1.3458 to 1.3847. The integrated sensor probe may prove useful in many chemical and biological sensing applications where highly sensitive refractive index monitoring is needed.

^{A series of ZnO thin film transistors (TFTs) using pyrochlore Bi_1.5Zn_(1+y)Nb_1.5O_(7+y) (BZN) thin films as gate insulators by RF sputtering has been fabricated. The relations between the zinc content and performance of BZN thin films and ZnO-TFTs are studied. The electrical properties of the ZnO-TFTs with BZN gate insulators as a function of Zn content are discussed. The research results showed that excess Zn (5 mol.%) can significantly enhance the performance of BZN thin films and ZnO-TFTs, which is mainly attributed to the compensation of Zn volatility during fabrication of BZN thin films. At an applied electric field of 250  kV/cm, the leakage current density of BZN thin films with 5 mol.% excess Zn is approximately four order of magnitude lower than that of BZN thin films without excess Zn. The subthreshold and surface state density of ZnO-TFTs were decreased from 684 and 350  mV/dec to 4.5×1012 and 2×1012 cm−2, respectively, as Zn content was increased.}

A series of Fe-doped TiO₂ trilayer films were prepared successfully by using the ion-implantation technique. The aim of the ion implantation was to enhance charge transfer and to reduce charge recombination. A maximum conversion efficiency of 4.86% was achieved in cells using Fe-ion-implanted electrodes with the illumination of 6×10¹⁵  atom/cm². It is 14.1% higher than that of the cells without ion implantations. The significant improvement in conversion efficiency by Fe-ion implantation could be contributed to the enhancement of dye uptake and charge transfer, as indicated from the incident photon-to-collected electron conversion efficiency and ultraviolet-visible measurements. Furthermore, the implanted Fe-ions introduce impurity levels in the bandgap of TiO₂, and this improves the electron injection efficiency from lowest unoccupied molecular orbital of excited N719 into the conduction band of TiO₂.