Light beams with helical phase-fronts are attractive for many optical applications, such as optical tweezers, particle manipulation, and other optical applications. A superoscillatory metalens has been designed for an azimuthally polarized wave at a wavelength λ  =  632.8  nm. Numerical simulation demonstrates that a superoscillation hollow dark spot is generated. The transverse inner FWHM is 0.358λ overcoming the diffraction limit, while the sidelobe ratio can be compressed to 18.3%. More importantly, for such incidence with different orbital angular momentum (OAM) ℓ  =  1, 2, and 3, the proposed metalens also presents elegant subwavelength tight focusing performances (the FWHMs are 0.469λ, 0.352λ, and 0.737λ, respectively). Extensive investigations have been made to present the unique characteristics of the generated foci, the phase distribution patterns are given for different OAM values. Small size and ultrathin thickness make such lenses suitable for various optical applications.

The stokes vector (SV) has been known since Stokes used it to represent the state of polarization (SOP) of light. While the polarization of light has been represented in the form of a SV since 1852, it has more recently found many applications in polarization management in optical communication. Our review begins with the mathematical representation of the SV. The system capacity in the optical transmission system is increased by employing a polarization diversity scheme. The rapidly changing SOP of the modulated optical signal is acquired using a Stokes vector-based direct detection (SV-DD) scheme at the receiver. A silicon photonics integrated circuit employs a SV-based polarization management scheme to control the on-chip SOP of light. Space-division multiplexed systems are a promising solution to the problem of capacity crunch. The multidimensional SV concept is employed to study signal propagation in these systems. The application of the SV in DD, silicon photonics, and space-division multiplexed systems is discussed in detail in our review. The conclusion provides a summary of the importance of SV-based polarization management in optical communication and its future prospects.

This guest editorial introduces the Special Section on Active Spectroscopy.

To obtain a real-time noninvasive analysis of the content of the aqueous humor (AH) in the eye, a confocal dark-field ophthalmic probe was designed and optimized by ray-tracing software. The confocal technique allows Raman spectrometric sampling of the AH with suppressing signals from the surrounding tissues. Dark-field illumination prevents the excitation light to reach the vulnerable retinal tissue directly. In order to evaluate the optical performance and safety of this method, a functional prototyped eye contact probe has been designed and tested on ex vivo rabbit eyes.

A trace chemical detector is described that combines external-cavity quantum cascade lasers and a mercury cadmium telluride camera to capture hyperspectral images of the diffuse reflectance from a target surface in the long-wave infrared. The system is able to generate individual hypercubes in <0.1  s. When raster scanning the laser beam over the target surface, areal coverage rates of >60  cm²  /  s have been achieved. Results are presented for standoff distances ranging from 0.1 to 25 m. Hyperspectral images generated by the system are analyzed for spectral features that indicate the presence of trace surface contaminants. This approach has been found to be highly capable of detecting trace chemical residues on a wide variety of surfaces, and we present a collection of detection results to demonstrate the capabilities of this technology. Examples include the detection of 10  μg of saccharin powder on a wide range of substrates, 0.2  μg of an explosive residue on a computer keyboard, residual pharmaceuticals within a plastic baggie, and a contaminated fingerprint on cell phone case.

Gas chromatography (GC) is a staple analytical technique used to separate chemical mixtures (analytes) prior to identification with a hyphenated technique, such as mass spectrometry or Fourier transform infrared (IR) spectroscopy. Traditionally, analytes elute through the GC separation column where they are detected when they exit. We have developed a technique to perform in situ IR spectroscopy during the process of separating the analytes along the GC column. This is achieved by spin coating the stationary phase onto a germanium prism and actively probing the stationary phase in an attenuated total reflectance configuration with a quantum cascade laser.. The GC column is formed by pressing a molded epoxy lid, with grooves that form the tubular column, onto the stationary phase coated prism.

Since solids are only sometimes seen en masse in a pure bulk form, and for liquids other than water almost never, a capability to model reflectance spectra from analytes deposited on various substrates would be highly advantageous. If available, the real, n  (  ν  )  , and imaginary, k  (  ν  )  , components of the complex refractive index,   n∼    =  n  +  ik, can be used to simulate infrared spectra, accounting for reflection, refraction, and absorption phenomena as a function of wavelength. We focus on using the Pacific Northwest National Laboratory (PNNL) derived n  /  k vectors for solid and liquid analytes deposited as thin layers on different types of substrates including conductors, such as aluminum, and inorganic dielectrics, such as glass. The model is an adaptation of the Monte Carlo ray trace modeling program, TracePro, extended through the use of its macrolanguage. The model is tested using thin films of organic liquids including silicone oil and no. 2 diesel fuel, as well as organic solids such as caffeine and acetaminophen on aluminum and glass. The predicted spectra for the solid films were compared to experimental hemispherical reflectance data measured using a Fourier transform spectrometer with an integrating sphere. The thickness of the calculated layer is a parameter for predicting the (transflectance) spectra and is obtained using the areal density measured from gravimetric methods to generate the thin-layer samples. Comparison of the calculated spectra with experimental hemispherical reflectance data shows excellent agreement, indicating promise for the use of measured n  /  k data to synthesize reference spectral data. In particular, accounting for the inhomogeneity of the deposits greatly improved the match with experimental data. Finally, the theoretical modeling shows that for thicker layers (ca. 20 to 100  μm) of typical organics possessing moderately strong k values, the longwave infrared features are often saturated and better spectral contrast is obtained from the overtone/combination bands in the shortwave infrared.

We describe an optical bench design that efficiently projects broadband infrared energy from a supercontinuum (SC) source onto an extended target and collects the backscattered return. The optical system enables standoff spectral measurements of surfaces at up to 15 m away with a continuous spectral range from 3.6 to 11  μm. The goal of the optical system is to maximize energy transmission and capture under various fundamental and practical constraints imposed by the SC fiber laser source (broad spectrum, high divergence exiting the fiber), spectrometer (light collimation), detectors (étendue limits), and the overall system (size, weight, and cost). The design effort led to an all-reflective, off-axis configuration for both the transmitter and receiver, using custom-designed enhanced gold surfaces. We are able to successfully capture >90  %   of the source energy and transmit it with low loss to the remote target while maintaining a nearly diffraction-limited Gaussian beam. The receiver telescope matches the detector étendue while providing uniform (±2.5  %  ) off-axis signal collection over the target area to enable scanning of the SC laser spot.

The development of a field-portable fiber-laser based hyperspectral imaging system for standoff detection and identification of trace levels of explosives is presented. The prototype system combines a hybrid of frequency and time-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy techniques with spatial scanning for rapid detection of microscopic traces of explosives on liquid or solid surfaces. An all-fiber light source architecture enables generation of three CARS excitation beams from a compact, single-unit femtosecond fiber laser and a highly nonlinear optical fiber. The light backscattered from the target surface is collected by a spectrometer, and a high-resolution automated stage system allows for two-dimensional microscopic CARS imaging. We demonstrate trace explosive detection in <100  ms.

In a homeland security setting, the ability to detect explosives at a distance is a top security priority. Consequently, the development of remote, noncontact detection systems continues to represent a path forward. In this vein, a remote detection system for excitation of infrared emissions using a CO₂ laser for generating laser-induced thermal emission (LITE) is a possible solution. However, a LITE system using a CO₂ laser has certain limitations, such as the requirement of careful alignment, interference by the CO₂ signal during detection, and the power density loss due to the increase of the laser image at the sample plane with the detection distance. A remote chopped-laser induction system for LITE detection using a CO₂ laser source coupled to a focusing telescope was built to solve some of these limitations. Samples of fixed surface concentration (500  μg  /  cm²) of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) were used for the remote detection experiments at distances ranging between 4 and 8 m. This system was capable of thermally exciting and capturing the thermal emissions (TEs) at different times in a cyclic manner by a Fourier transform infrared (FTIR) spectrometer coupled to a gold-coated reflection optics telescope (FTIR-GT). This was done using a wheel blocking the capture of TE by the FTIR-GT chopper while heating the sample with the CO₂ laser. As the wheel moved, it blocked the CO₂ laser and allowed the spectroscopic system to capture the TEs of RDX. Different periods (or frequencies) of wheel spin and FTIR-GT integration times were evaluated to find dependence with observation distance of the maximum intensity detection, minimum signal-to-noise ratio, CO₂ laser spot size increase, and the induced temperature increment (ΔT).

We are using active infrared (IR) spectroscopic imaging to detect trace explosives on surfaces at proximal distances up to a few meters. The technology comprises an IR quantum cascade laser (QCL) for illumination and an IR focal plane array (FPA) sensor to collect signal backscattered from surfaces of interest. By sweeping the wavelength of the QCL while collecting image frames with the FPA, we generate an active hyperspectral image (HSI) cube. The HSI cube contains both spatial and spectral information, where the spectrum of a pixel, or region of interest within the image, can be extracted and compared against a known threat library. These cubes are fed into a convolutional neural network (CNN) trained on purely synthetic data to identify chemicals in the field of view. The CNN identifies chemicals by their IR signature and identifies their location within the image.

Standoff detection of hazardous materials using infrared backscattering spectroscopy shows promise due to its speed of detection, sensitivity, chemical specificity, eye safety, and the ability to perform detection in a stealthy manner. However, infrared diffuse reflectance spectra of trace particles on substrates exhibit a strong dependence on substrate type, particle size, and mass loading. This has a negative impact on the performance of detection algorithms and false alarm rates when using commercially available spectral databases. We present two models that are quickly computed yet capture most of the observed spectral features. Model I is best suited for calculating the diffuse reflectance spectra of trace amounts of particles on relatively smooth substrates, while model II extends the applicability of model I to particles on very rough substrates. The models can be used for algorithm development and training. The main inputs to the models are the analyte and substrate optical constants as a function of wavelength, and the particle size distribution and average mass loading. The accuracy of the models was checked by comparing to experimentally measured diffuse reflectance spectra of several carefully prepared samples.

Mid-infrared (MIR) laser spectroscopy measurements coupled with multivariate analysis (MVA) routines were applied to detection and classification of high explosives (HEs) deposited on aluminum and acrylonitrile butadiene styrene substrates. An MIR quantum cascade laser (QCL) was optically coupled to a grazing angle probe (QCL-GAP) mount to operate in diffuse reflectance mode at an incidence angle of 82 deg from the surface normal. The experimental conditions enabled reflection–absorption infrared reflectance spectroscopy of the laser light by the surface deposits of HEs and allowed obtaining spectra with high signal-to-noise ratios in a very short time. This optical arrangement permitted collecting data for identifying the vibrational signatures of the HEs to be detectable even at low surface concentrations. The HEs used included pentaerythritol tetranitrate, 1,3,5-trinitroperhydro-1,3,5-triazine, and 2,4,6-trinitrophenylmethylnitramine. Using principal components analysis, 100% of the HE samples deposited on the reflective and nonreflective substrate were correctly classified, avoiding false-positive responses in the analyses. The combination of QCL-GAP spectroscopy with MVA presented will also allow obtaining highly confident detections/identification of hazardous materials and other chemical/biological agents residing on both reflective and nonreflective surfaces. The methodology can easily be extended to other types of applications.

Trace chemical detection and classification in stand-off reflection-based spectroscopic data is challenging due to the variability of measured data and the lack of physics-based models that can accurately predict spectra. Most available models assume that the chemical takes the form of spherical particles or uniform thin films. A more realistic chemical presentation that could be encountered is that of a nonuniform chemical film that is deposited after evaporation of the solvent that contained the chemical. We present an improved signature model for this type of solid film. The proposed model, called sparse transfer matrix, includes a log-normal distribution of film thicknesses and is found to reduce the root mean square error between simulated and measured data by about 25% when compared with either the particle or uniform thin film models. When applied to measured data, the sparse transfer matrix model provides a 10% to 28% increase in classification accuracy over traditional models.

Pupil densification is a technique to improve contrast in a direct imaging interferometer. By enlarging the size of each aperture in a sparse array, the fraction of energy contained in the central point spread function peak can be increased. However, the effective field of view is reduced as the contrast improves. Our work investigates the optimal strategy for pupil densification in the context of extended source imaging. Analytical expressions are derived to optimize the signal-to-noise ratio at the edge of the field of view for a variety of hypertelescope configurations. Free-space beam transport and fiber-optic beam transport are both considered. The relative advantages and disadvantages between the considered hypertelescope configurations are discussed.

We propose a logic optical projection tomography (LOPT) method to realize full-view, three-dimensional (3-D) shape parameters measurement and visualization for opaque objects. The main innovation of LOPT is use of a logic process to obtain high-contrast and homogeneous boundary projection signals. According to the reconstructed 2-D cross-section images, the structure characteristic points in each slice are independently extracted and combined to produce a full-view 3-D image. Further, the structural shape parameters of individual parts of object such as the inscribed cylinder, center line, and double threads are obtained and visualized simultaneously. Our experimental results demonstrate that LOPT can be potentially developed as an economical optical measurement and visualization tool for some industrial application.

The problems of low-contrast, blurred images, and low-edge details are encountered in infrared images. These undesirable features affect infrared imaging applications in military, science, medicine, and other fields. Therefore, a method of edge detection based on heat conduction matrix (HCM) has been proposed to show or sharpen the edges of objects in infrared images. First, the heat conduction formula of solid and still fluids was applied on the infrared image, and a feature matrix called HCM was obtained. Second, the HCM feature matrix was passed through a threshold to obtain distinct and noise-free edge images. The results of the numerical and visual applications were compared with the Sobel, Canny, and Laplacian of Gaussian methods. The results of our study showed that the proposed method is powerful in detecting edges in infrared images. The most important feature of the method is its high emphasis on important edges as well as thin and distinct edges.

A digital method is proposed whereby spatial speckles created when an optically rough surface is illuminated by a diffuse coherent laser beam are used to generate slope contour fringes. This is done by photographing the speckles contained in a parallel plane in front of the plate before and after deformation. The resulting photographs are then fast Fourier transformed twice and numerically compared with the digital speckle correlation software called computer-aided speckle interferometry to produce slope contours of the plate. The slope contour data are then stored in a computational array. The curvature contours are generated by numerical differentiation of the slope data; similarly, the deflection contours are generated by numerical integration of the slope data. The curvature data can then be further used in conjunction with plate theory to obtain the bending and twisting moments the plates experience as well as the stress distribution in the surface of the plate. The method is simple and straight forward. A comparison with several other techniques is presented.

Spatial interpolation using radial basis functions (RBFs) is compared with standard natural neighbor interpolation for the purpose of populating dead zones in optical path difference (OPD) wavefront data collected through aero-optics turbulence. Both methods, and some other commonly used basis functions, are described in detail. These candidates are applied to experimental subscale turbulent wind tunnel data collected with a conformal window hemispherical turret over a range of turret look-back angles spanning 60 deg to 135 deg. Interpolation performance is evaluated using both OPD magnitude and fractal (Hurst exponent) analysis. Interpolating aero-optical OPDs using the thin-plate RBF reduces the symmetric mean absolute percent error of the OPD magnitude and the Hurst exponent by an average of 28% and 50%, respectively, compared with the natural neighbor interpolation method. The improved interpolation accuracy should likewise increase the quality of subsequent postprocessing analysis requiring uniformly populated rectangular regions as inputs, such as optical flow algorithms. A MATLAB implementation of the pseudocubic spline RBF is provided.

The single collimating lens-based dual-beam exposure system is a holographic exposure system that is suitable for fabricating long-period gratings. The interference wavefront aberration in this system requires optimization. We analyzed the coma of a single collimating lens-based dual-beam exposure system, and the linear relationships between the coma and the positions of the pinholes and substrate are obtained. Based on these relationships, we can eliminate the coma by adjusting the pinholes or substrate along the optical axis of the collimating lens. Consequently, the defocus aberration, astigmatism, low-order spherical aberration, and coma are all simultaneously optimized by adjusting the exposure system to meet the pinhole symmetry condition and coma optimization condition. Because the optimized interference wavefront aberration includes only high-order aberrations with low peak to valley values, a high-quality exposure wavefront can be obtained. Furthermore, a 5-μm period grating exposure system is constructed with a spherical collimating lens of 0.6λ spherical aberration. Through the optimization process, a low spacing error of ∼0.04λ within an aperture of 65  ×  65  mm² is achieved (the theoretical value is 0.002λ).

High-speed laser shadowgraphy is crucial in range testing. We report the design, development, and application of a dual-sequence large-scale high-speed laser retroreflective shadowgraph system (DLH-LRSS) to monitor large-scale transient phenomena on a field-test range. The DLH-LRSS is based on the retroreflective technique. The narrow pulse width laser can effectively freeze ultrahigh-speed processes, thereby allowing the investigation of ultrafast phenomena. Multiple shooting modes contribute to the flexibility of the test. Three typical experiments were conducted at the Nanshan Test Center, and good quality sequential images were captured by DLH-LRSS. We demonstrate that DLH-LRSS has large-scale, ultra-fast recording and quantitative study capabilities. The DLH-LRSS can be used in the rapid diagnosis and measurement in range testing and similar observation applications.

A temperature-compensated optical fiber twist sensor based on cascaded long-period fiber gratings (LPFG) and fiber Bragg grating (FBG) is proposed and experimentally demonstrated. The LPFG is sensitive to twist and its transmitted light is twist modulated and reflected by the FBG. In this way, a single-ended twist sensor is realized by monitoring the reflection spectrum of the cascade grating. Both the wavelength and amplitude of the reflection spectrum are detected. The wavelength is sensitive to temperature but not to twist, so it is used for temperature compensation in twist measurement. Experimental results show that the proposed sensor improves the twist accuracy by 2 orders of magnitude in the temperature range of 15°C to 35°C.

We present an optical set up that significantly reduces the ghost images appearing due to multiple reflections on both surfaces of the combiner (typically the windshield) of head-up display (HUD) systems. The method is based on an optimized polarization correcting optics attached to the HUD projector. For the quantitative investigation of the ghost images, we calculate the ghost image suppression ratio as a function of viewing direction using a custom algorithm based on ray tracing and Jones-vector formalism. We also present experimental results on the proposed configurations exhibiting very good agreement with calculation results. As a result, the ghost image free viewing area of polarization-type HUDs can be increased more than fourfold.

High powered optical amplifiers (HPOAs) are a key subassembly in free-space optical communication systems. As these free-space optical communications systems are being introduced into space applications, it is critical to assess the impact of the radiation environment present in space on the reliability of these systems. One key component in HPOAs is multimode laser diodes, which are used to pump gain fibers in the HPOA. The reliability of these laser diodes is very dependent on optical output power, with increases in optical power causing dramatic reductions in lifetime. At the same time, radiation darkening is a well-known problem that reduces the efficiency in HPOAs operating in environments with high radiation levels. We use Monte Carlo simulation to evaluate the impact of radiation darkening on the reliability of multimode pump ensembles over time. At radiation fields related to geostationary earth orbits (2 to 5  krad  /  year) radiation effects reduce multimode pump lifetime by a factor of 7-220 under typical use conditions. Use of redundant pumps operating under derated conditions can mitigate the impact of radiation.

We present a design for a unit magnification imaging relay that is compact. The design uses two mirrors, and one is used by light twice for a three-reflection system. The mirrors are specified with freeform surfaces. There are several tradeoffs of performance depending on field of view, F  /  #, and wavelength range. One example provides a 30-mm circular field of view at F  /  2.8, and another example provides a circular field of view of 40 mm in diameter at F  /  4 for a length of ∼400  mm. Adding a third nearly flat mirror allows a system to provide a 50-mm field of view at F  /  4 for a length of 420 mm. A last example includes a beam splitter cube to allow accessing the full field of view. These systems are diffraction limited at 550 nm.

Terahertz time-domain spectroscopy (THz-TDS) is one of the main research platforms for terahertz (THz) science and technology. However, a traditional system is not only bulky but also has some limitations in detecting micro and liquid samples because of the large THz beam spot dimensions and the strong THz-wave absorption by water. We have developed a THz-TDS measurement chip by directly fabricating THz transmission lines and photoconductive antenna electrodes on a low-temperature GaAs epitaxial wafer. This chip integrates THz-waves generation device, detection device, and the transmission line in the size of 1  mm  ×  20  μm and uses the between the THz evanescent field sample interaction near the transmission line to obtain the spectral information from the trace or liquid samples. Using a self-built measurement beam path, we tested the chip with 1550-nm femtosecond laser and obtained THz spectroscopy in the 0.1 to 0.7 THz range, thereby verifying the feasibility of the chip.

Antireflection (AR) coatings allow increased substrate transmission and reduce surface reflection over a specific wavelength range. We proposed a blue-blocking (BB) AR coating that allows maximal visible-light transmittance while minimizing the transmittance of blue light, owing to selective absorption. Admittance diagrams of BB Cr₂O₃  /  SiO₂ coatings and traditional AR Ta₂O₅  /  SiO₂ coatings were shown to demonstrate that small absorption affects the reflectance much less than it affects the transmittance. Therefore, by controlling the degree of absorption, the lower residual reflection can be obtained by optimizing the thickness of different layers of the overall stacked structure. Coatings were fabricated using remote plasma sputtering to meet the requirements of low residual reflectance (<1.16  %  ), high visible-light transmittance (exceeding 90%), and short-wavelength transmittance attenuation by ∼20  %  . These promising results indicate that this type of BB coatings is a good candidate for vision protection applications, such as eyeglasses, desktop and mobile digital screens, and solar control films used in architectural glass.

The distribution and nonuniformity of the electric field have been simulated in the active medium of CO₂ laser using successive-over-relaxation numeric method. The different configurations of UV preionization have been considered as spark array, surface corona, plasma cathode, and sliding discharge. The electric field distribution and field deviation (FD) is compared in various configurations to obtain an insight into the electrical field uniformity. The numerical simulation results show that the minimum FD is related to the plasma cathode and sliding discharge configurations regarding boundary conditions. For the sliding configuration, a slight increase in the FD was observed in comparison with plasma cathode. The FD of surface corona configuration is about two percent lower than the spark array.

We propose an all-fiber configuration of tunable bandpass filter (TBPF), consisting of a mode selective coupler (MSC) and an acoustically induced fiber grating (AIFG). Either the MSC or the AIFG can act as high-order mode converters in a few-mode fiber. The working wavelengths of the MSC and the AIFG are chosen to achieve complete conversion cycle between the core-mode linearly polarized (LP₀₁) mode and the LP₁₁ mode. The tunability of the TBPF is achieved by changing the frequency of the radio frequency signal. An all-fiber tunable ring laser based on the TBPF is used to generate a stable continuous-wave laser with 3-dB bandwidth of around 0.8 nm. The tunable wavelength ranges from 1530 to 1580 nm. Furthermore, the TBPF is introduced into the cavity of a mode-locked fiber laser. Wavelength-tunable mode locking pulses are obtained at central wavelengths from 1530 to 1560 nm.

A two-sectioned InAs/InP quantum dash laser structure is proposed and investigated as a monolithic broadband multimodal tunable laser with an integrated semiconductor optical amplifier. The optical power-injection current and spectral characteristics of the device at different operating conditions demonstrated a total wavelength tunability of ∼15.8  nm in the extended-L-band (∼1615 to ∼1630.8  nm) window with ∼2.0 times −3  dB bandwidth enhancement. Furthermore, due to the unique tunability mechanism of forward biasing the amplifier section, the device exhibits simultaneous wavelength tuning as well as optical amplification features, with an estimated gain of ∼8.5  dB affirmed by an increase in the wall-plug efficiency up to 6.8% from 3.9%, shown by its single-section counterpart. This demonstration paves a potential platform for the deployment of broadband quantum-dash laser-amplifiers as unified light sources in next-generation optical access networks.

We demonstrate a technique for fabricating microstrip patch antennas using femtosecond laser patterning followed by ultraviolet beam and chemical treatment. Initially, we design the physical parameters of both single-slot and double-slot microstrip patch antennas and simulate them using high-frequency structure simulator for optimization. Simulation results exhibit a return loss of −26  dB at the resonant frequency of 22.9 gigahertz (GHz) for single-slot microstrip patch antenna and −18.3  dB at 24.03 GHz for double-slot microstrip patch antenna. The three-dimensional polar plot and far-field radiation pattern of the microstrip patch antennas confirm excellent directivity of the antennas. Furthermore, we investigate the return loss of the fabricated microstrip patch antennas. For single-slot microstrip patch antenna, experimental result shows a return loss of −21.25  dB at 22.7 GHz. In contrast, double-slot microstrip patch antenna shows a return loss of −27  dB at 24.1 GHz. In addition, we compare the performance of the double-slot microstrip patch antenna fabricated using femtosecond laser-assisted technique and photolithographic technique and find better performance in the femtosecond laser-fabricated microstrip antenna. The proposed femtosecond laser-based technique is simple and shows promises in precise fabrication of high-quality microstrip antennas.

Hermitian symmetry (HS) is essential for attaining a real-valued signal for intensity modulation and direct detection of visible light communication (VLC). However, HS results in an increase of the peak-to-average power ratio (PAPR). We propose an HS free direct current optical-universal filtered multicarrier (HSF DCO-UFMC) system for VLC. In this technique, the conventional complex UFMC signal is transformed by juxtaposing the real and imaginary components to attain a real valued signal. Therefore, HSF DCO-UFMC offers a great reduction of the PAPR. We also address the nonlinear companding transforms for further reduction of the PAPR of the HSF DCO-UFMC signal. A new companding transform called two μ-law that reduces the PAPR and provides more design flexibility by employing two optimum companding levels and a threshold value is proposed for the HSF DCO-UFMC system. However, this scheme raises the average power level of the signal. So, to maintain a constant average power level, an enhanced two μ-law is proposed to reduce the PAPR effectively. Simulation outcomes exemplify that the proposed HSF DCO-UFMC provides better PAPR reduction. In addition, the proposed two μ-law and enhanced two μ-law can provide an improved PAPR reduction, suppressed side-lobe levels, and better bit error rate performance than the original HSF DCO-UFMC signal.

Compressive sensing (CS) is an attractive technique to mitigate the high peak-to-average power ratio (PAPR) in optical orthogonal frequency division multiplexing (OFDM) systems. The lower computational complexity, easy implementation, and lack of side information requirement make signal compression an appropriate approach for PAPR reduction in both original and optical OFDM systems. We propose the CS approach for compressing the time-domain DC-biased optical OFDM (DCO-OFDM) signals at the transmitter side and multiple measurement vectors (MMV) based on orthogonal matching pursuit (OMP) algorithm to reconstruct the original signals at the receiver end. Simulations are conducted for QPSK and 16-QAM modulated symbols, and the results indicate that the PAPR of compressed signals is significantly reduced compared with the PAPR of the original DCO-OFDM signals. Furthermore, using the MMV-OMP algorithm and only about 1 dB additional signal-to-noise ratio, the original symbols are reconstructed at the receiver end without deteriorating the bit error rate performance.

We developed a compact 2-μm intracavity-pumped Ho  :  YVO₄ laser using a diode-pumped Tm:YAP laser at room temperature. Before applying the Ho laser, the lasing characteristics of the a-cut Tm:YAP laser were studied, and a maximum output power of 2.76 W at 1951.6 nm was obtained under an incident laser diode pump power of 25 W with a slope efficiency of 15.8%. Under the same cavity structure, the continuous-wave Ho  :  YVO₄ laser yielded a maximum output power of 1.7 W at 2052.7 nm for an incident diode pump power of 29 W, corresponding to a slope efficiency of 10.4%. The beam quality factor (M²) of the Ho  :  YVO₄ laser at the maximum output power was estimated to be 1.16 and 1.26 in the horizontal and vertical directions, respectively, which indicates the occurrence of near-diffraction-limited beam propagation.

We propose and experimentally demonstrate visible-light positioning (VLP) systems using silicon photovoltaic cells (Si-PVCs) and machine learning and neural network algorithms. Both angle-of-arrival (AOA)-based and received-signal-strength (RSS)-based VLP systems are evaluated and compared. The Si-PVC could also provide energy harvesting to store received optical power for the mobile unit. Here, second-order linear regression machine learning (RML) model and two-layer neural network are implemented in both AOA-based and RSS-based VLP systems to enhance the positioning accuracy. The root mean square (RMS) average positioning error of the AOA-based VLP system is reduced from 7.22 to 3.46 and to 2.99 cm when using the RML and neural network, respectively. The RMS average positioning error of the RSS-based VLP system is reduced from 7.07 to 3.01 and to 2.60 cm when using the RML and neural network, respectively. The experimental results clearly illustrate that the proposed schemes can significantly improve the positioning accuracy.

The photoresponse mechanism of graphene/InSb heterojunction middle-wavelength infrared (MWIR) photodetectors was investigated. The devices comprised a graphene/InSb heterojunction as a carrier-injection region and an insulator region of graphene on tetraethyl orthosilicate (TEOS) for photogating. The MWIR photoresponse was significantly amplified with an increase in the graphene/TEOS cross-sectional area by covering the entire detector with graphene. The graphene-channel dependence of the MWIR photoresponse indicated that the graphene carrier density was modulated by photocarrier accumulation at the TEOS/InSb boundary, resulting in photogating. The dark current of the devices was suppressed by a decrease in the graphene/InSb carrier-injection region and the formation of the heterojunction using an n-type InSb substrate. The results indicate that photocarrier transportation was dominated by the formation of a Schottky barrier at the interface of the graphene/InSb heterojunction and a Fermi-level shift under bias application. The high-responsivity and low-dark-current photoresponse mechanism was attributed to the graphene/InSb heterojunction diode behavior and the photogating effect. The devices combining the aforementioned features had a noise equivalent power of 0.43  pW  /  Hz^1/2. The results obtained in our study will contribute to the development of high-performance graphene-based IR image sensors.

We report a simple refractive index (RI) sensor based on a twin-hole fiber (THF). The THF is fused with the single-mode fiber to realize sensing by the multimode interference. The mode transmission characteristics in the THF are numerically investigated. In the sensing experiment, when the surrounding RI changes, the wavelength shift of the transmission spectrum shows approximate linearity in the low RI region and nonlinearity in the high RI region. When a THF with the length of 10 mm is used, the highest sensitivity can be up to 1351  nm  /  RIU in the RI range from 1.430 to 1.445. Moreover, the effect of the temperature on the transmission spectrum relative to the RI is much smaller when the external temperature changes from 30°C to 80°C.

An ultracompact transverse electric (TE)-pass/transverse magnetic (TM)-stop power divider using subwavelength gratings (SWGs) and hybrid plasmonic waveguides (HPWs) for 1.55  /  2  μm is proposed and analyzed, where SWGs are, respectively, embedded in a partially etched taper within the input waveguide to form an SWG transition and input taper/output inverse tapers in the bottom layer to construct an SWG coupler, and a silicon nitride layer with a metal cap is placed right above the bottom SWG coupler to form an augmented silicon nitride-guiding HPW. For both wavelengths of 1.55 and 2  μm, the injected TE mode is transmitted from the input waveguide to the bottom SWG coupler by the SWG transition and further undergoes a strong coupling from the bottom input taper into two adjacent inverse tapers due to the cutoff condition. As to the input TM mode, it is transferred into the eigenmode that is strongly confined in the silicon nitride layer of the HPW and is greatly absorbed by metals. Consequently, both power dividing and polarization handling for a dual-wavelength operation are achieved in the proposed single device simultaneously with a compact length of 5.2  μm. Results show an extinction ratio (ER) of 22.44  /  19.87  dB for TM mode and an insertion loss (IL) of 0.25  /  0.57  dB for TE mode are obtained at 1.55  /  2  μm; its bandwidth of IL  <  0.31  /  0.83  dB and ER  >  18  /  17  dB exceeds 290  /  260  nm around the wavelength of 1.55  /  2  μm. In addition, the fabrication tolerances and mode-field evolutions are also given.

To serve the changing needs of road traffic control, the road space and road structure surrounding an intersection have evolved into complex forms. The redesign of the trajectories, though complex, can be accomplished by the application of methods for navigation, guidance, and combination of expert knowledge of road traffic control of vehicles, using a concept of request/response in a two-way-to-way traffic light-controlled crossroad. The communication between the infrastructures and the vehicles (I2V), between vehicles (V2V), and from the vehicles to the infrastructures (V2I) is performed through visible light communication (VLC) using the street lamps and the traffic signaling to broadcast the information. Vehicle headlamps and taillights are used to transmit data to other vehicles or infrastructures allowing digital safety and data privacy. Data are encoded, modulated, and converted into light signals emitted by the transmitters. Tetra-chromatic white sources are used providing a different data channel for each chip. As receivers and decoders, silicon/carbon wavelength division multiplexer devices, with light filtering properties, are used. The primary objective is to control the arrival of vehicles to an intersection and schedule them to cross at times that minimize delays. A further objective is to allocate delays between left-turns and forward movements, moderating the speed and slot between vehicles traveling in these directions, maintaining a safe distance from one to another. Pedestrians and bicycles are incorporated. A I2V2V2I traffic scenario is established. A phasing traffic flow is developed as a proof of concept. The experimental results confirm the cooperative VLC architecture showing that communication between connected cars is optimized.

A Mach–Zehnder interferometer based on S-taper embedded in long-period grating (LPG) for simultaneous measurement of strain and temperature is proposed. The LPG is written using CO₂ laser in a single-mode fiber. Then an S-taper is made by putting the LPG in a fusion splicer and applying off-axis pull at the middle point. The spectral characteristics of strain and temperature are investigated theoretically and experimentally. The experimental results show that the maximum strain sensitivity is 53.6  pm  /  με within a strain range of 0 to 373.13  με. In additionally, the proposed sensor has a maximum temperature sensitivity of 70  pm  /    °  C when the temperature varies from 30°C to 90°C. Further analysis shows that this sensor could be applied for simultaneous measurement of strain and temperature.