A phenomenon, referred to as differential synthetic aperture ladar (DSAL) image inclination, is reported. The phenomenon states that in a stably operating DSAL system, the high-resolution image generated by DSAL technique is possibly inclined in the range direction. The range-inclined DSAL image does not change its resolution but makes the DSAL image differ from the real target structure, resulting in a kind of geometric deformation. Theoretical analysis shows that this inclination phenomenon originates from the geometric setting of the two receiving subapertures in the DSAL hardware and is independent of the target distance. By properly setting the subapertures, this phenomenon could be minimized. Detailed DSAL experimental results verified the theoretical interpretation. Methods to correct the inclined DSAL image are suggested.

Entropy-based measures are popular for objective image fusion quality assessment due to a small parameter set for implementation and independency of ground-truth image as the reference for evaluation. We focus on Tsallis entropy and consider mutual entropy and entropic distance as the two entropic measures for image fusion quality assessment. To perform an in-depth analysis over quality measures and evaluate to what extent they are able to fulfill desired behaviors that are expected from ideal image fusion quality measures, we separately conduct theoretical analysis for each of them. To this goal, we employ an image formation model to obtain a closed-form expression for quality while weighted averaging is used as fusion algorithm. Our study shows that the so-called measures do not always satisfy the expected desired behaviors. We also provide explanations for unexpected behaviors that can improve the accuracy of image fusion quality measure in application. Investigations on real images are also performed, and the results verify the output of theoretical analysis.

A mathematical description of the intersection of a transmitted beam and a beam-like field of view between two stationary, coplanar, circularly scanning light detection and ranging (lidar) devices is presented. This beam intersection information establishes when scattered signal interference between two lidar devices can occur. Such interference may result in erroneous range data. The fraction of time over which the scanners’ beams intersect is analyzed as a function of rotational frequencies and their reference phases. It is shown that the minima and maxima of this fraction are 0 and 1  /  2, with most configurations resulting near 1  /  4. A procedure to adjust the scan rates and phases to minimize this intersection time (including zero intersection) thus reducing the risk of mutual interference is also outlined. Four configurations that produce zero intersection between two scanners are presented. Configurations of three and four scanners are also presented for which no intersections between the scanners’ transmission paths occur.

Nowadays, stereoscopic devices are widely used for precise three-dimensional (3-D) measurements of hard-to-reach elements in industrial and biomedical applications. The most common approach for their implementation is the utilization of prism-based optical systems, which allows simultaneous acquisition of two images from different viewpoints on a single sensor. Normally, they are equipped with a wideband (WB) white-light source, but contrast visualization of the inspected object and, therefore, accurate quantitative characterization of its parameters, often requires narrow-band (NB) spectral imaging. We show that the standard geometrical calibration may lead to significant measurement errors when obtained using WB illumination and applied to NB images. As the criterion to evaluate the calibration error, we have chosen the difference between reference and measured lengths of the 1 mm segment along transverse x, y, and longitudinal z axes. If WB calibration data are applied to NB images, the measurement bias increases from the middle of the working spectral range to its edges and can reach significant values: up to 0.1 mm along x axis and 0.15 mm along z axis in 10 to 25 mm distance range. To overcome this, we propose the calibration and image processing procedures based on a proper choice of a few spectral bands for calibration and interpolation of the calculated calibration parameters. Results of multiple experiments using stereo video endoscope confirmed that the proposed technique allows a decrease in the measurement bias by three times in comparison to conventional WB calibration for all wavelengths of the visible range, which essentially improves the measurement accuracy. The impact of WB calibration on random errors of measurements and the quality of image rectification was also analyzed and shown to be insignificant.

A method based on the measurement of out-of-plane displacement by the phase-shifting digital speckle pattern interferometry is proposed to detect the edge of internal defects. The out-of-plane displacement is scanned by the scanning method of gradient streamlines first, which can provide paths through the defect edge with the best signal-to-noise ratio everywhere. Then, continuous wavelet transform (CWT) combined with the weighted structural intensity of the wavelet maxima is applied to detect the hidden singularities of the gradient streamlines that form the defect edge later. Moreover, the weighted structural intensity method is optimized to adapt to the detection of the hidden singularity by CWT. The results of theory, simulation, and experiments agree well, and the proposed method is efficient in internal defect detection.

A sample of lithium niobate crystal with known thickness is set on a vertical rotary stage, placed in between two crossed plane polarizers and illuminated by a collimated beam of white light. The sample is rotated to an appropriate position, and interference fringes observed behind an analyzer are recorded by a fiber-optic spectrometer. The recorded channeled spectrum is then analyzed, and from measured positions of interference minima (dark fringes) and known crystal’s thickness, the group birefringence of the crystal sample is obtained as a function of wavelength. The fringe order versus position of the fringe within the spectrum is fitted by appropriate dispersion function, and as the result phase birefringence as a function of wavelength is found. The measurement is performed for a sample declared as undoped, a sample doped with 0.025 wt. % of Fe, and a sample doped with 0.025 wt. % of Fe and 0.075 wt. % of Mn in the wavelength ranges (470 to 780) nm and (900 to 1700) nm. A good agreement between group birefringence dispersion obtained from positions of interference minima and known sample’s thickness and that calculated from phase birefringence dispersion given by Sellmeier dispersion model (used for undoped sample) and Cauchy model (used for doped samples) is found.

Cracking resistance or fracture toughness of brittle materials, such as optical glasses, can be determined by Vickers indentation, which allows generating cracks along the diagonals of the indent whose length depends of the loading rate. Subsequently, a relationship between the crack length and the corresponding indentation load is applied to calculate the indentation fracture toughness. As a result, the precision of the fracture toughness will depend on the accuracy of the crack length measurement. To minimize the uncertainties of this parameter, we propose its measurement more precisely by a profilometric method. On the other hand, as the glass surface roughness is recognized to have an important influence on the crack growth, different surface preparations are studied to minimize this roughness effect. In this work, four types of glasses are studied, namely Classical Crown K5, Borosilicate Crown BK7, Dense Flint SF2, and Ceramic Zerodur^® glasses. The surface polishing is performed by using CeO₂ pellets, which allow obtaining a roughness value for R_a to be <10  nm. Vickers instrumented indentation tests, allowing the plot of the applied load as a function of the indenter displacement, are performed with various maximum applied loads of up to 90 N. Different relationships based on assumptions on the crack shape (half-penny/median or Palmqvist cracks) are then used to calculate the indentation fracture toughness. Additionally, to accurately measure the crack length, we present an original approach based on the analysis of the interferometry map obtained with an optical profilometer using a monochromatic wavelength.

In the interferometric subaperture stitching, the subaperture-overlapped regions are often inaccurately positioned due to positioning error. To achieve better stitching effect, we propose a robust subaperture stitching method based on three-dimensional surface data matching. Speeded up robust features algorithm is used to calculate the relative displacements of adjacent subapertures, and subsequent pose adjusting is performed according to the displacements. The method needs neither auxiliary measures nor a known initial displacement value, which can eliminate the dependence on the precision of the mechanical device of the subaperture stitching. Experiments show that the proposed method can obtain smooth, mosaic-free surface results without dislocation.

We propose a method to correct the nonlinearities introduced by the projector and camera system in one-shot fringes projection profilometry. It is shown that the gamma value can be calculated with high precision employing Fourier techniques. Fourier and phase shifting phase extraction experiments are carried out to show the applicability of the method.

We present the performance of the portable wind light detection and ranging (LIDAR) system based on the 1.55-μm all-fiber technology in the atmospheric boundary layer. The LIDAR is 23.9 kg in weight, 50 cm in height, 35 cm in width, and 27 cm in depth, and the system’s local oscillator (LO) light power, pulse energy, and pulse width are adjustable. The LO light power is optimized to 3 mW, to minimize the effect of the relative intensity noise. The transmitting pulse energy is reduced to 19  μJ, to minimize the system’s power consumption while covering a detection height of >1  km in clear-sky condition. The pulse width is variable from 100 to 400 ns corresponding to a minimum resolution from 15 to 60 m. The signal-to-noise ratio performance experiment shows that this system can detect as high as 2.1 km. Field experiments compared with radiosonde and anemometer show that this system presents a detection accuracy of better than 1  m  /  s and 10 deg.

The possibility of using the phase of second-harmonic interference fringes as a position marker for detecting the zero optical path difference (OPD) in a nonlinear pulse-train interferometer is investigated. A theoretical analysis showed that the phase of second-harmonic fringes can be used as a position marker for detecting the zero OPD, and compared to the phase of the fundamental wave, it provides a higher coefficient of sensitivity to the change in distance. In a nonlinear pulse-train interferometer, the phases of the fundamental and second-harmonic fringes were measured, and the positions of their zero-crossing points near the fringe envelope peak were compared. A difference between these two positions and the effectiveness of the proposed method were confirmed. The concept and analysis reported herein pave the way for the development of higher-accuracy, absolute distance measurements.

Due to the lack of an economic midinfrared laser, especially wavelength around 3  μm, light sources are critical to trace gas sensing for the gases without absorption in the near-infrared zone. We present a trace ethane photoacoustic (PA) sensor using a midinfrared light-emitting diode (MIR LED). MIR LEDs are very attractive sources for PA technology since they are relatively high power spectral density, easy modulation, low power consumption, and low cost. The proposed PA system with a 3.36-μm MIR LED shows good linearity to ethane concentration and reaches a detection limit of 10 parts per million by volume easily. Ideas of further enhancement of the sensitivity are discussed.

We propose an approach for three-dimensional (3-D) refractive index (RI) measurement of special optical fiber by phase-shifting digital holographic microscopy based on optical vortex. The experimental system consists of a modified Mach–Zehnder interferometer and related lab-developed software. The phase shifts are obtained by loading different computer-generated spiral phase holograms on a liquid-crystal spatial light modulator with digital blazed grating, which is more accurate than the traditional phase-shift methods. The optical vortex generated in +1 diffraction order of the diffraction field is utilized as the reference beam. Eight digital holograms with the phase-shift increments of 0, π  /  2, π, and 3π  /  2 are recorded by a CCD before and after the tested optical fiber is placed in the experimental system. Then, a four-step phase-shifting algorithm and phase unwrapping algorithm are adopted to extract the phase distribution from the holograms. Based on a filtered backprojection algorithm, the 3-D RI distribution of the tested fiber is obtained at high accuracy. Experimental results demonstrate the feasibility of the proposed method for 3-D RI measurement of special optical fiber.

Wavelets shrinkage is the most illustrative of wavelets transform for speckle noise reduction. We aim to study the performance of a monogenic wavelet transform to reduce the speckle noise in digital speckle pattern interferometric fringes. The proposed method is implemented on simulated and experimental speckle fringe patterns and its performance is appraised on the basis of peak signal-to-noise ratio (PSNR) and quality index (Q). The ability to reduce the speckle noise by the proposed method is compared with other classical speckle denoising methods. The obtained results corroborate the effectiveness of the proposed method for speckle noise reduction in speckle fringes in terms of PSNR and Q. It is also observed that the method provides better qualitative and quantitative results. Furthermore, the proposed method preserves the edge information of the speckle fringes, a feature that is quantified by the edge preservation index.

Projecting defocused binary pattern method can overcome the nonlinear gamma effect of the projector, as well as filter high harmonics, but it is sensitive to defocus levels. Therefore, it makes it difficult to generate both comparable performance to ideal sinusoidal fringe patterns and good quality Gray code when only a small number of phase shift patterns are used. To overcome this problem, a phase unwrapping approach combined Gray code with phase-shifting fringe patterns generated by triangular pulse width modulation (TPWM) technique based on slight defocusing is proposed. The phase is retrieved using a four-step phase-shifting method, and the corresponding phase order is obtained by Gray-code patterns. Because of defocusing projection, the boundaries of two adjacent Gray-code patterns become blurred, which make the recovery of accurate Gray-code patterns difficult. To solve this problem, we propose to project an all-white pattern and an all-black pattern to obtain threshold values, which are used to execute the binarization of Gray-code patterns. This approach is robustness to different defocus levels of projector. Experiments are submitted to verify that the proposed technique can be utilized to high-accuracy three-dimensional shape measurement of complex and/or discontinuous objects.

The bit error rate (BER) performance of the incoherent synchronous pulse position modulation optical code division multiple access (PPM-OCDMA) network has been analyzed using a multilength weighted modified prime code (ML-WMPC) for three different multimedia services, such as data, voice, and video. Each service has a different bit rate than the other services and, hence, a different WMPC code length. In the network performance calculations, the multiple access interference (MAI) is included and considered as the only source of noise. In addition, to improve the network performance, the Manchester encoding and the MAI cancellation techniques are used with the WMPC at a higher number of active users. The network performance calculations are introduced in terms of BER for three kinds of PPM-OCDMA receivers, such as a simple receiver, receiver with MAI cancellation (MAIC), and receiver with MAIC and Manchester encoding technique. Finally, the BER calculation has been investigated at different bit rate according to the type of service provided by the network in the presence of the ML-WMPC and the third PPM-OCDMA receiver.

Since holograms are very costly content, security against holograms is very important. Hologram watermarking has been researched for a long time as a security solution for the digital hologram distributed on the network as the hologram is digitized. We deal with a digital watermarking for digital hologram contents as ownership protection tool. Since holograms have different optical characteristics from natural images, a watermarking technique using these characteristics should be needed. We therefore selected Fresnel diffraction for this property. Both of the host holograms and the watermarks are diffracted more than once for each. The results are refracted to concentrate the diffracted data into the localized regions of the transformed plane so that the region occupied one region among them. From this process, we change and select the region of the watermark. We experimented our scheme with various test images for various attacks on data-manipulating attack and geometric attack. It is apparently recognizable for eye inspection. From the results, we expect the proposed watermarking scheme to be an appropriate solution for protecting the ownership of holograms.

In the phase measurement deflectometry, sinusoidal fringe patterns are separately projected on a specular test surface, and the distorted fringes reflected by the surface are recorded by a charge-coupled device (CCD) camera. The phase shift algorithm is used to find the position of the light source on the display corresponding to the pixel point on the CCD, thereby calculating the slope of each point on the element to be tested, and finally retrieving the surface shape of the test element by using the surface reconstruction algorithm. The phase information is usually obtained by traditional 16-step phase-shifting algorithm, which involves certain complicated and time-consuming procedures, for instance, fringe projection in the horizontal and vertical directions. Therefore, to avoid these procedures, a four-step phase shift algorithm based on crossed fringes is proposed. Based on Gram–Schmidt orthonormalization method, only four crossed fringe patterns are needed to determine the phase in both directions simultaneously. Both numerical simulation and experiment are conducted to verify the validity of the algorithm.

To obtain a compound eye glass with good optical performances, optical simulation is carried out on the TracePro ray tracing software. The influence factors include the arrangement gap length (D), the chord height ratio (L  /  H), and the size of compound eyes (R). Compared to the conventional flat photovoltaic (PV) glass, the compound eyes PV glass has an increment of 6.41% on collecting radiant power, when the compound eyes arrange gapless with the L  /  H of 2.5 and R of 20.50 mm. Experimentally, the compound eyes PV glass samples fabricate with D of 0.584 mm, L  /  H of 2.5, and R of 4.1 mm and cover on silicon solar cells. In order to simulate the slanting of sunlight at early morning and later afternoon, the angle of incident light is changed by adjusting the detect platform angle under the standard AM1.5 simulator, and the absolute photoelectric conversion efficiency increment of the as-fabricated compound eyes PV glass achieves 1.03% in maximum, compared to that of the conventional flat PV glass. Moreover, the compound eyes glass does not cause obvious temperature rise. Therefore, the compound eye glass provides a promising enhancement on sunlight collection for PV modules.

Spectrally tunable light sources for general lighting have recently attracted much attention as versatile solutions that can be used in humancentric lighting implementations provided with excellent color rendering and increased user perception. However, temperature and age-dependent color shifts and flux variations in the light-emitting diode (LED) emission are nonresolved challenges that need to be overcome in order to be used in final applications. We demonstrate two strategies that can be used to efficiently and precisely generate arbitrary spectral power distributions (SPDs) using multichannel LED engines. First, we introduce different methods to match a given SPD and select an algorithm (simulated annealing) in virtue of its speed (in the milliseconds range) and accuracy (color shifts Δu^′v^′  <  5  ×  10^−  4). Then, we propose a closed-loop feedback control (PID) to compensate for spectral shifts due to temperature changes or lumen decay of the LEDs. Both methods can be used independently, but only a combination of them (which uses the output of the first method as an initial guess for the second) offers fast computational times and high spectral accuracy and precision. Computation times are important because these algorithms are intended to be executed on dedicated microprocessors integrated in the LED modules, often sharing scarce memory and processing resources. The results presented here are aimed to be universal and hold for different implementations of the light engine and any number of LED channels.

We have recently introduced the mass-polariton (MP) theory of light to describe the coupled dynamics of the field and matter when a light pulse propagates in a transparent medium. The theory is based on combining the electrodynamics of continuous media and continuum mechanics, which are both widely used standard theories in their fields of physics. The MP theory shows that a light pulse propagating in a transparent medium is accompanied by a mass density wave (MDW) of atoms set in motion by the optical force density of the light pulse. In the corresponding quantum picture, the covariant coupled state of the field and matter is described as the MP quasiparticle, which has coupled field and medium components. We study a schematic experimental setup that would enable measurements of the atomic displacements and the excess mass density related to the MDW of a Gaussian MP pulse propagating in an optical fiber made of fused silica.

We present a method for obtaining a passive Q-switched Nd  :  YAG  /  Cr^4  +  :  YAG laser with tunable pulse width. The output pulse width is tuned based on the delay time between the pumping pulse and trigger pulse. An auxiliary laser with a narrow pulse is used to trigger the laser pulse generation at different delay times during the pumping pulse duration. It is found that the output pulse width can be continuously adjusted from 1415 to 562 ps with an output peak power ranging from 44 to 240 kW at a fixed pumping power. The beam quality factor M² is kept <1.5 with changing of the pulse width.

Visible light positioning has a good application prospect because of its simultaneous lighting, convenience, and security. However, most existing visible light communication (VLC) positioning systems cannot achieve real three-dimensional (3-D) positioning but just the small range approximation or fail to provide satisfactory positioning precision and speed. We propose a 3-D indoor localization system based on VLC using improved bacterial colony chemotaxis algorithm. The positioning problem is transformed into a global optimization problem. We reduce the dimensions of the positioning algorithm from 3-D to one-dimensional and propose a fitness function based on the simple geometric relationship between light-emitting diodes’ projection circles in the horizontal plane. In addition, we improve the traditional bacterial colony chemotaxis algorithm by adopting self-adaptive reception scope to improve global convergence and introducing differential evolution operator to overcome the algorithm premature. Our simulation results show that the mean positioning error is 0.73 mm in an indoor space of 5  m  ×  5  m  ×  6  m and the positioning time for a single point is 21.8 ms. Also, the positioning results show the advantages of the proposed positioning algorithm with high precision and high speed. The proposed positioning algorithm is practical and efficient, showing great application prospects in indoor positioning.

We study the propagation properties of optical bottle beams in turbulent atmosphere. By allowing the mathematical expression of source plane to cover both the symmetric and asymmetric forms, the beam is propagated through turbulence using random phase screens. On the source plane, the intensity profile of the bottle beam resembles a dark hollow beam with an outside ring for symmetric cases, whereas it becomes divided into two or more separate lobes for the asymmetric cases. During propagation, both symmetric and asymmetric beams concentrate the intensity toward the center, eventually assuming a Gaussian shape, where this process seems to be more rapid for the former beam types. The rising trend of the kurtosis parameter of bottle beams changes to a falling trend when the refractive index structure constant is reduced from ^{10  −  14} to ^{10  −  13}  m^−  2/3.

A 200-Gb  /  s wavelength-division multiplexing transmission system using multiwavelength directly modulated transmitter optical subassemblies for data center has been proposed and demonstrated experimentally. The 8  ×  25  Gb  /  s PAM-4 optical transmission system over 10 km of standard single-mode fiber is tested. The penalty variation within 1 dB for eight channels indicates that colorless transmission operation in the C-band is feasible with stable performance. Clear eye openings without any distortion show a good linearity characteristic for the optical transmission system. The advantages of integration, channelization, miniaturization, and low power consumption of the proposed scheme present a promising result for data center interconnection.

A Nd  :  YAG  /  Cr^4  +  :  YAG composite crystal passively Q-switched laser with high-output pulse energy is studied both theoretically and experimentally. According to the characteristics of flash lamp pump, the rate equations of nonuniform pump are established. The pulse interval, pulse energy, and pulse width of the output pulse train are analyzed accurately by nonuniform pump. The experimental parameters are optimized by numerical simulation, with Cr^4  +  :  YAG saturable absorber initial transmittance of 14%, output coupling mirror reflectivity of 30%, and cavity length of 9.7 cm. Under the same conditions, the maximum static energy is 954.34 mJ without Q-switch. An output pulse train with four pulses can be obtained at the same pump condition in experimental study. The four pulses energy are 156.5, 151, 149.74, and 145.76 mJ in turn with corresponding intervals of 20.81, 24.16, and 46.03  μs. The average pulse width is 5.72 ns. The Q-switch efficiency is 63.2%. The energy of the four pulses decreases, the pulse interval increases, and the pulse width is basically unchanged. The nonuniform pump theoretical and experimental results are in good agreement.

We numerically present the generation of four types of pulses using nonlinear Schrödinger equations in an all-normal-dispersion dual-pump mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Transition state (TS) between dissipative soliton (DS) and dissipative soliton resonance (DSR), typical DSR, DS, noise-like pulse (NLP), and nonmode-locked state are investigated in a two-dimensional pump power space. Different from previous research, we first find that the pulse peak power and width vary dependently and nonmonotonously under asymmetric coupling ratios in the NALM. Under different coupler ratios and amplifier powers, the shift and distortion of simulated NALM transmittivity curve which are caused by the pump power intercoupling and the amplifier saturation are demonstrated. The ratio of incident power and the saturable power of NALM (when the ratio is >2, 1 ∼2, ∼1, <1) directly determines the pulse states (NLP, DSR, unstable, DS, or TS) in the cavity. Overall, the utilization of NALM could effectively enlarge the saturable power as well as the transmittivity, which provide a road to increase the DSR peak power.

We represent the effect of nonlinear propagation in mode-division multiplexed multimode fiber (MMF) link. The effect of intermodal four-wave mixing (IMFWM) and intramodal FWM for waves with different spatial modes (LP₀₁, LP₀₂, LP₁₁, and LP₁₂) over MMF has been analyzed with the help of an optical spectrum analyzer using VPI Photonics/VPI transmission Maker™ version 9.7 simulator. It can be concluded that high launch power and low dispersion generated sidebands in the cases of both intermodal and intramodal FWM. These FWM products grow in number with variation in wavelength of probe as well as these products grow toward the central pump frequency of that particular mode. It mathematically represents the multimode nonlinear propagation of different mode groups (containing N spatial modes in each group) in MMF due to Kerr effect. Thus, nonlinear interaction between modes results in interference that degrades the performance of multichannel mode-division multiplexed systems. To achieve full-phase matching and reduce the effect of IMFWM, chromatic dispersion has been increased in each spatial mode throughout the link.

A triple frequency- and phase-tunable optoelectronic oscillator based on stimulated Brillouin scattering effect and carrier phase-shifted single sideband (CPS-SSB) modulation is proposed and theoretically analyzed. In the structure, three pump lights are used to generate an oscillator signal in which the frequency is equal to triple the Brillouin frequency shift through the Brillouin gain–loss compensation. Benefiting from the wavelength-dependent characteristic of Brillouin shift, the triple frequency-tunable signal can be realized by directly tuning the laser source. The CPS-SSB modulation signal lies in the combination of a dual-parallel Mach–Zehnder modulator (DPMZM) and a tunable bandpass filter. The phase of the output microwave signal is changed by controlling the direct current bias voltage of DPMZM. Ultimately, a microwave signal, with a tunable frequency range from 26.3925 to 28.3425 GHz and a tunable phase from 0 deg to 360 deg, is obtained in theory. In addition, the theoretical simulation of the phase noise is below −110  dBc  /  Hz at the 10-kHz offset when tunable laser source is set at 1550 nm.

Mitigation of image distortion under modified von Karman-type phase turbulence has been examined by using acousto-optic (A-O) chaos waves generated via feedback in a Bragg cell. The chaos wave transferred to the optical carrier either over an image-bearing transparency (with the image reconstructed with appropriate lensing), or alternatively the two-dimensional image (both static as well as dynamic) is embedded in the chaos wave and propagated over the turbulent layer. These investigations demonstrate that the properties of chaos waves including relative immunity from variations in the propagation path enable reduction or mitigation of the image distortion under turbulent conditions. Mitigation of image distortion is explored using propagation through a turbulent atmospheric layer characterized by gamma–gamma type intensity fluctuations. Standard weak, moderate, and strong turbulence conditions are applied for the numerical analysis based on the corresponding structure parameters. Following standard approach, the relevant probability density functions are generated using small- and large-scale eddies (α and β numbers) incorporated into the turbulence model. In addition, mitigation of image distortion due to gamma–gamma turbulence is examined for propagation along a low-altitude slanted path with a fixed slope and different elevation angles using A-O chaos along with the Huffnagel–Valley model. Stationary images are transmitted under nonchaotic and chaotic conditions, and the corresponding distortions in the received images are measured using the conventional metric of bit error rates. The system performances under nonchaotic and chaotic transmissions are compared with the intent to establish that packaging a signal within a chaos wave offers a degree of distortion mitigation.

Increasing usage and higher demand for wireless traffic are causing a critical need for increased bandwidth and capacity communication networks. Accordingly, multipoint free-space optical communication (FSOC) links are beginning to draw significant attention as a valuable and promising solution for satisfying new network requirements. Extensive interest has focused on investigating and developing hardware design solutions to perform multiuser FSOC. However, to meet the low size, weight, and power needs of this technology, we propose using a robust and solid signal-processing method, namely independent component analysis (ICA), to implement an optical multiple access technique. A high-speed, multipoint free space optical system composed of two independent users and two receiving sensors is introduced. FastICA and joint approximate diagonalization of eigenmatrices algorithms are exploited for multiuser detection, and extensive analysis of power ratio, data rate difference, computational complexity, achievable system capacity, turbulence generation accuracy, and turbulence severity effects on signal reconstruction is performed. Experimental results demonstrate that ICA provides suitable source separation for all tested configurations. Additionally, transmission parameters that maximize signal reconstruction performances were identified.

The concentration of iodide (I⁻) ions was determined by a fluorescence quenching method; a polymer-tipped optical fiber (PTOF) was used as the sensing head. The polymer tip, made by a free-radical photopolymerization method, combined the advantages of high sensitivity of the tapered fiber with a flexible and simple fabrication process. A sol–gel encapsulated fluorescein, used as the fluorescence indicator, was dip coated on the surface of the PTOF. Experimental results indicate that the backscatter fluorescence intensity collected by PTOF is 9.60 times higher than that of the flat-cut fiber. When the I⁻ ions concentration changed from 6 to 36 mM, the backscatter fluorescence intensity collected by PTOF decreased linearly with a response time of 2 s. We demonstrate a method that can detect remote ion concentration with high repeatability, with stability, and which is simple to carry out. This method can be used in applications such as chemical production and water quality monitoring.

Channel-select filtering technology is a very promising solution for realizing reconfigurable demultiplexing components required in a flexible coarse wavelength-division multiplexing system. Here, we propose and demonstrate an electro-optic tunable broadband filter based on an asymmetric Mach–Zehnder interferometer formed on lithium niobate for this application. The counter-tapered directional coupler is introduced to replace the conventional one formed with uniform waveguides so that a splitting ratio of 50%:50% is much easier to be realized over a wide wavelength range and, hence, increase bandwidth of the filter. In addition, to reduce the bending loss in the long-wavelength region, the waveguide width and the height of the involved S-bend waveguides are optimized experimentally. Our fabricated filter, which has a total length of ∼30  mm, exhibits an extinction ratio of ∼15  dB and a wide tuning range of >40  nm with an electrical wavelength tuning sensitivity of ∼4.5  nm  /  V.

Design of a silicon-based ultracompact polarization splitter (PS) with high extinction ratio (ER) is proposed for the midinfrared-operating wavelength of 4.47  μm. It is based on a directional coupler (DC) that consists of a horizontal slot waveguide (WG) and a strip WG with Si as the core material and CaF₂ as the slot and substrate material. By choosing appropriate structural parameters, we have designed the DC-PS such that only the TM polarization satisfies the phase-matching condition for coupling at the targeted wavelength of 4.47  μm; achieved ERs are ∼41.6  dB and ∼37.1  dB for the cross-coupled and the throughput ports, respectively. Corresponding estimated insertion losses (ILs) are ∼0.25  dB and ∼0.07  dB in a device length of just ∼6.6  μm. Over a broad bandwidth of 200 nm (4.37 to 4.57  μm) in the mid-IR, our designed PS exhibited ER of >37  dB (with IL of <0.5  dB) for the cross-coupled port and an ER of >17  dB (with IL of <0.1  dB) for the throughput port. Numerical estimations revealed excellent tolerance to the potential fabrication errors.

An enhanced optical fiber-based surface plasmon resonance (SPR) biosensor is proposed utilizing a two-dimensional phosphorene heterostructure as an interacting layer with the analyte. This is the first model so far of a phosphorene-based fiber-optic SPR biosensor. High sensitivity and figure of merit (FOM), the two desirable parameters, are analyzed to investigate sensor performance. The analysis indicates that the proposed sensor provides significantly higher sensitivity in comparison to conventional fiber-optic SPR sensors. Numerical study shows that on using 10 layers of phosphorene on the Ag layer, the fiber-optic biosensor exhibits extremely high sensitivity of 3725  nm  /  RIU with a FOM of 61  RIU^−  1. FOM reaches its maximum value of 64  RIU^−  1 with six layers of phosphorene. Effects of the number of heterostructure layers and light wavelength on sensitivity are also analyzed. The results will open a new way for using phosphorene heterostructure in an optical fiber-based SPR sensor for biosensing applications.

A numerical illustration of a hybrid design and numerical analysis of graphene-coated fiber-optic surface plasmon resonance (SPR) biosensor for BRCA-1 and BRCA-2 genetic breast cancer detection is provided. Two specific mutations named 916delTT in BRCA-1 gene and 6174delT in BRCA-2 gene are being selected for detection of breast cancer numerically. This sensor is based on attenuated total reflection (ATR) method to detect individual point mutations in BRCA-1 and BRCA-2 genes. Based on the numerically obtained results, a momentous change is present in the SPR angle (minimum 35% more) and surface resonance frequency (SRF) (minimum 36% more) for probe DNA with various concentrations of target DNA corresponding to the mutation of the BRCA-1 and BRCA-2 genes. The variation of the SPR angle and SRF for mismatched DNA strands in BRCA-1 and BRCA-2 genes is quite negligible, whereas that for complementary DNA strands is considerable. This considerable change is essential for proper detection of genetic biomarkers (916delTT and 6174delT) for early breast cancer. To the best of our knowledge, this is the first demonstration of such an adept biosensor for detecting BRCA1 and BRCA2 genetic breast cancer. Here, we used graphene as bimolecular acknowledgement element for improving sensor performance. At the end of the article, the performance in terms of sensitivity is analyzed. Therefore, the proposed biosensor opens a window toward detection of early detection of BRCA-1 and BRCA-2 genetic breast cancers.

Relative humidity (RH) is an important factor in the field of structural health monitoring, especially during the early stage of corrosion. Many methods have been proposed for humidity sensing, and one of the attractive sensors is fiber-optic long-period grating (LPG) sensors. Unfortunately, the current sensing sensitivity of this kind of sensors is limited. A thin fiber-optic LPG sensor with a self-assembled thin film of PAH⁺  /  PAA⁻ is proposed and demonstrated for humidity measurements. The LPG sensor is inscribed in a single-mode fiber using a CO₂ laser, and the cladding size is reduced to about 27  μm. It shows that an LPG sensor with a reduced cladding size has an enhanced refractive index sensing characteristic, compared to those with normal cladding size due to the enhancement of the evanescent field. In a next step, selectivity is added to the LPG sensor coated with a film of PAH⁺  /  PAA⁻ for functionalization to be sensitive to humidity. The resonance spectral responses of LPGs are experimentally investigated with respect to its sensitivity to a change in humidity that modifies the index of the nanolayer and the cladding, leading to a resonant wavelength shift. The experimental results show that the coated thin LPG has a highly sensitive resonance wavelength shift of −220.75  pm  /    %  RH for an RH variation from 25% to 80%, of which the sensitivity is enhanced thrice compared to those with a normal cladding size. The proposed sensing setup opens LPG structures for a variety of sensing and detection applications.

Different well widths of m-plane Mg_0.19Zn_0.81O  /  ZnO multiple quantum wells (MQWs) have been fabricated on the two-step pregrown MgZnO-buffered m-plane sapphires by a pulsed laser deposition and have a narrower linewidth on x-ray diffraction than ZnO-buffered MQWs. By using the variation method to calculate the band diagram and by corroborating the results of temperature-dependent photoluminescence, we confirm the binding energies of free exciton and basal-stacking-fault-bound exciton, which is composed of electrons in the barrier and confined hole in the narrow-width MQWs. The coupling of A1(LO) phonon with excitons in m-MQWs shows more sensitivity toward a decrease in well width than E2(low), due to the crystal symmetry and the reduced Bohr radius.

This erratum corrects an error in a figure.