Laser-based additive manufacturing (LBAM) is a group of advanced manufacturing processes used to produce metal components and functionally graded products. Production in LBAM is either limited to the formation of thin or thick coatings on a substrate by laser metal deposition or the production of a fully functional metallic product by selective laser melting. In every case, LBAM fabricated components require optimization for the process parameters to avoid defects, such as porosity, crack holes, thermal deformation, and mechanical strength. As a key link in the laser additive manufacturing (LAM) process, laser scanning path planning is an effective strategy for balancing the temperature field of the formed part, avoiding stress concentration, and preventing deformation and cracking. Efficient, accurate, and reasonable planning of the laser scanning path is of great significance for improving the processing efficiency of the process data, prolonging the life of the laser scanning system, and improving the forming quality of the specimen. Through many studies, it was found that the scanning pattern of the lasers has a significant impact on the mechanical properties and deformations caused by a thermal mismatch during the process. Therefore, it is essential to have in-depth knowledge about path planning in LBAM. Our review mainly focuses on the influence of scanning patterns on deformation, temperature, and mechanical properties in LBAM. Finally, our paper discusses the current study limitations and some future studies in LAM technology.

The prime limitation of optical sensors is the need for external sources of illumination while capturing the scene. This prevents them from recognizing objects in extreme conditions, such as insufficient illumination or severe weather (e.g., under fog or smoke). The thermal imaging sensors have been introduced to circumvent this deficiency, which acquires the image based on thermal radiation emitted by the objects. The technological advancement in thermal imaging enables the visualization of objects beyond the visible range that promotes its use in many principal applications, such as military, medical, agriculture, etc. However, hardware point of view, the cost of a thermal camera is prohibitively higher than that of an equivalent optical sensor. This led to employ software-driven approaches called super-resolution (SR) to enhance the resolution of given thermal images. We propose a deep neural network architecture referred to as “ThermISRnet” as the extension of our earlier winner architecture in the Perception Beyond the Visible Spectrum (PBVS) thermal SR challenge. We use a progressive upscaling strategy with asymmetrical residual learning in the network, which is computationally efficient for different upscaling factors such as ×2, ×3, and ×4. The proposed architecture consists of different modules for low- and high-frequency feature extraction along with upsampling blocks. The effectiveness of the proposed architecture in ThermISRnet is verified by evaluating it with different datasets. The obtained results indicate superior performance as compared to other state-of-the-art SR methods.

Complex wave retrieval (CWR) from an off-axis digital Fresnel hologram proposed by Liebling et al. is a single-exposure scheme in which a non-linear holographic process is modeled using simultaneous linear equations. This linear model fits well into the compressive sensing (CS) framework. We propose a method in which the CS is applied to a CWR method with Fresnelet sparsification. This approach compensates for the noise in the retrieved Fresnel field that is aroused due to the linear modeling approximations involved in the reconstruction process. As a result, a superior intensity and phase reconstruction were obtained by this method. Four methods including the direct CWR (conventional) method, compressive CWR method without sparsification, compressive CWR method with Haar wavelet, and compressive CWR method were compared with Fresnelet sparsification. The simulation and the optical experimental results converge to the same conclusion: the Fresnelet sparsified case is the best choice for quantitative phase imaging using compressive off-axis digital Fresnel holography.

Full-field optical coherence tomography (FFOCT) can be used to acquire cellular-level morphological images of nonfixed and unstained biological tissues. However, the speckle noise and vignetting of FFOCT images are drawbacks in terms of pathological application. We present a novel adaptive nonlocal image enhancement algorithm that uses a fluorescence microscopy (FM) image of the nucleus to achieve cytoplasm image enhancement under FFOCT. The algorithm comprises a denoising module and an adaptive nonlocal enhancement module. The denoising module is an end-to-end residual network with skip connections and dilated convolution. The adaptive nonlocal enhancement module enhances images to segment tissues, nontissues, and transition areas so that the images retain the details of the original cell structure but do not contain noise. The proposed algorithm was implemented on a dual-modality scanner system that combines FFOCT and FM. A total of 12 pig liver and pig kidney tissues were sectioned into 10,236 samples for experiments. The peak signal-to-noise ratio and structural similarity index value increased from 30.8 and 0.58, respectively, for the aforementioned system without the proposed algorithm to 34.2 and 0.86, respectively, for the aforementioned system with the proposed algorithm. Ten samples of human breast tissues were used to validate the proposed algorithm. The average contrast-to-noise ratio was 1.69 and 11.73 when the aforementioned system was used with and without the proposed algorithm, respectively. The results of this study revealed that the quality of scanned images considerably improved when the proposed system was used and that the proposed system can facilitate clinical pathological diagnosis.

High-resolution images are essential in many remote sensing applications such as weather prediction, deforestation, crop monitoring, land mapping, urban growth, etc. In these applications, it is necessary to use multiple high-resolution images for proper analysis. These images carry a lot of information; therefore, multiplexing plays an important role for efficient storage of the images. Hence, we provide an idea of multiplexing high-resolution images using amplitude grating. Initially, images are modulated with variable spatial frequencies and orientation angles followed by the addition of modulated spectrums to form a single spectrum plane. The resultant spectrum plane is filtered before transmission to eliminate noise. To optimize storage space, only a part of the filtered spectrum is transmitted. A pixel intensity graph is plotted to identify the location of maximum information of the images, which is required during the retrieval process. The quality of the output images is analyzed by peak signal-to-noise ratio, structural similarity index measurement, and correlation coefficient methods. The entire work is done by simulation software, and we prove that our proposed multiplexing method is suitable for efficient storage of high-resolution images in remote sensing applications.

A full quantum secure direct communication system is implemented in MATLAB environment using linear optical devices and single-photon detectors. MATLAB has the advantage of integrating the simulation of the whole system, which has optical and electronic blocks, all together into one script. Each block of the system (transmitter, receiver, and security check) is physically and mathematically explained, and the idea behind the code is presented. Results from running the whole simulation are in accordance with those obtained either from the published practical systems or from using state-of-the-art commercially available optical simulators. Our approach opens the way to simulate with confidence hyperentangled systems where experimental data are not yet widely available.

Imaging of transparent phase samples using a wire grid polarizer (WGP)-based shearing interferometer is demonstrated. The proposed setup employs a WGP as a beam splitter in a triangular cyclic interferometer setup, so that the two counterpropagating beams are rendered linearly polarized in orthogonal directions as they emerge from the interferometer. These sheared waved fronts are made to pass through the phase sample. The magnitude of shear is affected by controlled angular shifts of one mirror about the reference X and Y axes. For each of these directional shears, the polarization phase shifting is implemented so as to generate a three-dimensional phase pattern, which is digitally combined and integrated to yield the sample phase. The proposed setup is robust, sufficiently tolerant to ambient vibrations, and shows very encouraging experimental results.

Wavefront sensing from an extended object is a challenging task since the phase to be sensed is disturbed by the one generated from the structure of the extended object. To address this problem, a general wavefront sensor is proposed. The configuration of the proposed sensor consists of a field lens, a collimating lens, a lenslet array, and a camera. The field lens, the first element of this sensor, ensures that each lenslet collects the same information of the extended object, which is a basis for the reconstruction of the distorted wavefront from the extended object using the proposed sensor. The fundamental strategy of the sensor is converting the scenario of wavefront sensing from an extended object into the traditional one from a point source by eliminating the Fourier spatial spectrum of the extended object in the frequency domain. As a result, the distorted wavefront can be reconstructed using the algorithm of the Shack–Hartmann wavefront sensor. Numerical simulations and experiments both verify the feasibility and accuracy of the proposed sensor for direct wavefront sensing from any types of extended objects.

Online three-dimensional (3D) measurement plays an important role in industry. When phase-shifting profilometry is employed in online 3D measurement, pixel matching is an important step to keep objects at the same coordinate value. However, the correlation operation and marker feature matching algorithms may take a long time, increasing the complexity. So a fast and robust online 3D measurement based on feature correspondence is proposed. In this method, only one frame of the sinusoidal fringe pattern is projected onto the measured object, and image correction technique is employed to rectify pixel size. Then five frames of deformed patterns with equivalent displacement are captured by the camera, and the corresponding modulation patterns are extracted. The oriented fast and rotated brief feature algorithm is used to extract the matching pair of feature points, and the improved grid-based motion statistical feature algorithm can better eliminate the false match to achieve pixel matching. In this way, five frames of deformed patterns with an equivalent shifted-phase can be extracted. Finally, the 3D shape of the measured object is reconstructed by the five-step equivalent phase-shifting algorithm. Experimental results verify the effectiveness and feasibility of the proposed method.

A method based on the Zernike coefficients transformation is proposed for the wavefront sensing of a sparse aperture system. We deduce a matrix that allows the Zernike transformation between the sub-apertures’ wavefronts and the global counterparts in a sparse aperture system. With the matrix and the given global aberrations, the aberrations of the sub-apertures can be calculated through the transformation. The simulation results indicate that the increase of filling factor of the sparse aperture system could lead to the increase of orders of the Zernike polynomials, which describe the sub-aperture’s wavefront. The root mean square value of the detected Zernike coefficients of the sub-aperture is determined by its azimuth angle when the filling factor is fixed.

To meet the proposed science goals of the Visible Emission Line Coronagraph on board the Aditya-L1 mission, the total scattered light should be less than 5 ppm (parts per million) of solar disk brightness over the field of view (FOV). Major contributors to the scatter light are microroughness and contamination over the surface of the primary mirror (M1). A detailed study to derive the microroughness specifications of the M1 provides a fair idea of the effect of the contamination on the mirror in terms of the scatter generated from the surface: 1.53 Å band-limited RMS microroughness on the M1 surface contributes toward a total scatter of 4 ppm over the FOV. This restricts the scatter over the FOV due to contamination being less than 1 ppm, and it is equally apportioned to scatter due to both particulate and molecular contamination. Hence, the scatter over the FOV due to particulate contamination should be limited to 0.5 ppm. Our paper provides a detailed explanation of the studies carried out to determine the maximum allowable particulate contamination levels on the M1.

A three-dimensional (3D) face profilometry based on facial landmarks and a multi-view system is proposed. Theoretically, at least three fringe patterns are required in phase-shifting profilometry, but the stereo relative phase pair obtained by a spatial unwrapping algorithm cannot be used for stereo matching directly because the starting points of phase unwrapping are not on the same fringe period order, so it will result in incorrect disparity. We proposed an innovative method that utilizes facial landmarks as prior knowledge to adjust the relative phase pair to the same phase reference for accurate stereo matching through a phase adjustment algorithm to determine the accurate fringe order difference. The three-step phase-shifting patterns are projected in a looping sequence, and each three-adjacent fringe stereo pair is treated as a group to obtain one frame of a 3D face, which means that the 3D face output of our proposed method can achieve the same frame rate as the camera. The accuracy evaluation and dynamic 3D facial expressions reconstruction verify the success of the proposed method.

Rotary optical encoders are widely used in all areas of industry, robotics, and special equipment. The accurate measurement of their characteristics is a necessary task for their successful implementation. We attempt to find a unified approach to determining the accuracy of rotary encoders. The proposed approach is based on statistics and code reliability, which characterizes the probability that the measured code corresponds to the specified code. To determine the code reliability, one has to measure the turn angle with a goniometric device and compare the measured values with the encoder codes. The paper considers the dynamic goniometer for determining the error of absolute angle encoders, which allows for highly accurate measurements in a wide range of rotation rates. The measurement is made at certain moments of time, depending on the electrical interface of the encoder. In the case of a parallel interface, the time points are determined by the moment when the encoder code changes. This data collection allows for obtaining a general population. In the case of an encoder, with the sequential approach to reading the code, it is not possible to obtain the general population in a wide range of rotation rates. We analyze the above-mentioned case in detail and present experimental data obtained with a dynamic goniometer and the results of code reliability calculations.

We report a complete design and simulation of a quasi-optical millimeter wave imaging system using ZEMAX and FEKO software, respectively. A Fresnel lens and a horn antenna are combined in this design. Compared to spherical and aspherical lenses, a Fresnel lens can be fabricated much easier at millimeter wavelengths. For focusing millimeter wave radiation, a Fresnel lens can be used to reduce the thickness of the focusing element and to lighten its weight from 25 to 4.5 kg. A horn antenna with a Gaussian profile and corrugated walls is designed for feeding this system at a central frequency of 94 GHz. The symmetrical radiation pattern of the designed corrugated Gaussian horn in E- and H-orthogonal planes, its wide bandwidth, and low side lobe levels make it a good candidate for feeding a W-band millimeter wave imaging system. The designed quasi-optical imaging system is light-weighted, has high resolution, and can be used in detecting hidden objects within a distance of 5 m with a 30-mm resolution in W-band at a central frequency of 94 GHz.

Guided-mode resonance (GMR) displays a wide range of optical properties that could prove useful for many applications in photonics. We report the enhancement of laser emission from the GMR sparse grating-based dye laser. The enhancement is due to an introduction of the low-refractive-index (low-n) separation layer, which causes a strong interaction between the local electric field and dye molecules. Finite-difference time-domain method is used to study how the structure’s geometry, including the thickness of the separation layer and the fill factor of the grating, affects the electric field and laser intensity. The results show that the emission can be enhanced by a factor of up to 8 compared with high-refractive-index SU-8 as the separation layer. The laser intensity reaches to the maximum value when the fill factor of the grating is 0.2, i.e., the sparse grating is used. The threshold of the low-n GMR dye laser is also examined. This design can effectively improve the performance of pumped photonic band-edge lasers.

This study integrated the use of a centering machine with an automatic optical axis measuring technique to improve the centering process for short-focus lenses, which are widely used in interferometric inspection, microscopy, and spectrometry. A major concern of the centering process is coma aberrations during the axis centering of a lens, which leads to deformation of the image system. Because of the small size and high curvature of short-focus lenses, high optical axis error and unstable grinding quality are highly problematic within the high-precision centering process. To reduce optical axis error and improve manufacturing quality, an on-line optical axis measuring system that applies convolutional neural network (CNN) machine learning for the evaluation of centering stability was developed. According to experimental results, the CNN achieved 95% accuracy. With the use of trace classification and optical axis measurements, the optical axis error was controlled to <150  μrad, the range of cracks to <E0.1, and the circularity error to <0.1  mm.

The interference method is used to achieve all-optical logic gate operations. This has been realized with a single design of photonic crystal ring resonator at different wavelengths rather than designing separate for all the logic gates. Such a single design can be used to perform all optical operations as AND, OR, and XOR logic gates at different wavelengths. Further operating wavelength, contrast ratio (CR), response time (RT), and bit rate have been analyzed for all three logic gates. The highest CR is obtained as about 22.06 dB with fast RT about 0.14 ps and high bit rate of 7.14  Tb  /  s for proposed XOR logic gate in comparison to all the other logic gates with single-design structure, which may lead us to construct on-chip logic circuits.

A polymer-dispersed liquid crystal (PDLC) device is fabricated to serve as a diffuser for random phase encoding of reference beams in holographic storage system. By tuning the applied voltage on the PDLC device, three individual random phase reference waves which are orthogonal to each other can be generated. Therefore, holographic multiplexing scheme can be performed using voltage modulation on the PDLC diffuser. In addition, shift multiplexing technique still can be performed in the storage system. When compared with conventional shift multiplexing, using electrically tunable PDLC diffuser offers additional multiplexing mechanism in the presented technique. Using the proposed PDLC diffuser, the multiplexing numbers can be enhanced three times owing to voltage modulation. A holographic storage system implemented using shift multiplexing and voltage modulation on PDLC diffuser is demonstrated.

Calcium fluoride and zinc sulfide crystals exhibit broad multispectral transparency and are used for a multitude of optical applications. However, they both belong to a soft brittle crystal category and are mechanically challenging to process. Thus, a fabrication protocol using a single-point diamond turning method was developed, which can yield surfaces with optical quality for these materials. By using a diamond turning tool with low waviness (≤100-nm waviness) and by using feed rates of 1 and 0.2  μm  /  rev, CaF2 and ZnS surfaces with surface roughness of 6 and 5 nm were manufactured, respectively. Design and fabrication of a miniature objective with 0.5 NA are also presented, which was developed for the ultrafast laser microsurgery application. The objective consists of CaF2 and ZnS lenses to withstand the high power from ultra-short laser pulses and to provide a tightly focused beam with 1.23  μm 1  /  e2 spot radius for laser ablation. Based on the spot size measurement of the objective, its performance was considered to be near diffraction limited.

Tracking the interfacial evolution in Ni/Ti multilayers is a crucial aspect of fabricating neutron supermirrors. According to the Debye–Waller factor, the reflectivity of neutron supermirrors is modulated by the interface width, which includes interface roughness and diffusion characteristics. Significant effort has been devoted to shrinking the interface width using mixed working gases or targets during the sputtering process. To elucidate the interfacial evolution, NiC/Ti and Ni/Ti multilayers with different periods were fabricated using direct-current magnetron sputtering. The NiC layers were deposited by co-sputtering Ni and C targets. The interfacial structure and evolution within the multilayers were characterized by grazing incident x-ray reflectivity and transmission electron microscopy (TEM), respectively. The Ni layers in the Ni/Ti multilayers underwent a higher degree of crystallization, resulting in rough interfaces. Moreover, TEM images of the Ni/Ti multilayers suggest that the interface diffusion in the Ni-on-Ti layers reduces the interface smoothness. However, C doping suppresses the crystallization of the Ni layer to achieve smoother Ni/Ti interfaces, thereby improving the interface quality.

We propose a method to compute the axial distribution of non-diffracting beams. The approach stands on simple ray tracing and energy conservation principles. The proposed method is applied for four different Bessel beam generators, such as the refractive axicon, to compute the output axial intensity profile for a given input beam. We show great agreement between the results and the CODE V diffractional simulations. The method can also be used to calculate analytically the incident illumination pattern needed for a target output profile or to design an optical element surface to reach the same goal.

A highly sensitive monolithic optical receiver, applicable in optical fiber and wireless communication is presented. The proposed chip is capable of processing data rates of up to 10  Mb  /  s and offers compatibility to standard CMOS logic by creating full 3.3-V output swing. The photodetector of the system is carried out as a large-area fully integrated avalanche photodiode with a diameter of 800  μm and a high responsivity to red light with a wavelength of 675 nm. The receiver features pseudo differential signal processing with offset cancellation feedback to achieve desensitization to background light irradiation usually appearing with optical wireless communication. Additionally, common-mode feedback was implemented to prevent pulse width distorted output signals. Measurements at 2.5, 5, and 10  Mb  /  s reveal high sensitivities of −50.4, −47.65, and −42.7  dBm, respectively, for BER  <  ^{10  −  9} in dark conditions. For the design, a 0.35-μm CMOS process was chosen.

This study describes the performance of a randomly coupled erbium-doped optical fiber amplifier (EOFA) using machine learning. A reinforcement learning-based artificial neural network (RL-ANN) is used to study various parameters of EOFA, such as gain, noise figure, and inter-core crosstalk for the 12-core EOFA. In addition to this, the relationship between mode-dependent loss, a penalty of the EOFA, and the average bending loss (BL) for three operating wavelengths, such as 1550, 1580, and 1600 nm, has been studied. To predict the performance of RL-ANN, three different machine learning algorithms, namely, K-nearest neighbor (KNN), ANN, and support vector machine (SVM) are used. The performance of the proposed RL-ANN is evaluated using five statistical error criteria: mean absolute error, root mean square error, scatter index, coefficient of variation, and overall index measurement. The statistical test result shows that the proposed RL-ANN provides the best performance compared to KNN, ANN, and SVM. The performance of the RL-ANN-based erbium-doped fiber amplifier (RA-EOFA) is studied and the results show that the gain and estimated NF based on a wavelength range from 1555 to 1565 nm provides a maximum gain. A comparison study was carried out for the proposed RA-EOFA with other learning methods. The results show that the proposed RA-EOFA provides the best performance in terms of less BL (0.25 dB), mode-dependent loss (0.5 dB), computational time (36.8 s), and a success rate of 98.39%.

Circular photonic crystal fiber consisting of doped amethyst and big and small round air holes is designed to support transmission of 118 orbital angular momentum (OAM) modes in the range of 1.2 to 1.65  μm. Numerical analysis by the finite element method (FEM) reveals that the effective index difference of all the modes are larger than 1  ×  ^{10  −  4}, which can propagate OAM modes stably. The nonlinear coefficient is 0.583  W^−  1 km^−  1 at 1.55  μm, which is very low and helps to reduce mode coupling. In addition, the confinement loss of the eigenmode is between ^{10  −  12} and 10^−  8  dB  /  m. The dispersion is very flat for the low-order mode HE_1,1, for which the dispersion variation is just 1.459  ps  /    (  km nm  )  . This fiber has great potential in stable and large-capacity mode propagation in optical communication.

We propose an artificial neural network-embedded visible light positioning (VLP) scheme based on a conventional monitoring system in which the transmitter and receiver comprise a single illuminating LED and two standard surveillance cameras, respectively. Experimental results show that this simple system achieved millimeter-level positioning accuracy and exhibited good antitilt ability and robustness. The VLP system achieved high-precision three-dimensional positioning with a mean positioning error (MPE) of 0.76 cm, even if the object was inclined at different angles. Moreover, the MPEs of the single-light system were only 0.65 and 0.67 cm when the tilt angle was 15 deg and −15  deg, respectively. This scheme is a low-cost solution for accurate positioning that could be applied in marine vessel operations, mining operations, shopping center, supermarket, factories, and warehouses where monitoring systems presently exist.

An ultra-short polarization beam splitter (PBS) based on a dual-core photonic crystal fiber (PCF) with the surface plasmon resonance effect is proposed. The finite-element method is used to investigate the coupling characteristics between the core mode and surface plasmon polariton mode. The influences of the PCF structure parameters on the coupling length and coupling length ratio are also investigated. The normalized output powers of the x-polarization and y-polarization are calculated, and the optimized PBS achieves an ultra-short length of 62.5  μm. The splitting bandwidth of 110 nm (1.51 to 1.61  μm) is achieved when the extinction ratio (ER) is less than −20  dB. The minimum ER reaches −71  dB at the wavelength of 1.55  μm. The proposed PBS has an important application in high-speed optical communication systems.

Femtosecond laser filament induced breakdown spectroscopy is a promising technique that can be used in the remote sensing of air pollutants. Raising the laser energy saturation point below which the laser filament fluorescence increases exponentially with the incident laser energy will significantly improve the sensing sensitivity and detection distance. In this work, a saturation point raising scheme is proposed in which a quarter-circle π phase plate is used to form four optical filaments in air. The extrapolation based on the experimental results infers that the employment of the quarter-circle π phase plate and its induced multiple filaments can lead to the increase of the laser energy saturation point by four times, which is proportional to the number of sub-regions in the phase plate.

All-optical non-inverted half-subtracter, half-adder, comparator, and decoder are pointed out based on a single semiconductor optical amplifier. We have replaced the continuous wave probe light with the clock pulse sequences light, which guarantees the non-inverted code output. All-optical AND, NOR, XNOR, XOR, A  •    B¯  , and   A¯    •  B logic gates are realized by four-wave mixing and cross-gain modulation of SOA, which are used to drive half-subtracter, half-adder, comparator, and decoder with 10  Gb  /  s return-to-zero code. Extinction ratio (ER) and optical signal-to-noise ratio (OSNR) are improved by a depolarizer. Compared with no depolarizer, for A  =  B output of comparator, it corresponds to XNOR; ER and OSNR are improved by about 1 dB. For d₀ output of decoder, it corresponds to NOR; ER and OSNR are improved to 1.1 dB compared with the original value. However, for borrow of half-subtracter, greater than and less than of comparator, d₁ and d₂ of decoder, it corresponds to A  •    B¯   and   A¯    •  B; ER and OSNR show only slight deterioration about 0.4 dB. For carry of half-adder and d₃ of the decoder, it corresponds to AND; OSNR has hardly changed and ER has improved by about 2 dB. It has potential applications in more complex optical computing and optical signal processing systems.

We present a numerical analysis of the average intercore crosstalk (IC-XT) of wavelength-division multiplexed (WDM) optical channels in a homogeneous two-core fiber system. This analysis is performed considering cores with zero-dispersion wavelengths at 1550 nm. In the analysis, we consider 11 WDM channels spaced 100 GHz apart transmitted in three different schemes, one centered at 1510 nm with negative dispersion D  =    −  3.5  ps  /  nm  ·  km, one centered at 1550 nm with D  =  0, and one centered at 1590 nm with D  =    +  3.5  ps  /  nm  ·  km. This selection allows for the observation of how the IC-XT of WDM channels is modified using positive, zero, and negative dispersion parameters. To analyze more realistic scenarios of IC-XT in multicore fibers, we considered random bending and twisting perturbations along the fiber. In addition, we considered fiber nonlinearities such as four-wave mixing (FWM) among WDM channels. The results show that FWM produces a power transfer among the transmitted WDM channels that depends on the dispersion parameter D at core 1, and this effect is transferred to the average crosstalk of the WDM channels at core 2. Therefore, the average IC-XT of WDM channels can be modified in a controlled way by selecting an adequate dispersion parameter D in combination with FWM nonlinearity. These results provide valuable information for understanding the wavelength dependence of the average IC-XT of homogeneous multichannel MCF systems working around a zero-dispersion wavelength.

We investigate a transparent wavelength conversion based on four-wave-mixing (FWM) in semiconductor optical amplifier. We verify the effectiveness of the proposed scheme by analyzing the spectra and bit error rate. Then, we set up a 10-Gb  /  s single-user and 4  ×  10  Gb  /  s multiuser transmission by Optisystem and discuss their performance. The simulation results show that the FWM effect between the two waves achieves a 2.4-nm wavelength conversion. At BER  =  10  −  3 level, for single-user transmission the sensitivities of downstream (DS) and upstream (US) reach −23.6 and −23.4  dBm, and for multiuser transmission the sensitivities of DS and US reach −23.9 and −23.8  dBm, respectively.

The wavefront distortion caused by atmospheric turbulence leads to the decrease of the coupling efficiency of spatial light to optical fiber. Adaptive optics technology was employed to improve the coupling efficiency of free-space coherent optical communication. Accordingly, the effect of the discrete distorted wavefront on the coupling efficiency could be determined. The experimental results showed that the average coupling power was −31.97, −31.42, −30.79, −32.05, and −35.31  dBm when turbulence strength D  /  r0 was 0.2, 0.5, 1, 2, and 5, respectively. After eliminating the noncommon path aberration, the fiber coupling optical power increased from −35 to −27  dBm. After the optical wavefront was transmitted at indoor (0.5 m), 600 m, 1, 5, 10, and 100 km, the coupling efficiency increased from 10.1%, 1.3%, 11.4%, 2.8%, 7.2%, and 8.2% to 95.2%, 83.4%, 93.5%, 56.7%, and 42.4%, respectively, after wavefront correction. This will improve the performance of free-space coherent optical communication system.

Ultraflattened dispersion of optical fibers is urgently needed for ultrawideband and ultralong-haul transmission in dense wavelength division multiplexing (DWDM) systems. A systematic solution is finely developed to adjust and control the ultraflattened dispersion curves in high accuracy. The validity is verified by designing two photonic crystal fibers (PCFs) with ultraflattened chromatic dispersion across the whole telecom band (1260 to 1625 nm). Numerical results confirm that line PCF with near-zero, all-positive, and ultraflatten dispersion ranging 0.32  ±  0.17  ps  /    (  nm  ·  km  )   can support 1600-km line transmission in DWDM, whereas the dispersion-compensating PCF with low negative and ultraflattened dispersion ranging −15.57  ±  0.37  ps  /    (  nm  ·  km  )   has a high dispersion compensation efficiency of up to 33 times for line PCF. An improved PCF structure is proposed by introducing three extra small air-holes between the standard lattices of odd rings. Moreover, other fiber parameters such as nonlinear coefficient (<1.1  w^−  1  ·  km^−  1), numerical aperture (>0.189), and confinement loss (<0.09  dB  /  km for line PCF) implied that both PCFs presented can support long-haul linear scheme in the whole telecom band.

Abnormal optical absorption phenomena were observed in as-modified few-layer graphene thin films by 10.6-μm CO₂ laser under ambient conditions. Saturable absorption (SA) responses were measured using z-scan technology. The graphene samples showed SA before laser modification, whereas they exhibited reverse SA behavior after laser modification due to the effects of thinning and damage to samples. The shift of 2D peak and appearance of D peak from Raman scattering spectra further confirm the effects of laser irradiation on graphene.

Using multi-beam interferometry, a photonic structure with a graded intensity distribution is designed and obtained experimentally. Based on the structure, we choose different exposure thresholds and background materials to design a graded photonic crystal (GPC) lens with a graded air hole diameter. The transmission and focusing characteristics of the GPC lens with bismuth silicate crystal (BSO) and polymethyl methacrylate (PMMA) as background materials are investigated in the visible-light band. Comparatively speaking, the graded photonic crystal lens of BSO with a large refractive index had better performance. In addition, the focal intensity does not decrease significantly with increasing the angle of incidence. The focus point intensity remains above 93% of parallel incidence for both lenses.

We have designed a silicon detector based on an avalanche photodiode for detecting vacuum ultraviolet radiation. It was demonstrated that the detector has a photon detection efficiency superior to 0.65  electrons  /  photon, in the wavelength range from 114 to 170 nm, with an external quantum yield from 49 to 7000  electrons  /  photon at a reverse-bias voltage from 190 to 315 V, respectively. The detector’s active area diameter is 1.5 mm.

An overhead ground wire is used for both communication and power transmission and is crucial for the long-term operation of power grids. The temperature variations in overhead ground wires under a lightning strike were analyzed using an optical frequency-domain reflectometer. During lightning, the relationship between the peak temperature and the time can be considered a double-exponential distribution. The temperature distribution over time for the overhead ground wire was considered a Gaussian distribution, where the R-squared coefficient was ∼0.95. Moreover, we used the −3  dB bandwidth of the temperature curve at each moment as the lightning temperature influence range for the overhead ground wire. The temperature influence was considered a quadratic function distribution, where the R-squared coefficient was ∼0.98. The characterization of the temperature variation of the lightning struck-overhead ground wire can provide a basis for distributed optical fiber measurement systems to accurately monitor the location and intensity of lightning strikes and can be potentially used to ensure the safety of transmission lines.

Detection of ammonia concentration in exhaled human breath is one of the early diagnoses of renal disease. A photonic crystal fiber (PCF) gas sensor is designed, which gives the highest relative sensitivity from the wavelength of 1.1 to 1.7  μm. The structure of the proposed PCF consists of five layers with elliptical holes rotated circularly around a core region. The core region consists of two layers with circular holes, which have a hexagonal layer and a central hole. The finite element method is used to investigate the optical properties of PCF. The results show that the proposed PCF gives the highest relative sensitivity of 81.05% at the wavelength of 1.544  μm, which is the absorption line of the ammonia gas. Other optical properties of the proposed PCF, such as confinement loss and effective area, are calculated at the wavelength of 1.544  μm, which equal to 0.31  dB  /  m and 8.71  μm², respectively. The impact of optical properties of the proposed PCF is investigated when the diameter of holes in the core and cladding region is changed. The results show that this structure shows good reliability for the manufacturing errors.

This erratum corrects an error in Fig. 3.