Terahertz (0.1-10 THz) band communication is envisioned as a key technology of future wireless communication because of its technical characteristics, such as super large bandwidth resources and super high communication rates. Herein, ray-tracing techniques and Beckman-Kirchhoff scattering theory are used to modify a unified multi-ray channel model in the terahertz band. The model combines direct, reflection, and scattering, and is supported by experimental data from the literatures. Furthermore, the wideband channel capacity usage is characterized using equal power and water-filling power allocation strategies. The simulation results show that the importance of resource allocation in exploiting the terahertz spectrum due to its extremely high frequency-selectivity. The study will provide some guidance for future research and design of the terahertz communication systems.

The optical fiber magnetic field sensors have the advantages of high sensitivity, small size, corrosion resistance, and anti-electromagnetic interference, overcoming the drawbacks of the traditional magnetic field sensor, and play a crucial role in many industries, including military, business, and power grid. Two different types of long-period fiber gratings (LPFGs) have been fabricated on both traditional single-mode fiber and photosensitive fiber using a carbon dioxide laser. The magnetic field sensors can be created by submerging the LPFG into the magnetic fluid. The resonance wavelength of the LPFG will be shifted when being applied to an external magnetic field. The attained greatest sensitivity of the wavelength shift was 126.5 pm/mT when the magnetic strength varies in the range of 1.6-25.5 mT. The proposed magnetic fiber optic sensor has potential applications in magnetic or electric field systems.

A carbon fiber reinforced polymer (CFRP) cable has the advantages of high strength, lightweight, and good durability. However, the durability of its anchoring structure in a long-term wet service environment is doubtful. In this study, a CFRP cable was subjected to a 2 million fatigue cycle test with 200-MPa stress amplitude and a stress upper limit of 0.65, the low-end anchor was placed in a water environment, and fiber gratings were attached at both ends of the CFRP tendons to measure changes in the axial strain. The test results show that under cyclic loading, microcracks are generated between the CFRP tendons and the anchoring filler, and the stress redistributes inside the anchor; water accelerates the internal stress of the anchor. The redistribution of stress aggravates the bond fatigue failure. The rule is divided into two stages: the first stage has a higher rate, which is mainly concentrated in the first 500 thousand cycles, and the second stage is from 500 thousand to 2 million cycles. The stress redistribution gradually stabilized. The CFRP cable under study safely passed the fatigue test, but the remaining strength was reduced by 17%, thereby indicating that the impact of the water environment on the durability of the CFRP cable anchoring structure cannot be ignored.

To address the issue of information islands in the visible light communication technology (VLC) and improve the communication requirements of the uplink, power line communication (PLC) is used as an effective supplement for the traditional VLC and enables connection to the uplink backbone network while supplying power. To assure the efficient transmission of signals in the two-level channel of the PLC and VLC technology cascade system, central node demodulation and modulation (CNDM) model based on orthogonal frequency division multiplexing (OFDM) is proposed in this paper. According to the multipath fading of PLC and the reflection properties of VLC, the channel model of the cascaded system is developed. Combined with the signal characteristics of OFDM and the cascaded system, the transmission strategy of a direct current biased optical OFDM system appropriate for both PLC and VLC is devised. To suppress the cascaded channel interference effectively, a multisegment joint control signal frame structure is designed. The experimental results show that under the simulation conditions, the cascade system may satisfy the communication requirements using various modulation schemes with acceptable signal quality, and the low bit error rate can reach 10-4. Under the real-world measurement conditions, the cascade system can successfully transmit and receive signals reliably, with an overall minimal bit error rate of 0.

The optical fiber early warning system has been widely used in the intrusion detection and early warning of oil and gas pipelines. The current technical difficulty is still how to improve the accuracy of multi-class recognition of optical fiber intrusion signals. In this paper, gradient boosting decision tree (GBDT) algorithm is used to train the multi-classification model of fiber intrusion signal, and a feature extraction and recognition algorithm based on Fourier decomposition method (FDM) and GBDT algorithm is proposed. The algorithm uses FDM to preprocess the fiber intrusion signal, extracts the approximate entropy, energy and spectral entropy characteristics of the signal, and then uses the GBDT algorithm to train the model to identify and classify the fiber intrusion signal. In order to test the performance of the algorithm, use support vector machine and AdaBoost algorithms to train the models and conduct comparative experiments. The results show that the algorithm can effectively identify four types of optical fiber intrusion signals, namely, knocking, trotting, passing and picking, with an average accuracy of 92.5%.

Developing improved communication models for expanding wireless communication capacity in spatial dimensions is an important research goal. However, there are two main theoretical concerns with the construction of orthogonal channels in spatial dimensions: one is ill-conditioned problem, and the other is sparseness. Addressing these two issues, we propose a new communication model for spatial-dimension expansion. Considering the acousto-optic effect, a laser is used to expand the spatial dimension; a sparse array layout is designed by an optimization algorithm; and the optimal distribution is determined by a clustering algorithm to complete the model layout design. Through the simulation calculation model example, it can be obtained that its condition number is 25. After cluster analysis, only 4 laser receivers can replace the original 88 unit antennas to form a receiving array. This model plays an important role in improving wireless communication capacity, provides a new concept for building underwater lightweight long-distance and high-capacity integrated communication systems, and contributes basic research for the theoretical development of future wireless communication models.

An iterative exponential filtering(EF) inverse projection reconstruction algorithm is proposed for data reconstruction in the field of optical tomography. The proposed algorithm combines the advantages of the high reconstruction quality of iterative reconstruction algorithm and high reconstruction speed of the EF inverse projection reconstruction algorithm. The filter function adopts the exponential function, and therefore, its antinoise performance is better than those of traditional filter functions. The algorithm adopts the normalized mean square distance d and normalized mean absolute distance r between the reconstructed and real images as the optimization goals. In addition, it adjusts the filter function exponential factor to reduce the impact of projection data noise and establishes an iterative calculation model. The simulation experiments show that the EF function has better reconstruction accuracy than traditional functions. The image reconstruction quality of the proposed algorithm is higher than that of the EF inverse projection reconstruction algorithm, r decreases by 20%. A refractive index optical tomography is conducted. The refractive index reconstruction of the measured data is performed using the proposed algorithm and EF inverse projection reconstruction algorithm. The reconstruction results are then compared with the calibration results obtained using the instrument. The results show that the proposed algorithm exhibits high reconstruction accuracy, and that the maximum error with the calibration results obtained using the instrument is 7.9×10-6. Compared with the EF inverse projection reconstruction algorithm, the reconstruction accuracy of the proposed method is improved by approximately 21%.

In this study, we theoretically studied and simulated the spatial resolution characteristics of a pulse dilation framing camera using Monte Carlo, finite difference, and finite element methods. The electronic pulses produced by the photo-cathode (PC) are first axially stretched through the pulse dilation device and then imaged on a microchannel plate using an imaging system consisting of three short magnetic lenses. When the imaging ratio is 1∶1 and the position of the electronic pulse emission is within 15 mm of the diameter of the PC surface, the spatial resolution is better than 10 lp/mm. Furthermore, we studied the relationship between spatial resolution and electronic pulse emission position, PC bias voltage, and the number of short magnetic lenses. Our results show that the spatial resolution is positively correlated with the emission position of electron pulse, PC bias voltage, and the number of short magnetic lenses.

The determination of physiological parameters of plant leaves is of great importance to the study of plant growth and development. In order to realize the low-cost and high-efficiency monitoring of physiological parameters such as plant photosynthesis, a monitor with automatic opening and closing and handheld leaf chamber is designed in this paper, which can collect and manage experimental data by connecting nodes and computing nodes. The data collection such as CO2 concentration, air temperature and humidity, atmospheric pressure and real-time calculation of photosynthetic physiological parameters are completed through the access node. The data storage and networked transmission of the human-computer interaction interface and remote monitoring system are realized through computing nodes. The results show that there is no significant difference between this instrument and similar monitoring systems abroad, and it can meet the requirements of automatic monitoring of plant photosynthesis. In addition, the monitor has fast response to automatic monitoring of plant physiological parameters, stable and reliable measurement results, and low cost. It plays an important assistant role in monitoring crop growth and development, guiding crop fertilization, and preventing plant diseases and insect pests.

The vibration signal of the gearbox contains all the characteristic information during gear operation. The current running state of the equipment can be mastered by collecting and analyzing vibration signals. There are many methods for collecting gearbox vibration signals. The noncontact measurement method, based on the principle of laser self-mixing interference, has extremely unique advantages. First, this method can collect the early characteristic signals of gearbox fault and take corresponding measures in time. Second, the method can obtain the required characteristic information without damaging the gearbox’s surface. Third, compared with the installation of an acceleration sensor, the influence of this method on the reflector attached to the gearbox is very small and can be ignored. The planetary gear wear fault is artificially made, and the fault characteristic signal is collected during the experiment. Furthermore, the fault vibration signals of planetary gears are extracted and the intrinsic mode function (IMF) components of each order and Hilbert spectrum are analyzed using the Hilbert-Huang transform (HHT) signal processing method. The Hilbert spectrum can be analyzed during the period of gear wear fault. The results are in good agreement with the theoretical values, showing that this method can effectively detect gearbox vibration faults.

On one hand, high dynamic range (HDR) laser display can show high contrast ratio scenery that is close to the dynamic range of the human eye. Moreover, it can also improve the color gamut of the display. Consequently, it is an important development direction for next generation display technologies. This study proposes a novel architecture based on local dimming technology to achieve a high-brightness, high-efficiency, and low-cost HDR laser display. Using an optical fiber array, the laser source modules output an array light field to the spatial light modulator (SLM). When displaying an image, each laser will dynamically control its brightness based on the grayscale value of its corresponding image area, and the SLM will modulate the pixels based on the dynamic light field and the source image. Furthermore, this study presents a local dimming algorithm that can eliminate the chromatic aberration caused by the laser wavelength difference. The main principle is that the modulating value of each pixel on the SLM is calculated based on the color coordinates of a mixed three-primary illuminating light. This algorithm enables the display of HDR images without chromatic aberration.

As an emerging coherent detection technology, terahertz time domain spectroscopy (THz-TDS) technology has promoted the progress and innovation of safety testing and nondestructive testing technologies, and has made outstanding contributions to the protection of people's life and health and property safety. In order to obtain better imaging results and detection sensitivity, it is necessary to increase the detection speed of terahertz signals while ensuring that the signal-to-noise ratio of the acquired signal meets the requirements. Based on the involute reflector optical delay line device, this paper analyzes the optical delay line shape parameters, and simulates a model with a delay distance of 71 mm and a delay time of 236.7 ps. The error analysis is made in three aspects: the eccentric error of the axis of the rotating mirror, the installation error of the planar mirror, and the outgoing spot distortion of the rotating optical delay line, and the method of reducing the error is discussed. It provides a theoretical and simulation basis for the device to increase the time delay, obtain a more complete terahertz spectrum, and further improve the performance of the THz-TDS system.

A transposition pose calibration method integrating scale factor is proposed for the problem of scale difference in the combined optical scanning system. The mathematical model of the combined optical scanning system is established. Then, the changing trend of global measurement accuracy relative to scale difference is explored based on the numerical simulation. The calibration model of transposition pose considering the scale factor is established. With the Kronecker product expansion, the theoretical solution of the transposition pose is given. The system parameters and scale factors are optimized. The effectiveness and accuracy of the calibration method are verified by simulation and experiment. The results show that the combined positioning accuracy is better than 0.1 mm; the transposition pose considering the scale factor significantly improves the overall scanning accuracy of the combined optical scanning system.

In this paper, a modulation transfer spectroscopy (MTS) of the hyperfine components at R67(15-1) transition of the iodine molecule is obtained with a 583 nm semiconductor frequency-doubling laser. For frequency stabilization of cooling laser in ultracold erbium experiment, the Pound-Drever-Hall (PDH) technique is used as the prefeedback to stabilize the laser to a Fabry-Perot cavity, and the MTS of iodine molecular is used as the secondary feedback to overcome the inevitable long-term drift of the optical reference cavity. The long-term drift data during 4 h show that, compared with the drift in frequency stabilization using only PDH technique (205 kHz), the maximum frequency fluctuation is within ±12 kHz for the two-stage laser stabilization. This meets the long-term stable operation requirements of the ultracold erbium atomic experimental system. The scheme expands the application of iodine molecular spectroscopy in 583 nm laser frequency stabilization, and provides a strategy for laser stabilization of cooling light in cold atom experiment with elements such as europium and thulium.

Oscillating laser welding provides an effective technical method for welding aluminum alloy sheets due to its outstanding role in suppressing pore defects. The process parameters for oscillating laser welding have a direct impact on welding quality and carbon emissions. This paper quantifies the carbon emission of lap oscillating laser welding of aluminum alloy sheets to achieve low carbonization of aluminum alloy sheet lap welding. To characterize the welding quality and carbon emission according to the process parameters, it constructs a surrogate model based on a multi-output Gaussian process. The optimal welding objectives are found using the non-dominated sorting genetic algorithm (NSGA-Ⅱ), and the corresponding optimal process parameters. The process test results show that the weld forming quality corresponding to the best process parameters is effectively improved, and the welding defects such as porosity, molten pool collapse, and concave are restrained. The carbon emission in the welding process is effectively reduced by 12.99%, while the maximum bearing capacity of the joint is only reduced by 2.47%, which can significantly reduce carbon emissions in the welding process while maintaining welding quality.

Aiming at the difficulty of forming high strength aluminum alloys by selective laser melting technology, this paper conducts finite element analysis based on ANSYS software to study the selective laser melting of aluminum alloys, and reveals the temperature field distribution and basic characteristics during the forming process. And the temperature gradient of the scan track is studied through the temperature distribution cloud map. At the same time, the influence of laser power and scanning speed on the thermal behavior of the molten pool was studied, and the influence of the scanning process parameters on the shape of the molten pool was studied by means of simulation. The results show that in the simulation forming process, the size of the molten pool has a nonlinear relationship with the scanning speed, and is positively related to the linear energy density of the laser input. As the energy density increases, the size increases. The simulation results are verified by experiments, and this study can provide a theoretical reference for subsequent forming analysis.

In this paper, to improve the problem of low hole injection efficiency in the active region of a deep-ultraviolet laser diode (DUV-LD), a hole reservoir layer (HRL) is introduced based on the conventional DUV-LD and the HRL is changed to a five-step HRL with a decreasing Al mole fraction from n-region to the p-region. The HRL is numerically examined using Crosslight's Lastip software, which revealed that the application of the structure can achieve a higher carrier radiation recombination rate and a lower threshold current. When a five-step HRL with decreasing Al mole fraction is located between the last quantum barrier layer of DUV-LD and the upper waveguide layer, the electron leakage is minimal and the device performance is significantly improved.

In order to obtain the optimal process parameters of 304L/316 dissimilar steel sheet laser welding, the joint morphology, geometric deformation, and tensile mechanical properties were studied by using the grey correlation analysis method and Taguchi L9 orthogonal test scheme with laser power, scanning speed, and defocusing amount as the main influencing factors. The results show that the angular deformation of laser welded sheet decreases with the increase of defocusing amount. The bending deformation increases with the increase of laser power. The tensile strength increases first and then decreases with the increase of defocusing amount. When the laser power is 1500 W, the scanning speed is 15 mm/s, and the defocusing amount is + 2 mm, the tensile strength of the 304L/316 dissimilar steel welded sheet is the strongest and the deformation is the smallest, and the joint strength is higher than that of the base metal. After melting, the filler powder penetrates along the butt gap to the bottom of the sheet, forming a metal filler joint containing fine columnar crystals and a small amount of equiaxed crystals. Finally, the filler powder and the base metal form an effective metallurgical combination.

To study the microstructure characteristics and mechanical properties of the CoCrMoW alloy fabricated using laser melting deposition, we studied the microstructure evolution, phase composition, and mechanical property of CoCrMoW multi-track coatings at different laser powers. Multi-track and multi-layer samples were prepared by selecting better process parameters. Furthermore, we investigated the microstructure of multi-layer samples and processed the tensile samples to study the mechanical properties of CoCrMoW alloy. The results show that the phase composition of the CoCrMoW alloy coating consists of particles with different orientations, γ-Co and ε-Co compositions, and grain size coarsened gradually with the increase in laser power, decreasing the microhardness from 346.14 HV0.2 (400 W) to 309.62 HV0.2 (1000 W). The microstructure of the CoCrMoW alloy multi-layer sample is mainly composed of equiaxed crystal and columnar crystal structures, and the microhardness at the top of the block is slightly higher than that at the bottom. Both the average tensile and yield strengths of the deposited CoCrMoW alloy are 993.4 MPa and 492.987 MPa, respectively, showing brittle fracture. The fracture morphology is composed of a smooth section and cleavage surface, with an obvious river pattern and cleavage fracture mechanism.

Based on gold nanoparticle and gold nanorod film substrates, surface-enhanced Raman spectroscopy (SERS) technology is used to detect the content of ciprofloxacin (CIP), providing a new mode of detecting ciprofloxacin residues in food. The gold nanoparticle colloid is prepared by using sodium citrate to reduce chloroauric acid, whereas the gold nanorod colloid is prepared by the seed growth method, and then applied to the SERS reinforced substrate. SERS detection of CIP is conducted using different excitation light wavelengths, and the optimal laser wavelength is determined to be 780 nm. The calibration-set CIP standard solution is used to establish a working curve of CIP concentration-SERS signal intensity, and the validation-set standard samples are used to observe the predictive ability. The results show that the recovery rates of SERS detection for CIP using gold nanoparticle and gold nanorod substrates are approximately 97.1%-105.0% and 96.3%-121.8%, respectively. This confirm that SERS has advantages of high sensitivity and rapid detection potential in the field of detecting CIP antibiotics.

In this paper, in order to solve the problem that the existing star simulators cannot simulate the sky background change, a star simulator with variable background is proposed to test the performance of the star sensor. Off-axis reflection collimating optical system is designed, by introducing a semi-inverse semi-lens in the main optical path to produce two conjugate focal planes, the star-point reticule and the stray diaphragm are illuminated by two integrating spheres in the stellar source simulator and the background source simulator, respectively. Real-time monitoring of the irradiance at the outlet of the integrating sphere, and the output power of the light source can be controlled according to the needs of star and background brightness. After the two beams merge, they become parallel light through the collimating optical system and are finally received by the star sensor, thus realizing the simulation of stars with variable starry sky background and apparent magnitude. The test results show that the apparent magnitude of the system can be adjusted continuously from -2.00-6.00, the apparent magnitude simulation accuracy is 0.30, and the uniformity of the background light source is 97.3%. The system can well simulate the background changes of stars when the star sensor is running, and can effectively improve the performance test accuracy of the star sensor.

In this paper, a linear variable optical filter (LVOF) in the visible spectral region based on TiO2 and SiO2 is prepared by combining a multi-target magnetron sputtering coating method with a self-made one-dimensional displacement baffle sample stage. The transmittance spectra are characterized by a spectrophotometer and a hand-operated one-dimensional translation stage. The effects of annealing temperature and time on the LVOF transmission spectrum are primarily evaluated in terms of functional dependency. The results show that the workable spectral range of the LVOF sample is 510-700 nm, and the transmission spectral peaks at different positions on the sample have a consistent linear relationship with the sample position. After vacuum-annealing at 300 ℃ for 1 h, the transmittance of the samples is significantly improved, better than 60%. This indicates that the annealing treatment can significantly improve the performance of the LVOF.

Most machine vision-based roughness measurement methods either build a prediction model based on roughness correlation indices or build an index-free prediction model using deep learning networks. However, both these models have disadvantages. The artificial designed index has a complicated calculation process, which is not conducive to inline detection. In comparison, deep learning models rely heavily on big data. It is difficult to train an effective model when the amount of data is insufficient. To address the above problems, this study proposes a graph neural network-based method for measuring the roughness of milling surfaces. This proposed approach acquired the ability to learn autonomously during the training phase. Thus, only a few milling samples were required to measure the roughness of the milling workpieces. The experimental results show that the proposed method can automatically extract features on roughness measurement of milling workpieces with high accuracy and good robustness of lighting environment.

The laser-induced damage threshold of optical elements is a key indicator for measuring laser damage resistance. Optical elements with periodic surfaces have advantageous optical characteristics and potential applications in high-power laser systems. It is important to determine the laser-induced damage threshold accurately. In this paper, the main sources of uncertainty are analyzed, a calculation formula for the uncertainty of laser-induced damage threshold is established, and the processing methods for reducing the uncertainty of laser-induced damage threshold are provided. The results show that when the spot radius is 400 μm, the error is 10 μm, the laser energy error is 5%, and the uncertainty introduced by energy density is zero. Then, the main factors contributing to the uncertainty of the laser damage threshold are the uncertainty of the damage probability and that of the linear fitting. The precision of the laser-induced damage threshold can be further improved by increasing the number of measurements for each energy level.

Three-dimensional (3D) emission computerized tomography (ECT) is superior to the "slicing" process of the traditional two-dimensional ECT technology and reconstructs the test zone as a whole. As the projections are not limited to the same horizontal plane, 3D ECT can solve the problems of limited detection positions and assembly errors encountered during actual combustion tests, which is necessary for 3D combustion imaging and testing. In this study, a 3D weight matrix calculation algorithm was developed using a mathematical model of camera imaging in a 3D space and the linear interpolation theory to reduce the data amount and improve the calculation accuracy and efficiency of the weight matrix for 3D ECT. The accuracy of the algorithm was verified via numerical simulations using an algebraic reconstruction algorithm for tomography reconstruction. An ECT system with multiple cameras was established, and the proposed algorithm was used to reconstruct the combustion flame. The results have a significant reference value for improving the accuracy and efficiency of tomographic reconstruction.

Aiming at the problems of high decoding complexity and unequal error protection of Spinal codes in free space optical communication, a SCB-Spinal code scheme is proposed, in which cyclic redundancy code (CRC), Spinal codes, and BCH codes are concatenated. Premature termination of the segmented CRC check reduces the amounts of decoding calculations and the complexity of decoding, and also cascades the BCH code at the tail to perform error correction protection for error-prone tail information. The simulation results show that the SCB-Spinal code effectively reduces decoding complexity and obtains better bit error rate performance under different turbulence intensities. Compared to the traditional Spinal code, the SCB-Spinal code is approximately 62% lower with respect to signal-to-noise ratio and turbulence. In addition, it exhibited a performance rate of 0.04-0.17 bit/symbol. With respect to moderate and strong turbulence intensities, the complexity under SCB-Spinal code is reduced by 50%-60%, facilitating efficient application of Spinal code in free space optical communication.

Laser direct deposition technology can be used for producing large metal parts due to its unlimited capacity for manufacturing size and high material freedom. However, conventional laser direct deposition technology makes it challenging to consider both printing efficiency and printing quality, thereby restricting the popularization and application of this technology. Therefore, high-speed laser direct deposition technology has gained research popularity. In this paper, the effects of scanning speed and laser power on the process characteristics of Inconel 718 laser direct deposition are discussed, and the changes in surface morphology, microstructure characteristics, internal defects, and mechanical properties of conventional and high-speed laser direct deposition are analyzed. Finally, the challenges of applying high-speed laser direct deposition technology in three-dimensional additive manufacturing are discussed, which offers guidance for the development of high-speed laser direct deposition technology in the future.

The phase-sensitive optical time domain reflectometer (φ-OTDR) is characterized by distributed sensing, fast response, simple structure, long detection distance, and anti-electromagnetic interference. However, due to the use of a light source with a long coherence length, the backward Rayleigh scattering of the pulsed light interferes inside the optical pulse, thereby affecting the φ-OTDR by factors, such as coherent fading, polarization fading, and common mode noise, which sharply reduce the signal-to-noise ratio. Therefore, this article discusses the working principle of φ-OTDR, summarizes and analyzes recent methods for reducing various types of noise in φ-OTDR system, and proposes future development directions for noise reduction in φ-OTDR systems.

With the rapid development of mobile communication and multimedia services, the bandwidth demand for the next generation access network demonstrates an explosive growth trend. The passive optical network (PON) is a key technology of optical access that provides huge bandwidth resources and long-distance links, but its flexibility is severely limited by optical fiber laying. However, the radio over fiber (RoF) technology can combine wireless access and optical fiber communication to benefit from low loss and high bandwidth, and thus, promote the integration and development of the two. Therefore, making full use of the huge bandwidth provided by optical fiber and the flexible access of wireless communication, the integration of RoF technology and PON can better generate, process, and transmit wired signals and microwave signals to meet people's needs for multi-services. This paper reviews the subsystems and key technologies of converged RoF-PON in recent years. The photon generation technology of microwave signal, RoF system, and next generation-PON (NG-PON2) are also summarized. Furthermore, the paper emphasizes the converged schemes of RoF technology and wavelength division multiplexing-PON (WDM-PON), time/wavelength division multiplexing-PON (TWDM-PON), and orthogonal frequency division-PON (OFDM-PON). The proposed four-channel RoF-WDM-PON and RoF-TWDM-PON system transmission capacity after 20 km of optical fiber transmission is 10 Gbit/s, with a bit error rate of 10-9. The RoF-WDM-OFDM-PON can realize a downstream system capacity of 50 Gbit/s, 50 km of optical fiber transmission, and passive optical network unit (ONU). Finally, the feasibility, advantages, and disadvantages of these systems are compared and analyzed in terms of transmission distance and bit error rate. In a nutshell, the converged RoF-PON system based on microwave photonics can not only reduce the system cost and provide high bandwidth and multi-services but also effectively improve the capacity and link quality of future access networks.

Electromagnetic waves have linear momentum, orbital angular momentum, and spin angular momentum. The orbital angular momentum carried via vortex beams has considerable application potential in contemporary optical technology and has important application value in the fields of classical communication, quantum communication, optical manipulation, and rotation detection. Recently, domestic and foreign researchers proposed various techniques for detecting beam orbital angular momentum, including diffraction grating, interferometry, Doppler analysis, and metasurface methods. Compared with other methods, the metasurface method has the advantages of high efficiency and strong light manipulation. This paper primarily introduces several methods for measuring beam orbital angular momentum using metasurfaces, as well as their research progress and development status.

Terahertz (THz) radiation is an electromagnetic radiation with a frequency that lies between the microwave and infrared regions of a spectrum. It has various unique properties, including transmission, transience, broadband, coherence, and low energy and can reveal the weak interactions between molecules. The THz application became possible due to the development of THz sources and detectors. Accordingly, with the development of machine learning, THz applications are being expanded to test many commonly used nonpolar dielectric materials and mixtures with a similar fingerprint. In this work, we introduce the theory of the optical system and the analytical method of a spectrum. We discuss the advanced applications of the THz technology in agriculture, including agricultural bio-molecular material detection, crop physiology inspection, soil detection, pesticide and antibiotic residue detection, and agricultural product and seed quality detection. In summary, we outlook the prospect of the development of terahertz time domain spectroscopy to promote its application in agriculture.

To avoid chromium and nickel series stainless steel being regarded as ordinary steel, it is necessary to perform classification when recovering stainless steel. In this study, a stainless steel fine classification model based on laser-induced breakdown spectroscopy (LIBS) and the random forest algorithm is proposed. Four grades of stainless steel 201, 304, 316, and 430 were selected as experimental samples, and 12 analytical spectral lines of Cr, Ni, Mn, Mo, and Fe were used as input characteristic quantities. The results show that the average recognition accuracy of 100 classification experiments on 300 groups of data is 98.28%, the modeling time is 0.418 s, the standard deviation is 0.20%, and the average classification time of a single group of data is only 0.019 s, indicating that the proposed classification model has good stability and efficiency. Owing to LIBS technology, the proposed model can be used in the field of online rapid classification of stainless steel.

Raman spectra of the typical chemical vapor deposition (CVD) synthetic diamonds were compared when excited with 405, 532, and 785 nm wavelengths, respectively, at room temperature. The results show that: 1) at the same excitation wavelength, the Raman peaks corresponding to the intrinsic structure or optical defects become more obvious with the increase of laser power, and the corresponding peak positions do not shift with the increase of laser energy. Moreover, there are some spectral differences even though these diamonds all belong to the type II, including the occurrence characteristics of impurity elements and the methods of treatment, which result to some differences for the Raman spectra during these diamonds are excited by the same wavelength; 2) the characteristic of sharp peak locates about 1332 cm-1 all existed in the Raman spectra of each of diamonds when excited under different wavelengths. Meanwhile, the intrinsic peaks of diamond do not shift with the change of excitation wavelength. On the contrary, the appearance of the optical defects caused by impurities or post-treatment is closely related to the selection of excitation wavelength. This work provides important guidance for the characterization of the structural information of diamond by Raman spectroscopy and the identification of intrinsic peak and optical defect characteristic peak of diamond.

To address the current problem that the printability of water-based inks is difficult to detect and control accurately, a method based on visible/near-infrared spectroscopy combined with chemometrics is proposed to accurately detect the type and concentration of water-based ink additives. First, a micro-spectrometer (380-980 nm) was used to obtain spectral data of water-based ink samples containing different concentrations of alcohol, toning red, and toning yellow additives, and by extracting the characteristic bands that can completely characterize the spectral information of the samples through principal component analysis, redundant information is reduced and accurate identification of the auxiliary types is achieved. Then, the prediction models constructed by six different pretreatment methods combined with partial least squares (PLS) and interval partial least squares (iPLS) for each additive concentration were compared and discussed. The experimental results showed that the cumulative contribution rate of the first two principal components in the spectral band of 617-726 nm was as high as 99.909% by principal component analysis, and the accurate identification of water-based ink additives types was achieved. Among them, the determination coefficient and root mean square error of alcohol additive prediction model were 0.9798 and 0.0223, while the determination coefficient and root mean square error of toning yellow additive were 0.9870 and 0.0075, and the determination coefficient and root mean square error of the toning red additive were 0.9948 and 0.0038. Experimental results prove that the model established by the optimal pretreatment method combined with the iPLS method is generally better than the single PLS model, which can accurately detect the type and concentration of water-based ink additives, meet the application needs of water-based ink detection and control in the printing production process, and provide a technical basis for the later realization of online detection of water-based ink.

When a gated framing camera is operating, the gating pulse transmission attenuation effect will greatly weaken the gain of microchannel plate (MCP) microstrip line and affect the camera performance. The attenuation model of gate voltage pulse transmitting on MCP microstrip line was established, and the gain uniformity of MCP was simulated. As the results, amplitude of voltage pulse decays exponentially in MCP microstrip. Under the attenuation coefficient of 0.0041 Np/mm, the voltage amplitude attenuates to 85% of the original value and the MCP gain attenuates to 29% of the original value at the position of 40 mm. Effects on MCP gain uniformity caused by three methods (voltage compensation, microstrip end reflection compensation, and width gradient microstrip line compensation) were simulated. The three compensation methods were combined to obtain a microstrip line compensation model. In the model, for the minimum width of microstrip is 4 mm, the voltage transmission amplitude is maintained above 94%, and the MCP gain is maintained at 66%.