Non-line-of-sight imaging can reconstruct images of scenes outside the line-of-sight. Different from traditional imaging, it imports the indirect signal returned from the hidden scene into the reconstruction algorithm to realize the reconstruction of the target scene, which is important in the fields of national defense, biomedicine, automatic driving, aerospace, and post-disaster search and rescue. This paper summarizes the research progresses of non-line-of-sight imaging technology, and introduces three non-line-of-sight imaging modes, including non-line-of-sight imaging based on time-of-flight, and non-line-of-sight imaging based on coherent information (including speckle pattern and spatial coherence methods), and non-line-of-sight imaging based on intensity information. Focusing on the characteristics and limitations of hardware parameters, reconstruction algorithms, reconstruction time, and image resolution of coherent information and intensity information imaging modes, the development trend of non-line-of-sight imaging is analyzed and discussed.

ObjectiveIn a previous work, the transmission spectrum of long-period fiber gratings (LPFGs) was simulated and analyzed based on traditional coupled mode theory. However, for a strongly modulated few-mode LPFG, traditional coupled mode theory cannot be used to accurately analyze the coupling process between the core guide modes in the strong modulation region. This is because of its strong angular refractive index modulation and the obvious correction of the mode field distribution. In addition, the problem of change in the angular refractive index in the fiber core was not considered in the previous strongly modulated model, which is inconsistent with the change in actual gratings. This can easily result in the simulation spectrum not matching the actual spectrum. Therefore, based on local coupled mode theory, this study analyzed the coupling process between the fiber core fundamental mode and the high-order angular mode in an LPFG grating. This study also established a model for a strongly modulated few-mode LPFG, introduced asymmetric and angular modulation parameters into the model, realized a transmission spectrum simulation of the grating, and matched the existing experimental results. A method was thus realized for analyzing the coupling process between the guide modes of the fiber core in a strongly modulated few-mode LPFG, which provides a reference for preparing gratings with different experimental requirements.MethodsBased on local coupling mode theory, a model of a strongly modulated LPFG written by a CO2 laser was constructed to analyze the coupling process between the core guided modes in the modulated region of a few-mode grating. With asymmetric and angular refractive index modulations introduced into the model, the strong angular refractive index variation in grating modulation was characterized, and a more accurate mode coupling coefficient and propagation constant were derived. These were substituted into the coupling equation between the fundamental and third-order azimuthal modes to solve, and a simulation of the grating transmission spectrum was realized. The simulation and experimental results were then matched. In addition, based on changes to the grating model parameters (grating period, the number of periods, and modulation depth), the change in the corresponding transmission spectrum was studied, thereby providing a reference for designing and fabricating higher-order-mode strongly modulated LPFGs.Results and DiscussionsWe first established a model for a strongly modulated few-mode LPFG by adding asymmetric and angular refractive index modulations. Figure 4 shows a comparison of the unmodulated and modulated mode fields, which reflects the necessity of angular modulation and enhancement of coupling between modes. Figure 5 shows a comparison between the simulation and experimental data. The simulation parameters of the grating were consistent with the experimental parameters. It can be seen that the shape, depth, and resonant wavelength of the experimental resonant peak are basically consistent with those of the simulation. Based on changes to the parameters of the established model (grating period, the number of periods, and modulation depth), the change rule of the grating transmission spectrum characteristics was analyzed, as shown in Figs. 6, 8, and 9. Thus, the simulation model can be applied to different optical fibers to guide the design and preparation of gratings with different requirements.ConclusionsIn this study, a simulation model of an LPFG is established, with emphasis given to analysis of the LPFG. Based on local coupled mode theory, the refractive index and coupling coefficient changes in the modulation region of the grating are calculated, and asymmetric and angular refractive index modulations are introduced. The spectral characteristics of a strongly modulated few-mode LPFG are obtained by numerically solving the local coupled-mode equations. The spectra of strongly modulated LPFG fabricated by using CO2 laser in the experiment and the simulation results are in agreement. These results and the effects of grating period, the number of periods, and modulation depth on the transmission spectrum of the grating were analyzed. This simulation method is more suitable for analyzing LPFGs prepared by high-power local laser (such as CO2 laser) irradiation methods and more accurately characterizes the coupling process between the fundamental and high-order angular modes in the fiber core area of a few-mode grating. This method has a reference value for realizing higher-order angular mode conversion and preparing LPFGs with different requirements. Compared with other methods, this method can more accurately analyze a series of LPFGs based on local high-intensity laser irradiation and heat treatment.

ObjectiveWhen laser propagates in gas medium, the gas absorbs the laser energy and causes the refractive index to change, forming the gas thermal effect and reducing the beam quality. The laser has a high power density in the inner channel of the system, so the attenuation of Gaussian beam transmitted in the inner channel is far greater than that transmitted in the outer atmosphere. In addition, the thermal blooming effect of high-energy laser is not only closely related to the beam shape, but also very complex due to the interaction between the air flow and the absorption of laser energy. We hope to establish a more comprehensive thermal coupling effect model of laser transmission, which will provide strong support for the design and performance evaluation of the high-energy laser system. In this paper, the theory of optical-fluid-thermal coupling effect is introduced, and the simulation model of the thermal coupling effect of combined transmission of the elliptical Gaussian laser beams is established.MethodsTo solve the thermal effect of laser transmission in gas medium, flow field calculation, optical transmission calculation and optical-thermal coupling calculation are required. The optical transmission satisfies Maxwell equations. When simulating the electromagnetic field propagation of optical structures with long distance in a large size space, ray tracing is performed by solving the ray position and wave vector. Fluid flow and heat transfer follow the three conservation laws of mass, momentum and energy. The energy attenuation of laser transmission can be calculated according to Beer’s law, while the refractive index change of gas medium due to temperature and density changes satisfies Gladstone-Dale relationship. The wavefront aberration of the laser propagating in the enclosed space of the system is obtained by solving the above theoretical model with the finite element method, and the beam quality is calculated by comparing the results with those of the ideal Gaussian beam. The established model of laser thermal coupling effect is in good agreement with the experimental law, and can predict the degradation of beam quality caused by the thermal effect of gas medium.Results and DiscussionsThrough numerical simulation, the natural convection phenomenon induced by laser heating in a cuboid enclosed space and the change process of the influence of the flow field on the beam propagation are analyzed, and the influential factors of the thermal effect are studied. After the light comes out, the surface temperature of the optical elements will continue to rise, while the air temperature will not change within a few seconds (Figs. 4 and 5). The optical path difference (OPD) distribution is elliptical Gaussian type at first, with obvious defocusing aberration, and its peak-to-valley (PV) value and beam quality show a trend of first increasing, then decreasing, and then stabilizing (Figs. 7 and 9). With the increase of the absorption coefficient, the OPD PV value caused by the thermal effect of gas increases (Fig. 12), but the absorption of the element surface has a low contribution to the thermal effect of gas medium (Fig. 13). The gas thermal effect can be suppressed and the beam quality can be improved by certain gas replacement methods (Fig. 15). The flow field distribution of each region in the spectral beam combining system is different, and the path of each sub-beam is different, so the aberration is also different. The number and power of sub-beams, the shape and distribution, the combining method, the transmission distance and the system layout will all affect the thermal aberration difference of the medium in the process of combined beam transmission.ConclusionsIn this paper, the theory of optical-fluid-thermal coupling effect is introduced, and the thermal coupling effect model of laser beam combining transmission in an enclosed space is established. The absorption of laser energy by the gas medium will affect the laser transmission, resulting in optical axis deflection, beam quality degradation, and changes in the shape of far-field spot. In this study, the influences of medium absorption, optical element absorption and gas replacement on the thermal effect of internal transport gas are analyzed. In the spectral beam combining system, all sub-beams will interfere with each other, the wavefront distribution will be different, and every sub-beam will have a dispersion trend. The model can analyze the thermal effects of different systems according to the actual situation, and the influence of structural parts and electronic devices on the flow field and laser transmission can also be considered, providing an effective reference for the design and performance evaluation of high-energy laser systems.

ObjectiveBased on cavity-enhanced absorption spectroscopy (CEAS) and wavelength modulated spectroscopy (WMS) technology, a cavity-enhanced spectrum measurement system was built to measure the volume fraction of CO gas. In an experiment, a distributed feedback laser (DFB) laser with a central wavelength of 2.3 μm was used as the light source, and an optical cavity with a base length of 30 cm was constructed with two highly reflective mirrors with a reflectivity of 99.8%. An effective absorption path of 147.15 m was achieved. On this basis, CO was detected using the CO absorption spectral line at 4297.705 cm-1 as the sensing target. In the experiment, the measurement accuracy of the system was verified using CO and N2 gas mixtures with different volume fractions of CO. The measured value was consistent with the reference value, and the measurement error was approximately 0.2%, which confirmed the measurement accuracy of the system. The detection limit of the system was analyzed using the second harmonic signal of CO gas with a volume fraction of 3×10-6, and the lowest detectable CO volume fraction with the system was 138×10-9.MethodsThe optical cavity in the sealed box was composed of two highly reflective mirrors. The length of the cavity was 30 cm, diameter of the highly reflective mirrors specified by the manufacturer was 25.4 mm, and radius of curvature was 1 m. Their reflectivity reached 99.8% in a wavelength range of 1.9-2.3 μm. In the experiment, the laser output from the 2.3 μm DFB laser was divided into two parts using a fiber beam splitter. One part was passed through an oscilloscope (Bristol 671) to obtain the real-time scanning output wavelength of the laser, and the other part passed through the sealed box made of tempered glass. The laser beam entered the optical cavity through the sealed box. Then, after multiple reflections in the cavity, it was transmitted from the other end and converged in the InGaAs detector through a lens with focal length of 5 mm for detection. After the detector signal was sent to the input port of a lock-in amplifier, it demodulated the signal according to the specific setting parameters. The demodulated signal was collected by a data acquisition card (National Instruments USB-6361) and stored in the computer for subsequent data processing. A triangle wave with a frequency of 2 Hz generated by a function generator and a sine wave with a frequency of 880 Hz generated by a phase-locked amplifier were superimposed together by an adder, and the superimposed signal was sent to the laser controller to achieve scanning and modulation of the laser wavelength.Results and DiscussionsThe experiment was carried out under the fixed conditions of 101325 Pa and 300 K, and the second harmonic (2f) signals of different volume fractions of CO were measured. The 2f signals of CO with volume fraction of 3×10-6, 10×10-6, 30×10-6, and 50×10-6 are shown in Fig. 5. Figure 6 shows the linear relationship between the peak height of the 2f signal of each volume fraction of CO and the volume fraction. The fitting results show that the linearity, R2, between the data points was 0.998, while the relative standard deviation was 0.34%. It can be seen that the two maintained a good linear relationship within the volume fraction range measured in the experiment.In this study, the measured CO volume fraction was compared with the known reference value during the gas distribution. This comparison is shown in Fig. 7. The linearity, R2, between the data points in the figure was 0.998, which shows that the measured CO volume fraction was consistent with the reference value, and there was a good linear relationship between them. The red line in the figure was obtained by the linear fitting of the scatter plots. The slope of the fitting line was 1.00282 ± 0.02261. It can be seen that the measurement accuracy of the experimental system was maintained at approximately 0.2%, which confirmed that the measurement of the experimental system had high accuracy.The system was used to conduct a 500 s time series measurement of CO gas with a volume fraction of 3×10-6. Each data sampling time was 1 s. Thus, a total of 500 volume fraction points of CO gas were obtained, as shown in Fig. 8 (a). Figure 8 (b) shows the histogram distribution of the 500 data points. The histogram shows a good Gaussian distribution, and the half-height width (HWHM) of the curve could be used to evaluate the measurement accuracy of the system. The results show that the CO volume fraction measurement accuracy of the system was 286×10-9. The Allan variance was used to analyze the detection sensitivity of the system under the environmental conditions of a temperature of 300 K and pressure of 101325 Pa, as shown in Fig. 9. It can be seen from the figure that when the system time was approximately 1850 s, the Allan variance was the lowest, which meant that this time was the best detection time for the system, and the detection limit at this time could reach 138×10-9. Before the optimal detection time of 1850 s, the Allan variance of the system was higher than the minimum detection limit, which was due to the influence of the white noise of the system. The noise after this was mainly caused by the instability of the system. Therefore, when measuring a stable flow field, the noise could be reduced and the measurement accuracy could be improved by averaging the collected data for many measurements within a period of 1850 s.ConclusionsThe CO absorption spectrum line at 4297.705 cm-1 was selected in the experiment. Based on cavity-enhanced absorption spectroscopy (CEAS) technology, combined with wavelength modulated spectroscopy (WMS) technology, a cavity-enhanced spectrum measurement system was built with a self-made sealed box. The reflectivity of the cavity mirror at 2.3 μm was calibrated to be 99.8%, which was consistent with the data given by the manufacturer. At the same time, the effective absorption optical path of the system was calibrated to be 147.15 m, and the optimal modulation amplitude was determined to be 360 mV. During the experiment, a DFB laser with a central wavelength of 2.3 μm was used as the light source. Under the conditions of 101325 Pa and 300 K, the linear relationship between the peak height of the 2f signal of each each volume fraction of CO and the volume fraction was obtained by measuring the 2f signals of the CO with different volume fractions prepared using the CO and N2 mixture gas, and a good linear relationship between the two was determined within the measured volume fraction range. Finally, the CO volume fraction measured by the system was compared with the known value during the gas distribution, and the measurement error was approximately 0.2%, which verified the reliability and accuracy of the system.

ObjectiveThe ripple index is one of the most crucial parameters of superluminescent diodes (SLDs). A low ripple index is necessary for the application of SLDs in sensing areas such as fiber-optic gyroscopes. Therefore, reduced reflectivity of the facet is required, and the main strategy involves coating with antireflective (AR) films. The reflectivity of the AR films is generally less than 0.1%, which is a strict requirement. Although the AR film designed using the plane wave method (PWM) is widely used, its performance in SLDs is not ideal, and the actual reflectivity deviates from the design value. Therefore, the purpose of this study is to design and fabricate AR films for SLDs that can effectively reduce the ripple index.MethodsThe first part is the simulation method. The finite difference time domain (FDTD) method is used to analyze the optical properties of the films, and a perfect matching layer (PML) is used as the boundary condition. In addition, to reduce the resource requirements of the FDTD method, a simplified simulation model is used, which highlights the main influencing factors. To optimize the films, parameters such as film thickness and refractive index are scanned by FDTD method to determine the parameter range of low reflectivity, and the particle swarm algorithm is used to obtain the optimal parameters within this range. For the coating process, the optical film is manufactured by reactive magnetron sputtering, and an AR ion beam assists the coating process. In addition, the film thickness is monitored and controlled online using a crystal oscillator control system. Reflectivity measurement of thin films is important, including direct measurement of the accompanying films and indirect measurement of the facet of the SLD. The SLD is indirectly measured by spectral ripple, which is also the mainstream reflectivity measurement method for other semiconductor optoelectronic devices.Results and DiscussionsThe simulation results show that the design of the PWM film faces many problems. For the single-layer and double-layer AR films, the reflection curve becomes blue-shifted, and the shift exceeds 150 nm. In addition, the reflectivity is more than one order of magnitude higher than the designed value, and the deviation is further enlarged at a large angle. Therefore, the AR film designed by PWM does not meet the requirements and cannot be used for SLD. To solve these issues, the optical film is optimized. For single-layer AR films, the average reflectivity is less than 0.11% and the lowest reflectivity is 0.04%. The optimized design of the double-layer film provides better results: the average reflectivity is less than 0.05% and the lowest reflectivity is only 0.01%. The optimized design effectively reduces the reflectivity, particularly for the optimized double-layer AR film, which has evident advantages over the film designed by PWM.The double-layer AR films are prepared by reactive magnetron sputtering. After optimization, the reflectivity of the AR film on the companion substrate is 0.12%, and the low-reflectivity bandwidth is greater than 200 nm, which verifies the coating process. The design and measurement curves do not fit because the size and structure of the companion substrate are different from those of the SLD. The reflectivity of the AR film on the SLD is indirectly measured. After optimization, the average reflectivity of the AR film decreases by 50%, indicating that the optimized design effectively reduces the reflectivity. The spectrum of the SLD shows that the intensity and number of ripples are clearly reduced, and the average ripple index is only 0.019, which is 44.5% of that before optimization. The average modulation index decreases from 4.79×10-3 to 2.30×10-3, a decrease of more than 50%. In addition, under a driving current of 100 mA, the output power of the SLD chips remains above 10 mW, maintaining a high output power and high efficiency.ConclusionsIn this study, the AR film designed by the PWM is analyzed, particularly its poor performance in inclined-cavity SLD. The FDTD method is used to analyze and determine the deviation of the reflection curve and high reflectivity. Therefore, the AR film design is optimized and the film is coated by reactive magnetron sputtering; this is verified by reflectivity and spectrum measurements. After optimization, the average reflectivity of the double-layer AR film is controlled within 0.1% and the lowest value is only 0.05%. The AR film effectively suppresses the ripple, and the ripple and modulation index are only 0.019 dB and 2.30×10-3, respectively, with a decrease of more than 50% compared with those of the traditional PWM film. The prepared SLD chips still maintain a 10-mW output power under a current of 100 mA. The AR film developed in this study can effectively reduce the reflectivity of the facet, and the fabricated weak-ripple SLD is prepared by using the film. The research results provide a reference for the development of optical films of SLD and other semiconductor optoelectronic devices.

ObjectiveHigh-strength 2195 aluminum-lithium (Al-Li) alloy exhibits excellent strength and fracture toughness both at room and low temperatures and is mainly used in the cryogenic storage tanks of space launch vehicles to satisfy the weight reduction requirements of key structures in the aerospace sector. Laser welding technology is a high-energy beam connection method with high energy density, good welding quality, high precision, high production efficiency, significant weight reduction, and other characteristics. Laser welding is an ideal joining technology for spacecraft tank structures. Alloying elements influence the type, size, volume fraction, and distribution pattern of precipitates in aluminum alloys, while precipitates and microstructures determine the mechanical properties of these alloys. Therefore, optimizing the composition of welded joints fabricated from 2195 Al-Li alloy can considerably improve the joint properties. In this study, 4047 filler wires and 2319 filler wires are used to conduct laser welding experiments on 2195 Al-Li alloy to compare and analyze the effects of different filler elements on the microstructure, alloy element distribution, and mechanical properties of laser-welded joints.MethodsThe dimensions of welded parts used in this work were 100 mm×50 mm×2 mm (Fig. 1). The utilized wires included 4047 filler wires with a diameter of 1.2 mm and 2319 filler wires with 2% (mass fraction) TiC particles. Al-Li alloy laser fillet welding was performed using a laser. Laser welding was conducted with a 6-axis robot, and the welded specimens were clamped using a special welding fixture. Wire-filling welding was performed using a wire feeder. Scanning electron microscopy (SEM) was conducted to observe the microstructure of the joint cross-section and tensile fracture morphology. The obtained tissue morphology was utilized to study the microstructural characteristics of joints with different welding wires and their tensile fracture mechanism. Chemical compositions of different areas in joint cross-sections were characterized by energy-dispersive X-ray spectroscopy (EDS) to determine elemental distributions and their influence on the joint properties.Results and DiscussionsLaser self-melting welding and laser welding with 4047 filler wire produce the microstructure with a fusion line passing close to the equiaxial fine crystal zone (EQZ), columnar crystal zone, and central dendrite zone (Figs. 4 and 5). The presence of EQZ near the fusion line is caused by the presence of Zr and Li elements in the alloy. The laser wire filling welding using 2319 significantly improves the properties of the welding seam (Fig. 6) containing fine equiaxial crystals owing to the addition of TiC particles to the center of the melt pool. This increases the number of nucleation centers and substantially compresses the growth space for columnar crystals, thus promoting the transformation of columnar branch crystals to equiaxial crystals. Grain boundary segregation and elemental burnout strongly influence the weld, leading to the redistribution of its elements and accumulation of Cu atoms at the grain boundaries (Tables 2-5). The 2195 aluminum-lithium alloy laser-welded joints undergo significant softening with a reduction in the number of strengthening phases due to the strong lithium and copper elemental burnout in the weld area and significant strength loss without wire filling. After wire filling, the mechanical properties of the joint are significantly improved owing to the refinement of weld grains and increase in the number of weld strengthening phases. In particular, using 4047 wires as a filler considerably increases the tensile strength of the laser-welded joints produced from 2195 aluminum-lithium alloy (Fig. 11).ConclusionsThe self-melting welding joint of 2195 aluminum-lithium alloy and joint by laser wire filling welding with 4047 consist of equiaxed fine crystals, columnar crystals, and equiaxed dendritic crystals. The laser wire filling welding joints using 2319 consist of equiaxed fine crystals and equiaxed dendritic crystals. In contrast to laser self-melting welding, the weld by laser wire filling welding using 2319 contains a large fraction of Cu atoms distributed at the grain boundaries with a Cu element supplementation rate of 6.5%. After 4047 wire filling, the 2195 aluminum-lithium alloy laser welding process is supplemented with Si atoms; the weld tissue grain boundaries contain a large number of Si atoms, and the Si phase strengthening effect is enhanced. Compared with laser self-melting welding, laser filler welding is more energetically intense, and its Li element burnout is more significant. Both 4047 filler wires and 2319 filler wires increase the tensile strength of the laser-welded joints fabricated from 2195 aluminum-lithium alloy with a stronger effect observed for the 4047 filler wires. As a result, the tensile strength of the 2195 Al-Li alloy laser-welded joints is 14.19% higher than that of the joints produced via laser self-melting welding.

ObjectiveAs a reliable technology for material joint processing, laser-MIG hybrid welding (MIG welding, melt inert-gas welding) has been applied to various fields of the product manufacturing industry for decades. Due to its characteristics such as deep penetration, high welding speed, and high-quality shaping, laser-MIG hybrid welding has become the research focus. However, all kinds of defects troubling many scholars often occur in laser-MIG hybrid welding, and root hump is one of the common defects. Unlike instantaneous defects such as undercut and non-penetration, root hump defects are caused by the accumulation of molten metal flowing to the end of the pool over a a period of time. During the formation of the root hump, the weld quality is continuously affected by it. When the molten metal has solidified to form a hump, the new molten metal will continue to accumulate in the next position to form a new hump, resulting in the periodic occurrence of the root hump within a certain range. This study presents an online detection of root hump based on invariable moment characteristics of the tail molten pool, which can detect accurately root hump defect in the strong noise environment of laser-MIG hybrid welding. We hope that our innovative approach could provide the basis for the online detection of defects in laser-MIG hybrid welding.MethodsThe laser-MIG hybrid welding process detection system is established by a high-speed camera, six-axis robot, arc welding machine, high-power fiber laser, and image processing computer. During laser-MIG hybrid welding, the images of the molten pool outlines are collected by the high-speed camera. To reduce the gray difference between the two sides of the molten pool when the arc is retracted or released, the multi-scale Retinex (MSR) enhancement method based on Retinex theory is used. After threshold segmentation and morphological processing, the binary images of the tail molten pool are obtained. Whereafter, the four kinds of invariant moments of the tail molten pool images are calculated. For suppressing the interference of local noise caused by random error on the tail molten pool invariant moments, the moving average method is adopted to reduce the influence of noise. The one-dimensional convolution neural network model using the improved dynamic learning rate algorithm is established, and the moving average values of the four normalized invariant moments from the tail molten pool images are used as input. The model is successful to realize the online detection of hump defects at the root of the weldment based on images of the weldment surface during laser-MIG hybrid welding.Results and DiscussionsAccording to the comparison of the moving average values of the four normalized invariant moments from the tail molten pool images between root hump and full penetration samples, the moving average values of root hump samples are higher than those of full penetration samples. The values of the root hump are almost higher than the specific moving average value, and the full penetration is lower than it (Fig.5). The occurrence of the root hump defect in the welding process can be preliminarily judged by the moving average values of the invariant moment. To accurately detect the root hump defects in the laser-MIG hybrid welding process, the one-dimensional convolution neural network model using the improved dynamic learning rate algorithm is established. The best accuracy of training set from training samples is 99.73%, and the best accuracy of the validation set from training samples even reaches 99.88% (Fig.8). A continuous weld bead, whose the first half of the weld bead has root hump and the second half is normal, is used to verify the reliability of the model. The samples are detected as root hump defect samples in the first 3604.5 ms. The false detection occurs in 2750-2900 ms. The reason for false detection is that this position is close to the boundary between the root hump area and no defect area. At this time, the moving average values of invariant moment decrease. In the latter part, the detection result alternates between 0 and 1 in 3950-4100 ms (Fig.9). A weak hump on the back of the weld bead leads to this false detection. Although the model has some detection errors, it can still accurately detect most root hump defects with 94.7% accuracy (Table 2).ConclusionsThis study adopts invariable moment characteristics of the tail molten pool to detect root hump in laser-MIG hybrid welding. Aiming at the problem of uneven illumination on both sides of the molten pool, the MSR enhancement method based on Retinex theory is adopted to reduce the gray difference on both sides of the molten pool. The moving average values of the four normalized invariant moments from the tail molten pool images coming from the image process can be used to judge the occurrence of the root hump defects. It is observed that the moving average values of the root hump samples are higher than those of the full penetration samples. A one-dimensional convolution neural network model with an improved dynamic adjusting learning rate algorithm is established to detect the root hump defects. The experimental result shows that the accuracies of the training set and the verification set can reach 99.73% and 99.88% respectively. The model is applied to detect root hump defects in continuous weld bead, whose accuracy reaches 94.7%. The root hump defects in laser-MIG hybrid welding are detected accurately, which provids a new idea for the realization of welding status and welding quality detection in laser-MIG hybrid welding.

An improved helmet detection algorithm for YOLOv4 (SMD-YOLOv4) is proposed to effectively detect whether construction workers are wearing helmets in complex scenes and reduce safety hazards. First, the SE-Net attention module is used to improve the ability of the model backbone network to extract effective features. Next, a dense atrous space pyramid pooling (DenseASPP) is used instead of spatial pyramid pooling (SPP) in the network to reduce information loss and optimize the extraction of global contextual information. Finally, the scale of feature fusion is increased in the PANet part and deep separable convolution is introduced to obtain detailed information about small targets in complex contexts without slowing down the network inference speed. The experimental results show that the mean average precision (mAP) of SMD-YOLOv4 algorithm reaches 97.34% on the self-built experimental dataset, which is 26.41 percentage points, 6.44 percentage points, 3.25 percentage points, 1.49 percentage points, and 3.19 percentage points higher than that of the current representative Faster R-CNN, SSD, YOLOv5, YOLOx, and original YOLOv4 algorithms, respectively, and can meet the real-time detection requirements.

We propose a novel flame generation algorithm, called fire-GAN, based on the HistoGAN algorithm to solve the issues of low quality and complex color control of flame images produced by a generative adversarial network. First, flame image segmentation is introduced in the image preprocessing link to remove background interference from the network, reduce the flame shape distortion and color distortion. Second, the roundness loss function is suggested to increase the focus of the network during training on the intricacy of the flame contour. Finally, data enhancement is implemented in the generator and discriminator to maintain the network stability during training and prevent gradient explosion. The experimental results demonstrate that the average RGB error between the flame generated by fire-GAN and the target flame is 2.6%, the Fréchet inception distance (FID) is 59.23, and the inception score (IS) is 2.81. The outcomes demonstrate the feasibility of the fire-GAN to produce a flame image with color, definition, and authenticity levels quite comparable to the target flame image.

A lightweight network model based on multiscale feature information fusion (MIFNet) is developed in this study owing to the imbalance among the parameter amount, inference speed, and accuracy in many existing semantic segmentation network models. The MIFNet is constructed on the encoding-decoding architecture. In the encoding part, the split strategy and asymmetric convolution are flexibly applied to design lightweight bottleneck structure for feature extraction. The spatial attention mechanism and Laplace edge detection operator are introduced to fuse spatial and edge information to obtain rich feature information. In the decoding part, a new decoder is designed by introducing a channel attention mechanism to recover the size and detail information of the feature map for a complete semantic segmentation task. The MIFNet achieves accuracies of 73.1% and 67.7% on the Cityscapes and CamVid test sets, respectively, with only approximately 0.82 M parameters. Correspondingly, it reaches up to 73.68 frame/s and 85.16 frame/s inference speed, respectively using a single GTX 1080Ti GPU. The results show that the method achieves a good balance in terms of the parameter amount, inference speed, and accuracy, yielding a lightweight, fast, and accurate semantic segmentation.