ConclusionsIn the present study, a metamaterial solar absorber with ultra-wideband and high absorption is designed based on the characteristics of combined resonant structure and impedance matching theory. The absorber unit cell is composed of a cross-shaped and metal/dielectric/metal stack structure, and the combined resonant structure effectively expands the absorption bandwidth. The finite difference time domain method is used to analyse the absorption characteristics, and numerical results reveal that the absorber has an average absorptivity of 94.9% in the 300-4000 nm band with an absorption bandwidth of 3700 nm, which can effectively cover the visible and infrared regions. The absorber exhibits certain polarisation-independent characteristics over the entire absorption band range and maintains an average absorption of 93% at a large angle of 60° oblique incidence. It can be seen from the electromagnetic field distribution at the resonance wavelength, that the broadband and high-absorption characteristics of the absorber are mainly derived from Fabry-Perot resonance, slow-wave effect, surface plasmon resonance, localised surface plasmon resonance, and hybridisation coupling between multiple resonances. Whereas, with the development of nano-processing technology, other similar metamaterial absorber structures have also been manufactured for the test, which also provides the basis for the fabrication of the proposed absorber structure. This work provides a reference value for the design of invisible equipment and solar cells and provides new ideas for the high-efficiency absorption of solar energy.

ConclusionsThe echo complexity of different air targets at different angles is determined by the influence of the photon detection mechanism and the diversity of the target structure and material. Using the photon detection echo theory, this study proposed the concept of photon echo posture affect levels to describe the photon echo of target attitude sensitivity. This method was verified using the modelling and simulation data, quantitative research on the susceptibility of three typical air moving target attitudes, comparison between their characteristics and differences, and experiments. The results show that the obvious difference in the influence level of the photon echo attitude of similar and dissimilar air targets can be used to distinguish the target, providing a new method and data support for target recognition with high theoretical and application values. The other angle range and specific application conditions of the theory for characterising air target movements need to be further discussed.

ConclusionsThe influence of temperature on the magnetic field sensor based on the magneto-refractive effect of an Er-doped fiber and an optical-fiber Mach-Zehnder interferometer is studied in this paper. In theory, the microstructure model of the Er-doped fiber material is established based on DFT. The structural parameters and atomic magnetic moments of the model are calculated at different temperatures. The numerical results indicate that as the temperature increases, the Er—O bond length and O—Er—O bond angle change, and the atomic magnetic moment increases monotonously, enhancing the magneto-refractive effect of the fiber. In experiment, an Er-doped fiber is used as the sensing arm of an optical-fiber Mach-Zehnder interferometer to fabricate the magnetic field sensor. The experimental results indicate that the magnetic field sensitivity increases with the increase of temperature. The sensitivities of the sensor to magnetic field are 12.63 pm/mT at 16.7 ℃ and 25.53 pm/mT at 43.5 ℃, respectively. The study on temperature characteristics of the magnetic field sensor is helpful to improve the accuracy of the sensor, and is of great significance to its practical application.

ConclusionsThis paper proposes a TLS point cloud and MVS point cloud fusion method based on the graph-cuts algorithm and the guided point cloud filtering algorithm. The graph-cuts algorithm is used to fuse the geometry and colour information of heterogeneous point clouds, and the neighbourhood relationship is considered to ensure the consistency of dense matching point clouds after segmentation. Then, using a neighbourhood point selection strategy, an appropriate proportion of the guide point cloud filtering neighbourhood points are chosen for the dense point cloud near the boundary of the LiDAR point cloud, to realise the guided point cloud filtering weighted by the surface curvature. The experimental results of fusing heterogeneous point clouds with different accuracy, smoothing the gap at the junction of mixed point clouds, and correcting stratification are realised. Experiments show that the proposed method can effectively remove the overlapping and redundant parts of the MVS point cloud and the LiDAR point cloud, as well as improve the accuracy and completeness of the dense matching point cloud, which benefits surface reconstruction.

ConclusionsIn this article, the influence of pump symmetry on the dynamic drift of the beam in a large-aperture chip amplifier is introduced. The beam dynamic drift characteristics of two large-aperture slap amplifiers and amplification configurations are compared by the numerical simulation calculations, and a large-aperture high-profile pump based on a pulsed xenon lamp with a diameter of 37 mm and asymmetrical arrangement of multiple lamps is studied experimentally. The dynamic drift characteristics of the beam caused by the pump of the gain slap amplifier are studied. When the symmetry of the pump field of the neodymium glass sheet is about 1.038∶1, the pump-induced beam dynamic wavefront tilt PV value is about 0.98λ (6 sheets accumulation), and the dynamic drift angle of a single laser passing through a single neodymium glass plate is about 0.32 μrad in 375 mm beam aperture, which is well consistent with the result of the theoretical model. This can provide reference for the design of slap amplifiers with different pump configurations as well as the design of multi-path optical paths of laser devices and the design of spatial filter apertures.

ConclusionsIn present study, a novel coupling phase shift detection technology of a single-mode fiber coupler based on the hybrid Sagnac interferometer is proposed. According to the valley wavelength characteristics of its reciprocal port and non-reciprocal port output spectra, the detection of the coupling phase shift of the single-mode fiber coupler is realized, and the detection accuracy can reach 0.0136°, which is an order of magnitude higher than the existing coupling phase shift detection accuracy (0.229°). The experimental system is built to realize the detection of the coupling phase shifts of 3×3 single-mode coupler with the fixed splitting ratio and 2×2 single-mode couplers with different splitting ratios. The detection results are consistent with theoretical estimates, and the maximum standard deviation of multiple detection is 0.0036°. The proposed detection method is also used to measure the coupling phase shift change of the 2×2 single-mode coupler at different temperatures for the first time. The research shows that the proposed detection method has advantages in simplicity, detection accuracy, and adaptability. It can be used for the quantitative and accurate measurement and analysis of coupling phase shift of various multi-port single-mode fiber couplers, and provides an important technical support for the development and screening of phase stabilizer couplers.

ConclusionsThe proposed Laguerre Gaussian mode and orbital angular momentum beam demultiplexing hybrid using multiplane light conversion simultaneously achieved mode demultiplexing and 90° hybrid , with good performance and wavelength characteristics. The designed and implemented Laguerre Gaussian mode demultiplexing hybrid using multiplane light conversion achieved good performance and a feasible scheme, thereby providing a technical reference for designing mode multiplexing and demultiplexing in space division multiplexing technology.

ConclusionsIn this paper, a seven-core erbium-doped fibre, to the best of our knowledge is fabricated for the first time with a China-based MCVD technology combined with drilling. A seven-core erbium-doped fibre amplifier is constructed with core pumping using fan in/out devices. For the signal, a 31 channel C-band DWDM light is used, with a total power of 0 dBm. The gain fibre is pumped by a 980 nm light source with a power of 350 mW. The maximum gain of 17 dB, average noise index of less than 6 dB, and the gain difference between individual fibre cores of less than 5 dB are obtained. Seven channels of signals are amplified simultaneously. After compensating for the passive loss of the whole system, the gain as high as 21 dB is able to meet the commercial application of erbium-doped fiber amplifier (EDFA). The disadvantage lies in the high complexity of the amplification structure system of the core pumping, and thus, the cladding pumping scheme is an effective way to reduce the structural complexity and improve the energy utilisation rate of the future MC-EDFA system.

ConclusionsA compact fiber sensing probe is proposed, and the magnetic field and temperature are simultaneously measured. The cascaded magnetic and non-magnetic polymer films ae integrated with the fiber to form the sensing probe, which is obtained using simple dipping and UV curing technologies. The temperature sensitivities of dips A and B are 0 and 381.7 pm/℃, and the magnetic fields sensitivities are 48.9 pm/mT and 55.4 pm/mT, respectively. The simultaneous measurements of magnetic field and temperature are obtained using the method of sensitivity matrix. The sensing probe may find a widespread application in aerospace space, geophysical research, and medical fields due to its easy fabrication process, high sensitivity, compactness, and good stability.

ConclusionsFrom the Φ-OTDR test environment built in the UHV DC control and protection engineering system, we analyze the characteristics and statistical law for fault detection signal in both the frequency and time domains along the optical cable link. The expressions of short-time over-level rate and addition short-time over dual-level rate for the signal in the time domain are given. Furthermore, we propose a synthesis algorithm of the time-domain multiplication short-time over dual-level rate and frequency domain-segmented band RSMD. Results show that the short-time dual-level rate is the best, and the SNR of the Φ-OTDR system can be improved to 9.41 dB, 9.02 dB, 7.60 dB, and 3.50 dB, under the fault positions of 2.3, 5.0, 10.8, and 16.0 km, respectively, along the cable link. The SNR of the Φ-OTDR system under the fault position of 2.3 km is further improved when N=8 and the peak value of the frequency band RSMD highlighted the weak fault position, which is a little different from the actual situation. Combining both algorithms can reduce the signal processing time and the weak fault location accuracy along the optical-fiber link to ±2 m. Additionally, the proposed algorithm improves the SNR of the Φ-OTDR optical cable link fault detection signal in the UHV DC control and protection engineering system, which identifies weak faults along the optical cable link and realizes the accurate positioning of weak fault events.

ConclusionsThe propagation and focusing of sound waves in liquid are simulated using finite element software. We find that delayed focusing can effectively reduce the echo time difference generated by different excitation sources. Additionally, the phase delay offset can effectively reduce the side-lobe amplitude of the focused beam. The increase in the laser excitation sources makes the beam directivity range narrowed to increase the energy amplitude at the focus; thereby, improving the imaging resolution and quality. Compared with a single laser source, the signal amplitude of eight laser source arrays can be improved by 3.46 times. Using post-processing focusing, multiple weak diffraction LIA signals are delayed and superimposed. The energy convergence is enhanced to realize higher precision detection at the target. The combination of the LIA-SAFT imaging method and PSM algorithm has good ability for acoustic beam synthesis, imaging resolution, and imaging quality. This study provides a new idea for LIA-SAFT imaging and shows a potential way of detecting underwater targets.

ConclusionsWe used dichroic mirror dense spectral beam combing to model and analyse the impact on incident angle, centre wavelength and thin-film transmission. Further, we developed an experimental study on three sub-beam combing wavelengths of 969, 976, 981 nm, achieving an output power of 311.9 W, a beam combining of 95.88%, a brightness of 58.42 MW/(cm2·sr), a spectral width of 12 nm, and a beam quality deterioration of less than 1.06 times. Compared to traditional beam combing based on the dichroic mirror, we can decrease the spectral width from several tens of nm to only 7 nm, and the combing channels substantially increased, allowing us to achieve higher brightness and power LD output relatively easily. In addition, when compared with dense spectral beam combing of about 0.5 nm, this type of beam combing has the disadvantages of a wider spectral interval, but this method can play the role of secondary beam combing and the output from dense spectral beam combing can be regarded as sub-beam. These findings are important in terms of increasing the power and brightness of direct diode lasers.

ConclusionsA 444 nm blue laser diode and a 469 nm blue laser diode, whose maximum pump powers are 1.4 W and 1.5 W, respectively, are used as the pumping source. The two blue laser diodes are fixed in the π polarization direction. A 45° combiner is used to combine the pump light of the two blue laser diodes. An aspherical lens with a focal length of 4 mm is used as a collimator to collimate the pump light emitted by the blue laser diode. A spherical lens with a focal length of 12 mm is used as a focusing lens to focus the pump light emitted by the blue laser diode. The cavity structure of the laser is V-shaped. The Fabry-Perot etalon is used to select a single-longitudinal mode. The Pr∶YLF crystal with a size of 3 mm×3 mm×5 mm and doping concentration (mass fraction) of 0.5% is used as the gain medium. The type-I phase-matched BBO crystal with a size of 3 mm×3 mm×3 mm is used as the frequency-doubling crystal. By optimizing the resonator parameters and the parameters of two Fabry-Perot etalons, the maximum output power of 261 nm single-longitudinal ultraviolet laser is 110 mW when the incident pump power is 2500 mW.

ConclusionsIn this study, the miniaturized demonstration system shows the performance of the low noise background. By inserting specific test modules, it can be utilized to test and evaluate the key technologies towards the next-generation earth gravitational detection mission, such as the noise performance of an ultra-stable optical bench, the dynamical adaptability of the detector and the phase meter, and the data post-processing algorithm for the suppression of the optical phase locking. Therefore, the high-precision miniaturized demonstration system is a powerful tool in the future research of the space-based laser ranging interferometer.

ConclusionsThe roughness measurement model based on the light section method can automatically process images and calculate roughness values with the advantages of low cost and high efficiency. The measurement errors are less than those of the stylus method. The average error of the seam-driven stitching image method is reduced by 1.42%, especially for the machined workpiece with large irregularity spacing, which improves the sampling length and measurement accuracy of the traditional light section method.

ConclusionsThis study proposed a one-dimensional convolutional encoder-decoder neural network called Dense-1D-U-Net based on the encoder-decoder structure with our design of dense blocks and added skip connections. Dense-1D-U-Net can adapt to various studies by modifying neural network parameters and changing weights initialization methods. Here, it is used in the SRSI method based on deep learning. End-to-end learning of the relationships between spectral interference fringes and real spectral phases utilizes input information without intermediate calculation, which is the advantage of deep learning. The fitting ability of the neural network is significantly improved using our design of dense blocks. The added skip connections can make good use of the primary information. The accuracy of spectral phase measurement using Dense-1D-U-Net is at least about one order of magnitude improved more than that of the traditional SRSI algorithm. It is verified that Dense-1D-U-Net, trained by simulated data, can calculate measured data (Fig. 10). However, laser pulses are more diverse in practice. In future studies, we will consider various conditions of laser pulses to enhance the dataset to adapt to the specific situation. The advantage of Dense-1D-U-Net is that it is robust and can adapt to different studies by training it on different datasets and initializing its weights in different ways. This neural network can be extended to ultrafast spectroscopy and related studies based on one-dimensional information.

ConclusionsThis paper proposes an integrated interference signal detection system based on four-quadrant detectors and wave plate arrays. The Jones matrix is used to establish a model for simulating errors caused by the alignment errors of the wave plate array, and the influence of the alignment errors of the wave plate on the measurement results is analyzed. An experiment is set up for verification and research. The errors introduced by the wave plate increases with the increase of alignment errors. Through experiments, the measurement error range of the quarter wave plate in the range of -20°-20° is 0-13 nm, and the measurement errors of the half-wave plate is 0-2.4 nm. From the above analysis, it can be seen that the quarter-wave plate alignment errors have a significant impact on the measurement results, and the half-wave plate has a smaller impact on the measurement results than the quarter-wave plate. In order to improve the measurement accuracy of the single-frequency interferometry system, the alignment direction of the fast axis of the delay wave plate should be adjusted in place to minimize the alignment errors. The high-performance wave plate should be used as much as possible to reduce the measurement errors. In summary, the wave plate array error model established in this paper provides a theoretical basis for the quantitative analysis of the accuracy of the single-frequency laser interferometer system, and has an important reference for the integration of the single-frequency laser interferometer system and the analysis and elimination of nonlinear errors.

ConclusionsHerein, a planar mirror-assisted single-camera DIC system is proposed, which expands a single-camera DIC system to a four-virtual-camera DIC system. The plane’s mirrored images can perform panoramic imaging on the front and back surfaces of the measured object from different angles, and both optical path systems can perform panoramic imaging on the front and back surfaces of the measured object. The optical path separation allows for 3D stereo measurements, which compensates for the common single-camera DIC system’s ability to only measure in-plane deformation. In the tensile experiment of the metal specimen, the four subregions have similar strain changes, strain is generally distributed at approximately 0.0125, and the displacement precision increases as the tensile value increases; the 3D reconstruction experiment of the decision currency has good 3D stereo reconstruction effect, and there is no obvious difference compared with the traditional four-camera DIC system. The system has a simple structure, meets the requirements for 3D panoramic dynamic deformation measurement, and has a promising future application. Meanwhile, future works will implement the optical path system to better adapt to the needs of various application environments.

ConclusionsThe multi-spectral temperature measurement is a radiation-temperature measurement method with great potential, especially in the absence of information on the emissivity spectrum of the measured object. In this paper, the photo-elastic modulation interference technology is used to collect radiation signals and the accelerated NUFFT algorithm is used to restore the spectrum of the collected interference signals. Accurate temperature measurement is obtained using the multi-spectral temperature measurement method and the standard black body served for verification. Results show that photo-elastic modulation has the advantages of short acquisition time of interference signals, large spectral range, and is a valuable new method. For future work, further in-depth study, which will provide a new method for temperature measurement in special occasions will be considered.

ConclusionsBecause of the similar chemical structure of various fluorophore groups and cell culture medium, the spectra heavily overlap, making it difficult to conduct the qualitative and quantitative analysis of single components directly. To begin, we use Rhodamine B and a calibration standard baffle fluorescence spectrophotometer to solve the problem. Moreover, the water Raman unit (RU350) is used for fluorescence intensity normalization, which increases the dynamic detection range of the instrument, and then the parallel factor analysis algorithm, which inherits the advantage of "second order" or "mathematical separation, " decomposes the entire dataset into four fluorescent components (TRY, NADH, FAD, and VB6), without the complex "chemical separation." The average recoveries of these fluorophores are in the range of (99.5%±6.5%)-(100.5%±6.4%), and the detection limits are in the range of 0.003-0.013 μg/mL, indicating that the method reaches the detection level of trace analysis. This method can meet the demand for the quantitative detection of metabolic fluorescent components in a culture medium, indicating a significant advantage in cell metabolism and immune research, medical diagnosis in vitro, and cell sorting.

ConclusionsIn this paper, the laser-induced breakdown spectroscopy and random forest are combined to study the GCr15 steel samples with different hardness. Through the linear relationship between the spectral line intensity ratio (IFe Ⅱ 316.786/IFe Ⅰ 375.745 and ICr Ⅱ 482.413/ICr Ⅰ 302.067) and the sample hardness, the experimental results show that the spectral line intensity ratio of the matrix element to the alloy element and the hardness of the sample present a linear correlation, in which the linear correlation between IFe Ⅱ 316.786/IFe Ⅰ 375.745 and hardness is higher, indicating that the method of the spectral line intensity ratio has a certain dependence on the selection of spectral lines. The LIBS-RF method is proposed to estimate the hardness of the sample. First, PCA is used to reduce the dimensionality of the original data, and subsequently a random forest model is established. It is found that the hardness of the sample can not be effectively predicted. Then, the random forest is used to select feature spectra based on the importance of the variables to establish a random forest model. The results show that the prediction accuracy of the LIBS-RF model based on the full-spectrum data is lower than that based on the partial characteristic spectrum data. This is because the full-spectrum data also contains a lot of noise and other redundant information, which also participates in the training of the model and results in a decrease in model accuracy. In addition, it is found that as the number of decision trees and the number of random features increase, the prediction accuracy of the model increases to a certain value and remains relatively stable. Based on this, the parameters of the model can be adjusted reasonably while satisfying the accuracy requirements, the complexity of the algorithm is reduced, and the efficiency of the algorithm is improved. Above all, as a novel hardness measurement technology, LIBS-RF has the advantages of simpler and faster than the traditional LIBS hardness measurement, and the results here provide a theoretical basis for engineering practice applications.