Multilayer coated gratings have excellent diffraction performance and can be an important dispersion optical element in synchrotron radiation soft X-ray band. In order to measure the diffraction efficiency of multilayer coated gratings, a miniaturized device for measuring diffraction efficiency of the grating was designed and installed in Shanghai Synchrotron Radiation Facility. The diffraction efficiency of the grating can be detected quickly in the existing spectroscopic microscopic experimental chamber. In the photon energy range of 480--730 eV, the zero-order diffraction efficiency peak of the multilayer grating is 1.11%, while the 1st order diffraction efficiency peak is 0.52%. The factors affecting the diffraction efficiency of multilayer coated gratings were analyzed. The diffraction efficiency of multilayer coated gratings was calculated numerically by differential theory, proving the rationality of the experimental scheme. The diffraction efficiency measurement and simulations will contribute to the preparation and application of multilayer coated gratings.

Slave optical transceiver control technology in a laser communication network was assessed to improve the efficiency of space laser communication and realize a space laser communication network. First, the principle of a "one-to-four" system in a space laser communication network was evaluated, and the control mode of a slave optical transceiver was analyzed. Second, a mathematical model of the strap-down control technology of a slave optical transceiver was established, and a nonlinear tracking differentiator was designed. The azimuth axis of the slave optical transceiver was simulated and analyzed by using MATLAB software. Finally, a network communication system was constructed to test the slave optical transceiver. The experimental results show that after introducing the nonlinear tracking differentiator, the azimuth-tracking accuracy of the slave optical transceiver is reduced from 105 μrad(3σ) to 31 μrad(3σ), and the system accuracy is improved significantly.

Optical interconnects have advantages of low power consumption and large bandwidth and can achieve a substantial increase in the number of nodes and switching capability for data centers. An arrayed waveguide grating router (AWGR) based architecture for high capacity optical interconnects is proposed in this paper. The tunable wavelength converter and AWGR are utilized to provide wavelength routing, and a distributed control is used to achieve fast configuration and low latency. Packet contention may occur at the buffer-less optical switches, and the optical buffering approach based on fiber delay lines (FDLs) is introduced for contention resolution. Two implementations are employed to support strictly non-blocking network and large-scale interconnection. In this paper, we analyze and compare the performance of the proposed architecture in terms of the network size, traffic mode, and buffer capacity. Simulation results indicate that the proposed architecture can interconnect 32768 nodes while providing low latency and high throughput.

Under low-cost intensity modulation with direct detection (IM-DD) optical inter-connection scenarios, we demonstrate high-speed mode division multiplexing (MDM) transmission experiment over 1000 m length few-mode fiber (FMF) using two degenerate LP11 modes enabled by multiple-input multiple-output (MIMO) equalizer based on a neural network (NN). With the help of the NN-based nonlinear MIMO equalizer, 2×100 Gbit/s MDM transmission with 30 G-class optical devices is achieved without the optical amplifier, and the single channel rate is approximately 100 Gbit/s. Four-pulse amplitude modulation (PAM-4) (2×50 Gbit/s) signals reached the 7% hard-decision forward error correction (HD-FEC) threshold with a sensitivity of approximately -10 dBm. Furthermore, a novel MIMO equalizer based on decision feedback neural networks (DFNN) is proposed from feature engineering, and the equalization performance is improved. This proposed method provides potential solutions for the future evolution of short-reach optical links.

Because of the difficulty in locating error bits in polarization code decoding caused by atmospheric turbulence in free-space optical communication, this paper proposes an LSTM-SCFlips decoding method in free-space optical communication. First, one-hot precoding is performed on the log-likelihood ratio (LLR) information sequence of polarization codes in serial cancellation (SC) decoding, and the characteristics of the information sequence are analyzed and learned in different training step sizes. The root mean square error and computational complexity of the neural prediction model are comprehensively considered, and an appropriate training step size is selected. After the accuracy of the prediction results is improved, the phenomenon of overfitting the prediction results is further eliminated. The LSTM neural network model is used to locate the first error bit of SC decoding and single-bit or multi-bit flips of the SC decoding algorithm according to the error probability are performed. The simulation results show that in different weak atmospheric turbulence intensities, the proposed decoding method can better identify the optimal flip position and reduce the computational complexity on the premise of sacrificing few computing resources. In addition, better bit-error-rate performance is achieved. When the bit error rate is 10 -4, the correct recognition rate of the optimal flipped bit of the LSTM-SCFlips decoding method is increased by 7 percentage points, and the coding gain of 0.3 dB--1.2 dB is generated.

To tackle the uneven settlement monitoring problem of building structures, we design a new static level with double fiber Bragg gratings based on equal strength beams. According to the basic theory of the communicating vessel principle, the sensing principle of the static level is explained, and it is concluded that the sum of wavelength changes in the two fiber Bragg gratings has a linear relationship with settlement changes. The level mainly includes a cylindrical container, a connecting rod, a cylindrical float, a cantilever beam with equal strength, fiber Bragg gratings, and a bottom support, and a leak-proof connecting device is used at the water pipe joint. The static characteristics of the static level are analyzed with the calibration data. The sensitivity is 15.765 pm·mm -1, the linearity error is 2.959%, the hysteresis error is 0.375%, the repeatability error is 1.744%, the correlation coefficients are all above 0.99, and the static error is 3.455%. The results above meet the requirements of structural health monitoring in civil engineering and are suitable for the long-term remote monitoring of uneven settlement of various structures. The designed double fiber Bragg grating static level has the advantages of convenient disassembly and reassembly and can eliminate the errors caused by temperature changes and liquid level changes, improving the accuracy of uneven settlement monitoring. The application of the double fiber Bragg grating static level to building monitoring proves its ability to monitor uneven settlement.

The division-of-focal-plane polarization imaging system has a compact structure, small size, and high real-time performance, and simultaneously can achieve light intensity response of multiple polarization directions in single imaging. It is one of the research hotspots of polarization imaging. The planar structure reduces the spatial resolution of the image. To reconstruct a polarized image at full resolution and reduce the influence of the instantaneous field of view error, interpolation of polarized images is essential. To protect the tensor structure of polarized images, the algorithm for interpolation of polarized images based on non-negative sparse tensor factorization is proposed. First, according to the non-negative sparse coding theory, the four-channel polarization image block is tensor-decomposed. Second, the sparse representation is solved using nonlocal self-similarity constraints. Finally, the reconstructed image blocks are inverted mapped according to the sampling matrix to obtain a full resolution polarized image. Experimental results show that quantitative indicators and image reconstruction effects of the proposed algorithm are more accurate than that of current mainstream algorithms.

Regarding difficult quantitative analysis, low automation, and intermediate link transformation in reverse design based on industrial computerized tomography (CT) images, this paper proposes a method for direct conversion of industrial CT images into triangular mesh finite element models. After the contour recognition of a CT image, the inner and outer boundaries of the finite element model are generated. According to the coordinate information of image pixels, the image is divided into several elements. Then, the initial triangular mesh finite element model is generated with the improved triangulation method, and the initial finite element model is optimized. The results show that the maximum side ratio of the generated triangular meshes is between 1 and 2, and the maximum twist angle is between 0° and 2°, which meets the evaluation standard of practical engineering application. Experiments are carried out on the sample and cylinder head model, and the results prove the feasibility of the proposed method.

In this paper, the uncertainty of convex lens focal length measured based on the Gauss formula method (real object-real image method) is analyzed comprehensively considering the factors of the depth of field, thickness, and spherical aberration of the lens, and a method to correct the systemic measurement deviation caused by lens thickness and spherical aberration is proposed. For the selected convex lenses with a nominal focal length of 25.0 mm, the measurement uncertainty decreases first and increases afterward with the increase of the object distance, and the minimum value before correction is 1.26 mm, where the spherical aberration of the lens has the most prominent influence on the measurement uncertainty. After the system measurement deviation being corrected, the change trend of the measurement uncertainty remains unchanged and the minimum value is reduced to 0.10 mm, which indicates that the deviation correction improves the accuracy of the measurement results greatly. The results of uncertainty analysis show that the measurement results are more reliable when the object point is placed near the twice focal length of the lens, providing a guide for experimental operation.

Phase shifting profilometry (PSP) based on crossed grating projection can acquire two orthogonal phases by a phase shifting algorithm in a specific direction (the phase shifting direction is usually controlled by a phase shifting adjustment parameter). However, this technique is sensitive to the nonlinear response of the projector-camera system and has not been studied in depth yet. In this paper, we examine the effect of the nonlinear response of the system on PSP based on crossed gratings, derive phase expressions with nonlinear errors, and analyze the reason for the phase crosstalk in two directions. On this basis, we study the active and passive correction methods for the suppression of nonlinear errors. To be specific, a double five-step PSP method based on the derived mathematical expressions is proposed to passively suppress the second-order nonlinear errors. The Gamma correction method based on statistical analysis is also studied, by which the projected crossed gratings are pre-encoded to actively offset the effect of the nonlinear errors. The two methods are compared in terms of practicality. The simulation and experimental results show that the Gamma correction method is more practical to improve measurement accuracy.

The semiconductor laser was used to achieve the transmission welding of polycarbonate (PC) parts. The influences of two kinds of laser absorbers, carbon black (CB) and copper film with carbon black (CWCB), on the weldability of PC parts were analyzed, and the effects of CWCB width on weld morphological characteristics, weld seam deformation, residual stress, and welding strength were further studied. The results show that when CWCB is used as laser absorber instead of CB, the number of bubbles within weld seams is significantly reduced and there occurs obvious deformation of CWCB. The depth and area of deformation increase with the increase of CWCB width. When the CWCB width is 2.5 mm, the depth and area of deformation start to decrease, and the overflow height is stable at 386.32--392.26 μm. In addition, the residual stress and weld width increase as the CWCB width increases. The weld parts have the largest strength of 21.5 MPa when the CWCB width is 1.0 mm, and then the welding strength begins to decrease with the increase of CWCB width.

In order to systematically study the effect of laser irradiation on the interfacial bonding and compactness of SLD-Ti6Al4V coatings and to reveal the relationship between the interfacial bonding and wear resistance of coatings, the SLD-Ti6Al4V coatings were prepared on the Ti6Al4V substrate with different laser irradiation powers. The microstructures, phase compositions, micro-hardness and wear-resistant properties of these coatings were investigated by SEM, EDS, XRD,micro-hardness tester and wear tester. The results show that laser irradiation can induce in-situ nitriding reaction at the interface of particles in SLD-Ti6Al4V coatings to form titanium nitride ceramic phase, and the size and content of titanium nitride increase with the increase of laser irradiation power. When laser irradiation power is 800 W, the width of titanium nitride ceramic phase can reach 4.5 μm, and the area fraction of ceramic phase in the coatings can reach 28.06%. The porosity of SLD-Ti6Al4V coatings can be reduced from 8.28% to 1.85% due to the closing effect of relative pores of the titanium nitride ceramics at the interface. The micro-hardness of SLD-Ti6Al4V coatings can reach 360-370 HV due to the existence of hard ceramic phase. In addition, the SLD-Ti6Al4V coatings have better wear resistance than the Ti6Al4V substrate and CS-Ti6Al4V coatings. This is due to the in-situ formation of nitride reaction layers including Ti2N and TiN at the particle interface under the synergistic effect of high-energy laser heating and high-pressure nitrogen gas source, which not only improves the compactness of the coatings, but also facilitates metallurgical bonding between particles, and thus improves the interfacial bonding strength. These in-situ formed and well-bonded titanium nitride ceramic phases significantly improve the wear resistance of SLD-Ti6Al4V coatings.

In recent years, ultrafast lasers have become the main source of laser precision manufacturing. The further improvement of laser processing efficiency and capability requires ultrashort pulse lasers with higher average power and single pulse energy. In this paper, two picosecond thin-disk laser regenerative amplifiers are developed based on one thin-disk module and two thin-disk modules connected in series, respectively. For the regenerative amplifier with one thin-disk module, on the basis of a seed source with a pulse width lower than 10 ps, the pulse laser with an average power of 44.2 W, a pulse width of 9.3 ps, and single pulse energy of 220 μJ is output, and the optical-optical efficiency of the amplifier is about 13.4%. For the regenerative amplifier based on two thin-disk modules connected in series, a seed source with a pulse width of 800 ps is adopted; then, we obtain a laser output of 126 W at a repetition frequency of 200 kHz and maximum single pulse energy of 0.96 mJ at a repetition frequency of 100 kHz. Considering the effect of amplified spontaneous emission, the dynamic process of the regenerative amplifier with one thin-disk module is theoretically modeled, and the results are consistent with the experimental ones. Further simulations show that the low single-pass gain makes the optical-optical efficiency of the thin-disk regenerative amplifiers greatly affected by the intra-cavity loss, and reducing the intra-cavity loss is one of the keys to achieving a high-efficiency thin-disk regenerative amplifier.

The beam adjustment method has been widely used in oblique aerial image orientation tasks as a traditional image pose calculation method. However, it highly depends on a good initial value. This dependence often results in slow convergence or convergence failure in flight missions with missing navigation systems or limited navigation accuracy. Therefore, to effectively iterate when the initial value is missing or of poor quality, an oblique image orientation method based on a local-to-global optimization strategy is proposed in this paper. This strategy improves the robustness of the optimization iterative solution. In the proposed method, first, a “local map” (consisting of a nadir and four oblique images) according to the maximum overlap principle is constructed, and then, a “global map” is formed by optimizing these local maps. In the local map, the point of the image with the same name is used as the observation value of the constrained optimization to obtain the unknown parameters and corresponding weight matrix (information matrix) of each local map. In the global map, the estimates of variables and the corresponding weight matrix of the local map constitute the observations and weights of the global optimization problem. The proposed method was tested using a set of large simulated dataset and a set of small, real international public test dataset. The experimental results show that compared with the traditional methods, the proposed method can converge faster and more stably under the premise of no loss of accuracy.

A new bud-shaped moth-eye structure (BSMS) with a parabolic center section was designed based on the excellent anti-reflection performance of the traditional parabola-shaped moth-eye structure (PSMS). With the finite-difference time-domain (FDTD) method, we determine the best bottom diameter to be 200 nm by comparing the average reflectance of PSMSs with different bottom diameters and heights. On this basis, the reflectance and cross-sectional electric field intensity distribution of PSMS, BSMS, and a cone-shaped moth-eye structure (CSMS) at different heights were analyzed. Furthermore, the anti-reflection performance of BSMS was assessed with the equivalent medium theory. The results show that the average reflectance of BSMS is lower than that of both PSMS and CSMS in the wavelength range of 300--1200 nm and the height range of 300--1000 nm; when the bottom diameter is 200 nm and the height is 800 nm, the average reflectance of BSMS is as low as 0.19%, and its anti-reflection effect is about 3.5 times that of PSMS and 3.8 times that of CSMS. BSMS demonstrating excellent and stable anti-reflection performance provides a reference for the further design and optimization of anti-reflection structures.

Metamaterial absorber (MA) is a hot research topic recently. To our best knowledge, it is difficult to achieve dual-wavelength high absorption and narrow bandwidth in the near-infrared band at the same time. In this paper, we propose a dual-channel narrow bandwidth MA with a simple structure, which is composed of three layers: a silica substrate, an Au thin layer, and an asymmetric periodic metal grating. Finite-difference time-domain simulations show that the proposed MA has high absorption efficiency at wavelengths λ1=1.1005 μm and λ2=1.19024 μm, and the full width at half maximum (FWHM) is only 0.21 nm and 3 nm, respectively. After analyzing the magnetic field distributions of MA, we find that the narrow bandwidth and high absorption at λ1 are mainly attributed to surface plasmon polariton (SPP) resonance; at λ2, both SPP resonance and Fabry-Pérot (FP) resonance take effect. Finally, the influence of structural parameters of the MA on its absorption characteristics is assessed, and it is found that the change in MA parameters can achieve the tuning of λ1 and λ2.

We proposed an image generation method of ophthalmic optical coherence tomography (OCT) based on the variational auto-encoders to alleviate the problem of insufficient images in deep learning and improve the performance of computer-aided diagnosis algorithms in ophthalmology. We created a generation network of OCT images based on the variational auto-encoders. In addition, we constructed three kinds of retinal OCT image datasets of age-related macular degeneration, diabetic macular edema, and normal situation based on the two public retinal OCT image datasets to train the network and obtain the generation models, respectively. The effectiveness of the image generation method was verified by subjective visual observation and objective experiments. Both the subjective visual observation and the objective experiments show that our method can effectively generate three kinds of retinal OCT images.

As an emerging two-dimensional material, MXene is a potential nonlinear optical material. For this purpose, Nb2C nanosheets are prepared and their absorption spectrum is measured with a spectrophotometer. Firstly, Nb2C nanosheets are prepared by hydrofluoric acid etching and liquid phase exfoliation. Then, the morphology of Nb2C nanosheets is characterized by scanning electron microscopy. Finally, it is discussed through experiments. Experimental results show that the micro-nano fiber device modified by MXene can achieve four-wave mixing in the communication band, and the conversion efficiency of four-wave mixing is increased by 5 dB; for a constant pump light, the wavelength range of signal light is about 4 nm; when the wavelength interval is 0.4 nm, gradually increase the amplification power of the signal light and pump light, the conversion efficiency of the four-wave mixing will gradually increase, the conversion efficiency and the incident power are linear, and the conversion efficiency does not appear to be saturated, so there is room for further improvement.

The novel cascaded optical imaging system designed herein is based on a fore monocentric symmetric objective and a relay lens array, enabling wide-coverage high-resolution aerial imaging. To satisfy ultralow-altitude requirements for precision agriculture monitoring and pesticide application, we propose a cascaded optical imaging system for ultralow-altitude flight focusing on the optimal mid-curved surface shape and location. Considering the light and small load capabilities of unmanned aerial vehicles (UAV), an aspheric surface is integrated into the system to balance the off-axis aberration, reduce the number of required lenses, and shorten the tube length. The designed optical system comprising a UAV-borne ultralow-altitude wide-coverage camera adopting the proposed cascaded optical structure requires a low altitude of 20--80 m, a focal length of 60 mm, an F-number of 3.4, and a field-of-view of 132°. As expected, the proposed system provides a wide ground coverage and excellent imaging performance under ultralow-altitude flight, thus demonstrating its potential for accurate and efficient agricultural situation monitoring of agricultural UAV.

The development of low-cost concentrators is one of the important ways to reduce the cost of solar thermal utilization on a large scale. In order to reduce the material cost, a new design idea of water-injected Fresnel condenser is proposed. The water-injected Fresnel condenser adopts a hollow triangular prism composed of thin flat glass, and the use of lens materials is reduced by injecting pure water into the hollow part, thereby reducing the cost. The design theory of the concentrator is studied and the corresponding optical model is established. Monte Carlo ray tracing method is used to study the optical performance of the concentrator under different sun incident angles, and compared with the traditional Fresnel concentrator. Build a test prototype and conduct a preliminary light gathering experiment. The research results show that, compared with the traditional Fresnel concentrator, although the water-injected Fresnel concentrator has a certain loss of concentrating effect, the average loss of concentrating efficiency is 5.40%, but the experimental prototype has obvious concentrating effect, the light spot width is also in good agreement with the theoretical situation, and it is feasible for engineering applications.

In the ground experiment of spatial infrared detection equipment, an infrared scene simulator needs to be used to generate an infrared image that simulates the infrared characteristics of a space target under laboratory conditions, and the infrared image is projected onto the device under test through a projection optical system. Therefore, this paper designs an optical projection system operating in the range of 100--300 K. The system with a coaxial Cassegrain structure has a working wavelength of 3.7--5.5 μm, a focal length of 638 mm, a field of view of 3.4°×3.4°, an exit pupil diameter of 102 mm, and an exit pupil distance of 660 mm. The Zenike polynomial is employed to fit the thermal deformation data of the mirrors, and the ZEMAX software is applied to evaluating the imaging quality of the system. The simulation results show that the modulation transfer function of the projection system at different temperatures meets the design requirements, and the projection system has been successfully applied to the ground cryogenic experiment of a spatial infrared detection device.

A high quality factor (Q-value) terahertz (THz) metamaterial sensor based on double ellipse structure is designed in this paper. Each cell of the sensor contains two 0.2 μm thick metal ellipses at a certain angle and located on the polymer substrate. The sensor structure is optimized by the finite integral time domain method, and the destruction of in-plane inversion symmetry leads to the excitation of vertically incident THz waves by the metasurface composed of the metal elliptical array. Experimental results show that the Q-value of the sensor is as high as 348. When the sensor surface is covered with a test object with a thickness of 20 μm, its sensitivity is 293 GHz/RIU (refractive index unit), and the sensor can be applied to high-sensitivity detection, trace biological sample detection, and early disease diagnosis.

This paper built a spectral response model of the “Jilin-1” night light remote sensing camera to analyze the reasons of the out-band spectral response and proposed a method for the out-band spectral response correction of the night light remote sensing satellite. According to the relationship between the spectral response of the remote sensing camera in each band, the out-band response correction coefficient is obtained by calculating the standard light sources with different spectral characteristics. The out-band spectral response of the target light source in the night light remote sensing images is corrected, and the radiance of the target light source is inversed through on-orbit experiments. The corrected radiance difference of the night light remote sensing satellite is significantly reduced, with an average decreasing from 10.53% to 4.88%, and the radiance difference in each band is less than 7%.

We simulated the reflectance of typical vegetation in the red-edge region before and after the channel center wavelength shift of hyperspectral remote sensors to quantitatively examine the effect of the channel center wavelength shift on the red edge spectra. The results show that the channel center wavelength shift leads to many "spike" and "shake" phenomena in the reflectance curves of the red-edge region, and the reflectance curves in the region without characteristic absorption become unsmooth. Moreover, there is a significant linear relationship between the channel center wavelength shift and the red edge position (REP) error (coefficient of determination R2=0.99). For a 10 nm hyperspectral remote sensor, when the channel center wavelength shifts are 1 nm, 3 nm, and 5 nm, the maximum errors in red-edge reflectance are 1.46%, 4.49%, and 9.57%, respectively, and the red edge spectra have the obvious "blue shift" phenomenon, with the REP shifting by 0.75 nm, 2.60 nm, and 5.52 nm, respectively. The channel center wavelength shift will lead to the pseudo-shift of the red edge or cause the "covering" or "strengthening" effect on the red edge shift induced by vegetation stress, which will directly affect the monitoring accuracy of vegetation stress based on hyperspectral remote sensing data. Channel center wavelength shift is an important source of the red edge shift. Accurate spectral calibration is the important basis of quantitative application related to the red edge spectra of vegetation.

The flow velocity is an important limiting factor behind dynamic light scattering (DLS) measurements for flowing aerosols. In this paper, the autocorrelation function (ACF) of scattering intensity under the laminar flow condition is used to retrieve the simulated and measured DLS data of flowing aerosols, and the restriction of flow velocities on the measurement of the particle size distribution (PSD) for aerosols is analyzed. The results show that the strong influence of velocity increase on the PSD retrieval results cannot be expressed in the ACF model through the contribution of flow velocities. The reason for the difficulty in particle size inversion is that the velocity increase aggravates the ill condition of the ill-conditioned equation where the ACF is located, which is manifested as the increase in the condition number of the kernel matrix in the equation. From the perspective of signal analysis, the increase in flow velocities reduces the amplitude of particle size information in ACF. The influence of flow velocities is related to the particle size of the measured aerosols, and this relevance to particle sizes can be evaluated by comparing the diffusion characteristic time in Brownian motion with the translation characteristic time in aerosol flow. The characteristic time ratio of diffusion to translation can not only characterize the different effects of flow velocities on aerosols with different particle sizes in DLS measurements, but also provide a basis for velocity selection according to the measured objects in actual measurements.

To identify the phytoplankton community by discrete three-dimensional fluorescence spectra, we studied a method based on a self-weighted alternating trilinear decomposition (SWTATLD) algorithm for five common alga species (Microcystis aeruginosa, Scenedesmus obliquus, Nitzschia sp., Peridinium umbonatum var. inaequale, and Cryptomonas obovata.). Then, the results were compared with those of the parallel factor (PARAFAC) algorithm. The recovery of the PARAFAC algorithm is 92.73%±13.99% for Microcystis aeruginosa, 105.51%±11.58% for Scenedesmus obliquus, 89.25%±13.68% for Nitzschia sp., 109.48%±13.47% for Peridinium umbonatum var. inaequale and 88.76%±13.60% for Cryptomonas obovata. The recovery of the SWTATLD algorithm is 96.70%±3.94% for Microcystis aeruginosa, 98.07%±4.48% for Scenedesmus obliquus, 101.71%±3.97% for Nitzschia sp., 97.26%±4.11% for Peridinium umbonatum var. inaequale, and 103.57%±4.34% for Cryptomonas obovata. The recovery results based on the SWTATLD algorithm were closer to the real concentration and have smaller deviations than those based on the PARAFAC algorithm. Our results provide a method for the effective identification and quantitative analysis of phytoplankton communities.

2, 6-Diamino-3, 5-dinitropyrazine (ANPZ) is an important insensitive high explosive of pyrazine type. To further understand the crystal structural characteristics of ANPZ, we employed terahertz time-domain spectroscopy (THz-TDS) to study the spectral evolution of ANPZ from 24 ℃ to 150 ℃. The solid-state density functional theory (DFT) was adopted to calculate and analyze the THz vibration characteristics of ANPZ. The results show that ANPZ has obvious absorption from 0.3 THz to 2.1 THz and its vibration mode originates from the interaction between the nitro groups and adjacent atoms, which is helpful for the understanding of ANPZ structures and the design of new derivatives. Moreover, the absorption at 1.14 THz and 1.92 THz shows different evolution characteristics as the temperature rises, which suggests that the interaction between the two nitro groups of ANPZ and surrounding atoms responds differently to thermal stimuli. Our results can guide the comprehension of the physical mechanism for ANPZ under high-temperature loading and even final combustion or explosion.

A compact double-gas sensing system was constructed for the simultaneous sensitive detection of CH4 and CO2,which combines quartz-enhanced photoacoustic spectroscopy (QEPAS) and frequency division multiplexing technique. Two continuous wave distributed feedback (DFB) diode lasers operating at central wavelengths of 1654 nm and 2004 nm were employed as the light sources. The photoacoustic signals were first excited by simultaneously injecting sinusoidal modulation currents with independent frequencies near the resonant frequency of the quartz tuning fork into the lasers, and were then demodulated to obtain the corresponding second harmonic components of CH4 and CO2,and finally the simultaneous detection of CH4 and CO2 was realized. The experimental results showed that there was no disturbance between the photoacoustic signals of these two gases. The relationships of CH4 and CO2 concentrations with their corresponding second harmonic signals were calibrated, and a good linear response result was obtained, in which the linear correlation coefficients were all larger than 0.994. The long time measurement of CH4 with volume fraction of 500×10-6 and CO2 with volume fraction of 2000×10-6 was conducted and the system performance was further evaluated through the Allan deviation analysis. The results indicated that the minimum detection limits of CH4 and CO2 for the system were obtained as 0.58×10-6 and 1.32 ×10-6, corresponding to the normalized noise equivalent absorption coefficients of 7.2×10-9 cm-1·W·Hz1/2 and 9×10-9 cm-1·W·Hz1/2, respectively.