ConclusionsAn ohmic alloy system with a Cr barrier layer was optimized for GaAs ohmic contacts in this study. At 420 ℃/60 s annealing, the new alloy system, Au/Cr/AuGe/Ni, has a low specific contact resistivity of 2.63×10-6 Ω·cm2, whereas the p-type alloy system, Ti/Cr/Au, has a resistivity of 6.99×10-7 Ω·cm2 at 440 ℃/60 s annealing. Simultaneously, both structures achieved a low specific contact resistivity over a relatively wide temperature of 420-460 ℃. The XRD analysis indicates that the Cr barrier layer effectively achieves a diffusion barrier and improves the ohmic contact quality. SEM and AFM characterization results indicate that the Au/Cr/AuGe/Ni alloy system with a Cr barrier layer has a uniform and flat surface, and the Ti/Cr/Au system has good surface properties. The Au/Cr/AuGe/Ni alloy system improves significantly in ohmic contact performance compared with the conventional Au/Ni/AuGe/Ni alloy system; the Ti/Cr/Au alloy system is comparable with the Ti/Pt/Au alloy system. Thus, ohmic contact electrodes with a Cr barrier layer meet the requirements of semiconductor devices and facilitate batch use.

ConclusionsThe composite Ti-Co alloy cladding layer (20% Stellite6+ 80% Ti) is well combined with the Ti-6Al-4V substrate (the micro-shear strength is up to 615 MPa). Because of the low Co content and abundant β stable elements, the main phases of the cladding layer are solid solution β-Ti and CrTi4 without intermetallic compounds. 80% of the grains are smaller than 30 μm in diameter. During the heat preservation and cooling processes, the β-Ti separation reaction takes place, which implies that the β′ phase precipitates as the black dispersed particles. The hardness of the cladding layer is increased by approximately 80 HV, and the wear loss is reduced by 38%. The increase in the hardness and wear resistance is mainly due to the solid solution strengthening of β-Ti solid solution, fine grain strengthening, and the diffusion strengthening of the second phase β′.

ConclusionsAiming at the defects of the traditional differential zero-crossing method, this paper proposes a two-threshold cloud detection algorithm based on cluster analysis. In this algorithm, the lidar signal is processed with the improved thresholds of peak-to-bottom ratio and background noise, and the cloud information is extracted accurately. In addition, for the layered cloud processing, this paper adopts the ISODATA algorithm for the cluster analysis of differential zeros to achieve accurate layered cloud processing. Experimental results show that compared with the traditional differential zero-crossing method, the proposed algorithm significantly improves the inversion accuracy, effectively eliminates the interference of aerosol signals, extracts cloud information, and has a good detection effect on the clouds with different heights. It is found that on the basis of hierarchical cloud processing, the echo signal characteristics with different types of clouds and different cloud structures can be used to distinguish cloud types such as cumulus, stratus, and cirrus, as well as cloud phase states such as ice cloud, water cloud, and ice-water mixed cloud.

ObjectiveMicrowave phased array antennas (PAAs) have attracted considerable interest owing to their advantages of rapid and accurate beam steering. Traditional PAAs based on electronic phase shifters suffer from the disadvantages of steering angle and instantaneous bandwidth of the radio-frequency (RF) signal due to the beam squinting problem. Integrated optical beamforming networks (OBFNs) use optical true time delay lines (OTTDLs) to broaden the instantaneous bandwidth. The processing RF signal in the optical domain also brings the advantages of low loss, compact size, light weight, and zero electromagnetic interference. Therefore, they can play an important role in broadband large-scale PAA systems. Moreover, the OBFNs can be integrated with other microwave photonic systems, such as optoelectronic oscillators, photonic analog-to-digital converters, and optical channelizing filters to realize fully integrated microwave photonic radars. The most important components in the integrated OBFNs are the OTTDLs. Integrated OTTDLs with a broad bandwidth and large delay tuning range are in high demand. Among the various integrated OTTDL structures, cascaded tunable microring resonators (MRRs) are promising owing to their continuous and broadband delay tuning. Previously, we demonstrated a cascaded-MRRs-based OTTDL chip working at the anti-resonant wavelength. It exhibited the advantages of broadband response with low delay fluctuation, high scalability, and simple control schemes. In this work, we proposed a 1×N binary tree OBFN chip based on cascaded MRRs (Fig. 1). The binary tree topology interlaces the power splitters with the OTTDLs, which effectively reduces the number of delay elements. Due to the random phase errors of MRRs and the thermal crosstalk upon tuning, we proposed and demonstrated an automatic calibration method to facilitate the state calibration of OTTDLs.MethodsFirst, we theoretically investigated the working principle of the OBFN using the transfer matrix method. From the simulated delay spectra, we determined that the anti-resonant MRRs have lower delay fluctuation over a broad bandwidth compared with the on-resonant MRRs (Fig. 2). Moreover, the bandwidth of the OBFN is dependent only on the bandwidth of the continuously tuned MRRs. Therefore, we presented a delay tuning method to increase the system bandwidth by reducing the number of continuously tuned MRRs (Fig. 3). In each stage of the binary tree OBFN, only one MRR was continuously tuned for the delay residual, and the other MRRs were digitally tuned to two specific states (coupling coefficient K=0 or K=1).To correct the random initial states of the MRRs, we developed a control system to automatically perform delay state calibration and characterization (Fig. 5). The calibration procedure includes the following two parts: the optical calibration stage and microwave delay calibration stage (Fig. 6). In the optical calibration stage, we first measured the power efficiency of the MRR phase shifters. Next, the MRRs were calibrated to K=0 and K=1 using the resonance extinction ratio method and the spectrum mean square error method, respectively (Fig. 7). Thereafter, the continuously tuned MRRs were calibrated by aligning the anti-resonant wavelengths to the operating wavelength at various coupling coefficients. The linear power relationship of the MRR tunable coupler and the ring phase shifter was extracted. Considering the inter- and intra-MRR thermal crosstalk, the power relationship could be remedied using a point-slope method (Fig. 8). In the microwave delay calibration stage, the group delay responses of continuously and digitally tuned MRRs were measured. By iterative control of the phase shifters based on the measured delay spectra, we effectively eliminated the influence of thermal crosstalk between the MRRs and obtained a flat delay response in the operating bandwidth.Results and DiscussionsWe experimentally verified the proposed algorithms by testing the longest path of a silicon nitride 1×8 OBFN chip. The chip was fabricated on the TriPLex? platform. The measured delay spectra of a continuously tuned MRR verify the accuracy of the optical calibration (Fig. 9). Due to the thermal crosstalk, the microwave delay calibration procedure is needed for multiple MRRs. Using the proposed method, all the 21 MRRs in the path are digitally tuned and the measured maximum delay is 560 ps with a delay fluctuation of less than 11.2 ps. Moreover, we successfully realize continuous delay tuning of three MRRs. The delay variation is less than 7.5 ps in the bandwidth of 08 GHz (Fig. 10). Comparing our work with other MRR-based OBFN and OTTDL chips, our OBFN controls the largest number of cascaded MRRs for a widely tunable delay range (Table 1). In addition, with the automatic calibration system based on the optical and microwave joint optimization, our proposed OBFN allows for a broad operating bandwidth and a low delay fluctuation.ConclusionsWe proposed a 1×N binary tree OBFN based on tunable cascaded MRRs working at the anti-resonant wavelength. The delay tuning method that utilizes one continuously tuned MRR in one stage expands the operating bandwidth of the OBFN. An automatic calibration system was experimentally verified by testing the longest path of a 1×8 OBFN chip. With the joint optimization algorithm, we realized the accurate calibration of 21 MRRs automatically. This method, with reduced system complexity, can be further used to calibrate the entire OBFN chip.

ConclusionsIn present study, aiming to the temporal and spatial evolutional characteristics of SSD beam focal spots and based on the optical imaging system of a streak camera, we carry out the beam test experiment with time resolution on the high-power laser device. Experiments demonstrate the influence of SSD beam control parameters on the dynamic evolutional characteristics of focal spots. The focal spots along the SSD scanning direction have typical CPP shaped speckle patterns. The dynamic evolutional modes presented by different modulation frequencies are different. The overall spatial-temporal distribution of intensity modulated by high frequency is more uniform, and a better smoothing effect can be achieved in short smoothing time. The spatial distribution and temporal evolution of the focal spot intensity in the vertical SSD sweep direction become very chaotic, showing the specific differences of SSD beams in two-dimensional space. The focal spot of the SSD beam has strong amplitude modulation, and the modulation has an obvious regional effect. Modulation depth is generally related to the SSD beam control parameters. Our results provide a better understanding of the dynamic evolution of the spot details of SSD beams, which can be combined with the mechanism analysis of LPI to support the optimization of SSD smoothing performances. The inconsistency of SSD beams in two dimensions can be used to optimize the sweeping direction of SSD, and the modulation structure of SSD beams can assist in the selection of modulation frequencies. An accurate description of focal characteristics provides important boundary conditions for optimizing the location of focal trajectory points.

ConclusionsCA-RepVGG can be used in clinical practice. The simplicity of the model and the small amount of calculation ensure the feasibility and reliability of CA-RepVGG. In this paper, CA-RepVGG is used to test and evaluate the classification effect of DR images in two datasets. At the same time, VGG-16, Inception-V3, ResNet-50, and ResNext-50 are compared with our model, and the accuracy, precision, and sensitivity of the network demonstrate the advanced nature of our model. The experimental results show that the model is not only feasible but also superior in classification. In the future, if our proposed model is applied to clinical practice, it can enhance the diagnostic efficiency of professional ophthalmologists regarding ophthalmic diseases, especially in remote and poor areas, ensuring that more patients can be treated in time and avoid losing their eyesight. If more datasets can be used to train the model in the future, the accuracy of automatic classification can be further enhanced and better results can be achieved in clinical practice.

ConclusionsThis study presents a method for achieving rapid beam alignment using image calibration at the transmitting end and a two-dimensional mirror control at the receiving end, thus, addressing the problem of long time-consuming beam alignment in traditional wireless optical communication. The image calibration coarse tracking at the transmitting end can ensure that the spot effectively covers the two-dimensional mirror, and the coupling spot coarse tracking at the receiving end can effectively suppress the beam drift caused by atmospheric turbulence; the uplink can be established simultaneously with the downlink. The non-common sight axis control avoids the inconvenience of transmitting control instructions from the receiving end to the transmitting end, and it is not necessary to use a space stable platform for the moving base, which greatly facilitates the promotion of wireless optical communication.

ConclusionsA noniterative detection method is proposed in this paper to reduce the nonlinear interference of the LACO-OFDM system in nonlinear channels. Maximum likelihood detection in the frequency domain of the proposed noniterative receiver is not required, which can avoid the propagation of nonlinear noise from the lower to the upper layer of LACO-OFDM. Simulation results show that the PAPR of LACO-OFDM signals of Layer 1 is the highest in the nonlinear amplitude clipping channel, which has the highest level of nonlinear interference. Each layer signal is independently demodulated in the noniterative detection receiver, and the frequency domain detection of the first layer signal is not performed. Therefore, the nonlinear noise of Layer 1 is not transmitted to the signal of the upper layer, and the ACO-OFDM signal of Layers 2, 3, and 4 is almost unaffected by the nonlinearity. Compared with the iterative detection receiver, the performance of the proposed receiver exceeds that of the iterative detection method with the enhancement of nonlinearity. The SNR gain of the proposed receiver is 1 dB-3 dB at the 7% FEC limit.

ConclusionsThis research investigated a dual-star parallel synchronous fiber laser network system. It theoretically proves the network parallel node cluster synchronization and the cross synchronization with dual-star, forming the theoretical mode of the network "star-node" synchronous cluster, and successfully predicting the parallel existences of "star-node" synchronization cluster and double "star-node" synchronous cluster. The synchronization of two groups of independent parallel nodes of the single-ring erbium-doped fiber laser and the corresponding laser ring as the network dual-star dual-ring erbium-doped fiber laser is simulated successfully. The single "star-node" synchronization cluster, chaotic synchronization cluster, quasi-period synchronization cluster, parallel double "star-node" synchronization cluster are found. The realization of a dual-star parallel synchronous fiber laser network under different parameters is analyzed. Compared with a single star network, the dynamic behavior of the network’s structure becomes rich, and its double send terminal breakthroughs the limitation of the single terminal send parallel network and shows the obvious advantages and can transfer information efficiently. This research has significant implications for optical networks, fiber laser synchronization networks, and applications.

ConclusionsIn this study, the highest photon conversion efficiency of the first Stokes laser is 82.4%, which is also the highest conversion efficiency of deuterium gas stimulated Raman scattering pumped by Nd∶YAG 1064 nm wavelength in free space. The phenomenon of Raman gain hole-burning is discovered and explained. Appropriately increasing the gas pressure of Raman medium is conducive for inhibiting Raman gain hole-burning, which is useful in maintaining pump laser high conversion efficiency at the stimulated Raman wavelength shifting of large pulse energy laser. To further improve the pulse energy of the Raman laser, we reduce the pressure of the Raman medium and increase the waist size of the pump laser beam to achieve maximum single pulse energy of 245 mJ. Additionally, we employ LBO crystal to obtain the second-harmonic generation with an output wavelength of 780 nm, maximum energy of 57.3 mJ, and conversion efficiency of 36.1%. The measurement of beam quality confirms that the gas thermal effect degrades the Raman laser beam quality, which reduces the laser energy conversion efficiency of frequency doubling. Raman medium circulating or cooling technology is a method for increasing the pulse energy of the laser.

ConclusionsIn this study, two kinds of strain-compensated MQW SESAM structures with a total thickness of 150 and 300 nm of InGaAs absorber layer are designed. The epitaxial material of MQW and DBR is grown by MOCVD. The features of epitaxial material are characterized by photoluminescence spectrometer, high-resolution X-ray diffractometer, and spectrophotometer; its growth parameters are optimized based on the characterization results. Further, epitaxial materials of SESAM are grown and characterized using the same test based on MQW and DBR epitaxial growth parameters. The two SESAMs are applied to the linear cavity Yb-doped fiber laser, and stable mode-locking is achieved. The experimental results show that SESAM with the thickness of the InGaAs absorber layer of 300 nm is more likely to achieve stable mode-locking and obtain pulse output with narrow pulse width.

ConclusionsIn this paper, we propose a broadband and frequency interval switchable multi-wavelength Brillouin random fiber laser. Due to the dual-open cavity configuration, the frequency interval of the output multi-wavelength laser can be switched between single and double Brillouin frequency shifts by adjusting the power ratio between the forward and backward BPs. The results show that when RP power is 831.8 mW, 253 Stokes lines with double Brillouin frequency shift interval (~0.176 nm) in a wavelength range of 44.5 nm (1528-1572.5 nm) and 483 Stokes lines with single Brillouin frequency shift interval (~0.088 nm) in a wavelength range of 42.5 nm (1532-1574.5 nm) are obtained. The frequency interval switchable multi-wavelength Brillouin random fiber laser is expected to greatly broaden the application and increase the flexibility of multi-wavelength laser in fields such as optical communication and sensing.

ConclusionsA semiconductor laser bar with low series resistance and relatively high internal quantum efficiency is designed for low-temperature operation. The structure is optimized elaborately with a low Al content AlxGa1-xAs in the waveguide. The peak electro-optical conversion efficiency of the optimized structure increases to 82.3% at -65 ℃. The constituents of power loss at the peak of the efficiency under different temperatures were also studied. It is observed that the main factor affecting the electro-optical conversion efficiency at low temperatures is the series resistance. By continuously reducing the device resistance, high efficiency at low temperature will be obtained if carrier leakage and optical loss can be controlled.

ConclusionsThis paper demonstrated a 6.02 kW narrow-linewidth fiber laser based on a one-stage MOPA fiber laser system. At the maximum power, the 3 dB and 10 dB linewidths are 0.36 and 1.47 nm, respectively. The peak signal-to-noise ratio is ~27 dB compared with the Raman Stokes light. The beam quality (M2 factor) is ~2.7. To the best of our knowledge, this is the highest power of narrow-linewidth fiber lasers that have ever been reported. In the following work, we will continue to optimize the temporal properties of the seed and the structure of the amplifier to further improve the output power and beam quality of the laser.

ConclusionsWe have fabricated a 5-tube Nested HC-ARF with an outstanding optical performance. This Nested HC-ARF possesses an average attenuation value of 0.38 dB/km from 1545 nm to 1660 nm. The excellent modal purity of the fiber is also experimentally verified. We believe that this fiber is attractive to many applications such as long-haul data transmission.

ConclusionsThis study presents a clustering model of indoor point cloud density based on an exponential function. First, the cutoff distance function model is developed according to the distance and angular resolution of point clouds. Second, the local density model based on the exponential function is constructed by analyzing the number of points and distance mean and standard deviation. Third, according to the distance between the point cloud and boundary, the constraint distance density of judging a wall is obtained. Similarly, the density function of the z value is constructed according to the amplitude distribution and the exponential function of the z value. Combined with the local density, the density clustering model of walls, ceilings, and floors is obtained. For indoor objects, the constraint distance is determined according to the local density within the cutoff distance. In addition, the clustering center can be determined according to the product of the constraint distance and local density. Finally, indoor targets are clustered according to the clustering attribute of each point. Based on the density clustering model, walls, ceilings, floors, and objects in the room can be extracted. The proposed method is compared with other clustering algorithms in different indoor scenarios, and the results show that the number of objects extracted using the proposed method is greater than that extracted using the CFDP and DPC methods. In addition, when there are a few noise points between adjacent targets, the extraction effect of the proposed method is better than that of the CFDP and DPC methods. Furthermore, accuracy, recall, and F1-score are used to evaluate the object extraction performance of the proposed method, which varies with types of rooms. The results show that the proposed method is more suitable for rooms with non-adjacent objects, and its performance is related to the closeness of adjacent objects. Given the shortcomings of the proposed method, future research work will focus on the extraction of objects close to each other. In addition, a future clustering algorithm can accurately extract some small items on other objects, such as books or cups on a table.

ConclusionsIn the present study, PCWD is proposed to measure the surface profile based on FDA. PCWD uses the constant value of the linear-fitted phase in the wavenumber domain to compensate for the noise-induced error in the slope of the linear-fitted phase. Further, a simulation is performed to study the characteristics of PCWD. Unexpectedly, some kinds of noise will increase the measurement error for PCWD. However, the negative effect cannot deny the good performance of PCWD in measuring the entire surface. The experimental result also agrees with the simulation, which proves that PCWD has better measurement accuracy and stability. Our study provides the white-light scanning interferometer with a method for determining the surface profile based on FDA with good noise resistance and helps the white-light scanning interferometer adapt to poor environments.

ObjectiveUltrafast optical parametric amplifiers (OPA) providing femtosecond laser pulses with a tunable wavelength have broad applications in ultrafast sciences, high-field physics, generation of table-top X-ray, or extreme ultraviolet radiations. In a standard optical parametric amplification scheme, a microjoule laser pulse propagates in a solid transparent nonlinear medium to generate supercontinuum seed light and a specific red-shifted infrared spectral range is selected for further stages of amplifications. However, the conversion efficiency of the red-shifted component is considerably lower than that of the blue-shifted component because of the nonlinear optical effects, such as self-steepening and electron-hole plasma generation during supercontinuum generation, requiring a high-input laser intensity close to the damage threshold to obtain red-shifted spectral components with a wavelength of approximately 1600 nm. Thus, the solid nonlinear medium is vulnerable to laser damage and the system reliability is degraded. To solve this problem, we propose taking advantage of the high conversion efficiency of the blue-shifted spectral component in the supercontinuum and preamplifying it with the second harmonic generation of the pump laser pulse for infrared tunable idler wave, which is further amplified in the following power amplification stages. In this scheme, the requirement of the input laser intensity for supercontinuum seed light generation is relaxed, reducing the possibility of solid nonlinear medium damage and improving the system reliability.MethodsIn this study, the ultrafast OPA-based on second harmonic generation pumped-preamplification has three stages: the white light supercontinuum generation seed, second harmonic generation pumped-preamplification, and fundamental wave pumped power amplification stages. The incident laser pulse (1 kHz repetition rate, 800 nm wavelength, 37 fs pulse duration, and 295 μJ pulse energy) is split using beam splitters, and approximately 10%, 20%, and 70% of the pulse energy are coupled into the seed, preamplification, and power amplification stages, respectively. In the seed stage, a 5 mm thick c-cut sapphire crystal is used for white light supercontinuum generation. In the preamplification stage, a 2 mm thick, type-I (cutting angle of 28.5°) barium metaborate (BBO) crystal converts the fundamental pump pulse to its second harmonic. Then, the frequency-doubled pump is further chirped through a 2 mm thick SF11 glass medium to reduce group velocity mismatch between the pump and signal pulses. The 400 nm pump pulse preamplifies the blue-shifted supercontinuum seed light in a 3 mm type-I (cutting angle of 28.5°) BBO crystal for infrared idler beam. In the power amplification stage, the fundamental (800 nm) laser pulse amplifies the infrared idler output of the preamplification with a 2 mm type-Ⅱ (angle of 27.3°) BBO crystal.Results and DiscussionsThe input pulse power for the supercontinuum seed stage is less than 1 mW, yielding a blue-shifted plateau region (500-700 nm) in the output spectrum. The white light supercontinuum generated by the sapphire crystal forms a stable plateau region in the range of 500-700 nm (Fig. 2). The blue-shifted spectral component has a conversion efficiency of ~5%, and the power is less than 50 μW. In the preamplification stage, the second harmonic (400 nm) pump pulse energy is 7 μJ, which is converted from 36 μJ of the 800 nm fundamental pulse. The wavelength tuning is achieved by controlling the temporal delay between the second harmonic pump light and the blue-shifted supercontinuum seed. Then, different spectral components (520-610 nm) of the supercontinuum seed are amplified as the signal beam and the wavelength of the corresponding idler beam ranges from 1800 to 1200 nm. The gain in the preamplification stage is approximately 2000 times (Fig. 3) and homogeneous for different phase-matching conditions. Using the idler output of the preamplification stage as the seed light for the power amplification stage, the signal pulse in the spectral range of 1160-1800 nm and the idler pulse in the range of 1440-2500 nm are obtained (Fig. 5(a)-(d)). The beam quality factors M2 of the final output signal and idler pulses are 1.91 (X-direction), 1.71 (Y-direction), 1.92 (X-direction), and 1.69 (Y-direction), respectively (Fig. 5(e)-(f)). For the optimized wavelengths of the signal pulse (1450 nm) and idler pulse (1785 nm), corresponding to the crystal cutting angle, the maximum conversion efficiency is 26.6% at 205 μJ pump energy (Fig. 6) and the output power stability (RMS) is approximately 1.8%. We have measured the pulse durations of the output signal (53.2±1.3) fs and idler (58.7±1.5) fs pulses from the power amplification stage (Fig. 7) using a home-built scanning autocorrelator, retaining the ultrashort pulse width of the pump pulse.ConclusionsWe have experimentally demonstrated an ultrafast optical parametric amplification system based on a second harmonic pumped-preamplification stage, realizing the wavelength tunability of the infrared signal pulse in the range of 1160-1800 nm and the idler pulse in the range of 1440-2500 nm. The energy conversion efficiency is approximately 26%, and the pulse width is 50-60 fs. The benefits of this scheme are three folds. First, it reduces the damage of the supercontinuum generation solid medium, improving the reliability of the entire system. Second, it leads to a uniform spectrum of the infrared idler output in the preamplification stage, thus improving the system wavelength tunability in the power amplification stage. Finally, this scheme can provide carrier-envelope phase stabilized-infrared idler output over a broadened spectral range from the power amplification stage.

ConclusionsThis paper presents a method for system design and core device selection, a physical optical-mechanical system is developed for the realization of a rubidium potassium hybrid two-dimensional magneto-optical trap, and the system analysis and experimental verification are carried out. The results show that the light spot size of a long window cooling repumping beam is 13.10 mm, and the divergence angle is 0.39 mrad. The light spot size of a short window cooling repumping beam is 13.27 mm, and the divergence angle is 0.41 mrad. The light spot size of a buffer beam is 10.00 mm, and the divergence angle is 0.13 mrad. The focal spot size of a push beam at the differential pipeline is 2.02 mm. The root mean square radius and the geometric radius of the imaging system are both smaller than those of the Airy spot, and the modulation transfer function of the optical system is very close to the diffraction limit transfer function, indicating that the system has an excellent optical performance. The V-GROOVE design scheme of an optical fiber is proposed to let the four-way beams input optical port at the same time, and the overall physical optical-mechanical structure design also meets less parts used in an optical group, short total optical path, small volume, low power consumption, and light total weight, greatly improving the integration of all optical components and at the same time promoting atomic capture efficiency and the quality of the groups. The atomic loading rate of 1.89×108 /s in the 3D-MOT is achieved.

ConclusionsThis study proposes a spectral simulation method for calibrating the light source of TVM based on a digital micromirror. The skew nature of the spectral simulation unit is analyzed, and Bigaussian, Asym2Sig, and ECS functions are selected to fit the positive, normal, and negative skew distribution spectral simulation units, respectively. The spectral simulation comparison experiment between using the Gaussian function and combining the Bigaussian, ECS, and Asym2Sig functions is performed; the spectral simulation errors of the two functions are 33.4% and 25.2%, respectively. This shows that the Gaussian function can be replaced with a combination of three functions for spectrum simulation. The analysis reveals that the main factors influencing the spectral simulation errors are the presence of superimposed regions of spectral simulation units on the surface of the digital micromirror array and the lack of independent control of each simulation unit. The correctness of the spectral correction method based on the maximum simulation error is demonstrated using the local band of 410-460 nm. The final simulation error of the 2700 K color temperature spectrum in the band of 380-780 nm is 6.2%, which is 4.06 times higher than the overall spectral modulation capability before the correction; simulation accuracy of 2700K color temperature spectrum corrected by experiment satisfies the ICAO observation accuracy requirements for the simulation error of 2700 K color temperature spectra at 50 m baseline and provides a research basis and technical support for establishing TVM calibration and traceability chain that satisfies the definition of MOR and the standard of ICAO.

ObjectiveNitrate is an ionic nutrient that can flow into rivers, lakes, and seas through agricultural fertilizers, organic nitrogen-containing soils, industrial and domestic wastewater, as well as other sources, leading to the eutrophication of water bodies. Therefore, the detection of nitrate content is an important indicator of water quality. The main detection methods for nitrate include ion chromatography and spectrophotometry. Ion chromatography requires professional instruments, resulting in a high cost, whereas spectrophotometry requires pretreatment of the sample, resulting in a complicated and time-consuming process. Surface-enhanced Raman spectroscopy (SERS) has high sensitivity, does not require pretreatment of the sample, and does not experience interference from water. Thus, it is applicable to detect substances in aqueous solution. Presently, nitrate detection in water using SERS has low detection limits, which cannot meet the national environmental quality standards for groundwater. In this study, a composite SERS substrate is reported with positively charged cysteamine modified around negatively charged gold nanoparticles. The reported substrate can directly detect nitrate while reaching the detection standards of Class-I water.MethodsIn this paper, Au-cysteamine composite SERS substrates were prepared using a combination of chemical reduction and self-assembly methods. First, gold nanoparticles were prepared through the reduction of chloroauric acid using trisodium citrate. Second, cysteamine, having strong coordination with metals and charged groups, was used to functionally modify the surrounding negatively charged gold nanoparticles to increase the affinity of gold nanoparticles to nitrate anions and improve the signal-detection sensitivity. Finally, the morphology of the cysteamine and AuNPs-cysteamine composite SERS substrates were characterized by scanning electron microscopy (SEM), and the effects of cysteamine stacking density and gold nanoparticle density on the performance of the substrates were investigated to improve the signal sensitivity to nitrate anions.Results and DiscussionsThe AuNPs-cysteamine composite SERS substrate prepared in this paper has high sensitivity to nitrate anions in water and can meet the detection standard for Class-I water in the national groundwater environmental quality standard. SEM showed a coral-like network formed by the cysteamine soaked in deionized water, which had a uniform distribution and was tightly and uniformly bound to the gold nanoparticles (Fig. 2). The self-assembled coverslip with cysteamine was immersed in 20 mL of gold sol, and the bound cysteamine and gold nanoparticles were more uniformly distributed, which produced more hotspots (Fig. 3). The AuNPs-cysteamine composite SERS substrate performed better in experiments when soaked in deionized water than otherwise, owing to the uniform distribution of cysteamine. The AuNPs-cysteamine composite SERS substrate prepared by the same batch of cysteamine modified with 20 mL of gold sol had moderate detection performance owing to the content of gold nanoparticles, whereas the best performance was obtained after binding with cysteamine (Fig. 4). The detection limit of the AuNPs-cysteamine composite SERS substrate prepared by self-assembly using cysteamine soaked in deionized water and 20 mL of gold sol was 0.01 mg/L (Fig. 5) with an enhancement factor of 2.14×105 (Fig. 6), and the relative standard deviation (RSD) of the potassium nitrate signal on eight different AuNPs-cysteamine composite SERS substrates was 10.36% (Fig. 7), which meets the detection standard of Class-I water.ConclusionsIn this paper, an AuNPs-cysteamine composite SERS substrate for the detection of nitrate in water is prepared. To increase the affinity of gold nanoparticles toward nitrate anions, gold nanoparticles are prepared by chemical reduction, and the signal-detection sensitivity is improved by functionalizing the surrounding negatively charged gold nanoparticles with positively charged cysteamine. The experimental results show that the best enhancement performance is obtained when AuNPs-cysteamine composite SERS substrate is prepared by soaking coverslips with 10-3 mol/L cysteamine for 3 h using deionized water and then modifying 20 mL of gold sol, with an enhancement factor of 2.14×105. The RSD of potassium nitrate SERS signal on eight different AuNPs-cysteamine composite SERS substrates was 10.36%; moreover, the detection limit for standard nitrate solution is 0.01 mg/L. Therefore, the development of AuNPs-cysteamine composite SERS substrate has laid the foundation for the detection of nitrate nitrogen in water by Raman spectroscopy and has a great prospect of application.

ConclusionsIn this study, a white light generation device based on four different lenses is constructed according to the principle of white-light continuum generation and the different focusing characteristics of the achromatic lens and spherical lens at the focal position are discussed. Through theoretical simulation and experimental demonstration, it is concluded that: 1) a more blue-shifted white-light continuum can be achieved using an achromatic lens. When the beam is focused, the achromatic lens can considerably reduce the influence of dispersion, resulting in a smaller focal point radius and uniform photon distribution. When focusing on the sapphire crystal, the photon density at the focal spot is higher than that in the spherical lens at the same energy and the blue shift is visible. 2) Using a short focal lens will afford a blue shift and better spectral stability of the white-light continuum. For the same lens, different focal length affords different focal spot radii and depths at the focus position. When acting on sapphire crystal, the white-light continuum generated by a short focal lens has an obvious blue shift because the short focal lens can afford highly focused energy and uniform photons compared to a long focal lens. Because the short focal lens operating range is shorter and the monofilament channel created in the sapphire crystal is more stable, the white-light continuum has superior stability. The transient absorption of Rhodamine B was studied using four different types of white-light continuum-generating equipment. The blue-shifted white-light continuum is found to be more useful for measuring Rhodamine B's stimulated absorption signal. Therefore, in a transient absorption experiment, a short-focal achromatic lens can be used to assess Rhodamine B sample.

ConclusionsWe have extended the photocurrent model on THz radiation generation from an external DC electric field biased and single-color femtosecond laser induced Ar-gas micro-plasma. The properties of THz radiation under different conditions have been analyzed. The simulation results show that the THz radiation from laser-induced plasma can be significantly enhanced by the introduction of an external DC electric field, which is linearly dependent on DC field. Furthermore, the THz polarization direction follows the direction of the external DC field. The electrons ionized and accelerated by both the laser field and the external DC electric field result in THz radiation, in which the external DC electric field has a dominant contribution. The theoretical and numerical investigations help us deeply understand the THz radiation generation process from DC electric field biased and femtosecond laser ionized gas plasma.