Acquisition and Tracking Technology for Space Laser Communication

The divergence angle of laser beam used in space laser communication is usually no more than 100μrad.Using laser beam with small divergence angle to achieve acquisition and tracking for space laser link has always been a difficult problem.In addition,the random nature of the atmosphere will affect the satellite-ground laser link,which increases the difficulty of the acquisition and stable tracking for the laser link.Thus,taking into account the above challenges for satellite-ground laser communication,an acquisition and tracking scheme of using both beacon beam and signal beam was designed for the Laser Communication Terminal(LCT)of Shijian 20 satellite.In-orbit test results indicated that under the condition of moderate atmospheric turbulence(atmospheric coherence length r0≈3 cm),the process of acquisition and tracking for the satellite-ground laser link can be completed within 1 s after the initial pointing between the LCT and Optical Ground Station(OGS)is performed,and the tracking error was less than 1μrad(3σ).In addition,the laser link can be re-established quickly once being interrupted by unsteady atmospheric turbulence,and can be maintained for a long time under moderate twurbulence conditions,which lays a foundation for future application of satellite-ground laser communication.

Three-dimensional simulation method of multipactor in microwave components for high-power space application

Based on the particle-in-cell technology and the secondary electron emission theory, a three-dimensional simulation method for multipactor is presented in this paper. By combining the finite difference time domain method and the panicle tracing method, such an algorithm is self-consistent and accurate since the interaction between electromagnetic fields and particles is properly modeled. In the time domain aspect, the generation of multipactor can be easily visualized, which makes it possible to gain a deeper insight into the physical mechanism of this effect. In addition to the classic secondary electron emission model, the measured practical secondary electron yield is used, which increases the accuracy of the algorithm. In order to validate the method, the impedance transformer and ridge waveguide filter are studied. By analyzing the evolution of the secondaries obtained by our method, multipactor thresholds of these components are estimated, which show good agreement with the experimental results. Furthermore, the most sensitive positions where multipactor occurs are determined from the phase focusing phenomenon, which is very meaningful for multipactor analysis and design.

Optical Nonlinearity of Violet Phosphorus and Applications in Fiber Lasers

A D-shaped fiber is coated with a new two-dimensional nanomaterial,violet phosphorus(VP),to create a saturable absorber(SA)with a modulation depth of 3.68%.Subsequently,the SA is inserted into a fiber laser,enabling successful generation of dark solitons and bright–dark soliton pairs through adjustment of the polarization state within the cavity.Through further study,mode-locked pulses are achieved,proving the existence of polarization-locked vector solitons.The results indicate that VP can be used as a polarization-independent SA.

Developing polydopamine modified molybdenum disulfide/epoxy resin powder coatings with enhanced anticorrosion performance and wear resistance on magnesium lithium alloys

Epoxy resin powder coating has been successfully applied on the corrosion protection of magnesium lithium alloys.However,poor wear resistance and microcracks formed during the solidification have limited it extensive application.There are limited approaches to exploit such anti-corrosion and mechanical properties of magnesium lithium alloys.Herein,the epoxy resin powder coating with polydopamine modified molybdenum disulfide(MoS_(2)@PDA-EP powder coating with 0,0.1,0.2,0.5,1.0 wt.%loading)was well prepared by melt extrusion to investigate its anticorrosion performance and wear resistance.The results revealed that the addition of MoS_(2)@PDA enhanced the adhesion strength between coatings and alloys,wear resistance and corrosion protection of the powder coatings.Among them,the optimum was obtained by 0.2 wt.%MoS_(2)@PDA-EP powder coating which could be attributed to well dispersion and efficient adhesion with coating matrix.To conclude,MoS_(2)@PDA-EP powder coating is meaningfully beneficial for the anticorrosive and wear performance improvement of magnesium lithium alloys.

Fano Resonance Enabled Infrared Nano-Imaging of Local Strain in Bilayer Graphene

Detection of local strain at the nanometer scale with high sensitivity remains challenging.Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal,intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency.The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon.The activated phonon further interacts with continuum electronic transitions,and generates a strong Fano resonance.The resulted Fano resonance features a very sharp near-field infrared scattering peak,which leads to an extraordinary sensitivity of-0.002%for the strain detection.Our results demonstrate the first nano-scale near-field Fano resonance,provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.

Validation and accuracy analysis of wind products from scatterometer onboard the HY-2B satellite

The Chinese marine dynamic environment satellite HY-2B was launched in October 2018 and carries a Ku-band scatterometer.This paper focuses on the accuracies of HY-2B scatterometer wind data during the period from November 2018 to May 2021.The HY-2B wind data are validated against global moored buoys operated by the U.S.National Data Buoy Center and Tropical Atmosphere Ocean,numerical model data by the National Centers for Environmental Prediction,and the Advanced Scatterometer data issued by the Remote Sensing System.The results showed that the wind speeds and directions observed by the HY-2B scatterometer agree well with these buoy wind measurements.The root-mean-squared errors(RMSEs)of the HY-2B wind speed and direction are 0.74 m/s and 11.74°,respectively.For low wind speeds(less than 5 m/s),the standard deviation of the HY-2B-derived wind direction is higher than 20°,which implies that the HY-2B wind direction for low wind speeds is less accurate than that for moderate to high wind speed ranges.The RMSE of the HY-2B wind speed is slightly larger in high latitude oceans(60°–90°S and 60°–90°N)than in low latitude regions.Furthermore,the dependence of the residuals on the cross-track location of wind vector cells and the stability of the HY-2B scatterometer wind products are discussed.The wind stability assessment results indicate that a clear yearly oscillation is observed for the HY-2B wind speed bias which is due to seasonal weather variations.In general,the accuracy of HY-2B winds meets the operational precision requirement and is consistent with other wind data.

Satellite Autonomous Integrity Monitoring of BDS and Onboard Performance Evaluation

With the development of satellite navigation technology,the user demands for the integrity of Global Navigation Satellite System(GNSS)have increased more and more.A ground-based monitoring system can hardly report an alarm message to GNSS users during the valid alarming period due to the satellite-Earth propagation delay.It is beneficial to monitor abnormal events and report the corresponding alarms from orbit.Adopting this approach,which is an important feature for future GNSS integrity monitoring,the time needed to provide an alarm is shorter and the system integrity capability is strengthened.The BeiDou Navigation Satellite System(BDS)new generation satellites have the capabilities of satellite autonomous integrity monitoring(SAIM).This paper presents the technical scheme of SAIM,and proposes the integrity monitoring method of both navigation signals and the clocks onboard.The proposed method was verified through the onboard test on the BDS satellites.In addition,we analyzed the integrity telemetry data from the new generation of BDS satellite,including signal delay,power,carrier phase measurement,correlation peak,consistency of pseudo-code and carrier phase,clock phase and frequency step.The analysis results indicated that the quality of the data on orbit met the requirements,and SAIM could monitor effectively any abnormal change of satellite clocks and navigation signal,generate rapidly an alarm message,and transmit it to the user.The alarm time was less than 6 s through the message,and 2 s through non-standard code(NSC).Finally,we present future opportunities for improving the SAIM technology of BDS.

Method of evaluating interface traps in Al2O3/AlGaN/GaN high electron mobility transistors

In this paper, the interface states of the AlGaN/GaN metal–insulator–semiconductor(MIS) high electron mobility transistors(HEMTs) with an Al2 O3 gate dielectric are systematically evaluated. By frequency-dependent capacitance and conductance measurements, trap density and time constant at Al2 O3/AlGaN and AlGaN/GaN interface are determined.The experimental results reveal that the density of trap states and the activation energy at the Al2 O3/AlGaN interface are much higher than at the AlGaN/GaN interface. The photo-assisted capacitance-voltage measurements are performed to characterize the deep-level traps located near mid-gap at the Al2 O3/AlGaN interface, which indicates that a density of deep-level traps is lower than the density of the shallow-level states.

Effects of O_(2)adsorption on secondary electron emission properties

The surface adsorption of gas molecules is a key factor limiting the secondary electron yield(SEY)of a material in many areas of applied physics.The influence of O_(2)adsorption on the SEY of metallic Ag is investigated in this work.To account for the particle distribution,we propose a BET theory based on multilayer O_(2)physisorption model.Furthermore,based on the phenomenological model of secondary electron(SE)emission and by taking into account the different scattering processes between electrons and particles in the adsorbed layer,we develop a numerical model of SEY in the adsorbed state using Monte Carlo simulations.The relationships among O_(2)adsorption,adsorption layer thickness,and SEY variation characteristics are then examined through a series of experiments.After 12-h exposure to O_(2),the clean samples increases12%-19%of the maximum value of SEY and 2.3 nm in thickness of the adsorbed layer.Experimental results are also compared with the results from the MC model to determine whether the model is accurate.

Analysis of secondary electron emission using the fractal method

Based on the rough surface topography with fractal parameters and the Monte–Carlo simulation method for secondary electron emission properties, we analyze the secondary electron yield(SEY) of a metal with rough surface topography. The results show that when the characteristic length scale of the surface, G, is larger than 1 × 10^(-7), the surface roughness increases with the increasing fractal dimension D. When the surface roughness becomes larger, it is difficult for entered electrons to escape surface. As a result, more electrons are collected and then SEY decreases. When G is less than 1 × 10^(-7),the effect of the surface topography can be ignored, and the SEY almost has no change as the dimension D increases. Then,the multipactor thresholds of a C-band rectangular impedance transfer and an ultrahigh-frequency-band coaxial impedance transfer are predicted by the relationship between the SEY and the fractal parameters. It is verified that for practical microwave devices, the larger the parameter G is, the higher the multipactor threshold is. Also, the larger the value of D,the higher the multipactor threshold.

Effect of Cu doping on the secondary electron yield of carbon films on Ag-plated aluminum alloy

Reducing the secondary electron yield(SEY)of Ag-plated aluminum alloy is important for high-power microwave components.In this work,Cu doped carbon films are prepared and the secondary electron emission characteristics are studied systematically.The secondary electron coefficientδ_(max) of carbon films increases with the Cu contents increasing at first,and then decreases to 1.53 at a high doping ratio of 0.645.From the viewpoint of surface structure,the higher the content of Cu is,the rougher the surface is,since more cluster particles appear on the surface due to the small solid solubility of Cu in the amorphous carbon network.However,from viewpoint of the electronic structure,the reduction of the sp2 hybrid bonds will increase the SEY effect as the content of Cu increases,due to the decreasing probability of collision with free electrons.Thus,the two mechanisms would compete and coexist to affect the SEY characteristics in Cu doped carbon films.

Design and VHDL simulation of a special high speed spaceborne bus controller

The application of high speed data bus on the satellites having several remote sensors could not only satisfy the demand of high speed data transfer,but also could reuse the equipment at both the com-ponent and subsystem levels for other space missions,thus reducing the costs of on-board processing system integration.In this article,a special high speed data bus with simple interfaces is chosen for transferring the data produced by remote sensors and other instrumentations.First,the function and the structure of the data bus are discussed carefully.And then,the bus controller,which is a key mod-ule of the data bus,is designed.At last,the function and performance of the bus controller are verified by VHDL simulation.

Detection of trace metals in water by filamentand plasma-grating-induced breakdown spectroscopy

Filament-and plasma-grating-induced breakdown spectroscopy(F-GIBS)was demonstrated as an efficient technique for sensitive detection of metals in water,where plasma gratings were established through synchronized nonlinear interaction of two noncollinear filaments and an additional filament was generated with another fs laser beam propagating along their bisector.A water jet was constructed vertically to the three coplanar filaments,overcoming side effects from violent plasma explosion and bubble generation.Three distinct regimes of different mechanisms were validated for nonlinear couplings of the third filament with plasma gratings.As the third filament was temporally overlapped with the two noncollinear filaments in the interaction zone,all the three filaments participated in synchronous nonlinear interaction and plasma grating structures were altered by the addition of the third filament.As the third filament was positively or negatively delayed,the as-formed plasma gratings were elongated by the delayed third filament,or plasma gratings were formed in the presence of plasma expansion of the ahead third filament,respectively.Using F-GIBS for trace metal detection in water,significant spectral line enhancements were observed.

Observation of regular pulse train in a narrow-band optoelectronic oscillator

We have experimentally observed a new operating state of a regular pulse train in a narrow-band optoelectronic oscillator(OEO) system, where the direct-current(DC) bias of the Mach-Zehnder modulator is set at the maximum value of the transmission transfer function instead of the usual quadrature point. The observed quasi-steady-state pulse train is distinctly periodic, with a period of 10.5 μs and a center frequency of 10 GHz, and resembles a mode-locked OEO in its waveform. The formation of regular pulses here may arise from the dynamic balance of nonlinearity and narrow-band filter effects, with the transient characteristics of the pulses arising mainly from instabilities between the gain and cavity loss. Our results are of great importance for deepening the understanding of the nonlinear dynamical processes in OEO systems.

Theoretical and experimental studies of retro-reflective antenna array for microwave power transmission from space solar power satellite to earth

The development of space solar power satellites aims to harvest solar power by artificial satellites over the earth’s geostationary orbit and then deliver the harvested power to the earth wirelessly.The retro-reflective antenna array technique is believed to be a close-to-optimal technical approach to achieve efficient wireless power transmission from a geostationary satellite to the earth,as it is capable of generating a microwave power beam aiming at a ground station on the earth via analyzing a pilot signal broadcasted by the ground station.In this paper,some of our preliminary research outcomes on retro-reflective antenna array for space solar power applications are reported.In the theoretical part of this paper,closed-form formulations with precision better than the classic theory of phased array are derived to analyze the performance of retro-reflective antenna array when the far zone condition is not satisfied between the space solar power satellite and the ground station.In the experimental part of this paper,a bench-scale retro-reflective antenna array with physical dimensions of about 0.6 m by 0.6 m is fabricated and tested.The theoretical and experimental results demonstrate that the microwave beam generated by a satellite-borne retro-reflective antenna array could be adjusted in real time to aim at the location from which the pilot signal stems.Based on the theoretical and experimental studies of this paper,systematic research endeavors are being conducted on the retro-reflective antenna array for space solar power applications.

Visible light communication system at 3.59 Gbit/s based on c-plane green micro-LED

Visible light communication(VLC)based on the micro light emitting diode(micro-LED)has attracted increasing attention owing to its high bandwidth,low power consumption,and high security.Compared with semi-polar or non-polar micro-LEDs,the commercial polar micro-LED has the advantages of low cost and more mature epitaxy technique.In this study,green micro-LEDs with different indium tin oxide(ITO)sizes are fabricated based on the commercial c-plane LED epitaxial wafer.The transmission performance of 80,100,and 150μm devices has been studied in detail.A partial pre-equalization scheme is utilized to increase data rates.Finally,the VLC system with a 100μm green micro-LED as the transmitter could achieve a maximum data rate of 3.59 Gbit/s.Such a result will be beneficial to promote the further development of low-cost,high-speed VLC devices in the future.

A reliable sunlight communication system

A sunlight communication system is proposed that uses Sr2Si5N8:Eu2+phosphors to concentrate sunlight signals in strong background light noise;thus,a wide spectrum sunlight communication system is converted into a narrow spectrum one.A communication method is proposed to enable compression to the dark line H-α(656.28 nm)spectrum.A 50% solar energy conversion efficiency is achieved with a 0.3μs code delay,a0.2μs code rise time(20%–80%),and a 96% optical transmittance.Experimental results show that phosphors enhance the sunlight intensity 1.5 times with the same distance.This method has immense potential in future long-distance sunlight communication.

Efforts towards a low-temperature-sensitive physics package for vapor cell atomic clocks

Strong environmental dependence is an intractable problem for vapor cell clocks,for which the high-temperature sensitivity of the physics package is considered one of the dominant reasons.In this paper,we report the design and realization of a low-temperature-sensitive physics package for vapor cell clocks.The physics package comprises three layers of magnetic shields,three layers of heating ovens,and the cavity-cell assembly.The cavity-cell assembly employs a compact magnetron-type cavity and a Rb vapor cell sealed with N2-Ar mixed buffer gas.The dependence of the clock frequency on temperature fluctuation is evaluated to be 2×10^(−11)/℃.In pursuit of the stable temperature,a three-stage temperature regulator is implemented on the physics package.It adopts a combination of open andclosed-loop control to address the problem of significant thermal coupling between the heating ovens.Under a laboratory environment,the measured Hadamard deviation of the temperature variation is 4×10^(−5)℃in 1 day of averaging.