The second fusion of laser and aerospace-an inspiration for high energy lasers

Since the first laser was invented,the pursuit of high-energy lasers(HELs)has always been enthusiastic.The first revolution of HELs was pushed by the fusion of laser and aerospace in the 1960s,with the chemical rocket engines giving fresh impetus to the birth of gas flow and chemical lasers,which finally turned megawatt lasers from dream into reality.Nowadays,the development of HELs has entered the age of electricity as well as the rocket engines.The properties of current electric rocket engines are highly consistent with HELs’goals,including electrical driving,effective heat dissipation,little medium consumption and extremely light weight and size,which inspired a second fusion of laser and aerospace and motivated the exploration for potential HELs.As an exploratory attempt,a new configuration of diode pumped metastable rare gas laser was demonstrated,with the gain generator resembling an electric rocket-engine for improved power scaling ability.

Improved Measurement Performance of Inorganic Elements in Coal by Laser-Induced Breakdown Spectroscopy Coupled with Internal Standardization

Laser-induced breakdown spectroscopy was employed to determine the inorganic elements in coal. To improve the measurement＇s accuracy and precision, a new internal standardization scheme, which we named changed internal standardization, was compared with the traditional internal standardization and no internal standardization for the analysis of inorganic elements. The new internal standardization scheme used the atomic line of carbon at 247.86 nm and the molecular band of CN at 388.34 nm and C2 at 516.32 nm to normalize the lines of inorganic elements that were distributed in the same spectral channel. The performance of the utilization of the new internal standardization scheme was evaluated using a set of coal samples, including twenty calibration samples and five validation samples. The results show that the coefficients of determination R2 and the slope of calibration models coupled with changed internal standard- ization are better than that of the calibration models coupled with fixed internal standardization and no internal standardization. Moreover, the measurement accuracy and reproducibility of changed internal standardization for the analysis of five validation samples also yielded further improvement. The results that we obtained suggest that changed internal standardization could compensate for the matrix effects, as well as the influence of the difference in the spectral response of the light collection system.

Reduced graphene oxide aerogel decorated with Mo_(2)C nanoparticles toward multifunctional properties of hydrophobicity,thermal insulation and microwave absorption

Reduced graphene oxide(rGO)aerogels are emerging as very attractive scaffolds for high-performance electromagnetic wave absorption materials(EWAMs)due to their intrinsic conductive networks and intricate interior microstructure,as well as good compatibility with other electromagnetic(EM)components.Herein,we realized the decoration of rGO aerogel with Mo_(2)C nanoparticles by sequential hydrothermal assembly,freeze-drying,and high-temperature pyrolysis.Results show that Mo_(2)C nanoparticle loading can be easily controlled by the ammonium molybdate to glucose molar ratio.The hydrophobicity and thermal insulation of the rGO aerogel are effectively improved upon the introduction of Mo_(2)C nanoparticles,and more importantly,these nanoparticles regulate the EM properties of the rGO aerogel to a large extent.Although more Mo_(2)C nanoparticles may decrease the overall attenuation ability of the rGO aerogel,they bring much better impedance matching.At a molar ratio of 1:1,a desirable balance between attenuation ability and impedance matching is observed.In this context,the Mo_(2)C/r GO aerogel displays strong reflection loss and broad response bandwidth,even with a small applied thickness(1.7 mm)and low filler loading(9.0wt%).The positive effects of Mo_(2)C nanoparticles on multifunctional properties may render Mo_(2)C/r GO aerogels promising candidates for high-performance EWAMs under harsh conditions.

Spectroscopic and radiation-resistant properties of Er,Pr:GYSGG laser crystal operated at 2.79 μm

We demonstrate the spectroscopic and laser performance before and after 100 Mrad gamma-ray irradiation on an Er,Pr：GYSGG crystal grown by the Czochralski method. The additional absorption of Er,Pr：GYSGG crystal is close to zero in the 968 nm pumping and 2.7-3 μm laser wavelength regions. The lifetimes of the upper and lower levels show faint decreases after gamma-ray irradiation. The maximum output powers of 542 and 526 mW with the slope efficiencies of 17.7% and 17.0% are obtained, respectively, on the GYSGG/Er,Pr：GYSGG composite crystal before and after the gammaray irradiation. These results suggest that Er,Pr：GYSGG crystal as a laser gain medium possesses a distinguished antiradiation ability for application in space and radiant environments.

Theoretical analysis of the optical rotational Doppler effect under atmospheric turbulence by mode decomposition

The optical rotational Doppler effect associated with orbital angular momentum provides a new means for rotational velocity detection.In this paper,we investigate the influence of atmospheric turbulence on the rotational Doppler effect.First,we deduce the generalized formula of the rotational Doppler shift in atmospheric turbulence by mode decomposition.It is found that the rotational Doppler signal frequency spectrum will be broadened,and the bandwidth is related to the turbulence intensity.In addition,as the propagation distance increases,the bandwidth also increases.And when C_(n)^(2)≤5×10^(-15)m^(-2/3)and 2z≤2 km,the rotational Doppler signal frequency spectrum width d and the spiral spectrum width d_(0)satisfy the relationship d=2d_(0-1).Finally,we analyze the influence of mode crosstalk on the rotational Doppler effect,and the results show that it destroys the symmetrical distribution of the rotational Doppler spectrum about 2l·Ω/2π.This theoretical model enables us to better understand the generation of the rotational Doppler frequency and may help us better analyze the influence of the complex atmospheric environment on the rotational Doppler frequency.

Efficient single-pixel imaging encrypted transmission based on 3D Arnold transformation

Single-pixel imaging(SPI)can transform 2D or 3D image data into 1D light signals,which offers promising prospects for image compression and transmission.However,during data communication these light signals in public channels will easily draw the attention of eavesdroppers.Here,we introduce an efficient encryption method for SPI data transmission that uses the 3D Arnold transformation to directly disrupt 1D single-pixel light signals and utilizes the elliptic curve encryption algorithm for key transmission.This encryption scheme immediately employs Hadamard patterns to illuminate the scene and then utilizes the 3D Arnold transformation to permutate the 1D light signal of single-pixel detection.Then the transformation parameters serve as the secret key,while the security of key exchange is guaranteed by an elliptic curve-based key exchange mechanism.Compared with existing encryption schemes,both computer simulations and optical experiments have been conducted to demonstrate that the proposed technique not only enhances the security of encryption but also eliminates the need for complicated pattern scrambling rules.Additionally,this approach solves the problem of secure key transmission,thus ensuring the security of information and the quality of the decrypted images.

Analytical Expressions of Dual-Frequency Plasma Diagnostic Theory

Based on the fundamental ideas concerning microwave attenuation in plasma, we obtain a new expression of transmission attenuation of microwaves as a function of the incident wave frequency. And with reasonable hypothesis, analytical forms of the electron density and the electron-neutral collision frequency are derived from the equations of the transmission attenuation of microwaves at two near frequencies. This method gives an effective and easy approach to diagnose the unmagnetized plasma.

Dimensional Gradient Structure of CoSe2@CNTs-MXene Anode Assisted by Ether for High-Capacity,Stable Sodium Storage

Recently,abundant resources,low-cost sodium-ion batteries are deemed to the new-generation battery in the field of largescale energy storage.Nevertheless,poor active reaction dynamics,dissolution of intermediates and electrolyte matching problems are significant challenges that need to be solved.Herein,dimensional gradient structure of sheet-tube-dots is constructed with CoSe2@CNTs-MXene.Gradient structure is conducive to fast migration of electrons and ions with the association of ether electrolyte.For half-cell,CoSe2@CNTs-MXene exhibits high initial coulomb efficiency(81.7%)and excellent cycling performance(400 mAh g^-1 cycling for 200 times in 2 Ag^−1).Phase transformation pathway from crystalline CoSe2-Na2Se with Co and then amorphous CoSe2 in the discharge/charge process is also explored by in situ X-ray diffraction.Density functional theory study discloses the CoSe2@CNTs-MXene in ether electrolyte system which contributes to stable sodium storage performance owing to the strong adsorption force from hierarchical structure and weak interaction between electrolyte and electrode interface.For full cell,CoSe2@CNTs-MXene//Na3V2(PO4)3/C full battery can also afford a competitively reversible capacity of 280 mAh g^−1 over 50 cycles.Concisely,profiting from dimensional gradient structure and matched electrolyte of CoSe2@CNTs-MXene hold great application potential for stable sodium storage.

Analysis of optical properties of bio-smoke materials in the 0.25–14 μm band

At present, research into optical properties of bio-smoke materials mostly concentrates on single band or single germplasm. Herein, we measured the spectral reflectance of three eukaryotic bio-smoke materials and three prokaryotic bio-smoke materials in the waveband from 0.25 μm to 14μm. Based on the Kramers-Kroning algorithm, the complex refractive index m(λ) was calculated and the Fourier-transform infrared(FTIR) spectra of materials were analyzed. The results show that n(λ) of bio-smoke materials varies between 1.1-2, and n(λ) values in the visible light to near-infrared wavebands are significantly larger than those in other wavebands. The k(λ) of bio-smoke materials varies between 0-0.4.At 6-6.5 μm, k(λ) of prokaryotic materials is 3 times that of eukaryotic materials, which is caused by C=O stretching vibration of amide I and C-N stretching vibration of amide Ⅱ in proteins. At 2.5-3 μm and 9.75 μm, k(λ) values of eukaryotic bio-smoke materials are nearly 2 times that of prokaryotic ones. The absorption peak at 2.5-3 μm is mainly triggered by C-H stretching vibration in lipid and O-H stretching vibration in bound water. The absorption peak at 9.75 μm is mainly caused by symmetric stretching vibration of PO2-in nucleic acids.

Influence of moderate-to-strong anisotropic non-Kolmogorov turbulence on intensity fluctuations of a Gaussian–Schell model beam in marine atmosphere

The scintillation index（SI） of a Gaussian–Schell model（GSM） beam in a moderate-to-strong anisotropic nonKolmogorov turbulent atmosphere is developed based on the extended Rytov theory. The on-axis SI in a marine atmosphere is higher than that in a terrestrial atmosphere, but the off-axis SI exhibits the opposite trend. The on-axis SI first increases and then begins to decrease and saturate as the turbulence strength increases. Turbulence inner and outer scales have different effects on the on-axis SI in different turbulent fluctuation regions. The anisotropy characteristic of atmospheric turbulence leads to the decline in the on-axis SI, and the rise in the off-axis SI. The on-axis SI can be lowered by increasing the anisotropy of turbulence, wavelength, and source partial coherence before entering the saturation attenuation region. The developed model may be useful for evaluating ship-to-ship/shore free-space optical communication system performance.

An All-Fiber Gas Raman Light Source Based on a Hydrogen-Filled Hollow-Core Photonic Crystal Fiber Pumped with a Q-Switched Fiber Laser

A gas Raman light source based on a H2−filled hollow-core photonic-crystal-fiber cell with a Q-switched fiber laser followed by a fiber amplifier as the Raman pump source is demonstrated.The Stokes frequency-shift lasing line is observed at 1135.7 nm with the Q-switched pump pulses at 1064.7 nm.Our experimental results show that the generated Stokes pulse is much narrower than the pump pulse,and the generated Stokes pulse duration is increased with the single pulse energy for the same duration pump pulses.For the 125 ns pump pulses with a repetition rate of 5 kHz,the Raman threshold pump energy and the conversion efficiency at the Raman threshold are 2.13µJ and 9.82%.Moreover,by choosing narrower pump pulses,the Raman threshold pump energy may be reduced and the conversion efficiency may be improved.

CLUSTERING OF DOA DATA IN RADAR PULSE BASED ON SOFM AND CDBW

Clustering is the main method of deinterleaving of radar pulse using multi-parameter.However,the problem in clustering of radar pulses lies in finding the right number of clusters.To solve this problem,a method is proposed based on Self-Organizing Feature Maps(SOFM) and Composed Density between and within clusters(CDbw).This method firstly extracts the feature of Direction Of Arrival(DOA) data by SOFM using the characteristic of DOA parameter,and then cluster of SOFM.Through computing the cluster validity index CDbw,the right number of clusters is found.The results of simulation show that the method is effective in sorting the data of DOA.

Noise-like rectangular pulses in a mode-locked double-clad Er:Yb laser with a record pulse energy

Generation of noise-like rectangular pulse was investigated systematically in an Er–Yb co-doped fiber laser based on an intra-cavity coupler with different coupling ratios.When the coupling ratio was 5/95,stable mode-locked pulses could be obtained with the pulse packet duration tunable from 4.86 ns to 80 ns.The repetition frequency was 1.186 MHz with the output spectrum centered at 1.6μm.The average output power and single pulse energy reached a record 1.43 W and1.21μJ,respectively.Pulse characteristics under different coupling ratios(5/95,10/90,20/80,30/70,40/60)were also presented and discussed.

High-power microwave propagation properties in the argon plasma array

The argon plasma induced by the L-/C-band high-power microwave(HPM) is investigated theoretically and experimentally. Influences of the microwave power, pulse width, polarization and the plasma electron density on the protection performance of the plasma array against HPM are studied. The results show that the effect of HPM is caused by energy accumulation, with the gas breakdown emerging only after a short time. The attenuation of the wave by the plasma array with the tubes off can reach approximately 23 dB at 1.3 GHz. It can also be obtained that the protection performance of the plasma array against the TE wave is better than that against the TM one. The plasma array shows better protection performance in the L-band than in the C-band. In addition,the attenuation of 5.6 GHz HPM can reach 30 dB when the tubes are turned on in the experiment.The research shows that the plasma array has protection ability against HPM.

Research on Complex Refraction Indices of Expanded Graphite

The expanded graphite (EG) with a low density and better extinction performance can be used in military as passive jamming material in IR and MMW bands. Its complex refractive index is a significant parameter for the extinction property. This paper presents a method to calculate the complex refractive index of EG. The reflection spectra of EG pellets were measured in the 0.24-2.6μm and 2.5-25μm bands, respectively. Based on the measurement results, the complex refractive index of EG in 5-10μm band was calculated by using Kramers-Kronig(K-K) relation and Bruggeman effective medium theory, and then the errors were analyzed. The results indicate that it is feasible to calculate the complex refractive index of EG based on its IR reflection spectra data.

Evaluation of infrared camouflage effectiveness via a multi-fractal method

This paper reports an alternative approach to the evaluation of infrared camouflage effectiveness via a multi-fractal method. By calculating multi-fractal spectra of the target region and the background regions in an infrared image, the spectrum shape features and the discrete Frechet distances among these spectra were used to analyze the camouflage effectiveness of the target qualitatively and quantitatively,and the correlation coefficients of the spectra were further used as the index of camouflage effectiveness.It was found that the camouflaged target had better camouflage effectiveness than the target without camouflage in the same one background, and the same one camouflaged target had different camouflage effectiveness in different backgrounds. On the whole, the target matching well with its background had high camouflage effectiveness value. This approach can expand the application of multi-fractal theory in infrared camouflage technology, which should be useful for the research of infrared camouflage materials, the design of camouflage patterns as well as the deployment of military equipment in battlefield.

Design and Realization of a Novel Compact Electromagnetic Band-Gap Structure

A novel compact electromagnetic bandgap （EBG） structure constructed by etching two reverse split rings （RSRs） and inserting interleaving edge （IE） on the patch of conventional mushroom-like EBG （CML-EBG） is investigated. Simulated dispersion diagrams show that the proposed EBG structure presents a 13.6% size reduction in the center frequency of the bandgap. Two comparisons have been carried out for the analysis of the effect of the RSRs and IE configuration. Then, a sample of this novel EBG is fabricated and tested, further experimental data agree well with the simulated results. Thus, this EBG structure makes a good candidate to decrease mutual coupling in compact microstrip patch array.

Two-dimensionally controllable DSR generation from dumbbell-shaped mode-locked all-fiber laser

An all-fiber dumbbell-shaped dual-amplifier mode-locked Er-doped laser that can function in dissipative soliton resonance(DSR)regime is demonstrated.A nonlinear optical loop mirror(NOLM)and a nonlinear amplifying loop mirror(NALM)are employed to initiate the mode-locking pulses.Unlike conventional single-amplifier structure,the output peak power of which remains unchanged when pump power is varied,the proposed structure allows its output peak power to be tuned by changing the pump power of the two amplifiers while the pulse duration is directly determined by the amplifier of nonlinear amplifying loop mirror.The entire distribution maps of peak power and pulse duration clearly demonstrate that the two amplifiers are related to each other,and they supply directly a guideline for designing tunable peak power DSR fiber laser.Pulse width can change from 800 ps to 2.6 ns and peak power varies from 13 W to 27 W.To the best of our knowledge,the peak power tunable DSR pulse is observed for the first time in dumbbell-shaped Er-doped all-fiber mode-locked lasers.

Simulation of propagation of the HPM in the low-pressure argon plasma

The propagation of the high-power microwave（HPM） with a frequency of 6 GHz in the lowpressure argon plasma was studied by the method of fluid approximation.The two-dimensional transmission model was built based on the wave equation,the electron drift-diffusion equations and the heavy species transport equations,which were solved by means of COMSOL Multiphysics software.The simulation results showed that the propagation characteristic of the HPM was closely related to the average electron density of the plasma.The attenuation of the transmitted wave increased nonlinearly with the electron density.Specifically,the growth of the attenuation slowed down as the electron density increased uniformly.In addition,the concrete transmission process of the HPM wave in the low-pressure argon plasma was given.

Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser

We fabricate a pair of fiber Bragg gratings(FBGs)by a visible femtosecond laser phase mask scanning technique on passive large-mode-area double-cladding fibers for multi-kilowatt fiber oscillators.The bandwidth of high-reflection(HR)and lowreflection(LR)FBG is~1.6 nm and 0.3 nm,respectively.The reflection of the HR-FBG is higher than 99%,and that of the LR-FBG is about 10%.A bidirectional pumped all-fiber oscillator is constructed using this pair of FBGs,a record output power of 5027 W located in the signal core is achieved with a slope efficiency of~82.1%,and the beam quality factor M2is measured to be~1.6 at the maximum power.The FBGs are simply fixed on a water cooling plate without a special package,and the thermal efficiency of the HR-FBG and the LR-FBG is 2.76℃/kW and 1℃/kW,respectively.Our research provides an effective solution for robust high-power all-fiber laser oscillators.