Development and applications of gain-switched fiber lasers[Invited]

We briefly review the development of gain-switched rare-earth-doped fiber lasers and their applications in wavelength conversion to mid-IR,supercontinuum generation,and medicine in recent years.We illustrate the similarities between gain-switching and Q-switching techniques that will provide tools for the design and optimization of the gain-switched fiber lasers.From the nature of the gain-switched fiber lasers,benefits of this kind of lasers to 2-μm region and in-band-pumped(two-level system)laser systems are obvious.Advantages of in-band-pumped 2-μm lasers are discussed and analyzed with a simple numerical simulation in terms of Tm-doped fiber lasers.We also propose the key factors in the development of the gain-switched fiber lasers and predict the future tendency.

Numerical analysis of hypersonic thermochemical non-equilibrium environment for an entry configuration in ionized flow

A theoretical methodology for thermochemical non-equilibrium flow combing with the HLLC(Harten-Lax-van Leer Contact) scheme was applied to study the hypersonic thermochemical non-equilibrium environment of an entry configuration in ionized flow. A two-temperature controlling model was utilized and the Gupta’s 11 species(N 2, O2, NO, O, N, NO+, N+2, O+2, N+, O+, e)thermochemical non-equilibrium model was taken. Firstly, numerical calculations of hypersonic thermochemical non-equilibrium environments for different aerodynamic shapes were carried out to verify the reliability of the method above. Then, the method was used to research the effects of ionization and wall catalysis on the hypersonic thermochemical non-equilibrium environment of the entry configuration in ionized flow. The shock stand-off distance can be reduced by thermochemical reactions but doesn’t continue to decrease significantly when ionization occurs. The shock stand-off distance calculated by the 11 species model is 4.2% smaller than that calculated by the 5 species(N2, O2, NO, O, N) thermochemical non-equilibrium model without considering ionization.Ionization reduces wall heat flux but increases wall pressure a little. The effect of ionization on aerothermal loads is greater than that of aerodynamic loads. The thermochemical reactions of electrons and ions catalyzed at the wall increase wall heat flux significantly but make a small change in wall pressure. The maximum wall heat flux obtained by only considering the electrons and ions catalyzed at the partially catalytic wall condition is 11.8% less than that calculated at the supercatalytic wall condition.

一种基于OCT图像的深度学习I和II型wAMD辅助分型方法

目的:探讨基于光学相干断层扫描(OCT)图像的深度学习算法用于湿性年龄相关性黄斑变性(wAMD)的分型的可行性以达到辅助眼科疾病诊断的目的。方法:数据使用2018年6月至2019年6月在宁波市眼科医院门诊确诊为wAMD患者39例(46眼)。首先将资深眼科专家提供的每例患者AMD等级作为分型算法的金标准,再使用Resnet34模型结构输出预测分型结果并与金标准对比,不断进行参数微调直到损失收敛,最终实现自动判断患者疾病等级。结果:使用已训练的模型进行所有测试病例的预测,基于正确的病例数与所有测试病例数的比例作为算法最终的准确率,结果表明深度学习网络方法分型准确率,与普通医师wAMD分型准确率相比,高出20%。通过梯度加权的类激活映射可视化模型诊断依据可作为临床医师诊断的参考。结论:深度学习经OCT图像数据训练后对wAMD分型的准确率明显高于普通医师对wAMD的分型准确率。基于深度学习算法的wAMD分型结果可用于疾病的辅助诊断,缓解国内专业眼科医师紧缺的现状。

Toward photocatalytic hydrogen generation over BiVO_(4) by controlling particle size

Owing to excellent light absorption and high activity fo r oxygen evolution,monoclinic bismuth vanadate(BiVO_(4)) is regarded as an ideal candidate for photocatalytic water splitting.However,its application is limited by the large particle size in micrometer scale,as well as the slightly positive conduction band.In this work,we successfully synthesized nano-BiVO_(4) with particle size ranged from 27 nm to 57 nm by wet chemical method based on electrostatic spinning method.Unlike bulk BiVO_(4),the nano-sized BiVO_(4) possesses the ability to generate hydrogen by water splitting,and the activity could reach up to1.66 μmol h^(-1) g^(-1) with the assistance of Pt.The enhanced activity is mainly attributed to the improvements resulted from reduced particle size,which includes elevated conduction band,enlarged specific surface area and promoted charge separation.This work provides a simple method for synthesizing photocatalyst with small particle size and high yield.

Frequency-domain parametric downconversion for efficient broadened idler generation

An opposite-chirped frequency-domain optical parametric amplification(OC-FOPA) design is demonstrated and numerically verified. This scheme combines both an ultrabroad seeding generation and the subsequent effective amplification in one single optical parametric amplification stage. Based on a slightly asymmetrical 4-f optical system, the spectral contents of both pump and signal waves are spectrally dispersed with opposite spatial chirps,to broaden the initial idler seeding. Via a properly designed fan-out periodically poled LiNbO_3 chip, nearly perfect quasi phase matching can be realized across the full spectrum, whereby each individual spectral pair precisely maps to its required grating period. Full-dimensional simulations based on commercial ~110 fs(FWHM) nearinfrared(near-IR) lasers at 790 and 1030 nm are quantitatively discussed, and few-cycle mid-IR laser pulses(~60 fs at 3.4 μm) plus a high conversion efficiency exceeding 50% are theoretically predicted. By means of a high-power pump source, the OC-FOPA scheme can be also applied to directly produce high-intensity carrier-envelope-phase-stabilized mid-IR idler pulses.

Microwave Catalytic Depolymerization of Polyethylene Terephthalate Plastic to the Monomers

Plastic waste management has emerged as a critical environmental issue due to the exponential increase in plastic consumption worldwide.Polyethylene terephthalate(PET)is extensively used in the production of water bottles,which constitutes a significant fraction of the plastic waste.PET recycling is a challenging task due to the lack of efficient and cost-effective depolymerization methods.In this study,we developed a microwave(MW)catalytic depolymerization method for PET recycling using modified zinc oxide loaded with manganese oxide as a cocatalyst.The modified Mn_(3)O_(4)/ZnO catalyst presents high efficiency in depolymerizing PET into its monomers with only 0.4 wt% ratio of the catalyst to PET at 175℃ for 5 min,resulting in 100% conversion of PET and 88% selectivity toward bis-hydroxyethyl terephthalate monomers.It is believed that Mn_(3)O_(4)provides additional Lewis acid sites,promoting the dissociation of glycol from PET,and the MW irradiation plays a crucial role in rapidly heating the ethylene glycol and the catalyst,thereby accelerating the PET depolymerization process.In addition,the heterogeneous nature of the catalyst facilitates its easy separation from the reaction mixture for reuse,simplifying the catalyst recovery process and enabling costeffective and sustainable PET recycling.Thus,this study provides an innovative and sustainable solution for PET recycling,contributing toward the circular economy and mitigating the environmental impact of plastic waste.