GaN基蓝绿光LED电应力老化分析

对InGaN/GaN多量子阱蓝光和绿光LED进行了室温20,40,60 mA加速电流下的电应力老化研究,发现蓝光与绿光样品经过60 mA电流老化424 h后,其电学性能表现出一定的共性与差异性:在小测量电流下,绿光样品的光衰减幅度较蓝光样品大~9%;而在较大测量电流 (20 mA)下,两者的光衰减幅度基本相同 (18%)。同时,蓝绿光样品的正向电学性能随老化时间的变化幅度基本一致,反映出它们具有相似的退化机制,绿光样品老化后增多的缺陷大部分体现为简单的漏电行为,而并非贡献于非辐射复合中心。在此基础上对GaN基外延结构进行了优化,优化后的LED长期老化的光衰减幅度较参考样品降低了3%。     

Si衬底GaN基蓝光LED钝化增透膜研究

在Si衬底GaN基蓝光LED芯片上生长了一层SiON钝化膜,使器件的光输出功率提高12%且有效降低了器件在老化过程中的光衰。对有、无钝化膜的样品进行性能比较,结果表明SiON钝化膜能有效隔离环氧树脂与高温芯片,缓解环氧树脂的老化变黄;又能部分弛豫环氧树脂对芯片的张应力,降低非辐射复合中心产生的几率;有效减小器件的侧壁漏电通道,降低器件的光衰和漏电流,提高器件的可靠性。     

GaN基大功率白光LED的高温老化特性

对大功率GaN基白光LED在85℃下进行了高温加速老化实验.经6500 h的老化,样品光通量退化幅度为28％ ～33％.样品的Ⅰ-V特性变化表明其串联电阻和反向漏电流不断增大,原因可归结为芯片欧姆接触的退化及芯片材料中缺陷密度的提高.样品的热特性变化显示出各结构层热阻均明显增大,这是由散热通道上各层材料的老化及焊料层出...     

GaN基白光发光二极管失效机理分析

对小功率白光GaN基发光二极管(LED)在室温、40 ℃和70 ℃下进行温度加速老化寿命实验,通过对老化前后不同时间段器件的电学、光学和热学特性进行测量来分析器件的失效机理,着重分析器件的芯片和荧光粉的失效机理.器件老化前后的I-V特性表明:老化过程中,器件的串联电阻和低正向偏压下的隧道电流增大,这是由于器件工作时其芯片的欧姆接触退化和半导体材料的缺陷密度升高而引起的.器件的热特性表明:高温度应力下器件的热阻迅速变大,封装材料迅速退化,这是器件退化的主要原因;光谱曲线表明温度加速了器件的     

低色温无荧光粉LED光源的可靠性研究

为了对比研究新型低色温无荧光粉LED光源和低色温荧光粉LED光源的可靠性,本文进行了85℃加速老化和温度步进应力实验,测试了各光电参数在老化过程中的变化规律,并分析了有无荧光粉低色温光源的老化衰减机理。实验结果表明,在20 A/cm2工作电流密度、85℃的加速老化条件下,两种类型的光源都表现出很好的稳定性,但随着功率的增加,无荧光粉光源展现出更好的光通量稳定性;随着温度的升高,无荧光粉光源在高达200℃左右仍表现出良好的可靠性,而荧光粉光源则在175℃后出现了明显的光通量衰减和色温升高。电流应力或温度应力提升以后,无荧光粉光源展现出更优异的可靠性。     

功率型白光LED光学特性退化分析

将GaN基蓝光芯片涂敷YAG荧光粉和透明硅胶制成额定功率为1 W的白光发光二极管(LED),对其施加900mA的电流应力,在老化过程中测量白光LED的主要光学参数,考察其光学特性的退化情况。经过4 200 h的老化,样品光通量退化为初始值的15%~18% 。样品的漏电流明显增大,表明芯片有源区缺陷密度提高,但光谱分布图中蓝光部分的辐射量未减少,仅观察到黄光部分辐射量的减少,推断出YAG荧光粉的转换效率降低。同时,从原理上分析了样品色温逐渐增大,显色指数基本不变的原因,对大功率白光LED在照明领域的应用有一定的借鉴意义。     

电-热应力对GaN基白光LED可靠性的影响

研究了电流和温度应力因子及其共同作用时对Ga N基白光LED可靠性的影响,并从LED结构方面探究了各应力下LED的失效机理。结果表明,以电流作为加速应力时,荧光粉退化为其主要的失效方式,同时LED出光的相关色温上升,红色比减少;在热应力下,主要是LED芯片结构发生变化,峰值波长蓝移,光通量衰减,同时支架出现老化现象。电流和温度应力共同作用时,温度应力对LED光通量的影响大于电流应力。电-热应力下的光通量衰减大于电、热应力单独作用时的衰减之和,即电-热应力作用时,光衰不具有线性叠加性。     

近紫外LED封装器件的热稳定性及可靠性

紫外发光二极管(紫外LED)已经在许多超越照明领域展示出了特殊的应用优势,例如健康医疗、消毒杀菌、环保及传感等领域。本文采用仿真和实验相结合的方法研究了具有不同封装结构的近紫外LED封装器件的热稳定性,并对其进行了高温老化可靠性测试评估。研究结果表明:LED器件的辐射功率和正向电压随温度的升高而下降,其中,具有倒装结构的器件下降趋势明显小于正装结构,这表明其热稳定性较好;经过55℃恒温额定电流条件下的可靠性测试发现具有倒装结构的器件光-色特性衰减速率小于正装器件。通过本文研究可以得出结论:具有低热阻、小尺寸等优点的的倒装封装结构有助于提高近紫外LED器件的光-热稳定性和可靠性。     

Si衬底GaN基蓝光LED老化性能

报道了芯片尺寸为500μm×500μm硅衬底GaN基蓝光LED在常温下经1000h加速老化后的电学和发光性能,其光功率随老化时间的变化分先升后降两个阶段;老化后的反向漏电流和正向小电压下的电流均有明显的增加;老化后器件的外量子效率(EQE)比老化前低;老化前后EQE衰减幅度在不同的注入电流下存在明显差异,衰减幅度最小处出现在发光效率最高时对应的电流密度区间。     

新型AlGaInP系发光二极管饱和特性与寿命的研究

针对AlGaInP系发光二极管(LED)电极阻挡出光、衬底吸收、全反射角小导致器件出光效率低、热积累大、饱和特性差等问题,提出了一种具有复合电流输运增透窗口层、复合DBR反射镜和电流阻挡层结构的新型LED,并测试了其饱和特性和寿命.电流分布模拟显示:新型LED电极下仅存在极小的无效电流;实验结果表明新型LED出光效率高,饱和电流大,饱和电流时光强约为常规LED的3倍,光电性能明显提升.器件饱和特性和老化实验研究显示:新型LED寿命长达17.8×104h,器件内部发热量低,具有高饱和特性和高可靠性,适合在大电流大功率下工作.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.