Photonic crystal LEDs – designing light extraction

Photonic crystals (PhCs) have attracted much attention during the last decade as a solution to overcome the low extraction efficiency of as‐grown light‐emitting diodes (LEDs). In this review we describe the underlying physics and summarize recent results obtained with PhC LEDs. Here, the main focus is on diffracting PhC. In order to quantify the benefit from the incorporation of PhCs for diffracting light a comparison by simulations between a PhC LED and a standard state‐of‐the‐art LED is carried out. Finally, the impact of the PhC on the LEDs emission characteristics will be discussed with respect to étendue‐limited applications.     

三片式硅基液晶激光投影显示中光学引擎的设计

 介绍了激光硅基液晶芯片的显示原理,针对硅基液晶光学引擎的特点,采用光学扩展量表征系统的光能利用率,并分析了光学扩展量的变化与光能利用率的关系。利用光学软件Zemax和Tracepro设计了三片式激光硅基液晶光学引擎。实验结果表明:此光学引擎体积小,光学扩展量小,照度均匀性高达到92%以上,能量利用率达到56%以上,满足设计要求。     

Planar concentrators near the étendue limit

Recently proposed aplanatic imaging designs are integrally combined with nonimaging flux boosters to produce an ultracompact planar glass-filled concentrator that performs near the étendue limit. Such optical devices are attractive for high-efficiency multijunction photovoltaics at high flux, with realistic power generation of 1 W from a 1 mm2 cell. When deployed in reverse, our designs provide collimation even for high-numerical-aperture light sources.     

LED光学参数测试方法研究

通过分析LED（发光二极管）发光机理和封装特点，选择了LED需要测试的光学参数：光通量、亮度、发光强度、空间光强分布、相对光谱功率分布及色度，进而研究了每个光学参数的特点。根据其特点，制定了每个光学参数的测试原理以及相应的计算公式，最终确定了各参数的测试方法。设计了LED光通量测量系统、亮度和发光强度测量系统、发散角及空间光强分布测量系统、相对光谱功率分布及色度测量系统。实际应用表明：这些系统均能满足目前工作需要。     

Optical characteristics of LGP depending on the scattering pattern orientation for flat-type LED lighting

In flat-type light-emitting-diode (LED) lighting systems, a planar light is formed using a luminance source positioned on the side of the system and light guide panel (LGP) or reflecting plates. Thus, such systems are favorable for their thinness, which allows a relatively small number of LEDs to be used. However, the application of a high-power LED light to a large-area lighting system yields Lambertian luminaires; therefore, a point or a discomfort glare is produced, which generally causes degradation of the luminance efficiency and uniformity. In this study, we solved the problems of luminance non-uniformity and inefficiency by adjusting the orientation of an applied LGP scattered pattern and removing the remaining glare. Through computer simulation, optical characteristics that increase the efficiency even in the case of low-output LEDs were found. Specifically, a scattered pattern vertically oriented relative to the direction of the incident light improves the luminance uniformity at the side of the system, while a scattered pattern oriented parallel to the direction of the incident light plays the role of a waveguide. We implemented a flat-type LED lighting system by fabricating a large-area LGP based on the computer-simulation results and using an extremely sensitive laser. The optical characteristics observed using the laser-processed LGP were identical to those obtained in the computer simulation. Therefore, for large-area flat-type LED lighting systems, we confirmed that adjusting the orientation of the LGP scattered pattern can increase the luminance efficiency and uniformity.     

基于光源近场模型的LED光学扩展量测量方法

基于精确的LED光源近场模型,提出了一种LED光学扩展量测量方法.通过追迹LED近场光源模型中的光线数据,可获得LED光功率关于光学扩展量的关系曲线,直观反映出LED光源的光学扩展量特性和光能利用率等信息.以紫外曝光系统中的UV-LED阵列面光源为例,对三款不同型号的UV-LED进行了实际测量.通过测量得到的光学扩展量和光能利用率曲线,可以对UV-LED的光束质量作出判断,并为阵列面光源的优化设计提供帮助.     

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。     

利用单层密排的纳米球提高发光二极管的出光效率

在发光二极管(LED)的透明电极层上制作单层六角密排的聚苯乙烯(polystyrene, PS) 纳米球, 研究提高GaN基蓝光LED的出光效率. 采用自组装的方法在透明电极铟锡氧化物层上制备了直径分别约为250, 300, 450, 600和950 nm的PS纳米球, 并且开展了电致发光的研究. 结果表明, 在LED的透明电极层上附有PS纳米球能有效地提高LED的出光效率; 当PS纳米球的直径与出射光的波长比较接近时, LED的出光效率最优. 与参考样品相比, 在20 mA和150 mA工作电流下, 附有PS纳米球的样品的发光效率分别增加1.34倍和1.25倍. 三维时域有限差分方法计算表明, 该出光增强主要归因于附有PS纳米球的LED结构可以增大LED结构的光输出临界角, 从而提高LED的出光效率. 因此, 这是一种低成本的实现高效率LED的方法.     

AlGaInP大功率发光二极管发光效率与结温的关系

目前,AlGaInP大功率发光二极管(LED)存在的主要问题是大电流工作时发热严重,主要是由于电流扩展不均匀、出光面电极对光子的阻挡和吸收以及器件材料与空气折射率之间的差距引起的全反射现象,这些因素造成大功率LED出光受到限制、发光效率低、亮度不高.提出了一种复合电流扩展层和复合分布式布拉格反射层(DBR)的新型结构LED,使得注入电流在有源区充分地扩散,同时提高了常规单DBR对光子的反射率.结果显示,这种新型结构LED比常规结构LED的性能得到了很大的提升,350 mA注入电流下两者的输出光功率分别为4 关键词： 复合电流扩展层 复合分布式布拉格反射层 出光效率 结温     

Comparison between blue lasers and light‐emitting diodes for future solid‐state lighting

Solid‐state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light‐emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state‐of‐the‐art input‐power‐density‐dependent power‐conversion efficiencies; potential improvements both in their peak power‐conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.     

Electroluminescence from hybrid organic–inorganic LEDs based on thermally evaporated CdS thin films

Hybrid organic–inorganic light emitting devices combine the color purity and durability of inorganic light emitting diodes (LEDs) with high efficiency, flexibility and low processing cost of organic LEDs (OLEDs). A significant challenge is to incorporate inorganic nanocrystals inside the OLED structure. In the present work, thin films of CdS were successfully incorporated inside standard OLED structure using vacuum thermal evaporation technique. For the characterization of these films, they were deposited on plain glass plates at room temperature and studied using structural (XRD and TEM), morphological (SEM and AFM) and optical (UV and PL) techniques. The films were found to be composed of nanocrystals of CdS in which the size of the crystals increased with the increase in film thickness. The hybrid organic–inorganic LEDs showed improved luminance and efficiency as compared to the organic LED without CdS layers.     

注入电流对绿光高压LED光电特性的影响

设计并制备了12 V 的GaN基绿光高压发光二极管（LED）,并对其进行了变电流测试。研究了绿光高压LED的正向电压、峰值波长、光功率以及光效等重要参数随注入电流的变化关系,电流变化范围为3~50 mA,测试温度为25 ℃。实验结果表明：电流对绿光高压LED的光电特性有很大影响。在驱动电流为20 mA时,对应电压为14 V。随着注入电流的增大,峰值波长蓝移了2 nm。随着电流的增大,光功率近似于线性增加。在注入电流从3 mA增大到20 mA的过程中,光效降低了约61%;在注入电流从20 mA增大到50 mA的过程中,光效降低了约39%。这说明高压LED在大电流驱动时,光效降低的幅度比较缓慢。上述结果对 GaN基绿光高压 LED 的改进优化具有一定的参考价值。     

基于白光LED照明光源的室内VLC系统

LED照明与可见光通信技术相结合,构建出基于LED可见光无线通信系统。对室内VLC(Visible Light Communication)系统的白光LED光源特性和系统信道模型分析,提出照明光源布局设计与接收光功率分布的关系;对强度调制直接检测方式的室内VLC系统中信噪比和多径效应引起的码间串扰分析,提出采用光分集接收技术克服码间串扰和提高信噪比,并给出光检测器阵列布局的模型。建立VLC系统仿真模型,给出OOK-NRZ (On-Off Keying & Non-Return Zero)和OOK-RZ(On-Off Keying & Return Zero)调制方式的误码率和均方根时延扩展之间的关系曲线。仿真结果表明,接收光功率相同时,均方根时延扩展时间大于1.0 ns时,OOK-RZ特性优于OOK-NRZ。     

GaN基LED倒装芯片蓝宝石衬底半球型图形优化设计

为了提高GaN基LED芯片的光提取效率,以GaN基LED芯片为研究对象,建立了在蓝宝石衬底出光面和外延生长面上具有半球型图形的LED倒装芯片模型,并利用光学仿真软件对图形参数进行优化设计。实验结果表明:在蓝宝石衬底的出光面和外延生长面双面都制作凹半球型图形对芯片光提取效率的提高效果最好,并且当半球的半径为3 m,周期间距为7 m时,GaN基LED倒装芯片的最大光提取效率为50.8%,比无图形化倒装芯片的光提取效率提高了115.3%。     

LEDs and photodetectors for wavelength-division-multiplexed light-wave systems

The very wide transmission window, from 0.8 to 1.6 μm wavelength, in present low-loss optical fibres offers opportunities for wavelength-division-multiplexing (WDM) in light-wave systems. This paper reviews light emitting diode (LED) light sources and photodetectors capable of meeting the power (distance) and bandwidth (bit rate) requirements of such systems. Particular attention will be paid to the new multiplexing dual wavelength LEDs and demultiplexing photodetectors which point a way toward simpler, perhaps more economical, WDM light-wave systems in wide-band loop and inter-office trunking applications.     

快速实现矩形准直光束的高集光效率LED透镜设计

为了实现LED矩形准直光束,提出一种快速构建高集光效率LED透镜的设计方法。基于分步法、边缘光线定理和几何光学定律,分步设计两个自由曲面轮廓线,快速获取两个自由曲面并构建透镜。结果表明:当LED距透镜内曲面尺寸与LED尺寸的比值为6时,系统的半峰全宽为2.3°×1.15°,集光效率为82.6%,可以有效地实现矩形准直光束。随着比值的增大,透镜的尺寸变大,但是半峰全宽变小,透镜集光效率变高。根据设计参数加工了透镜并对仿真结果进行了实验验证。该方法为实现LED矩形准直光束提供了一种有效途径。     

LED阵列光源的舞台灯变焦透镜系统

 针对国内大功率LED阵列光源舞台灯具在光束角度不可变和光能利用效率低等问题,提出利用透镜组变焦原理来设计阵列光源变焦透镜系统。基于透镜组变焦原理设计了单颗LED光源的变焦透镜组,高级光学系统分析模拟软件ASAP的计算结果显示：调焦范围为0~10 mm时,出光角度（1/10峰值角）的变化范围为18.5°~38.7°,光能利用效率在调焦距离为10 mm时达到78%以上。在此基础上,将单颗LED光源的变焦透镜组扩展为Red、Green、Blue各12颗共36颗LED的变焦透镜系统,进一步分析36颗LED阵列光源在不同排布方式下的出光角度及混色效果。ASAP计算结果显示：在调焦范围与单颗LED相同的情况下,LED阵列光源变焦透镜系统的出光角度（1/10峰值角）的变化范围为21°~38.6°,且3种颜色LED交叉排列的混色效果较好。由LED阵列的计算结果可知,与国内现有的大功率LED舞台灯具相比,在出光角度的变化范围和光能利用效率方面都得到了提高。     

A model for LED spectra at different drive currents

A mathematical model for the spectra of monocolor light-emitting diodes （LEDs） and phosphor-coated white LEDs at different drive currents is established.The simulation program of the color rendering of a white light LED cluster is developed based on this model.The program can predict not only the spectral power distribution and color rendering index （CRI）,but also the number of LEDs,drive currents,input power,and luminous efficacy of a white light LED cluster at a given color temperature according to the requirement of the luminous flux.The experimental results show that the relative spectral power distributions （SPDs） and chromaticity coordinates of the model LED are very close to that of the real LED at different drive currents.Moreover,the correlated color temperature （CCT）,CRI,special color rendering index （R9） luminous flux,input power,and luminous efficacy of the white light LED cluster predicted by simulation are also very close to the measured values.Furthermore,a white/red cluster with high rendering （CCT=2903 K,CRI=91.3,R9=85） and a color temperature tunable warm-white/red/green/blule LED cluster with high rendering （CCT=2700 6500 K,CRI 〉 90,R9 〉 96） are created.     

功率型GaN基发光二极管芯片表面温度及亮度分布的物理特性研究

基于电流连续方程、欧姆定律及定性三维热流传导模型研究发光二极管(LED)芯片 的电流密度分布、热量、温度之间的相互交叉关系,进而测试分析GaN基蓝光 LED电流扩展效应和亮度分布的关系,认为芯片亮度变化趋势可作为判别电流扩 展性能的有效手段. 由于芯片表面温度、亮度分布和电流密度之间存在紧密的联结 关系,通过测试芯片表面温度或亮度分布就可定性了解器件电流扩展性能, 从而为优化电极结构提供一种判定依据. 在不同电流和热沉温度下,进一步 讨论了电流密度非均匀性和亮度分布的关系, 电流密度拥挤将导致芯片局 部区域热量堆积,非辐射复合作用增强,限制出射光子数目, 因此热 量是影响亮度分布的重要因素之一. 通过载流子传输机理进一步说明 温度影响亮度均匀性的原因,并通过实验说明理论分析的可行性. 通过优化电极结构能改善器件的电流扩展效应以及亮度均匀性, 对提高大功率LED的可靠性具有重要作用.     

发光二极管材料与器件的历史、现状和展望

文章介绍了发光二极管材料和器件的研究、开发的历史，概述了发光二极管技术的发展现状和进展．通过与其他类型光源的比较，向读者展示了发光二极管未来的重要地位和光明前景．发光二极管的最近的成就是实现了有色光方面的成功应用．高功率白色发光二极管已开始应用于便携式和特殊照明．而在通用的照明领域要成功地应用发光二极管，则需要通过性能和价格方面的继续突破来实现．