Silicon thin‐film solar cells with rectangular‐shaped grating couplers

As an alternative to randomly textured transparent conductive oxides as front contact for thin‐film silicon solar cells the application of transparent grating couplers was studied. The grating couplers were prepared by sputtering of aluminium‐doped zinc oxide (ZnO) on glass substrate, a photolithography and a lift‐off process and were used as periodically textured substrates. The period size and groove depth of these transparent gratings were tuned independently from each other and varied between 1 and 4 μm and 100–600 nm. The optical properties of rectangular‐shaped gratings and the opto‐electronic behaviour of amorphous and microcrystalline silicon solar cells with integrated grating couplers as a function of the grating parameters (period size P and groove depth h_g) are presented. The optical properties of the gratings are discussed with respect to randomly textured substrates and the achieved solar cell results are compared with the opto‐electronic properties of solar cells deposited on untextured (flat) and randomly textured substrates. Copyright © 2005 John Wiley & Sons, Ltd.     

Tuning Bragg wavelength by writing gratings on prestrained fibers

Bragg gratings at a specific wavelength are made using an excimer KrF laser and a phase mask. The wavelength can be varied for fine tuning and multiplexing applications by straining the optical fiber during UV illumination. When the strain is removed, the grating formed is at a smaller wavelength than that dictated by the phase mask for the unstrained fiber. This technique was demonstrated by writing two gratings located at the same point in the optical fiber. The transmission from the first grating was used as a means for in situ absolute wavelength tuning. The second grating made with approximately 0.2% axial strain was at 1534.54 nm. Once the grating was made, the strain was removed from the fiber leaving the second grating tuned to 1532.04 nm, a wavelength shift of 2.5 nm     

光通信波段亚波长光栅偏振器的设计与分析

亚波长周期结构光栅具有传统光栅所不具有的特殊特性。基于矢量衍射理论-耦合波分析法对矩形亚波长光栅的衍射效率进行了理论计算,针对光通信中的1 550nm波长设计了一种基于SOI衬底的亚波长偏振光栅,分析了光栅周期、光栅深度、占空比和光栅结构的变化对其偏振特性的影响。仿真结果表明,当光栅的周期为960nm,槽深为230nm,占空比为24%时,可使TM模式的透射率大于95%,TE模式的透射率小于5%,且矩形的光栅结构相对于三角形和圆形的光栅结构具有更好的偏振性能,可有效用于光开关、光隔离器、激光器、光探测器等半导体光电子器件。     

808 nm半导体分布反馈激光器的光栅设计与制作

半导体分布反馈（ DFB）激光器的核心工艺之一是分布反馈光栅的制作，设计了808 nm DFB激光器的一级光栅结构。利用纳米压印技术与干法刻蚀附加湿法腐蚀制作了周期为120 nm的梯形布拉格光栅结构，使用MATLAB和Pics3D软件模拟了一次外延结构的光场分布和能带图。通过优化湿法腐蚀所用腐蚀液各组分比例、腐蚀温度、腐蚀时间等条件，得到了理想的湿法腐蚀工艺参数。扫描电子显微镜表征显示，光栅周期为120 nm，光栅深度约为85 nm，占空比约为47%，光栅边缘线条平直，表面平滑，周期均匀。创新型的引入湿法腐蚀工艺和腐蚀牺牲层使光栅表面的洁净度得到保证，提高了二次外延质量的同时，也为进一步制作DFB激光器高性能芯片奠定了良好的基础。     

A pixellated grating array using photoelectrochemical etching on aGaAs waveguide

We have demonstrated and evaluated a grating array outcoupler fabricated by photoelectrochemical (PEC) etching, a manufacturable and practical approach for fabrication of grating-based III-V semiconductor waveguide devices. An array of submicron period gratings was etched into photolithographically delineated areas in a single PEC step. The fabricated devices are: 10-μm wide rib waveguides with 0.35-μm first-order outcoupling gratings; and 10-μm wide rib waveguides with 10 μm×10 μm pixellated areas of gratings. Device characterization demonstrates the effectiveness of outcoupling grating fabrication using PEC and that the pixellated grating outcoupler is an effective and simple means of generating an optical beam array     

纳米压印制作半导体激光器的分布反馈光栅

分布反馈(DFB)光栅的制作是半导体激光器芯片的关键工艺,通过纳米压印技术在InP基片表面涂覆的光刻胶上压印出DFB光栅图形,并分别通过湿法腐蚀和干法刻蚀技术将光栅图形转移到InP基片上。所制作的DFB光栅周期为240nm(对应于1 550nm波长的DFB激光器),光栅中间具有λ/4相移结构。采用纳米压印技术制作的DFB光栅相对于通常双光束干涉法制作的光栅具有更好的均匀性以及更低的线条粗糙度,而且解决了双光束干涉法无法制作非均匀光栅的技术难题。相对于电子束直写光刻法,采用纳米压印技术制作DFB光栅具有快速与低成本的优势。采用纳米压印技术在InP基片上成功制作具有相移结构的DFB光栅,为进一步进行低成本高性能的半导体激光器芯片的制作奠定了良好基础。     

Fabrication of sidelobe-suppressed InP-InGaAsP vertical coupleroptical filter using pair grating structure

In this letter, we report -19-dB suppression of sidelobes in a grating-assisted codirectional coupler (GACC) optical filter, which is the lowest to the best of our knowledge. Pair grating structures has been employed to control the coupling strength with a raised cosine functional form. A unit pair grating is composed of two unit conventional gratings and the fundamental period of pair gratings is fixed at 2Λ, where Λ is the grating period of conventional GACC filter. The distance between the two gratings inside a pair grating is linearly shifted to control the coupling strength. This structure provides a spatially weighted coupling strength without any change of averaged effective optical index or propagation constant     

一种基于散焦光栅的光束质量M2因子实时测量技术

设计出相位型正交散焦光栅,实现了一种新的光束质量M2因子实时测量技术。将散焦光栅和短焦透镜密接使用,透镜提供主要的聚焦能力,散焦光栅产生9个光轴,且在每个光轴上对焦距进行微调,使之具有不同的等效焦距,这样利用单一的CCD就可以在透镜焦平面上同时测量待测光束9个不同位置处的光强分布图样。利用二阶矩的方法计算束宽,经双曲线拟合得到光束的M2因子。对He-Ne激光器的输出光束进行了测量,得到其光束质量因子M2=5.775,与传统测量方法得到的结果相比,误差为5.1%。     

All-fiber WDM optical crossconnect using ultrastrong widely tunableFBGs

We demonstrate an all-fiber wavelength-division-multiplexed optical crossconnect using ultrastrong fiber Bragg gratings (FBGs) with wide tunability. These FBGs have 0.2-nm bandwidths and can be tuned over ~52 nm due to a novel grating preparation technique to reduce mechanical strength degradation. There is negligible distortion in the transmission spectra while tuning the grating, and the crossconnect architecture uses cascades of these gratings for WDM operation. The ability to tune these >98% reflectivity gratings completely out of the entire WDM signal band minimizes the leakage of unwanted signal power and leads to the improvements of >10-dB crosstalk and >0.6-dB power penalty compared with conventional gratings     

深刻蚀高密度熔融石英光栅

深刻蚀高密度熔融石英光栅是一种新型高效的衍射光学元件,具有衍射效率高、成本低、抗损伤,能在高强度激光条件下工作等优点。给出了利用感应耦合等离子体(ICP)技术制作熔融石英深刻蚀光栅的详细过程,并在一定的优化条件下制作了一系列不同周期、开口比和深度的高质量深刻蚀石英光栅。实验得到的最大刻蚀深度为4μm,并且在600 l/mm的高密度条件下得到了刻蚀深度为1.9μm的高深宽比石英光栅。光栅侧壁陡直,表面平整,没有聚合物沉积。所制作的熔融石英光栅元件在高强激光环境、光谱仪、高效滤波器和波分复用系统等领域中有非常广泛的用途。     

向列液晶中的光控永久光栅

液晶中的光折变光栅在数据存储、实时全息术、干涉测量、光放大、相位共轭、光学图像处理和光束操控等领域具有潜在的应用价值。通过二波耦合的方法在掺C60的向列液晶中记录了永久光栅,该光栅的衍射特性能够被外加直流电场或以任意角度入射的、与记录光强度相当的一束平面光波控制。为了进一步了解光栅建立的机制,研究了在恒定外加直流电场下的电流动态过程,从而证明该光栅源于表面电荷的光调制。     

Wavelength-interleaved OADMs incorporating optimized multiple phase-shifted FBGs for fiber-radio systems

We demonstrate that a single fiber Bragg grating (FBG)-based optical add-drop multiplexer can be used to improve the optical spectral efficiency in a mm-wave WDM-fiber wireless network. The technique employs wavelength interleaving in conjunction with optical single-sideband modulation, together with a single multiphase-shifted fiber Bragg grating. Wavelength optical add-drop features are also demonstrated using the grating, and our results show the performance of the novel grating in terms of channel isolation and add-drop functionality. We perform simulations to optimize placement of the phase shifts when an apodization profile is applied to the grating, in an attempt to minimize in-band and out-of-band transmission-amplitude ripples. Experiments are performed using cascade configurations both with the apodized gratings with optimal phase-shift positions and with the original unoptimized gratings. The experimental results demonstrate how these novel gratings can be used in fiber-radio networks incorporating multiple optical add-drop multiplexers.     

Multiplexed identical broad-band-chirped grating interrogationsystem for large-strain sensing applications

We report here a simple and low-cost technique for extreme strain measurement. The overlapping of a chirped sensor grating with an identical reference device provides a novel mechanism for measuring strain. This system permits a linear relationship between strain and resultant optical power. The maximum sensing range of the system is determined by the bandwidths of the gratings employed. A 20-nm chirped grating facilitates a sensing range up to 20 000 μϵ. This paper also demonstrates an arrangement for multiplexing up to four gratings, which can be rapidly and truly simultaneously interrogated     

Measurement of wave-front aberrations in high-resolution opticallithographic systems from printed photoresist patterns

A new method of measurement of wave-front aberrations in high-resolution optical lithographic systems used for semiconductor manufacturing is proposed. The method is based on the measurement of the positional shifts and focus offsets of a set of printed photoresist grating patterns with different periods and orientations, produced under nearly fully coherent illumination condition. The proposed experimental procedures are described in detail and various sources of systematic and random errors are discussed. It is estimated that a measurement precision of λ/50 for the wave-front aberrations at selected points on the lens pupil can be achieved with this method     

二次X射线曝光制作凹面光栅

介绍了一种通过微细加工技术即二次X射线曝光制作凹面光栅的方法。首先利用X射线截面光强遵循高斯分布的性质，无LIGA掩模版曝光显影后在PMMA上得到凹面，凹面深度可由曝光时间控制；再用带有光栅图案的LIGA掩模版第二次曝光显影在凹面上制作光栅。利用不同的掩模版，非常方便地制作了一维和二维凹面光栅，其粗糙度RMS值小于20nm。     

双光栅结构薄膜太阳能电池的优化

为了提高单晶硅薄膜太阳能电池短路电流密度和转换效率, 采用在单晶硅薄膜太阳能电池正背面分别集成硅介质光栅和铝金属光栅的方法, 并利用有限时域差分法软件仿真研究了两种光栅的周期、厚度、占空比对单晶硅薄膜太阳能电池短路电流密度和光转换效率的影响。结果表明, 通过优化可得当正背面光栅都处于最优值时(介质光栅占空比F=0.8、介质光栅周期P=0.632μm、介质光栅厚度h_g=0.42μm; 金属光栅占空比F₁=0.9、金属光栅周期P=0.632μm、金属光栅厚度h_m=0.005μm), 短路电流密度可达35.15mA/cm2, 转换效率为43.35%;将最优光栅单晶硅薄膜太阳能电池与传统单晶硅薄膜太阳能电池对比, 无论是光程路径还是吸收效率, 光栅单晶硅薄膜太阳能电池都有显著的提高。这为以后制备高性能薄膜太阳能电池提供了理论指导。     

Analysis of grating-coupled radiation in GaAs:GaAlAs lasers and waveguides

Grating-coupled radiation in GaAs:GaAlAs lasers and waveguides is analyzed. A general formulation is developed for arbitrary-shaped gratings which need not be small in size. Two methods are used to solve the resulting equations in the case of rectangular-shaped gratings. The first is a perturbation technique and the second is iterative in nature. The iterative procedure converges to a numerical exact solution in many cases of practical interest and indicates that the perturbation results are quite accurate. Curves are presented for radiated power from traveling waves as a function of grating tooth height, tooth width, refractive index, waveguide thickness, and period for rectangular gratings in heterostructure waveguiding geometries. It is shown that radiation is not a monotonically increasing function of tooth height, but rather maxima occur when the teeth are half the optical wavelength in the material. Also, in particular geometries with an air:GaAs grating interface, radiated power of a mode can exceed 100 cm^-1.     

Fabrication and characterization of annealed proton exchanged long period waveguide grating in x-cut LiNbO3

In this paper, a long-period waveguide grating was fabricated in x-cut lithium niobate substrate by patterned annealed proton exchange waveguide fabrication process. The waveguide mode characteristic was evaluated using a charge-coupled device (CCD) camera. It shows that the waveguide is single mode transmission at a wavelength of 1 550 nm. The transmission spectra of the long period waveguide gratings were measured by optical spectrum analyzer (OSA) and show an extinction ratio of ~17 dB and a 3 dB bandwidth of ~10 nm at the resonant wavelength. The resonant wavelength moves toward to the long wavelength direction as the waveguide width-difference increases in the same period, and also shifts toward to the long wavelength direction with the increase of the period in the case of the same waveguide width-difference. The method of fabricating a long period waveguide grating based on a patterned annealed proton exchange technique simplifies the fabrication process, and at the same time, reduces the fabrication cost.     

Gain clamping in semiconductor optical amplifiers with second-order index-coupled DFB grating

A systematic investigation of gain-clamped semiconductor optical amplifiers (GC-SOAs) based on the second-order index-coupled DFB gratings is carried out by way of simulation. In particular, we focus on the main effects of the radiation loss caused by the first-order diffraction of the gratings on the amplifier performance. The magnitude of the total complex coupling coefficient is the main factor to determine the level of gain clamping. We demonstrate that a high-performance GC-SOA can be realized by using purely loss-coupled second-order DFB gratings with more relaxed tolerance on grating strength and period. It is shown that, in the presence of weak reflection-related coupling, the parasitic radiation loss associated with the second-order grating always helps to expand the linear amplification region and to reduce the longitudinal spatial hole burning along the cavity. Further, we demonstrate through comparison that the GC-SOAs have higher saturation power and much shorter carrier lifetime than the conventional SOAs. An improved design by longitudinal variation of the grating duty cycle is proposed such that the noise performance of the amplifier can be enhanced without much sacrifice on the linear amplification regime.     

部分相干光倾斜照明成像研究

本文用部分相干光成像理论研究了提高分辨力及增大焦深的原理．建立了二元光栅倾斜照明成像数理模型。对几种倾斜照明方式进行了模拟计算．对模拟结果作了分析比较。结果表明，用二元光栅照明较好，既能较大程度提高投影光刻曝光系统的分辨力及焦深，又能克服其他照明方式的能量利用率低等缺点。