窗口型极小孔激光器的研制

报道了一种窗口型的用于高分辨率近场光学存储领域的极小孔激光器.这种窗口结构的引入解决了出射端面处由于金属膜的存在而导致的pn结短路问题,同时一定程度上抑制了激光器腔面处的COD效应.简化了极小孔激光器的工艺,降低了制备难度,提高了激光器的输出特性.通过FIB设备制备出了小孔大小为4 0 0 nm,工作电流在31m A时的出光功率约0 .3m W的极小孔激光器     

带有非吸收窗口的大功率915 nm半导体激光器

为了实现大功率输出,应用无杂质空位诱导量子阱混合(IFVD)方法制备带有非吸收窗口结构的915 nm半导体激光器单管.通过实验确定促进和抑制量子阱混合的Si02和SiaN4薄膜的厚度分别为300和500 nm,退火条件为800℃,90 s.最终制备出的带有非吸收窗口的激光器,与普通激光器的阈值电流和斜率效率几乎一样.但普通激光器在电流为10 A时发生灾变性光学损伤(COD)并失效,而带有非吸收窗口的激光器在电流达到13A时仍然可以正常工作,相比普通激光器其最大输出功率增加了15％.每种器件各20个在20℃,电流为9A时进行直流老化试验,普通激光器在老化时间达到100 h时全部失效,而带非吸收窗口器件在老化200 h时仅有两个失效,这表明非吸收窗口结构显著提高了器件的抗COD能力.     

气动激光窗口的光束传输特性

本文分析和实验研究了HAW型气动激光窗口所形成的小孔光阑效应和类透镜效应对光束传输的影响,阐明了在高功率气体激光器中使用气动激光窗口的优越性。     

纳米孔径垂直腔面发射激光器的制备及特性

在850 nm波长垂直腔面发射激光器的基础上制备了纳米孔径垂直腔面发射激光器.当小孔尺寸为400 nm×400 nm时,在25 mA驱动电流下,其最大输出光功率达到了0.3 mW,功率密度约为2 mW/μm2.文中介绍了纳米孔径垂直腔面发射激光器的制备工艺,并对它的光谱特性和寿命特性进行了分析.     

纳米孔径垂直腔面发射激光器的制备及特性

在850 nm波长垂直腔面发射激光器的基础上制备了纳米孔径垂直腔面发射激光器.当小孔尺寸为400 nm×400 nm时,在25 mA驱动电流下,其最大输出光功率达到了0.3 mW,功率密度约为2 mW/μm2.文中介绍了纳米孔径垂直腔面发射激光器的制备工艺,并对它的光谱特性和寿命特性进行了分析.     

微小孔阵列垂直腔面发射激光器的研究

在980nm波长的大功率垂直腔面发射激光器(VCSEL)的基础上制备了高输出功率的微小孔阵列半导体激光器,其最大输出光功率达到了1mW。介绍了针对微小孔阵垂直腔面发射激光器的特殊制备工艺,并对其特性进行了分析。     

带非吸收窗口的大功率657 nm半导体激光器

在激光器腔面处制作非吸收窗口(NAW)可以有效地减少光吸收,防止激光器过早出现光学灾变损伤(COD),是提高大功率半导体激光器的功率特性的重要手段之一.采用金属有机化学气相沉积(MOCVD)技术二次外延生长了大功率657 nm红光半导体激光器结构,通过闭管扩散Zn的方法在腔面附近制作了非吸收窗口.实验发现扩散温度550 ℃,扩散时间[20 min时,得到的非吸收窗口最为有效,激光器连续工作的无扭折输出功率大于100 mW,超过常规的无窗口结构激光器的最大输出功率的两倍,激光器的斜率效率提高了23%.测量该类器件的温度特性发现,环境温度为20～70 ℃时,其输出功率均可大于50 mW,计算得到激光器的特征温度约为89 K,波长增加率约为[0.24 nm/℃.     

硅油对CO_2激光器NaCl窗口的防潮作用

对小孔耦合输出的CO_2激光器,需要在小孔反射镜的后面贴一块能透过波长10.6微米的红外材料.NaCl晶体的透过率较好,但因其在空气中极潮解,所以影响使用.采用硅油作NaCl晶体保护膜,作为窗口使用能够承受较大的功率密度,且有良好的防潮作用;缺点是硅油易粘污灰尘,但由于涂膜方法简便,可克服这方面的不足.     

改进激光器窗口

美国海军研究所制备的溴化钾单晶在1.06和10.6微米波长处的体吸收系数为0.000005厘米-1。其纯度比目前用作高功率激光器窗口的氯化钾或碲化镉的体吸收差不多降低十倍。     

底部出光纳米孔径面发射激光器的研究

在普通大功率垂直腔面发射激光器(VCSEL)基础上,制备出了底部出光纳米孔径VCSEL。利用聚焦离子束刻蚀技术完成纳米孔径的制作。当小孔直径为480nm×480nm时,测量得到器件的远场输出光功率为0.07mW。并分析了温度的变化对该器件远场输出光功率的影响。     

二维非线性时域有限差分法分析微孔激光器出射光的近场特性

微孔激光器(VSAL)作为应用于近场光存储系统中的一种新型光源,它的出射光斑的近场特性对于近场光存储是十分重要的。运用二维非线性时域有限差分法(2D-NL-FDTD)分析VSAL出射端即微孔金属膜的近场光学性质,模拟计算了不同参数的微孔金属膜的光强近场分布,对所得结果加以分析,从光学性质的角度,讨论其在近场光存储中的应用,并给出反映其近场光学特性的相关数据。     

光子带隙加窗技术在功率放大器中的应用

利用光子带隙的带阻特性,抑制功率放大器的二次谐波分量,可提高功率放大器的输出性能的特点,制作了加窗光子带隙电路并进行了测量.结果与传统光子带隙结构相比,有效改善了在通带内射频传输特性的平坦度.     

Optimal tracking of time-varying channels: a frequency domainapproach for known and new algorithms

In this paper, we developed a systematic frequency domain approach to analyze adaptive tracking algorithms for fast time-varying channels. The analysis is performed with the help of two new concepts, a tracking filter and a tracking error filter, which are used to calculate the mean square identification error (MSIE). First, we analyze existing algorithms, the least mean squares (LMS) algorithm, the exponential windowed recursive least squares (EW-RLS) algorithm and the rectangular windowed recursive least squares (RW-RLS) algorithm. The equivalence of the three algorithms is demonstrated by employing the frequency domain method. A unified expression for the MSIE of all three algorithms is derived. Secondly, we use the frequency domain analysis method to develop an optimal windowed recursive least squares (OW-RLS) algorithm. We derive the expression for the MSIE of an arbitrary windowed RLS algorithm and optimize the window shape to minimize the MSIE. Compared with an exponential window having an optimized forgetting factor, an optimal window results in a significant improvement in the h MSIE. Thirdly, we propose two types of robust windows, the average robust window and the minimax robust window. The RLS algorithms designed with these windows have near-optimal performance, but do not require detailed statistics of the channel     

On Nonuniform Windowing M-ary FSK Data in a DFT-Based Detector

Nonuniform amplitude windowingM-ary frequency shiftkeying (FSK) data prior to demodulation reduces the crosstalk normally caused by Doppler and oscillator drift. A major disadvantage of such nonuniform windowing is the loss in signal detectability as a result of the mismatch between the nonuniform windowed signal and the uniform windowed detectors. This paper is concerned with the tradeoff of nonuniform windowing to combat frequency offset degradation and signal detectability losses caused by the use of such windows in a DFT (discrete Fourier transform)-based detector of noncoherentM-ary FSK. What is shown here is that if a frequency control system can maintain frequency to within a certain deviation (which depends on error rate and nonuniform window), nonuniform windowing is not warranted; on the other hand, if the frequency deviation becomes too large, nonuniform windowing is advised. A technique is developed that will determine this crossover deviation.     

Bias correction and forward-backward averaging in frequency/DOA tracking problems

In this paper, we discuss some issues relevant to frequency and direction of arrival (DOA) tracking problems. First, we develop a linear Frequency Modulated (FM) signal model for accurately describing windowed, slowly time varying narrowband signals that typically occur in tracking problems. We then derive first order bias expressions for the peak locations of a Discrete Time Fourier Transform (DTFT) spectrum of a windowed, slowly time varying linear FM signal. We also show that Forward-Backward (FB) averaging is generally inappropriate for nonstationary data, but that it is appropriate when applied to tracking the frequencies of windowed, slowly time varying narrowband signals. A major motivation for using FB averaging is to increase the efficiency of subspace based frequency/DOA estimation in tracking problems. Finally, simulations confirm our first order bias expressions, and show that FB averaging does not significantly alter (or degrade) the time varying MUSIC based frequency estimation performance over that of Forward only averaging.This research was supported in part by the National Science Foundation Grant MIP-9203296 and Texas Advanced Research Program Grant 009741-022 and 009741-065.     

Calculation of the local cross-correlation function on the basis ofthe Laguerre transform

A simple and computationally efficient mechanism for calculating a running or local cross-correlation function of two time-domain signals is presented. In order to obtain a running cross-correlation function, the signals must be windowed. It is argued that an appropriate window for a local cross correlation is an exponential function. To obtain a computationally efficient mechanism, the windowed functions are decomposed in a series of orthogonal functions. The set or orthogonal functions is matched to the chosen window and is a Laguerre-Fourier series. The cross correlation of the windowed functions is equal to a weighted summation of cross-correlated pattern functions. The weights are determined by cross correlating the Laguerre coefficients     

一种新的带窗重叠自适应滤波器

基于一种带窗重叠自适应滤波器,将重叠滤波思想引入LMS算法。利用重叠滤波的平滑性,将加窗重叠滤波和LMS算法相结合,给出了窗加权重叠LMS(WO-LMS)算法。与传统的LMS算法相比,WO-LMS算法既提高了收敛速度又可以得到较低的稳态均方误差。理论分析了算法的收敛性,通过与LMS算法的比较,验证了WO-LMS算法的优越性。     

Windowed linear canonical transform and its applications

In this paper, we generalize the windowed Fourier transform to the windowed linear canonical transform by substituting the Fourier transform kernel with the linear canonical transform kernel in the windowed Fourier transform definition. It offers local contents, enjoys high resolution, and eliminates cross terms. Some useful properties of the windowed linear canonical transform are derived. Those include covariance property, orthogonality property and inversion formulas. As applications analogues of the Poisson summation formula, sampling formulas and series expansions are given.     

全相位半带滤波器设计与实现

半带滤波器是一种特殊的低通FIR滤波器,在信号的分析与完全重构中应用广泛。本文在分析全相位滤波器频率取样特性基础上,首先得到无窗、单窗全相位恒通过而双窗则低于频率取样点的结论,其次利用全相位双边带对称传输特性H设计三种类型的半带滤波器且均具有严格零相位特性,类型I和II具有实系数单位冲激响应,类型III则具有复系数冲激响应而使半频点处幅值不等于0.5。第三,分别实现全相位九种半带滤波器并与三种传统设计方法进行比较,结果表明,全相位设计半带滤波器通带及阻带波纹数更少,且衰减降低近3dB。