二维色散和各向异性磁化等离子体光子晶体色散特性研究

采用平面波展开法和时域有限差分法研究了二维色散和各向异性磁化等离子体光子晶体的色散特性.当波矢在周期平面时,由于外加磁场的作用使TE模的色散曲线出现两个不同区域的平带,改变磁场的大小不但可以控制平带的位置,而且可以控制光子带隙的位置和大小.增大背景材料的介电常数,可以形成全方向光子带隙,随着背景材料介电常数的增加,带隙的中心位置降低但带隙宽度增加.当波矢偏离周期平面时,色散曲线不再分为TE和TM模,随着非周期平面波矢的增加,带隙位置上移,带隙宽度先增加随后基本保持不变.     

含负磁导率或双负材料的一维光子晶体的全方位带隙中波的反射相位特性

运用传输矩阵法研究了含负磁导率或双负材料的一维光子晶体结构在全方位光子禁带中波的反射相位谱.研究发现"负磁导率材料-正折射率材料"光子晶体和"双负材料-正折射率材料"光子晶体,在所述带隙中,对TE波而言,反射相位随入射角的增大而增大;对TM波而言,禁带频率范围内波的反射相位随入射角的增大而减小;TE波和TM波,两种结构的反射位相均随介质层的厚度缩放因子增大而增大,且都与周期数无关.     

旋转对二维正方晶格介质柱内空结构光子晶体禁带的影响

采用平面波展开的方法计算了三种旋转操作下二维正方晶格各向异性材料(Te)介质柱内空结构光子晶体TE,TM模式能带.讨论了三种旋转操作对TE,TM模式带隙及完全光子禁带的影响.发现TM模式高频带隙与结构的旋转对称性有着密切的关系.而TE模式的带隙不仅受到晶体旋转对称性的影响同时也受到介质在x-y平面分布情况的影响. 关键词： 二维光子晶体 内空结构 旋转操作 光子带隙     

旋转对二维正方晶格介质柱内空结构光子晶体禁带的影响

采用平面波展开的方法计算了三种旋转操作下二维正方晶格各向异性材料(Te)介质柱内空结构光子晶体TE,TM模式能带.讨论了三种旋转操作对TE,TM模式带隙及完全光子禁带的影响.发现TM模式高频带隙与结构的旋转对称性有着密切的关系.而TE模式的带隙不仅受到晶体旋转对称性的影响同时也受到介质在x-y平面分布情况的影响.     

二维复式晶格完全带隙光子晶体缺陷频率的分析

提出了一种具有完全带隙的二维复式晶格光子晶体,该晶体是在二维正方形格子中,旋转截面为正方形的柱子,同时在每个原胞中心引入圆形截面的柱子而形成的,并在其中引入点缺陷。运用平面波展开法并结合超晶胞理论分析此缺陷态光子晶体的频率特性。仿真结果表明,通过调节缺陷的尺寸、角度等结构参量,可以改变缺陷态频率的位置,使TE和TM模缺陷态频率一致,TE模和TM模同时谐振,处于缺陷态频率的入射光就能够完全耦合进点缺陷,具有较高的耦合效率。这种结构的复式晶格完全带隙光子晶体为制作完全带隙光子晶体谐振腔提供了理论基础。     

基于晶格效应的光子晶体带隙特性研究

对10种类型阿基米德晶格按不同晶格点阵进行分类,利用平面波展开法数值计算了光子晶体的能带结构,讨论归一化半径对其带隙的影响,比较了低频带隙相对宽度值.计算结果表明:单胞含多个"原子"的类正方晶格和类三角晶格光子晶体均存在TM模各向同性带隙和低频带隙,还容易产生TE模和TM模的完全带隙,进一步分析发现(32.4.3.4)晶格和(3.6.3.6)晶格打开TM模的低频第1带隙下边界值为最小;选择合适归一化半径分别为0.2和0.26,类正方晶格和类三角晶格光子晶体都能够得到较宽TM模低频带隙的最大值,(32.4.3.4)晶格和(3.6.3.6)晶格获得低频带隙为最宽.典型复式晶格空气环/硅光子晶体能够获得较宽的完全带隙.研究结果为设计新型光学器件提供了理论依据.     

微波段左手材料光子晶体带隙特性研究

利用电磁理论和传输矩阵法仿真得到了在0.3~6.935GHz微波波段具有负折射率的左手材料,并分别对右手材料和左手材料构成的光子晶体带隙特点进行了分析.结果表明:右手材料光子晶体结构的带隙对周期数变化不敏感,而对层厚度比、入射角度变化敏感;左手材料光子晶体带隙随层厚度比的增大,带隙位置出现蓝移,随着入射角度的增大,TM波主带隙的上带边会出现红移,而TE波带隙特性对入射角度变化不敏感.研究结果对微波技术中全方位反射器等器件的设计有一定的参考意义.     

含负折射率材料一维光子晶体的全方位带隙和缺陷模

运用光学传输矩阵理论,研究了含负折射率材料一维二元光子晶体的禁带特性和局域模特性,发现了一种新型全方位光子带隙.与传统的Bragg带隙相比,这种新型全方位光子带隙的中心频率和带宽对入射角的变化不敏感.讨论了引入缺陷层后,入射角变化和各层介质厚度做一定比例的缩放时对缺陷模位置的影响.这种特性在具有固定带宽的全方位反射器和微波技术中全方位或大入射角滤波器方面有重要的应用价值.     

一维光子晶体缺陷模偏振特性的研究

利用一维光子晶体的透射率公式,计算出一维光子晶体掺杂后TE波和TM波缺陷模的波长随入射角的响应曲线、缺陷模透射峰随入射角的响应曲线、缺陷模透射峰随入射波长的响应曲线.研究发现,TE波和TM波的缺陷模透射峰均随入射角的增加而向短波方向移动；TE波缺陷模透射峰的半高宽度(FWHM)和峰值随入射角的增加而减小,而TM波缺陷模透射峰的半高宽度(FWHM)和峰值确随入射角的增加而增加；对TM波其波长为λ0的缺陷模也存在明显的“广义布儒斯特角”现象, TE波的缺陷模不存在“广义布儒斯特角”现象.     

二维光子晶体禁带特性研究

采用平面波展开法对二维光子晶体分别在E和H极化下的带隙进行了计算. 考虑了填充比、晶格结构、介电常数对最大绝对帯隙的影响. 结果表明,不论是正方晶格还是三角晶格,TM模在介质柱型光子晶体中更容易形成带隙;TE模在空气孔型光子晶体中更容易形成带隙. 填充比一定,最大绝对帯隙宽度并非随着介电常数增大总是增大,而是存在一个峰值. 相对介电常数一定,最大绝对帯隙宽度随填充比的变化也存在一个峰值. 不论空气孔型还是介质柱型结构,三角晶格比正方晶格更容易形成帯隙. 关键词： 平面波展开法 TE模 TM模 最大绝对帯隙     

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.