四波混频过程中差频与和频的竞争研究

在以惰性气体Xe为非线形介质,利用双光子共振四波混频差频(2PR4WDM,ω-=2ωUV-ωT)技术,产生可调谐相干真空紫外激光(VUV)的实验中,观察到产生的VUV在一些特定波长附近出现共振衰减.这些波长对应着伴随过程四波混频和频的双共振增强.对四波混频过程进行了比较粗略的定量的处理,考虑到泵浦光在传播途中的衰减,得到的结论能很好地解释观察到的差频输出的衰减现象,差频输出的衰减是由于双共振四波混频和频(ω+=2ωUV+ωT)与差频竞争的结果.     

High-resolution Absorption Spectra of Acetylene in 142.8-152.3 nm

利用双光子共振四波混频差频技术产生高分辨、可调谐的真空紫外(VUV)激光,在142.8-152.3nm波长范围内测量了超声射流冷却乙炔分子的吸收光谱. 由于射流冷却的效果和VUV激光高分辨的特性,当前的吸收光谱显示出较以往实验光谱更清晰的光谱振动峰结构,其中主要的三个振动谱峰序列对应了乙炔分子C1Ⅱu态的C-C对称伸缩振动激发(v2=0-2). 此外,148.2 nm处的肩峰则被归属为反对称弯曲振动v4的第一泛频激发,同时由于Renner-Teller效应分裂和跃迁选律的限制,当前的吸收光谱中观测到420(μ1Ⅱu)和420(к1Ⅱu)两个组分. 由此获得了各振动子带的带源位置和半高峰宽. 随着振动的激发,各振动峰逐渐加宽,能级寿命减小.     

射流冷却的OCS分子高分辨VUV吸收光谱研究

以双光子共振四波混频差频产生的高分辨、可调谐真空紫外激光(VUV)为光源,在151～156nm内测量了超声射流冷却的OCS分子1Σ+-1Σ+电子跃迁带的吸收光谱.由于采用射流冷却和高分辨激光,得到与前人吸收谱测量结果不同的光谱线线形,观察到两个呈现明显非对称结构的振动峰,通过Fano线型拟合,获得了这两个振动峰的半高峰宽和非对称参数q值,与前人光碎片激发谱测量结果基本一致.实验中,为了消除VUV激光强度波动的影响,通过记录参比光的方法,有效地提高了吸收光谱的信噪比,使得吸收测量的灵敏度达到1%左右.     

基于氧化镁晶体中级联四波混频过程的紫外飞秒光脉冲产生

紫外波段飞秒激光脉冲是研究超快化学和超快物理相关过程的重要工具,实现波长可调谐的宽带紫外飞秒光脉冲将有助于推动超快动力学及相关领域的研究.本文报道了以两束400 nm的飞秒光脉冲作为级联四波混频的抽运源,在氧化镁晶体中产生9阶频率上转换和5阶频率下转换边带信号的实验结果.边带波长范围从350 nm到450 nm连续可调谐,这些边带信号的发散角和波长与级联四波混频理论预测结果吻合.紫外边带相对于入射光的整体转化效率约为1.2%.同时,高阶边带的光谱形状呈现高斯型,其谱宽理论上支持傅里叶转换极限脉宽为20—50 fs.本文展示了一种高效产生波长可连续调谐的紫外飞秒光脉冲的便捷方法,为基于紫外超短脉冲的相关研究提供了有效工具.     

稀土元素铕(Fu)的单光子共振和双光子共振激光增强电离光谱

本文首次报道稀土元素铕(Eu)的激光增强电离光谱.用紫外可调谐激光器检测到EuI 287.779um,EnI 287·887nm,EuI 289.254nm,Eul 289.303nm,EuI 289.383nm的单光子共振激光增强光谱.用可见区可调谐染料激光器检测到EuI的双光子共振激光增强电离光谱.对铕(Eu)原子激发和离化机理进行了讨论.稀土元素铕的检测限达10ng/ml.     

向列相液晶染料可调谐激光器的研究

对向列相液晶染料的可调谐激光器进行了光学特性研究. 以650 nm为中 心波长设计了SiO₂和TiO₂多层膜的一维光子晶体, 以激光染料与向列相液晶的混合物作为增益介质层, 制备了波长可调谐激光器.用Nd: YAG倍频脉冲激光器输出的532 nm激光抽运所制备的激光器样品得出如下光学特性: 激光发射波长随温度调谐范围为605.5---639.8 nm, 达到34.3 nm, 随电压调谐范围为634.5---619.5 nm, 达到15 nm. 发射激光每脉冲的阈值能量为12.3 μJ, 激光线宽小于1 nm.     

窄带可调真空紫外激光——气相中的非线性过程

本文评述了气相中的非线性过程产生VUV激光的最新进展、在稀有气体和Hg蒸汽中,通过四波和六波混频过程,可产生可调范围为λ_(VUV)=60～200nm,光谱宽度为E=0.01～1cm,~(-1),光强为2×10~θ～ 3×10~(13)光子/脉冲的VUV激光、这一光源具有技术简单,使用方便等特点,对原子、分子的激光光谱以及激光化学等方面的深入研究起了重要作用。     

窄带可调真空紫外激光——气相中的非线性过程

本文评述了气相中的非线性过程产生VUV激光的最新进展、在稀有气体和Hg蒸汽中,通过四波和六波混频过程,可产生可调范围为λ_(VUV)=60～200nm,光谱宽度为E=0.01～1cm,~(-1),光强为2×10~θ～ 3×10~(13)光子/脉冲的VUV激光、这一光源具有技术简单,使用方便等特点,对原子、分子的激光光谱以及激光化学等方面的深入研究起了重要作用。     

一种新型有机染料HEASPS在不同波长时的双光子吸收和频率上转换性质研究

报道了一种新型双光子吸收染料 ,即反式 - 4- [4′- (N-羟乙基 - N-乙基胺基 )苯乙烯基 ]- N-甲基吡啶对甲苯磺酸盐的非线性光学性质 ;测试了染料在 72 0～ 110 0 nm波段的非线性透过率曲线。结果发现 :双光子吸收最强波长相对线性吸收峰波长的两倍处有明显蓝移 ;计算出的相应波长的双光子吸收截面在 930 nm处染料有最大双光子吸收截面(2 .0 6× 10 - 4 7cm4·s/ photon) ;测量了染料在 90 0～ 110 0 nm波段的上转换效率 ,在 10 2 0 nm处有最高效率 (5 .1% ) ,最高激射效率的波长相对最强双光子吸收的波长有明显红移     

CS2+多光子光解产生CS+的动力学研究

在射流气束条件下 ,利用第一束 4 83.2nm的电离激光使中性CS2 分子通过 (3+1)共振增强多光子电离 (REMPI)制备出纯净的CS2 + 分子离子 ;用第二束解离激光在 385～ 4 35nm扫描 ,由获得的光解离碎片激发(PHOFEX)谱研究了光解CS2 + 产生CS+ 的两种动力学途径 .当第一束电离激光和第二束解离激光在时间上有约6 0ns的延迟 (远大于激光脉宽约 5ns)时 ,光解CS2 + 母体离子产生CS+ 碎片离子有明显的阈值效应 ,由PHOFEX谱确定了CS+ 的绝热出现势 (5 .85 2± 0 .0 0 5 )eV (从CS2 + 的 X 2 Πg ,3 / 2 (0 ,0 ,0 )能级位置算起 ) ,测量了 4 72 0 0～5 0 4 0 0cm-1双光子能量范围内碎片离子的分支比CS+ /S+ (从 0逐渐增加到略大于 1) .提出了这种情况下CS2 +产生CS+ 碎片离子的 [1+1]共振增强多光子解离机理 :通过单光子激发产生CS2 + ( X 2 Πg)→CS2 + ( 2 Πu)跃迁、 和 X高振动能级耦合使得可以产生到CS2 + ( B2 Σ+ u)的单光子跃迁 ,再经由 B态与4Σ-和2 Σ-排斥态耦合使CS2 + 解离为CS+ (X2 Σ+ )和S(3 P) .但是 ,当电离激光和解离激光时间上重合时 ,不再能分辨出CS+ 的出现阈值 .这表明 ,除了存在着上述的产生CS+ 的 [1+1]共振增强多光子解离机理外 ,在激光波长长于 4 2 3.8nm时还存在着 [1+1+1’]、[1+1     

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.