飞秒激光泵浦Ⅰ类BBO晶体中自发参量下转换的研究

实验研究了飞秒脉冲激光泵浦Ⅰ类BBO晶体中自发参量下转换效应,以及产生的彩色锥形辐射现象.系统分析了各参量对二次谐波转换效率及彩色锥形辐射现象的影响.研究结果表明:蓝绿色锥形辐射具有最大的发散角,这与相位匹配理论模拟结果吻合;正入射时,增大泵浦光强及晶体厚度均会引起彩色锥形辐射亮度增加,且最大出射角中心波长往长波移动;...     

飞秒激光泵浦I类BBO晶体中自发参量下转换的研究

实验研究了飞秒脉冲激光泵浦I类BBO晶体中自发参量下转换效应,以及产生的彩色锥形辐射现象.系统分析了各参量对二次谐波转换效率及彩色锥形辐射现象的影响.研究结果表明:蓝绿色锥形辐射具有最大的发散角,这与相位匹配理论模拟结果吻合|正入射时,增大泵浦光强及晶体厚度均会引起彩色锥形辐射亮度增加,且最大出射角中心波长往长波移动|不同光束偏振态下可依次观察到彩色锥形辐射、超连续现象.     

泵浦光的入射角对参量下转换的影响

利用参量下转换制备相关光子实现对各类探测器的量子效率的定标,是近年来兴起的新型定标技术。由于参量下转换的光子转换效率低,探测器输出信号信噪比小。采用短波激光器泵浦PPLN周期极化性晶体,可获得较大功率的参量光。分析了泵浦光在PPLN晶体端面的入射角对参量光的转换效率、空间分布、参量带宽及晶体周期的影响,并进行了数值模拟。分析发现当泵浦光正入射晶体端面时,参量光功率转换大,带宽小,发散角大;随着泵浦光入射角度逐渐增大,参量下转换的转换效率降低,相关光子的带宽变小,发散角变小且参量光的中心波长的空间位置会发生相对偏移。该研究结果可为相关光子探测、模拟探测器量子效率的高精度定标提供理论依据。     

4.1μm高功率中红外内腔光参量振荡器

提出了一种紧凑型、声光调Q、高功率的4.1μm中红外内腔式氧化镁掺杂的周期性极化铌酸锂晶体光参量振荡激光系统.基于内腔单谐振光参量振荡器动力学模型,对内腔光参量振荡器的阈值倍数及下转换效率的影响因素进行分析,提出实验中对光参量振荡阈值调节的办法,优化了大功率情况下光参量振荡的下转换效率.引入共振泵浦与单端键合晶体方式提高了大功率泵浦的热稳定性.基于光场传输理论与谐振腔稳定性理论,并考虑增益介质热效应,数值模拟了大功率泵浦注入时腔内基频光、信号光及闲频光的三波光场模式匹配,以确保光参量振荡器在高功率下稳定运转.对光参量振荡阈值进行调节,提高了参量光的下转换效率,最终得到4.125μm的中红外高重频脉冲瓦级激光输出,激光重复频率1～100kHz可调,脉冲宽度小于9ns,最高单脉冲能量36.7J,最高峰值功率4.257kW,最高输出功率为1.12 W,其对应的下转换效率为29.7%,最大光光转换效率为4.26%.     

Ⅰ类自发参量下转换光谱圆环空间分布

理论模拟了I类自发参量下转换产生的相关光子光谱圆环的空间辐射角度及光强分布,搭建了相关光子圆环光强分布的实验测量装置,该装置利用355 nm连续激光器泵浦BBO晶体产生I类自发参量下转换相关光子圆环,使用CMOS传感器作为相关光子圆环探测单元,通过2×2维点阵扫描方法探测了完整的相关光子圆环,测量的光谱分布为430～860 nm.实验结果表明:测量的相关光子圆环直径与理论值符合较好;Ⅰ类自发参量下转换过程产生的相关光子在空间呈同心圆环分布;相关光子光谱具有宽光谱分布特点.     

I类自发参量下转换光谱圆环空间分布

理论模拟了I类自发参量下转换产生的相关光子光谱圆环的空间辐射角度及光强分布,搭建了相关光子圆环光强分布的实验测量装置,该装置利用355nm连续激光器泵浦BBO晶体产生I类自发参量下转换相关光子圆环,使用CMOS传感器作为相关光子圆环探测单元,通过2×2维点阵扫描方法探测了完整的相关光子圆环,测量的光谱分布为430~860nm.实验结果表明:测量的相关光子圆环直径与理论值符合较好;I类自发参量下转换过程产生的相关光子在空间呈同心圆环分布;相关光子光谱具有宽光谱分布特点.     

周期极化铌酸锂中通信波段纠缠双光子的波长管理方法

理论研究了周期极化铌酸锂晶体中通过自发参量下转换产生的纠缠双光子的波长管理方法.简并自发参量下转换产生的单色偏振纠缠双光子的波长,可以通过调整晶体的极化周期或工作温度自由地调节,特别是在加工好的晶体中极化周期是确定的,因而调整晶体的工作温度更加便捷.对于双色偏振纠缠双光子来说,不需改变入射的泵浦光以及晶体的极化结构,仅通过调节晶体的工作温度就可以实现o光光子和e光光子波长的严格交换.     

LD端面泵浦折叠腔Nd∶YVO4/LBO激光器

对端面泵浦Nd∶ YVO4构成的四镜Z型折叠腔结构进行了理论研究,合理调整谐振腔的参量关系,使谐振腔能够适应不同泵浦功率下激光晶体热焦距的变化,同时所设计的折叠腔还具有腔参量调整灵活等特点.以LBO晶体为倍频晶体,采用Ⅰ类角度调节位相匹配技术,在双端泵浦光功率为26 W时,成功地获得了4 W稳定的连续绿光输出.Nd∶ YVO4/LBO绿光激光器输出为4 W时的稳定性为1.3%,其光-光转换效率达到13%.     

双共振准相位匹配光学参量振荡器的调谐特性

利用全固化单频Nd：YVO4激光器泵浦双共振难相位匹配铌酸锂连续光学参量振荡器，实验研究了该光学参量振荡器下转换光的调谐特性。通过改变PPLN晶体的温度及OPO的腔长，下转换光的调谐范围分别为189nm和175nm，通过改变泵浦光频率，信号光频率连续调谐375MHz。实验结果与理论计算值基本吻合。     

多波段相关光子光谱分布与时间相关性测量实验研究

基于相关光子的定标技术能够实现“无标准传递”绝对定标,研究相关光子的光谱辐射特性及时间相关特性对遥感器在宽波段的辐射定标具有重要意义。为满足光电探测器在宽谱段量子效率定标需求,基于相关光子的定标技术有必要从单一波段向更多波段扩展。根据自发参量下转换所满足的相位匹配条件,推导出相关光子在晶体内的非共线角计算公式,通过数值模拟相关光子光谱辐射角度分布规律,优化晶体相位匹配角,使得自发参量下转换产生的相关光子具有宽光谱分布,并且相关光子辐射角度与光谱波长能够一一对应。根据光谱分布数值模拟结果,建立了多波段相关光子的光谱分布和时间相关性测量实验系统,利用该系统测量了四对相关光子的光谱分布、符合计数、相关时间以及偏振特性。其中,测量的光谱分布范围为633~808 nm,最大光谱分布测量偏差为1.51 mm,光谱分布实验测量结果与数值模拟结果符合一致;测量了四对相关光子对的相关时间,最小相关时间为0.32 ns,并在实验中观察到了“符合三峰”现象;相关光子单光子计数及符合计数与泵浦光的偏振方向呈正弦函数关系。实验研究表明,相关光子对具有可见光~近红外宽波段分布、时间相关及偏振特性。论文研究结果在国内外尚属首次报道,该研究结果有望应用于光电探测器在多波段的辐射定标。     

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.