双光子吸收的Franz-Keldysh效应

从实验上证实Hg_0.695Cd_0.305Te 光电二极管空间电荷区中存在双光子吸收的Franz-Keldysh效应.利用一个皮秒Nd:YAG激光器抽运的光学参量产生器和差频产生器作为激发光源,测量了入射波长为λ₀=7.92μm的脉冲激光所激发的光响应随入射光强的变化关系.脉冲光响应峰值强度随入射光强的增大呈现二次幂函数增强趋势.采用等效RC电路模型将脉冲光伏信号峰值与入射光强相关联,得到空间电荷区中强电场下单光束 关键词： Franz-Keldysh效应 碲镉汞 双光子吸收 脉冲光伏信号     

光泵重水气体产生THz激光的半经典理论分析

从半经典密度矩阵理论出发，采用三能级系统模型对光泵重水气体产生太赫兹激光进行了理论分析，求解得到了脉冲光泵重水气体分子产生太赫兹激光过程中激光信号增益系数G_s和抽运光信号被吸收系数G_p的表达式，通过迭代法对太赫兹激光信号的输出光强进行了数值计算，理论计算得到的频谱特性曲线完全符合受激Raman辐射的频谱特性，即频谱宽度较大、输出光强随抽运失谐量的改变而变化明显等特征.在脉冲激光抽运受激Raman辐射过程中，工作介质D₂O气体分子的偶极矩由于受到抽运脉冲光场的扰动发生变化，在频谱特性曲线中表现为受激辐射THz信号的谱线发生了分裂.理论计算结果与已报道的实验结果能较好地相符.     

光时分复用(OTDM)系统中基于电吸收调制器(EAM)的短脉冲光源及解复用器的设计考虑

研究了电吸收调制器(EAM)的衰减随外加反向电压增加而指数增加的情形下,短脉冲光源的脉冲输出和解复用器的开关窗口对EAM的消光效率、反向DC偏置电压以及正弦RF驱动信号的幅度等参量的依赖关系.在基于EAM的短脉冲光源中,输出脉冲的消光比等于EAM消光效率η与正弦驱动电压峰峰值V_pp的乘积,输出光脉冲的消光比和脉宽均与EAM的反向偏置电压无关,但输出脉冲的峰值功率与η、V_pp和V_b都有关系.在基于EAM的解复用器中,为了使解复用器的开关窗口近似为矩形,可利用EAM的削波效应,使V_pp/2>V_b.在EAM的消光效率η已知时,通过仔细设计反向DC偏置电压V_b和正弦驱动电压的峰峰值V_pp,达到OTDM解复用器所需要的开关窗口形状、宽度和消光比.     

香蕉叶片Chla荧光动力学参量对激发光强的反应

在蓝色激发光(λ_m＝660nm)作用下,香蕉叶片的叶绿素a(Chla)发出波长为720～740nm的荧光信号,荧光强度随时间呈现有规律的变化.随着激发光强的增加,荧光动力学参量F₀和F_VI/F_V呈上升趋势,F_V/F₀与F_V/F_M逐渐达到饱和,而CA和T_1/2呈下降趋势.研究结果表明,保证香蕉叶片光合机构高速有序运转的适宜光强在20～60μmol·m^-2·s^-1之间,过量的光强将造成叶片光合机构的损伤.讨论了香蕉叶片诸荧光动力学参量的生物学意义及其随激发光强度增加而变化的原因.     

LiNbO3晶体中屏蔽光伏孤子自偏转的 时空演化与可控因素

基于带输运模型理论建立了 LiNbO₃ 晶体屏蔽光伏孤子的时空演化动力学方程, 用有限差分方法求解发现, LiNbO₃ 晶体中明、暗屏蔽光伏孤子存在大的自偏转, 并且光孤子形状变得具有不对称性, 偏转方向的曲线斜率绝对值变大, 偏转反方向的曲线斜率绝对值变小. 分析研究表明影响其自偏转度和形变的因素包括受主浓度 N_A, 暗辐射强度 I_d 和外加电场 E₀ . 其他条件不变的情况下N_A 越大, 明孤子的自偏转度与形变越小, 暗孤子的自偏转度与形变反而越大; 对于 I_d , 它对明暗孤子的影响是相同的, I_d 越小, 晶体里诱导出的空间电荷场越容易达到饱和, 当信号光中心光强与暗辐射强度之比为 10^-1时无饱和现象产生; 随着 E₀ 数值的增大, 明孤子的自偏转度和形变减小, 而暗孤子的自偏转度和形变反而增大.     

LD抽运被动调Q固体激光器的脉冲稳定性

报道了二极管激光抽运Cr,Nd∶YAG被动调Q Nd∶YVO₄ 1064nm激光输出.为了提高被动调Q激光的输出稳定性，谐振腔采用长腔，增益介质位于谐振腔中间位置.实验中获得了稳定的被动调Q激光输出.在最高入射抽运功率为17.5W时，脉冲重复频率达到71.3kHz，脉冲宽度为0.4μs.调Q脉冲幅度不稳定性低于±10%，脉冲时间波动性低于±3.5%.     

复宗量Laguerre-Gauss光束在强非局域非线性介质中的传输

研究得到了偏离束腰入射的复宗量Laguerre-Gauss光束在强非局域非线性介质中传输的解析表达式,并且得到了其二阶矩束宽的解析解.通过例子研究了偏离束腰入射的复宗量Laguerre-Gauss光束在强非局域非线性介质中传输性质.结果表明：非(0, m)模的复宗量Laguerre-Gauss光束的光束形状随着传输而发生改变,并以Δz=πz_c为周期做周期性演化.而(0,m)模复宗量Laguerre-Gauss光束在演化过程中则形状保持不变,仅改变光束宽度;不论功率多大,在偏离束腰入射条件下总是表现为呼吸子;只有当其为束腰入射,并且入射功率等于临界功率时才能形成孤子.     

紧凑的非链式脉冲HF激光器

研究了气体组分和峰化电容对紫外光预电离脉冲HF激光器性能的影响.实验发现最佳气体混合比为SF₆/C₂H₆=20:1,峰化电容和主放电电容的最佳比为C_p/C_s=0.3,在最佳气体混合比时,最大激光脉冲峰值功率和对应的总气压随充电电压的提高而增大.同时得到了E/P值和激光能量的关系.整个器件的最大激光输出为400mJ,脉冲峰值功率为1.5MW,最大电光转换效率约为2.2%.     

静电场对高次谐波时频特性的影响

通过数值求解含时薛定谔方程，发现在静电场的作用下，对原子高次谐波谱截止区附近的谐波进行叠加可以得到脉冲间隔为一个光周期的阿秒脉冲链. 利用这种性质，用12fs波长为800nm的激光附加强度为激光强度0.25%的静电场脉冲驱动He原子可以得到脉宽为220as的单个脉冲. 当静电场与激光场幅度之比达到0.39时，原子谐波谱的截止区可以被拓宽到I_p+9.1U_p，同时，次数高于I_p+0.7U_p的谐波的相位几乎是锁定的，因此可以得到谱宽大于40eV且相位锁定的谐波. 对这部分谐波进行叠加就可以得到脉宽小于100as的阿秒脉冲链.     

空气簇射切伦科夫光观测

用快、慢光电倍增管分别构成两种探测装置,记录了广延空气簇射(EAS)切伦科夫(C)光的脉冲形状和光强.测量了海平面EAS(能量E₀>5×10¹⁴eV)C光脉冲的半高全宽(FWHM)分布和光子密度谱.两种光强探测结果的比较表明:由GDB100慢光电倍增管和峰值ADC组成的光强测量系统,对于测定EAS C光的光强是一种简单和经济的好方法.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.     

Fiber Modal Index Measurements Based on Fiber Gratings

A novel method based on fiber gratings for measuring the effective indices of fiber modes is proposed. The effective index difference between the core mode and a cladding mode was obtained by analyzing the interference fringe of a pair of cascaded long-period fiber gratings. In order to extract the core mode index from the measured index difference, an index matching oil immersion method is proposed. By analyzing the interaction between the cladding mode and the oil applied on the cladding surface, the mode order and the effective index of the involved cladding mode might be calculated. Experimental results about the interference fringe shifts induced by the oil index and the oil-applied length are also presented.     

Optical Pulse Compression Schemes That Use Nonlinear Bragg Gratings

Nonlinear optical pulse compression of picosecond pulses typically requires long lengths of optical fiber and multiple components. Periodic structures, such as fiber Bragg gratings, are highly dispersive at wavelengths outside of the photonic bandgap. This implies that such gratings can be used as very short all-fiber compressors. In this paper a number of such compression schemes are reviewed involving uniform and nonuniform fiber Bragg gratings, relying on both soliton and nonsoliton compression principles. Experimental results supporting the corresponding underlying theory are also presented. Finally, an extension of one of the compression schemes is shown, which allows the generation of adjustable high-repetition rate soliton trains.