超声调制漫射光子自相关的Monte Carlo模拟

本文首次用Monte Carlo方法研究了超声调制生物介质中漫射光子的时间自相关性质,讨论了超声参量、运动参量和散射参量对自相关函数的影响.正常生物组织和病变生物组织的自相关函数有明显的差别,超声调制自相关函数为光学医学诊断提供一种新参考.     

计算加速度相关Lagrange函数的方法

研究从运动微分方程构造加速度相关Lagrange函数的问题,给出一种由方程的自伴随形式计算加速度相关Lagrange函数的方法.举例说明结果的应用. 关键词： 分析力学 加速度相关Lagrange函数 自伴随性 Lagrange力学逆问题     

动态光散射颗粒测量中相关函数最优截断点的确定

从动态光散射信号中反演纳米颗粒粒度分布,结果准确性和重复性受测量的自相关函数数据点影响,数据点长度不同会导致不同的反演结果.为了解决该问题,提出了一种根据拟合自相关函数的均方根误差来截断自相函数的方法,该方法通过设置拟合误差阈值来自适应地选择最佳自相关函数数据点数.实验结果表明,使用均方根误差阈值方法获得的颗粒粒度分布...     

一种无旁瓣相位编码信号脉冲压缩方法

本文介绍了一种相位编码波形,其自相关函数的实部的旁瓣为零.因此,以自相关函数的实部作压缩脉冲时,压缩脉冲无旁瓣.本文分析了这种方法的原理及实现框图,并给出了计算机模拟结果.     

光纤环相干性分析

运用部分相干光理论,得到了光纤环自相关函数,并利用自相关函数推导出了光谱透射因子,它把光纤环输入、输出光场的光谱密度函数直接联系起来,代表了光纤环的相干性.应用光谱透过因子,结合光纤环耦合输出系数、光学长度、损耗分析讨论了光纤环相干性.     

光注入下外光反馈半导体激光器输出自相关特性研究

基于半导体激光器(SL)受到外部扰动(光反馈和光注入)下的速率方程组,研究了反馈系数、延迟时间、注入强度和频率失谐对半导体激光器输出混沌信号自相关特性的影响.研究表明：上述四个参量对SL输出混沌信号的自相关函数曲线的半高全宽(FWHM)以及边峰抑制比都有影响;通过合理选择各参量,可以使SL输出的混沌信号具有尖锐的自相关函数曲线分布,其FWHM可降到0.02 ns,比已有相关报道提高了一个数量级. 关键词： 半导体激光器 光反馈 光注入 自相关函数     

自相关在双向剪切激光准直检校中的应用

对基于双向剪切干涉的激光准直检校方法进行了研究.采用CCD对干涉条纹进行采集,提出了用自相关比较条纹宽度的方法,解决了人眼直接观察会引入主观判断误差且不适用于非可见光的问题.理论分析和实验表明：条纹灰度分布的自相关函数会以条纹宽度值为周期出现峰值;比较两半场条纹自相关函数峰值出现的位置即能得知入射激光的准直与否.     

纵向运动双散射散斑场的动态特性研究

研究了慢纵向运动双散射散斑场的统计性质,得出纵向运动双散射散斑振幅自相关函数、强度涨落自相关函数,并由此得出,夫朗和费衍射区内的接收平在上任意点上的自相关函数是时间的非平稳随机过程,而在光轴附近小区域内,强度涨落自相关函数是时间的平稳随机过程,相关时间与散射体纵向运动速度或反比,理论和实验结果相一致。     

漫散射光自相关用于生物介质成象的实验研究

本文用背散射光时间自相关函数研究了随机介质中隐一生物组织时的成象,由相关时间τ_c确定出隐含物体的位置.还研究了生物组织在随机介质内不同深度时自相关函数的性质.     

偏振模色散模拟器的特性

通过对微分群时延差(DGD)统计特性和偏振模色散(PMD)矢量自相关函数的数值模拟，分析比较了目前常用的三种PMD模拟器的性能. 结果表明：在DGD分布方面，由DGD发生器构成的模拟器，一个DGD发生器就可与实际光纤PMD的统计特性相符合，而由保偏光纤和可旋转的连接器构成的模拟器则至少需要15段. 在自相关函数方面，基于保偏光纤的模拟器取决于各段光纤的延时量，而基于DGD发生器的模拟器在中心频带外的自相关值很平稳，可视为常数. 关键词： 光纤通信 偏振模色散 偏振模色散模拟器 自相关函数     

Two-color cross-correlation of fs-laser pulses by two-photon induced photoconductivity for near and far field optical measurements

Two-color two-photon induced photoconductivity in a GaAsP diffusion type photodiode is demonstrated by measuring femtosecond cross-correlation functions for widely separated wavelength pairs of 775 and 1300 nm. Results are obtained for a range of tunable wavelengths and average powers of the incident lasers by measuring the two-photon induced photocurrent as a function of the optical delay between the pulses. The temporal autocorrelation of femtosecond laser pulses in the near-field of a small diameter aluminum coated optical fiber tip is also obtained with the same photodiode method for single colors.     

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP comprises three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. This device is very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers due to its resonantly enhanced optical nonlinearity and sub-ps time resolution. We report on interferometric autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). The intense FEL radiation further allows us to study the saturation properties of two-photon QWIPs at liquid nitrogen temperature and their detection properties at 300 K. The device is well suited for standard diagnostics of the FEL pulse shape via interferometric autocorrelation.     

Two-photon fluorescence measurements in a hexagonal CdS crystal

Measurements of two-photon fluorescence (TPF) in a CdS crystal using a dye laser pumped by a Q-switched ruby laser are reported. The TPF is measured as a function of source intensity and wavelength. The total fluorescence shows linear and non-linear dependence on the source intensity; from these measurements the TPF part is determined. The measured transition of the crystal's fundamental gap is 2.5 ± 0.02 eV and another forbidden one-photon but allowed two-photon transition is found at 3.24 ± 0.02 eV.     

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     

Femtosecond pulse duration measurement by two-photon fluorescence detection

The two-photon absorption induced fluorescence in a methanolic rhodamine 6G solution is used to measure the pulse duration of a cw femtosecond Kerr–lens mode-locked Ti:sapphire laser by a Michelson-interferometer-type autocorrelator arrangement. Interference-free autocorrelation traces are obtained by correlating perpendicular polarized pulses. The two-photon polarization parameter of rhodamine 6G is determined by contrast ratio measurement. The time resolution limits are discussed.     

Parallel excitation pathways in ultrafast interferometric pump-probe correlation measurements of hot-electron lifetimes in metals

Hot electron (E-E_Fermi=0.75 to 1.55 eV) lifetimes for cesiated Cu(100) and Cu(111) surfaces are measured via interferometric time-resolved two-photon photoemission with a 19-fs intensity FWHM mode locked Ti:sapphire laser at 1.55 eV. The data are analyzed using the optical Bloch equations and a laser pulse characterized in situ via surface second-harmonic generation interferometric autocorrelation. It is found that the retrieved hot-electron lifetimes are unphysically fast, and have a strong dependence on the temperature of the sample and the polarization of the laser. A simple explanation for the data is that the measured signal consists of contributions from transitions through both virtual and real intermediate states. Received: 26 July 2000 / Accepted: 8 September 2000 / Published online: 12 October 2000     

Autocorrelation measurement of femtosecond laser pulses by use of a ZnSe two-photon detector array

We demonstrate autocorrelation measurements of 85-fs Ti:sapphire laser pulses, using a 32-pixel ZnSe detector array in a single-shot geometry. The two-photon photoconductor is fabricated by deposition of an array of interdigitated gold fingers on a single-crystal ZnSe substrate.     

Autocorrelation measurement of 6-fs pulses based on the two-photon-induced photocurrent in a GaAsP photodiode

We experimentally characterize the two-photon response of a GaAsP photodiode by use of a femtosecond Ti:sapphire laser tuned below the diode bandgap. The photodiode is shown to be highly suitable for real-time second-order autocorrelation measurements of pulses as short as 6fs in duration and with energies as small as a few picojoules.     

Production of doubly charged helium ions by two-photon absorption of an intense sub-10-fs soft x-ray pulse at 42 eV photon energy

We report on the observation of doubly charged helium ions produced by a nonlinear interaction between a helium atom and photons with a photon energy of 42 eV which are generated with the 27th harmonic of a femtosecond pulse from a Ti:sapphire laser. The number of ions is proportional to the square of the intensity of the 27th harmonic pulse, and thus two-photon double ionization should be dominantly induced as compared with other nonlinear processes accompanying sequential ionization via a singly charged ion. This phenomenon is utilized to measure the pulse duration of the 27th harmonic pulse by using an autocorrelation technique, for the first time to our knowledge, and as a result a duration of 8 fs is found.     

实时测量飞秒激光脉冲的二次谐波相关技术研究

本文介绍了一种具有飞秒分辨率实时监测激光脉宽的二次谐波自相关技术。该技术既可进行强度自相关测量,又可进行自标定的干涉自相关测量。干涉自相关测量精度达1fs,强度自相关测量相对误差为3.6%。