“Spreading” of a biphoton in a group-velocity-dispersion medium and two-photon interference

It is shown that two-photon Bell states can be prepared by “spreading” of a two-photon wave packet (biphoton) in a dispersive medium without compensating for group delays between photons with orthogonal polarizations or using narrow-band filters but by selecting the time correlation function. This is possible because two-photon interference effects are manifested in the shape of the time correlation function of intensity due to its spreading.     

飞秒超短脉冲的双光子干涉

利用钛宝石飞秒激光超短脉冲抽运BBO晶体,产生Ⅰ型自发光参量下转移的双光子对,在实验上得到双光子对的四阶干涉现象,证明了双光子纠缠态的关联特性。     

Three-wave interference with participation of polaritons

Second-order interference with respect to the field strength during spontaneous parametric light scattering was studied under the conditions of absorption at the frequencies of idler (polariton) modes. An expression is derived that describes the scattered light intensity as a function of the scattering angle for an arbitrary shape of the nonlinear interaction region. The interference was experimentally studied for the light scattering on polaritons in a nonlinear crystal (lithium iodate, iodic acid), with a double slit placed into a pumping beam in front of the crystal.     

Polarization entanglement generation at 1.5 μm based on walk-off effect due to fiber birefringence

In this Letter, a linear scheme to generate polarization entanglement at 1.5 μm based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four-wave mixing. A 90 deg offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single-side and two-photon interference in two nonorthogonal polarization bases are demonstrated. A two-photon interference fringe visibility of 89±3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly a efficient and stable fiber based polarization-entangled quantum light source at the optical communication band.     

Detection of two-mode compression and degree of entanglement in continuous variables in parametric scattering of light

Generation of “twin beams” (of light with two-mode compression) in single-pass optical parametric amplifier (a crystal with a nonzero quadratic susceptibility) is considered. Radiation at the output of the nonlinear crystal is essentially multimode, which raises the question about the effect of the detection volume on the extent of suppression of noise from the difference photocurrent of the detectors. In addition, the longitudinal as well as transverse size of the region in which parametric transformation takes place is of fundamental importance. It is shown that maximal suppression of noise from difference photocurrent requires a high degree of entanglement of two-photon light at the outlet of the parametric amplifier, which is defined by Federov et al. [Phys. Rev. A 77, 032336 (2008)] as the ratio of the intensity distribution width to the correlation function width. The detection volume should be chosen taking into account both these quantities. Various modes of single-pass generation of twin beams (noncollinear frequency-degenerate and collinear frequency-nondegenerate synchronism of type I, as well as collinear frequency-degenerate synchronism of type II) are considered in connection with the degree of entanglement.     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

Dispersion spreading of biphotons in optical fibers and two-photon interference

We present the first observation of two-photon polarization interference structure in the second-order Glauber correlation function of two-photon light generated via type-II spontaneous parametric down-conversion. In order to obtain this result, two-photon light is transmitted through an optical fiber and the coincidence distribution is analyzed by means of the start-stop method. Beyond the experimental demonstration of an interesting effect in quantum optics, these results also have considerable relevance for quantum communications.     

双光子抽运随机激光器中辐射光能量的演化

基于非相干反馈随机激光器的扩散方程理论，将扩散方程中的增益项改写为饱和形式，并利用有限差分方法，研究了双光子抽运随机激光器中辐射光能量随时间演化特性.对扩散方程的数值求解给出了系统辐射光能量由自发辐射开始，经历小信号增益状态得到放大，再到由于增益饱和形成稳态输出的整个演化过程.表明介质的无序导致了在较低能量情况下也能获得饱和增益这样的非线性效应.结果为研究获得双光子抽运随机激光器的稳态输出提供了理论依据. 关键词： 随机激光器 双光子抽运 饱和效应 无序介质中的光学效应     

双光子失谐对慢光和光存储影响的实验研究

利用电磁感应透明(EIT)效应在87Rb热原子气室中进行了慢光和光存储的实验研究,在单光子红失谐650 MHz处测量了双光子失谐对光脉冲延迟和光存储的影响.结果表明:在双光子失谐0—0.5 MHz范围内存在显著的光脉冲延迟和光存储恢复信号,其慢光波形与理论计算结果基本相符;而恢复光脉冲信号随着双光子失谐的变化出现形变,这是由于多个EIT子系统之间的干涉引起的.这一研究结果为连续变量光场在热原子系综中的存储提供了实验参考.     

一种全光开关及任意比能量输出光分束器的设计

基于光子晶体非线性Kerr效应设计了一种结构简单的任意比能量输出光分束器, 通过控制抽运光的强度来控制端口能量输出比. 该结构还可以实现控制光传输的动态开关作用, 并且具有插入损耗低、无串扰、信号光的开/关两个状态下的透射率相差非常大的特点, 因此该光开关具有很高的开关比. 关键词： 全光开关 光分束器 非线性Kerr效应 光子晶体     

Prediction of multichannel polarization-entangled photon pairs in a single periodically poled lithium niobate with a monochromatic pump

A method is presented for the first time to our knowledge for generating multiple-channel polarization-entangled photon pairs simultaneously based on type II quasi-phase-matched spontaneous parametric downconversion pumped by monochromatic light in a single periodically poled lithium niobate (PPLN) crystal. The expression for the count of the polarization-entangled photon pairs is analytically obtained. It is predicted that one-, two-, and even three-channel polarization-entangled photon pairs can be simultaneously generated just by suitably choosing the PPLN grating period and the pump frequency.     

基于啁啾脉冲干涉仪的高精度定位方案设计分析

提出利用啁啾脉冲干涉仪进行高精度空间定位的新方案,该方案利用2个具有强的经典频率关联的正负啁啾脉冲,入射到分束器后经和频晶体产生和频干涉信号,通过探测所得干涉信号精确确定经过不同路径的2个光信号的相对延迟时间,从而精确确定目标位置,实现高精度定位,理论上可以达到微米量级的定位精度。研究正负啁啾脉冲不完全对称即正负啁啾系数大小的差异对定位结果的影响。结果表明:正负啁啾系数大小的差异对定位结果影响很小,定位结果对这种差异有很强的容忍度。该方案除具有量子定位中HOM(Hong-Ou-Mandel)干涉仪的所有优点外,还具有装置简单,抑制损耗等优点。     

Optimization of self-frequency doubling in periodically and quasi-periodically poled nonlinear crystals

The theory of self-frequency doubling in a periodically poled nonlinear crystal is generalized to the case of varied domain thickness (quasi-periodically poled crystals). A particular statistical model of quasi-periodicity is considered. The effect of quasi-periodicity on the output emission intensity is calculated. The degree of quasi-periodicity allowable for practical applications is estimated, and the advantages of quasi-periodically poled crystals are discussed. The problem of maximization of the output emission intensity for periodically and quasi-periodically poled crystals is considered. The dependence of the output emission intensity on the crystal length is investigated, and an upper estimate for the maximum intensity is obtained. A formula for the optimal reflection coefficient of the output mirror as a function of the crystal length is derived.     

Transverse compression of two-photon wave packets

A method for generating two-photon wave packets (biphotons) with a small width of the spatial intensity correlation function (correlation radius) has been considered. We propose to prepare a two-photon wave packet inhomogeneously broadened in the angle by means of spontaneous parametric down-conversion in an inhomogeneous crystal. Such biphotons are not diffraction limited; i.e., their correlation radius is much larger than the inverse width of the spatial spectrum. However, the correlation radius of such a biphoton decreases as the biphoton propagates in free space, and transverse compression occurs at a certain distance from the crystal; i.e., the biphoton becomes diffraction limited.     

Two-photon path-entangled states in multimode waveguides

We experimentally show that two-photon path-entangled states can be coherently manipulated by multimode interference in multimode waveguides. By measuring the output two-photon spatial correlation function versus the phase of the input state, we show that multimode waveguides perform as nearly ideal multiport beam splitters at the quantum level, creating a large variety of entangled and separable multipath two-photon states.     

基于六角格子光子晶体波导的高效全光二极管设计

提出了一种基于六角格子光子晶体波导微腔和Fabry-Perot(FP)腔非对称耦合的全光二极管结构, 它由一个包含非线性Kerr介质的高Q值微腔与一个光子晶体波导中的FP腔组成. 通过有限时域差分方法对其传输特性进行了仿真, 发现通过两腔的非对称耦合可以实现在特定光强度下的正向传输、反向截止的功能. 在靠近微腔方向光入射时, 特定强度的光可以激发非线性微腔的Kerr效应, 改变了Fano腔的共振频率, 从而变成透射状态. 而远离微腔方向光入射, 由于这个不对称的结构造成场局域的分布不对称, 激发微腔Kerr效应的光强还不够, 所以光不能透射. 所设计的全光二极管结构具有良好的性能参数: 最大透射率高和高透射比、光强阈值低和易于集成等.     

光子晶体光纤非线性光谱特性的理论与实验研究

光子晶体光纤具有特殊的导光机制和结构可调性,可以产生奇异的色散特性及高非线性,为非线性光纤光学领域的研究提供了新的条件。受多种非线性光学效应的共同作用,在不同泵浦光脉冲参数条件下,不同结构参数及传输特性的光子晶体光纤能产生丰富的非线性光谱。利用分步傅里叶方法求解非线性薛定谔方程,模拟飞秒激光脉冲在光子晶体光纤中的传输过程,获得输出光谱与入射光脉冲参数(泵浦光峰值功率P、泵浦光波长λ、光脉冲形状、光脉冲宽度T_FWHM)、光纤结构参数(孔间距Λ、空气填充比d/Λ、光纤长度z)、传输特性(色散、非线性系数)的关系,分析拉曼孤子、色散波、自相位调制等非线性效应产生的光谱特性。利用光子晶体光纤包层节区进行非线性光学实验研究,获得了孤子波和色散波的宽带光谱输出。理论分析与实验测量的光谱中都包括了波长0.5 μm附近可见光波段的蓝移色散波、0.82 μm波段的剩余泵浦光、1.1 μm波段的孤子波、2 μm附近的红移宽带色散波。理论分析与实验测量结果一致,阐明光子晶体光纤中非线性光谱产生的物理原理,实现了对宽带光谱的可控输出,为高非线性光子晶体光纤的结构设计、制备及非线性光谱的应用研究奠定基础。     

基于光纤电流传感器的韦尔代常数优化测量法

韦尔代常数(Verdet)是决定光纤电流传感器(FOCT)灵敏度的重要因素之一。根据萨格纳克(Sagnac)干涉原理计算得到校准信号,并测量FOCT的实际输出信号,通过比较这两组信号建立目标函数,基于单纯形算法进行参数优化,从而得到石英光纤的韦尔代常数。实验结果与经典模型计算结果基本吻合。FOCT的输出信号经过测量电路会引入相位差;另外受到调制器和外界环境的影响,干涉回路的工作点会产生漂移,导致输出信号不对称而产生直流量。考虑到以上因素,提出的这种方法还能同时测量出电路的相位差、干涉回路的工作点以及反映非线性畸变的直流量。     

基于虚拟仪器的高精度Z扫描实验系统设计

设计了一个表征样品非线性特性的基于虚拟仪器的z扫描实验系统。利用Labview控制数字示波器TDS3012采集两路光强信号，并结合对微位移平台的控制实现Z扫描。以Nd：YAG飞秒放大器产生的800nm的飞秒激光作为探测光，经透镜聚焦后通过薄介质样品；用数字示波器测量样品在透镜焦点附近的透射光强，经过数据处理得到样品的非线性折射率和非线性吸收系数。利用该实验系统，对标准非线性吸收样品Rhodamine 6B进行了测量，计算得到其双光子吸收截面为σ^2=6．8GM（GM：10^-50cm4s photon^-1 molecule^-1），该结果与其他文献利用不同测量方法得到的结果接近，证明该实验系统的测量结果是正确的，可满足非线性测量的精度要求。     

三维光强有序分布的获得：从结构导出相干光束的配置

以体心四方为例，叙述了由相干结构导出入射光配置的原则和方法用相干光强的分布模拟晶格点阵。选择这样几组晶面，使它们的密勒指数较低且具有对称性。通过建立这些晶面与相干光束之间的几何关系，就可能获得光束配置的解析解。