电光调制对外部光反馈垂直腔表面发射激光器输出矢量混沌偏振的操控

基于周期性极化铌酸锂晶体的线性电光效应耦合波理论,数值研究了电光调制对外部光反馈垂直腔表面发射激光器(VCSEL)输出矢量混沌偏振模的操控.研究结果表明,VCSEL输出的偏振度随着电光晶体的长度或施加于电光晶体的外电场强度成周期性转换,控制一定的施加外电场强度和晶体的长度,激光器的不同参数下引起初始混沌偏振态都可以转换为其他任意混沌偏振态.特别是合理选择一定的施加外电场强度或晶体长度,VCSEL输出的任意混沌偏振模可以转换为完全一致的两线性混沌偏振模(x和ŷ偏振),即两线性混沌偏振模的能量能够达到稳定和完全均衡.     

Polarization-insensitive all-optical wavelength conversion of 320 Gb/s RZ-DQPSK signals using a Ti:PPLN waveguide

Polarization-insensitive wavelength conversion of a single channel 320 Gb/s RZ-DQPSK data signal using a Ti:PPLN waveguide in a bi-directional loop configuration with less than 0.5 dB polarization sensitivity is reported. The conversion efficiency with polarization scrambling of the signal was ?21 dB, which includes 9.2 dB of passive losses in the whole Ti:PPLN subsystem. In BER measurements error-free operation with 2 dB OSNR penalty for the converted signal was achieved. Theoretical and experimental investigations of the temporal shape and chirp of the converted data pulses show only very little broadening and chirping, indicating the potential for wavelength conversion of even much higher data rates.     

Temperature and wavelength dependent refractive index equations for MgO-doped congruent and stoichiometric LiNbO<Subscript>3</Subscript>

We present wavelength- and temperature-dependent refractive index equations for 5% MgO-doped congruent PPLN and for 1% MgO-doped stoichiometric PPLN crystals valid for a wide spectral and temperature range. The dispersion equations were derived from quasi-phase-matched nonlinear interactions with these two crystal compositions in the near and mid-infrared. The results show a good agreement with previously published frequency conversion experiments. PACS 42.65.Ky; 42.65.Yj; 42.70.Mp     

Theoretical investigation of phonon-polariton modes in undoped and ion-doped PPLN crystals

The properties of phonon-polaritons in one-dimensional undoped and ion-doped periodically poled superlattice lithium niobate (PPLN) crystals are studied theoretically. In this paper, we focus on the case that the electric field vector of the electromagnetic wave is parallel to the acoustic propagation vector. The phonon-polaritons are products of the coupling between electromagnetic and acoustic waves of mutually perpendicular propagations. Based on the theory of the phonon-polariton in PPLN, we investigate the phonon-polariton in ion-doped PPLN. The effects of the duty cycle and the impurities upon the properties of phonon-polaritons are discussed in detail. Some potential applications are also discussed.     

布里渊光时域分析传感器的消偏振衰落技术

在布里渊光时域分析(BOTDA)光纤传感器中,由于受激布里渊散射的偏振相关性的影响,激励光和探测光的偏振态随机变化引起的信号起伏严重影响布里渊散射光信号的稳定测量。通过建立与偏振态有关的激励光和探测光的耦合方程,推导出接收探测光光强关于激励光和探测光偏振态的理论公式,并由此提出偏振扩展接收技术。理论分析表明,偏振扩展接收技术可以有效地抑制偏振相关信号衰落。通过对耦合方程进行模拟,验证了理论分析的正确性和偏振扩展接收技术的可行性。最后给出了采用偏振扩展接收的布里渊光时域分析实验结果,与理论分析和模拟具有较好的一致性。     

Wave coupling theory of quadratic electro-optic effect and its applications

Wave coupling theory of quadratic electro-optic effect is developed by considering the third-order nonlinearity as a perturbation in lossless nonmagnetic centrosymmetric medium. A general solution of the resultant equations is given to describe the quadratic electro-optic effect for the electromagnetic wave propagating along an arbitrary direction with applied external electric field. The effect of the polarization state on electro-optic coupling is studied in paraelectric phase KLTN crystals. A novel application of wave coupling theory of quadratic effect is described, namely polarization rotator, which can rotate the polarization of the electromagnetic wave with an arbitrary angle.     

Three‐Dimensional,Time‐Resolved Profiling of Ferroelectric Domain Wall Dynamics by Spectral‐Domain Optical Coherence Tomography

We apply here spectral‐domain optical coherence tomography (SD‐OCT) for the precise detection and temporal tracking of ferroelectric domain walls (DWs) in magnesium‐doped periodically poled lithium niobate (Mg:PPLN). We reproducibly map static DWs at an axial (depth) resolution down to ～ 0.6 μm, being located up to 0.5 mm well inside the single crystalline Mg:PPLN sample. We show that a full 3‐dimensional (3D) reconstruction of the DW geometry is possible from the collected data, when applying a special algorithm that accounts for the nonlinear optical dispersion of the material. Our OCT investigation provides valuable reference information on the DWs’ polarization charge distribution, which is known to be the key to the electrical conductivity of ferroelectric DWs in such systems. Hence, we carefully analyze the SD‐OCT signal dependence both when varying the direction of incident polarization, and when applying electrical fields along the polar axis. Surprisingly, the large backreflection intensities recorded under extraordinary polarization are not affected by any electrical field, at least for field strengths below the switching threshold, while no significant signals above noise floor are detected under ordinary polarization. Finally, we employed the high‐speed SD‐OCT setup for the real‐time DW tracking upon ferroelectric domain switching under high external fields.     

Single photon level spectrum measurement at fiber communication band using frequency up-conversion technology

We have developed a polarization independent (PI) spectrometer based on frequency up-conversion technology for single photon level spectrum measurement at the fiber communication band. To overcome the polarization dependence of the frequency up-conversion process, we use two periodically poled lithium niobate (PPLN) waveguides with a polarizing beam splitter. We experimentally study the sensitivity and resolution of the PI up-conversion spectrometer. We demonstrate the spectrometer by way of a spectrum measurement of a single photon level signal in the communication band.     

Consistency in formulation of spin current and torque associated with a variance of angular momentum

Based on the Noether's theorem, we develop systematically and rigorously the spin-dependent formulation of the conservation laws. The effect of the electronic polarization due to the spin-orbit coupling is included in the Maxwell equations. The polarization is related to the antisymmetric components of spin current, and it provides a possibility to measure the spin current directly. The variances of spin and orbit angular momentum currents imply a torque on the "electric dipole" associated with the moving electron. The dependencies of the torque on the polarization and the force on the motions of spin-polarized electrons in a two-dimensional electron gas with the Rashba spin-orbit coupling are discussed.     

Effects of the Rashba spin-orbit coupling on Hofstadter's butterfly

We study the effect of Rashba spin-orbit coupling on the Hofstadter spectrum of a two-dimensional tight-binding electron system in a perpendicular magnetic field. We obtain the generalized coupled Harper spin-dependent equations which include the Rashba spin-orbit interaction and solve for the energy spectrum and spin polarization. We investigate the effect of spin-orbit coupling on the fractal energy spectrum and the spin polarization for some characteristic states as a function of the magnetic flux α and the spin-orbit coupling parameter. We characterize the complexity of the fractal geometry of the spin-dependent Hofstadter butterfly with the correlation dimension and show that it grows quadratically with the amplitude of the spin-orbit coupling. We study some ground state properties and the spin polarization shows a fractal-like behavior as a function of α, which is demonstrated with the exponent close to unity of the decaying power spectrum of the spin polarization. Some degree of spin localization or distribution around +1 or -1, for small spin-orbit coupling, is found with the determination of the entropy function as a function of the spin-orbit coupling. The excited states show a more extended (uniform) distribution of spin states.     

Electromagnetically induced transparency-like transmission in periodically poled lithium niobate with a defect

Electromagnetically induced transparency (EIT)-like transmission is observed in an electro-optic (EO) tunable periodically poled lithium niobate (PPLN) with a central defect. When an electric field is applied, light satisfying the phase matching condition keeps its original polarization, although it experiences polarization rotation then turns back. Therefore the corresponding light always may pass though freely but with an EO tunable phase and dispersion. On the other hand, light with a neighboring wavelength is blocked. An EIT-like spectrum is thus obtained with tunable group delay. A low-voltage bulk phase shifter with over two orders of magnitude larger index change is obtained.     

光纤拉曼放大器中增益的偏振相关特性研究

提出了一种改进的、可用于计算偏振相关拉曼增益的光纤拉曼放大器的非线性耦合波方程。建立了单模双折射光纤拉曼增益的数学模型,分析了线偏振抽运光以与光纤快轴成45°激励的单模双折射光纤拉曼放大器模型与实际具有随机双折射的光纤拉曼放大器的拉曼增益偏振相关特性的等价性。基于上述模型,提出了一个可定量表征单模光纤偏振模色散统计特性的拉曼增益偏振相关因子,用以替代常规的光纤拉曼放大器非线性耦合波方程中的偏振相关因子。计算结果与已报道文献的实验数据非常吻合。同时对抽运增益在同向和反向抽运方式下截然不同的增益偏振相关特性给出了合理的解释。     

Polarization mode coupling in long single-mode fibres

In this paper we look at the polarization state evolution of light propagating in long, birefringent, single-mode fibres in the presence of mode coupling. We show first that there always exist two orthogonal linear input polarizations which lead to linear output polarization for monochromatic light. The applicability of the coupled power equations is discussed and it is shown that they cannot be used with only two propagating modes. A more detailed analysis is then carried out for a uniform coupling and the effect of the spectral width of the source is investigated. Some insight into the time-dependent propagation is also given. Experimental results illustrate the theoretical considerations.     

拉曼效应对低双折射光纤偏振特性的影响

在低双折射光纤中,利用线偏振光满足的包含拉曼效应的非线性耦合模传输方程,通过引入斯托克斯参量,导出了斯托克斯参量所满足的耦合模传输方程.利用庞加莱球图示法,描述了拉曼增益效应作用下光波偏振态的演化,研究分析了拉曼效应对低双折射光纤中光波偏振态演化规律的影响.结果表明,当输入功率与运动常量满足一定关系时,拉曼增益效应改变了光波传输时其偏振态演化周期和偏振态的椭圆率.     

On the Maxwell equations in a Riemann-Cartan space

Considering the vacuum polarization effect in the presence of torsion, the modifications of the Maxwell equations are explicitly computated to first order in the spin-torsion coupling constant.     

The influence of pump beam polarization on the signal beam, in TWM in photorefractive crystals: Transmission geometry

Two-wave mixing (TWM) in transmission geometry of photorefractive (PR) crystals of 23 symmetry such as BSO, BGO is investigated by solving a set of coupled wave equations and taking their optical activity as one of the effective parameter. Coupled wave equations are solved numerically by fourth order Runge-kutta method and results are presented in graphical form. The effects of various parameters such as crystal thickness, optical activity, coupling coefficient, absorption, and input polarization angle of pump beam has been studied on the input beam polarization components. It has been observed that by varying the behaviour of pump beam polarization, signal beam can be modified. This can be used in various applications such as spatial light modulation and optical information processing, etc.     

Core polarization effects in the Hartree-Fock-random phase approximation schemes

Core polarization effects in odd nuclei are investigated in the framework of the Hartree-Fock and random phase approximation schemes. The results of the particle vibration coupling model are recovered by linearizing the equations of motion in the interaction Hamiltonian between the external and the core particles. The formalism is used to study the renormalization of diagonal and off-diagonal M1 matrix elements. It is found that M1 polarization effects exhibit a very strong dependence on the range of the force.     

双折射晶体中旋光效应的耦合波理论

与传统的旋光效应理论不同，采用三阶赝张量κ⁽²⁾_jkl描述旋光现象，并将相关的电极化强度作微扰处理，直接从Maxwell方程组出发推导出了双折射晶体中自然旋光效应的耦合波方程组并得到解析解.该解析解包含了已有的旋光效应宏观理论结果，可方便地用于描述任意偏振态的单色光波在任意点群的双折射旋光晶体中沿任意方向的传播行为.最后，以石英晶体为例,通过对出射光波的偏振态分析，研究了波矢失配对旋光效应的影响. 关键词： 耦合波理论 旋光效应 非线性光学     

Linear and nonlinear optical properties of excitons in semiconductor quantum wells and microcavities

Linear and nonlinear light propagation in single and multiple quantum wells and in semiconductor microresonators are studied on the basis of Maxwell’s equations. The treatment includes radiative broadening of quantum-confined excitons, radiative coupling between quantum wells as well as coupling of quantum wells to the cavity field of a microresonator for steady state or ultrashort pulse excitation. The dynamical evolution of the coherent quantum-well polarization under the influence of many-body effects is studied within a microscopic model. The theory is used to investigate the influence of exciton saturation and dephasing on pulse propagation and excitonic normal-mode coupling.     

Thomas-Fermi description of nuclear systems including spin-orbit interactions

The Thomas-Fermi statistical method is generalized to include spin-orbit interactions. The momentum distributions are given by toroids, different for two particle spin orientations. A system of two coupled differential equations is derived by a variational procedure for the densities of the two populations. From these equations the polarization at the surface of nuclear matter is calculated, as well as the change of the nuclear surface tension due to spin-orbit coupling. Within the statistical framework the coupling strength of the spin-orbit potential is found to be in reasonable agreement with experiment by using only the experimental single-particle level order of the shell model which implies an excess of states with spin parallel to the orbital angular momentum.