非局域自散焦克尔介质中空间光暗孤子成丝的理论与实验研究

对非局域自散焦克尔介质中的空间光暗孤子成丝进行了研究. 理论上从非局域非线性理论模型出发, 数值模拟研究了非局域程度和吸收系数对暗孤子成丝的影响. 当入射背景光强一定时, 非局域程度越大成丝起始点越远、成丝数量越少; 而当入射背景光强与临界光强之比一定时, 非局域程度基本不影响成丝起始点以及成丝数量, 且非局域下的成丝数量与局域下一样. 此外, 当入射背景光强一定时, 吸收系数越大成丝数量越少. 实验上通过改变染料溶液的浓度以及背景光斑的椭圆率, 分别研究了样品浓度和背景光斑椭圆率对暗孤子成丝的影响. 当入射背景平均光强一定时, 样品浓度越小成丝数量越少, 背景光斑椭圆率越小成丝数量越少; 而当入射背景平均光强与临界光强之比一定时, 样品浓度基本不影响成丝数量. 在实验中还观察到了光学冲击波现象.     

一种基于非线性光纤环镜开关特性的超短光孤子产生方法

提出了一种利用非线性光纤环镜的开关特性将连续波同时转化为亮孤子和暗孤子的新方法 ,即让连续波和另一波长的调制脉冲串共同耦合入光纤环镜 ,交叉相位调制使得一部分连续波被环镜透射 ,其余部分被反射 ;再让透射波和反射波分别在反常色散光纤和正常色散光纤中传输 ,自相位调制和群速度色散之间的相互作用使得透射波和反射波分别演化为亮、暗孤子。数值计算表明 ,该方法不仅可产生脉宽比调制脉冲窄、重复频率比调制脉冲高的亮孤子和暗孤子 ,而且几乎可将全部的连续波能量转化为孤子能量。     

Solitons in photovoltaic photorefractive media with an external electric field

We use the classical Lie-group method for studying the evolution equation in photovoltaic photorefractive media with an external electric field, reducing it to some similarity equations firstly, and then obtain some exact analytical solutions including the soliton solution, the period solution and the oscillatory solution. We also obtain the bright soliton, dark soliton, gray soliton from these similarity equations with the numerical method. Furthermore, we investigate what factors contribute to the beamwidth of these solitons with the numerical method and know the beamwidth of these solitons are associated with the external electric field, the photovoltaic field and the intensity ratio of the incident soliton.     

Experimental Observation of Bright and Dark Solitons Mode-Locked with Zirconia-Based Erbium-Doped Fiber Laser

We demonstrate the generation of dark and bright solitons with our homemade zirconia-based erbium-doped fiber and graphene oxide(GO) saturable absorber in anomalous dispersion region.The GO is fabricated using an abridged Hummer's method,which is combined with polyethylene oxide to produce a composite film.The film is sandwiched between two optical ferrules and embedded in the laser cavity to enhance its birefringence and nonlinearity.The self-starting bright soliton is easily generated at pump power of 78 mW with the whole length cavity of 14.7 m.The laser produces the bright pulse train with repetition rate,pulse width,pulse energy and central wavelength being 13.9 MHz,0.6 ps,2.74 p J and 1577.46 nm,respectively.Then,by adding the 10 m of single mode fiber into the laser cavity,dark soliton pulse is produced.For the formation of dark pulse train,the measured repetition rate,pulse width,pulse energy and central wavelength are 8.3 MHz,20 ns and 4.98 p J and1596.82 nm,respectively.Both pulses operate in the anomalous region.     

拉曼增益对高双折射光纤中暗孤子俘获的影响

从高双折射光纤中含有拉曼增益的耦合非线性薛定谔方程出发, 利用拉格朗日方法, 推导出了暗孤子俘获的阈值, 并利用快速分步傅里叶变换, 模拟了孤子的两个正交偏振分量的演化, 对比了两种方法得到的阈值, 探究了暗孤子俘获受拉曼增益的影响. 研究发现解析解所得阈值比数值解偏小, 且群速度失配越小时, 二者符合得越好; 并且拉曼增益减小了暗孤子的俘获阈值, 当平行拉曼增益增大时, 俘获阈值减小加快.     

Soliton generation via continuous stokes acoustic self-scattering of hypersonic waves in a paramagnetic crystal

A new mechanism is proposed for continuous frequency down-conversion of acoustic waves propagating in a paramagnetic crystal at a low temperature in an applied magnetic field. A transverse hypersonic pulse generating a carrier-free longitudinal strain pulse via nonlinear effects is scattered by the generated pulse. This leads to a Stokes shift in the transverse hypersonic wave proportional to its intensity, and both pulses continue to propagate in the form of a mode-locked soliton. As the transverse-pulse frequency is Stokes shifted, its spectrum becomes narrower. This process can be effectively implemented only if the linear group velocity of the transverse hypersonic pulse equals the phase velocity of the longitudinal strain wave. These velocities are renormalized by spin-phonon coupling and can be made equal by adjusting the magnitude of the applied magnetic field. The transverse structure of the soliton depends on the sign of the group velocity dispersion of the transverse component. When the dispersion is positive, planar solitons can develop whose transverse component has a topological defect of dark vortex type and longitudinal component has a hole. In the opposite case, the formation of two-component acoustic “bullets” or vortices localized in all directions is possible.     

光纤中基于拉曼放大与脉冲压缩的超短光孤子产生

提出一种在单模光纤负群速色散枢由弱脉冲产生高强度超短光孤子的新方法。     

钛宝石飞秒激光器中孤子分子的内部动态探测

孤子是自然界中一种基本的非线性波动传递形式,孤子间的相互作用能够映射出复杂非线性系统的多体动力学过程,具有重要的基础研究价值.被动锁模激光器是研究孤子相互作用的理想平台.光孤子之间的吸引、排斥作用能够形成孤子分子,而时间拉伸色散傅里叶变换(TS-DFT)技术使得实时探测孤子分子动力学成为可能.基于TS-DFT技术,本文实验研究了钛宝石飞秒激光器产生的孤子分子的内部动态,通过改变抽运功率,分别观察到了间隔为180 fs的稳定的孤子分子和间隔为105 fs的具有微弱相位振荡的孤子分子,后者的振动幅度仅为0.05 rad.实验发现受到环境影响,稳定态的孤子分子还能够转变为相位滑动状态.这些间隔为百飞秒量级的光学孤子分子对于研究孤子的近程非线性相互作用具有突出的意义.     

Soliton formation from a gaussian pulse in an optical fiber

Propagation of a frequency-modulated Gaussian pulse along an optical fiber with a gradient refractive index is studied. It is shown that, after propagating a certain distance, the pulse undergoes compression. If, at the point of maximum compression, the pulse intensity is exactly equal to the threshold intensity, then either a bright or dark vortex soliton can form.     

Dark incoherent soliton splitting in biased photorefractive-photovoltaic crystals

Using the coherent density approach, we study the dark incoherent soliton splitting in biased photorefractive-photovoltaic crystals. We show that when the full width half maximum (FWHM) of the optical beam’s intensity is increased, the odd incoherent dark beam splits into an odd-number sequence of multiple dark stripes, whereas the even incoherent dark beam splits into an even-number sequence of multiple dark stripes. We find that when more incoherent solitons are generated, the separations between adjacent dark stripes become smaller and the stripes far away from the center become less visible and that for a given physical system and for a given splitting, the separations between adjacent dark stripes decrease with an increase in the intensity FWHM of the optical beam. On the other hand, the dark incoherent soliton splitting in biased photorefractive-photovoltaic crystals is the dark incoherent screening soliton splitting when the bulk photovoltaic effect is neglectable and the dark incoherent closed- and open-circuit photovoltaic soliton splitting when the external bias field is absent.     

Different operation states of soliton pulses in an erbium-doped fibre ring laser

We report on the experimental observation of soliton pulses in an erbium doped fibre ring laser. The passive mode-locking is achieved using the nonlinear polarization rotation technique. By adjusting the pump power and the intracavity polarization controllers, a normal soliton, a stable 8th harmonic mode-locked pulse and a noise-like pulse have been observed in our laser. The experimental results revealed that the noise-like pulse is not suitable for the optical telecommunication, and in order to obtain the stable harmonic mode-locked soliton, a strong unstable CW laser field is necessary to mediate global soliton interaction. The formation mechanism of the harmonic mode-locked pulse has also been analysed.     

Computer simulation and observation of anomalous light emission from nonlinear photonic crystal with various geometry of its elements

Recently, in papers [1, 2] were described the effect of long-time emission of photonic crystal (synthetic opals) under the action of nanosecond laser pulse. The duration of the luminescence is of the order of seconds. Despite on the other phenomena, which were considered in these papers, below we focus our attention on possible explanation of long time emission from nonlinear photonic crystal. The basis of our consideration is papers [3, 4], those deal with soliton formation in several layers of 1D nonlinear photonic crystal. Because of this, the light wave can exist in photonic crystal a long time (in ideal case—infinite time interval). Taking into account a relation between soliton duration and its maximum intensity, the leaving of laser energy from the layer of photonic crystal takes place due not full reflection from boundaries of layer. The time of light emission for this case depends on nonlinear susceptibility of photonic crystal and the intensity of laser pulse. Hence, soliton formation inside the nonlinear elements of photonic crystal can be one of the reasons of long time emission. We got in computer simulation a long time emission of 1D (layered) and 2D (circular or rectangular elements) nonlinear photonic crystal under the action of laser pulse with femtosecond duration under the conditions of soliton formation in some elements of photonic crystal. Input intensity of laser pulse, at which a soliton appears in nonlinear photonic crystal, can be ten times less due to effects of enhancement of optical intensity in periodic structure [4, 5].     

在光纤零群速色散区传输的光孤子波

通过对超短光脉冲在单模光纤中传输方程的分析研究,给出了在零群速色散传输方程的亮,反波解。结果表明,超短光脉中在光纤的零群群速色散仍能以亮,暗孤波的形式传输,且不存在孤子自频移现象。     

电磁诱导透明系统中的暗孤子

利用电磁诱导透明效应提供的高色散和非线性系数, 研究暗孤子的形成环境以及孤子演化与环境参数的关系. 为了提高电磁诱导透明的稳定性和可操作性, 用双势阱半导体作为基质材料. 将量子理论和经典场理论结合, 获得了非线性薛定谔方程. 以非线性薛定谔方程为基础, 研究暗孤子的形成条件, 以及孤子演化与环境参数的关系. 研究结果表明: 当介质为反常色散同时交叉相位调制为负时, 在该介质中可以形成和传播暗孤子; 暗孤子演化中, 脉宽、灰度与相位相互关联, 脉宽越小、灰度越大, 相位增长越迅速. 此外, 研究了系统的调制不稳定性, 探讨了在调制不稳定下的增益谱.     

Optoacoustic solitons in Bragg gratings

Optical gap solitons, which exist due to a balance of nonlinearity and dispersion due to a Bragg grating, can couple to acoustic waves through electrostriction. This gives rise to a new species of "gap-acoustic" solitons (GASs), for which we find exact analytic solutions. The GAS consists of an optical pulse similar to the optical gap soliton, dressed by an accompanying phonon pulse. Close to the speed of sound, the phonon component is large. In subsonic (supersonic) solitons, the phonon pulse is a positive (negative) density variation. Coupling to the acoustic field damps the solitons' oscillatory instability, and gives rise to a distinct instability for supersonic solitons, which may make the GAS decelerate and change direction, ultimately making the soliton subsonic.     

Temporal behavior of dark spatial solitons in closed-circuit photovoltaic media

We show that the time-dependent nonlinear wave equation in closed-circuit photovoltaic media can exhibit quasi-steady-state and steady-state spatial solitons. We demonstrate that the formation time of open-circuit quasi-steady-state and open-circuit steady-state dark solitons decreases with an increase in the intensity ratio of the soliton, which is the ratio between the soliton peak intensity and the dark irradiance. We find that for the time-dependent nonlinear wave equation that exhibits only an open-circuit steady-state dark soliton, changing the electric current density J₀ does not generate quasi-steady-state dark solitons and affects the formation time of steady-state dark solitons and that for the time-dependent nonlinear wave equation that exhibits an open-circuit quasi-steady-state dark soliton, changing J₀ gives rise to three different time evolution regimes of the full width half maximum of the soliton’s intensity. The first regime shows that the formation time of steady-state dark solitons increases with J₀ whereas the formation time of quasi-steady-state dark solitons is independent of J₀. The second regime shows that the formation time of steady-state dark solitons decreases with an increases in J₀ and the formation time of quasi-steady-state dark solitons increases with J₀. The third regime shows that changing J₀ enables only steady-state dark solitons in the time-dependent nonlinear wave equation, of which the formation time increases with J₀.     

Oscillating dark cavity solitons

We show that oscillating dark solitons exist on a stable continuous plane-wave background in a cavity with a saturable defocusing nonlinearity. These oscillations can be attributed to a linear internal mode of the soliton with a complex eigenvalue emerging above a definite input field intensity. Using a simple analytical model, we find that the self-oscillations appear as a consequence of an interaction between the center of the soliton and the surrounding ring.     

Increase in the efficiency of the shear wave generation in gelatin due to the nonlinear absorption of a focused ultrasonic beam

Experimental results and theoretical estimates are presented to demonstrate the prospects of using the acoustic nonlinearity of a gel-like medium for increasing the efficiency of the shear wave generation in it by a pulsed ultrasonic beam. The experiment is based on the propagation of a focused beam of longitudinal acoustic waves at a frequency of 1.1 MHz in a gelatin sample and on the detection of shear waves by the optical method [1]. It is demonstrated that the amplitude of the shear wave excited by a nonlinear acoustic pulse can be increased by an order of magnitude owing to the formation of shock fronts in the profile of this pulse.     

脉冲离散效应对光纤中基于交叉相位调制的脉冲压缩的影响

通过数值模拟，分析了脉冲离散效应对单模光纤正群速色区基于暗孤子交叉不相位调制的脉冲压缩的影响，并提出了减轻这种影响的办法，作者发现，即使暗孤子与被压缩脉冲的中心波长相差很小，离散效应对脉冲压缩的影响也很大，离散效应不仅导致脉冲压缩比的减小和压缩后脉冲峰值功率的降低，而且还导致脉冲压缩所需最佳光纤长度的增加以及压缩后的脉冲呈现不对称性，还发现，若采取本文提出了的非同步耦合法，则可以明显地弥补离散效应     

Self-deflection of bright soliton in a separate bright-dark screening soliton pair based on higher-order space charge field

Based on the interaction of the separate soliton pair, the self-deflection of the bright screening soliton in a bright-dark pair is studied by taking the higher order space charge field into account. Both numerical and analytical methods are adopted to obtain the result that the higher order of space charge field can enhance the deflection process of the bright soliton and varying the peak intensity of the dark soliton can influence the self-deflection strongly. The expression of the deflection distance with the dark soliton's peak intensity is derived, and some corresponding properties of the self-deflection process are figured out.