Stable new bound soliton pairs in a 10 GHz hybrid frequency modulation mode-locked Er-fiber laser

We report what is, to our knowledge, the first experimental observation of stable new bound soliton pairs at the 10 GHz repetition rate in a hybrid FM harmonic mode-locked Er-fiber laser (1177 soliton pairs simultaneously in the laser cavity). The two solitons in the soliton pair have the identical pulse shape and are with the antiphase (pi phase difference). Their time separation is about three times the FWHM soliton width and varies with the phase modulation strength. The corresponding mechanism for explaining the formation as well as the superior stability of these closely bound soliton pairs is also given.     

Nonlinear Dynamics of Circularly Polarized Laser Pulse Propagating in a Magnetized Plasma with q ‐Nonextensive Velocity Distributions

The nonlinear dynamics of a circularly polarized laser pulse propagating in magnetized plasma contains hot nonextensive q ‐distributed electrons and ions is studied theoretically. A nonlinear equation which describes the dynamics of the slowly varying amplitude electromagnetic wave is obtained using the relativistic two‐fluids model. Some nonlinear phenomena include modulational instability, self‐focusing, soliton formation, and longitudinal and transversal evolutions of laser pulse in nonextensive plasma medium are investigated. Results show that the nonextensivity of particles can substantially change the nonlinearity of medium. The external magnetic field enhances the modulation instability growth rate of right‐hand polarization wave but for the left‐hand polarization the growth rate decreases. The spot size of the laser pulse is strongly affected by the plasma nonextensivity. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of higher order non-linearity in frequency variation of self-phase modulation in optical fiber communication

In optical soliton propagation through a single mode optical fiber,it is established that self-phase mod ulation is maintained by the third order non-linearity of the silica-based glass material of the fiber.In this paper we show that the fifth order non-linearity has also some contribution in frequency variation of self-phase modulation.     

激光脉冲在等离子体中的压缩分裂

通过数值求解一维非线性薛定谔方程,研究了圆偏振入射激光脉冲在初始密度范围为1/4到略低于1倍临界密度的等离子体中的自压缩和分裂现象. 提高等离子体密度和入射激光强度以及减小脉冲宽度可以在更短的传输距离获得有效的激光脉冲压缩,压缩后的脉冲半高宽度可达到初始脉冲半高宽度的1/35,甚至更小. 这种压缩是激光脉冲在等离子体中形成高阶孤子的过程中产生的,可以获得比在稀薄等离子体中更好的压缩比例. 数值计算的结果给出了该情况下激光脉冲在等离子体中自压缩后形成的高阶孤子分裂. 利用一维粒子数值模拟程序(particle-in-cell,PIC)也观察到了脉冲的压缩和分裂现象,得到了与数值计算一致的结果. 关键词： 非线性薛定谔方程 自压缩 脉冲分裂 粒子模拟     

Superluminal optical soliton via resonant tunneling in coupled quantum dots

We demonstrate the formation of superluminal optical soliton in an symmetry semiconductor double quantum dot (QD) driven coherently by a weak pulsed laser using the tunnel coupling. It is shown that the group velocity of the soliton can be larger than the vacuum light speed c, i.e., superluminal soliton can be produced. The results obtained can be used for the development of new types of nanoelectronic devices for realizing high-speed optical modulation and rapidly responding quantum switching.     

Femtosecond filament induced birefringence in argon and in air: Ultrafast refractive index change

The birefringence of a linearly polarized femtosecond laser filament in gases has been previously established. In this work, we report the time-dependent refractive index measurements of the filament based upon the spectral modulation of a weak probe pulse in air and in argon gas at 1 atmospheric pressure. The polarization dependence of the refractive index modulation induced by the delayed molecular alignment and by the electronic Kerr effect is highlighted. A numerical simulation of the refractive indices, which takes into account the molecular alignment, the electronic Kerr non-linearity and the plasma, is in good agreement with the measurements.     

Soliton shaping of dispersive waves in a passively mode-locked fibre soliton ring laser

We have studied the mechanism of soliton formation in a passively mode-locked fibre ring laser when the gain is increased. We show experimentally that new soliton pulses in the laser are generated by soliton shaping of dispersive waves. Our observation is further confirmed by numerical simulations where an improved model is used that takes into account explicitly the polarization dispersion effect and discrete saturable absorber effect.     

Observation of dip-type sidebands in a soliton fiber laser

We report on the experimental observation of dip-type spectral sidebands on the soliton spectra of a passively mode locked fiber laser. We point out that the formation of the dip sidebands is due to a four-wave-mixing type of parametric process between the soliton and dispersive waves induced by the periodic soliton parameter variation in the laser cavity. Numerical simulations have well reproduced the experimental results.     

激光加速质子束对电磁孤立子的照相模拟研究

激光在次稠密等离子中传输, 由于频率下移而被俘获, 从而产生电磁孤立子. 根据先前理论及PIC 模拟给出的孤立子的演化过程, 对不同阶段孤立子的电磁场分布进行了建模. 使用Geant4蒙特卡罗程序, 模拟研究了激光加速产生的能量为几个MeV的质子束对后孤立子的照相. 分析了质子能量, 质子源尺寸等因素对照相结果的影响, 并利用了TNSA加速产生质子束的分幅特性, 开展了时间分辨的孤立子照相模拟研究. 模拟给出的质子照相结果验证了文献中给出的孤立子静电场模型, 为以后在实验上探测孤立子提供了理论依据. 关键词： 超短激光 质子照相 孤立子 蒙特卡罗方法     

Observation of negative alkali ions from alkali halides during 248-nm laser irradiation

Below laser fluences where a plasma is formed (the so-called plasma or plume formation threshold) a number of fundamental phenomena can occur where particles such as atomic and molecular ions, atoms and molecular neutrals, and electrons can be emitted. An understanding of such processes is necessary to develop predictive models for material removal from laser irradiated surfaces—at the foundation of laser etching, machining, and pulsed laser deposition. We have reported on a number of the mechanisms for such emission processes. Here, due to space limitations, we present a summary of our studies on the formation of negative alkali ions from single crystal KCl during exposure to pulsed 248-nm radiation at fluences well below the threshold for plasma formation. Despite the high electron affinities of the corresponding halogen atoms, negative halogen ions were not detected. Significantly, the positive and negative alkali ion distributions overlap strongly in time and space, consistent with K formation by the sequential attachment of two electrons to K⁺. Negative alkali ions are also observed under comparable conditions from LiF, NaCl, and KBr. In each material, the strong overlap between the positive and negative alkali ion distributions, and the lack of detected negative halogen ions, suggest that negative ion formation involves a similar mechanism.     

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.     

The role of Cairns–Gurevich distributed electrons on obliquely propagating ion-acoustic waves

We used a new distribution of electrons in a two-component magnetized plasma to study the non-linear ion-acoustic solitary structures. The distribution called “Cairns–Gurevich distribution” describes simultaneously the evolution of the energetic electrons and those trapped in the plasma potential well. A modified KdV equation describing the non-linear comportment of the ion-acoustic wave (IAW) was found by using the standard reductive perturbation technique and the appropriate independent variables. The behaviour of the soliton by changing the plasma parameters has been investigated, and we demonstrated that by decreasing the non-thermality parameter, the soliton solution amplitude is enhanced. In addition, we have discussed the growth rate of the solitary waves by calculating the instability criterion. Through discussion, we have conferred how different plasma parameters, such as the trapping, non-thermality, Mach number, obliqueness via the angle of propagation, and magnetic field via the ion-cyclotron frequency, can affect the solitary wave structures. This kind of theoretical studies can be relevant to understand the non-linear propagation of IA solitary waves plasmas of electrons and particles in laser-plasma interaction, pulsar magnetosphere, the auroral zone, and the upper ionosphere, where plasma with trapped and energetic electrons are often present.     

Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.     

All optical quasi-steady-state photorefractive spatial solitons

We have created a one- (rather than two-) dimensional quasi-steady-state photorefractive spatial soliton induced by a laser beam instead of an applied electric field. The formation of this type of spatial soliton depends on the intensity of the self-trapped beam, which is different from quasi-steady-state spatial solitons reported in the literature. The optically induced spatial soliton can be exploited in all optical switching applications.     

One-dimensional spatial solitons in AlGaAs waveguides

We present experimental results on one-dimensional (1-D) spatial solitons in AlGaAs waveguides. Three distinct types of spatial solitons have been observed: namely the fundamental soliton, the Manakov soliton, and the vector soliton. The fundamental soliton is the simplest form of 1-D soliton which consists of a single polarization. The properties of waveguiding and ‘robustness’ are experimentally studied. Vector solitons which result from the complex interplay between the two orthogonally polarized beams due to self-phase modulation, cross-phase modulation, and four-wave mixing effects are studied. The complex beam dynamics and polarization behaviour of the vector solitons are experimentally studied. Manakov solitons which are a special case of the vector soliton exist when the ratio between the self-to-cross-phase modulation is one and the four-wave mixing effects becomes zero are demonstrated experimentally and the basic properties discussed. Finally, some soliton interactions such as trapping and dragging are reported and possible applications of soliton interactions are discussed.     

飞秒强激光脉冲在等离子体中的自压缩

文章从激光等离子体相互作用的非线性薛定谔方程出发,理论研究了飞秒强激光脉冲在等离子体中的自压缩行为.结果表明在一定范围内随着激光脉冲宽度、激光强度的增大以及等离子体密度的减小,飞秒强激光脉冲在等离子体中传播的自压缩现象越明显.另外通过适当设定参量得到了近似稳定传播的基孤子.     

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].     

Soliton dynamics of megawatt ultrashort light pulses in a hollow photonic-crystal fiber: Effect of high-order dispersion and retarded nonlinearity

The properties of megawatt optical solitons in hollow photonic-crystal fibers (PCFs) are studied. We demonstrate regimes where high-power laser pulses can be compressed to a few cycles of light field and identify the main tendencies in the soliton dynamics of ultrashort laser pulses in hollow PCFs originating from dispersion properties and limited transmission band of such waveguides. The influence of retarded nonlinearity on the soliton dynamics of high-power laser pulses in a hollow PCF is analyzed. In a multisoliton regime, retarded nonlinearity and high-order dispersion are shown to result in the formation of high-power pulses displaying no oscillations of their temporal envelope, which would be typical of standard high-order solitons.     

基于增益诱导光纤激光器的实验研究及理论分析

利用非线性偏振旋转技术,实现了正色散掺铒光纤激光器的自启动锁模,产生了一种新型的增益诱导孤子脉冲输出.该类型脉冲的光谱近似为矩形,半高全宽光谱宽度可达19 nm以上,脉宽为皮秒量级.增益诱导孤子脉冲的形成是增益饱和、光谱滤波效应、腔内色散和各种非线性效应共同作用的结果.同时还分析了脉冲啁啾特性及峰值功率等参数.当抽运功率增加时,该激光器还可以实现双脉冲模式下运转. 关键词： 光纤激光器 增益诱导孤子 啁啾     

Direct generation of a 10 GHz 816 fs pulse train from an erbium-fiber soliton laser with asynchronous phase modulation

By employing the technique of asynchronous mode locking, we have successfully demonstrated direct generation of stable 10 GHz 816 fs pulse trains with a supermode-suppression ratio >70 dB from a hybrid mode-locked Er-fiber laser. When the modulation frequency deviates from the cavity harmonic frequency by 15-40 kHz, stable femtosecond soliton pulses are formed. Our results demonstrate that asynchronous mode locking can act as an effective mechanism for achieving a shorter pulse width and for stabilizing high-repetition-rate pulse trains in soliton fiber lasers.