超短激光脉冲在水中的非线性传输特性研究

基于扩展的非线性薛定谔方程及与其耦合的电子密度演化方程,理论研究了超短激光强脉冲在水中的非线性传输特性.理论模型综合考虑了衍射、正常群速度色散,多光子吸收、自聚焦及激光诱导产生的等离子体对光脉冲的自散焦效应.采用有限差分法模拟得到超短脉冲在传输过程中轴向上光功密度和等离子体密度的时空分布.系统分析了超短脉冲在水中传输的动力学过程,讨论了非线性自聚焦和等离子体自散焦效应在脉冲传输过程中的竞争关系.同时研究了不同入射激光能量,脉宽和聚焦条件对等离子体丝的时宅结构和脉冲能流横向分布的影响.该研究将有助于理解和推动超短光脉冲在激光医学、激光安全防护和水中激光加工中的应用.     

Spatiotemporal Evolution of Intense Short Gaussian Laser Pulses in Weakly Relativistic Magnetized Plasma

The weakly relativistic regime of propagation of a short and intense laser pulse in the magnetized plasma is investigated. By considering relativistic nonlinearity and using non‐linear Schrödinger equation with paraxial approximation, two second‐order coupled differential equations are obtained for the longitudinal pulse width parameter (in time) and for the transverse pulse width parameter (in space). The simultaneous evolution of spot size and length of a relativistic Gaussian laser pulse in a magnetized plasma can be calculated by the numerical solution of the equations. The effect of magnetic field is investigated. It is observed that in the presence of magnetic field both the self‐compression and the self‐focusing can be enhanced. Furthermore, the interplay between the longitudinal self‐compression and the transverse self‐focusing in a magnetized plasma is investigated. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

稠密等离子体表面不稳定性的入射光偏振态效应

用二维粒子模拟程序研究了相对论强激光和稠密等离子体相互作用引起的表面不稳定。数值模拟表明,在s偏振光作用下,等离子体表面出现了类瑞利泰勒不稳定性。形成的不稳定结构随时间发展进一步深入到等离子体内部,最终使等离子体密度形成分层泡状结构,并向前传播。这种不稳定的产生与初始等离子体密度有密切关系,在高于20倍临界密度等离子体的表面没有明显观察到这种不稳定。在p偏振激光作用下,等离子体表面不能明显地形成这种结构。因此在三维几何结构下,这种等离子体表面不稳定性将呈现各向异性。这种表面不稳定将直接影响高次谐波产生和离子加速效率。     

磁光光纤中光脉冲的非线性传播特性研究

根据光纤中磁光效应与非线性效应的微扰理论,推导了磁光光纤中光脉冲的非线性耦合模方程,比较了修正的分步傅里叶算法中磁光效应的时域和频域处理方案,表明了在步长足够小的情况下,两种方案的结果一致.分析了磁光效应、光纤非线性以及色散对光脉冲传输特性的影响,通过改变磁光耦合强度,不但可以灵活控制脉冲形状,还可以改变非线性引起的频率啁啾大小,有助于实现基于光脉冲展宽的动态整形功能.本文给出的理论分析方法,有助于开发可用于光纤通信、光纤传感等领域的基于非线性磁光光纤的新型磁光信息处理器件.     

磁光光纤中光脉冲的非线性传播特性研究

根据光纤中磁光效应与非线性效应的微扰理论,推导了磁光光纤中光脉冲的非线性耦合模方程,比较了修正的分步傅里叶算法中磁光效应的时域和频域处理方案,表明了在步长足够小的情况下,两种方案的结果一致.分析了磁光效应、光纤非线性以及色散对光脉冲传输特性的影响,通过改变磁光耦合强度,不但可以灵活控制脉冲形状,还可以改变非线性引起的频率啁啾大小,有助于实现基于光脉冲展宽的动态整形功能.本文给出的理论分析方法,有助于开发可用于光纤通信、光纤传感等领域的基于非线性磁光光纤的新型磁光信息处理器件.     

Wavelength Switching of a Fabry-Perot Semiconductor Laser by Optical Input Pulses

Wavelength switchings of a Fabry-Perot semiconductor laser are experimentally demonstrated by applying externally optical input pulses detuned from a cavity-resonant wavelength of the semiconductor laser with a constant injection current. The wavelength region of optical switching ranges from less than 0.1 nm to several nm over gain spectrum, being locked to the wavelength of optical inputs. These behaviors are based on multistability and multi-split branches in the optical output versus detuned optical input characteristics, caused by the carrier-induced refractive index change accompanied by optical injections. The experimental results are believed to be useful for wavelength division multiplexing and exchange in optical communication.     

高重复频率激光对光学材料损伤特性的研究

研究了几种红外窗口材料的高重复频率Ｎｄ：ＹＡＧ激光和Ｃｕ蒸气激光体损伤规律，发现样品损伤有明显积累效应。经过分析，发现高重复频率激光损伤是一处微观损伤积累过程，并引入吸收元的概念建立了一个简单的损伤积累模型。     

Broadband Optical Active Waveguides Written by Femtosecond Laser Pulses in Lithium Fluoride

Broadband waveguiding through light-emitting strips directly written in a blank lithium fluoride crystal with a femtosecond laser is reported. Light guiding was observed at several optical wavelengths, from blue, 458 nm, to near-infrared, at 155nm. Visible photoluminescence spectra of the optically active F2 and F33 color centers produced by the fs laser writing process were measured. The wavelength-dependent refractive index increase was estimated to be in the order of 10-3-10-4 in the visible and near-infrared spectral intervals, which is consistent with the stable formation of point defects in LiF.     

Waveform Distortions in Femtosecond Laser Pulses Transmitting through Coated Optical Fiber Probes

We calculate the light field in metal-coated optical fiber probes under illumination of femtosecond laser pulses with the three-dimensional finite-difference time-domain method. By choosing the maximum of the calculated light field data in the time period of one light wave cycle at each spatial point, we obtain the amplitude distributions instead of the conventional light field distribution of a time instant. It is found that with an incident plane wave pulse of y-polarization, the output amplitude distributions of y-polarization have roughly a circled-cross-like pattern with two arc-like zero-amplitude zones. By analyzing the y-polarized waveforms versus time at the points near the arc-like zero-amplitude zones, we find that waveform distortions appear at points in the neighborhood of the zero amplitude lines and in the time intervals when two wave-packets overlap.     

Nonlinear Optical Absorption in GaSe upon Laser Excitation

Optics and Spectroscopy - The nonlinear absorption in GaSe crystals at high optical excitation levels is experimentally studied. A Nd:YAG laser (first and second harmonics, 1064 and 532 nm) and a...     

High-Efficiency Relativistic Generators of Nanosecond Pulses in the Millimeter-Wavelength Range

Radiophysics and Quantum Electronics - We propose and study a scheme of a microwave pulse generator operating in the frequency ranges near 37 and 73 GHz, in which the electron interaction with both...     

非线性光学限幅技术与激光防护

介绍了非线性光限幅技术的基本原理，评述了单一型光限幅技术的发展以及复合型光限幅技术的最新成果。     

能同时产生不同频率脉冲信号的双波长主动锁模光纤激光器

用两只光纤光栅（FBG）作波长选择器件，实现了一种能同时输出两个不同重复频率的光脉冲序列的双波长主动锁模环形腔光纤激光器。通过调速外加调制信号的频率，使其中一个波长工作在谐波锁模状态，另一个波长工作在有理数谐波锁模状态，实验在2GHz和2.5GHz的调制频率下都成功地获得了双波长且重复频率不同的光脉冲序列，脉冲宽度分别约为60ps和45ps。     

Optical Switch Formation in Antimony Super-Resolution Mask Layers Induced by Picosecond Laser Pulses

Sb is a classic material of a super-resolution near field structure （super-RENS） mask layer in which the optical switch formation is often realized by nanosecond laser pulse stimulation. We achieve fast and repeatable optical switching driven by picosecond laser pulses in a proper fluence range on Sb thin films. The optical properties of Sb thin films before and after switching are studied by surface-sensitive micro-area ellipsometry. The change of optical constants after switching is less than 2% in the whole visible range. The Sb mask layer is shown to be very promising for ultrafast super-resolution optical storage applications.     

Propagation of Solitary Pulses in Optical Fibers with Both Self-Steepening and Quintic Nonlinear Effects

Pulse dynamics and stability in optical fibers in the presence of both self-steepening and quintic nonlinear effects are analyzed. Propagating profiles of the quintic derivative nonlinear Schrodinger model are isolated via two invariants of motion. The resulting canonical equation admits exact periodic propagating patterns in terms of the Jacobi elliptic functions, and solitary pulses are recovered in the long wave limit, i.e. degenerate cases of periodic profiles where each pulse is widely separated from the adjacent ones. Two families of such exact wave profiles are identified. The first one has a precise constraint concerning the magnitude of self-steepening and quintic nonlinear effects, while the second one permits more freedom. The reduction to the well established temporal soliton in an optical fiber waveguide in the absence of self-steepening and quintic nonlinearity is demonstrated explicitly. Numerical simulations are performed to identify regimes of parameter values where robust propagation patterns exist.     

Generation of Multipetawatt Laser Pulses with a Small Number of Field Oscillations

Radiophysics and Quantum Electronics - We propose a method for two-stage compression of laser pulses, which makes it possible to shorten laser pulses of tens of picoseconds to an optical period...     

Nonlinear Laser Radiation Absorption Due to Relativistic Plasma Resonance in an Inhomogeneous Plasma

Journal of Experimental and Theoretical Physics - We have developed an analytic theory of the nonlinear resonant absorption of electromagnetic radiation in an inhomogeneous plasma with account for...     

Nonlinear Refraction of Dielectric Single Crystals under Excitation with Ultrashort Near-Infrared Laser Pulses

Optics and Spectroscopy - Nonlinear refraction in a number of crystals (KGW, KGW:Eu3+, Ba(NO3)2, BaWO4, PbMoO4, and CdWO4) that are promising for excitation of multifrequency stimulated Raman...     

A Sensitive Scheme to Observe Weak Photo-Refraction Effects in Some Nonlinear Optical Crystals Pumped by Ultrashort Optical Pulses

We present a sensitive scheme, for the first time to our knowledge, to observe photo-retraction （PR） ettects in some nonlinear optical crystals, e,g. β-BBO, LBO and BIBO, pumped by an intense ultrashort laser pulse chain. These quite weak effects are ＂amplified＂ by sensitive cw intraeavity loss modulation. Our results show that they are repeatable and are dependent on pumping power and wavelength, and their response time ranges from tens of seconds to several minutes. The recorded dynamical transitions between the self-focusing to the self-defocusing （or vice versa） induced by the PR effect may be critically important for us to give more insight into the stability of some cascade nonlinear frequency conversions, e.g. multi-stage optical parametric amplifiers.