基于类噪声方波脉冲的掺铒光纤激光器

搭建了长腔"8"字型掺铒光纤激光器,并对产生的谐波锁模方波脉冲进行了实验研究.结果表明,该脉冲具有光滑的宽带光谱,示波器上显示为方波脉冲,但自相关迹证实其为类噪声脉冲,即为类噪声方波脉冲;通过调节偏振控制器和泵浦功率等激光器腔内参量,获得了高达5阶的谐波锁模类噪声方波脉冲;该类噪声方波脉冲并非可以无限增大脉冲能量,而是在一定条件下产生脉冲分裂,出现谐波锁模方波脉冲现象.     

采用双脉冲提高谐波谱的谱线密度

通过计算线偏振双脉冲激光场与一维氢原子模型相互作用产生的高次谐波辐射谱的含时Schrdinger方程. 研究发现,选用适宜的双脉冲激光组合可以增加谐波谱的密度,这样的谐波谱特别适合作为产生孤立阿秒脉冲的光源. 详细地分析了谐波谱线密度增加的物理机理,给出不同脉宽与谐波谱线密度增加的关系,从而可以有针对性地控制谐波谱线的疏密程度. 关键词： 高次谐波 阿秒脉冲 双脉冲     

Temporal shaping of femtosecond solitary pulses in photoionized media

Femtosecond light pulses interacting with solids are studied. With a power close to the self-focusing threshold, an optical pulse evolves like a solitary wave with a temporal duration shrunk to a few femtoseconds through the defocusing action of multiphoton ionization. Self-steepening and space-time focusing are shown to form shock profiles, which do not prevent the pulse from keeping a shorter duration over one Rayleigh length. Theoretical estimates justify the key mechanisms leading to pulse compression, whose influence on the power spectra is finally discussed.     

拉伸分子高次谐波产生的阿秒控制

通过数值求解一维含时薛定谔方程,本文研究了高频阿秒脉冲对拉伸双原子分子模型与低频飞秒激光脉冲相互作用产生高次谐波谱的影响.研究表明,若阿秒脉冲在低频脉冲的第二个光学周期截止处注入时,只第二个平台截止位置IP 5.6UP(IP为电离势,UP为有质动力能)附近谐波的效率得到显著提高;若阿秒脉冲的包络峰值处于低频脉冲第三个光学周期T/4(为飞秒激光脉冲的光学周期)附近时,则谐波谱出现了明显的整体抬高现象.对此,本文根据拉伸分子谐波谱的四种产生机制及阿秒脉冲在飞秒激光不同相位注入时的电子电离曲线,对这些谐波谱的结构给出了合理解释.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Estimation of turbulence spectra parameters from log-amplitude fluctuations spectra of an optical wave

From the general formula for the temporal frequency log-amplitude fluctuations spectra the asymptotic expressions are found, valid for the geometric and wave approximations. A comparison of the plane and spherical wave formulas with those valid for the beam wave is made in all frequency regions. The difference between the on-axis and off-axis beam spectra is presented. The possibility of estimating from the log-amplitude scintillations the inner and outer scales of turbulence together with the exponent of turbulence spectra in the subrange is briefly discussed.     

Modal group time spreads in weakly range-dependent deep ocean environments

The temporal spread of modal group arrivals in weakly range-dependent deep ocean environments is considered. It is assumed that the range dependence is sufficiently weak that mode coupling is predominantly local in mode number. The phrase "modal group arrival" is taken here to mean the contribution to a transient wave field corresponding to a fixed mode number. There are three contributions to modal group time spreads which combine approximately in quadrature. These are the reciprocal bandwidth (the minimal pulse width), a deterministic dispersive contribution that is proportional to bandwidth and grows like range r, and a scattering-induced contribution that grows approximately like r(32). The latter two contributions are shown to be proportional to the waveguide invariant beta, a property of the background sound speed profile. The results presented, based mostly on asymptotic theory, are shown to agree well with full-wave numerical wave field simulations and available exact mode theoretical results. Simulations are shown that correspond approximately to conditions during the LOAPEX acoustic propagation experiment.     

ELECTRON BUNCHING CHARACTERISTICS IN THE GYROTRON TRAVELING-WAVE AMPLIFIER WITH TWO-STAGE INTERACTION CIRCUIT

Based on a simplified model, an analytic theory describing the bunching of the electron beam in a gyrotron traveling wave tube (gyro-TWT) with two-stage interaction circuit is presented. The analytic formulas of the traveling-wave bunching parameter, ballistic bunching parameter, and the density of the bunching beam are obtained. The bunching procedures corresponding to different harmonic operations are studied. It is shown that the maximum of harmonic component depends on the ratio of the harmonic number in the output stage to that in the input stage, not on their absolute values.     

Controlling the spacing of attosecond pulse trains from relativistic surface plasmas

When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.     

双色高频激光作用下原子低阶次谐波的理论研究

通过数值求解含时薛定谔方程,研究了高频双色激光脉冲与原子相互作用产生的光辐射.研究表明,光辐射谱中既有基频光的谐波,又可观测到谐波能量附近的多个频率的光辐射产生,且辐射的峰值强度随着入射激光强度的提高呈指数增强,相邻辐射频率差值为入射的两束激光脉冲频率差.     

Theory of a laser-plasma method for detecting terahertz radiation

A theory is developed for calculating the spectrum and the shape of a terahertz wave packet from the temporal profile of the energy of the second harmonic of the laser field generated during nonlinear interaction of laser and terahertz pulses in an optical-breakdown plasma. The spectral and temporal characteristics of the second-harmonic envelope and a terahertz pulse are shown to coincide only for short laser pulses. For long laser pulses, the second-harmonic spectral line shifts to the red and its temporal profile is determined by the time integral of the electric field of terahertz radiation.     

Theoretical Study of Millimeter Wave Harmonic Oscillator Stabilized with a Transmission Cavity

An equivalent circuit model of millimeter wave second harmonic oscillator stabilized with a transmission cavity has been proposed for constructing analytical formulations between performance parameters of the oscillator and parameters of the circuit. The model consists of an equivalent circuit of fundamental wave and that of second harmonic wave. Each of the circuits comprises circuit models of main cavity, transmission waveguide, and transmission cavity. Absorbing material placed between the transmission waveguide and the transmission cavity can suppress additional resonances originated from transmission cavity. The behavior of the second harmonic oscillator can be effectively described by the circuit model. Furthermore, based on this model, mechanical tuning characteristics have been studied at first, and then analytical formulas for quality factor and efficiency depending on circuit parameters have been derived. The circuit parameters can be conveniently extracted by electromagnetic field simulation. Hence the formulas exhibit both compact form and enough accuracy. Thereafter, general rules of performance parameters varying with circuit parameters have been deduced for the harmonic oscillators. Then some design considerations have been derived according to the corresponding analysis. The equivalent circuit model is useful for designing and adjusting millimeter wave second harmonic stabilizing oscillator with a transmission cavity.     

Interpretation of harmonic structure in solar type II radio bursts

Spectrographic and partly imaging observations of three Type II solar radio bursts displaying three drifting bands with frequencies related as 1¬2¬3 are discussed. The radio data of two of these events were simultaneousely recorded by the digital radiospectrograph of the Observatory of Solar Radioastronomy in Potsdam-Tremsdorf and the radioheliograph of the Paris-Meudon Observatory in Nançay. The data allow the brightness temperatures of radio emission in the three frequency bands to be determined. The second harmonic is traditionally explained as a result of coalescence of two plasma waves into an electromagnetic wave at twice the plasma frequency. Two nonlinear merging processes—the coalescence of three plasma waves, and of a plasma wave and an electromagnetic wave at twice the plasma frequency—are considered to explain the occurrence of the third harmonic on Type II dynamic spectra. The analysis shows that both processes can fit the observed brightness temperatures of the second and third harmonic. The first process acts preferably at low phase velocities of plasma waves and sharp electron density gradients in the source, and the second process dominates in the case of high plasma wave phase velocities. It is shown that the occurrence of the third harmonic in type II bursts due to nonlinear processes in the coronal plasma indicates not only a powerful event but also some specific conditions in the shock or foreshock region. Finally, we propose a method to distinguish between the two invoked nonlinear processes by a statistical investigation of Type II burst data.     

Pulse propagation in an elastic medium with quadratic nonlinearity (L)

This letter examines the propagation of an acoustic pulse in an elastic medium with weak quadratic nonlinearity. Both a displacement pulse and a stress pulse of arbitrary shapes are used to generate the wave motion in the solid. By obtaining the explicit solutions for arbitrary pulse shapes, it is shown that for a sinusoidal tone-burst, in addition to a second order harmonic field, a radiation induced static strain field is also generated. These results help clarify some confusion in the recent literature regarding the shape of the propagating static displacement pulse.     

On the Fröhlich charge density wave and Peierls transition

The exact solution of the system of nonlinear equations for Fröhlich charge density wave (CDW) in the continuum approximation is discussed, and the temperature behaviour of the CDW condensate is investigated in a framework of the mean-field approximation. It is shown that the CDW representation in the form of a conventional harmonic wave is only correct under certain conditions, namely, not too strong an electron-phonon interaction or not too low an electron density. According to the exact solution, the CDW is essentially a nonlinear wave represented by a harmonic series. This is confirmed by the experimental data on the CDW current oscillation spectra in which the overtones are present together with the principal harmonic (the so called “narrow band noise”). At strong enough electron-phonon coupling or at low enough electron density the CDW wave vector is temperature dependent, and the BCS relation between Peierls transition temperature and energetic gap in the electron spectra at zero temperature is violated.     

Spatial and temporal study of shock waves generated by laser ablation for Ti target

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Ionization phase-match gating for wavelength-tunable isolated attosecond pulse generation

High-order harmonic emission can be confined to the leading edge of an 800 nm driver laser pulse under moderately intense focusing conditions (7×10¹⁴ W/cm²) (Pfeifer et al. in Opt. Express 15:17120, 2007). Here, the experimentally observed curtailment of harmonic production on the leading edge of the driver pulse is shown to be controlled by an ionization-induced phase-matching condition. The transient plasma density inherent to the process of high-harmonic generation terminates the harmonic emission by an ultrafast loss of phase matching on the leading edge of the laser pulse. The analysis is supported by a reconstruction of the in situ intensity envelope of the driver pulse with attosecond temporal resolution, performed by measurements of the carrier-envelope phase dependence of individual half-cycle harmonic cutoffs. The method opens the way to wavelength-tunable isolated attosecond pulse generation.     

一维非线性声波传播特性

针对一维非线性声波的传播问题进行了有限元仿真和实验研究. 首先推导了一维非线性声波方程的有限元形式, 含有高阶矩阵的非线性项导致声波具有波形畸变、谐波滋生、基频信号能量向高次谐波传递等非线性特性. 编制有限元程序对一维非线性声波进行了计算并对仿真得到的畸变非线性声波信号进行处理, 分析其传播性质和物理意义. 为验证有限元计算结果, 开展了水中的非线性声波传播的实验研究, 得到了不同输入信号幅度激励下和不同传播距离的畸变非线性声波信号. 然后对基波和二次谐波的传播性质进行详细讨论, 分析了二次谐波幅度与传播距离和输入信号幅度的变化关系及其意义, 拟合出二次谐波幅度随传播距离变化的方程并阐述了拟合方程的物理意义. 结果表明, 数值仿真信号及其频谱均与实验结果有较好的一致性, 证实计算方法和结果的正确性, 并提出了具有一定物理意义的二次谐波随传播距离变化的简单数学关系. 最后还对固体中的非线性声波传播性质进行了初步探讨. 本研究工作可为流体介质中的非线性声传播问题提供理论和实验依据.     

Parametric phase conjugation for the second harmonic of a nonlinear ultrasonic beam

The effect of phase conjugation for the second harmonic of a focused ultrasonic beam was investigated experimentally and by numerical simulation. An ultrasonic pulse with the carrier frequency f=3 MHz was emitted into water and focused at a point between the source and the phase conjugating system. The phase conjugation for the second harmonic of the incident wave (2f=6 MHz) was performed in a magnetostrictive ceramic as a result of the parametric interaction of the incident wave with the pumping magnetic field (the pumping frequency was f_p=4f=12 MHz). The axial and focal distributions of sound pressure in the incident and conjugated beams were measured using a broadband PVDF membrane hydrophone. The corresponding calculations were performed by solving numerically the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation allowing for the nonlinearity, diffraction, and thermoviscous absorption. The results of measurements agreed well with the calculations and showed that the field of a conjugate wave adequately reproduces the field of the second harmonic of the incident wave. A certain advantage of focusing with the phase conjugation for the second harmonic was demonstrated in comparison with the operation at the doubled frequency of the incident wave. The results of this study can serve as a basis for the utilization of the phase conjugation of harmonics in ultrasonic tomography and nondestructive testing.     

Time-dependent physical spectra of Gaussian Schell-model pulses

The time-dependent physical spectra introduced by Eberly and Wódkiewicz [J. Opt. Soc. Am. 67 (1977) 1252], observable with a photon-counting detector placed behind a Fabry-Perot filter, are evaluated for a class of temporally partially coherent non-stationary fields known as Gaussian Schell-model plane-wave pulses. Numerical comparisons between source spectra and observed spectra are provided and discussed, and several general features such as initial spectral broadening at the arrival of the pulse and a temporal pulse tail are illustrated. Closed-form approximations are obtained for the time evolution of the observable spectrum by evaluating the leading terms in an appropriate series expansion.