Analysis of femtosecond pulsed beam propagation in dispersive directional coupler

An analysis of ultrashort-pulse interaction inside directional coupler structures having dispersive material is conveyed using the well-known finite difference time domain (FDTD) technique and time domain beam propagation method (TD-BPM). The implicit TD-BPM uses a higher-order non-paraxial equation suitable for femtosecond pulse. The result obtained by the one-way technique is verified with that of the widely used FDTD. A rigorous study is conveyed in order to distinguish material dispersion from intermodal dispersion. The analysis reveals that material dispersion causes energy loss in the device, in addition to extra pulse broadening and splitting that occur to a low extent in the case of intermodal dispersion.     

Analysis of ultra short pulse propagation in optical directional coupler structures

We employ an efficient time-domain beam propagation technique to study the effect of the propagation of ultra short pulse duration on the behavior of directional coupler operation. The technique used is based on higher order non-paraxial formulation that takes into account the spatiotemporal coupling effect which is crucial for the proper propagation of ultra short optical pulses. In this work the characterization of pulse spread and broad frequency content interactions of short pulse propagation have been analyzed. We validate in this rigorous analysis that the intermodal dispersion of the structure changes the behavior of the well known operation and breaks up the pulse during propagation which gives rise to distortion.     

Diffraction-free and dispersion-free pulsed beam propagation in dispersive media

The diffraction of pulsed beams of light is formulated as an anomalously dispersive phenomenon. In a dispersive material, the effects of material group-velocity dispersion and diffraction on pulsed beam propagation can mutually cancel if the transverse profile of the pulse is suitably chosen.     

色散补偿技术中超短光脉冲传输特性的研究

本文用变分法对色散补偿技术中超短脉冲的传输特性进行了理论和数值分析.首先给出了描述超短脉冲传输特性的耦合微分方程组,然后对其进行数值求解.计算中考虑了光纤非线性系数的差异,首次引入平均非线性系数参量.分析表明,要想达到最佳的传输性能,不同的色散补偿方案,对超短脉冲的入纤能量将有不同的要求.最后给出超短脉冲最佳的入纤能量与其脉宽、色散参量比、光纤长度,以及平均色散参量间的关系图.     

超常介质中超短电磁脉冲的传输特性研究

将超常介质的色散磁导率合并到非线性极化项中,借鉴常规介质中超短脉冲传输方程的推导方法,得到了非线性超常介质中超短脉冲的传输方程。在德鲁德(Drude)色散模型下,根据脉冲中心频率的不同在传输方程中出现了可正、可负、可为零的自陡峭系数,以及高阶非线性色散项。此外,利用矩方法对传输方程进行分析,得到了超常介质中超短脉冲传输方程的能量守恒定律表达式,揭示了色散磁导率导致的超短脉冲传输的新特性,发现二阶非线性色散使超短脉冲的能量、脉冲频移、脉冲宽度、中心位置和啁啾都随传输距离呈现振荡式变化。     

Acousto-optic mode coupling of partially coherent light in two-mode optical fibers

An acoustic pulse propagating on a two-mode fiber can act as a beam splitter in a scanning interferometer. When this device is employed in white-light interferometry, the effects of distributed coupling and dispersive interferometer arms need to be considered. A theory suitable for treating acousto-optic interaction of partially coherent light in a moving interaction region was developed. It was found that differential optical dispersion should be negligible and the acoustic pulse length short. Also the coherence time should be short but long compared to the intermodal group delay difference over a pulse length. Experiments with long acoustic pulses were performed, and fairly good agreement with theory was obtained.     

Transverse surface waves in a layered structure with a functionally graded piezoelectric substrate and a hard dielectric layer

As to an ideally layered structure with a functionally graded piezoelectric substrate (material parameters change continuously along the thickness direction) and a hard dielectric layer, the existence and propagation behavior of transverse surface waves is studied by analytical technique. The dispersion equations for the existence of the transverse surface waves with respect to phase velocity are obtained for electrically open and short circuit conditions, respectively. A detailed investigation of the effect of gradient coefficient on dispersion relation, electromechanical coupling factor and penetration depth is carried out. It is found by numerical examples that adjusting gradient coefficient makes the electromechanical coupling factor of the transverse surface waves achieve quite high values at some appropriate ratio values of the layer thickness to the wavelength, and at the same time, the penetration depth can be reduced to the same order as the wavelength.     

Ultra‐short and ultra‐intense X‐ray free‐electron laser single pulse in one‐dimensional photonic crystals

The propagation within a one‐dimensional photonic crystal of a single ultra‐short and ultra‐intense pulse delivered by an X‐ray free‐electron laser is analysed with the framework of the time‐dependent coupled‐wave theory in non‐linear media. It is shown that the reflection and the transmission of an ultra‐short pulse present a transient period conditioned by the extinction length and also the thickness of the structure for transmission. For ultra‐intense pulses, non‐linear effects are expected: they could give rise to numerous phenomena, bi‐stability, self‐induced transparency, gap solitons, switching, etc., which have been previously shown in the optical domain.     

（3＋1）-dimensional nonlinear propagation equation for ultrashort pulsed beam in left-handed material

In this paper a comprehensive framework for treating the nonlinear propagation of ultrashort pulse in metamaterial with dispersive dielectric susceptibility and magnetic permeability is presented. Under the slowly-evolving-wave approximation, a generalized （3＋1）-dimensional wave equation first order in the propagation coordinate and suitable for both right-handed material （I~HM） and left-handed material （LHM） is derived. By the commonly used Drude dispersive model for LHM, a （3＋1）-dimensional nonlinear Schrodinger equation describing ultrashort pulsed beam propagation in LHM is obtained, and its difference from that for conventional RHM is discussed. Particularly, the self-steeping effect of ultrashort pulse is found to be anomalous in LHM.     

Linear wavelength sweep of a semiconductor laser and its consequence for dispersion measurements and interference noise in optical fibers

The wavelength shift of a semiconductor laser during triangular pulse modulation has been investigated, using both a Michelson interferometer and direct spectral measurements. This property is used to establish a method for high-resolution dispersion measurements limited to 10 ps, and not affected by material dispersion or detector rise time. A theoretical investigation shows under what conditions the method is equivalent to the standard dispersion measurement setup. The technique is applied to measure polar mode dispersion in single-mode fibers. Examples of polarization noise caused by polarization anisotropy and the variation of source wavelength are presented.     

Linear wavelength sweep of a semiconductor laser and its consequence for dispersion measurements and interference noise in optical fibers

The wavelength shift of a semiconductor laser during triangular pulse modulation has been investigated, using both a Michelson interferometer and direct spectral measurements. This property is used to establish a method for high-resolution dispersion measurements limited to 10 ps, and not affected by material dispersion or detector rise time. A theoretical investigation shows under what conditions the method is equivalent to the standard dispersion measurement setup. The technique is applied to measure polar mode dispersion in single-mode fibers. Examples of polarization noise caused by polarization anisotropy and the variation of source wavelength are presented.     

Relaxation dynamics of a broadband nanosecond acoustic pulse in a bubbly medium

The first experimental observation of the propagation dynamics of a short broadband acoustic pulse in a resonance medium with gas bubbles is carried out. The probing pulse is generated using the optoacoustic effect. It is shown that the theory of short pulse propagation in media with generalized resonance relaxation adequately and accurately describes the dynamics of short pulse dispersion. A possibility to determine the relaxation and resonance parameters of media by the pulsed testing technique is demonstrated.     

Coherent matter waves for ultrafast laser pulse characterization

A technique for the characterization of ultrashort laser pulses using coherent matter waves is demonstrated. We emphasize the analogy between matter wave packets and electromagnetic wave packets propagating in dispersive media. Due to quadratic dispersion the wave packets are stretched and their temporal structure eventually converges to their spectrum, thus providing a possibility for energy measurements in conjugate space. This is demonstrated theoretically and experimentally and is the basis for our laser pulse characterization technique. We use energy resolved interferometrically recorded photoelectron spectra generated by above-threshold ionization in an autocorrelation setup to characterize ultrashort laser pulses at 800 nm wavelength. This approach is potentially applicable to the XUV wavelength region.     

A review of Ni-like X-ray laser experiments undertaken using the VULCAN laser at the Rutherford Appleton Laboratory

Recent progress using the VULCAN laser at the Rutherford Appleton Laboratory to pump X-ray lasing in nickel-like ions is reviewed. Double pulse pumping with ∼100 ps pulses has been shown to produce significantly greater X-ray laser output than single pulses of duration 0.1–1 ns. With double pulse pumping, the main pumping pulse interacts with a pre-formed plasma created by a pre-pulse. The efficiency of lasing increases as there is a reduced effect of refraction of the X-ray laser beam due to smaller density gradients and larger gain volumes, which enable propagation of the X-ray laser beam along the full length of the target. The record shortest wavelength saturated laser at 5.9 nm has been achieved in Ni-like dysprosium using double pulse pumping of 75 ps duration from the VULCAN laser. A variant of the double pulse pumping using a single ∼100 ps laser pulse and a superimposed short ∼1 ps pulse has been found to further increase the efficiency of lasing by reducing the effects of over-ionisation during the gain period. The record shortest wavelength saturated laser pumped by a short ∼1 ps pulse has been achieved in Ni-like samarium using the VULCAN laser operating in chirped pulse amplified (CPA) mode. Ni-like samarium lases at 7.3 nm.     

Group-velocity-dispersion-compensated femtosecond optical parametric amplifier

A tilted pulse front geometry is known to allow group-velocity-dispersion (GVD) compensated propagation of short pulses of different wavelengths in dispersive media. This scheme is shown to be applicable to an optical parametric amplifier (OPA) pumped at 450nm. A numerical calculation shows that a certain combination of the tilt of the pulse front and the angle between the interacting beams provides optimum GVD compensation for each wavelength. The parameters providing broadband tunability are identified. The tunability of the GVD-compensated OPA pumped with 55fs pulses was experimentally demonstrated between 545nm and 645nm. This revised version was published online in November 2006 with corrections to the Cover Date.     

周期量级飞秒脉冲电场在非线性克尔介质中的传输

采用时间转换法研究了周期量级飞秒脉冲电场在非线性克尔介质中的传输.由于周期量级飞秒脉冲电场的脉宽小于介质拉曼响应的特征时间,在传输过程中脉冲电场会发生剧烈的变形和分裂,并在频谱上观察到了强烈的拉曼感应频移和色散波.由于周期量级脉冲电场依赖于载波包络相位,发现在脉冲电场传输过程中,主脉冲电场和色散波电场的相位线性地依赖于初始脉冲的载波包络相位.     

Time-domain modeling of nonlinear distortion of pulsed finite amplitude sound beams

This work aims to validate a time domain numerical model for the nonlinear propagation of a short pulse of finite amplitude sound beam propagation in a tissue-mimicking liquid. The complete evolution equation is simply derived by a superposition of elementary operators corresponding to the 'one effect equation'. Diffraction LD, absorption and dispersion LAD, and nonlinear distortion LNL effects are treated independently using a first order operator-splitting algorithm. Using the method of fractional steps, the normal particle velocity and the acoustical pressure are calculated plane by plane, at each point of a two-dimensional spatial grid, from the surface of the plane circular transducer to a specified distance. The LA operator is a time convolution between the particle velocity and the causal attenuation filter built after the Kramers-Kroning relations. The LNL operator is a time-based transformation obtained by following an implicit Poisson analytic solution. The LD operator is the usual Rayleigh integral. We present a comparison between theoretical and experimental temporal pressure waveform and axial pressure curves for fundamental (2.25 MHz), second, third and fourth harmonics, obtained after spectral analysis.     

一种新的低非线性宽带色散补偿微结构光纤的设计

采用矢量光束传输法对空气孔包层呈正六边形分布的微结构光纤的色散和非线性特性进行了数值模拟。通过分别调节内三层空气孔的直径和包层空气孔节距,设计了一种低非线性宽带色散补偿微结构光纤。该光纤在波长1.55 μm处具有－3 235.8 ps/nm/km的大负色散,可在以1.55 μm为中心的100 nm宽带波长范围对相当于自身长度190倍的普通单模传输光纤进行宽带色散补偿(色散补偿率偏移在0.5%以内),同时该光纤可在此宽带波长范围内保持非线性系数低于5 W^-1·km^-1。     

时频分布技术在激光与等离子体相互作用粒子模拟诊断中的应用

在对超短超强激光与等离子体相互作用进行粒子模拟诊断的研究中,输出量普遍为时间或空间上的非平稳信号。将时频分布技术引入粒子模拟结果的诊断,指出用时频分布技术等现代信号处理技术诊断模拟结果中的非平稳信号具有重要意义。作为实例,考察了用线性啁啾激光打靶条件下,包含nc/4(nc为等离子体临界密度）的非均匀等离子体区域中的受激拉曼散射的演化,并用时频分布技术得到了清晰的物理图像。     

Difference frequency generation by optical rectification of femtosecond laser pulse in GaAs crystal with a periodic domain structure

We study the influence of spatial boundedness of a femtosecond laser pulse (FLP) on the efficiency of generation of difference frequency radiation (DFR) in a GaAs crystal with a periodic domain structure. It is shown that at propagation of DFR a spectral-angular filtration occurs in the transverse distribution of the beam. An expression is obtained for the frequency-angular spectrum of DFR. For a pump pulse with the duration of 100 fs, the beam radius of 24μm, the energy of 30 nJ, and the wavelength of 1.98μm propagating in 1.716-mm long GaAs crystal with the domain structure period of 74.6μm the efficiency of generation of DFR at the wavelength of 14μm is calculated to be close to 2×10⁻⁶.