Effect of intensity dependent higher-order dispersion on femtosecond pulse propagation in quantum well waveguides

We have studied theoretically the dispersion of ultrafast coherent pulses in GaAs/AlGaAs quantum well waveguide structures as a function of optical intensity. Semiconductor Bloch equations are used to obtain the polarization induced in the medium due to an incident Gaussian electromagnetic beam. The partial differential equation describing the pulse propagation in the presence of group velocity dispersion is used to analyze the role of higher-order dispersion on femtosecond pulse propagation in the waveguide. Due consideration has been given to the intensity dependent optical susceptibility of the medium. The results of the numerical analysis manifest significant influence of higher-order dispersion on femtosecond pulse propagation over short waveguide distance.     

Cubic-quintic saturable nonlinearity effects on a light pulse strongly distorted by the fourth-order dispersion

Abstract

We analyze a useful process able to safeguard the fundamental soliton light pulse stability in a strongly perturbed environment by the fourth-order dispersion (FOD). This optical pulse propagation is described by the nonlinear Schrödinger equation (NLSE) with cubic–quintic saturable nonlinearities. Some pulse parameters, called collective variables (CVs) such as amplitude, temporal position, width, chirp, frequency shift and constant phase are obtained analytically. Numerical evolution of CVs and their stability are studied under a typical example to verify our analysis.     

Axial intensity distribution of partially coherent light focused by a lens with spherical aberration

Abstract

The focusing of partially coherent light by a lens with spherical aberration (SA) is studied. The numerical calculation results are given, showing that the axial intensity distribution not only depends on the SA, but also on the coherence of partially coherent light and the Fresnel number of the lens. As the coherence decreases, the influence of the SA on the axial intensity distribution decreases, and the positions of maximum axial intensity shift towards the lens.     

腔内高强度聚焦超声肿瘤治疗的实验研究

本文作者对于利用高强度聚焦超声（ＨＩＦＵ）进行肿瘤腔内治疗作了许多理论计算和试验工作,比较了两种换能器的聚焦特性并研制了一种既能监视又能治疗的高强度聚焦超声换能器,利用此换能器对人工模及活体肿瘤进行了一系列加热实验,验证了高强度聚焦超声治疗肿瘤的可行性。为加热治疗机的开发提供了依据。     

Analysis of triboelectric charging of particles due to aerodynamic dispersion by a pulse of pressurised air jet

Triboelectric charging of powders causes nuisance and electrostatic discharge hazards. It is highly desirable to develop a simple method for assessing the triboelectric charging tendency of powders using a very small quantity. We explore the use of aerodynamic dispersion by a pulse of pressurised air using the disperser of Morphologi G3 as a novel application. In this device particles are dispersed by injection of a pulse of pressurised air, the dispersed particles are then analysed for size and shape analysis. The high transient air velocity inside the disperser causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause triboelectric charging of the particles. In this study, we analyse this process by evaluating the influence of the transient turbulent pulsed-air flow on particle impact on the walls and the resulting charge transfer. Computational Fluid Dynamics is used to calculate particle trajectory and impact velocity as a function of the inlet air pressure and particle size. Particle tracking is done using the Lagrangian approach and transient conditions. The charge transfer to particles is predicted as a function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. Particles experience around ten collisions at different velocities as they are dispersed and thereby acquire charges, the value of which approaches the equilibrium charge level. The number of collisions is found to be rather insensitive to particle size and pressure pulse, except for fine particles, smaller than about 30 µm. As the particle size is increased, the impact velocity decreases, but the average charge transfer per particle increases, both very rapidly. Aerodynamic dispersion by a gas pressure pulse provides an easy and quick assessment of triboelectric charging tendency of powders.     

Scattering of a tightly focused beam by an optically trapped particle

Near-forward scattering of an optically trapped 5-mum-radius polystyrene latex sphere by the trapping beam was examined both theoretically and experimentally. Since the trapping beam is tightly focused, the beam fields superpose and interfere with the scattered fields in the forward hemisphere. The observed light intensity consists of a series of concentric bright and dark fringes centered about the forward-scattering direction. Both the number of fringes and their contrast depend on the position of the trapping beam focal waist with respect to the sphere. The fringes are caused by diffraction that is due to the truncation of the tail of the trapping beam as the beam is transmitted through the sphere.     

Third-order dispersion in a pair of prisms

The general equation for third-order group velocity dispersion is derived in this paper, which contains all the terms of equation (11) in Arissian and Diels [1], as well as one extra term, which we have verified is not negligible. To verify our equation we have modeled the sum of Gaussian modulated frequencies. We show the electric field of bandwidth limited incident ultrashort pulses with a time duration of: 5 fs, 10 fs, 15 fs, 20 fs, 30 fs and 50 fs after propagating through a pair of identical isosceles prisms for a central frequency of 0.8 µm and 0.620 µm. The model is applied by using (a) the finite phase, (b) the phase approximated up to the second-order dispersion and (c) up to the third-order dispersion for each frequency component of the pulse. Graphs are presented for prisms made with fused silica and SF14.     

Influence of spatiotemporal coupling induced by an ultrashort laser pulse shaper on a focused beam profile

4f pulse shapers have been widely used to temporally manipulate femtosecond optical pulses by spectral filtering. When the temporal waveform is manipulated with a spatial light modulator consisting of segmented pixels, the spatial profile of the output beam also varies because of diffraction at the pixel array, which is known as a spatiotemporal coupling effect. This effect produces a complicated spatio-temporal profile near the focus of the ultrashort pulses, which may affect the interpretation of experimental results obtained with shaped ultrashort pulses. We investigate the spatial intensity distribution at the focus of temporally shaped pulses through ablation experiments. The three-dimensional space-time beam profile is also numerically calculated.     

Techniques and physical properties of 10MHz short pulse focused ultrasonic transducer

A focused ultrasonic transducer used for biomedical purposes with a fundamental frequency of 10MHz and a pulse width of one and a half periods is described in this paper. Its physical properties are given including (1) focused acoustic field recorded by an optical means, (2) electric waveform for triggering the transducer and the corresponding waveform of the wave received by another transducer, and (3) result of tests on a sample object.     

A high-order corrected description of ultra-short and tightly focused laser pulses, and their electron acceleration in vacuum

Field expressions are derived for ultra-short, tightly focused laser pulses up to the second-order temporal correction and seventh-order spatial correction. To evaluate the importance of these corrections, we simulate these fields and investigate the final energy of the accelerated electrons. We vary the order of the corrected expressions, the pulse duration, and the beam waist. We find that electron capture is still an important and generic phenomenon in ultra-short, tightly focused laser pulses. While small differences in the electron acceleration are obtained for various orders of the corrected field equations relative to the paraxial field equations, there is no qualitative difference in the behavior of the electron. Furthermore, the temporal and spatial corrections are found to be correlated.     

Temperature distribution on the surface of a crystal induced by a local heat pulse

We study the time evolution of the temperature field for small times in the presence of a local heat pulse on the surface of a solid.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 50, No. 6, pp. 1013–1017, June, 1986.     

10MHz短脉冲超声聚焦换能器的工艺及效果

文章报导了为生物医学研究用的聚焦换能器,可以在水中发出延续时间为一个半周期,主频为10MHz的超声脉冲。文中介绍了研制工艺,并给出换能器以下的物理特性：1．用光学法拍摄的换能器在水中的聚焦过程;2．激发换能器的电脉冲波形及接收电压的波形和幅值;3．对于试样的探测结果。     

用SPIDER法还原飞秒脉冲振幅和相位误差

基于光谱位相相干直接电场重构法(SPIDER)测量飞秒激光脉冲的基本原理和重构飞秒脉冲的算法,数值模拟了SPIDER技术重构飞秒脉冲的过程,分析了时间延迟τ、光谱剪切量Ω的选取原则.以宽度约为50 fs的高斯型线性啁啾脉冲为例,取不同的时间延迟τ、光谱剪切量Ω重构了飞秒脉冲,得到:重构出的飞秒脉冲强度和位相最接近时,时间延迟τ约为2 260 fs,相对光谱剪切量Ω/Δω约为10%.     

Application of the direct search in solving a problem of forming longitudinal distribution of intensity

This paper is devoted to the application of a well-known genetic algorithm for optimization of diffractive optical element forming pre-given axial intensity distribution. Computer simulation results and experimental research are presented.     

Enthalpy flow rate of a pulse tube in pulse tube refrigerator

An integration formula of enthalpy flow rate along a pulse tube in pulse tube refrigerators is described on the assumption of sinusoidal mass flow rate and sinusoidal pressure fluctuation. For ideal double inlet and ideal orifice pulse tube with helium as working medium, it is simplified to a polynomial formula. Polynomial formulas for roughly evaluating the volume of the pulse tube in ideal double inlet and ideal orifice pulse tube refrigerators are also given.     

Numerical calculation of the sound field focused by acoustic lens with an arbitrary axisymmetric sound speed distribution

The expressions that describe the sound field focused by an acoustic lens with different sound speed distribution (SSD) in lens are given. Numerical calculation results are presented, in which the axial and lateral sound pressure distributions and the -3 dB sound beam profile along the focused field are given out for three types of the acoustic focused lens with revolving-arc curved outer surface and four types' axisymmetric SSD. It is shown that, when sound beam is focused by a lens with SSD, the best focal point moves away from the lens and achieves a large focal region compared with the no-SSD case. The maximum axial focused intensity at the best focal point is smaller than that of the no-SSD case, but the focused beam width at the best focal point is larger than that of the no-SSD case. The larger the coefficient of the SSD, the larger the focused beam width and the larger focal region, but the divergent angle is almost the same.     

Realization of wavelength conversion with hyperbolic secant femtosecond pulse in normal dispersion regime

Realization of wavelength conversion based on second-order femtosecond dark solitons with hyperbolic secant pulse is presented. This is achieved by introducing localized dispersion perturbation along the optical fiber. We demonstrate that an initial 30 fs second-order pulse decays to similar sub-pulses by applying perturbation using a step increment of β₂ from 6.3 to 15.75 ps² km^?1. This shows that the realization of a 1 × 2 channel wavelength converter for femtosecond pulses is possible. Recent research shows the possibility of realizing wavelength conversion generated from picosecond solitons neglecting nonlinear effects. However, employing the same method for femtosecond pulses fails due to the manifestation of nonlinear effects. In this paper, pulse deformation under different levels of perturbation was tested, and appropriate perturbation leading to similar sub-pulses is achieved.     

Melting temperature of nickel by a pulse heating technique

The melting temperature of 99.98⁺ % pure nickel was measured by means of a subsecond duration pulse heating technique. The results, based on IPTS-68, yield a value of 1729 K for the melting temperature with an estimated maximum uncertainty of ±4 K.     

The melting point of palladium by a pulse heating method

The melting point of 99.95⁺% pure palladium was measured by means of a subsecond duration pulse heating technique. The average temperature at the melting point for three specimens was 1827 K with an average absolute deviation from the mean of 0.1 K. The total error in temperature is estimated to be not more than ±4 K.     

Modulation instability in a dispersion oscillating fibre pumped by a broad band pulse

We numerically investigate the modulation instability generated in a dispersion oscillating fibre pumped by a chirped pulse with a broad bandwidth. We highlight that the side bands are wide, not symmetric in frequency about the pump and several instantaneous side bands can spectrally overlap with each other in one side, while they are well located in the other side. We also show that the spectral distribution can be intuitively explained with an analogy in which the fibre is pumped by a tuneable continuous wave.