Shock wave in the flow field of a source and a vortex source in the energy supply zone at thermal crisis

Thermal crisis of a vortex source outflowing initially in regime I into a rarefied space (into vacuum) with a transition of the supersonic flow into the subsonic flow in the shock wave, and into a stagnant space in regime II with final stagnation is considered in the model of a perfect gas with a constant heat capacity. The shock wave can be located in the energy supply zone or outside the energy release zone depending on the preset total pressure at infinity. In the absence of circulation, a cylindrical source is compared with a spherical source. The dependences of energy parameters and temperature, as well as the total pressure and density, on the coordinates of the shock wave are considered. The dependences of the critical parameters of the flow in the wake behind the zone on the coordinate of the heat supply zone, its length, and gas circulation in the cylindrical vortex source are analyzed.     

Experimental investigation of the magnetohydrodynamic parachute effect in a hypersonic air flow

New data on experimental implementation of the magnetohydrodynamic (MHD) parachute configuration in an air flow with Mach number M = 6 about a flat plate are considered. It is shown that MHD interaction near a flat plate may transform an attached oblique shock wave into a normal detached one, which considerably extends the area of body-incoming flow interaction. This effect can be employed in optimizing return space vehicle deceleration conditions in the upper atmosphere.     

HEPS在轴注入冲击器系统及快脉冲电源样机研制

高能同步辐射光源(HEPS)是我国计划建造的下一代基于储存环的高亮度光源,束流自然发射度已经接近衍射极限。作为典型的低发射度储存环(LER),HEPS的动力学孔径远小于物理孔径,传统的离轴累积注入已经无法满足要求,只能采用基于strip-line kicker的在轴注入方案。为了实现逐束团操控,HEPS要求注入kicker脉冲电源底宽(3%~3%) < 10 ns,半高宽(50%~50%)>4.5 ns,幅度>±17.5 kV(50 Ω负载),重复频率>50 Hz。高能同步辐射光源验证装置(HEPS-TF)工程研制了一台基于DSRD的双极性快脉冲电源性能样机,在50 Ω负载上可以获得上升时间(10%~90%) < 2.6 ns,下降时间(90%~10%) < 3.2 ns,半高宽(50%~50%)>5 ns,底宽(3%~3%) < 10 ns,幅度>±18 kV的脉冲高压,可以满足HEPS注入基准方案——在轴置换注入的要求。     

不同来流马赫数下激光能量形成高超声速进气道虚拟唇口的特性

用基于三维非定常可压缩雷诺平均Navier-Stokes方程的有限体积法计算了马赫数低于设计值6时一种高超声速进气道的性能参数,发现其性能存在明显下降。为提升进气道性能,将功率为15kW的激光能量注入进气道固体唇口前的流场中,形成虚拟唇口,马赫数为4.5,5.0和5.5时,计算得到来流捕获率分别提高了34%,20.6%和15.6%。绘制了不同马赫数下来流捕获率达到峰值时的流场压强云图,说明了虚拟唇口的特性及形成机制。结果表明:来流马赫数越低,来流捕获率越小,但相对于无能量注入时的来流捕获率的提升程度越明显;在不同来流马赫数条件下,通过改变激光能量引致的激波结构和位置,可达到最优状态,即激波与进气道前缘斜激波相交后的透射波打在进气道肩部位置的状态。     

Coherent energy transfer from one atom to another under a selective excitation of one of them by a field of continuous optical radiation and optical size resonances

The existence of a new sort of optical size resonances formed due to the self-consistent coherent interaction of atoms when one of them is excited by a field of continuous optical radiation is proved. The processes of energy transfer from the thus-excited atom to large (of the order of several wavelengths) interatomic distances are considered. It is shown that these processes can be observed in the wave zone upon the interference of the optical fields formed by the oscillating dipole moments of the interacting atoms.     

Supercontinuum laser illumination applied to traditional optical flow imaging methods

The schlieren method of refractive flow visualization traditionally requires non-coherent illumination in order to avoid problems with coherent artifact noise and binary cutoff by the knife-edge. Related shadowgraph and Wollaston-prism schlieren-interferometer methods, however, work well with light sources of high spatial coherence. Here we apply a supercontinuum laser as a light source for these methods, providing spatially coherent white-light illumination. Excellent results are obtained with shadowgraphy and schlieren-interferometry, but the schlieren method still requires a modified cutoff in place of the traditional knife-edge for laser illumination. Once they become widely available, supercontinuum lasers will find frequent use in these optical instruments, especially the schlieren-interferometer.     

Stability characteristic of hypersonic flow over a blunt wedge under freestream pulse wave

To investigate the stability characteristic of hypersonic flow under the action of a freestream pulse wave, a high-order finite difference method was employed to do direction numerical simulation (DNS) of hypersonic unsteady flow over an 8° half-wedge-angle blunt wedge with freestream slow acoustic wave. The evolution of disturbance wave modes in the boundary layer under a pulse wave and a continuous wave are compared, and the wall temperature effect on the hypersonic boundary layer stability for a pulse wave disturbance is discussed. Results show that, both for a pulse wave and a continuous wave in freestream, the disturbance waves inside the nose boundary layer are mainly a fundamental mode; the Fourier amplitude of pressure disturbance mode in the boundary layer for a pulse wave is far less than that for a continuous wave, and the band frequency of the former is wider than that of the latter. All disturbance modes decay rapidly along the streamwise in the nose boundary layer. In the non-nose boundary layer, the dominant mode is transferred from fundamental mode into second harmonic. The transformation of dominant mode for a pulse wave appears much earlier than that for a continuous wave. Different frequency disturbance modes present different changes along streamline in the boundary layer, and the frequency band narrows around the second harmonic mode along the streamwise. Keen competition and the transformation of energy exist among different modes in the boundary layer. Wall temperature modifies the stability characteristic of the hypersonic boundary layer, which presents little effect on the development of fundamental modes and cooling wall could accelerates the growth of the high frequency mode as well as the dominant mode transformation.     

Ultrahigh-resolution optical coherence tomography with a fiber laser source at 1 microm

We report a compact, high-power, fiber-based source for ultrahigh-resolution optical coherence tomography (OCT) near 1 microm. The practical source is based on a short-pulse, ytterbium-doped fiber laser and on generation of a continuum spectrum in a photonic crystal fiber. The broadband emission has an average power of 140 mW and offers an axial resolution of 2.1 microm in air (<1.6 microm in biological tissue). The generation of a broad bandwidth is robust and efficient. We demonstrate ultrahigh-resolution, time-domain OCT imaging of in vitro and in vivo biological tissues.     

A picosecond near-infrared laser source based on a self-seeded optical parametric generator

We report on the design and development of a new type of near-IR laser source. The source comprises of an optical parametric generator (OPG) and a second harmonic generator (SHG) pumped by an 80-MHz, 1064-nm, 7-ps Nd:YVO₄ laser. The OPG is self-seeded with a fraction of its own signal output, which significantly enhances its conversion efficiency. The SHG doubles the frequency of OPG signal to produce a coherent output. The final output beam has a tunable wavelength near 800 nm, an average power of over 1 W, and a pulse duration around 5 ps. The M²-factor of the output beam can reach 1.1 after spatial filtering. With the new laser source, we have successfully demonstrated coherent anti-Stokes Raman scattering microscopy on 1 μm polystyrene beads, which shows that it has the potential to be a substitute for a picosecond optical parametric oscillator in certain microscopy or spectroscopy applications.     

低速热射流涡及其气动光学特性的测量

用37单元哈特曼 夏克波前传感器以419Hz的帧频对加热的扁喷管气流的气动光学特性进行了测量,采用模式法进行波前重构,从而得到观测孔径内该低速热射流所造成的光程差分布,进而计算出其Strehl比,时间序列结果反映了它们的动态过程。实验给出了4.3~8 m/s及44~85℃的中心平均空气流速和温度的几种不同气流参数下的光程差分布,用相关的方法得出剪切层中相干结构（涡）的流动速度,结果表明,剪切层中相干结构引起的光程差分布及Strehl比随气流参数而变化;当高于65℃时,温度的影响不明显。     

Receptivity to free-stream disturbance waves for hypersonic flow over a blunt cone

A high-order shock-fitting finite difference scheme is studied and used to do direction numerical simulation (DNS) of hypersonic unsteady flow over a blunt cone with fast acoustic waves in the free stream, and the receptivity problem in the blunt cone hypersonic boundary layers is studied. The results show that the acoustic waves are the strongest disturbance in the blunt cone hypersonic boundary layers. The wave modes of disturbance in the blunt cone boundary layers are first, second, and third modes which are generated and propagated downstream along the wall. The results also show that as the frequency decreases, the amplitudes of wave modes of disturbance increase, but there is a critical value. When frequency is over the critial value, the amplitudes decrease. Because of the discontinuity of curvature along the blunt cone body, the maximum amplitudes as a function of frequencies are not monotone. Supported by the National Natural Science Foundation of China (Grant Nos. 10632050 and 10502052)     

High-resolution optical time domain reflectmetry based on a broadband continuous wave multimode laser diode

A broadband light source can be obtained by operating a multimode laser diode with a bias current below threshold. We propose an optical time domain reflectometer for high-resolution with a broadband cw multimode laser. The emission spectrum of the multimode laser diode that affects an interference signal is investigated. With this method, the spatial resolution is measured as 11.5 μm, corresponding to that of 38.3-fs pulses, and a minimum detectable reflectivity < -90 dB is achieved.     

An optical device to homogenize a laser beam

Based on segmentation-recombination principle, a specific optical device is designed to homogenize a highpower CO2 laser beam which is used as a heating source.     

高功率,红光至中红外可调谐腔内和频光学参量振荡器

本文利用高重复频率,高平均功率大模场面积飞秒光纤激光器作为同步抽运源,抽运以多周期极化掺氧化镁铌酸锂为非线性晶体的单共振光学参量振荡器,获得了高功率可调谐红光至中红外光,信号光调谐范围为1450—2200 nm,闲频光调谐范围为2250—4000 nm,在2 W的抽运功率下,信号光输出波长为1502 nm时获得最大输出功率374 mW,转换效率为18.7%,脉冲宽度为144 fs,此时中红外输出中心波长为3.4μm,平均功率为166 mW.再利用BBO晶体对信号光进行腔内和频,获得和频光输出波长调谐范围为610—668 nm,在4.1 W抽运的情况下,最高平均功率为615 nm处的694 mW,转换效率达16.9%.     

The transient response of a transversely isotropic cylinder under a laser point source impact

The transient response of a transversely isotropic cylinder under a laser point source impact is solved theoretically. The radial displacement generated by the laser under the ablation regime is numerically calculated by introducing Fourier series expansion and two-dimensional Fourier transform. The validity of this theoretical solution is demonstrated on a fiber reinforced composite cylinder with a strong anisotropy. Experimental displacements are detected at the cylinder surface by the laser ultrasonic technique, and are analyzed by the ray trajectories. Corresponding theoretical displacements are calculated numerically and compared to the experimental signals. Good agreement is found. The diffraction effect caused by the cusp is observed in both theory and experiment.     

Spatial redistribution of energy in a nanosecond laser pulse by an organic optical limiter

The spatial redistribution of energy resulting from the interaction between a near-diffraction-limited nanosecond laser pulse and the nonlinear absorbing optical limiting dye silicon naphthalocyanine is described, for what is to our knowledge the first time, in an optical geometry that is likely to be found in practical applications. For input fluences above that required for nonlinear absorption but below that for bubble growth, a plane wave or Gaussian spatial input evolves unexpectedly to a sharp central spike and a well-defined outer ring. The observed energy redistribution is thought to rely on a combination of nonlinear processes, since a pure absorptive process alone cannot explain the profiles presented. A model involving nonlinear absorption and nonlinear refraction qualitatively reproduces the observed spatial profiles. It is clear from the results that the performance of optical limiting dyes in representative optical geometries, even at fluences well below that required for bubble growth, cannot be described solely by nonlinear absorption.