A comparison between degenerate paramagnetic oscillators and two-photon correlated-spontaneous-emission lasers

The degenerate parametric oscillator (DPO) is analyzed with the nonlinear material treated quantum mechanically. The source of squeezing in the DPO is the same as in the two-photon correlated-spontaneous-emission laser (CEL) — the level coherence. In this sense, they are similar. The amount of squeezing in DPO is the same as in the two-photon CEL at two-photon resonance, but the gain in DPO is usually much smaller than that in the two-photon CEL at two-photon resonance.     

A Heuristic Analysis of Quantum Noise Quenching in the Two-Photon Correlated Emission Laser

We demonstrate how the two-photon correlated emission laser (CEL) can be understood from a simple physical picture in a quasirigorous fashion. We use semiclassical arguments to derive correct expressions for the phase and amplitude diffusion in a simple way. We then illustrate how noise suppression is achieved in the two-photon CEL.     

Laser peening of aluminum alloy 7050 with fastener holes

The fatigue properties of laser peened aluminum alloy 7050 specimens with fastener holes were investigated.The surface profile and residual stress induced in the shock affected zone were characterized.Then,the fatigue specimens with notch were treated by laser peening(LP),and the fatigue lifes of LP-treated specimens were measured and compared with base materials without LP.The results indicated that LP improved the fatigue lifes of all tested specimens.The average fatigue lifes of specimens treated by LP before hole-drilling were 173% longer than those of untreated samples and had better effects than those specimens treated by LP after hole-drilling.     

Analysis of the impedance mismatch effect in foam-solid targets compressed by laser-driven shock waves

The impedance mismatch effect in a two-layer (low density plastic foam, and solid aluminum, respectively) plane target compressed by a laser driven shock wave is considered. In such targets the ablative pressure generated by absorption of laser light in the foam layer is amplified when crossing the foam-aluminum interface. In this paper an analytical model is developed to evaluate the shock pressure in the aluminum layer as a function of the density and thickness of the foam layer and of the laser parameters. The model is in good agreement with previously published experimental results [A. Benuzzi et al., Phys. Plasmas 5, 2827 (1998)]. Received 20 January 2000 and Received in final form 16 May 2000     

Thermal evolution of residual stress in IN718 alloy subjected to laser peening

The thermal relaxation behaviors of residual stresses induced by laser peening (LP) in IN718 alloy were investigated using an integrated numerical simulation and experimental approach. LP and heat treatments (HT) were carried out after which the X-ray diffraction (XRD) technique was employed in measuring the residual stresses. Micro-structures were observed using an optic microscope (OM) and transmission electron microscope (TEM). Dislocations induced by LP were also observed by TEM and characterized using the XRD technique. The effects of the applied temperature and the exposure time on residual stress and micro-structures were investigated. The results show that the extent of the residual stresses relaxation increased accordingly with the increase in the applied temperature. The relaxation rate was initially high and tended to stabilize for a longer exposure time. Grain size evolution during the process was subsequently discussed. Furthermore, a conceivable mechanism of residual stresses thermal relaxation behavior was obtained.     

Generation of high quality ion beams through the stable radiation pressure acceleration of the near critical density target

In order to generate high quality ion beams through the stable radiation pressure acceleration(RPA) of the near critical density(NCD) target, we propose a new type of target where an ultra-thin high density(HD) layer is attached to the front surface of an NCD target, which has a preferable self-supporting property in the RPA experiments than the ultra-thin foil target. It is found that in one-dimensional particle-in-cell(PIC) simulation, by the block of the HD layer in the new target,there emerges the hole-boring process rather than propagation in the NCD layer when the intense laser pulse impinges on this target. As a result, a typical RPA structure that the compressed electron layer overlaps the ion layer as a whole is formed and a high quality ion beam is obtained, e.g., a circularly polarized laser pulse with normalized amplitude a_0= 120 impinges on this new target and a 1.2 GeV monoenergetic ion beam is generated through the RPA of the NCD layer. Similar results are also found in the two-dimensional PIC simulation.     

不同偏振飞秒激光经块状材料传输后的脉宽压缩

实验研究了线偏振和圆偏振状态下的飞秒强激光脉冲在块状材料中的传输过程。不同偏振的激光脉冲在传输过程中得到了不同程度的光谱展宽,经色散补偿后,脉冲时域宽度均得到了压缩。详细分析了压缩脉冲的脉宽以及啁啾情况与入射激光脉冲能量之间的关系,比较了飞秒激光在线偏振及圆偏振情况下的不同压缩效果。在线偏振入射光情况下得到了最短21fs的压缩脉冲宽度,在圆偏振情况下得到的最短脉冲宽度为22fs。实验结果表明,这种光谱展宽与色散补偿方式对圆偏振光同样适用,而且圆偏振的入射激光将更有利于对更高能量的脉冲进行压缩。在色散补偿量相同的情况下,压缩效果随入射脉冲能量变化的规律符合理论估计。     

短脉冲激光产生的等离子体X射线辐射

此文着重研究短脉冲强激光作用下的等离子体Ｘ射线辐射强度随入射激光能量或功率的变化，等离子体辐射主要由激光功率决定，实验显示了辐射曲线的突变区域和他和区域结构。此外，本结果表明在激光以２０Ｐｓ入射条件下的等离子体相对论吸收效应还不太明显，在等离子体中主要是逆韧致吸收。     

Fiber laser with combined feedback of core and cladding modes assisted by an intracavity long-period grating

We present a fiber laser made in a single piece of conventional doped-core fiber that operates by combined feedback of the fundamental core mode LP((0,1)) and the high-order cladding mode LP((0,10)). The laser is an all-fiber structure that uses two fiber Bragg gratings and a long-period grating to select the modes circulating in the cavity; the laser emits at the coupling wavelength between the core mode LP((0,1)) and the counterpropagating cladding mode LP((0,10)) in the Bragg gratings. This work demonstrates the feasibility of high-order mode fiber lasers assisted by long-period gratings.     

Effect of phase fluctuations on entanglement generation in a correlated emission laser with injected coherence

This proposal investigates the influence of the phase fluctuations on the time evolution of entanglement generation in a three-level correlated emission laser (CEL) with injected coherence. The atoms are injected in the cavity with an initial partial coherent superposition of the upper and lower levels. The corresponding phase φ is considered to be randomly distributed around some fixed phase φ₀. The fluctuations in phase φ modifies the coherence between the two corresponding atomic levels. Therefore, we observe strong influence of phase fluctuations on the entanglement generation using the injected coherence CEL system.     

Quasistatic magnetic fields generated by a nonrelativistic intense laser pulse in uniform underdense plasma

Quasistatic magnetic fields generated by nonrelativistic intense linearly polarized (LP) and circularly polarized (CP) laser pulses in an initially uniform underdense plasma in the collision-dominated limit are investigated analytically. Using a selfconsistent analytical model, we perform a detailed derivation of quasistatic magnetic fields in the laser pulse envelope in the collision-dominated limit to obtain exact analytical expressions for magnetic fields and discuss the dependence of magnetic fields on laser and plasma parameters. Equations for quasistatic magnetic fields including both axial component B_z and the azimuthal one B_θ are derived simultaneously from such a selfconsistent model. The dependence of quasistatic magnetic field on incident laser intensity, transverse focused radius of laser pulse, electron density and electron temperature is discussed.     

利用倾斜光纤光栅的可开关双波长光纤激光器

报导了一种在四波混频作用下利用倾斜光纤布拉格光栅进行波长选择的可开关双波长掺铒光纤激光器。通过将倾斜光纤光栅与单模光纤进行横向错位焊接,使光栅的反向LP01和LP11两个模式具有相近的有效反射率,从而可以用来进行激光器的双波长选择。接入腔内的一段高非线性光子晶体光纤引入的四波混频效应克服了模式竞争,使得双波长激光在室温下稳定振荡。腔内起偏器和偏振控制器的联合作用可产生依赖于波长的损耗,以补偿光纤光栅两反射峰峰值的大小差异。基于以上原理,通过调节腔内的偏振态,该激光器实现了室温下稳定的双波长输出,也实现了在两波长之间的转换。两波长激光均有超过45 dB的信噪比,最大的功率波动为0.8 dB     

Ultrashort-pulse investigation of the propagation properties of the LP(11) mode in 1.55-mum communication fibers

The coupling of an ultrashort laser pulse into a single-mode optical communication fiber gives rise to two propagating pulses as a result of the excitation of two guided modes, the fundamental, LP(01) , and the leaky, LP(11) . Such a phenomenon provides a new approach to the study of the propagation properties of the LP(11) mode. An experiment with tunable 110-fs pulses at a wavelength near 1550 nm is described. Information about the group velocity, the polarization-rotation length, the attenuation coefficient, and the cutoff wavelength of the LP(11) mode is obtained in a simple and direct way for various fibers.     

Numerical characterization of time delay signature in chaotic vertical-cavity surface-emitting lasers with optical feedback

We have discussed numerically the characterization of time delay (TD) signature in a chaotic vertical-cavity surface-emitting laser (VCSEL) subject to optical feedback, where the results for polarization-preserved optical feedback (PPOF) and polarization-rotated optical feedback (PROF) are presented comparatively. It is found that, when the feedback strength is moderate, TD signature can be retrieved successfully by analyzing the total intensity, intensities of x-linearly polarized (x-LP) mode, and y-LP mode in a VCSEL with PPOF; while for PROF case, the intensity amplitude peaks in autocorrelation function (ACF)/delayed mutual information (DMI) of the two orthogonal LP modes are located at multiple values of TD. Furthermore, the influence of the feedback strength and time delay has been investigated. The results show that, a further increase of the feedback strength (roughly>40 ns^?1) leads to the successful identification of the TD signature in the two LP mode intensities of VCSELs with PROF. It is also observed that the estimation of TD depends significantly on the value of the spin–flip relaxation rate (γ_s). For low relaxation rates the TD signature could be much easier to be revealed by the two LP modes than that for large values of relaxation rate in VCSELs with PROF. Finally the influence of injection current and the device linear anisotropies is investigated, and the results of TD estimation are confirmed by calculating permutation entropy (PE).     

飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象,证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律.实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂.此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象. 关键词： 超短激光脉冲 脉冲压缩 非线性传输     

Optical probing of laser-induced shock waves in air

Shock Waves (SW) were produced in air by focusing the (0.25 J, 6 ns) second-harmonic ( = 532 nm) Nd : YAG laser light into a stainless-steel cylindrical cell at a pressure from 200 to 760 Torr. The laser fluence at the focal point is > 5 GW/cm². The spatial variation and consequently the time evolution of the radial propagation velocityU of the generated shock waves were measured via a simple optical system utilizing a HeNe laser beam triply intersecting the propagating shock wave at three successive positions. Using a reflector, we were able to probe the traveling SW in six consecutive positions during its round trip. Good agreement was obtained between the experimental results and the predictions of the point strong explosion theory. It is shown that this method is simple with a fairly good precision. It therefore appears to be useful for the determination of the SW dynamic parameters, namely its Mach number, the pressure at the SW front, the thickness of the compressed air layer and the energy consumed in producing this layer.     

强激光和超薄等离子体薄膜相互作用产生高次谐波

通过一维粒子（particle in cell,简称PIC）模拟，研制了超薄等离子体薄膜在两列旋转方向相反的圆偏振激光脉冲作用下的高次谐波辐射。在10^21W/cm^2强度的激光作用下，可以获得200次以上的高次谐波。当激光脉冲宽度较短时，由于等离子体薄膜被压缩所导致的多普勒频移对高次谐波产生了显著的频率调制。而这种多普勒频移可以通过对抽运光脉冲引入频率啁啾来补偿。     

利用整形激光脉冲压缩薄膜靶得到中子源

研究了整形激光脉冲对冰冻氘氚靶的压缩。通过数值分析,发现利用分步激光产生的系列激波压缩氘氚靶可以获得较高密度和较低温度的等离子体靶。初始激光强度的选取将影响到压缩后的等离子体密度,继而影响到产生中子的数量。通过调节初始激光强度可以使压缩后的氘氚靶温度处在反应率比较高的范围内,从而得到优化结果。当初始归一化激光振幅为0.5,最终为32时,压缩后的氘氚靶密度可达到18416倍的临界密度,温度达到16 keV,每焦耳入射激光能量可得到109个中子,这个中子产额比现有其他方法所得到的中子产额大4个数量级。     

Comparison of surface films formed on titanium by pulsed Nd:YAG laser irradiation at different powers and wavelengths in nitrogen atmosphere

The nitridation of titanium (Ti) caused by a Q-switched Nd:YAG laser under nitrogen gas atmosphere was investigated in situ using X-ray photoelectron spectroscopy (XPS). A laser having a wavelength of 1064 nm and 532 nm (SHG mode) was irradiated on a titanium substrate in an atmosphere-controlled chamber, and the substrate was then transported to an XPS analysis chamber without exposing it to air. The characteristics of the surface layer strongly depend on the laser power. When the power is relatively low, a titanium dioxide layer containing a small amount of nitrogen is formed on the substrate. Laser irradiation beyond a certain laser power is required to obtain a stoichiometric titanium nitride (TiN) layer. A TiN layer and an oxynitride layer with a TiO_xN_y-like structure are formed as the topmost and the lower surface layer, respectively, when the laser power exceeds this threshold value. The threshold laser power strongly depends on the wavelength of the laser, and this threshold value for the 532-nm laser is quite lower than that for the 1064-nm laser.     

Generation of cylindrical vector beams in a mode-locked fiber laser using a mode-selective coupler

We experimentally obtain cylindrical vector beams(CVBs) in a passively mode-locked fiber laser based on nonlinear polarization rotation. A mode-selective coupler composed of both a single-mode fiber(SMF) and a twomode fiber(TMF) is incorporated into the cavity to act as a mode converter from LP01 mode to LP11 mode with broad spectral bandwidth. CVBs in different mode-locked states including single-pulse, multi-pulse, and bound pulse are obtained, for the first time to our best knowledge. The ultrafast CVBs with different operation states have potential applications in many fields such as laser beam machining, nanoparticle manipulation, and so on.