BBOⅠ类相位匹配非共线参量产生中的群速自匹配

利用重复频率为1kHz的钛宝石激光的倍频蓝光抽运Ⅰ类相位匹配的偏硼酸钡(BBO)晶体，获得了可见光区的非共线参量超荧光.给出了最强光强处参量超荧光的非共线角，波长随相位匹配角的变化.理论计算了在参量超荧光中三波的群速失配随非共线角和相位匹配角的变化规律.结果表明，Ⅰ类非共线相位匹配的参量超荧光关于抽运光中心对称.并且沿光强最强的参量超荧光方向三波的群速失配最小，有效互作用长度最大，即参量超荧光的强度角分布是三波的相速和群速自匹配的结果.该理论和实验研究为减小飞秒光参量放大中三波的群速失配提供了一个理想的几何模型，为获得高增益、窄脉冲宽度的参量光输出提供了理论依据和实验验证. 关键词： 群速自匹配 参量超荧光 非共线相位匹配 飞秒光参量放大     

试论王立军实验的意义

 2000年王立军等人在《自然》上发表了一篇重要论文“具有增益的超光速传播”,文中认为他们已成功地实现了光脉冲的超光速传播和从铯室(Ｃｓｃｅｌｌ)中的出射光脉冲在时间上超前于入射光脉冲。图1是实验的示意图:图1王立军实验(ＷＫＤ实验)示意图图1中,入射光脉冲于实验室参照系的时刻ｔ0进入铯室,于实验室参照系的时刻ｔＬ从铯室出去.在铯原子气的反常色散区群速指数ｎｇ=ｎ(ｖ)+ｖｄｎ(ｖ)ｄｖ=-310±5<0(1)光脉冲群速为负速度,即ｖｇ=Ｃｎｇ=-Ｃ310<0.(2)光脉冲在铯室内的通行时间为Δｔ(Ｃｓ)=ｔＣｓＬ-ｔ0=     

飞秒光参量放大中三波群速失配的补偿

为了消除群速失配对参量放大的不利影响，描述了利用脉冲波面倾斜与非共线相位匹配相结 合，完全补偿飞秒光参量放大（OPA）中三波群速失配的新方法.计算了在BBOⅠ类、Ⅱ类相 位匹配条件下， 三波实现群速匹配时，相位匹配角、脉冲波面倾斜角以及非共线角随信号 光波长的变化.并分析了三波群速匹配对空间走离长度、参量增益和参量带宽的影响.结果表 明，在BBOⅠ类、Ⅱ类相位匹配条件下，利用该方法均能实现飞秒OPA连续调谐时三波的群速 匹配，从而大大增加了三波的有效互作用长度，为能够获得高增益，窄脉宽的参量光脉冲提 供了理论依据和指导. 关键词： 群速匹配 脉冲波面倾斜 非共线相位匹配 飞秒光参量放大     

非线性光学 非线性光学器件

O437.42006010281BBOⅡ类共线相位匹配结构中群速失配的补偿=Compen-sating group velocity mismatch in BBO type-Ⅱcollinearphase-matched configuration[刊,中]/马晶(天津大学精密仪器与光电子工程学院超快激光研究室,光电信息技术科学教育部重点实验室.天津(300072)),章若冰…∥中国激光.—2005,32(7).—913-916为了消除群速失配对飞秒光参量放大(OPA)的不利影响,描述了在Ⅱ类共线相位匹配的飞秒BBO光参量放大中,同时倾斜抽运光和信号光的脉冲波面完全补偿三波群速失配的方法。理论计算了三波群速匹配时,相位匹配角、抽运光和信…     

考虑不同脉冲形状时群速失配效应在二次谐波自相关法中的影响

本文讨论了在考虑不同脉冲形状时群速失配效应对二次谐波自相关法测量超短光脉冲宽度的影响.结果表明了脉冲形状不同时,群速失配效应对干涉自相关法和强度自相关法的影响不同.     

考虑不同脉冲形状时群速失配效应在二次谐波自相关法中的影响

本文讨论了在考虑不同脉冲形状时群速失配效应对二次谐波自相关法测量超短光脉冲宽度的影响.结果表明了脉冲形状不同时,群速失配效应对干涉自相关法和强度自相关法的影响不同.     

非线性光学 位相共轭、自聚焦等相关效应

O437 2005042689 飞秒光参量放大中三波群速失配的补偿=Compensation of group velocity mismatching of three pulses in femtosec- ond optical parametric amplification[刊,中]／马晶(天津大学精密仪器与光电子工程学院超快激光研究室,光电信息技术科学教育部重点实验室(天津大学),天津(300072)), 章若冰…／／物理学报,-2005,54(2),-755-762 为了消除群速失配对参量放大的不利影响,描述了利用脉冲波面倾斜与非共线相位匹配相结合,完全补偿飞秒光参量放大(OPA)中三波群速失配的新方法。计算了在 BBOⅠ类、Ⅱ类相位匹配条件下,三波实现群速匹配时,相位匹配角、脉冲波面倾斜角以及非共线角随信号光波长的     

利用周期信号驱动Chua电路实现相同步

我们在实验上和理论上对周期信号驱动的混沌电路的相同步问题进行了研究。我们选择三类周期信号作为驱动信号，包括Chua电路产生的周期信号，正弦信号和脉冲信号。改变驱动信号的频率、振幅和脉冲信号的占空比，在周期信号和混沌信号存在小的频率失配的条件下，驱动信号的振幅在适当的变化范围内，以及脉冲信号的占空比在适当的变化范围内，都可以获得相同步。甚至当占空比小到3%的情况下，仍然可以得到相同步。特别是这类脉冲信号的控制在实际中是有意义的。     

在掺铒光纤中直接观测慢光和超光速信号的演化

最近几年中,慢光和超光速的研究取得了很多进展.慢光和超光速产生的物理机制作为慢光和超光速研究领域的一个重要方面也吸引了研究者的关注.设计了一个新颖的实验,在掺铒光纤中观测了慢光和超光速信号的演化.分别在慢光(0相似文献   

反常色散介质“超光速”现象研究的新进展

文章介绍了王力军等人的光脉冲在反常色散介质传播实验的测量结果，说明了实验测量的不完全性，因而它不能在是否超光速的问题的答案，文章还介绍了近一年来相应的理论研究工作，文章作者依据物理概念和理论研究结果论证了该实验所得的负群速度本身并不超光速；如果把负群速度看成是某种能量的传播那私超光速出现在负群速度开始时间间，因而它是超距作用，违反能量守恒或动量守恒，因此，负群速度只是表观速度，能流的研究结果也证明了这一点。这就是说，该实验并没有观察 到超观察到超光速传播。     

Negative and Superluminal Group Velocity Propagation with Narrow Pulse in a Coaxial Photonic Crystal

We investigate the propagation of electric signal along a spatially periodic impedance mismatched transmission line group. Anomalous dispersion is caused by the periodically mismatched impedance structure and a forbidden band appears near 8 MHz in transmission. The group velocity of the amplitude-modulated signal is augmented up to infinity, even -3.89c （c the speed of light in vacuum） in the forbidden region with the amplitude of the modulated signal increasing. When the carrier sinusoid signal is modulated in amplitude by the modulating sinusoid signal, of which the peak is superimposed with a narrow pulse at fivefold frequency, the superluminal group velocity also occurs. The experiment failed to show whether the propagation velocity of narrow pulse exceeds c or not.     

Emission of a spatially modulated resonant medium excited with superluminal velocity

Specific characteristics of the radiation of a resonant medium excited by an ultrashort light pulse propagating through the medium with a superluminal velocity are considered. The medium is assumed to consist of identical linear harmonic oscillators with a spatial density periodically modulated along the direction of propagation of the superluminal excitation. The field of radiation of the resonant medium under these conditions is calculated. It is shown that, under the superluminal excitation, the radiation spectrum of the medium shows, along with the fundamental frequency of the oscillators, new frequencies that depend on the spatial frequency of the distribution of oscillators and on the angle of observation. Possible application of the effect is discussed.     

超光速群速度与信息传输的有效速度

研究了反常色散介质中脉冲形变对超光速群速度的影响,发现即使光脉冲完全不产生形变群速度仍会超过真空中的光速。但波包的群速度并不等同于信号的传输速度,采用信息论方法,定义了信号的有效传输速度,并用于解释WKD(Wang,Kuzmich,Dogariu)实验。通过计算入射光与出射光信号所携带的信息量,发现由于光的波动衍射及光子散粒噪声的影响,出射光所携带的信息量会损失,使得光信号的有效传播速度不会超过真空中的光速。     

Group velocity manipulation in active fibers using mutually modulated cross-gain modulation: from ultraslow to superluminal propagation

We propose and experimentally demonstrate the propagation of slow/fast light in an erbium-doped fiber (EDF) using mutually modulated cross-gain modulation. The group velocity of the light signal can be manipulated by the effect of gain cross-saturation modulation by a saturating light at an arbitrary wavelength in the gain bandwidth of the EDF. The ultraslow propagation with a small group velocity of 5.6 × 10?3c (c is the light speed in free space) and superluminal propagation with a negative group velocity of -1.1 × 10?3c has been observed under different modulation phases.     

Superluminal light attenuated by strong dispersion of complex refractive index

The propagation of narrow packets of electromagnetic waves(EMWs) in frequency dispersive medium with the consideration of the complex refractive index is studied. It is shown that counting in the dispersion of the complex refractive index within the context of the conventional expression of the group velocity of narrow wave packets of EMWs propagating in a dispersive medium results in the appearance of additional constraints on the group velocity, which dictates that the physically acceptable group velocity can only be realized in the case of a negligible imaginary part of the group index. In this paper, the conditions that allow one to realize the physically acceptable group velocity are formulated and analyzed numerically for the relevant model of the refractive index of a system of two-level atoms in the optical frequency range. It is shown that in the frequency band where superluminal light propagation is expected, there is a strong dispersion of the refractive index that is accompanied with strong absorption, resulting in a strongly attenuated superluminal light.     

Average energy flow of optical pulses in dispersive media

The arrival time of a light pulse at a point in space is defined using a time expectation integral over the Poynting vector. The delay between pulse arrival times at two distinct points is shown to consist of two parts: a spectral superposition of group delays (inverse of group velocity) and a delay due to spectral reshaping via absorption or amplification. The result provides a context wherein group velocity is always meaningful even for broad band pulses and when the group velocity is superluminal or negative. The result imposes luminality on sharply defined pulses.     

Simultaneous wavelength tuning and amplitude modulation in QPM optical parametric amplifiers

We repot simultaneous optical parametric amplifiers (OPA) and amplitude modulation of signal pulse in a monolithic periodically poled KTP (PPKTP) by electrically controlling the relative phase between the pump, signal and idler waves in a KTP dispersion section sandwiched between two PPKTP gratings, theoretically. The controlling electric field can be arbitrary direction. For an ultrashort pulse group velocity mismatch, group dispersion and diffraction must be regarded, simultaneously. The solutions of the truncated equations, including the above effects in the two PPKTP gratings of simultaneous frequency tuning and amplitude modulation OPA, were analyzed. It is shown that the intensity of output signal pulse of OPA depends on the thickness of the dispersion section, the magnitude and azimuth of controlling electric field and one evidences periodicity. For the certain angular dispersion there exist the largest frequency bandwidths of optical parametric amplifiers. Certain angular dispersion of a signal pulse can increase the frequency bandwidth, and broadband amplification of ultrashort pulse can be performed effectively.     

Absorption-free superluminal light propagation in a quantum-dot molecule

The propagation of a weak probe field in a three-level quantum-dot molecule is investigated by employing the tunnel coupling. It is shown that in the absence of tunnel coupling, the propagation of light pulse is superluminal, similar to a simple two-level system. A high-resolution dip appears in optical spectra due to the presence of tunnel coupling. We show that this narrow dip leads to the subluminal light propagation with doublet absorption, so the group velocity of a light pulse can be controlled by interdot tunnel coupling. It is also demonstrated that by applying an indirect incoherent pumping field to the probe transition, the absorption doublet switches to the gain doublet and then the absorption-free superluminal light propagation is appeared.     

Absorption-free superluminal light propagation in a quantum-dot molecule

The propagation of a weak probe field in a three-level quantum-dot molecule is investigated by employing the tunnel coupling. It is shown that in the absence of tunnel coupling, the propagation of light pulse is superluminal, similar to a simple two-level system. A high-resolution dip appears in optical spectra due to the presence of tunnel coupling. We show that this narrow dip leads to the subluminal light propagation with doublet absorption, so the group velocity of a light pulse can be controlled by interdot tunnel coupling. It is also demonstrated that by applying an indirect incoherent pumping field to the probe transition, the absorption doublet switches to the gain doublet and then the absorption-free superluminal light propagation is appeared.     

Measurement of net group and reshaping delays for optical pulses in dispersive media

We experimentally demonstrate the direct measurement of net group and reshaping delays for arbitrary optical pulses in dispersive media, verifying the earlier prediction of Peatross et al. [Phys. Rev. Lett. 84, 2370 (2000)]. Incoherent pulse propagation in an absorptionless system is well described by net group delay; even the medium causes a great deal of deformation in the transmitted pulse. However, in the case of phase modulated chirping pulses in a resonant absorber, the so-called superluminal or subluminal propagation velocity is strongly influenced by the reshaping delay.