用于开环液晶自适应光学系统的模式预测技术研究

时间延迟误差是液晶自适应光学系统的一个最主要的误差源. 本文提出了一种利用智能模式预测-----迭代最小二乘(RLS)模式预测算法来克服其对成像分辨率的影响. 首先, 介绍了具有RLS模式预测能力的开环液晶自适应光学系统的结构和工作原理. 其次, 详细讨论了RLS模式预测算法的实现过程. 再次, 设计和搭建了一套带有液晶湍流模拟器的开环液晶自适应光学系统, 对RLS模式预测算法的预测效果进行了分析, 并和直接开环校正做了比较. 分析结果表明: 当系统处于中等强度湍流条件(大气相干长度r₀=6 cm, Greenwood频率f_G=35 Hz)和只有时间延迟误差情况下, 经过RLS预测后, 残差波面的RMS值由直接校正的0.26波长(1波长=785 nm)降低到了0.15波长, 校正效果提高了42%. 最后, 对预测前后自适应光学系统的成像效果进行了对比试验. 实验结果显示, 经过预测以后, 系统的成像分辨率由直接开环校正的25.4 cycles/mm提高到了32.0 cycles/mm, 成像分辨率提高了26%, 达到了0.9倍的衍射极限分辨率. 因此, RLS模式预测技术可以有效的提高开环液晶自适应系统的成像分辨率.     

Sky implementation of modal predictive control in adaptive optics

We present what we believe are the first experimental results obtained with modal linear predictive control of an adaptive optics system operating on sky at a visible wavelength on a 1.52-m telescope (Observatoire de Haute-Provence, France). This system yields substantially better results than optimized integral control: a 30% relative gain in Strehl ratio is measured, even in conditions of low wind speed.     

系统增益对光混沌通信系统性能影响的研究

针对强注入开环结构的外腔反馈半导体激光器混沌通信系统,数值计算得到系统增益与注入系数的定量关系曲线.对比是否使用系统增益对接收系统输出进行校正两种情况下系统的解调性能,表明使用增益校正的相减解调法获得的信息更接近原始信息.通过采用不同频率的正弦信号进行调制,表明信息频率对系统增益没有影响;用不同调制方法对0.2?Gbit/s数字信号加密,结果表明系统增益与信号类型及调制方法无关. 关键词： 光混沌通信 强注入 系统增益 同步     

开放四能级系统中相对位相对探测响应的影响

利用具有自发辐射诱导相干开放四能级系统的定态数值解,从不同角度分析了探测场和驱动场之间的相对位相Φ对探测场增益(吸收)以及粒子数差的影响.研究结果表明：通过对相对位相的控制,可以实现探测场无反转增益和色散的周期性放大和减弱,并能得到无吸收高色散;Φ=α的色散曲线与Φ=α+π/2的增益曲线相似;在探测场共振条件下,适当调节相对位相,可以使无反转激光（LWI）增益达到最大.还研究了粒子注入和退出速率的变化对位相相关的LWI增益的调制作用.     

Gain Characterization of All-Optical Gain-Clamped PDFFA for WDM Networks Using the Feedback Theory

Using feedback theory, we analyze the gain of an all-optical gain-clamped praseodymium-doped fluoride fiber amplifier (AOGC PDFFA) for eight-channel multiwavelength operation. We demonstrate by numerical simulation that gain stabilization is achieved when the open-loop gain is higher than the feedback path attenuation. If this condition is fulfilled, the gain stabilizes to a value equal to the feedback path attenuation and is not influenced by the pump power, input signal power, number of channels, doped fiber length, or any other doped fiber parameter. In our setup the AOGC PDFFA noise figure is 0.12 dB lower than the open-loop PDFFA noise figure.     

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).     

Two-pulse nuclear echo signal in magnetically ordered media

An analytical expression is obtained for the amplitude of a nuclear echo signal generated in magnetically concentrated materials upon resonant excitation of the nuclear subsystem by two pulses with an equal amplitude. It is revealed that, when both the inhomogeneous broadening of the spectroscopic transition and the inhomogeneous gain distribution are taken into account in the theory simultaneously, the spin echo signal can consist of eight satellites symmetrically located around the central peak. The analytical expressions obtained are used to determine relations between the relative amplitudes and the instants of generation of satellites, on the one hand, and microscopic parameters, such as the inhomogeneous half-width of the spectral line, the halfwidth of the gain distribution function, and the average gain, on the other hand. The results of theoretical calculations and the experimental data obtained for the FeV (2 at % V) alloy are in good agreement.     

非相干泵浦对原子系统位相相关吸收性质的影响

研究发现在具有真空诱导相干性的开放和封闭ladder 型原子系统中,探测场和驱动场之间相对位相对吸收性质有重要的调制作用,同时非相干泵浦对位相相关的吸收性质有显著的影响.在这两种系统中,通过调节非相干泵浦,可以实现从吸收到增益和从粒子数反转激光到无粒子数反转激光的转化,并可获得最大的无反转激光增益.但是,当非相干泵浦不存在时,对相对位相的任意值,在封闭系统中只出现吸收而不能得到无反转激光增益;然而在开放系统中,对于适当的相对位相值甚至可以产生比有非相干泵浦时更大的无反转激光增益.     

基于斜三角开口对环的宽带低耗左手材料

本文将磁谐振环-金属线共面结构与磁谐振环相结合, 提出了一种基于斜三角开口对环结构的新型宽频带低耗左手材料结构. 该结构能产生两个磁谐振, 实现两个双负频段. 研究表明, 调节单元尺寸可使两个双负频段移动重合形成一个宽频带. 理论分析、仿真和测试结果均表明, 本文提出的新形结构在9.3—13.2 GHz频段同时具有负的磁导率和负的介电常数, 相对带宽34.7%, 损耗性能系数347.9, 实现了宽带低耗左手材料. 其研究结果对多频段、宽带低耗左手材料的设计具有重要的指导意义.     

Effects of relative phase on transient evolution in an open resonant ladder-type atomic system

In an open ladder-type resonant atomic system, variation in relative phase between probe and driving fields does not affect the transient evolution of populations, but it has remarkable effects on gain and dispersion of the probe field. No matter whether an incoherent pump is present or absent, transient and stationary gains without inversion (GWI) always can be obtained by choosing an appropriate value of the relative phase. When the incoherent pump is absent,the values of transient and stationary GWIs are much larger and the time interval required to reach the stationary value is longer than those when the incoherent pump is present. Varying the exit rate and the ratio between injection rates can obviously change the phase-dependent GWI. In addition, in the transient evolution process, the phenomenon of high dispersion (refractive index) without absorption occurs at some values of relative phase. In the corresponding closed system, the stationary GWI can be obtained by choosing an appropriate value of relative phase only when incoherent pump exists, moreover the gain is smaller than that in the open system.     

三能级闭环λ原子系统中吸收和色散的相位依赖特性

在微波场闭合的三能级Lambda系统中计算了弱探测场的吸收和色散特性,发现闭合原子系统中的量子干涉导致弱探测场的吸收和色散特性依赖于施加场的相对位相。通过调节相对位相,获得了大的无反转增益和零吸收的高折射率。在三光子非共振的情况下,探测增益和吸收呈现随时间的振荡行为。     

开放V型无反转激光系统相位涨落的影响

通过对从开放的V型双泵浦系统的定态解析解得到的数值计算结果的分析表明:1)驱动场相位将涨落导致一个有限线宽;一般说来,被探测场耦合的上下两能级间的粒子数差随线宽的增加而减小,当线宽增加到一定值时,系统将发生从传统的反转激光到无反转激光的转化;线宽的增加将使无反转激光增益降低;当线宽不为零时,仍可产生无吸收高色散(折射率)2)线宽变化对增益、色散和粒子数差影响的性质和大小与系统其它参量的取值密切相关,只有适当选择线宽和系统其它参量的值才能获得最大的无反转激光增益和无吸收高色散(折射率).     

开环双脉冲液晶自适应光学视网膜成像系统

为了获得高分辨率视网膜图像,利用液晶空间光调制器作为波前校正器建立了一套开环液晶自适应光学视网膜成像系统。与闭环模式相比,采用开环模式后,系统的能量利用率提高了1倍。系统采用双脉冲照明方式,以减少人眼曝光量,保护人眼安全。在照明光学系统中加入了大小视场切换装置使成像视场由之前的0.8°增至1.7°。同时优化了系统的时序控制流程,对人眼像差连续校正的同时快速调节成像相机的前后位置至最佳像面。对于开环模式对动态人眼像差的校正精度进行了测量,实验测得,经开环校正后,残差波面的均方根值约为0.09λ;相应的斯特雷尔(Strehl)比高于0.70,系统分辨率接近光学衍射极限的分辨率。对两名志愿者进行了实验,获得了清晰的眼底视网膜细胞图像。     

螺旋天线步进电机控制系统仿真及运行曲线优化

为实现机械相控阵列天线的波束扫描,采用步进电机驱动螺旋天线单元转动来达到预定的辐射相位。为了优化步进电机的开环控制,研究运行曲线对步进电机控制性能的影响,提高步进电机控制速度和精度。根据步进电机及控制系统的工作原理,建立了基于Simulink的步进电机开环控制系统仿真模型,并在此基础上对梯形和抛物线两种运行曲线进行了仿真研究。仿真结果表明,该仿真模型能精确实现两种运行曲线对步进电机的控制,在100 ms的控制周期内,采用梯形运行曲线步进电机的最大无失步转动角度能够达到270°,而采用抛物线运行曲线时其转动角度能够达到360°,抛物线运行曲线在开环控制系统中具有更高的控制速度。     

模拟退火算法光纤放大器相干合成

提出了利用模拟退火算法实现相干合成的思路。对利用模拟退火算法实现多路光纤放大器相干合成进行了数值模拟,验证了方法的有效性,并分析了算法收敛速度与合成光束数目的关系。进行了两路W量级光纤放大器相干合成的实验,结果表明,模拟退火算法能够有效控制各路光纤激光的相位,系统闭环将目标圆孔内的能量提高了1.8倍,并使得目标圆孔内能量大于理想值80%的概率从19.4%提升到了51.3%,取得了较为明显的相干合成效果。     

Coherent beam combining of two W-level fiber amplifiers in turbulence atmospheric environment based on stochastic parallel gradient descent algorithm

We demonstrate coherent beam combining of two W-level fiber amplifiers based on stochastic parallel gradient descent (SPGD) algorithm at 8 m distance in a simulated turbulence atmospheric environment. A photodetector was used to get the coherent optical intensity of the main-lobe from a pinhole, based on the intensity and the SPGD algorithm, the phase controlling was performed by the digital signal processor. In order to simulate the atmospheric environment, turbulence was induced in the free space of the light path by fans and air-conditions in the lab. Experimental result shows that the system performs well for long-time both with and without the simulating turbulence in close-loop, combining efficiency as high as 84.25% with turbulence and 84.85% without turbulence were realized. Visibility increased from near zero in open-loop to 0.432 with turbulence and 0.505 without turbulence in close-loop and the residual phase error is controlled to be less than λ/18. The probability of energy encircled in the main-lobe to be more than 70% of its ideal value was increased from 18.66 to 93.71% without turbulence and from 14.80 to 92.49% with turbulence when the system evolves from open-loop to close-loop.     

Information-theoretic approach to the study of control systems

We propose an information-theoretic framework for analyzing control systems based on the close relationship of controllers to communication channels. A communication channel takes an input state and transforms it into an output state. A controller, similarly, takes the initial state of a system to be controlled and transforms it into a target state. In this sense, a controller can be thought of as an actuation channel that acts on inputs to produce desired outputs. In this transformation process, two different control strategies can be adopted: (i) the controller applies an actuation dynamics that is independent of the state of the system to be controlled (open-loop control); or (ii) the controller enacts an actuation dynamics that is based on some information about the state of the controlled system (closed-loop control). Using this communication channel model of control, we provide necessary and sufficient conditions for a system to be perfectly controllable and perfectly observable in terms of information and entropy. In addition, we derive a quantitative trade-off between the amount of information gathered by a closed-loop controller and its relative performance advantage over an open-loop controller in stabilizing a system. This work supplements earlier results (Phys. Rev. Lett. 84 (2000) 1156) by providing new derivations of the advantage afforded by closed-loop control and by proposing an information-based optimality criterion for control systems. New applications of this approach pertaining to proportional controllers, and the control of chaotic maps are also presented.     

流动微泡群瞬态空化强度时域分布的比例反馈调节*

声波、血流环境及流动微泡群的稳定性都会影响焦区内瞬态空化强度(ICI)在超声作用时间内的分布,从而影响基于瞬态空化的治疗效率和生物安全性。本文在搭建仿体中流动微泡群瞬态空化发生和实时测量系统的基础上,设计了基于LabView FPGA的比例反馈控制器,在保持脉冲重复频率和脉冲长度不变的条件下,通过选择适当的比例系数,依据当前周期的声波激励下实时测量的ICI,实时调节下一周期声波信号的峰值负压,以调控ICI在时间上的分布。研究表明,在最优比例系数(1 × 107)下,和开环系统相比,ICI的稳定率提升~2.31 倍,ICI的时域下降速率减小~94.41%;在超声作用时间内总ICI也基本达到期望水平。这些结果表明该比例反馈控制器在调控脉冲超声激励下流动微泡群ICI时域分布的有效性,有望改进瞬态空化在相关疾病治疗中的效率和安全性。     

Probe gain via four-wave mixing based on spontaneously generated coherence

We have studied the probe gain via a double-Λ atomic system with a pair of closely lying lower levels in the presence of two probe and two coherent pump fields. The inversionless gain can be realized by using nondegenerate four-wave mixing under the condition of spontaneously generated coherence(SGC) owing to near-degenerate lower levels. Note that by using SGC, two probe fields can be amplified with more remarkable amplitudes, and the gain spectra of an extremely narrow linewidth can be obtained. Last but not least, our results show that the probe gain is quite sensitive to relative phases due to the SGC presence which allows one to modulate the gain spectra periodically by phase modulation, and can also be influenced by all laser field intensities and frequencies, and the angles between dipole elements.     

High-gain lasing and polarization switch with a distributed optical-klystron free-electron laser

This Letter reports the first experimental results from the world's first distributed optical-klystron (DOK) free-electron laser (FEL), the DOK-1 FEL, at Duke University. The DOK-1 FEL is a hybrid system, comprised of four wigglers: two horizontal and two helical. With the DOK-1 FEL, we have obtained the highest FEL gain among all storage ring based FELs at 47.8% (+/-2.7%) per pass. We have also demonstrated that the FEL gain can be enhanced by increasing electron bunching using wigglers with a different polarization. Furthermore, we have realized controlled polarization switches of the FEL beam by a nonoptical means through the manipulation of a buncher magnet.