中国散裂中子源/快循环同步加速器束流负载效应的补偿

针对强流质子同步加速器中的束流负载效应,基于中国散裂中子源/快循环同步加速器射频系统样机,在不降低腔体Q值的前提下,以数字化低电平控制为主要技术手段,对束流负载效应进行补偿。提出完整的由多控制环路组成的束流负载效应补偿方案。该方案主要由引入自适应算法的束流前馈和高带宽低延时的射频直接反馈,以及在束流负载下对腔体谐振状态进行控制的腔体预失谐和动态调谐等组成。     

Single-longitudinal-mode dual-wavelength fiber ring laser by incorporating variable saturable absorbers and feedback fiber loops

A single-longitudinal-mode dual-wavelength fiber ring laser is proposed and demonstrated experimentally by introducing feedback fiber loops and a variable saturable absorber in a fiber ring cavity. A dynamic ultra-narrow bandpass filter is formed by the combined interactions of cascaded fiber Bragg gratings, feedback fiber loops and low-pumped erbium-doped fiber. Both ultra-narrow bandpass filter and accurate control for the cavity length are not required for the single-longitudinal-mode performance. The proposed scheme offers a possibility of generating terahertz waves by photomixing the lasing wavelengths in a high-speed photodetector.     

Quasi-common-path microchip laser feedback interferometry with a high stability and accuracy

We report on an improved scheme of quasi-common-path microchip laser feedback interferometry, which demonstrates a high stability and accuracy. An additional beam splitter is used to divide the external cavity into a measuring feedback cavity and a reference feedback cavity. With this scheme, the optical path lengths of the measuring and reference feedback light can be made nearly the same, thus greatly reducing the long-period phase fluctuation caused by the laser frequency drift. The final system performances are evaluated as followed: the short-term displacement resolution is better than 2 nm, the output fluctuation is less than 10 nm within a 40-min-long stability test, and the maximum error within the 100-μm range is 25 nm when calibrated with the Agilent 5529A dual-frequency laser interferometer.     

光腔衰荡光谱方法探测痕量一氧化碳气体

基于通讯波段的分布式反馈半导体激光器(DFB),搭建了一套光腔衰荡光谱仪(CRDS)。衰荡光腔由一对反射率高于99.997%的高反镜组成,衰荡腔长约为130 cm,空腔衰荡时间约为150 μs。当光谱平均次数达到1 000次时,光谱仪灵敏度(最小可探测吸收系数)达到5×10-12 cm-1。利用热隔绝的方式稳定衰荡腔长,并使用衰荡光腔自身作为光学标准具,来标定光谱的频率：利用反馈式光谱扫描程序步进改变激光器频率,使之与衰荡腔的纵模频率逐一匹配,从而实现所测得光谱的自动标定。通过测量一氧化碳分子在1.565 μm附近的吸收光谱,测定气体中一氧化碳的含量。将光谱测量结果和标准样品中的一氧化碳含量进行对比,对装置的定量精度进行了检验,表明其对一氧化碳的探测极限达4 ppbv。利用该装置对实际大气中一氧化碳的含量进行了实时监测。     

Sensitive birefringence measurement in a high-finesse resonator using diode laser optical self-locking

We demonstrate a new method to measure weak birefringence of dielectric mirrors with excellent spatial resolution and sensitivity (<10^-7 radians). We exploit a well-known optical feedback scheme for line-width narrowing and frequency locking of a diode laser to a high-finesse cavity. Feedback comes from the intracavity field which builds up at resonance, selected by its change in polarization with respect to the incident field. This change, due to the residual birefringence of the cavity mirror coatings, was already exploited for birefringence measurements using an active laser-locking scheme. Here we measure the optical feedback rate as a function of rotation angle of one of the cavity mirrors (around the cavity axis). A stable feedback signal is obtained since the laser, as soon as it locks to a cavity resonance, effectively behaves as a monochromatic source. By fitting the data with a theoretical expression, we determine quantitatively the local birefringence vectors of both mirrors, which are around 10^-6 radians. Our scheme is simple, works with cavities of very high finesse (F∼10⁵), and is promising for measuring birefringence in gases induced by external fields. Received: 18 July 2001 / Final version: 14 March 2002 / Published online: 8 May 2002     

ACTIVE CONTROL OF PRESSURE FLUCTUATIONS DUE TO FLOW OVER HELMHOLTZ RESONATORS

Grazing flows over Helmholtz resonators may result in self-sustained flow oscillations at the Helmholtz acoustic resonance frequency of the cavity system. The associated pressure fluctuations may be undesirable. Many solutions have been proposed to solve this problem including, for example, leading edge spoilers, trailing edge deflectors, and leading edge flow diffusers. Most of these control devices are “passive”, i.e., they do not involve dynamic control systems. Active control methods, which do require dynamic controls, have been implemented with success for different cases of flow instabilities. Previous investigations of the control of flow-excited cavity resonance have used mainly one or more loudspeakers located within the cavity wall. In the present study, oscillated spoilers hinged near the leading edge of the cavity orifice were used. Experiments were performed using a cavity installed within the test section wall of a wind tunnel. A microphone located within the cavity was used as the feedback sensor. A loop shaping feedback control design methodology was used in order to ensure robust controller performance over varying flow conditions. Cavity pressure level attenuation of up to 20dB was achieved around the critical velocity (i.e., the velocity for which the fundamental excitation frequency matches the Helmholtz resonance frequency of the cavity), relative to the level in the presence of the spoiler held stationary. The required actuation effort was small. The spoiler peak displacement was typically only 4% of the mean spoiler angle (approximately 1′). The control scheme was found to provide robust performance for transient operating conditions. Oscillated leading edge spoilers offer potential advantages over loudspeakers for cavity resonance control, including a reduced encumbrance (especially for low-frequency applications), and a reduced actuation effort.     

Quasideterministic generation of entangled atoms in a cavity

We present a scheme to generate a maximally entangled state of two three-level atoms in a cavity. The success or failure of the generation of the desired entangled state can be determined by detecting the polarization of the photon leaking out of the cavity. With the use of an automatic feedback, the success probability of the scheme can be made to approach unity.     

双信道偏振复用保密通信系统的完全混沌同步的操控性研究

针对双信道偏振复用保密通信系统,利用线性电光效应,提出了一种新的完全混沌同步的控制方案.在该方案中,每一个线性偏振模的完全混沌同步质量随外加电场成准周期性变化.其变化规律:完全混沌同步?剧烈振荡;固定一定的外加电场,电光调制促使其完全混沌同步质量对偏置电流和反馈强度的稳健性获得极大的加强.在较大的偏置电流和反馈强度范围内,每一个线性偏振模能够实现完全混沌同步,并且调制到每一个线性偏振模的加密信号基本上能够恢复.     

Time-delayed phase-control for suppression of the flow-induced noise from an open cavity

A flow over an open cavity causes a cavity resonance, which is a feedback mechanism between the acoustic waves and the pressure fluctuation of the cavity flow. Previous research on the reduction of the cavity resonance has focused on suppressing the flow disturbance. This paper presents a time-delayed phase-control method to reduce the global noise of the cavity. Acoustic feedback of the cavity noise, which amplifies the flow disturbance, can be generally reduced by this control method, regardless of the flow physics. The positions of the sensors and the actuator are determined to increase the control efficiency. Experiments show that this control method reduces the peak of the flow oscillation by suppressing the acoustic feedback.     

Using coherent feedback loop for high quantum state transfer in optomechanics

We investigate the coherent feedback loop scheme to improve the quantum correlations transfer from optical to mechanical degrees of freedom in a double cavity optomechanical system. We use the Duan criterion to determine the separability of the two-mode mechanical states. The logarithmic negativity is employed to quantify the amount of the entanglement between mechanical modes in steady and dynamical regimes. We show that the entanglement can be significantly enhanced by a coherent feedback using a suitable tuning of the reflectivity parameter of the beam splitter located in each cavity. We also show that this enhancement is influenced by the temperature, the light squeezing parameter and the gain of the parameter amplifier. The entanglement dynamics in presence of the coherent feedback loop is also analyzed.     

Frequency stabilised grating feedback laser diode for atom cooling applications

The free running linewidth of an external cavity grating feedback diode laser is on the order of a few megahertz and is limited by the mechanical and acoustic vibrations of the external cavity. Such frequency fluctuations can be removed by electronic feedback. We present a hybrid stabilisation technique that uses both a Fabry–Perot confocal cavity and an atomic resonance to achieve excellent short and long term frequency stability. The system has been shown to reduce the laser linewidth of an external cavity diode laser by an order of magnitude to 140 kHz, while limiting frequency excursions to 60 kHz relative to an absolute reference over periods of several hours. The scheme also presents a simple way to frequency offset two lasers many gigahertz apart which should find a use in atom cooling experiments, where hyperfine ground-state frequency separations are often required.     

Frequency doubled diode laser in alternative extended cavity

We report on an alternative extended cavity scheme used with a 180 mW quantum well GaAlAs diode laser, operating near 850 nm. A dispersing prism and a thin glass plate are employed to enforce stable single-mode operation in the strong feedback regime, with no need for laser AR coatings. Compared to other configurations where a grating is used, the lower loss in the extended cavity allows higher fundamental power available for second harmonic generation. For example, by frequency doubling using potassium niobate in a power enhancement cavity, enough power can be generated from a single diode laser to decelerate and trap calcium atoms with radiation at 423 nm.     

Controlling Spatiotemporal Chaos with a Generalized Feedback Method

The usual linear variable feedback control method is extended to a generalized function feedback scheme. The scheme is applied to high-dimensional spatiotemporal systems. By a combination of local generlized feedback control and the spatial coupling effect among elements, turbulent motion can be successfully eliminated.     

具有自动跟踪功能的新型光纤光栅振动传感器

振动传感技术是分析机械动力学的重要手段之一,结合光纤Bragg光栅（FBG-Fiber Bragg Grating）传感系统的特点,提出了一种新型的光纤Bragg光栅振动传感器的设计,该传感器的新颖之处在于提出了一种用反馈电路实现的对光纤Bragg光栅反射波长移位进行自动践踏的探测方法。该反馈电路用以驱动法布里-珀罗腔（F-P-Fabri-Perot）。利用当法布里-珀罗腔满足特定条件时其透射光可达到最强或最弱原理,建立了F-P腔解调系统模型。     

A mirror based on stimulated brillouin scattering with a double feedback loop in a laser cavity

The possibility of switching an external mirror based on stimulated Brillouin scattering (SBS) with a double feedback loop to a laser cavity is shown experimentally. A significant increase in the output laser power is obtained with conservation of the diffraction quality of the initial radiation. Experimental comparison with an ordinary SBS mirror, excited in a single pass through a nonlinear medium, is performed and the advantage in the energy parameters of the scheme with a double feedback loop is shown. The threshold characteristics of the SBS mirror under study are calculated and good agreement with the experimental data is obtained.     

Generation of Entangled Multi-atom Dicke States in Cavity Quantum Electrodynamics

We propose a scheme to generate two-atom maximally entangled state in cavity quantum electrodynamies （QED）. The scheme can 5e extended to generation of entangled multi-atom Dicke states if we control the interaction time of atoms with cavity modes. We use adiabatically state evolution under large atom-cavity detuning, so the scheme is insensitive to atomic spontaneous decay. The influence of cavity decay on fidelity and success probability is discussed.     

Quantum feedback cooling of two trapped ions

We present a sub-Doppler cooling scheme of a two-trapped-ion crystal by quantum feedback control method. In the scheme, we obtain the motional information by continuously measuring the spontaneous emission photons from one single ion of the crystal, and then apply a feedback force to cool the whole chain down.We derive the cooling dynamics of the cooling scheme using quantum feedback theory and quantum regression theorem. The result shows that with experimentally achievable parameters, our scheme can achieve lower temperature and faster cooling rate than Doppler cooling.     

双反馈半导体激光器的混沌特性研究

实验利用双反馈半导体激光器获得了关联维数为3.8的高维混沌光.同时对比分析了双反馈与单反馈两种不同模式产生混沌的区别.结果表明：在反馈强度均为-26 dB时,双反馈产生混沌的关联维数高于单反馈产生混沌的关联维数2.6;双反馈可获得带宽为11 GHz的混沌光,为单反馈产生混沌带宽5.5 GHz的两倍.当双反馈的两个外腔长度不相等时,混沌的自相关曲线能很好的隐藏外腔长度信息,可提高混沌通信的保密性. 关键词： 双反馈 半导体激光器 混沌 带宽     

One-step generation of qutrit entanglement via adiabatic passage in cavity quantum electrodynamics

A scheme is proposed for generating a three-dimensional entangled state for two atoms trapped in a cavity by one step via adiabatic passage. In the scheme, the two atoms are always in ground states and the field mode of the cavity excited is negligible under a certain condition. Therefore, the scheme is very robust against decoherence. Furthermore, it needs neither the exact control of all parameters nor the accurate control of the interaction time. It is shown that qutrit entanglement can be generated with a high fidelity.     

利用纠缠交换制备原子-原子最大纠缠态

基于腔量子电动力学(QED)提出一种利用两对纠缠的级联型三能级原子与单模腔场系统制备原子-原子最大纠缠态的简单方案,最初两原子之间、两腔场之间互不纠缠,使其中一个原子与一个腔场发生作用,即纠缠交换,该过程仅需对单个腔场态测量就可实现从未有直接作用的两个原子之间的纠缠,精确控制原子与腔场的相互作用时间可获得具有最大保真度的纠缠态.该方案可以延长腔的有效泄漏时间,从而能有效克服光腔的消相干的影响,这样大大降低了系统对腔的品质的要求.