A Movable-Cavity Cold Atom Space Clock

We present an experimental scheme of a cold atom space clock with a movable cavity. By using a single microwave cavity, we find that the clock has a significant advantage, i.e. the longitudinal cavity phase shift is eliminated. A theoretical analysis has been carried out in terms of the relation between the atomic transition probability and the velocity of the moving cavity by taking into account the velocity distribution of cold atoms. The requirements for the microwave power and its stability for atomic π/2 excitation at different moving velocities of the cavity lead to the determination of the proper working parameters of the rubidium clock in frequency accuracy 10^-17. Finally, the mechanical stability for the scheme is analysed and the ways of solving the possible mechanical instability of the device are proposed.     

Ionization in an intense field considering Coulomb correction

We derive a simple ionization rate formula for the ground state of a hydrogen atom in the velocity gauge under the conditions:ω1 a.u.(a.u.is short for atomic unit) and γ1(ω is the laser frequency and y is the Keldysh parameter).Comparisons are made among the different versions of the Keldysh-Faisal-Reiss(KFR) theory.The numerical study shows that with considering the quasi-classical(WKB) Coulomb correction in the final state of the ionized electron,the photoionization rate is enhanced compared with without considering the Coulomb correction,and the Reiss theory with the WKB Coulomb correction gives the correct result in the tunneling regime.Our concise formula of the ionization rate may provide an insight into the ionization mechanism for the ground state of a hydrogen atom.     

Uniting Precision Measurement and Quantum Control

Phase control of a single-frequency continuous-wave laser and the electric field of a mode-locked femtosecond laser has now reached the same level of precision, resulting in sub-optical-cycle phase coherence being preserved over macroscopic observation times exceeding seconds. The subsequent merge of CW laser-based precision optical- frequency metrology and ultra-wide-bandwidth optical frequency combs has produced remarkable and unexpected progress in precision measurement and ultrafast science. A phase-stabilized optical frequency comb spanning an entire optical octave （ 〉 300 THz） establishes millions of marks on an optical frequency ＂ruler＂ that are stable and accurate at the Hz level. Accurate phase connections among different parts of electromagnetic spectrum, including optical to radio frequency, are implemented. These capabilities have profoundly changed＇ the optical frequency metrology, resulting in recent demonstrations of absolute optical frequency measurement, optical atomic clocks, and optical frequency synthesis. Combined with the use of ultracold atoms, optical spectroscopy and frequency metrology at the highest level of precision and resolution are being accomplished at this time. The parallel developments in the time domain applications have been equally revolutionary, with precise control of the pulse repetition rate and the carrier-envelope phase offset both reaching the sub-femtosecond regime. These developments have led to recent demonstrations of coherent synthesis of optical pulses from independent lasers, coherent control in nonlinear spectroscopy, coherent pulse addition without any optical gain, and coherent generation of frequency combs in the VUV and XUV spectral regions. Indeed, we now have the ability to perform completely arbitrary, optical, waveform synthesis, complement and rival the similar technologies developed in the radio frequency domain. With this unified approach on time and frequency domain controls,     

The spectral changes of partially coherent light focused by an apertured lens with chromatic aberration

The spectral changes of a partially coherent polychromatic light focused by an apertured lens with chromatic aberration are investigated. It is demonstrated that the spectrum in the focused field is different from that in the aperture. Comparing with the spectrum in the aperture, the spectrum in the focused field shifts to lower or higher frequency, which is defined as a spectral shift. The influence of chromatic aberration of the lens, the coherence of the partially coherent light in the aperture, the radius of the aperture, and the spectral width of the spectrum of the aperture on the spectral shift are investigated in detail. The numerical results show that these parameters affect the spectral shift noticeably.     

Double coherence resonance of the FitzHugh Nagumo neuron driven by harmonic velocity noise

The effect of noise frequency on the FitzHugh–Nagumo neuron is investigated by the use of the harmonic velocity noise, which has a direct frequency parameter and no zero frequency part of the power spectrum. It is shown that the neuron has the resonance characteristic strongly responding to the noise with a certain frequency at fixed power, and there is double coherence resonance related to the frequency and the intensity. If the harmonic velocity noise lacks low frequency ingredients, there is no synchronization between the frequency of the neuron and that of the noise. Thus the low frequency part of the noise plays an important role in creating the synchronization.     

Development of an all-fiber heterodyne lidar for range and velocity measurements

A 1550-nm all-fiber monostatic lidar system based on linear chirp amplitude modulation and heterodyne detection for the measurements of range and velocity is presented. The signal processing method is given, after which the relationship between the peak frequency values in the final signal spectrum, the target＇s range, and the line-of-sight velocity is obtained in the presence of the fiber end-face-reflected signal plaguing many monostatic lidar systems. The range of an electric fan as well as the line-of-sight fan speed of different levels is tested. This proposed system has a potential application for the space-borne landing system.     

Fourier analysis of elastic light scattering spectrum of epithelial cell nuclei

<正>A new method for simultaneously determining the size and refractive index of epithelial cell nuclei is presented.The function of the modified elastic light scattering spectrum is regarded as a function of wave number factor,Q=2λ~(-1)sin(θ/2).The modified spectrum has a constant oscillation period with its frequency proportional to the average diameter of cell nuclei.To the same average diameter,the different relative refractive indexes of epithelial cell nuclei only induce the horizontal shift of the spectra.Both the oscillation frequency and the horizontal shift are quantified by the fast Fourier transform on the modified spectra.The average diameter can be figured out through the peak frequency divided by the value of the refractive index of the surrounding medium.The phase angle of the peak frequency has an approximate linear relationship with the relative refractive index of epithelial cell nuclei.     

Beat note analysis and spectral modulation of terahertz quantum cascade lasers with radio frequency injection

We demonstrate the electrical beat note analysis and radio frequency(RF) injection locking of a continuous wave(cw) terahertz quantum cascade laser(QCL) emitting around 3 THz(~100 μm). In free running the beat note frequency of the QCL shows a shift of ~180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II,and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more efficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.     

An Accurate Frequency Control Method and Atomic Clock Based on Coherent Population Beating Phenomenon

An accurate frequency control method and atomic clock based on the coherent population beating(CPB) phenomenon is implemented.In this scheme,the frequency difference of an rf and an atomic transition frequency can be digitally obtained by measuring the CPB oscillation frequency.The frequency measurement resolution of several milli-hertz can be achieved by using a 10 MHz oven controlled crystal oscillator as the reference.The expression of the Allan deviation of the CPB clock is theoretically deduced and it is revealed that the Allan deviation is inversely proportional to the signal-to-noise ratio and proportional to the line-width of coherent population trapping spectrum.We also approve that the CPB atomic clock has a large toleration of the drift of the local oscillator.In our CPB experimental system,a frequency instability of 3.0×10~(-12) at 1000 s is observed.The important feature of high frequency measurement resolution of the CPB method may also be used in magnetometers,atomic spectroscopy,and other related research.     

Investigation of the spatial correlations of flow noise in vector hydrophone towed linear array

Following the wall pressure spectrum of the turbulent boundary layer developed by Corcos, a method in the frequency-wavenumber space was presented to analyze the flow noise in the vector hydrophone towed linear array. The general forms of the acoustic pressure and particle velocity in the flow noise field were obtained, and the spatial correlations of the flow noise were calculated. The numerical analysis results based on wavenumber integration show that： （1） The spatial correlations of flow noise drops rapidly with increasing axial separation between the elements, so the flow noise received by different vector hydrophones usually sampled in a half-wavelength rate can be considered as independent; （2） The flow noise is highly correlated in the radial direction at low frequency, and only those of high frequency componet can be neglected.     

SPECTRUM OF THE MICROMASER INJECTED WITH Λ-TYPE THREE-LEVEL ATOMS

The spectrum of the micromaser injected with A-type three-level atoms is studied. The spectrum has a Lorentzian lineshape and its central frequency has a shift. The effects of various parameters (for example, the atomic center-of-maas velocity, the cavity-length, the one-photon detuning, the ratio of the atomic injection rate to the cavity decay rate and the number of thermal photons) on the linewidth and the frequency shift are examined. The spectrum shows very different features in different velocity regimes, i.e., in the fast-atom regime, in the intermediate-velocity-atom regime, and in the slow-atom regime.     

Floquet study of ionization and high-order harmonic generation for a hydrogen atom in high-frequency laser fields

In this paper, we investigate the dynamics of a hydrogen atom in high-frequency (several atomic units) super strong (up to several tens of atomic units) laser fields within the high frequency Floquet theory framework. The ionization rate, ionization spectrum, angular distribution and high-order harmonic generation are all investigated. Our studies reveal the universal behavior of the total ionization rate, excess-photon ionization spectrum and angular distribution of the ionization rate in the stabilization regime, and achieve a deep insight into the dynamics of high-order harmonic generation in the stabilization regime. Received 6 June 2001 and Received in final form 31 August 2001     

Light-shift induced by two unbalanced spontaneous decay rates in EIT (CPT) spectroscopies under Ramsey pulse excitation

Light shift is important and inevitably affects the long-term stability of an atomic clock. In this work, considering two unbalanced branches of the spontaneous decay rate in a three-level system, we studied the frequency shifts of electromagnetically induced transparency (EIT) and coherent population trapping (CPT) clocks operating under the pulse sequence regime by numerically solving the Liouville density matrix equations. The results show that the frequency shifts are larger when the two branches of spontaneous emission rate are not equal compared to the equal case. In addition, in EIT-Ramsey, the effect of the unbalanced branches of the spontaneous decay rate and relaxations of low-energy states on the frequency shift is greater than that of Rabi frequency. In CPT-Ramsey, the relaxations of low-energy states play a dominant role in frequency shift.     

Relaxation shift of the laser action frequency

It is found that the change of the atomic spectrum by a laser field leads to a modification of atomic relaxation. The modification of the dissipative part of the atomic kinetic equations results in a frequency shift of the laser action.     

Generation of VLF emissions with the increasing and decreasing frequency in the magnetosperic cyclotron maser in the backward wave oscillator regime

We study the mechanisms of the formation of falling tones in the dynamic spectrum of whistler-mode waves generated by energetic electrons in the Earth’s magnetosphere when the backward-wave oscillator (BWO) regime is realized in the magnetospheric cyclotron maser. As was shown earlier, this regime allows one to explain many features of ELF/VLF chorus emissions in the magnetosphere, in particular, the generation of elements with discrete frequency spectrum, characterized by a large growth rate and a fast frequency drift. On the basis of numerical simulations of a simplified system of nonlinear equations describing the magnetospheric BWO dynamics under the assumption of small efficiency of wave-particle interactions we show that the falling tones are generated in the case where the generation region is shifted from the equatorial plane (geomagnetic-field minimum) upstream with respect to the motion of energetic electrons. In this case, the resonant electrons move towards the decreasing magnetic field in the process of generation; hence, their longitudinal velocity increases, which corresponds to a decrease in the cyclotron-resonance frequency. Two mechanisms of the shift of the generation region from the equator are considered, i.e., (i) an increase in the linear instability growth rate (e. g., due to an increase in the energetic-electron density), and (ii) persistence of the phase bunching of the particles coming back to the generation region due to the bounce oscillations. We show that both of these mechanisms can result in the formation of falling tones, but the properties of the generated emissions such as the frequency drift rate and characteristic time interval between the elements are different. The conditions of preserving the phase bunching due to the bounce oscillations are discussed. Probably, this mechanism can operate in the case where the length of the generation region along the magnetic field is close to the characteristic bounce-oscillation length of energetic electrons which is realized for a sufficiently high cold-plasma density in the generation region.     

Velocity Distribution of Effective Atoms in a Small Optically Pumped Cesium Beam Frequency Standard

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Velocity Distribution of Effective Atoms in a Small Optically Pumped Cesium Beam Frequency Standard

In this paper, the velocity distribution of effective atoms in a small optically pumped cesium beam frequency standard has been achieved from the Fourier transforms of the experimentally recorded Ramsey patterns. The result fits well with the theoretical calculation. The second order Doppler shift correction of the small cesium atomic clock is obtained from the velocity distribution of effective atoms.     

保偏光纤中在不同频率区域拉曼效应和参量放大增益谱

根据考虑拉曼效应后的双折射光纤所满足的非线性相干耦合薛定谔方程, 推导出了当沿两个偏振轴入射两束不同波长的激光脉冲时所产生的增益表达式. 通过与入射相同频率的光脉冲所产生增益的对比, 在考虑拉曼效应的情况下, 讨论了入射不同频率光脉冲对增益谱的影响. 结果表明, 在正常色散区和反常色散区, 当输入两束激光脉冲频率不同时, 增益谱较输入相同频率激光脉冲时产生了明显的变化, 其外侧的斯托克斯部分和反斯托克斯部分增益峰, 随着群速度失配的增加强度明显加强、偏离中心频率, 可以用于提取太赫兹脉冲.当两偏振模处于不同色散区时, 增益谱与不考虑拉曼效应时也存在明显的不同, 增益谱的对称性遭到破坏, 斯托克斯部分的增益峰强度要明显高于反斯托克斯部分. 关键词： 不同频率区域 保偏光纤 拉曼效应 参量放大     

Forces and spatial ordering of driven atoms in a resonator in the regime of fluorescence suppression

An atom in a high-Q cavity, which is coherently driven at the frequency of a cavity mode, exhibits strong suppression of fluorescence when the atomic decay rate exceeds the cavity linewidth. This effect is due to destructive interference of cavity and pump field, such that at the atomic position the total field intensity has a local minimum. For atomic ensembles the magnitude of the interference effect grows with atom number and depends on the relative atomic positions. It is strongest for a wavelength spaced array of atoms placed at the antinodes of the cavity mode. This suppresses fluorescence and enhanced collective scattering into the cavity mode. We analyze the mechanical forces in the regime where the interference condition is fulfilled. We show that the atomic pattern is mechanically stable whenever the driving frequency is red detuned with respect to the cavity frequency, irrespective of the atomic transition frequency. Hence atomic selforganization, as predicted in [6] can also occur in the parameter regime where superradiant scattering is suppressed by collective interference. PACS 32.80.Pj; 42.50.Pq; 42.50.Fx     

基于原子吸收光谱的碱金属气室内多种混合气体压强测量方法

碱金属气室是基于原子无自旋交换碰撞弛豫的超高灵敏惯性和磁场测量装置的核心敏感器件。碱金属气室内气体的含量会对原子的弛豫以及系统其他参数的选取产生很大的影响,因此精密测量气室内混合气体各自的压强具有重要的意义。当气室内存在气体时谱线会出现压力展宽和频移,且压力展宽远大于自然展宽和多普勒展宽,因此仅考虑压力展宽。利用压力展宽、频移的大小与气体压强存在的函数关系,提出一种基于原子吸收光谱的碱金属气室内多种混合气体压强测量方法。通过扫描碱金属原子的吸收光谱,得到光学深度曲线,并用洛伦兹函数对其拟合,测得多种混合气体引起的单种碱金属原子的混叠压力展宽和频移,再根据已知的单种、单位压强气体引起的单种碱金属原子的压力展宽和频移,联立计算得到多种气体各自的压强。当存在n种碱金属时,最多可以测量4n种混合气体的压强。仿真结果表明,该方法适用于入射激光未被原子完全吸收的情况;激光功率和频率的波动在1%～10%的数量级时,测量精度影响低于0.4%的数量级,而温度波动在1%～10%的数量级时,测量精度影响高达30%的数量级。