A low-noise,high-SNR balanced homodyne detector for the bright squeezed state measurement in 1–100 kHz range

We report a low-noise, high-signal-to-noise-ratio(SNR) balanced homodyne detector based on the standard transimpedance amplifier circuit and the inductance and capacitance combination for the measurement of the bright squeezed state in the range from 1 kHz to 100 kHz. A capacitance is mounted at the input end of the AC branch to prevent the DC photocurrent from entering the AC branch and avoid AC branch saturation. By adding a switch at the DC branch,the DC branch can be flexibly turned on and off on different occasions. When the switch is on, the DC output provides a monitor signal for laser beam alignment. When the switch is off, the electronic noise of the AC branch is greatly reduced at audio-frequency band due to immunity to the impedance of the DC branch, hence the SNR of the AC branch is significantly improved. As a result, the electronic noise of the AC branch is close to -125 dBm, and the maximum SNR of the AC branch is 48 dB with the incident power of 8 mW in the range from 1 kHz to 100 kHz. The developed photodetector paves a path for measuring the bright squeezed state at audio-frequency band.     

相位不连续点数目随湍流强度的变化

激光在湍流大气中传输时,若光束波前中出现相位不连续点,自适应光学的校正能力将明显降低,利用4维程序计算了激光在大气中传输时产生的相位不连续点的数目。通过选取不同的相位屏数、计算网格和网格间距,得到了畸变光场中产生的相位不连续点的数目随湍流强度变化的过程,即:相位不连续点的数目随湍流的增强有一个从无到有,从缓慢增加到近似线性的急剧增加,而后又缓慢增加,最后趋于饱和的变化过程。另外,在不同的传输条件下,相位不连续点的数目及其变化存在很大差异。     

Study on sound propagation in long enclosures with a vertical or inclined branch of different dimensions

As there may be some branches in long enclosures, such as high-speed railway and freeway tunnels, it will make a difference in the sound propagation in long enclosures if we give more consideration to different designs of the branches at the onset of their construction. However, most researches on the sound propagation in long enclosures available are concerned with straight long enclosures. In this paper, the sound pressure level (SPL) attenuation, early decay time (EDT), and reverberation time (RT30) of long enclosures with a vertical or inclined branch of different dimensions have been studied by comparing experimental results from physical scale models of such enclosures with those of the straight long enclosure. This experimental investigation gives interesting results on the behavior of sound propagation in long enclosures with a vertical or inclined branch of different dimensions. When conducting engineering design for long tunnels of the high-speed railway or freeway, it would be appropriate to consider designing the branch of the tunnels into a widened left inclined or right inclined one to provide a better relative SPL than that of other branches with different dimensions. This study further reveals that with an inclined branch of different dimensions, the EDT of the long enclosure will be different. At both 500 and 1000 Hz 1/3 octave bands, the EDT is the shortest when the long enclosure has a left inclined branch and an extended left inclined branch, while the EDT is the longest when the long enclosure has a widened vertical branch. Generally speaking, in the near field (i.e. the source-receiver distance is shorter than the threshold distance), the EDT measurements are similar, while in the far field (i.e. the source-receiver distance exceeds the threshold distance), the EDT is the longest for the straight long enclosure, second longest for the long enclosure with a widened inclined branch, third longest for the long enclosure with a vertical branch, and the shortest for the long enclosure with an extended inclined branch, which is similar to the trend with RT30.     

非共振激发条件下微腔中激子自发辐射的时间分辨光谱

低温下,测量了InGaAs量子阱平面微腔的时间分辨光谱。在非共振激发条件下,观察到上下两支腔极化激元光荧光的衰退时间与失谐无关;下支腔极化激元光荧光的上升时间也与失谐无关;而上支腔极化激元光荧光的上升时间却与失谐有强烈的依赖关系,随着从负失谐到正失谐的增加,上升时间逐渐减小。对实验结果的物理根源进行了讨论。     

带有矩形过渡区的非对称Y分支光波导

提出了带有矩形过渡区的非对称Y分支光波导的两种设计方案。分别通过设置两分支波导不同的宽度和改变分支波导与矩形中心的轴偏移量,去获得非对称分支比。采用有限差分光束传播法（FD-BPM）对两种结构进行模拟,得出它们的分支比和损耗。分支比分别最高可达809／6和72．9％,其相对应的损耗分别低于0．12dB和0．23dB,充分证明了方案的可行性,为以后非对称Y分支光波导的制作提供了实验基础。     

Asymmetric moving localized structures in a wide-aperture nonlinear interferometer

The existence of a branch of asymmetric solitary localized radiation structures moving in the transverse direction with a constant velocity is demonstrated for the slit (transversely one-dimensional) scheme of a wide-aperture interferometer with a threshold nonlinearity which is excited by external coherent radiation. This branch is shown to split off the branch of symmetric motionless structures at a certain bifurcation value of the external-radiation intensity. The generalization to other schemes, for which the existence of dissipative optical solitons is known, is discussed.     

Whistling of a pipe system with multiple side branches: Comparison with corrugated pipes

Corrugated pipes are widely used because they combine local rigidity with global flexibility. Whistling induced by flow through such pipes can lead to serious environmental and structural problems. The whistling of a multiple side branch system is compared to the whistling behavior of corrugated pipes. The study has been restricted to cavities with sharp edges which are convenient for theoretical modeling. The side branch depth is chosen to be equal to the side branch diameter, which corresponds to cavity geometries in typical corrugated pipes. The low frequency resonance modes of the multiple side branch system have been predicted by means of acoustic models, of which the validity has been tested experimentally. Several experiments have been carried out for characterizing the whistling behavior of the system. While the behavior of a multiple side branch system is interesting on its own it can be compared to that of corrugated pipes. These experiments show that the multiple side branch system is in many aspects a reasonable model for corrugated pipes. Advantage of the multiple side branch system is that it is an experimental setup allowing easy modification of cavity depth. We used this feature to identify the pressure nodes of the acoustic standing wave along the main pipe as the regions where sound is produced. This contradicts recent publications on corrugated pipes. Another interesting aspects is that the system appears to whistle at the second hydrodynamic mode of the cavities rather than at the first hydrodynamic mode. A prediction model for the whistling behavior is proposed, consisting of an energy balance, based on the vortex sound theory. The model predicts the observed Strouhal number but overestimates the acoustic fluctuation amplitude by a factor four.     

Studies on self-sustained reaction-rate oscillations: I. Real-time surface infrared measurements during oscillatory oxidation of carbon monoxide on platinum

Time-resolved infrared absorption features in the 1800–2400 cm^?1 region during a typical cycle in the oscillatory oxidation of CO over a platinum foil were obtained by Fourier transform infrared reflection absorption spectroscopy. Pretreatment of the foil in an oxidizing environment at high temperatures was found to be necessary to induce large-amplitude, stable oscillations. The oscillations are approximately square-wave in shape, with a high and a low reaction-rate branch. The level of chemisorbed CO in the high reaction-rate branch is typically below the noise level, while in the low reaction-rate branch substantial substantial surface coverages of CO can be observed. No evidence for CO bridge-bonded to the platinum substrate or chemisorbed in the presence of a subsurface Pt oxide could be found at any time during the oscillation cycle. Evidence is presented for the existence of CO islands in the low reaction-rate branch. It is also shown that the low reaction rate realized in this branch is not due to blocking of the surface by chemisorbed CO.     

Current-Driven Kink-Like Instability in Collisional Plasmas

The stability of left-hand circularly polarized waves propagating along an external magnetic field with wavelengths much larger than the ion Larmor radius is studied for fully-ionized collisional plasmas carrying a field-aligned current. It is found that, in the presence of electron-ion collisions, this "kink-like" instability has two branches of unstable wavenumbers: a main branch and a resistive branch. The resistive branch owes its existence to electron-ion collisions, but its growth rate is much smaller than that of the main branch, which is typically some fraction of the ion cyclotron frequency. The effect of collisions on the main branch is to reduce its maximum growth rate while extending the range of unstable wavenumbers to larger values. However, these changes are significant only when the electron-ion collision frequency is comparable to the electron cyclotron frequency. The dispersion relation is solved numerically for plasma and magnetic field parameters appropriate to the UCLA arcjet plasma. The results show that, within the framework of an infinite and homogeneous theory, the kink-like instability should occur in this plasma device.     

Lower branch coherent states in shear flows: transition and control

Lower branch coherent states in plane Couette flow have an asymptotic structure that consists of O(1) streaks, O(R(-1)) streamwise rolls and a weak sinusoidal wave that develops a critical layer, for large Reynolds number R. Higher harmonics become negligible. These unstable lower branch states appear to have a single unstable eigenvalue at all Reynolds numbers. These results suggest that lower branch coherent states control transition to turbulence and that they may be promising targets for new turbulence prevention strategies.     

A novel algorithm for branch cut phase unwrapping

Branch cut method is a powerful noise-immune algorithm for correct phase unwrapping of noisy phase maps. The shortest branch cut length promises the optimal unwrapping result of the wrapped phase maps. A new algorithm is proposed to search for the shortest branch cut length by simple exchange operation. Although the algorithm is on the basis of stochastic search techniques, it has a high probability of finding the shortest branch cut length or an approximation of it. Compared with the traditional algorithms, the algorithm is fast and competitive.     

Model experiment and analysis of pressure waves emitted from portals of a tunnel with a branch

A model experiment was performed to investigate pressure waves generated by a train passing by a branch and pulse waves radiated from portals of a main tunnel and the branch. For the experiment, the train speed was set as 400–500 km/h. The cross-sectional area ratio of the branch to the main tunnel was 0–0.5. The cross-sectional area ratio of the branch to the main tunnel was identified as a dominant factor in determining the magnitude of the pressure waves in the tunnel and the pulse waves radiated from the portals. Closed form expressions for the magnitude of the pressure changes generated by a train passing by a branch were derived using low Mach number approximation. Correlation between the pressure waves in the tunnel and the pulse waves radiated from the portals was clarified using simple acoustic theory. The overall tendency of the experimental results is explainable based on analytical results.     

一种可稳定计算Pekeris波导中声场的波数积分方法

提出一种可稳定计算Pekeris波导中声场的波数积分方法,并在此基础上开发出一个数值模型,可用于提供Pekeris波导中声场的精确、稳定的数值解。在这个方法中,由于与深度有关的波动方程齐次解中所有的上行波与下行波均采用了合理的归一化表示,从而得到的系统方程是无条件稳定的。在简正波方法中,割线积分一般只对近场有显著影响。因此,传统的简正波模型一般都忽略割线积分对声场的贡献。但是,如果某号简正波离割线非常近,则割线积分对非常远距离的声场仍可能有显著影响。在这种情况下,传统的简正波模型由于忽略割线积分的贡献,从而得到的声场结果是不准确的。本文通过数值算例比较本文提出的波数积分模型与传统的简正波模型。数值结果表明,本文提出的模型可以提供精确、稳定的Pekeris波导中声场的数值解,而在某些情况下传统的简正波模型得到的声场结果是不准确的。因此,本文提出的模型可以作为Pekeris波导中声传播问题的标准模型使用。      

湍流大气中的准直激光束III.分形结构与相位不连续点饶瑞中

 利用数值方法研究了不同起伏条件下准直激光束在湍流大气中的传播，分析了光斑的分形维数以及相位不连续点数目的统计特征。结果表明：随着起伏条件的增大，光斑的分形维数以及相位不连续点数目增大；光斑的分形维数与锐度（描述光斑质量的参量）有一定的关系，但相位不连续点数目与光斑质量不存在确定的关系。在本文的计算条件下，分形维数一直随起伏条件的增大而增大，没有出现类似于闪烁饱和的现象。在一定的起伏条件下，相位不连续点数目具有一定的统计分布，而不是一个确定的值，并且具有相当的发散性。     

Picosecond x-ray studies of coherent folded acoustic phonons in a multiple quantum well

Coherent folded acoustic phonons in a multilayered GaSb/InAs epitaxial heterostructure were generated by femtosecond laser pulses and studied by means of ultrafast x-ray diffraction. Coherent phonons excited simultaneously in the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the refractive index due to photogenerated free carriers.     

一种解电路網络的方法

本文提出一种解电网的方法,用这种方法来解电网,可以避免解联立方程之麻烦,以节省劳动力。本文分下列两部分:1.一般电网解法——凡是一切直线律电网,不论其中含有若干电动势,都可应用这解法。这解法是首先将电网中的电动势代换以等值的电源电流,其次将电网中的结点逐点化去,使电网化简为一等值支路,以求该支路两端点的电势差,然后再返原为原来的电网,以求其中各结点的电势,如此,最后就很易计算出各支路的电流。2.只含一个电动势电网的特殊解法——凡是一切直线律电网,若其中只含有一个电动势,则用这解法更为简便。这解法与一般电网解法所不同的地方就是可将电网返原的这一步骤省去。为了要省去这一步骤,只须在电网化简前,电网各结点上均虚设一定量的电源电流,如此,当电网化简为一等值支路时,就能直接求得原来电网中各结点的电势。再者,本文所提出的解电网方法对于交流电网及直流电网均能适用,而本文是按解交流电网的方式叙述。     

吸收型双光子光学双稳态的时间行为

利用多重时间尺度微扰分析方法,研究了一个半经典吸收型双光子光学双稳系统的时间演化行为。一级近似结果表明,在高透射分支附近,系统的长时间行为由输出场振幅的方程所支配,其它动力学量除绝热地跟随场振幅变化而外,还有快速振荡。而在低透射分支附近,所有动力学变量都通过长时间尺度变化,没有与短时间尺度相联系的振荡行为。稳定态在高透射分支与低透射分支之间的跃变可以由简单的方程描述。 关键词：     

An efficient method for switching branches of period-doubling bifurcations of strongly non-linear autonomous oscillators with many degrees of freedom

Popular algorithms for switching branches at a bifurcation point of strongly non-linear oscillators are generally quite involved as they require the computation of the tangent of a new branch and second derivatives. In this paper, a simple but efficient algorithm is presented by using a perturbation-incremental method for switching branches at a period-doubling bifurcation of strongly non-linear autonomous oscillators with many degrees of freedom. To switch to a new branch at a bifurcation point, a parameter is simply turned on from zero to a small positive value so as to obtain an initial solution on the emanating branch for subsequent continuation. The parametric value at a period-doubling bifurcation can also be determined accurately. Furthermore, limit cycles of period 2k(k?1) can be calculated to any desired degree of accuracy.     

Exciton-like excitations in liquid helium

The energy spectrum of elementary excitations in superfluid helium is derived on the basis of arguments based on a model for the liquid structure. The spectrum consists of two branches: the Landau-Feynman branch and a new branch, above the Landau-Feynman branch, whose central part has been observed experimentally by Iwamoto.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 64–67, June, 1974.     

Self-sustained oscillations in pipe systems with multiple deep side branches: Prediction and reduction by detuning

Flow-induced pulsations are frequently observed in pipe networks. In the present work we focus on the case of flow-induced pulsations in a pipe system composed of six equally spaced deep closed side branches. These pulsations are self-sustained aeroacoustic oscillations driven by the instability of the flow along the closed branches. The prediction of pulsations in such complex systems has not yet been proved to be possible, indeed the methods proposed in the literature have only been applied to relatively simple geometries, mainly single or double side branch systems. We propose a prediction model of the self-sustained oscillations in multiple deep side branch systems. This has been established by means of an analytical model for the acoustic wave propagation in which a semi-analytical source model is included. Detuning of the acoustic resonator is often considered as a possible remedial measure to suppress pulsations. Although this countermeasure appears to be very effective for double side branch systems in cross configuration, its effectiveness has never been assessed for different geometries. The effectiveness of the length-detuning on the six side branch system appear to be limited and depends on the upstream and downstream acoustic boundary conditions of the main pipe.