Carrier shock and frequency conversion of a few—cycle pulse laser propagating in a non—resonant two—level atom medium

We have studied the spectral behaviour of few-cycle soliton pulses in a non-resonant two-level atom medium by solving the full Maxwell－Bloch equations. It is demonstrated further that the carrier effects play an important role in the propagation of the few-cycle pulse laser. When the frequency detuning is not very large, both the population distribution and the refractive index of the medium follow the oscillatory carrier field instantaneously; in this case, carrier-wave compression or carrier shock occurs, and a supercontinuum broader than that in the resonant medium may be generated. When the frequency detuning is large, the carrier shock is weak and the spectrum is not continuous, only showing an odd harmonic radiation.     

Time-modulation of entangled two-body weak decays with massive neutrinos

In recent experiments at the GSI, Darmstadt, time-modulated orbital Electron Capture (EC) decays of H-like ¹⁴⁰Pr⁵⁸⁺, ¹⁴²Pm⁶⁰⁺, and ¹²²I⁵²⁺ ions with one electron in the K-shell, coasting in the ESR storage ring with velocity β=0.71, were observed. The EC-branches show exponential decay curves time-modulated with period T=7.06(8) s and amplitude a=0.18(3) for ¹⁴⁰Pr decays, T=7.10(22) s and a=0.23(4) for ¹⁴²Pm decays, and T=6.04(6) s and a=0.19(3) for ¹²²I (preliminary) decays in the laboratory frame. The simultaneously measured β⁺ branch of ¹⁴²Pm shows no modulation with a<0.03. We discuss here as origin of the modulation quantum beats produced by the superposition of massive neutrino, mass eigenstates emitted in the entangled two body weak decay. From the modulation frequency a value for the difference of the quadratic mass values is deduced, which is 2.9 times larger than the value derived by the KamLAND antineutrino oscillation experiment. The origin of the small modulation amplitudes is discussed as the result of a partial restoration of the interference terms which are expected to cancel for the usual assumed unitarity of the neutrino flavor mixing matrix.     

水下对转桨无空化噪声调制理论分析与试验研究

水下对转桨无空化噪声由流场-桨叶相互作用引起,调制特性是水下对转桨无空化噪声的重要特征,本文研究了水下对转桨无空化辐射噪声调制机理。首先利用广义声类比方法得到了无空化条件下水下对转桨的远场声压谱,建立了水下对转桨无空化噪声的调制模型,然后数值仿真了模型对转桨无空化噪声的功率谱和调制谱,最后在空泡水筒中进行了模型对转桨的无空化噪声测量试验,数值仿真结果和试验验证了调制模型的准确性。该模型对于水下对转桨无空化噪声调制特性预报及目标识别具有重要价值.     

Modulation of homogeneous shear turbulence laden with finite-size particles

The dynamics of homogeneous shear turbulence laden with spherical finite-size particles is investigated using fully resolved numerical simulations to understand how the presence of particles modulates turbulent shear flows. We focus on a dilute flow laden with non-sedimenting particles whose diameter is slightly smaller than or comparable with those of vortex cores in turbulence. An immersed boundary method is adopted to represent a spherical finite-size particle. Numerical results show that the presence of particles augments the viscous dissipation of turbulence kinetic energy, which leads to a slower increase in the turbulence energy. Although the augmentation of energy dissipation occurs predominantly inside viscous layers surrounding particles in an initial period, the contribution from their outside becomes more significant due to the modification of turbulence structures as turbulence develops. It is found that the particles exhibit weak tendency to accumulate in vortex layers. The particles approaching and colliding with vortex layers induce large velocity fluctuations, which leads to the generation and shedding of thin vortex tubes. Newly generated vortex tubes interact with developed vortex tubes and layers, and modify the entire structure of the vorticity field.     

激光羽烟透过率测试技术研究

为了准确评估固体推进剂的性能,研制了红外激光羽烟透过率测量系统。系统采用透射的方法:在发射端对激光信号采用内调制,由光学系统送入接收端,并将接收到的信号进行光电转换、信号处理和A/D转换后送入计算进行数据的处理和显示。现场测试结果表明该系统的精度、响应速度和稳定度达到设计要求。     

Terahertz laser velocity modulation spectroscopy of ions

Velocity modulation spectroscopy has been investigated in the terahertz region, employing pure rotational transitions of ArH⁺ and rotation-tunneling transitions of H₃O⁺ to study the competition between pressure broadening and Doppler broadening on the lineshapes, neutral suppression, and modulation efficiency. Velocity modulation is demonstrated to be effective to frequencies as low as 60 cm⁻¹, yielding S/N values for ArH⁺ (770/1) that surpass published terahertz FM results by an order of magnitude.     

Generation of few-cycle laser pulses:Comparison between atomic and molecular gases in a hollow-core fiber

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation(SPM), we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation(MPM) effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.     

Eye Opening Degradation due to Sideband Suppression in Direct Detection Systems for Different Signaling Formats

Abstract

The intensity distortion resulting from sideband suppression in direct detection systems is investigated analytically and through numerical simulation for unipolar binary and alternate mark inversion formats. A method to calculate the sequence leading to the highest eye opening penalty is proposed, based on the characterization of the distortion originated by the Hilbert transform of an isolated pulse and constructive/destructive addition of the distortions of pulses in different time slots. The application of the proposed method to a practical transmitter is assessed, and accurate results are shown for signals without complete sideband suppression.     

有源耦合腔激光器锁模特性研究

本文在理论上首次发现有源耦合腔激光器的自启动锁模和时域调制特性,并对其进行了全面的分析和计算.对有源耦合腔激光器的自启动锁模机制和时域调制特性给予详尽的数值计算结果和清晰的物理图象解释,提出在有源耦合腔锁模激光器中可用连续泵浦源代替锁模泵浦源,而且有产生更短的光脉冲可能性.     

Electromagnetically induced grating in a four-level tripod-type atomic system

An electromagnetically induced grating in a four-level tripod-type atomic system is studied theoretically. By virtue of a weak standing-wave signal field, the phase modulation effectively diffracts a weak probe field into the first-order direction. By changing the weak signal field, the diffraction of the weak probe field can be modulated in real time, and a first-order diffraction efficiency of more than 32% can be obtained with proper parameters. Such a system has a potential application in an all-optical switch controlled by a weak optical signal.