混合模激光的模式拟合

通过研究混合模激光的模式分布,提出模阶数之和（２ｐ＋ｌ＋１）相同的模在混合模中占相同比例的假设,用计算机模拟的方法实现了混合模的模拟,理论模拟结果与实验测量的混合模分布很好地吻合,由此方法可模拟估算出混合模中各高阶模的强度比例。     

Frequency Characterization of a Terahertz Quantum-Cascade Laser

In this paper, the frequency characteristics of a terahertz quantum-cascade laser (QCL) operating around 2.5-THz in the pulsed regime have been studied by means of a heterodyne detection by using an optically pumped molecular laser as the local oscillator. The absolute frequency of the QCL can be determined with an accuracy of 1 MHz due to the pulse-to-pulse jitter of the QCL. The frequency tunability as a function of the injection current of the QCL is estimated to be around +2 GHz/A     

Mode Competition in a Semiconductor Ring Laser

In a homogeneously-broadened ring laser strong coupling between the counter-propagating travelling waves leads to stable oscillation in one direction at a time. Such behaviour has never been reported for semiconductor ring lasers in spite of the fact that their gain saturation has a homogeneous character. To explain this different nature of the mode competition, we have calculated the strength of the nonlinear coupling between the counter-propagating travelling waves in a semiconductor ring laser. Crucial elements of the calculation are the carrier diffusion in the amplifying medium and the large mismatch losses at the amplifier facets; the latter lead to a spatially non-uniform amplitude of the eigenmodes of the ring cavity. Additionally, we show how reflection from one of the facets leads to a linear coupling of the counter-propagating waves with a mixed conservative-dissipative nature. The final result of the mode competition depends on the interplay of linear and nonlinear coupling. Experimental results are presented for both a double-sided and a single-sided AR-coated semiconductor amplifier in a ring cavity. For the first ring the observed behaviour agreed well with theory, for the second ring only partial agreement was found.     

Spin-Photocurrent in p-SiGe Quantum Wells Under Terahertz Laser Irradiation

A detailed study of the circular photogalvanic effect (CPGE) in SiGe structures is presented. It is shown that the CPGE becomes possible because of the built-in asymmetry of quantum wells (QWs) in compositionally stepped samples and in asymmetrically doped structures. The photocurrent arises due to optical spin orientation of free carriers in QWs with spin splitting in k-space. It is shown that the effect can be applied to probe the macroscopic in-plane symmetry of low dimensional structures and allowing to conclude on Rashba or Dresselhaus terms in the Hamiltonian.     

Operation of a Wideband Terahertz Superconducting Bolometer Responding to Quantum Cascade Laser Pulses

We make use of a niobium film to produce a micrometric vacuum-bridge superconducting bolometer responding to THz frequency. The bolometer works anywhere in the temperature range 2–7?K, which can be easily reached in helium bath cryostats or closed-cycle cryocoolers. In this work the bolometer is mounted on a pulse tube refrigerator and operated to measure the equivalent noise power (NEP) and the response to fast (μs) terahertz pulses. The NEP above 100?Hz equals that measured in a liquid helium cryostat showing that potential drawbacks related to the use of a pulse tube refrigerator (like mechanical and thermal oscillations, electromagnetic interference, noise) are irrelevant. At low frequency, instead, the pulse tube expansion-compression cycles originate lines at 1?Hz and harmonics in the noise spectrum. The bolometer was illuminated with THz single pulses coming either from a Quantum Cascade Laser operating at liquid nitrogen temperature or from a frequency-multiplied electronic oscillator. The response of the bolometer to the single pulses show that the device can track signals with a rise time as fast as about 450?ns.     

胶印机串墨性能的仿真研究

以海德堡速霸胶印机的输墨系统为例,采用计算机仿真的方法给出串动量对油墨流布的影响和不同墨区的油墨经串动后的油墨流布状况。研究发现,随着串动量的增加,保留在本墨区的油墨呈线性降低;一个墨区的油墨经串动后会呈正态分布地向邻近墨区流开,但中间墨区和两端墨区的流布状态不同。     

Spin-Photocurrent in p-SiGe Quantum Wells Under Terahertz Laser Irradiation

A detailed study of the circular photogalvanic effect (CPGE) in SiGe structures is presented. It is shown that the CPGE becomes possible because of the built-in asymmetry of quantum wells (QWs) in compositionally stepped samples and in asymmetrically doped structures. The photocurrent arises due to optical spin orientation of free carriers in QWs with spin splitting in k-space. It is shown that the effect can be applied to probe the macroscopic in-plane symmetry of low dimensional structures and allowing to conclude on Rashba or Dresselhaus terms in the Hamiltonian.     

通气超空泡自激振荡和受迫振荡的数值仿真研究

根据Logvinovich独立膨胀原理发展了一种用于计算非定常通气超空泡形态的计算方法,并运用该方法对通气超空泡自激振荡和受迫振荡过程进行了数值仿真研究.研究表明:当超空泡发生自激振荡时,空泡内部压力和空泡长度发生周期或准周期振荡.超空泡自激振荡模式主要与气体弹性,通气率和泄气率有关.当超空泡发生受迫振荡时,空泡内部压力和空泡长度也会发生周期或准周期振荡,受迫振荡模式与扰动方式以及自激振荡模式有关.     

利用聚合经验模态分解抑制振动信号中的模态混叠

传统EMD易于造成分解模态的混叠,混叠后的IMF分量失去原有物理意义。聚合经验模态分解(ensemble empirical mode decomposition,EEMD),是一种将噪声辅助分析应用于经验模态分解中的新方法,可以较好的抑制EMD分解中产生的模态混叠现象,将其应用于振动信号的模态提取中,取得较好的工程效果。     

Bloch Analysis of Photonic Lattices that Incorporate Vertical Cavity Surface-Emitting Laser Arrays

We analyzed one-dimensional photonic lattices that incorporate mirror-modulated vertical cavity surface-emitting laser arrays utilizing the Bloch formalism. First, infinitely long arrays are considered. The in-phase mode (with a main central lobe at the far field) and antiphase mode (with two main symmetrically-located lobes at the far-field) are examined. A comparison of the modal losses of the in-phase and the antiphase modes, resulted in the discovery of regimes in which the in-phase mode is dominant. Considering lattices of finite length, we compared the results of the Bloch model to the exact solutions. It is shown that the boundary conditions in these lattices select a specific mode from the continuous spectrum in the infinite case. Consequently, the lattice's length affects the eigenmodes and the corresponding eigenvalues in a periodic manner.     

Polarization-Dependent Periodic Pulse Oscillation in a Diode-Laser-Pumped and Intracavity Frequency-Doubled Nd:YVO 4 Laser

We demonstrate that two cross-polarized longitudinal modes can have 50% higher conversion efficiency than two parallel-polarized longitudinal modes in a diode-laser-pumped and intracavity frequency-doubled Nd:YVO(4) laser when operated under periodic pulse oscillation. Through simulations of the rate equations for primary frequency intensities and gains, we also verify that this effect can be attributed to gain competition and complementary conversion coefficient between second-harmonic and sum-frequency generations.     

Numerical Simulation of Self-Excited Oscillation of a Turbulent Jet Flowing into a Rectangular Cavity

The example of a plane jet flow into a rectangular cavity (“dead end”) is used in comparing the capabilities of different approaches to numerical simulation of self-oscillatory turbulent flows characterized by global quasi-periodic oscillation of all flow parameters. The calculations are performed for two flow modes, of which the first one is statistically steady according to the available experimental data, and the second one is self-oscillatory. In both cases, three approaches are used to describe the turbulence, namely, the method of large eddy simulation (LES) in combination with the subgrid model of Smagorinsky, and steady and unsteady Reynolds averaged Navier-Stokes equations (SRANS and URANS) with two well-known differential models of turbulence. In the case of the first flow mode, all three approaches produce qualitatively similar and quantitatively close results. In the case of the second (self-oscillatory) mode, a steady-state solution of Reynolds equations may only be obtained in half the domain using the symmetry boundary conditions; within the framework of the other two approaches, the solutions turn out to be unsteady-state. In so doing, their characteristics calculated using the LES and URANS methods differ significantly from each other; in the case of URANS, they further depend on the model of turbulence employed. The best results as regards the accuracy of prediction of the amplitude-frequency characteristics of self-oscillation are produced by the use of the LES and three-dimensional URANS methods. A similar inference may be made with respect to the mean flow parameters. From this standpoint, the worst results are those obtained from calculations involving the use of the symmetry boundary conditions on the geometric symmetry plane of the flow.__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 4, 2005, pp. 568–579.Original Russian Text Copyright © 2005 by D. M. Denisikhina, I. A. Bassina, D. A. Nikulin, and M. Kh. Strelets.     

Oscillation in a nonequilibrium superconductor

Using the ^*-model and the Rothwarf-Taylor equations, this paper predicts that a superconductor will oscillate between two nonequilibrium superconducting states with different energy gaps when quasiparticles are injected at the edge of the energy gap through the tunneling effect. The main sources of origin of this oscillation are the quasiparticle injection at the gap edge and the existance of recombination phonons with energy = 2(n). Under certain nonequilibrium conditions, the recombination phonon system does not take part in Cooper pair-breaking processes and can further stimulate the quasiparticles to recombine into pairs. The uncertainty N_p of the number N _pof Cooper pairs has the same order of magnitude as N _pitself in such an oscillating superconductor, so that the phase of the wave function has a definite value. If superconducting weak coupling is formed between such two oscillating superconductors, this system should be described in the -representation.     

Asymmetric Mode of an Off-axis Diaphragmed Laser

Abstract

Diaphragmed resonators have been widely studied in the axially symmetric case. Here we present some experimental and theoretical results concerning the field distribution of an off-axis apertured laser. Measurements have been made with a commercial Argon laser based on a plano-concave cavity and operating at 488 nm. Calculations have been carried out using and extending a method presented previously for a symmetric resonator. Numerical results are given for the field distributions inside and outside the laser, the round-trip loss for the amplitude and the round-trip phase shift for various transverse misadjustments of the aperture. We find the asymmetric field distribution of the fundamental mode in both experimental and theoretical aspects.     

Theory of Passive Mode Locking of a Laser Having Two Laser Amplifiers of Unequal Relaxation Time with a Slow Saturable Absorber

This paper presents a closed-form solution for the pulses of a laser having two homogeneously broadened laser amplifiers of unequal relaxation time mode locked by a ‘slow’ saturable absorber. The pulse shape is found to be secant-hyperbolic. It is found that the gains of both amplifiers participate in time-modulating the pulse and that the pulses are bandwidth-limited by the amplifier with a relaxation time longer than the cavity round-trip time. We have also shown that the model of a ‘slow’ saturable absorber and an amplifier of relaxation time longer than the cavity round-trip time does not give self-starting, mode-locking solutions.     

Diode-Pumped Cr:forsterite Laser Mode Locked by a Semiconductor Saturable Absorber

We report the passive mode locking of a diode-pumped Cr:forsterite laser using a semiconductor saturable absorber. The laser generates 10-pJ pulses as short as 1.5 ps in duration with ~650 mW of absorbed pump power. We measured the transient reflectance of the saturable absorber mirror under different incident energy fluences by using a standard pump-probe method. The performance of this laser is also compared with a high-power fiber-laser-pumped femtosecond forsterite laser mode locked with the same absorber.     

Second Harmonic Generation as a Tool for Measuring Mode Coupling,Pulse Length and Mode Size in Short Laser Pulses

Abstract

We report on a detailed analysis of second harmonic generation with a multi-modal beam for a non-collinear geometry. As a result some improvements for a picosecond pulse width analyser are suggested. Experimental results relative to both a free electron laser and to a mode locked Nd: YAG laser are discussed. This modified version of the pulse analyser is able to monitor, in real time, the relative pulse-to-pulse change of mode size, pulse length and mode mixing effect independently of the pulse length from femtoseconds to milliseconds.