Experimental study of scenarios of transitioning to broadband generation in a laboratory model of a low-voltage vircator

The results from experimental investigations of the transition from single-frequency generation to broadband generation in a laboratory model of a low-voltage vircator are presented. It is found that the transition to broadband generation is realized via intermittency or through a cascade of period-doubling bifurcations.     

Semilinear coherent oscillator with reflection-type photorefractive gratings

The saturation intensity, oscillation spectrum, and temporal dynamics of oscillation are studied for a semilinear coherent oscillator with two counterpropagating pump waves and a photorefractive crystal with dominating reflection gratings. The instability of the single-frequency oscillation spectrum is revealed, similar to that known for an oscillator with dominating transmission gratings. The experimental manifestation of this transition in the output characteristics of the oscillator is favourably compared with numerical calculations. PACS 42.65.Hw; 05.45.-a; 42.65.Pc; 42.65.Sf     

Single-frequency regime of oscillation of a high-power He—Ne laser with a transverse SHF discharge

The possibility of producing a single-frequency regime of oscillation of a He−Ne laser with a transverse shf discharge by changing the pressure of the gas mixture is investigated. A single-frequency regime of oscillation with a high radiation power (10 mW) and a low level of amplitude noise (10⁻⁵ Hz^−1/2) has been produced at a pressure above 6 mm Hg. The laser power in the single-frequency regime comprises 60–80% of the power in the regime of multifrequency oscillation. Physics Department, Tashkent State University, Campus, Tashkent, 700095, Uzbekistan. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 116–118, January–February, 1997.     

Variability of the vertical interference structure of the sound field in a shallow sea

Dynamics of the spectral intensity oscillations that occur in the vertical plane because of the time variability of the medium along the propagation path is described. The errors arising in measuring the frequency shifts of the interference structure are considered. For low-frequency broadband signals received on a stationary propagation path, experimental data on the shifts of their frequency spectra due to the variation of the reception depth are presented. The number of interfering modes and their arrival directions in the vertical plane are estimated from the measured frequency shifts of the spectral intensity oscillation.     

Instability of single-frequency operation in semilinear photorefractive coherent oscillators

The transition of the single-frequency oscillation of a semilinear photorefractive coherent oscillator for sufficiently large coupling strengths into two-frequency oscillation is predicted and is observed experimentally. The critical value of the coupling strength at which the bifurcation occurs is a function of pump intensity ratio and cavity losses. The supercritical bifurcation in the oscillation spectrum is analogous to the second-order phase transition.     

Experimental study of complex dynamics in a delayed-feedback multiple-cavity klystron self-oscillator

The main characteristics of a delayed-feedback multiple-cavity klystron oscillator with various oscillation modes (single-frequency oscillations, as well as regular and chaotic self-modulations) are studied experimentally. Maps of dynamic modes on the beam current-accelerating voltage plane are presented. Basic scenarios of transition to chaos are considered. As the beam current and amount of feedback increase, regular and chaotic oscillation modes are found to alternate in a complex manner. It is shown that one can significantly increase the power of chaotic oscillations by appropriately tuning the control parameters.     

Spatio-temporal invariants of the time reversal operator

The decomposition of the time reversal operator, known by the French acronym DORT, is a technique to extract point scatterers' monochromatic Green's functions from a medium. It is used to detect, locate, and focus on scatterers in various domains such as underwater acoustics, medical ultrasound, and nondestructive evaluation. A limitation of the method arises from its single-frequency nature, when the signals used in acoustics are often broadband. Reconstruction of the broadband Green's functions from the single-frequency Green's functions can be very difficult when numerous scatterers are present in the medium. Moreover, the method does not take advantage of the axial resolution associated with broadband signals. Time domain methods are investigated here as an answer to these problems. It is shown that the time reversal operator in the time domain takes the form of a tensor. The properties of the invariants are discussed. It is shown they do not have all the expected properties. Another method is proposed that requires a priori information on the medium.     

Transition scenario to turbulence in thin vibrating plates

A thin plate, excited by a harmonic external forcing of increasing amplitude, shows transitions from a periodic response to a chaotic state of wave turbulence. By analogy with the transition to turbulence observed in fluid mechanics as the Reynolds number is increased, a generic transition scenario for thin vibrating plates, first experimentally observed, is here numerically studied. The von Kármán equations for thin plates, which include geometric non-linear effects, are used to model large amplitude vibrations, and an energy-conserving finite difference scheme is employed for discretisation. The transition scenario involves two bifurcations separating three distinct regimes. The first regime is the periodic, weakly non-linear response. The second is a quasiperiodic state where energy is exchanged between internally resonant modes. It is observed only when specific internal resonance relationships are fulfilled between the eigenfrequencies of the structure and the forcing frequency; otherwise a direct transition to the last turbulent state is observed. This third, or turbulent, regime is characterized by a broadband Fourier spectrum and a cascade of energy from large to small wavelengths. For perfect plates including cubic non-linearity, only third-order internal resonances are likely to exist. For imperfect plates displaying quadratic nonlinearity, the energy exchanges and the quasiperiodic states are favored and thus are more easily obtained. Finally, the turbulent regime is characterized in the light of available theoretical results from wave turbulence theory.     

大幅度增加弛豫振荡频率来实现毫米级外腔半导体激光器的外腔机制转换

混沌外腔半导体激光器输出明显存在弛豫振荡特征,弛豫振荡频率小于外腔振荡频率时,外腔半导体激光器输出态是短腔机制;反之,外腔半导体激光器输出态是长腔机制.首先对比分析了弛豫振荡频率为5.6 GHz,腔长对频谱有效带宽的影响.然后同时调节注入电流和载流子寿命来大幅度地增加弛豫振荡频率.最后在弛豫振荡频率为40 GHz、腔长为毫米级(4—20 mm)时,实现由短腔机制到长腔机制的转换,进而分析了外腔反馈率和外腔长对外腔半导体激光器频谱带宽的影响.分析结果表明:短腔机制下,输出混沌态不稳定,0.1 mm的偏差就会导致混沌态与非混沌态之间的转化;长腔机制下,输出混沌态稳定,输出混沌区域较大,证明长腔机制下更有益于获得宽带连续的混沌区域.在弛豫振荡频率为40 GHz、外腔长度为毫米级时,实现了外腔半导体激光器的长腔机制,从而增大了高带宽混沌的参数空间.     

水中密排圆柱体群的超声多重散射参数

以水中紧密排列的平行圆柱体群为对象,研究平面超声脉冲经多重散射后的透射波性质,通过分析其中头波和散射波的特征获得对应的多重散射参数.对直径随机分布、位置无序排列、数量密度约100个/cm2、面积占空比约0.53的非接触圆柱体群,采用中心频率2.5 MHz的宽带脉冲波入射。为解决透射信号在时域表现出随机性的问题,将散射体尺寸、分布都相同但位置分布不同的多个模型仿真的透射波叠加平均后用于分析.在频域对头波的宽带衰减系数进行分析,并在时域研究散射波声强的时间演化曲线,获得了系统的弹性平均自由程、传输平均自由程等多重散射参数。经多重散射后,透射波中的头波表现出相干性,由不相干近似理论可对其对应的散射参数进行定性描述;散射波是不相干的,其对应的多重散射参数可近似利用扩散近似理论获得。      

Waveguide acoustooptic devices for the real-time spectral analysis of broadband optical signals

Computer simulation and experimental testing of the main characteristics of a waveguide acoustooptic device for the real-time spectral analysis of broadband optical signals are discussed. A mathematical model of such a device is similar to the experimentally tested mathematical model of a waveguide acoustooptic spectral analyzer of radio signals. Two modifications of the spectrum analyzer based on Y-cut lithium niobate are developed and numerically studied. These modifications make it possible to process broadband optical signals in the red and IR ranges. The parameters of the devices are chosen based on real technological and topological requirements for the elements. Comparative analysis shows that the theoretical data on the first modification are in good agreement with the corresponding experimental results.     

Metal-insulator transitions in the periodic Anderson model

We solve the periodic Anderson model in the Mott-Hubbard regime, using dynamical mean field theory. Upon electron doping of the Mott insulator, a metal-insulator transition occurs which is qualitatively similar to that of the single band Hubbard model, namely, with a divergent effective mass and a first order character at finite temperatures. Surprisingly, upon hole doping, the metal-insulator transition is not first order and does not show a divergent mass. Thus, the transition scenario of the single band Hubbard model is not generic for the periodic Anderson model, even in the Mott-Hubbard regime.     

Modeling of dynamics of a femtosecond Kerr-lens mode-locked laser by the mode decomposition of the intracavity beam

A full spatio-temporal model is used for analyzing the features of generation of femtosecond pulses in a Kerr-lens mode-locked laser. The developed algorithm involves the field decomposition in terms of Laguerre-Gaussian functions which are the modes of empty space. Polarization of the medium is calculated from the Bloch equations for the two-level transition. With allowance for the frequency-dependent diffraction, such a method allows us to describe generation of pulses with a duration of several femtoseconds. It is shown that diffraction results in a shift of the carrier frequency of sub-10-fs pulses toward shorter wavelengths. A multiple-pulse oscillation regime can be realized near zero group-velocity dispersion in the cavity. It is shown that such a regime can be realized in the absence of higher-order dispersion. Strong coupling between the spatial and temporal characteristics of the field is observed for the pulses with a duration of several femtoseconds. This leads to a complicated dependence of the beam size on its power and, therefore, to a complicated variation in power-dependent losses. Due to this feature, regimes of generation of ultrashort pulses cannot be correctly described by models in which power-dependent losses are introduced artificially.     

Self-modulation lasing regimes in a fast-flow CO<Subscript>2</Subscript> laser

Numerical simulation of self-modulation lasing regimes has been carried out for a CO₂ laser with transverse flow of the active medium through an unstable resonator with inhomogeneous internal pumping. Two types of steady-state self-modulation oscillation differing in the feedback mechanism were observed. The type of lasing regime and the conditions of its realization depend both upon the pumping profile and upon the composition and pressure of the working mixture, which makes it generally possible to control temporal characteristics of lasing.     

Chain-length-dependent relaxation scenarios in an oligomeric glass-forming system: from merged to well-separated alpha and beta loss peaks

We have studied the relaxation dynamics of a homologous series of propylene glycol based dimethyl ethers in the supercooled regime by means of broadband dielectric spectroscopy. The system is chosen in order to minimize changes of the intermolecular interactions with varying molecular weight, M. A gradual transformation from a scenario of well-separated to one of merged alpha and beta loss peaks was observed with decreasing M. The results give strong evidence for the currently debated excess wing being due to an underlying beta relaxation. The study suggests that the main difference between glass formers with and without excess wings is the relaxation time at the merging temperature.     

Quasi-optical theory of coaxial and cylindrical relativistic surface-wave oscillators

In terms of a quasi-optical approach, a nonlinear nonstationary theory of surface-wave oscillators, coaxial and cylindrical multiwave Cherenkov oscillators (MCOs) fed by large-diameter tubular electron beams, is constructed. The small curvature of the waveguide walls allows one to appreciably simplify the MCO dynamics analysis by considering a quasi-plane model. In this model, local surface fields near the corrugated cylindrical wall are close to fields of a plane corrugated with the same depth and period and the cylindrical geometry is taken into account by introducing azimuthal periodicity conditions. The results obtained in terms of the averaged approach are compared with those of direct numerical particle-in-cell (PIC) simulation and experimental data. Remarkably, PIC simulation demonstrates the existence of a single-frequency oscillation regime at long perimeters in which the self-synchronization of different azimuthal modes takes place. As a result, an azimuthally asymmetric stationary field distribution sets in, which can be assigned to dissipative structures well known in the theory of self-sustained oscillation systems.     

Nonstationary Behavior of a Gyrotron in the Presence of Reflections

Nonstationary behavior of a gyrotron in the presence of reflections is investigated by means of our time-dependent self-consistent code. It is shown that the traditionally accepted scenario of the transition from the single-frequency stationary operation regime to the chaotic nonstationary regime due to an increase in the magnitude of the reflection coefficient is not correct if a realistic non-fixed field profile is used in the simulations. In this case, the increase of the reflection coefficient above a certain threshold value (R 0.4) leads to an excitation of a number of modes of the mismatched gyrotron cavity. Then the gyrotron shows a very complicated periodic or quasiperiodic behavior. For higher reflection coefficients (R 0.85), the competition between the modes of the mismatched cavity leads to the suppression of the main mode by another one operating at higher frequency. The influence of the distance to the reflecting load on the gyrotron behavior is also demonstrated.     

Coherent control of vacuum squeezing in the gravitational-wave detection band

We propose and demonstrate a coherent control scheme for stable phase locking of squeezed vacuum fields. We focus on sideband fields at frequencies from 10 Hz to 10 kHz, which is a frequency regime of particular interest in gravitational-wave detection and for which conventional control schemes have failed so far. A vacuum field with broadband squeezing covering this entire band was produced using optical parametric oscillation and characterized with balanced homodyne detection. The system was stably controlled over long periods utilizing two coherent but frequency shifted control fields. In order to demonstrate the performance of our setup the squeezed field was used for a nonclassical sensitivity improvement of a Michelson interferometer at audio frequencies.     

Broadband second harmonic generation of an optical frequency comb produced by four-wave mixing in highly nonlinear fibers

We demonstrate broadband second harmonic generation of low-energy pulses produced by injecting two single-frequency lasers into a highly nonlinear fiber. Full nonlinear conversion of the corresponding spectra, consisting of broadband (∼200 nm) optical frequency combs at ∼1580 nm, were obtained by using conventional birefringence phase-matching in two BIBO crystals (2-mm and 100-μm long) with a normal incidence configuration. The crystals were not tilted and the pulses were not compressed. This broadband conversion results from the large phase-matching bandwidth of the nonlinear BIBO crystals at ∼1550 nm, but also seems to be a consequence of a fundamental comb with small spectral phase variation.     

Acoustic scattering from fluid-filled finite cylindrical shell in water:Ⅱ.Experiment

Acoustic scattering from the submerged fluid-filled finite cylindrical shell insonified by an incident plane wave is studied experimentally and theoretically.A monostic broadband transducer with the sharp directivity is used in the experiment.The broadband LFM signal and the single-frequency narrow pulse are used to measure the backscattering field of the cylindrical shell.The measured results have a good agreement with the theory both in time and frequency domain.The theoretical and experimental results show that the resonances of several additional waves which are caused by the internal fluid are presented in the frequency domain.And a series of ’whispering gallery’ waves produced by the waves reflected back and forth in the internal fluid filled in the cylindrical shell are added.The reason for the clustering of the bowl-shape resonance curves in the frequency-angle spectrum is explained as the superposition of the first several modes of ’whispering gallery’ waves.