Single-mode Excitation Research Articles

Long Range Inspection of Rail-Break Using Ultrasonic Guided Wave Excited by Pulse String

Compared with single pulse excitation mode, ultrasonic guided wave excited by pulse string is more suitable for long range inspection of rail-break. Shear horizontal (SH) guided wave transducers with modified structure and pulse string emission circuit were designed for the purpose of achieving longer inspection distance. To solve the control difficulty of pulse string, the pulse string phase shift superposition (PSPSS) theorem was proposed and exhaustive algorithm together with genetic-like algorithm were also given to calculate the optimal phase shift parameters. Experiments of pulse string guided wave on rail were carried out. Result shows that the PSPSS theorem can improve wave energy significantly, and the maximum propagation distance of pulse string guided wave can be enhanced up to more than 600 meters.

The Faraday threshold in small cylinders and the sidewall non-ideality

AbstractIn this work we investigate, by way of experiments and theory, the Faraday instability threshold in cylinders at low frequencies. This implies large wavelengths where effects from mode discretization cannot be ignored. Careful selection of the working fluids has resulted in an immiscible interface whose apparent contact line with the sidewall can glide over a tiny film of the more wetting fluid, without detachment of its actual contact line. This unique behaviour has allowed for a system whose primary dissipation is defined by the bulk viscous effects, and in doing so, for the first time, close connection is seen with the viscous linear stability theory for which a stress-free condition is assumed at the sidewalls. As predicted, mode selection and co-dimension 2 points are observed in the experiment for a frequency range including subharmonic, harmonic, and superharmonic modes. While agreement with the predictions are generally excellent, there are deviations from the theory for certain modes and these are explained in the context of harmonic meniscus waves. A review of previous work on single-mode excitation in cylinders is given, along with comparison to the viscous model and analysis based upon the conclusions of the current experiments.

Cavity optomechanics with a trapped, interacting Bose-Einstein condensate

The dispersive interaction of a Bose-Einstein condensate with a single mode of a high-finesse optical cavity realizes the radiation pressure coupling Hamiltonian. In this system the role of the mechanical oscillator is played by a single condensate excitation mode that is selected by the cavity mode function. We study the effect of atomic s-wave collisions and show that it merely renormalizes parameters of the usual optomechanical interaction. Moreover, we show that even in the case of strong harmonic confinement – which invalidates the use of Bloch states – a single excitation mode of the Bose-Einstein condensate couples significantly to the light field, that is the simplified picture of a single “mechanical” oscillator mode remains valid.

Stability and excitations of a bilayer of strongly correlated dipolar bosons

We study correlation effects and excitations in a dipolar Bose gas bilayer which is modeled by a one-dimensional double well trap that determines the width of an individual layer, the distance between the two layers, and the height of the barrier between them. For the ground state calculations we use the hypernetted--chain Euler Lagrange method and for the calculation of the excitations we use the correlated basis function method. We observe instabilities both for wide, well-separated layers dominated by \emph{intra-layer} attraction of the dipoles, and for narrow layers that are close to each other dominated by \emph{inter-layer} attraction. The behavior of the pair distribution function leads to the interpretation that the monomer phase becomes unstable when pairing of two dipoles becomes energetically favorable between or within layers, respectively. In both cases we observe a tendency towards "rotonization", i.e. the appearance of a soft mode with finite momentum in the excitation spectrum. The dynamic structure function is not simply characterized by a single excitation mode, but has a non-trivial multi-peak structure that is not captured by the Bijl-Feynman approximation. The dipole-dipole interaction between different layers leads to additional damping compared to the damping obtained for uncoupled layers.

Subsurface imaging of carbon nanotube networks in polymers with DC-biased multifrequency dynamic atomic force microscopy

The characterization of dispersion and connectivity of carbon nanotube (CNT) networks inside polymers is of great interest in polymer nanocomposites in new material systems, organic photovoltaics, and in electrodes for batteries and supercapacitors. We focus on a technique using amplitude modulation atomic force microscopy (AM-AFM) in the attractive regime of operation, using both single and dual mode excitation, which upon the application of a DC tip bias voltage allows, via the phase channel, the in situ, nanoscale, subsurface imaging of CNT networks dispersed in a polymer matrix at depths of 10–100 nm. We present an in-depth study of the origins of phase contrast in this technique and demonstrate that an electrical energy dissipation mechanism in the Coulomb attractive regime is key to the formation of the phase contrast which maps the spatial variations in the local capacitance and resistance due to the CNT network. We also note that dual frequency excitation can, under some conditions, improve the contrast for such samples. These methods open up the possibility for DC-biased amplitude modulation AFM to be used for mapping the variations in local capacitance and resistance in nanocomposites with conducting networks.

Simple and reliable light launch from a conventional single-mode fiber into a helical-core fiber through an adiabatically tapered splice

We propose a simple and efficient light launch scheme for a helical-core fiber (HCF) by using an adiabatically tapered splice technique, through which we overcome its inherent difficulty with light launch owing to the large lateral offset and angular tilt of its core. We experimentally demonstrate single-mode excitation in the HCF in this configuration, which yields the coupling efficiency of around -5.9 dB (26%) for a ~1.1-μm light input when the splice joint is tapered down to 30 μm in diameter. To our knowledge, this is the first proof-of-principle report on the fusion-splice coupling between an HCF and a conventional single-mode fiber.

Part-per-trillion level SF_6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF(6) was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10(-10) W·cm(-1)/Hz(1/2).

Excitation of single multipolar modes with engineered cylindrically symmetric fields

We present a new method to address multipolar resonances and to control the scattered field of a spherical scatterer. This method is based on the engineering of the multipolar content of the incident beam. We propose experimentally feasible techniques to generate light beams which contain only a few multipolar modes. The technique uses incident beams with a well defined component of the angular momentum and appropriate focusing with aplanatic lenses. The control of the multipolar content of light beams allow for the excitation of single Mie resonances and unprecedented control of the scattered field from spherical particles.

Resonance Condition in Microtubules Using Ultrasound Plane Waves

Microtubules (MTs) are protein filaments forming a major part of the cytoskeleton of all eukaryotic cells which directly contribute to the process of cell division by forming mitotic spindles and providing force for the segregation of chromosomes. MTs exhibit mechanoelastic properties of considerable interest in view of their functional roles in cell division and cell shape. In this work by introducing a new method, we have analytically solved equations of motion for the vibrational dynamics of an MT that is attached at its two ends. This is especially relevant for MTs during mitosis when they attach to chromosomes and centrosomes at the plus and minus ends, respectively. Our analysis applies to MTs present inside a viscous solution and when driven by an ultrasound plane wave. We have shown that with using ultrasound plane waves the resonance condition for the MT treated as a rigid rod cannot be provided, and in order to achieve resonance we should excite a single mode of the MT with a harmonic number larger than a threshold value introduced in this paper. Having a large enough amplitude for the resonant vibration effect is crucial in order to maximize the bending moment of an MT. Attaining a high quality factor for a resonance is crucial to have a better control of the energy transfer to the MT. Finally, single mode excitation not only helps to transfer the minimum amount of energy to the surrounding medium compared with multi-mode excitation but it also allows for a simultaneous high-amplitude and high-quality factor which is impossible when using plane waves.

Decoding the spectra of low-finesse extrinsic optical fiber Fabry-Perot interferometers

A theoretical model is developed to address the fringe visibility and additional phase in the interference spectra of low-finesse extrinsic optical fiber excited Fabry-Pérot interferometers. The model described in the paper applies to both single-mode and multimode fiber excitations; according to the theory, the fringe visibility and additional phase term are primarily determined by the working wavelength and angular power density distribution outputting from the excitation fiber, rather than based on spatial and temporal degree of coherence. Under certain approximations, the output interference intensity and the spatial power density distribution projected onto the fiber axis form a Fourier-transform pair, which potentially provides a tool for spatial density distribution analysis of fiber output. With excellent agreement with experiments, the theory presented in this paper leads to design guidelines for Fabry-Pérot interferometric sensors and insightful physical understanding of such devices.

Piezoelectric transducer parameter selection for exciting a single mode from multiple modes of Lamb waves

Excitation and propagation of Lamb waves by using rectangular and circular piezoelectric transducers surface-bonded to an isotropic plate are investigated in this work. Analytical stain wave solutions are derived for the two transducer shapes, giving the responses of these transducers in Lamb wave fields. The analytical study is supported by a numerical simulation using the finite element method. Symmetric and antisymmetric components in the wave propagation responses are inspected in detail with respect to test parameters such as the transducer geometry, the length and the excitation frequency. By placing only one piezoelectric transducer on the top or the bottom surface of the plate and weakening the strength of one mode while enhancing the strength of the other modes to find the centre frequency, with which the peak wave amplitude ratio between the S0 and A0 modes is maximum, a single mode excitation from the multiple modes of the Lamb waves can be achieved approximately. Experimental data are presented to show the validity of the analyses. The results are used to optimize the Lamb wave detection system.

Tapered Multi-Mode Interference Waveguide for High-Power Self-Organizing Single-Mode Semiconductor Laser Arrays

A 1 × N, tapered, multi-mode interference (MMI) coupler is numerically studied for use in a self-organizing semiconductor laser-array system. The MMI structure is linearly tapered in a way such that the single-port side is wider than the N-port side. The single-port waveguide is N times wider than the other ports to maintain intensity similar to that in each of the N-port waveguides. Using such a device, the N-ports of a semiconductor laser array can be coherently combined in a self-organizing architecture. This modified MMI design increases the output power threshold for catastrophic optical damage while maintaining only single-mode excitation of a wide single port output. The device is nonadiabatic with a single-pass power transmission of 98%. A power imbalance up to 40% in a single port reduces the device efficiency by less than 1%, while a waveguide roughness as large as 70 nm rms detracts only 3% from the device efficiency. When designed to work at 1 μm wavelength, the structure has a full-width half-maximum pass-band of 10 nm, which can provide wavelength selectability when used in a self-organized semiconductor laser-array system.

Vibrational dynamics of bio- and nano-filaments in viscous solution subjected to ultrasound: implications for microtubules

In this paper, using a new analytical method, we solved the beam equation for a uniform bio- and nano-filament in a viscous solution. The filament is assumed to be attached at its two ends and driven by ultrasound plane waves. To obtain analytical solutions, we converted the beam equation to an equation that allows us the use of the method of separation of variables. We then reconstructed the solution of the original beam equation from the solution of the converted equation. Subsequently, we have used the parametric equations derived in this paper to investigate the resonance condition for a microtubule (MT) in an aqueous solution. We show that by using ultrasound plane waves, one cannot satisfy a resonance condition for MTs treated as rigid rods. In order to achieve resonance, a single mode of the MT vibration must be excited with a harmonic number larger than a threshold value found here. Single mode excitation not only helps to transfer a minimum amount of energy to the surrounding medium compared with multi-mode excitation, but it also allows for a simultaneous high amplitude and high mode quality that is impossible using plane waves. In order to overcome this difficulty, we propose to use an ultrasound generation device as a potential technical solution characterized by both frequency control and optimized energy transfer to the MT. Finally, the minimum required intensity of the ultrasound at the location of the MT in order to break it is shown to be on the order of 10(5)W/m(2), which corresponds to 170dB.

Controlled Coupling of a Single Nitrogen-Vacancy Center to a Silver Nanowire

We report on the controlled coupling of a single nitrogen-vacancy (NV) center to a surface plasmon mode propagating along a chemically grown silver nanowire (NW). We locate and optically characterize a single NV center in a uniform dielectric environment before we controllably position this emitter in the close proximity of the NW. We are thus able to control the coupling of this particular emitter to the NW and directly compare the photon emission properties before and after the coupling. The excitation of single plasmonic modes is witnessed and a total rate enhancement by a factor of up to 4.6 is demonstrated.

Enhanced switching law for synchronized switch damping on inductor with bimodal excitation

Piezoelectric shunt damping is an emerging field of research. In recent years, a multitude of different electrical circuits have been developed aiming to increase the damping performance and robustness. Synchronized switch damping on inductor (SSDI) is a semi-active control technique that utilizes a passive inductance to build-up a voltage on the piezoceramics that is synchronized with the mechanical vibration. For a single mode excitation the voltage inversion should occur at the moments of maximum deformation, but for multimodal vibrations such a switching law may not be optimal. In this paper a novel switching law for bimodal vibrations is presented using a modal observer. An enhanced voltage build-up is generated by utilizing the vibration energy of the second mode. The amplification of dissipated energy is calculated in an analytical way using normalized parameters, yielding a general result which includes the influence of the frequency and amplitude ratio of the excitation signal. Measurements on a clamped beam test rig are conducted in order to validate the proposed method. An increase of nearly 350 percent in energy dissipation compared to the classical SSDI has been achieved. Furthermore, the increase in energy dissipation is higher than for a previously suggested, comparable switching law.

Scattering of evanescent pipe modes form elastic spherical objects external to a fluid-filled, submerged cylindrical waveguide.

Investigation into the interaction of low-frequency fluid-borne, radially decaying pipe modes with elastic objects exterior to a cylindrical waveguide using the elastodynamic finite integration technique (EFIT). The system is an oil-filled steel pipe submerged in water. When the sound speed of the conveyed fluid is lower than the exterior fluid, there exist fluid/fluid modes possessing cutoff frequencies supported by both the interior and exterior fluids. These are in addition to the higher-velocity, steel-borne modes, and the low-velocity internal-fluid-borne mode without a cutoff frequency. Excitation of a fluid/fluid mode near its cutoff frequency results in its further penetration into the exterior fluid. An object close to the pipe wall allows for scattering of this evanescent mode. An advantage of EFIT, direct manipulation of material properties, allows for easy inclusion of objects. Simulation results show the scattering from elastic objects with and without air pockets that are compared to analytical plane wave solutions. A plastic sphere with a radius ¾ the pipe inner radius gives a backscattered signal 15 dB below and a forward scattering signal 10 dB below the incident field. In order to achieve mode separation over shorter propagation distances, the analytical mode solution is used for single mode excitation.

Magnetic excitations in theXY-pyrochlore antiferromagnetEr2Ti2O7

The XY-pyrochlore antiferromagnet ${\text{Er}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ is studied by heat-capacity measurements and electron-spin resonance spectroscopy performed on single-crystal samples. The magnetic phase diagrams are established for two directions of applied field, $H\ensuremath{\parallel}[100]$ and $H\ensuremath{\parallel}[111]$. In the magnetically ordered phase observed below ${T}_{N}=1.2\text{ }\text{K}$, the magnetic excitation spectrum consists of a Goldstone mode acquiring an isotropic gap in an applied field, and another mode with a gap softening in the vicinity of a field-induced phase transition. This second-order transition takes place at a critical field ${H}_{c}$ above which the magnetization process is accompanied by a canting of the magnetic moments off their local ``easy planes.'' The specific-heat curves for $H\ensuremath{\parallel}[100]$ $(H⪢{H}_{c})$ are well described by a model presuming a single dispersionless excitation mode with the energy gap obtained from the spectroscopic measurements.

Efficient fiber-to-chip grating coupler for micrometric SOI rib waveguides

Grating couplers are an efficient means for fiber to chip coupling, as they require no facet preparation and enable wafer scale testing. While grating couplers are commonly used in silicon wire waveguides, their application to micrometric silicon-on-insulator rib waveguides is complicated due to the presence of high-order Bloch modes. We study the Bloch modes behavior and their excitation determined by access waveguide design. The latter is implemented to enable single Bloch mode excitation. The use of a design process based on modal analysis is proposed. A grating coupler is proposed in silicon-on-insulator with 1.5 microm thick silicon layer that achieves a coupling efficiency of 65.6% at 1.55 microm. The structure, including interconnection waveguides, access waveguide and grating can be fabricated using a single lithography step.

Single-Mode Excitation in High-Power Gyrotrons by Controlling Gun Perveance

Present-day gyrotrons intended for plasma heating and current-drive experiments operate in high-order modes. Therefore, in such gyrotrons, in the process of voltage rise, as a rule, higher frequency parasitic modes are excited prior to the desired mode. This excitation can be avoided when electron beams with lower current density at intermediate voltages are used. Then, when the voltage passes through the region of excitation of high-frequency parasitic modes, the beam current can be made lower than the start current of those modes. At the same time, when the voltage reaches the excitation zone of the desired mode, the current can be high enough for exciting the desired mode. Such lowering of the beam current at intermediate voltages can be realized by reducing the ratio of the space-charge-limited to the temperature-limited emission. The method is illustrated by an example showing how the excitation of a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23, 6</sub> -mode can be avoided in a gyrotron designed for operation at the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22, 6</sub> -mode. Applicability of this method to millimeter-wave megawatt-class gyrotrons is discussed. The method proposed has additional advantages for gyrotrons operating in power-modulated regimes which are used for suppression of neoclassical tearing modes in large-scale tokamaks and stellarators.

Frequency dispersion of parametric array signal in shallow water.

Results of experimental test of parametric array application for marine shallow water waveguide excitation by sweep frequency modulated signal are discussed. Parametrical sound signal is forming in shallow water environment, which is stimulated by intensity modulated high frequency power acoustical pump. As a result the end‐fire parametric array is forming there, which excites sharp directional signal radiation at the modulation frequency. Such a low‐frequency signal, generated in the virtual end‐fire array by parametrical means, will propagate in shallow water waveguide independently from the pump radiation. Shallow water signal propagation obeys to waveguide dispersion. Sweep modulated signal compression is experimentally shown for single mode signal propagation in shallow water. Acoustical signal of 2‐ms duration is generated in frequency band of 7–15 kHz by parametric array. This signal is transmitted in single lobe of 2 deg width along the path of 5.6 km long in water layer from 2.5‐ to 3‐m depth. The directivity of the signal transmitted was constant in the whole frequency range. It was shown the single mode excitation of the shallow water waveguide takes place under this circumstance. [Work supported by ISTC, Project No. 3770.]