Dominant Transverse Mode Research Articles

Nonlinear dynamics of beam self-cleaning on LP11 mode in multimode fibers.

We investigate the modal energy flow of the femtosecond-pulsed beam self-cleaning on LP11 mode with the influence of different factors such as the initial fraction of LP11 mode, initial peak power, distribution of high-order modes and the numerical aperture of the fiber. It is interesting that there is a critical value of the initial peak power, Pcr, which is the watershed, not only in the quantitatively dominant transverse mode converting from LP11 mode to LP01 mode, but also in the behavior of HOMs of the transition from Attractor to chaos. Our simulation results may provide a novel perspective to understanding the beam self-cleaning on LP11 mode.

Flame response to transverse velocity excitation leading to frequency doubling and modal coupling

Transverse thermoacoustic modes may occur in gas turbines, aero-engines, and rocket engines. Various scenarios of flame excitation can be observed, depending on the type of transverse mode and the location of the flame relative to the mode shape. If an acoustically compact, symmetric flame is exposed to transverse velocity fluctuations of uniform strength and direction, the resulting modulation of the overall heat release rate is invariant to the direction of the velocity perturbation. It follows that the dominant flame response occurs at twice the forcing frequency, even for an infinitesimally small oscillation amplitude. The present study proposes a modeling framework for this inherently non-linear phenomenon, which relies on a second-order kernel of the Volterra series. As a possible realization of the Volterra series, an ad-hoc model is proposed and validated with CFD for mono-frequency forcing. Furthermore, a mechanism of modal interaction is established by which frequency doubling in the flame response causes an unstable transverse mode to drive a higher order stable mode, such that at near-resonance conditions the higher order mode exhibits elevated amplitude. This mechanism can explain the observations made by Urbano et al. (2016) in a small-scale rocket thrust chamber, where a radial mode appears at exactly twice the frequency of the dominant transverse mode. A simple representative setup of a cylindrical combustion chamber is used to explain this mechanism analytically.

Phase-locking of arrays of weakly coupled semiconductor lasers.

In this paper we study the conditions for achieving almost perfect phase locking in large arrays of semiconductor diodes. We show that decayed non-local coupling of diode lasers can provide the necessary conditions for robust phase synchronization of an entire diode laser array. Perfect global coupling is known to allow for robust synchronization, however it is often physically impossible or impractical to achieve. We show that when diodes are coupled via the decayed non-local coupling layout, the dominant transverse mode of the laser array has a uniform phase across the lasers and can be stable. This state is robust to noise and frequency disorder and can be realized under periodic (fixed-intensity limit cycle) continuous-wave and chaotic behavior of lasers.

Scalable Top-Down Approach Tailored by Interferometric Lithography to Achieve Large-Area Single-Mode GaN Nanowire Laser Arrays on Sapphire Substrate.

GaN nanowires are promising for optical and optoelectronic applications because of their waveguiding properties and large optical band gap. However, developing a precise, scalable, and cost-effective fabrication method with a high degree of controllability to obtain high-aspect-ratio nanowires with high optical properties and minimum crystal defects remains a challenge. Here, we present a scalable two-step top-down approach using interferometric lithography, for which parameters can be controlled precisely to achieve highly ordered arrays of nanowires with excellent quality and desired aspect ratios. The wet-etch mechanism is investigated, and the etch rates of m-planes {11̅00} (sidewalls) were measured to be 2.5 to 70 nm/h depending on the Si doping concentration. Using this method, uniform nanowire arrays were achieved over a large area (>105 μm2) with an spect ratio as large as 50, a radius as small as 17 nm, and atomic-scale sidewall roughness (<1 nm). FDTD modeling demonstrated HE11 is the dominant transverse mode in the nanowires with a radius of sub-100 nm, and single-mode lasing from vertical cavity nanowire arrays with different doping concentrations on a sapphire substrate was interestingly observed in photoluminescence measurements. High Q-factors of ∼1139-2443 were obtained in nanowire array lasers with a radius and length of 65 nm and 2 μm, respectively, corresponding to a line width of 0.32-0.15 nm (minimum threshold of 3.31 MW/cm2). Our results show that fabrication of high-quality GaN nanowire arrays with adaptable aspect ratio and large-area uniformity is feasible through a top-down approach using interferometric lithography and is promising for fabrication of III-nitride-based nanophotonic devices (radial/axial) on the original substrate.

Long-range orientation correlation in dipolar liquids probed by hyper-Rayleigh scattering.

Hyper-Rayleigh scattering (HRS) is sensitive to long-range molecular orientation correlation in isotropic liquids composed of dipolar molecules. The correlation functions that appear in the calculation of HRS mediated by the vector part of the first hyperpolarizability β are the same as the correlation functions for the homogeneous isotropic random vector fields that appear in the description of fluid turbulence. Recent experiments measuring the angle and polarization dependence of HRS from water find a dominant transverse mode contribution with amplitude independent of the scattering wavevector, and this observation of transverse mode HRS strongly constrains the form of the orientation correlation function. Analysis of these HRS results for water determines that the long-range molecular orientation correlation function varies as r(-3±ε) with |ε| < 0.03 on spatial scales up to 2000 nm.

Reduced-Order Model for Combustion Instability in a Two-Dimensional Duct with a Flameholder

A reduced-order model for combustion instability in a two-dimensional duct with a flameholder is presented. A linear flame-response function for a premixed flame of arbitrary mean shape is derived using the G-equation approach. Perturbations consisting of coupled axial and transverse modes are superimposed on system mean flow conditions and analyzed for instability. Test cases involving variations of duct geometry, flameholder location, and mean flame shape are considered for evaluating the reduced-order model. Mean flow and temperature conditions andmean heat-release rate, needed as inputs to themodel, are obtained from highly resolved two-dimensional largeeddy simulation data for the same geometry. The pressure power spectrum from large-eddy simulation is available for two cases: 1) with the flameholder located symmetrically between the top and bottomwalls of the duct and 2) with the flameholder offset vertically by a small distance relative to the duct height. For the first case, the large-eddy simulation pressure power spectrum shows a dominant transverse instability mode that is also captured by the model. In the second case, large-eddy simulation shows that a small vertical offset damps the dominant transverse mode found in the zero-offset first case. The model also qualitatively supports this trend. For additional test cases in which large-eddy simulation data are not available, the model is seen to produce qualitatively consistent trends.

Observation of Self-Amplified Spontaneous-Emission-Induced Electron-Beam Microbunching Using Coherent Transition Radiation

We report the measurement of electron-beam microbunching at the exit of a self-amplified spontaneous-emission free-electron laser (SASE FEL), by observation of coherent transition radiation (CTR).thinsp The CTR was found to have an angular spectrum much narrower than spontaneous transition radiation and a narrow-band frequency spectrum. The central frequency of the fundamental CTR spectrum is found to differ slightly from that of the SASE, a finding in disagreement with previously invoked CTR theory. The CTR measurement establishes the uniformity of microbunching in the transverse dimension, indicating the SASE FEL operates in a dominant transverse mode. {copyright} {ital 1998} {ital The American Physical Society }

Heat-pulse study of the phonon emission by hot two-dimensional holes in a gallium arsenide heterojunction

The process of energy relaxation by phonon emission of hot two-dimensional (2D) holes in a GaAs/AlGaAs heterojunction has been studied using the heat-pulse technique. The holes were heated above the substrate temperature by applying short current pulses to a device, and the emitted phonons were detected by superconducting bolometers on the opposite side of the GaAs substrate. At the same time, by making measurements of the device resistance during the excitation pulse, the hole temperature was determined as a function of the power dissipated. In contrast to earlier measurements on 2D electron devices, significant longitudinal mode phonon emission was seen as well as the dominant transverse mode. Theoretical calculations of the temperature, mode and angular dependence of the acoustic phonon emission by hot holes were made. The model was based on the theory of acoustic phonon emission by 2D electrons, with the appropriate change of parameters such as the effective mass and the deformation potential constant. Detailed comparison is made between the experimental and theoretical results. We obtained a value for the valence band deformation potential, . It was found that at hole temperatures above 55 K optic phonon emission takes over as the dominant energy loss process. The changeover occurs at a power dissipation of 25 pW/hole, which is about ten times greater than for a 2D electron system of similar areal density, and is attributable to the difference in effective masses of the holes and electrons. We determined the hole - optic phonon scattering time, . The angular dependence of the phonon emission has also been investigated using a spatially resolving superconducting bolometer. Evidence of a predicted abrupt cut-off of the emission at small angles, close to the normal to the 2D carrier gas, is observed for the first time.

Direct bandwidth and polarization control of an XeF unstable resonator laser

Several approaches to directly controlling the bandwidth and polarization of an unstable resonator without the use of an injection source are discussed. The techniques are based on the control of oscillations within (and around) the resonator core through the use of small, subaperture optical elements (e.g. etalons, Brewster-angle plates, Glan-air prisms) mounted at select locations within the system. The experiments were performed on a 4-J, e-beam pumped, XeF (B to X) laser operating at ambient temperature, with output predominantly at 353.2 nm. A spectral linewidth of less than 12 GHz (0.05 AA) and nearly perfect linear polarization were simultaneously achieved at high laser efficiency. Oscillation on a single, dominant transverse mode has been demonstrated. The increased cost and complexity of injection-controlled systems are avoided through the use of these direct, core-control approaches.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Continuous-wave garnet ring laser emitting single-mode linearly polarized radiation and intracavity second harmonic generation in LiIO3

An investigation was made of a method of selecting the TEM00 mode in a laser by using birefringence induced in an active element during pumping. The reasons for astigmatism of the thermal lens in an active element with Brewster-cut faces were determined and an investigation was made of the structure of the radiation field of a laser using such an element. It is found that the optimal conditions for efficient emission of linearly polarized radiation in the dominant transverse mode in a laser with a Brewster active element are achieved by using an active element of rectangular cross section with a specific ratio of the sides. A prototype YAG ring laser emitting linearly polarized radiation with a power of 15 W in the TEM00, mode is described. By using an LiIO3 nonlinear element, 0.43 W of cw radiation at the 0.53 μ wavelength is obtained.