共查询到20条相似文献,搜索用时 31 毫秒
1.
The self-mixing interference effects are investigated in birefringence dual-frequency Nd:YAG lasers at different optical feedback
levels. For weak feedback level, both of the intensities of the two modes are sinusoidally modulated by external cavity length
with a feedback period of half laser wavelength. While these two modes can only coexist in a part of feedback period at moderate
feedback level, i.e., one mode reaches its maximum intensity, the other one extinguishes. At high feedback level, complete
polarization switching (one mode oscillating, the other one extinguishing) between two polarized modes is observed, and every
switching corresponds to quarter-wavelength variation of external cavity length. Furthermore, two feedback schemes, i.e.,
isotropic optical feedback (IOF) and polarized optical feedback (POF) are taken into account. The theoretical analysis based
on a model of two-mode-coupled laser is presented, which is in good agreement with experimental results. Our results can advance
the research of self-mixing interferometer of orthogonally polarized dual-frequency lasers. 相似文献
2.
We investigate the switching between two surface plasmon polariton modes in a non-Hermitian system composed of a pair of graphene sheets using topological operations. The topological operation is implemented by suitably designing the waveguide geometry and the chemical potential of graphene, which is equivalent to changing the system parameters along a closed loop in the parameter space. Efficient and robust mode switching takes place as the loop encloses an exceptional point and the wave experiences a sufficiently large propagation length. Moreover, we show this mode switching is chiral, in the sense that the output modes are different for choosing different loop directions. The study provides a promising approach to robust mode switching on a deep-subwavelength scale. 相似文献
3.
Polarization switching in a quasi-isotropic microchip Nd:YAG laser induced by optical feedback 下载免费PDF全文
This paper demonstrates the influence of external optical
feedback on the polarization state of longitudinal modes in
quasi-isotropic microchip Nd:YAG lasers. Under optical feedback, the
polarization state of longitudinal modes in quasi-isotropic lasers
relies strongly on the intracavity anisotropy loss and mode
competition. When the intracavity anisotropy loss is small, external
optical feedback can cause polarization switching and strong mode
competition between two orthogonal linearly polarized eigenstates of
one laser longitudinal mode, which leads to the distortion of laser
intensity modulation waveform. The polarization switching is
independent of the initial external cavity length. By increasing the
intracavity anisotropy loss, one polarization eigenstate can be
suppressed and the laser works in single-polarization state. A
theoretical analysis based on the compound cavity model is
presented, which is in good agreement with the experimental results.
The results offer guidance to the development of laser feedback
interferometers. 相似文献
4.
Abel Faure-Beaulieu Thomas Indlekofer James R. Dawson Nicolas Noiray 《Proceedings of the Combustion Institute》2021,38(4):5943-5951
Annular combustion chambers of gas turbines and aircraft engines are subject to unstable azimuthal thermoacoustic modes leading to high amplitude acoustic waves propagating in the azimuthal direction. For certain operating conditions, the propagating direction of the wave switches randomly. The strong turbulent noise prevailing in gas turbine combustors is a source of random excitation for the thermoacoustic modes and can be the cause of these switching events. A low-order model is proposed to describe qualitatively this property of the dynamics of thermoacoustic azimuthal modes. This model is based on the acoustic wave equation with a destabilizing thermoacoustic source term to account for the flame’s response and a stochastic term to account for the turbulent combustion noise. Slow-flow averaging is applied to describe the modal dynamics on times scales that are slower than the acoustic pulsation. Under certain conditions, the model reduces formally to a Fokker-Planck equation describing a stochastic diffusion process in a potential landscape with two symmetric wells: One well corresponds to a mode propagating in the clockwise direction, the other well corresponds to a mode propagating in the anticlockwise direction. When the level of turbulent noise is sufficient, the stochastic force makes the mode jump from one well to the other at random times, reproducing the phenomenon of direction switching. Experiments were conducted on a laboratory scale annular combustor featuring 12 hydrogen-methan flames. System identification techniques were used to fit the model on the experimental data, allowing to extract the potential shape and the intensity of the stochastic excitation. The statistical predictions obtained from the Fokker–Planck equation on the mode’s behaviour and the direction switching time are in good agreement with the experiments. 相似文献
5.
基于圆波导TE11模的模式简并特性和微波在椭圆波导中传输两个正交TE11模式相速不同的性质,研制了一种带有椭圆波导结构的圆波导TE11模圆极化器。该圆极化器通过圆波导到椭圆波导的过渡段,将输入的线极化TE11模式分成两个等幅、正交的TE11模,然后调整椭圆波导长度,使得两个正交的TE11模式的相位差为90°,实现了TE11模式微波线极化到圆极化的转换。利用时域有限差分软件优化设计了该圆极化器,并按照优化的结构尺寸加工了一套实验装置进行了实验测试,测试结果表明:在工作频率9~10 GHz范围内,该圆极化器轴比小于1 dB,驻波比小于1.1,且功率容量大于1.6 GW。 相似文献
6.
7.
Strong optical feedback in a birefringent dual frequency He--Ne laser with a
high reflectivity feedback mirror has been investigated for the first time.
The output characteristics of two orthogonally polarized modes are
demonstrated in two different optical feedback cases: one is for both modes
being fed back and the other is for only one of the modes being fed back.
Strong mode competition can be observed between the two modes with strong
optical feedback. And when one mode's intensity is near its maximum, the
other mode is nearly extinguished. When both modes are fed back into the
laser cavity, the mode competition is stronger than when only one mode is
fed back. The difference in initial intensity between the two orthogonally
polarized modes plays an important role in the mode competition, which has
been experimentally and theoretically demonstrated. 相似文献
8.
The concept of adiabatic mode transformation between silicon waveguide and surface plasmon-polariton modes in subwavelength metal-dielectric slots is investigated. The mode transformer consists of a modulated-index slot region, which is bound by two metal slabs. Using the design scheme, we will show that the optical dispersion of a modulated-index metal slot waveguide can be engineered well above the silicon light-line on a large wavelength region, allowing direct phase-matching between surface plasmon polaritons and traditional waveguide modes. Based on full-field, finite difference simulations, we demonstrate that reversible mode conversion can be achieved within submicrometer length scales. 相似文献
9.
Mode characteristics of nested eccentric waveguides constructed by two cylindrical nanowires coated with graphene 下载免费PDF全文
A kind of nested eccentric waveguide constructed with two cylindrical nanowires coated with graphene was designed. The mode characteristics of this waveguide were studied using the multipole method. It was found that the three lowest modes (mode 0, mode 1 and mode 2) can be combined by the zero-order mode or/and the first-order modes of two single nanowires. Mode 0 has a higher figure of merit and the best performance among these modes within the parameter range of interest. The mode characteristics can be adjusted by changing the parameters of the waveguide. For example, the propagation length will be increased when the operating wavelength, the minimum spacing between the inner and outer cylinders, the inner cylinder radius and the Fermi energy are increased. However, when the outer cylinder radius, the dielectric constants of region I, or the dielectric constants of region III are increased, the opposite effect can be seen. These results are consistent with the results obtained using the finite element method (FEM). The waveguide structure designed in this paper is easy to fabricate and can be applied to the field of micro/nano sensing. 相似文献
10.
Falin Liu 《International Journal of Infrared and Millimeter Waves》1998,19(8):1103-1112
Based on the mode matching method, the generalized scattering parameters including the fundamental and higher order modes (multi-modes) are obtained for the discontinuity interface of a category of waveguide H-plane discontinuities. Then by using a general port-connection order-decrease method raised by the author, the whole discontinuity including the two interfaces and the intermediate part of finite length or zero length (as is the case for very thin inductive windows or irises) can be combined into one network, and the generalized scattering parameters of this network can be obtained from which the scattering parameters of the fundamental mode can be extracted. If needed, the equivalent circuit parameters can be calculated directly from the scattering parameters. Examples are given to show the validity and versatility of this new method for dielectric filled waveguide, inductive iris and/or window, E-plane metal and/or finline insert, and even the offset E-plane finline insert, etc.. Multiple inserts, windows and irises can also be analysed. 相似文献
11.
提出了一种新型同轴插板式模式变换器,可以实现同轴TEM到圆波导TE11模式的变换。介绍了这种模式变换器的工作原理:即通过在同轴波导中沿轴向插入金属板,将同轴TEM模变换成扇形截面波导TE11模,进而利用不同扇形截面波导中的相移改变电场分布的轴对称性,在同轴波导中形成同轴TE11模,最后将同轴TE11模转换为圆波导TE11模式。基于这一原理设计了一个中心频率为3.8GHz的同轴TEM-圆波导TE11模式变换器,并进行了数值模拟。模拟结果表明:这种模式变换器可以承受高功率,中心频率上转换效率为98.5%,转换效率大于90%的带宽超过10%,在3.5~4.1GHz的频率范围内反射损耗低于0.3dB。 相似文献
12.
13.
目前国际上提出了采用短脉冲射频(RF)信号实现相对论磁控管的模式切换,数值模拟经证实了可以采用几十kW到几百kW的RF信号实现相对论磁控管采用轴向提取功率的相邻模式以及同一模式的不同纵向模式之间的切换,这里假设所需的RF信号的能量已经馈入到相对论磁控管腔体内。提出了实验系统中采用扇形波导和探针天线来馈入前级微波源来提供模式切换所需的RF信号的能量的方法。该方法分为两个步骤,首先采用扇形波导来将前级微波源提供的能量馈入到相对论磁控管的阳极体中;然后利用探针天线将馈入的RF信号辐射至相对论磁控管腔体内,提供模式切换所需的能量。数值模拟证实了该方法在实际应用中具有可行性以及实用性。 相似文献
14.
A. W. M?bius J. A. Casey K. E. Kreischer A. Li R. J. Temkin 《International Journal of Infrared and Millimeter Waves》1992,13(8):1033-1063
Results are reported of a theoretical and experimental investigation of a quasi-optical mode converter for the transformation of whispering gallery mode gyrotron output into a linearly polarized Gaussian like beam. The mode converter consists of a helically cut waveguide launcher, similar to that originally proposed by Vlasovet al, followed by a focusing mirror. Theoretical results using aperture field methods indicate that the length of the waveguide launcher is of critical importance in providing a confined radiation pattern. Experimental results on the radiation pattern were obtained for several launcher lengths using a 0.6 MW, 149 GHz pulsed gyrotron operating in the TE16,2 mode. Radiation pattern results for the optimum launcher length agree well with theoretical calculations using the Stratton-Chu aperture radiation theory for unperturbed waveguide modes. A mirror focusing in the azimuthal direction was designed by a geometrical optics approach to focus the radiation coming from the launcher. Good focusing with 91.4% efficiency (power in the focused beam divided by gyrotron power) was found experimentally using the combined launcher and mirror with the pulsed gyrotron. These results indicate that quasi-optical antennas are useful for transforming high order, high frequency gyrotron modes into directed beams in free space. 相似文献
15.
We propose nondegenerate four-wave mixing mirrorless oscillation in a multimode silicon nonlinear waveguide. Thanks to the large modal dispersion between two spatial modes caused by the high-index-contrast waveguide structure, two counterpropagating pumps of one spatial mode can generate two new optical waves of the other spatial mode at different frequencies. The phase-matching condition can be satisfied with the higher-order modes involved; therefore, frequencies of the newly generated light can be tuned by simply changing the pump frequency. The threshold power and conversion efficiency of the proposed mirrorless oscillation are investigated under different waveguide parameters. 相似文献
16.
The effects of the nonlinear interplay between modes of a lithium niobate channel waveguide are numerically investigated. Based on the nonlinear phase-shift obtainable via up- and down-conversion of each mode at the input frequency into and from the second-harmonic modes generated through Type I and Type II interactions, the waveguide can operate as an all-optical transistor with large small-signal-gain, and as a mode-mixing device capable of spatial switching and routing. It exhibits good switching contrast at 1.55 m with low driving powers. 相似文献
17.
Two mode-evolution-based low-loss compact-size integrated polarization rotators and transformers are presented. They are designed to fulfill the tasks of rotating mode polarizations between Gaussian-like strip waveguide modes and non-Gaussian-like slotted waveguide modes, as well as the mode transforming between horizontal and vertical slotted waveguides. Consisting of several adiabatic tapers, the presented devices can achieve complete polarization rotations and transformations between different classes of waveguide modes with a device length of tens of micrometers and with a negligible insertion loss. 相似文献
18.
结合理论和实验研究了掩埋光栅一级分布反馈太赫兹量子级联激光器中的模式竞争和功率特性。理论计算得到掩埋光栅腐蚀深度与两个带边模式的波导损耗、光学限制因子、辐射损耗以及辐射效率的关系。理论计算表明,掩埋光栅分布反馈结构可以通过改变腐蚀深度,保证激光器稳定单模工作在高频带边模式的同时,调节激光器的阈值增益以及辐射效率。实验和测试结果表明,激光器辐射波长和掩埋光栅的周期成正比,激光器可以在整个动力学范围内稳定单模工作。单模激光器的波长范围可覆盖86.2~91.7μm的范围,边模抑制比可达25 dB,最大输出功率为9.1 mW。该工作有助于高性能单模太赫兹激光器及锁相耦合激光器阵列的研制。 相似文献
19.
The non-local optical interaction of two semiconductor microdisks with a waveguide bridged at radial direction is proposed and studied by three dimensional finite-difference time-domain (FDTD) electromagnetic simulations. The strong and weak optical interactions between two microdisks are observed and ascribed to the internal coupled modes with different coupling ratios. The vertical radiation losses and the related mode quality factors are modulated by waveguide length and present oscillation characteristics for the resonant modes. In addition, the optical leakage of coupling system is affected by the etching depth of disks due to tile emission of minor components of electric field. 相似文献
20.
Cai LW Dacol DK Calvo DC Orris GJ 《The Journal of the Acoustical Society of America》2007,122(3):1340
Multiple scattering of acoustic waves in a planar horizontal waveguide by finite-length cylinders is considered. Cylinder height equals the waveguide depth, and both are vertically constrained by the pressure-release boundaries. An analytically exact solution is obtained via normal mode expansion method in conjunction with the concept of the T matrix. The problem is decomposed into an infinite number of two-dimensional multiple scattering problems, modulated by waveguide mode shapes. Examples are presented for an isovelocity waveguide in which the medium is uniform and the waveguide depth is constant. It is found that, in numerical computations, including one or two evanescent modes captures the essence of the evanescent modes. Multiple scattering in the waveguide is compared with the corresponding two-dimensional case. It is concluded that, in low frequencies, the wave patterns in the two cases are very similar, with a shift in the frequency. The similarity diminishes when there are multiple propagating modes. Despite the mode mixing, some key features in the scattering as observed in the two-dimensional problem remain observable in the waveguide case. 相似文献