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Near-Field Acoustic Microscopy
Published in J. David, N. Cheeke, Fundamentals and Applications of Ultrasonic Waves, 2017
Rabe presents interesting experimental observations using a laser interferometer of the higher bending modes of a silicon cantilever up to n = 13 for 19 cantilevers from two different wafers (Figure 18.5). Deviations are observed, most of which could be explained by a more detailed model. In some cases, deviations were observed, which could be attributed to multiple peaks caused by mode coupling. So, there are many refinements, which would need to be considered to get systematic quantitative agreement, but the basic picture of the model seems to be confirmed by these results.
Modes and Mode Conversion in Microwave Devices
Published in R A Cairns, A D R Phelps, P Osborne, Generation and Application of High Power Microwaves, 2020
Perturbation of the waveguide geometry leads to mode coupling. Depending on the perturbation amplitude ϵ • a (Figure 2.1), where a is the radius of the unperturbed waveguide, and the characteristic perturbation length Ls, different numerical methods of CAD for waveguide mode converters are appropriate (Thumm 1986): ϵ ≪ 1 and Ls ≫ λPerturbation Method:Numerical integration of the coupled-wave equations(generalized telegrapher’s equations).ϵ ≈ 1 and Ls < λ (axisymmetric perturbations)Scattering Matrix Method:Mode matching employing the decomposition of the perturbed waveguide in short homogeneous sections. Inclusion of cutoff-modes (evanescent modes).ϵ ≈ 1 and Ls < λ (non-axisymmetric perturbations)Finite Element (FE) - Finite Difference (FD) - and Finite Integration Method: These methods, though universal, can be used in practice only to design moderately oversized mode converters.
Numerical investigation on the wave dissipating performance due to multiple porous structures
Published in ISH Journal of Hydraulic Engineering, 2021
where, and for and are the unknown parameters to be determined. The eigenfunctions in open water region and porous structure region is expressed in the form of for are same as defined in Equation (10) and the method of solution used in the analysis of a multiple porous structures placed upon step-type seabed in the presence and absence of the leeward wall is same as presented in Section 3.1. The no-flow condition due to rigid step, continuity of dynamic pressure and velocity across the vertical interface is applied along with mode-coupling relation to obtain the linear system of equations. The infinite series sums in the algebraic equations are truncated upto finite terms and wave reflection coefficient and transmission coefficient due to the multiple porous blocks kept on the stepped rigid bottom is obtained as described in Equation 19(a).
Optimizing the pump wavelength to improve the transverse mode instability threshold of fiber laser by 3.45 times
Published in Journal of Modern Optics, 2021
Yingchao Wan, Baolai Yang, Peng Wang, Xiaoming Xi, Hanwei Zhang, Xiaolin Wang
Fiber lasers are widely used in industrial processing, biological medical treatment, fundamental research and other fields due to their excellent beam quality, flexible operation, convenient thermal management, compact structure and other advantages [1–3]. For the past two decades, with the progress of the high brightness pump source and the technological level of various fiber devices, the power of fiber lasers has been rapidly improved [4–6]. However, transverse mode instability (TMI) in fiber lasers has been recently found to present a serious limiting for the power scaling in a good-quality laser beam [7]. TMI refers to the phenomenon that the output laser mode changes abruptly when the laser power exceeds a certain threshold value and causes beam quality degradation [8]. It is generally believed that the TMI is the mode coupling caused by the thermally induced refractive index grating caused by the quantum defect and other thermal effects in the laser conversion process, which is also a kind of TMI phenomenon that has been widely studied at present [9–11]. When TMI appears, the fiber laser will come out the decrease of optical-to-optical (O-O) conversion efficiency, the abnormal temperature increase of the cladding stripper and the fluctuation in the time–frequency domain, which may destroy the normal operation of the laser.
Adiabatic description of superfocusing of femtosecond plasmon polaritons
Published in Journal of Modern Optics, 2018
P. A. Golovinski, E. S. Manuylovich, V. A. Astapenko
For adiabatic propagation, the local taper length scale should be larger than the coupling length of corresponding modes (22). The local taper length scale is the height of the circular cone, and the coupling length of two modes is taken to be the beat length. It was shown (23) that the adiabaticity requirement is easily satisfied for small taper angles. The divergence of the effective refractive index with decreasing cone radius experienced by a SPP propagating towards the apex results in continuous transformation of cylindrical modes and thus in near-adiabatic SPP nanofocusing into the apex of the tip (24). Metal dissipation is approximately included in equations using the complex propagation constant. This is conceptually equivalent to subdividing each thin section of the tapered wire into a cylindrical absorption section, in which there is dissipation, but no mode coupling. Polarization anisotropy of apex-emitted light demonstrates the dependence expected of a point dipole at the apex (25).