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We proposed a new type of transverse electric (TE) polarized mode-order converter based on a deeply-etched polygonal slot on a silicon-on-insulator waveguide. Along the transverse direction of the waveguide, two irregular boundary surfaces of the slot can introduce high-contrast index modulation on guided modes, leading to multimode interference in the slot. Therefore, when the slot is optimized, we can achieve efficient mode conversions based on the multimode interference. As examples, mode converters from the fundamental TE mode (TE0) to the first-order TE mode (TE1) and to the second-order TE mode (TE2) have both been demonstrated with a short device length (97.6%), and a low modal crosstalk (

We have studied the transmission properties of odd/even one-way modes and their reversible conversion in a double-channel waveguide consisting of two magneto-optical photonic crystals (MOPCs) sandwiched with Al2O3 PC. There exist two pairs of even and odd modes, i.e., M1(even)/M2(odd) or M3(odd)/M4(even) modes, for the double-channel waveguides with one- or two-stranded coupling layer of Al2O3 rods, respectively. Among them, the M1, M2, and M3 modes are caused by the weak coupling strength of two sub-waveguides, while the M4 mode results from the strong coupling effect and supports dispersionless slow-light propagation. Furthermore, we realize the reversible conversion between odd and even modes (i.e., between M1 and M2 modes, or M3 and M4 modes) in the one- or two-stranded structure, respectively, by adjusting the length and position of the perfect electric conductor (PEC) defect properly to cause the desired significant phase delay along the upper and lower equivalent transmission paths. Additionally, we find that the robustness of the M1 even mode is poor because of extra excitations of counter-propagation modes near the right Brillouin boundary, while the other three modes have extremely strong robustness against PEC defects and their one-way transmittances are nearly 100%. These results hold promise for many fields, such as slow-light modulation and the design of topological devices.

A compact and robust in-fiber acoustically-induced Mach-Zehnder interferometer (AI-MZI) was demonstrated based on the phase mismatch in an acoustic wave driving fiber Bragg grating (FBG). The fiber structure of the AI-MZI was easy to fabricate and of a uniform diameter with larger refractive index difference in its arm part, which helped to reduce the length of the arm. In the experiment, the total length of the AI-MZI was less than 7 cm and the response time was about 120  μs. The bandpass isolation and resonant center wavelength could be optimized by changing the power and frequency of the radio frequency source, respectively. As its application, a tunable and individually switchable dual-wavelength fiber ring laser was realized and characterized. The proposed structure gains advantages of robustness, easy fabrication, compact size and fast tuning speed accordingly. At the same time, it provides another practical way to use an FBG to introduce phase mismatch into an acousto-optic fiber grating. [ABSTRACT FROM AUTHOR]

We present intra-fibre mode conversion from the antisymmetric LP11 mode to the circularly symmetric LP02 mode using a force-induced, long-period fibre grating accompanied by an in-line polarization controller. The field orientation of the incoming LP11 mode is aligned to the antisymmetric perturbation in the fibre grating to activate resonant mode coupling. Mode conversion is achieved by applying pressure with a spatial period of 430 μm along an interaction length of 50 mm. We verify the mode conversion process by observing the far-field patterns of the fibre output and comparing the transmission power with and without a high-numerical aperture, narrow-core fibre. The intra-fibre mode conversion system with erasable and reconfigurable natures exhibits an insertion loss of 0.48 dB and a 3-dB bandwidth of 52 nm. [ABSTRACT FROM AUTHOR]

In this study, a broadband, low-loss, 4-way waveguide power combiner operating in the W-band is designed and fabricated. The electromagnetic fields in the input waveguide are rotated by 90 degrees and distributed to the output waveguides by using the mode converter. In addition, four output waveguides are placed symmetrically along the input waveguide to minimize the imbalances in the insertion loss and phase shifts. It achieves low insertion loss and high power-handling capability, because it consists of metals without dielectric substrates or absorbers. The fabricated power combiner shows excellent performance, with an insertion loss of less than 0.99 dB and a return loss of more than 14.0 dB across the full W-band. [ABSTRACT FROM AUTHOR]

An optical switch based on silicon-on-insulator (SOI) technology is proposed that works in the C-band and switches by amorphous (Am) to crystalline (Cr) and Cr-to-Am phase transitions. The optical switch integrates the functions of polarization beam splitting and mode conversion, and consists of two asymmetric directional couplers (ADCs). The TM0 mode is converted to the TM1 mode through an asymmetric coupler to achieve the polarization splitting of the TM0 mode and TE0 mode. The output of the TE0 mode is then controlled by Ge2Sb2Se4Te1 (GSST). When the TE0 mode is input and the wavelength is 1550 nm, the insertion loss (IL) is lower than 0.62 dB and the crosstalk (CT) is lower than −9.88 dB for a directional coupler loaded with GSST that realizes the optical switch function in both amorphous and crystalline GSST. The extinction ratio (ER) of the two waveguides of the directional coupler is lower than −11.40 dB, simultaneously. When the TM0 mode is input and the wavelength is 1550 nm, the IL is lower than 0.62 dB for a directional coupler loaded without GSST.

A self-consistent system of equations (known as single-mode gyrotron equations) is extended to describe the beam-wave interaction in a cylindrical gyrotron cavity with mode-converting longitudinal corrugations, which produce coupling of azimuthal basis modes. The system of equations is applied to investigate the effect of corrugations on starting currents of the cavity modes. For these modes, eigenvalues, ohmic losses, field structure, and beam-wave coupling coefficients are investigated with respect to the corrugation parameters. It is shown that properly sized mode-converting corrugations are capable of improving the selectivity properties of cylindrical cavities for second-harmonic gyrotrons. [ABSTRACT FROM AUTHOR]

In this article, a novel waveguide-to-microstrip transition with a built-in dc/IF return path is proposed, in which a traditional E- plane probe is directly connected to the backshort by a thin metal wire along the axis of the waveguide. The proposed waveguide-to-microstrip transition implements an additional dc/IF return path while realizing mode conversion. Moreover, the presence of the dc/IF return path has little influence on the performance of the original E- plane probe. The operating principle is analyzed by theoretical analysis and full-wave simulation, and the design procedure of the transition is also presented. To implement the proposed dc/IF return path on different substrates, two approaches, including direct integration with planar circuits or with wire bonding technology, are developed. Furthermore, two back-to-back transition prototypes working in the millimeter and submillimeter bands were designed and characterized. The measured results are in excellent agreement with the simulations. Finally, as an application example, a 70-/140-GHz unbalanced frequency doubler adopting this structure for both dc biasing and dc grounding was designed and fabricated. Measured results show that the conversion loss of the fabricated doubler is less than 13 dB in the range from 125 to 145 GHz, and the minimum conversion loss is 11 dB at 140 GHz with 100-mW input power. [ABSTRACT FROM AUTHOR]

This article presents the results of theoretical and experimental investigations of characteristic changes of Lamb wave excitation and scattering by a strip-like horizontal delamination in a layered elastic waveguide for Lamb waves induced by a piezoelectric wafer active sensor. The boundary integral equation method is used to describe wave propagation in an infinite layered waveguide with a delamination, while the frequency domain spectral element method is employed to model the dynamic behaviour of the piezoelectric wafer active sensor, which allows to simulate debonding between the piezoelectric wafer active sensor and the waveguide. Experimental investigations of the dynamic interaction of the piezoelectric wafer active sensor with a layered plate containing a horizontal delamination is conducted for several damage scenarios, showing a good agreement with the results obtained using the developed mathematical model. The obtained mathematical model is employed to analyse alteration of the piezo-induced Lamb waves including modes' decomposition due to delamination. The conversion and/or conservation of the Lamb waves on account of a delamination is investigated. The electro-mechanical impedance of the piezoelectric transducer and the stress intensity factors of a delamination are analysed in dependence on the delamination location. [ABSTRACT FROM AUTHOR]

Metamaterials are a class of man-made composites that can realize extraordinary wave phenomena and can be designed through exploiting anisotropy of elastic media. Anisotropy, as is known, is also an inherent characteristic of classical composites. Therefore, there is a great potential that the anisotropy of classical composites can be utilized to achieve extraordinary manipulation of elastic waves. To this end, we propose a concept of fiber-reinforced composite metamaterials, which attains extraordinary wave properties with rationally designed ply angles. Specifically, we consider the phenomenon of transmodal Fabryâ€"Perot interference. This phenomenon, which can transfer wave energy between longitudinal and transverse modes solely and maximally, requires that the interferometer should attain unique anisotropy. Based on the transmodal interference theory and the classical lamination theory, the composite metamaterials are designed and validated numerically and experimentally. We note that the base material properties of each layer can significantly affect the metamaterial design, and an anisotropy index is introduced to investigate the effects. Our work broadens the applications of classical composites and offers a new way to design metamaterials. [ABSTRACT FROM AUTHOR]

Abstract We investigate the propagation characteristics of low‐altitude hiss in the ionosphere by numerical simulation with a two‐dimensional full‐wave model. The simulation results demonstrate that linear mode conversion from whistler to H+ band electromagnetic ion cyclotron wave and polarization reversal occur simultaneously where wave frequency matches the H+−He+ crossover frequency. This mode conversion efficiency shows sensitive dependence on wave normal angle and plays a significant role in the propagation of whistler emission near the local proton gyro‐frequency in the ionosphere by redistributing the wave energy below and above the H+−He+ cutoff frequency, which can explain the low‐altitude hiss observed by the Freja and Detection of Electromagnetic Emissions Transmitted from Earthquake Regions satellites, respectively. The energy of whistler‐mode low‐altitude hiss emission can be transferred to reflected left‐hand polarized electromagnetic ion cyclotron through mode conversion and the efficiency reaches a maximum for intermediate incident wave normal angle (of 45°).

Aiming at single mode of Underpan generator power supply, and the feature of being not meet the regular work of system under special working conditions, engine fuel exhaustion, unexpected faults, Design of Dual Mode Power Supply Distribution Circuit With Detection And Control Functions which includes Battery power, power distribution module, anti-diode, generator detection module, underpan on-off control module, Battery anti-loss detection module was proposed .The tests show that the generator power supply mode and battery power supply mode can convert freely, and the Supply Distribution Circuit is stable and reliable.

We propose and numerically demonstrate a design of bidirectional transverse electric (TE) polarized mode-order converter based on silicon-on-insulator platform. This converter is realized by introducing high refractive index material inlaid in a silicon slab waveguide. Simulated by three-dimensional finite-difference method time-domain, the forward (TE0 to TE1-like conversion) transmittance reaches approximately 88.2%, while the backward value (TE1 to TE0-like conversion) is about 89.4% at the wavelength of 1550 nm. The footprint of this converter is as small as 0.95 × 1.5 μm2. Fabrication tolerance analysis demonstrates satisfactory robustness. Moreover, we present a polarization-independent converter with slightly modified geometry. The transmittance keeps above 87.2% within the wavelength range from 1500 nm to 1600 nm for both TE and transverse magnetic modes. These devices are expected to contribute to the on-chip mode division multiplexing.

We have demonstrated direct generation of high-order orbital angular momentum (OAM) modes by chiral long-period gratings (CLPGs) written in a few mode fiber (FMF) via a CO2 laser. The high-order core modes (LP11, LP21, and LP31 modes) were realized by the CLPGs through high order diffraction modes with efficiencies of more than 99.69%. The -1st, −2nd, and −3rd order OAM modes can be generated directly by the CLPGs with various grating periods. The proposed method opens a new path to implement all fiber high-order OAM mode generation. The OAM mode converters can be adopted as wavelength tunable mode converters by adjusting twist rate of the CLPGs, which could be widely used in optical communication and fiber sensing system based on FMFs. [ABSTRACT FROM AUTHOR]

Featured Application: On-chip optical reading of magnetic memory processed as ultrathin magnetic claddings on photonic waveguides. Design and simulations of an integrated photonic device that can optically detect the magnetization direction of its ultra-thin (∼12 nm) metal cladding, thus 'reading' the stored magnetic memory, are presented. The device is an unbalanced Mach Zehnder Interferometer (MZI) based on InP Membrane on Silicon (IMOS) platform. The MZI consists of a ferromagnetic thin-film cladding and a delay line in one branch, and a polarization converter in the other. It quantitatively measures the non-reciprocal phase shift caused by the Magneto-Optic Kerr Effect in the guided mode which depends on the memory bit's magnetization direction. The current design is an analytical tool for research exploration of all-optical magnetic memory reading. It has been shown that the device is able to read a nanoscale memory bit (400 × 50 × 12 nm) by using a Kerr rotation as small as 0.2 ∘ , in the presence of a noise ∼10 dB in terms of signal-to-noise ratio. The device is shown to tolerate performance reductions that can arise during the fabrication. [ABSTRACT FROM AUTHOR]

We experimentally obtain a lower frequency shift based on mode conversion by using a mode-selective coupler (MSC) and an acoustically induced fiber grating (AIFG) for optical heterodyne micro-vibration measurement. Through efficient conversion between LP11 and LP01 core modes in a few-mode fiber, a mode conversion frequency shifter (MCFS) can be achieved and the magnitude of frequency shift is several hundred kHz, which is generally challenging for traditional bulk acousto-optic frequency shifter. Moreover, the signal-to-signal beating interference (SSBI) can be eliminated by using the MCFS without requirement of guard band. The scheme of optical heterodyne micro-vibration measurement based on the MCFS has a high-quality signal-to-noise ratio (SNR) over 85 dB. An all-fiber Bragg grating (FBG) scheme is also designed to improve the measurement sensitivity of heterodyne detection, which has a measurable vibration frequency range of 1 Hz to 300 kHz and minimal detectable amplitude of 0.019 nm. The all-fiber MCFS has the advantages of compactness, easy integration and can be applied directly to satellite ground station and laser feedback tomography. [ABSTRACT FROM AUTHOR]

The constantly growing demand for cost-intensive lightweight structures is accompanied by an increased demand for reliable non-destructive testing (NDT) methods. Previous investigations have shown that Lamb Waves (LW) are very well suited for the use in Structural Health Monitoring (SHM) due to their relatively undamped propagation over long distances. Therefore the numerical modelling of a non-contact and automated system for non-destructive testing of C-FRP laminates was investigated in the present study. The method is based on the propagation of LW, which are excited by an actuator-driven piezo transducer and interact with defects in the structure. In contrast to classical Structural Health Monitoring, a scanning inspection system is used instead of a sensor permanently integrated into the structure to detect the measurement signals. For the representation of the wave propagation a three-dimensional simulation model with the commercial FEM software PZFlex was modelled. The wave interactions in defective C-FRP coupons are investigated and visualized through different output formats like A-, B- and C-Scans. Additionally the model allows the determination of the dispersion characteristics of the LW in the examined laminate. The validation of the numerical results was successfully carried out at different levels with the help of experimental investigations. [ABSTRACT FROM AUTHOR]

Efficient and reliable mode converters with broadband operation and small footprint will enable high‐density silicon photonic integrated circuits to build high bitrate optical networks and handheld optical devices. Here, all‐dielectric nanoaperture metasurfaces in the silicon layer of a silicon‐on‐insulator platform are reported for mode‐order conversion with an ultrasmall footprint. Utilizing the advantages of an abrupt width change and metasurface, the mode conversion between TE00 and TE10 modes is realized with a conversion efficiency as high as ≈90% in simulations and 83.1% in experiments at λ = 1.55 µm within an ultrashort conversion length of about 2.42 µm, the shortest ever achieved. The incident TE00 mode is converted to TE10 mode with purity of more than 90% in a broadband (Δλ ≈ 230 nm) wavelength range from 1.42 to 1.65 µm. Moreover, optimizing the number, position, and dimension of nanoapertures, it is further demonstrated that the nanoaperture metasurface design can enable mode conversion between silicon waveguides of different thicknesses and different relative positions, and between TM modes. [ABSTRACT FROM AUTHOR]

In this study, a TE01 mode converter was developed to locate an axial crack on the inner surface of a metal pipe. Three-dimensional finite element simulation was conducted to evaluate the effect of inserted coaxial cables on the transmission characteristic of the mode converter. The result showed that the energy of TE mode microwaves leaned to transmit to one side when the cables penetrated with inclination. To demonstrate the effectiveness of the mode converter, experimental verification was conducted. The mode converter was fabricated based on the simulation result. Microwaves were emitted through the pipe wall of the converter to propagate on both sides ('right side' and 'left side') of a pipe with an artificial slit. Compared with the signals from each side, the reflection from the slit on the right side was more significant than that on the left side. This result is consistent with the numerical simulation result. [ABSTRACT FROM AUTHOR]

The mode conversion process responsible for radio wave generation has been studied for several decades; however, the properties of the condition required for an efficient conversion process are still unknown. The aim of this study is to determine the value of plasma frequency required for an efficient mode conversion process from slow Z-mode to left-hand ordinary (LO)-mode waves in the matching cases, where the two branches of the dispersion relation of the two modes are perfectly connected. We derive the dispersion relations for electromagnetic wave propagation in an inhomogeneous plasma considering Snell's law and investigated them in detail. We quantify the minimum variation of plasma frequency required for the efficient mode conversion process, which we call "the plasma frequency demand." We show that the condition required for the efficient mode conversion can be satisfied by waves propagating first toward the high-density region and then returning toward the low-density region before reaching the region where the wave frequency matches the cutoff frequency; therefore, a large inhomogeneity is not always required. We show that the angle between the background magnetic field and the density gradient has a significant effect on the plasma frequency demand. [ABSTRACT FROM AUTHOR]

Wave motion in pipe bends is much more complicated than that in straight pipes, thereby changing considerably the propagation characteristics of guided waves in pipes with bends. Therefore, a better understanding of how guided waves propagate in pipe bends is essential for inspecting pipelines with bends. The interaction between a pipe bend and the most used non-dispersive torsional mode at low frequency in a small-bore pipe is studied in this paper. Experiments are conducted on a magnetostrictive system, and it is observed that T(0,1) bend reflections and mode conversions from T(0,1) to F(1,1) and F(2,1) occur in the pipe bend. The magnitude of the T(0,1) bend reflections increases with increasing propagation distance and excitation frequency. The amplitude of the mode-converted signals also increases with increasing propagation distance, but it decreases with increasing excitation frequency. Because of their longer bent path, the test signals for a pipe bend with a bending angle of 180X are much more complicated than those for one with a bending angle of 90X. Therefore, it is even more difficult to scan a bent pipe with a large bending angle. The present findings provide some insights into how guided waves behave in pipe bends, and they generalize the application of guided-wave inspection in pipelines. [ABSTRACT FROM AUTHOR]

The mode-converted (longitudinal to transverse, L-T) ultrasonic scattering method was utilized to characterize the structural anisotropy of a phantom mimicking the structural properties of trabecular bone. The sample was fabricated using metal additive manufacturing from high-resolution computed tomography (CT) images of a sample of trabecular horse bone with strong anisotropy. Two focused transducers were used to perform the L-T ultrasonic measurements. A normal incidence transducer was used to transmit longitudinal ultrasonic waves into the sample, while the scattered transverse signals were received by an oblique incidence transducer. At multiple locations on the sample, four L-T measurements were performed by collecting ultrasonic scattering from four directions. The amplitude of the root mean square (rms) of the collected ultrasonic scattering signals was calculated for each L-T measurement. The ratios of rms amplitudes for L-T measurements in different directions were calculated to characterize the anisotropy of sample. The results show that the amplitude of L-T converted scattering is highly dependent on the direction of microstructural anisotropy. A strong anisotropy of the microstructure was observed, which coincides with simulation results previously published on the same structure as well as with the anisotropy estimated from the CT images. These results suggest the potential of mode-converted ultrasonic scattering methods to assess the anisotropy of materials with porous, complex structures, including trabecular bone. [ABSTRACT FROM AUTHOR]

In this letter, a comparative analysis is carried out between the mechanism of mode conversion in differential microstrip lines due to bend discontinuities on one side and trace asymmetry on the other side. With the help of equivalent modal circuits, a theoretical basis is provided for the idea to compensate the undesired common mode (CM), due to the presence of the bend, by intentionally designing asymmetric traces. As an application example, the proposed CM-reduction strategy is used in conjunction with another recently-presented wideband CM suppression filter for differential microstrip lines. It is shown that the proposed solution enhances the overall CM-reduction performance of the filter by some decibels, while preserving its transmission properties. [ABSTRACT FROM AUTHOR]

A Ku/Ka-band oversized waveguide bend, transmitting the power of TE01 mode at Ka-band and TE11 mode at Ku-band, is designed for the high-power transmission line. To suppress the generation of parasitic modes, a waveguide bend based on the elliptical waveguide is used to minimize the coupling capacity between the operating modes and the undesired modes. The mode coupling rules inside the elliptical waveguide bend are investigated. The optimized parameters, including the orientation of bend plane, the bend radius, and the eccentricity are chosen according to the analysis results. The calculation results show that the transmission efficiency of the TE01 mode is over 95% from 26 to 32 GHz, while the transmission efficiency of the TE11 mode is over 98.5% from 10 to 20 GHz. To validate the design, a prototype is fabricated and measured. Compared with the calculated results, the measured transmission efficiency has a 4% and 5%–6% drop in the Ka-band and Ku-band due to ohmic loss. The high output mode purity of the waveguide bend is verified by the near-field measurements. [ABSTRACT FROM AUTHOR]

We investigate the propagation of whistler mode waves near the local lower hybrid resonance (LHR) frequency in a dipole field with a two‐dimensional full‐wave model. First, we run a simulation in which a parallel whistler with frequency above the local LHR frequency is launched at the equatorial region in electron plasma. We find that whistler emission propagates along the dipole field line to a high latitude, turns quasi‐electrostatic where the wave frequency is close to the local LHR frequency, and continues to propagate until being absorbed. Then, the proton response is considered. We find that (1) a quasi‐electrostatic whistler reflects where the wave frequency is below the local LHR frequency and propagates to a larger L‐shell and lower latitude, (2) a strong standing‐wave pattern is formed in the LHR reflection region, and (3) the whistler emission turns from right‐hand circularly polarized to linearly polarized near the reflection region. Finally, we run a simulation in which a quasi‐electrostatic whistler is launched at a high latitude with a small‐scale density irregularity added as a depletion to the background plasma. We find that a small portion of quasi‐electrostatic whistler energy can be coupled to a parallel whistler, which can propagate to a much lower altitude while most of the wave energy experiences LHR reflection. Moreover, the mode coupling depends on the transverse and longitudinal sizes of the density irregularity. This makes a possible explanation of ground observations of nonducted whistler emission, which could have been reflected in the high‐latitude ionosphere and magnetosphere. Key Points: The effect of protons in whistler lower hybrid resonance reflection is verified with two‐dimensional full‐wave simulationIn the lower hybrid resonance reflection region, whistler waves become quasi‐electrostatic, accompanied with standing wave structure and wave polarization change from circular to linearLocalized density irregularities off the equator can produce quasi‐parallel whistler waves and thus avoid magnetospheric reflection [ABSTRACT FROM AUTHOR]

Mode conversion between higher-order modes is attained in a step-index four mode fibre using an in-line polarization controller followed by a force-induced long period fibre grating. By adjusting the field orientation of the LP11 mode to the antisymmetric fibre grating, the LP11 mode is converted to the LP21 mode with a spatially periodic force of 50 N along the fibre length of 50 mm. The mode conversion is evaluated by observing the far-field and near-field patterns of the fibre output and by comparing the transmitted optical power with and without the additional two-mode fibre. The phase-matching grating period is required of the accuracy of roughly ±5 μm and the radiation loss during the mode conversion is as low as 0.01 dB. [ABSTRACT FROM AUTHOR]

Rayleigh scattering in graded-index polymer optical fibers (GI-POF) is studied under the assumption that spherical scatterers are uniformly distributed within the layers of concentric ring structure. This approximation allows to describe typical imperfections occurring during the fabrication of GI-POFs. For the purpose of experimental characterization by side illumination, an analytical model of the light conversion into waveguide modes has been developed. This model describes the mode excitation by a multiplicative contribution of three different physical mechanisms. The corresponding experimental analysis of the light scattering has been performed and compared with numerical simulation results for the fibers with single-ring and multi-ring structures. Besides the application to GI-POFs, this article can be also applied to multi-step index fibers. [ABSTRACT FROM AUTHOR]

We demonstrate the fabrication of long-period fiber gratings (LPFGs) in an elliptical core few-mode fiber (e-FMF) by CO2 laser. The mode conversion from fundamental mode to high-order modes (LP11a, LP11b, and LP21a mode) can be realized by the LPFGs with high efficiency of more than 95%. The effect of bending, torsion, temperature, and strain on the resonance wavelength of the e-FMF based LPFG mode converters are experimentally investigated. The temperature and strain sensitivities of the LPFGs are characterized to be −36 pm/°C and −4.9 pm/μϵ, and the LPFGs are found to be insensitive to the bending and torsion. Due to the existence of trench in the e-FMF, the higher-order LP02 and LP31a mode can also be stimulated by the e-FMF based LPFGs with a grating period of 515 μm and 400 μm, respectively. The proposed e-FMF based LPFGs could have a promising prospect in the application of fiber sensing and mode division multiplexing optical communication system. [ABSTRACT FROM AUTHOR]

Motivated by the unexpected appearance of shear horizontal Rayleigh surface waves, we investigate the mechanics of antiplane wave reflection and propagation in couple stress (CS) elastic materials. Surface waves arise by mode conversion at a free surface, whereby bulk travelling waves trigger inhomogeneous modes. Indeed, Rayleigh waves are perturbations of the travelling mode and stem from its reflection at grazing incidence. As is well known, they correspond to the real zeros of the Rayleigh function. Interestingly, we show that the same generating mechanism sustains a new inhomogeneous wave, corresponding to a purely imaginary zero of the Rayleigh function. This wave emerges from 'reflection' of a bulk standing mode: This produces a new type of Rayleigh-like wave that travels away from, as opposed to along, the free surface, with a speed lower than that of bulk shear waves. Besides, a third complex zero of the Rayleigh function may exist, which represents waves attenuating/exploding both along and away from the surface. Since none of these zeros correspond to leaky waves, a new classification of the Rayleigh zeros is proposed. Furthermore, we extend to CS elasticity Mindlin's boundary conditions, by which partial waves are identified, whose interference lends Rayleigh-Lamb guided waves. Finally, asymptotic analysis in the thin-plate limit provides equivalent one-dimensional models. [ABSTRACT FROM AUTHOR]

We propose a compact silicon-on-insulator based device which is capable of arbitrarily converting optical modes TE $_\text{i}$ to TE $_\text{j}$ (i, j = 0, 1, 2, 3). The device consists of two symmetric 1 × 4 Y-junction couplers connected back-to-back with a 4 × 4 multimode interference coupler (MMI). While the two 1 × 4 Y-junction couplers are designed to divide and combine modes at the input and output of the device, the 4 × 4 MMI coupler is used to direct signals from its inputs to suitable outputs. The conversion of a pair of modes is decided by four phase shifters located at the input and output of the MMI coupler. Through beam propagation method (BPM) with effective index method (EIM) simulations, we show that by setting the phase shifters to 0° or 180°, 16 different conversion pairs of input-output modes (TE $_\text{i}$ –TE $_\text{j}$) are successfully realized with a mode conversion efficiency larger than 98% and a crosstalk smaller than −30 dB within a wide wavelength range from 1530 nm to 1565 nm. [ABSTRACT FROM AUTHOR]

We investigate the propagation characteristics of low‐altitude hiss in the ionosphere by numerical simulation with a two‐dimensional full‐wave model. The simulation results demonstrate that linear mode conversion from whistler to H+ band electromagnetic ion cyclotron wave and polarization reversal occur simultaneously where wave frequency matches the H+−He+ crossover frequency. This mode conversion efficiency shows sensitive dependence on wave normal angle and plays a significant role in the propagation of whistler emission near the local proton gyro‐frequency in the ionosphere by redistributing the wave energy below and above the H+−He+ cutoff frequency, which can explain the low‐altitude hiss observed by the Freja and Detection of Electromagnetic Emissions Transmitted from Earthquake Regions satellites, respectively. The energy of whistler‐mode low‐altitude hiss emission can be transferred to reflected left‐hand polarized electromagnetic ion cyclotron through mode conversion and the efficiency reaches a maximum for intermediate incident wave normal angle (of 45°). Key Points: Mode conversion from whistler to H+ band EMIC and polarization reversal occur simultaneously where wave frequency matches H+−He+ crossover frequencyThe wave normal angle dependence of mode conversion efficiency determines the features of low‐altitude hiss observed by low orbit satellite in the ionosphereLow‐altitude hiss emission of whistler mode can be converted to left‐hand polarized EMIC with efficiency maximized with wave normal angle near 45° [ABSTRACT FROM AUTHOR]

A drilled hole with a diameter of less than or equal to the wavelength of a surface acoustic wave on the subsurface of an aluminum plate is detected by laser probing. The holes that could not be detected by a conventional method are detected from the phase change of the Rayleigh wave included in the reflected laser light. Experiments involving 5 MHz Rayleigh waves propagating along the surface of an aluminum plate are performed by laser probing. Finite element method (FEM) analysis reveals mode conversion from a Rayleigh wave to an A0 mode Lamb wave between the drilled hole and the material surface. The phase change at the edge of the drilled hole that causes the difference in sound velocity between the Rayleigh and Lamb waves is also described. [ABSTRACT FROM AUTHOR]

A novel and generalized methodology of mode conversion based on the transmissive metasurfaces is studied in this article. The mathematic formulations, the design criterions, and procedures have been presented in detail. To further demonstrate the proposed methodology, four typical examples are designed. Subsequently, a prototype with the input mode of circular waveguide TM0,1 mode is designed, fabricated, and measured at ${X}$ -band. The simulated and measured results show that the input mode has been converted into the Guassian-like mode, which could verify the correctness of the proposed design methodology. In addition, due to the rich applications of the transmissive metasurfaces, the proposed methodology will start a new page to realize the mode conversion, especially for the high-frequency case. [ABSTRACT FROM AUTHOR]

The fundamental circumferential shear horizontal (CSH0) wave is of practical importance in monitoring corrosion defects in large-diameter pipes due to its virtually non-dispersive characteristics. However, so far, there have been limited CSH0 wave transducers which can be used to constitute a structural health monitoring (SHM) system for pipes. Moreover, the CSH0 wave’s capability of sizing the corrosion-like defect has not yet been confirmed by experiments. In this work, firstly, the mechanism of exciting CSH waves was analyzed. A method based on our previously developed bidirectional SH wave piezoelectric transducers was then proposed to excite the pure CSH0 mode and first order circumferential shear horizontal (CSH1) mode. Both finite element simulations and experiments show that the bidirectional transducer is capable of exciting pure CSH0 mode traveling in both circumferential directions of a 1-mm thick steel pipe from 100 to 300 kHz. Moreover, this transducer can also serve a sensor to detect CSH0 mode only by filtering circumferential Lamb waves over a wide frequency range from 100 to 450 kHz. After that, a method of sizing a rectangular notch defect by using CSH0 wave was proposed. Experiments on an 11-mm thick steel pipe show that the depth and circumferential extent of a notch can be accurately determined by using the proposed method. Finally, experiments were performed to investigate the reflection and transmission characteristics of CSH0 and CSH1 waves from notches with different depths. It was found that transmission coefficients of CSH0 mode decrease with the increasing of notch depth, which indicates that it is possible to monitor the depth change of corrosion defects by using CSH0 wave. [ABSTRACT FROM AUTHOR]