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Traditional mode conversion ultrasonic motors are typically composed of the superposition of multiple vibration modes, making them more complex than single-mode ultrasonic motors. This complexity makes it difficult to miniaturize. A micro radial-torsional vibration mode ultrasonic motor is proposed and tested in this work. It is excited by a single-phase excitation signal. The proposed motor utilizes the radial in-plane vibration mode of the stator to convert the torsional vibration mode of the inner ring through the beams. The torsional vibration of the inner ring causes the rotor to rotate due to the friction force between the inner ring and the rotor. Based on the designed structure and verified using the finite element method, a prototype motor with a mass of 2.6 g and overall dimensions of Ô20 mm in diameter and 15 mm in height is fabricated and characterized. During the mechanical output performance test, the prototype achieved a maximum no-load speed of 3300 r/min and a stall torque of 0.2 mN $\cdot $ m at the driving voltage of 100 V $_{\mathrm {p-p}}$ .

Undesired conversion between common-mode (CM) and differential-mode (DM) noise often occurs in modern electronic and electrical systems, posing challenges in terms of Electromagnetic Compatibility (EMC). Modal analysis represents a crucial tool in EMC investigation and provides insight into the mechanism underlying mode conversion. By inspecting CM and DM behaviors and their interconversion, it allows for understanding of conducted emission propagation mechanisms, drives electromagnetic interference (EMI) filter design towards optimal/tailored solutions, and enables the possibility to identify the main contributors to the radiated emission phenomenon. This paper offers a comprehensive review of mathematical methodologies and modelling strategies for EMC-oriented modal analysis, with particular emphasis on mode conversion phenomena. To this end, modal decomposition techniques and standard parameters for quantifying mode conversion are summarized and compared. Additionally, the paper provides an overview and in-depth discussion of different scenarios and test cases in which mode conversion occurs, with the final goal to achieve a systematic comprehension of its root causes and consequences within power and communication systems. Eventually, a survey of circuit modelling approaches for mode conversion is presented, offering insights into addressing this phenomenon effectively.

Implementing a compact optical integrated circuit by utilizing subwavelength plasmonic devices requires the design of compact and efficient photonic to plasmonic mode converters. Especially for plasmonic multiple-input devices such as logic gates that require multiple converters, the footprint can be largely penalized by the photonic waveguides, which should be considered in the design. In this work, we simulate and benchmark five photonic to plasmonic mode converter topologies for the application of multiple-input plasmonic devices. Our design includes both directional and end coupling schemes of plasmonic waveguide and Si photonic waveguides of wire and slot configurations. We optimize the performance of the converters considering the pitch mismatch between the photonic and plasmonic waveguides, the total footprint, and mode conversion efficiency.

In this article, a 90°-bent differential transmission line using the asymmetric periodic structure filter is proposed. The asymmetry of the periodic structure filter is used to reduce the differential-to-common mode conversion while the frequency response of the periodic structure filter is used to reflect the common-mode noise. As compared with the conventional 90°-bent differential transmission line, the differential-to-common mode conversion is greatly reduced from −3.04 to −20.42 dB while the time-domain differential-to-common mode transmission is greatly reduced from 0.094 to 0.006 V. Besides, the common-mode reflection coefficient is largely increased from −13.72 to −0.94 dB while the time-domain common-mode reflection is largely increased from 0.017 to 0.068 V. Furthermore, the performance of the eye diagram is upgraded, meaning that the signal integrity is improved. [ABSTRACT FROM AUTHOR]

In this letter, we propose a method to realize rectangular waveguide mode to spoof surface plasmon polaritons (SSPP) mode conversion using phase gradient metasurface (PGM). By applying a PGM with designed wavevector to the narrow wall of the rectangular waveguide, propagation mode and direction of the guided waves can be conversed and controlled due to wavevector matching. Such system can provide an excellent transition by matching the momentum of the rectangular waveguide mode and the SSPP mode. A smooth bridge between the rectangular waveguide system and the SSPP waveguiding system can be realized with potential advantages in conformal design, super-resolution imaging or signal manipulation. Simulated and experimental results, agreeing well with each other, validated the effectiveness of rectangular waveguide mode to SSPP mode conversion using PGM from 6.96-7.55 GHz. Furthermore, a dual channel rectangular waveguide to SSPP splitter is proposed and validated with applications for dual-channel power dividing and signal transmission, achieving an efficiency of 72.4%. [ABSTRACT FROM AUTHOR]

Highly oversized mode converters are the critical devices for high power millimeter-wave transmission systems, characterizing high power capacity and low loss. In order to improve the conversion efficiency from TE01 to TE11 mode, the mode coupling mechanism of oversized elliptical waveguides is explored, and an optimal ellipticity interval to obtain the high conversion efficiency is deduced by analyzing the mode coupling capacity. Considering that ellipticity can affect the power capacity, breakdown thresholds of different operating modes in elliptical waveguides are studied. Based on the analyses, a synthesis method to design the elliptical mode converter integrating the mode conversion efficiency and the power capacity is proposed. A highly oversized mode converter working in $W$ -band is designed, and the simulation results demonstrate that the mode converter achieves the high conversion efficiency over 95% in a relative bandwidth of 19.6%. In order to validate the design, a prototype of the mode converter is fabricated, and the high purity of outputted TE11 mode is verified by the measurement. The oversized mode converter designed by the proposed synthesis method features high efficiency, broad bandwidth, and high power capacity, enabling its full utilization of the potential of gyrotron traveling-wave tubes (gyro-TWTs). [ABSTRACT FROM AUTHOR]

Mode-converting metasurfaces are passive, lossless devices that can be designed to transform a set of incident modes to a desired set of transmitted modes. In this paper, mode-converting metasurfaces are utilized to synthesize arbitrary, azimuthally-invariant TM apertures. The methods presented in this work can be used to design antennas that can meet specific near-field and far-field criteria unlike most metasurfaces which solely manipulate the far field. The proposed antennas consist of a coaxially-excited, radial cavity topped by the mode-converting metasurface. The main role of the mode-converting metasurface is to establish the desired aperture by converting the modal distribution of the excitation to that of desired aperture. Its secondary role is to impedance match the coaxial feed to the radial cavity. Using modal network theory, an optimization-based design procedure is developed to synthesize the proposed metasurface antennas. The admittance profiles of the electric sheets that comprise the metasurface are optimized to establish the desired aperture profile. To illustrate the design procedure, a radial Gaussian beam antenna is synthesized at 10 GHz and its performance is verified using a full wave electromagnetic solver. The proposed antenna has a height and weight advantage over Gaussian beam horn antennas.

Shear Horizontal (SH) guided waves have been extensively used to estimate and detect defects in structures like plates and pipes. Depending on the frequency and plate thickness, more than one guided-wave mode propagates, which renders signal interpretation complicated due to mode mixing and complex behavior of each individual mode interacting with defects. This paper investigates the use of machine learning models to analyse the two lowest order SH guided modes, for quantitative size estimation and detection of corrosion-like defects in aluminium plates. The main contribution of the present work is to show that mode separation through machine learning improves the effectiveness of predictive models. Numerical simulations have been performed to generate time series for creating the estimators, while experimental data have been used to validate them. We show that a full mode separation scheme decreased the error rate of the final model by 30% and 67% in defect size estimation and detection respectively.

A tunable bandwidth add-drop filter in ideal quasi-critical coupling (QCC) state was analyzed in theory in detail and demonstrated in experiment, with two vector modes HE $^{\rm X}_{11}$ and HE $^{\rm Y}_{11}$ coupled single whispering gallery mode (WGM) as its basic configuration. In the experiment, the bandwidth tuning range at the Drop port was from 54.0 MHz to 775.8 MHz with the conversion efficiency maintained above 95%, and bandwidth tuning range at the Through port was from 6.6 MHz to 720.6 MHz with the insertion loss fluctuation less than 5%. The demonstrated filter gains advantages of stable efficiency, narrow band, and broad bandwidth tuning range, which will be helpful for its application in microwave photonics, narrow linewidth lasers and nonlinear optics. [ABSTRACT FROM AUTHOR]

An optimum approach for designing a TE01 mode oversized waveguide bend is proposed based on angular-deformed circular waveguide. Due to the deformation of the circular waveguide cross section, the degeneracy of the TE01 mode and the TM11 mode is removed, and the mode conversion inside the waveguide bend is weakened. Characterizations of the angular-deformed circular waveguide with different deformation and its influence on the mode spectrum, the transmission loss, and the power capacity are investigated. Based on the analysis results and the coupling-wave theory, a Ka-band broadband oversized deformed circular waveguide bend with the propagating TE01 mode is designed, fabricated, and measured. A good agreement between the calculation results and the S-parameter measurement of a prototype is achieved. The transmission efficiency is over 95% in the frequency range of 26.5–35.9 GHz (the relative band is over 30%). The near-field test results further demonstrate the high output purity of the TE01 mode in the working frequency band. [ABSTRACT FROM AUTHOR]

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 (

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]

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]

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]

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 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]

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]

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]

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]

In this paper, waveguide mode conversions in a parallel plate waveguide, rectangular waveguide, and circular waveguide are all demonstrated at terahertz frequencies. Different from the traditional ways to realize mode conversions, an ultracompact design method based on the inner-embedded subwavelength metallic gratings is provided. The principle of the method relies on modulating the phase velocities of spoof surface plasmon polaritons (SSPPs) supported on the gratings to generate π phase difference. Owing to the gradient matching the phase velocities of the waveguide modes and SSPPs, the efficiencies of the mode conversions are considerable. The output mode purities of the both demonstrations are very good. Besides, waveguide bend is also achieved by this method. The work in this paper hopes to establish a universal technique for compact waveguide mode control, especially in the terahertz frequency range.

Manipulation of orbital angular momentum (OAM) of light is essential in OAM-based optical systems. In particular, the OAM divider, which can convert the incoming OAM mode into one or several new smaller modes in proportion at different spatial paths, is very useful in OAM-based optical networks. However, this useful tool has rarely been reported. Here, we put forward a passive OAM divider based on coordinate transformation. The device consists of a Cartesian to log-polar coordinate converter and an inverse converter. The first converter converts the OAM light into a rectangular-shaped plane light with a transverse phase gradient, and the second converter converts the plane light into multiple diffracted light. The OAM of zeroth-order diffracted light is the product of the input OAM and the scaling parameter. The residual light is the output from other diffracted orders. Furthermore, we extend the scheme to realize equal N-division of OAM and arbitrary division of OAM. The ability of dividing OAM shows huge potential for OAM-based classical and quantum information processing.

In this article, we first discover and investigate the pattern diversity of electromagnetic fields within an antisymmetrically biased twin-ferrite-loaded substrate integrated waveguide (SIW) structure. A special scenario is identified in which two separate guided-wave channels can be created exclusively for each direction of wave propagation within the same ferrite-loaded SIW body. A detailed theoretical foundation is formulated and presented to develop the so-called “nonreciprocally dual-channelized waveguide (NDW)” in this article. Next, this NDW structure is used to devise and demonstrate a fully integrated fundamental nonreciprocal building element called “nonreciprocal mode-converting waveguide (NRMCW),” which manifests modal nonreciprocity. In other words, this ferrite waveguide interchangeably converts TE10 and TE20 modes within the SIW, only in one direction. A fundamental approach to design such a unidirectional mode-converting device is proposed through unsymmetrical perturbation loading inside an NDW. Furthermore, a class of four-port circulator is devised and demonstrated for the first time, in which the operation mechanism is based on the one-way mode conversion. The measurement results of the proposed circulator are in agreement with those of the simulation and mathematical models, thereby validating the theoretical foundation and the concept of “NRMCW. ” [ABSTRACT FROM AUTHOR]

This article proposes a novel method to design a 90° oversized TE01-to-TM11 mode converter for high-power transmission line. In order to obtain a full power conversion at 90° bend angle rather than an acute angle, the mode coupling process inside the oversized circular waveguide bend is analyzed, and a phase rematching method is deduced. Based on the analysis results, a phase rematching structure, consisting of two identical circular-to-elliptical transitions and a uniform circular waveguide which can transform the phase difference from $-\pi $ /2 into $\pi $ /2, is introduced. Two mode converters with the input/output radius of 14 mm are designed at the center frequency of 34 GHz to validate the proposed method. The results show that the mode converter with a limited height has a bandwidth of 4.6 GHz, while the mode converter with a limited width has 4.3 GHz with conversion efficiency over 95%. The mode converter with a limited height is manufactured and measured, and the high-purity outputs are validated by the obtained radiation field patterns at the output port of the mode converter from 32 to 36 GHz. The proposed mode converter provides a new approach for developing a compact, high-efficiency, and broadband high-power millimeter-wave transmission line. [ABSTRACT FROM AUTHOR]

A new design approach for developing balanced-to-single-ended (BTSE) power dividers (PDs) to achieve high-level and wide range of common-mode (CM) suppression and minimum mode-conversion level is presented. The suppression of CM signals and mode conversion is realized by a microstrip-to-slotline transition, which is due to the orthogonality between the electric field of the microstrip line and slotline. Filtering responses are included in the differential-mode transmission performance, and multiple transmission zeros are generated by loading shunted coupled-line stubs. A multimode slotline resonator is used to provide multiple resonances in the passband, and these resonances can be easily controlled so that the operating bandwidth can be varied in a large frequency range. Based on this design approach, several BTSE PDs with different bandwidths are simulated in the EM environment. Prototypes are fabricated and tested to verify the design. The experimental results reveal that 200% fractional bandwidth of CM suppression is obtained. The CM suppression and mode-conversion levels are below −35 dB at all frequencies, which is extremely desired in differential circuits and systems. [ABSTRACT FROM AUTHOR]

Guided waves have been studied for monitoring defects on switch-rails. However, few researchers study monitoring of spallings. Previous studies on switch-rail damage detection are not suitable for this kind of damage at the edge of the rail web. In addition, wired configurations have the challenges of power supply and more cost. This paper proposes a time-frequency analysis based algorithm that is used to locate spallings. The algorithm is verified through an improved wireless based structural health monitoring (SHM) platform. The excitation frequency of 108 kHz is chosen as a compromise between larger wave velocity differences and smaller detectable damage sizes. A total of 4 piezoelectric (PZT) devices make up one actuator-sensor array. 2 PZT devices are placed at 2 sides of the rail web and both of them have the same excitation direction. The other 2 devices are mounted at the top of the rail web to measure spalling induced waves according to the cloud charts. The arrival time, frequency, and wave velocity features of the main modes in transmission and reflection waves are extracted. Then these parameters are substituted in the proposed algorithm to predict the spalling location. Simulation and epoxy bonding based experimental results show that the proposed method can identify a 15 mm length spalling at different locations. Finally, attention is paid to the mode identification errors that influence spalling localization. The effects of curvature on guided waves in switch-rails are analyzed and considered to be the cause of the errors. [ABSTRACT FROM AUTHOR]

High-speed transmission lines are commonly routed as differential lines to control sensitivity to noise on the reference planes at higher speeds. Differential lines are typically characterized in terms of mixed-mode scattering parameters, as they provide insight into the behavior of differential and common signals, as well as the mode conversion among them. These mixed-mode scattering parameters can be mathematically obtained from single-ended parameters, which can, for example, be measured with a four-port vector network analyzer. There has been recent efforts to develop extended or modified versions of mixed-mode scattering parameters, especially for tightly-coupled lines. This can be a point of confusion in interpreting the behavior of differential lines. In this paper, we introduce the mixed-port scattering and hybrid parameters, which do not suffer from any such ambiguous definitions. Mixed-port hybrid parameters are the most natural way to represent any four-port differential circuit, as they are based on intuitive differential and common-port excitations of the network. They also enable extraction of the current division factor experimentally, which is a critical parameter for electromagnetic interference analysis of differential lines. Mixed-port scattering parameters are also defined based on common and differential port excitations, allowing a simpler interpretation than their mixed-mode counterparts, without the need for defining even, odd, common, or differential-mode impedances. As such, mixed-port scattering and hybrid parameters can be used to analyze the performance of a general differential network, certainly including coupled or asymmetrical lines, without any ambiguity. [ABSTRACT FROM AUTHOR]

In this work, a class of antisymmetrically biased ferrite-loaded substrate integrated waveguides (SIWs) is studied in detail along with the definition of three main regions of operation. With focus on one of those newly defined regions, a nonreciprocal mode waveguide (NRMW) is introduced for the first time and its related design procedure is explained and illustrated. Furthermore, a fundamental approach is proposed and devised for developing a fully integrated structure called nonreciprocal mode-converting waveguide (NRMCW). This ferrite-based device is capable of effectively converting TE10 mode into TE20 mode only in one direction, thereby suggesting that the original mode be preserved in the reverse direction. In other words, the mode conversion, in this case, is unidirectional, which has never been reported so far. To substantiate the usefulness of the proposed structure and physical mechanism, a class of circulators is designed, fabricated, and demonstrated on the basis of the introduced NRMCW, thus validating the formulated theory of NRMCW in this work and providing an application example of such devices. Both theoretical and experimental results are presented in support of the claims made in this work regarding the proposed circuit concept and its related foundation work. [ABSTRACT FROM AUTHOR]

In the fixed-field (cold) approximation, a generalized formula is derived for the starting currents of eigenmodes excited by an electron beam in a gyrotron cavity with the mode conversion. Tapered gyrotron cavities with gradual and abrupt transitions are considered as examples. It is shown that the effect of the mode conversion on the starting current consists mainly in the change of the diffractive quality factor and, therefore, is small for cavities with low diffractive losses. For such cavities, our formula is in good agreement with single-mode, self-consistent (hot) calculations of the electron beam–wave interaction. The agreement degrades for high-order axial modes having clear-cut distinctions of cold and hot fields. The derived formula is used to test the validity of the single-mode approximation for a complex gyrotron cavity with the abrupt change in radius. [ABSTRACT FROM AUTHOR]

A Ka-band broadband TE01-mode overmoded waveguide bend based on an elliptical waveguide structure is proposed for high-power transmission line in this paper. To investigate the mode conversion inside the waveguide bend, the mode coupling coefficients of elliptical waveguide bend are theoretically deduced. The influence of the cross section on the power-handling capacity and the transmission efficiency is studied. Calculation and simulation results present that the proposed overmoded waveguide bend has a propagation bandwidth of 7 GHz (relative bandwidth of 21%) with an efficiency over 95%. A prototype is fabricated and measured. The average transmission efficiency is over 93% in the range of 29–36 GHz according to the S-parameter measurement. The results of near-field test demonstrated that a TE01 mode with high purity is obtained at the output port of waveguide bend. The high-power performance is investigated by connecting it with a Gyrotron traveling wave tube, and the waveguide bend works stable under a maximum output power of 150 kW. The measurement results demonstrate that a novel approach is proposed successfully for the design of high-power waveguide bend. [ABSTRACT FROM AUTHOR]

In this paper, the LP11 mode evolution in a few mode-polarization maintaining fiber is experimentally investigated. Selective conversion from the LP11a/LP11b mode to the LP11b/LP11a , OAM+1, and OAM-1 modes are demonstrated with varying wavelengths and fiber lengths. This property may have nice applications in spatial division multiplexing (SDM) systems, and one of them, i.e., the all-fiber compact and robust scalar orbital angular momentum (OAM) generator, which is cascaded by a mode-selective coupler and a few mode-polarization maintaining fiber, is also demonstrated with expected performance for conversion between LP01 mode and OAM+1/OAM-1.

The advent of interconnected devices, sensors, and systems requires increasingly large data transmission rates. Optical communications has met these bandwidth needs due to the large information-carrying capacity of light, although new modulation and multiplexing approaches are required to keep up with increasing demand. To this end, mode-division multiplexing (MDM), in which light propagates in on-chip waveguides with distinct and orthogonal spatial mode shapes, may offer a promising solution. To date, however, most chip-scale MDM devices have relied on (de)multiplexing on-chip of fixed optical modes as opposed to tunable mode conversion. In this Letter we demonstrate a new approach to mode conversion and mode-division multiplexing (MDM) that takes advantage of MEMS-tunable optical phase matching. We show via simulations that the approach enables mode conversion with high purity. Measurements on fabricated devices demonstrate the feasibility of this new MEMS index perturbation and phase matching approach for on-demand and dynamic mode conversion for MDM data/communications links. [2020-0107] [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Metasurfaces offer exciting opportunities that enable precise control of microwave propagation, phase, and polarization, which has been recently studied to realize the versatility of such surfaces. However, there have been few attempts to realize mode conversion using them. In this communication, we demonstrate a novel method for mode conversion that consists of a mode generator, a wavefront transforming mirror, and a metasurface. The measured polarization content can reach 98.3% at the center frequency of 94 GHz using the proposed method, indicating that the circular TE0,1 mode has been successfully converted into linear polarization through the metasurface. The proposed method will be an important step forward in the advancement of millimeter communication and radar systems. [ABSTRACT FROM AUTHOR]

We report a mode splitting technique for LP01 and LP11 modes based on polarization manipulation in a few-mode fiber (FMF) by using a three-paddle two-mode fiber polarization controller (TMPC). In the characterization test, LP01 and LP11 modes are simultaneously excited in a two-mode fiber (TMF) based on core-offset splicing technique. By employing the TMPC, the LP01 and LP11 modes in the TMF are aligned into two perpendicular polarization states and then separated by a polarizing beam splitter (PBS). The output LP01 and LP11 modes from the PBS have high mode extinction ratio (>30 dB), good power stability (≤0.5 dB), and stable mode profile position over the wavelength range of 1530–1560 nm ($C$ -band). The proposed technique is a simple and efficient method for mode manipulation and management in various space-division multiplexing applications. [ABSTRACT FROM AUTHOR]

Shear horizontally (SH) guided waves, generated by periodic permanent magnet arrays, have been used previously in nondestructive evaluation of metal plates and pipes. When an SH guided wave interacts with a defect or a change in sample thickness, the incident SH wave may undergo mode conversion. Analysis of mode conversion is complicated, due to the interference of several propagating modes in the received signal that can often temporally overlap. This paper proposes a mode selection technique to help understand the interaction of SH guided waves with changes in sample thickness. Using an understanding of the propagation characteristics of the guided waves, SH guided waves are sequentially generated and detected on both surfaces of the plate, capturing four distinct waveforms. By superposition of the detected signals, symmetric modes can be clearly separated from antisymmetric modes in the processed received signals. For this method to work well, the transducers used should have very similar responses and be precisely positioned on exactly opposite positions either side of the plate. Finite element simulations are also performed, mirroring the experimental measurements, and the results correlate well with the experimental observations made on an 8-mm-thick plate with a region of simulated wall thinning machined into the sample. [ABSTRACT FROM AUTHOR]

Power distribution grids are exploited by power line communication (PLC) technology to convey high-frequency data signals. The natural conformation of such power line networks causes a relevant part of the high-frequency signals traveling through them to be radiated instead of being conducted. This causes not only electromagnetic interference with devices positioned next to power line cables, but also a consistent deterioration of the signal integrity. Since existing PLC channel models do not take into account losses due to radiation phenomenon, this paper responds to the need of developing accurate network simulators. A thorough analysis is herein presented about the conducted and radiated effects on the signal integrity, digging into differential mode to common mode signal conversion due to network imbalances. The outcome of this work allows each network element to be described by a mixed-mode transmission matrix. Furthermore, the classical per-unit-length equivalent circuit of transmission lines is extended to incorporate radiation resistances. The results of this paper lay the foundations for future developments of comprehensive power line network models that incorporate conducted and radiated phenomena. [ABSTRACT FROM AUTHOR]

In this paper, a perturbative technique and the related computational algorithms are presented for the prediction of differential mode (DM) to common mode (CM) conversion arising from asymmetry and nonuniformity in differential interconnects. It is shown that discontinuities evolving from nonuniformities and geometrical imbalance along the (usually desired uniform) lines give rise to equivalent distributed sources that couple the (ideally decoupled) DM and CM circuits. The prediction of modal quantities is then achieved numerically, as an iterative perturbative refinement of the frequency response of an ideal differential line. Different levels of approximation are introduced and discussed. Specifically, the assumption of weak imbalance allows analyzing the propagation of the two modes separately. Moreover, the assumption of weak nonuniformity allows avoiding CM refinements. The proposed technique applies to both ideally uniform and inherently nonuniform interconnects, and leads to a significant simulation speed-up compared to traditional approaches that analyze nonuniform lines by their subdivision into uniform sections. The methodology is illustrated based on two differential microstrip lines, one tapered and one with sinusoidally varying trace edges. Validation against full-wave simulations and measurements is provided. [ABSTRACT FROM AUTHOR]

Electromagnetic interference (EMI) is correlated with mode conversion, from differential-mode signals to common-mode currents and further to antenna-mode currents on the outside of cables or enclosures. This paper presents methods to quantify the mode conversion caused by discontinuities in the digital visual interface (DVI) signal link. These discontinuities are mostly present in the DVI connector, the DVI cable, and the connector–cable interface. A systematic approach was developed in this study to isolate and identify the different coupling paths in a high-speed interface (in this case, DVI is shown) and also to identify which discontinuity is most critical to mitigate EMI. The method developed in this study can be used for any high-speed interface in modern communication systems. [ABSTRACT FROM AUTHOR]

A compact and highly sensitive temperature sensor based on mode conversion in the ring-core fiber (RCF) incorporating an asymmetric long-period grating (aLPG) is proposed and theoretically simulated. The RCF is composed of a hollow core and a high refractive index ring. The aLPG inscribed in the upper half ring generates mode conversion between modes with different azimuthal orders within the ring. A composite resonant dip containing dual-resonant dips which are generated by mode conversion from HE 11x to \mathrmTE01 and HE 21x can be observed in the transmission spectrum. The hollow core is infiltrated with a refractive index liquid whose material refractive index can be modulated via temperature, so the wavelength of the resonant dip can be modulated by temperature, which displays a high sensitivity of 12.92 nm/°C ( 32.3~\mu \textm /RIU). Furthermore, the relationship between 3-dB bandwidth of the resonant dip and temperature is also obtained by polynomial fitting with a high fitting degree of 99.99%. [ABSTRACT FROM PUBLISHER]

This paper proposes novel mixed-mode S-parameters, which are dubbed as “extended mixed-mode S-parameters.” The extended mixed-mode S-parameters were applied to three-conductor transmission lines, resulting in the independent differential and common modes using the current division factor of three-conductor transmission lines. By contrast to the conventional mixed-mode S -parameters, there are no cross-mode S-parameters in the extended mixed-mode S -parameters, even in the asymmetrical conductor transmission lines. In addition, two new mode conversion matrices, which contain the current division factor, are also proposed to convert the extended mixed-mode S -parameters to the standard S-parameters. The validity of the proposed extended mixed-mode S -parameters and the mode conversion method were confirmed by comparing the S-parameters from the theory, EM-simulation, and experiment as well. [ABSTRACT FROM PUBLISHER]

We propose and demonstrate efficient \textLP01-\text{ LP}11 selective mode conversion over a wide spectral range based on tilted binary phase plate. Due to the extended optical path length of the transverse wave through the tilted phase plate, the matched wavelength for efficient mode conversion can be tuned toward longer wavelength by controlling the tilt angle. In the mode profile characterization, the mode purity of the output \textLP11 beam profile has been theoretically analyzed based on the difference in peak intensities between the two lobes of the \textLP11 mode. The presented model is used for determining the \textLP01:\textLP11 ratio of an impure \textLP11 mode in the experiment. The proposed instrument is a simple yet powerful tool for efficient mode conversion in SDM communication network. [ABSTRACT FROM PUBLISHER]

We propose and experimentally demonstrate LP11–LP01 mode conversion based on an angled-facet two-mode fiber (AFTMF). It is a simple device based on a standard two-mode fiber with an output facet angle of 43°. At a specific orientation, an incident LP11 beam on the angled-facet can be transformed into a transmitted beam with single-lobe mode profile. The single-lobe beam coupled into a target few-mode fiber (FMF) has successfully excited LP01 mode in the target FMF. The feasibility of the AFTMF to operate as a mode converter is demonstrated. The proposed device is a simple and low-cost mode conversion tool for the space division multiplexing system. [ABSTRACT FROM PUBLISHER]

The asymmetry/coating-induced mode conversion (ACIMC) effect in a transmission line system is discussed in this letter. This effect is proved only to be induced when asymmetry and coating exist simultaneously. The reason for ACIMC is that asymmetry and coating lead to the inconsistency of current division factors for electric and magnetic effects. Based on this idea, ACIMC is modeled by adding common-mode current-controlled voltage sources in differential circuits, and the results agree well with those obtained by the method of moments. Moreover, it is proved that balanced-state-matched terminal loads do not exist for a asymmetric and coated transmission line system, but approximate ones at low frequencies could still be found. [ABSTRACT FROM PUBLISHER]

A mechanical mode converter for mode division multiplexed systems over SSMF at 850 nm is proposed and evaluated experimentally by the transmission of OOK-modulated optical signals. The proposed mode converter is based on a periodic structure defined by the grating period ( $\Lambda $ ) parameter and the number of mode coupling points, N$ . A mechanical grating with N=50 points and a tunable grating period ( \Lambda $ form 440 to ) has been designed offering a 124-nm tuning bandwidth. The LP01 mode to LP11 mode conversion has been experimentally assessed and a conversion efficiency of 89 % has been achieved in the designed device. Optical transmission of OOK-modulated signals with 1.25 and 2.5 Gb/s bitrates on LP01 mode or a converted LP11 mode is demonstrated achieving BER <10^{-9} after 1-km optical transmission over SSMF at 850 nm. [ABSTRACT FROM PUBLISHER]

In this paper, waveguide mode conversions in a parallel plate waveguide, rectangular waveguide, and circular waveguide are all demonstrated at terahertz frequencies. Different from the traditional ways to realize mode conversions, an ultracompact design method based on the inner-embedded subwavelength metallic gratings is provided. The principle of the method relies on modulating the phase velocities of spoof surface plasmon polaritons (SSPPs) supported on the gratings to generate π phase difference. Owing to the gradient matching the phase velocities of the waveguide modes and SSPPs, the efficiencies of the mode conversions are considerable. The output mode purities of the both demonstrations are very good. Besides, waveguide bend is also achieved by this method. The work in this paper hopes to establish a universal technique for compact waveguide mode control, especially in the terahertz frequency range. [ABSTRACT FROM PUBLISHER]

Manipulation of orbital angular momentum (OAM) of light is essential in OAM-based optical systems. In particular, the OAM divider, which can convert the incoming OAM mode into one or several new smaller modes in proportion at different spatial paths, is very useful in OAM-based optical networks. However, this useful tool has rarely been reported. Here, we put forward a passive OAM divider based on coordinate transformation. The device consists of a Cartesian to log-polar coordinate converter and an inverse converter. The first converter converts the OAM light into a rectangular-shaped plane light with a transverse phase gradient, and the second converter converts the plane light into multiple diffracted light. The OAM of zeroth-order diffracted light is the product of the input OAM and the scaling parameter. The residual light is the output from other diffracted orders. Furthermore, we extend the scheme to realize equal N-division of OAM and arbitrary division of OAM. The ability of dividing OAM shows huge potential for OAM-based classical and quantum information processing. [ABSTRACT FROM PUBLISHER]