Open Waveguide Research Articles

A Versatile, Machine-Learning-Enhanced RF Spectral Sensor for Developing a Trunk Hydration Monitoring System in Smart Agriculture

This paper comprehensively explores the development of a standalone and compact microwave sensing system tailored for automated radio frequency (RF) scattered parameter acquisitions. Coupled with an emitting RF device (antenna, resonator, open waveguide), the system could be used for non-invasive monitoring of external matter or latent environmental variables. Central to this design is the integration of a NanoVNA and a Raspberry Pi Zero W platform, allowing easy recording of S-parameters (scattering parameters) in the range of the 50 kHz–4.4 GHz frequency band. Noteworthy features include dual recording modes, manual for on-demand acquisitions and automatic for scheduled data collection, powered seamlessly by a single battery source. Thanks to the flexibility of the system’s architecture, which embeds a Linux operating system, we can easily embed machine learning (ML) algorithms and predictive models for information detection. As a case study, the potential application of the integrated sensor system with an RF patch antenna is explored in the context of greenwood hydration detection within the field of smart agriculture. This innovative system enables non-invasive monitoring of wood hydration levels by analyzing scattering parameters (S-parameters). These S-parameters are then processed using ML techniques to automate the monitoring process, enabling real-time and predictive analysis of moisture levels.

Stabilizing Remote Entanglement via Waveguide Dissipation

Distributing entanglement between remote sites is integral to quantum networks. Here, we demonstrate the autonomous stabilization of remote entanglement between a pair of noninteracting superconducting qubits connected by an open waveguide on a chip. In this setting, the interplay between a classical continuous drive—supplied through the waveguide—and dissipation into the waveguide stabilizes the qubit pair in a dark state, which, asymptotically, takes the form of a Bell state. We use field-quadrature measurements of the photons emitted to the waveguide to perform quantum state tomography on the stabilized states, where we find a concurrence of 0.504−0.029+0.007 in the optimal setting with a stabilization time constant of 56±4 ns. We examine the imperfections within our system and discuss avenues for enhancing fidelities and achieving scalability in future work. The decoherence-protected steady-state remote entanglement offered via dissipative stabilization may find applications in distributed quantum computing, sensing, and communication. Published by the American Physical Society 2024

Development of Dispersion Calculator SAFEDC of Guided Wave with Complex Material and Waveguide

The propagation characteristics of guided waves (GW) are related to both the material, loading and geometry parameters. Correct understanding of GW is the key step towards further GW-based structural health monitoring. A dispersion calculator of GW propagating in the complex geometry with viscoelastic and anisotropic material parameter in the open waveguide is developed based on the semi-analytical finite element (SAFE) method and perfectly matched layer (PML). By assuming the simple harmonic motion along the propagation direction and adopting the mesh discretization in the cross section, SAFE features the capability of GW calculation in the complex waveguide at only a cost of limited calculation resources. Hence, besides the capabilities of the most off-the-shelf tools focusing on the analytical solution of GW in simple plate or pipeline, the developed algorithm further offers the solution of GW in open waveguide with laminates of arbitrary ply stacking sequences and in arbitrarily complex cross section. Quadratic one-dimensional and triangular elements are adopted to automatically discretize the plate structure and complex cross section, respectively. The obtained results of GW in plates are in a good agreement with the results obtained with the off-the shelf-tools that adopt analytical derivations, and show stronger robustness of mode searching and sorting by avoiding the complex root-finding procedure required in the analytical derivation. The most GW features, including phase velocity, group velocity, attenuation, wave structure in terms of displacement, stress, and strain, and animation of wave propagation are directly calculated. All these functions are integrated and updated in the previously developed freeware SAFEDC, whose 1st version SAFEDC v1.0 was published in September 2023.

Improved Numerical Estimation Method for Surface Wave Attenuation on Metasurfaces

AbstractComprehensive numerical tools to investigate and analyze the surface wave attenuation on a metasurface waveguide that consists of periodic patches deposited on a dielectric substrate are presented. The surface wave attenuation is evaluated in both waveguide and open‐space environments at high frequencies around the 28‐GHz band. Two approaches including a driven modal approach and an integrated eigenmode approach for investigating the attenuation behaviors in a metasurface waveguide operating in transverse electromagnetic (TEM) mode are employed. These results show different attenuation values depending on the waveguide height due to an unavoidable coupling issue despite the same metasurface design. Thus, an alternative method for more accurately evaluating the frequency‐dependent attenuation characteristics through field distribution in open space with a customized horn antenna is proposed. A relationship between the surface wave attenuation and the propagation distance by analyzing the analytical electric field distribution at different points on the propagation path in the open space is established. Thus, this study provides an improved method for assessing surface wave attenuation characteristics, which contributes to designing surface wave control for wireless communications, wireless power transfer, signal processing, and electromagnetic compatibility.

Secondary roughness effect of surface microstructures on secondary electron emission and multipactor threshold for PTFE-filled and PI-filled single ridge waveguides

Secondary electron yield (SEY) is a dominant factor in determining the multipactor threshold. In this study, we analyzed the secondary roughness effect of surface microstructures for plastic dielectric on SEY reduction and multipactor mitigation. A single ridge waveguide (SRW) operating in Ku-band, filled with polytetrafluoroethylene (PTFE) or polyimide (PI), was designed with a dielectric–metal multipactor gap. By employing a femtosecond laser, periodic microstructures were fabricated on PTFE and PI surfaces to suppress SEY. The SEY peak values of PTFE and PI decreased from 2.05 to 1.40 and 1.37 to 1.07 by the porous surface. The surface morphologies and cross-sectional images of the porous PTFE and PI demonstrated the existence of secondary roughness structures. Via simulation, we obtained multipactor thresholds of 8496 W, 12 374 W, and 9397 W for the SRWs filled with untreated PTFE surface, ideal porous surface (without secondary roughness), and real porous surface (with secondary roughness). Similar works were implemented for the PI-filled SRWs, resulting in simulated multipactor thresholds of 7640 W, 11 327 W, and 9433 W. The results indicate that the multipactor effect may not be effectively suppressed under the influence of secondary roughness structures such as plastic velvet and foam. Besides, simulation works indicated that the radio frequency electric field could extract secondary electrons from the microstructures, weakening the mitigation effect of microstructures on multipactor. The impact of surface charging on electron motion was also analyzed by considering energy distribution. It was suggested that the surface microstructures of plastic dielectrics lead to a decrease in the surface charge density and the electrostatic field strength, weakening the self-extinguishing effect and lowering the multipactor threshold. This study provides an in-depth analysis of the effect of secondary roughness on SEY and multipactor for organic dielectrics, which makes significant sense for the further investigation of dielectric multipactor.

An online tunable TE11 dual-circular polarizer applied for modern high-power radar system

Circular polarizers have essential applications in modern radar and communication systems. However, an online tunable wideband dual-circular polarizer with a power capability of more than 150 kW in the Ku band has not been reported yet, and it has recently become a fundamental problem for developing some high-power radar systems. This paper has designed an overmoded waveguide operating in the TE11 mode with a non-contact cutting-off choke to imply a high power capability. A high-precision stepping motor is used to control the rotation of an elliptical waveguide remotely, and then, the left-hand and right-hand circular polarizations can be switched online. The dual-circular polarizer has been fitted on a 150 kW gyro-TWT high-power test system, and the experimental results show a tremendous high-power capability. The microwave leakage caused by the non-contact gap is reduced to less than −35 dB. The polarization bandwidth exceeds 2.5 GHz in the Ku band, and the axial ratio is better than 1.3 dB. This design concept provides a reference for future high-power radar systems.

Performance Study of Polarization Beam Splitters Based on Horizontal Slot Waveguide Operating at 700nm Wavelength

Abstract: Polarization beam splitter (PBS) integrated waveguides are the key components in the receiver of quantum key distribution (QKD) systems. Their function is to analyze the polarization of polarized light and separate the transverse-electric (TE) and transverse-magnetic (TM) polarizations into different waveguides. In this paper, a performance study of polarization beam splitters based on horizontal slot waveguide has been investigated for a wavelength of . PBS based on horizontal slot waveguide structure shows a polarization extinction ratio for quasi-TE and quasi-TM modes larger than with insertion loss below and a bandwidth of . Also, the fabrication tolerance of the structure is analyzed.

Single- and two-photon absorption induced all-optical control of gallium selenide integrated silicon nitride photonic devices in the 700–800 nm wavelength range

In this work, we report single- and two-photon absorption (TPA) induced transmission and resonance modulation in a multilayer gallium selenide (GaSe) integrated silicon nitride (Si3N4) waveguide and ring resonator operating in the 700–800 nm wavelength range. Intensity dependent saturable absorption at low optical powers followed by TPA at higher power levels in GaSe integrated Si3N4 waveguides is observed at 785 nm pulsed laser excitation. A TPA coefficient of 0.117 cm/GW for the GaSe–Si3N4 composite waveguide and a three-photon absorption coefficient of 7.876 × 10−6 cm3/GW2 for the bare Si3N4 waveguide are extracted from intensity dependent transmission measurements. The single-photon absorption process induced by a blue laser incident on the multilayer GaSe transferred on top of the Si3N4 ring resonator is used for all-optical resonance tuning through the free-carrier refraction effect. A strong blue shift of the resonance by ∼12.3 pm/mW combined with resonance broadening is observed due to the free-carrier induced refractive index and absorption modulation. The TPA in the GaSe integrated Si3N4 ring resonator is also shown to result in a blue shift of the resonances excited using a 785 nm pulsed laser. This work demonstrates the all-optical control of 2D material integrated Si3N4 guided-wave structures operating in the shorter near-infrared wavelength range with potential applications in integrated quantum photonics, miniaturized sensing devices, and biomedical imaging.

The effect of docking defects on a metal-dielectric shielded waveguide in the EHF frequency range

Background. To improve the accuracy of navigation and radar systems, functional nodes of the EHF wavelength range are used. At the same time, an increase in the operating frequency increases the requirements for the accuracy of manufacturing and docking devices. Analysis of the propagation of electromagnetic waves, subject to the presence of defects in the waveguide paths, will allow determining sufficient manufacturing accuracy and evaluating physical processes in an irregular area. Aim. The aim of this work was to study the effect of sharp irregularities on the characteristics of the digital twin of a broadband shielded dielectric waveguide operating in the frequency range from 90 to 100 GHz. Methods. Using a numerical experiment, two variants of irregularity were investigated. They are presented in the form of displacement of sections of shielded dielectric waveguides in the vertical and horizontal planes. Results. According to the values of the transfer characteristic of the waveguide path, the regularities of the attenuation of the electromagnetic wave energy from the magnitude of the irregularity are revealed and the permissible manufacturing accuracy is proposed. Conclusion. To solve the problems of sewerage of weak signals, the authors recommend using a non-jointed manufacturing technology.

Sc2O3 on sapphire all-crystalline grating–waveguide resonant reflectors

This paper reports the fabrication and first demonstration of all-dielectric crystalline grating–waveguide reflectors comprising a Sc2O3 waveguide grown on a sub-wavelength-patterned sapphire substrate. Rigorous coupled-wave analysis is employed to simulate the operation of the structure, suggesting a 100% resonance reflectivity in theory. Structuring of the sapphire substrate is achieved using inductively coupled plasma etching, whilst pulsed laser deposition is used for epitaxial growth of the Sc2O3 crystalline waveguide. Devices with distinct TE- and TM-polarisation resonances around 1030-nm for an angle of incidence near 10° are demonstrated, with reflectance approaching 90%. The discrepancy in reflectivity is attributed to the waveguide thickness variation and surface roughness. Refinement of the fabrication processes and tolerances should lead to improvement in the surface quality of the crystalline grating–waveguide structure and operation closer to the ideal resonance reflectivity.

Leaky Waves in Flatland

AbstractLeaky waves are complex eigenmodes compatible with radiation in terms of their momentum, which can be supported by open waveguiding structures and can be effective at modeling radiative phenomena from such systems. Here, a “flatland” analog of leaky‐wave radiation is introduced to describe a novel mechanism for in‐plane radiation leakage that can occur in artificial or natural low‐dimensional materials. Possible platforms are illustrated for its physical realization, in the form of suitably designed planar junctions of isotropic impedance surfaces that implement surface‐wave or line‐wave guiding structures. A simple and insightful semi‐analytical model for the radiation mechanism is developed and validated against full‐wave numerical simulations. These results provide a new tool for the advanced manipulation of surface waves at the nanoscale, which relies entirely on interface‐optics effects, and can find interesting applications in the emerging field of polaritonics.

On the scattering of a plane wave by a perturbed open periodic waveguide

We consider the scattering of a plane wave by a locally perturbed periodic (with respect to ) medium. If there is no perturbation, it is usually assumed that the scattered wave is quasi‐periodic with the same parameter as the incident plane wave. As it is well known, one can show existence under this condition but not necessarily uniqueness. Uniqueness fails for certain incident directions (if the wavenumber is kept fixed), and it is not clear which additional condition has to be assumed in this case. In this paper, we will analyze three concepts. For the limiting absorption principle (LAP), we replace the refractive index by in a layer of finite width and consider the limiting case . This will give an unsatisfactory condition. In a second approach, we require continuity of the field with respect to the incident direction. This will give the same satisfactory condition as the third approach where we approximate the incident plane wave by an incident point source and let the location of the source tend to infinity.

SYMBOLIC-NUMERICAL IMPLEMENTATION OF THE GALERKIN METHOD FOR APPROXIMATE SOLUTION OF THE WAVEGUIDE DIFFRACTION PROBLEM

In this paper, we construct a symbolic-numerical implementation of the Galerkin method for approximate solution of the waveguide diffraction problem at the junction of two open planar three-layer waveguides. The Gelerkin method is implemented in the Maple computer algebra system by symbolic manipulations; its software implementation is based on the scprod symbolic-numerical procedure, which enables the numerical calculation of scalar products for the Galerkin method based on symbolic expressions. The use of symbolic manipulations makes it possible to speed up the calculation of integrals in the Galerkin method owing to single-run symbolic calculation of integrals typical for the problem, rather than multiple numerical integration.

Scattering in a partially open waveguide: the forward problem

AbstractThis paper is devoted to an acoustic scattering problem in a 2D partially open waveguide, in the sense that the left part of the waveguide is closed, that is with a ‘bounded’ cross-section, while the right part is bounded in the transverse direction by some perfectly matched layers that mimic the situation of an open waveguide, that is with an ‘unbounded’ cross-section. We prove well-posedness of such scattering problem in the Fredholm sense (uniqueness implies existence) and exhibit the asymptotic behaviour of the solution in the longitudinal direction with the help of the Kondratiev approach. Having in mind the numerical computation of the solution, we also propose some transparent boundary conditions in such longitudinal direction, based on Dirichlet-to-Neumann operators. After proving such artificial conditions actually enable us to approximate the exact solution, some numerical experiments illustrate the quality of such approximation.

Single photons versus coherent-state input in waveguide quantum electrodynamics: Light scattering, Kerr, and cross-Kerr effect

While the theoretical studies in waveguide quantum electrodynamics predominate with single-photon and two-photon Fock-state (photon number states) input, the experiments are primarily carried out using a faint coherent light. We create a theoretical toolbox to compare and contrast linear and nonlinear light scattering by a two-level or a three-level emitter embedded in an open waveguide carrying Fock-state or coherent-state inputs. We particularly investigate light transport properties and the Kerr and cross-Kerr nonlinearities of the medium for the two types of inputs. A generalized description of the Kerr and cross-Kerr effect for different types of inputs is formulated using the first-order correlation function to compare the Kerr and cross-Kerr nonlinearity between two photons in these models.

75 GHz Relativistic Surface-Wave Oscillator of Planar Geometry

We present the results of simulations and experimental studies of a W-band surface-wave oscillator powered by a ribbon high-current relativistic electron beam produced by SINUKI accelerator (IAP RAS, Nizhny Novgorod, 1 kA / 650 keV / 17 ns). Planar geometry of the interaction space facilitates efficient mode selection over wide transverse coordinate by using open waveguide at fairly large oversize factor (the width of the waveguide is ~8 wavelengths). Stable oscillation regime is obtained at frequency of 75 GHz with a pulse duration of about 5 ns. The output power measured by the calorimetric method reaches 25 MW.

Design and Mode Matching Analysis of Stepped Substrate Integrated Waveguide and Filters

This paper presents a stepped substrate integrated waveguide. The generalized scattering properties of the cascaded H -plane discontinuity in a substrate integrated waveguide operating in various frequency bands are obtained using mode-matching techniques (MMT) from the respective field equations at four different step discontinuities. The scattering parameters ( S ) obtained from the individual regions are cascaded to obtain the S -parameters of the overall structure. The processor computation time and memory used by MMT are compared to those of commercial simulators like high-frequency structure simulator (HFSS) and computer simulation technology microwave studio (CST-MWS), and the S-parameters obtained using MMT are compared with HFSS and CST-MWS simulated and measured S-parameters. Due to periodicity in the designed structure, a filter-like property has been observed. The S -parameters generated from mathematical formulations, simulations, and measurements show very good agreement.

Single-photon scattering on a qubit: Space-time structure of the scattered field

We study the space-time structure of the scattered field induced by the scattering of a narrow single-photon Gaussian pulse on a qubit embedded in a one-dimensional open waveguide. For weak excitation power we obtain explicit analytical expressions for the space and time dependence of reflected and transmitted fields which are, in general, different from plane traveling waves. The scattered field consists of two parts: a damping part which represents spontaneous decay of the excited qubit and a coherent, lossless part. We show that for a large distance $x$ from the qubit and at times $t$ long after the scattering event our theory provides the result which is well known from stationary photon transport. However, the approach to the stationary limit is very slow. The scattered field decreases as the inverse powers of $x$ and $t$ as both the distance from the qubit and the time after the interaction increase.

Interaction between giant atoms in a one-dimensional structured environment

Giant atoms -- quantum emitters that couple to light at multiple discrete points -- are emerging as a new paradigm in quantum optics thanks to their many promising properties, such as decoherence-free interaction. While most previous work has considered giant atoms coupled to open continuous waveguides or a single giant atom coupled to a structured bath, here we study the interaction between two giant atoms mediated by a structured waveguide, e.g., a photonic crystal waveguide. This environment is characterized by a finite energy band and a band gap, which affect atomic dynamics beyond the Markovian regime. Here we show that, inside the band, decoherence-free interaction is possible for different atom-cavity detunings, but is degraded from the continuous-waveguide case by time delay and other non-Markovian effects. Outside the band, where atoms interact through the overlap of bound states, we find that giant atoms can interact more strongly and over longer distances than small atoms for some parameters -- for instance, when restricting the maximum coupling strength achievable per coupling point. The results presented here may find applications in quantum simulation and quantum gate implementation.

Spatiotemporal Search for Pulsed Radiation in the Photon Flux Detection Mode with Prolonged Observation Time of the Decomposition Element

The development of science and technologies of microwave frequencies (UHF) has contributed to the fact that the use of ultrawideband (UWB) pulsed signals has emerged as a separate scientific and technical area of radiophysics, requiring a deeper study of the methods of formation, emission, reception and processing of such signals. Aperture radiators are mainly used to transmit UWB signals. This type of emitters is a variety of modifications of mirror antennas, horn antennas of various shapes and profiles, open waveguides, planar and conformal antenna arrays, as well as lens antennas. The classical theory of analysis of radiation and propagation of electromagnetic signals is well known, but its application to the synthesis of UWB emitters in the time domain often becomes too cumbersome and incorrect. Progress in the fields of radar, communications, and radio astronomy in the 1960s stimulated the creation of aperture theory based on physical optics approaches. This theory makes it possible to analyze and calculate the characteristics of mirror radiators and antenna arrays with sufficient accuracy for practical application. It is often possible to use the aperture theory to analyze some horn emitters that have a large radiation surface relative to the spatial dimensions of pulsed signals.