Reconfigurable Guided‐Mode Conversion Using Cladding Medium Switching Strategy in Subwavelength Grating Waveguide

A mode conversion from the TE 10 in WR3 rectangular waveguide to the TE 53 in an oversized circular waveguide is required to perform the cold test of a Bragg resonator for a high power 250 GHz Cyclotron Auto-Resonance Maser (CARM) under development at the Frascati ENEA research centre. High operating frequency and oversized cylindrical cavity (exceeding any previous CARM experiment) make the design particularly challenging. An initial transition, aimed at transforming the TE 10 in WR3 rectangular waveguide into the TE 11 , in a circular waveguide, is performed using a linear taper, a well-assessed device. Then a serpentine mode converter in circular waveguide with average radius of 1.48 mm and appropriate length has been adopted for the conversion into the TE 01 mode. The subsequent transducers, for not-rotating (TE 0m ) and rotating (TE 53 ) modes, are provided by means of rippled wall mode converters with small periodic perturbations of waveguide radius. The ripple period corresponds approximately to the beat wavelength (X b = 2π/(β m − β n )) with β m and β n the wavenumbers of the two interacting modes. For not-rotating modes an efficient conversion has been obtained by cascading single step TE 01 to TE 02 , TE 02 to TE 03 and TE 03 to TE 04 mode converters. For each of them, profiles, radius and number of periods have been optimized in order to obtain low levels of unwanted spurious modes, low reflection and sufficient bandwidth. As the modes to convert (TE 04 to TE 53 ) present similar wavenumbers, the most appropriate method for the conversion consists in a three-periods rippled-wall mode converter made by a helically corrugated waveguide with an average radius of 4 mm and five azimuthal variations. The longitudinal profile of the ripples has been properly smoothed to avoid steps between the circular waveguide and the corrugated converter. In this paper the conversion chain will be described and the simulation results (S-parameters, efficiency and electric field) of each single converter will be reported. Finally some considerations about the high precision micromachining technique to be used, such as copper electroforming and Computer Numerically Controlled (CNC) milling, for the construction of these high frequency components, will be presented.

A novel mode converter that can transform coaxial transverse electromagnetic (TEM) mode into circularly polarized TE11 (CPT) mode was developed and experimentally studied with high-power microwave (HPM). The mode conversion is realized by two steps. First, the TEM mode is converted into linearly polarized TE11 (LPT) mode by the webbed-shaped balun structure. Second, the LPT mode is transformed into the CPT mode with the insertion of circular-arc diaphragms in a circular waveguide. In this paper, the principle of a mode converter working at 1.57 GHz is demonstrated, as well as the experimental results. The experimental and simulation results are in good agreement. At the central frequency, the conversion efficiency is more than 99.5%, the measured axial ratio approximates to 1.39 dB, and the power handling capacity is in excess of 1.5 GW. These measured results meet the demand for narrow-band HPM source application.

Joint USGS‐CUAHSI Workshop: In Situ Optical Water Quality Sensor Networks; Shepherdstown, West Virginia, 8–10 June 2011; Advanced in situ optical water quality sensors and new techniques for data analysis hold enormous promise for advancing scientific understanding of aquatic systems through measurements of important biogeochemical parameters at the time scales over which they vary. High‐frequency and real‐time water quality data also provide the opportunity for early warning of water quality deterioration, trend detection, and science‐based decision support. However, developing networks of optical sensors in freshwater systems that report reliable and comparable data across and between sites remains a challenge to the research and monitoring community. To address this, the U.S. Geological Survey (USGS) and the Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHSI), convened a 3‐day workshop to explore ways to coordinate development of standards and applications for optical sensors, as well as handling, storage, and analysis of the continuous data they produce.

Multi-core fiber has aroused considerable interest as one of potential candidates for space division multiplexing that provides an additional freedom degree to increase optical fiber capacity to overcome the transmission bottleneck of current single-mode fiber optical networks. Few-mode fiber is also under intense study as a means to achieve space division multiplexing. We propose a novel dual-mode large-mode-area multi-core fiber (DMLMAMCF), which uses multi-core structure to realize few-mode condition when pursuing large mode-area. The proposed fiber consists of 5 conventional silica-based cores in the center region and 14 air hole cores surrounding the center cores. The outer circle with 12 air hole cores, which function similarly to the fluorine doping region in the bend-insensitive fiber, can mitigate the bending loss when keeping large mode area. The symmetrically distributed two cores on both sides of the center core in central region can reduce the half second-order LP11 mode consisting of two degenerate HE11 modes, TE01 mode, two degenerate HE21 modes and TM01 mode, thus leading to the remaining four vector modes, i.e. two degenerate HE11 modes and two degenerate HE21 modes. That is the reason why we call it strict dual-mode. We focus on large-mode-area properties and bending characteristics of the dual-mode. The influence of structural parameters that include corepitch Λ, refractive index difference between core and cladding Δn, and fiber core radius a, on mode characteristics and mode area of HE11 mode and HE21 mode is investigated in detail. The results reveal that it is helpful to increase the effective area of fundamental mode when we increase the value of corepitch, reduce the refractive index and fiber core radius. The effective mode area of HE11 is about 285.10 μm2 under the strict dual-mode condition. In addition, the relationship between bending loss and bending radius, and the relationship between effective mode area and bending radius of two modes are both investigated. For the HE11 mode, the least bending loss is about 5×10-5 dB/m while the least effective mode area with bending radius larger than 0.6 m is about 285.10 μm2. The HE21 mode is more sensitive to bend effect. The least bending loss is about 0.028 dB/m and the effective mode area is larger than 280.00 μm2 except for resonant coupling points. Large effective areas of both modes with low bending loss can be realized. Larger effective mode area with larger corepitch, appropriate refractive index difference and fiber core radius can be achieved. This fiber may find its usage in high power fiber lasers and amplifiers.

A theoretical and experimental study of TE11-to-HE11 mode conversion in highly oversized waveguides is given. The balanced HE11 hybrid mode propagating in a circumferentially corrugated circular waveguide is in many respects ideal for effective electron cyclotron resonance heating of fusion plasmas with high-power gyrotrons. At an aperture antenna it exhibits desirable radiation characteristics such as linear polarization with very low cross polarization and axisymmetric power distribution with low sidelobe levels. This “Gaussian-like” mode can be generated from a TE11 mode by means of a corrugated mode converter in which the depth of the annular slots is tapered gradually from one-half to one-quarter wavelength. Computer-aided optimization of converter length, shape of corrugations, and of non-linear slot depth variation for 28 GHz- and 70 GHz-transducers was achieved with a scattering matrix formalism employing the modal field expansion technique (Modular Analysis Concept). In both cases the measured conversion efficiency of (98.5±0.5)% and the experimental values of the maximum cross polarization (below −28 dB) are in very good agreement with the predicted values. High-power operation was successfully demonstrated using pulsed 28 GHz- and 70 GHz-VARIAN gyrotrons (200 kW).

Mode converters (MCs) play an essential role in mode-division multiplexing (MDM) systems. Numerous schemes have been developed on the silicon-on-insulator (SOI) platform, yet most of them focus solely on the conversion of fundamental mode to one or two specific higher-order modes. In this study, we introduce a hybrid shape optimization (HSO) method that combines particle swarm optimization (PSO) with adjoint methods to optimize the shape of the S-bend waveguide, facilitating the design of arbitrary-order MCs featuring compactness and high performance. Our approach was validated by designing a series of 13 μm-long MCs, enabling efficient conversion between various TE modes, ranging from TE0 to TE3. These devices can be fabricated in a single lithography step and exhibit robust fabrication tolerances. Experiment results indicate that these converters achieve low insertion losses under 1 dB and crosstalks below -15 dB across bandwidths of 80 nm (TE0-TE1), 62 nm (TE0-TE2), 70 nm (TE0-TE3), 80 nm (TE1-TE2), 55 nm (TE1-TE3), and 75 nm (TE2-TE3). This advancement paves the way for flexible mode conversion, significantly enhancing the versatility of on-chip MDM technologies.

The mode converter, as a passive mode conversion device in transmission lines, is well-investigated and widely implemented in various electromagnetic systems. However, most traditional mode converters can only realize a single conversion mode. Thus, a mode converter achieving multiple controllable output modes is urgently needed. In this paper, a reconfigurable mode converter operating in the microwave range is achieved by embedding a deformable all-dielectric material with quadrilateral shape into a rectangular waveguide based on coupled-mode theory. It can achieve different target modes with controllable output for the same input by exciting the deformable all-dielectric material. The design principle of the mode converter is expounded concretely and simulation is carried out using HFSS software 2022 R2. Experimental results, consisting of the simulation results, demonstrate that the proposed mode converter can achieve various mode conversions with mode purity higher than 95%. This article innovatively applies deformable materials to waveguide mode conversion, expanding the application of deformable memory materials in electromagnetic devices.

Reconfigurable mode converters are essential components in efficient higher-order mode sources for on-chip multimode applications. We propose an on-chip reconfigurable silicon waveguide mode conversion scheme based on the nonvolatile and low-loss optical phase change material antimony triselenide (Sb2Se3). The key mode conversion region is formed by embedding a tapered Sb2Se3 layer into the silicon waveguide along the propagation direction and further cladding with graphene and aluminum oxide layers as the microheater. The proposed device can achieve the TE0-to-TE1 mode conversion and reconfigurable conversion (no mode conversion) depending on the phase state of embedded Sb2Se3 layer, whereas such function could not be realized according to previous reports. The proposed device length is only 2.3 μm with conversion efficiency (CE) = 97.5%, insertion loss (IL) = 0.2 dB, and mode crosstalk (CT) = -20.5 dB. Furthermore, the proposed device scheme can be extended to achieve other reconfigurable higher-order mode conversions. We believe the proposed reconfigurable mode conversion scheme and related devices could serve as the fundamental building blocks to provide higher-order mode sources for on-chip multimode photonics.

Optical bistability is a nonlinear phenomenon enabling stable switching between two optical states and has important applications in optical communication and photonic neural networks (PNNs). However, conventional bistable devices often suffer from fabrication imperfections and scattering losses, which limit their robustness and dispersionless performance. In this study, we numerically investigate optical bistability from a one-dimensional photonic topological hypercrystal (PhH) composed of alternating hyperbolic metamaterials (HMMs) and dielectric layers. By designing a center-inversed symmetric layered PhH structure and introducing Kerr nonlinearity into the localized dielectric region of maximum electric field intensity at the inversion center, we achieve a robust, angle-insensitive optical bistability for TM polarization through phase variation compensation mechanism. When applied as a nonlinear activation function in PNNs, the bistable PhH exhibits performance comparable to conventional digital activation functions such as ReLU and Sigmoid in image-recognition tasks. Our work paves the way for integrating topological bistable devices into next-generation PNNs.

A liquid crystal (LC) beam deflector with a microblazed grating produced by stepping photolithography and reactive ion etching (RIE) was reported. A homogeneously aligned nematic liquid crystal (NLC) materials are filled inside the microcavity of blazed grating, and sandwiched between two glass plates. An indium tin oxide (ITO) transparent electrode is deposited on it to provide the beam steering capability. Our LC-micro blazed grating device gives a high diffractive efficiency (about 95%) and a controllable large steering angle over 7.2 ° (for n e ) and 1.7° (for n o ), respectively. It was found that this type of non-mechanical beam steering without any moving parts is ideally suited for applications in optical communication and optical interconnection network.

There is a growing demand for higher computational speed and energy efficiency of machine learning approaches and, in particular, neural networks. Optical implementation of neural networks can address this challenge. Compared to other neuromorphic platforms, fibre-based technologies can unlock a wide bandwidth window and offer flexibility in dimensionality and complexity. Moreover, fibre represents a well-studied, low-cost and low-loss material, widely used for signal processing and transmission. At the same time, mode-locked fibre lasers offer flexibility and control, while the mode-locking effect can be crucial for unlocking ultra-short timescales and providing ultra-fast processing. Here, we propose a mode-locked fibre laser with a non-linear power threshold in both power and spectrum. The advantage of the proposed system is a spectrum width two-branch function dependent on the input signal power. The effect is caused by a transition between two operating regimes and is governed by the input signal power. The proposed design enables receiving a non-linear transfer function in amplitude with a power threshold as an optical analogue of biological neurons with the additional advantage of a non-linear two-branch transfer function in spectrum width. The latter property is similar to the frequency-varied response dependent on stimulus properties in biological neurons. Thus, our work opens new avenues in research into novel types of artificial neurons with a frequency spectrum width variable response and, consequently, spiking neural networks and neural-rate-based coding with potential applications in optical communications and networks with flexible bandwidth, such as 5G and emerging 6G.

In this simulation work, the wave vector and reflectance properties of a 1DPC made of dielectric/metamaterial multilayer structure employing TMM, for TE and TM modes are studied. For TE mode, it is observed that the obtained wide bandgap increases with increase in the angle of incidence, whereas the narrow bandgap also shows a minor increase with increase in the incident angle. It is noted that bandgap increases with increase in the incident angle in TE mode, while bears opposite behavior in TM mode and decreases with increase in the incident angle. If the DPS thickness is increased, the number of bands increases in both TE and TM modes, while the wide bandgaps for TE and TM decreases in width. In both cases of the increased DPS width, it is found that the narrow bandgap obtained in TM mode is showing enhancement with an increase in incident angle, and exhibits ultra‐tunable behavior. Some other researchers have reported earlier that a complementary split‐ring resonator based metamaterial with effective medium ratio has influence on the extraction of effective permittivity, permeability, and refractive index, where DNG regions exist in the ranges 6.34–7.39 and 8.20–9.98 GHz. Comparing with the proposed DNG, the study finds that the aforesaid values correspond to frequency regime taken in our case, 1–10 GHz, particularly the middle of the range 1–10 GHz, where broadband is obtained, which is satisfactory. The insights included in the article can be employed in designing microwave broadband reflectors, tunable mirrors, and can be proved to be useful in photovoltaic cells. Such PC can also be employed in designing super lenses as well as optical cloaking, and the miniaturization of antennas.

In this article, the conduction of transverse electric/transverse magnetic (TE/TM) propagation modes in nanomaterial graphene utilization of asymmetric dielectric slab waveguide was studied in terms of their applicability in optical fiber communication. Nano-graphene film was deposited on silica substrate and air cladding. Three layers were exposed to electromagnetic waves with a 1.55 µm wavelength and 1 µm thick nanographene film, silica, and air covering to study the performance and optical properties of graphene nanomaterials. The models were influenced by factors such as attenuation wavenumbers, cut-off frequency, mode number, propagation wavenumbers, skin depth, and angles of total internal reflection. The impact of these parameters was assessed through numerical analysis, revealing significant correlations. The modes generated ten numbers of TE and nine numbers of TM mode dependent upon the structure of nanographene, with low loss propagation wavenumber. As the TE/TM modes increase, the decay wavenumbers of the cladding and substrate parameters diminish. This indicates that the magnetic and electric fields undergo extremely rapid decay beyond the film region, which leads to the skin depth for higher TE/TM modes traveling longer distances in the cladding and substrate than do lower-order modes. Therefore, nanographene exhibits good optical properties due to its low absorption loss. The angles of internal reflection are greater in magnitude than the substrate and cladding key angles. Still, in the tenth TE mode, the internal reflection angles are extremely close to the critical angle as a result of the small substrate decay parameter and the near operating frequency. The tenth TE mode is weakly characterized. The characteristics of TE/TM modes in an asymmetric three-layer slab waveguide structure are realized for various mode orders and various parameters of nanographene material. The field profiles of the TE/TM modes are submitted to comprehend these characteristics.

A new dielectric slab waveguide with a left-handed material (LHM) cover and substrate is proposed. The dispersion relations and normalized effective thickness of the asymmetric LHM slab waveguide are investigated, in view of the normalized parameters. A number of unusual properties are found, for example, the fundamental and first-order modes do not exist and higher-order modes have double degeneracy. The propagation modes are absent at the low normalized frequency, and the cutoff frequencies of some LHM slab waveguide modes decrease with increase in the asymmetry measure. Unlike traditional slab waveguides, the V –H curves of the LHM slab waveguides are in one-to-one correspondence. Both TE and TM modes are discussed; in addition, the dispersion relations and normalized effective thicknesses of the TM modes are discussed in detail, when the difference in refractive indices of the film and the substrate is small. The results show that the region of mode coexistence taking place near the cutoff frequency becomes narrower with increase in the difference in refractive indices of the film and the substrate. The influence of this difference on the normalized effective thickness curves is different, and becomes smaller and smaller with increase in the value of the asymmetry measure, if different values of the refractive indices are employed.

In this paper, a high efficient fiber-to-waveguide mode converter is demonstrated on the SOI platform, which is composed of a suspended tapered SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> waveguide and overlapped Si nano-tapers. The overlapped Si nano-tapers are located in the center of suspended tapered SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> waveguide. With a refractive index (<;1.4) matching liquid, the coupling losses between the cleaved optical fiber and this converter are 1.5 dB/facet and 2.1 dB/facet for TE and TM modes, respectively. The 1 dB bandwidths for both TE and TM modes are more than 100 nm. The alignment tolerances for both TE and TM modes are ± 2.5 μm and 2.0 μm for 1 dB excess loss in horizontal direction and vertical direction, respectively.