Design and Test of Embedded Reconfigurable Mode Converter Based on Spontaneous Deformable Materials.

In this paper, a compact, high efficiency TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode converter is presented. In the proposed structure, dielectric material is loaded below a tapered centre conductor, in lower section of a circular waveguide. The input TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode is initially transformed to TEM mode by the tapered centre conductor. The loaded dielectric material is designed to provide the desired phase shift for the mode conversion. Different dielectric materials are analyzed for loading to achieve high conversion efficiency and wide bandwidth. The proposed mode converter is fabricated with PTFE as dielectric for mode conversion and the results are verified through far field radiation pattern measurements. The mode conversion efficiency is more than 90% for 2.95-3.60 GHz band. Due to compactness and high conversion efficiency for wide bandwidth, it is suitable for S-band high power microwave (HPM) applications.

Dissipative solitons (DSs), which are pulses with high pulse energy and a broad rectangular output spectrum, have several applications. In a fiber system, the generation of higher-order DS requires a mode converter with a high and flat conversion efficiency that can cover the working waveband. The requirements on high and flat conversion efficiency as well as broadband working waveband is a great challenge for a traditional mode converter. This study presents an L-band mode-locked fiber laser with a 3-dB bandwidth of more than 47 nm and a first-order cylindrical vector (CV) mode DS output based on an ultra-broadband long-period fiber grating (LPFG). The fabricated mode converter is a novel cascading LPFG with an ultra-broadband working bandwidth and flat conversion efficiency which could meet the demand of DS mode conversion. The 15-dB bandwidth is 149.76 nm, from 1515.36 nm to 1665.12 nm. Furthermore, the conversion efficiency in the range of the output spectrum is 98.60 ± 0.23%. The single-, double-, and triple-soliton output characteristics and their real-time spectra are observed using time-stretched dispersion Fourier transformation (TS-DFT) technology. Experimental results demonstrate that the first-order CV mode DS laser can maintain its dynamics after mode conversion under the accuracy of detection system, implying that the LPFG works only as a mode converter. The broadband DS laser with CV mode output in the L-band has been presented for the first time in study, to the best of our knowledge. This study is also the first to detect real-time characteristics for a higher-order mode-locked fiber laser using the TS-DFT technology. The proposed type of LPFG is s good candidate for generating higher-order mode lasers with an all-fiber structure.

Gyrotrons are sources of high-power millimeter waves. Current interest in gyrotrons is mainly because they can act as a source of microwave power for electron-cyclotron-resonance heating (ECRH) for fusion experiments. However, the field pattern of these gyrotron modes is unsuitable for long path waveguide transmission or plasma heating. Here, it is desired to have a linearly polarized wave requiring the use of a mode conversion system which consists of many nonuniform waveguide components. Based on the theory of mode coupling, this paper discusses the circular waveguide mode conversion with waveguide axis curved and radius tapered in detail. It also carries out the optimized analysis about the geometry structure of TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0n</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode converter. Adopting different phase rematch techniques, the optimal geometry parameter can be obtained. The mode converter designed in this way owns high conversion efficiency exceeding 98%.

Small circular waveguide mode converters used in the 28 GHz, 200 kW ECH system for the Hitachi tokamak HT-1 have been developed. In order to convert from the TE02 to the TE11 mode, TE0202-to-TE01 and TE01-to-TE11 mode converters are used. Both converters use periodic waveguide wall perturbations, the case of the former, circularly symmetric perturbations, and in the case of the latter, curvature perturbations. The lengths of the mode converters were successfully shortened with high mode conversion efficiencies by decreasing the number of periods and then optimizing the periods using quite a small diameter for the converters. The 30ϕ TE02-to-TE01 mode converter is 103 · 34 mm long and its mode conversion efficiency measured by a scalar network analyser is 96% at 28GHz. The TE01-to-TE11 mode converter is 427 · 38 mm long and has an efficiency of 79%, which is 10% lower than the design value. This discrepancy resulted mainly from mode conversion from the TE11 to the TE12 mode, caused by the smaller number of...

In this paper, a non-constant serpentine TM/sub 01/ to TE/sub 11/ mode converter with short length for X-band high power microwaves applications is presented. The mode converter is designed to operate at 10 GHz and utilize the structure of circular waveguide with an excessive curvature in order to convert efficiently between two modes. To achieve optimum design, the basic parameters are calculated by the approximation method proposed in this paper and optimized by FIM simulator. The proposed mode converter with a length of 200 mm and more than 95% efficiency is simulated and measured.

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.

In a waveguide‐based coupled‐mode system, various mode conversions are achieved by designing mode evolution procedures. However, the differences of refractive indices among modes exceed the capability of conventional tuning mechanism, so most reported mode manipulation devices can only realize fixed mode conversions. This paper introduces the cladding medium switching strategy to the subwavelength grating waveguide‐based coupled‐mode system, and proposes a reconfigurable mode converter on silicon nitride platform. This work systematically analyzes the phase matching conditions on a coupled‐mode system. Results show that for both whole and partial upper‐cladding switchable, it is possible to achieve reconfigurable mode conversions. For whole cladding switchable case, this work demonstrates a double‐state mode converter, which converts the input mode to TE2 and TE1 modes under cladding refractive indices of 1 and 1.7. This work expands it to triple‐state mode conversion with window‐style cladding, which converts the input to TE2, TE1, and TE0 modes under cladding of 1, 1.54, and 1.7. The optical bandwidths are relatively large, and the conversion efficiencies exceed 56% for all cases. The mechanism provides a non‐volatile and energy‐efficient way for tunable mode conversions, which is promising for applications in optical networks, communications, and sensors, etc.

A mode converter for X-band located between a generator and an antenna to convert TM01 to TE11 mode with high efficiency for a high power transmitting system is presented. The proposed mode converter has extended radius and excessive curvature with symmetric structure in order to convert between two modes efficiently with short length. The measured and simulated results of the return loss, mode patterns and efficiency of the proposed mode converter with 200 mm length are provided.

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.

We experimentally investigate the generation of high-order modes (HOMs) with flexible switchable output. Cascaded acoustically induced fiber gratings (AIFGs) act as mode converters. A periodic micro-bending in the AIFG is produced by travelling flexural acoustic wave, and two modes with adjacent azimuthal numbers can be coupled to each other. Different HOMs (LP21a/b and LP11a/b) could be obtained just by tuning the frequency of the radio frequency (RF) driving signal. The dependence of RF frequencies on the two acousto-optic (AO) coupling stages is theoretically analyzed and experimentally verified. Based on cascaded AIFGs mode converter, a multiple-level intensity modulator and an all-fiber Yb-doped laser with flexible mode switching output of HOMs (LP21a/b and LP11a/b) are demonstrated. Moreover, by optimizing the outer diameter of the second part of the AO coupling regions, the mode conversion between LP01 and LP21 can be realized by applying a single frequency on the piezoelectric transducer (PZT).

An S‐band sectoral waveguide (SWG) mode converter was presented for simulation and electromagnetic analysis. A mode‐matching technique (MMT) for the electromagnetic analysis of an SWG mode converter has been proposed. The problem of trifurcation of the coaxial waveguide to shape SWGs and bifurcation of SWGs is discussed. In this framework, the selections of appropriate testing modes at each junction are justified. The simulation time has improved and the complexity of the proposed problem has reduced by the proposed approach. The numerical results of the MMT are compared against simulation results and are found to be in good agreement. The efficiency of both the simulated and analyzed results shows a mode conversion efficiency higher than 98.0%. An experimental evaluation shows the mode conversion of TM01 to TE11 mode using far‐field radiation patterns and has been compared with simulation results. Also, the MMT and simulation validate the mode conversion of TM01 to TE11 mode.

Computational inverse design techniques have shown potential to become reliable means for designing compact nanophotonic devices without compromising the performance. Much effort has been made to reduce the computation cost involved in the optimization process and obtain final designs that are robust to fabrication imperfections. In this work, we experimentally demonstrate TE0-TE1 and TE1-TE3 mode converters (MCs) on the silicon-on-insulator platform designed using the computationally efficient shape optimization method. These MCs have mode conversion efficiencies above 95%, and the insertion loss ranges from 0.3 dB to 1 dB over a wavelength span of 80 nm ranging from 1.5 µm to 1.58 µm. Maximum modal crosstalk found experimentally in the C-band is -19 dB. The conversion efficiency drops at most by 2.2% at 1.55 µm for 10 nm over/under etch, implying good robustness to dimensional variations. We present the mode conversion mechanism of these MCs by studying the simulated electromagnetic field patterns and validate with supportive data. We also demonstrate their performance in the time domain with a 28 Gbps OOK and a 20 GBaud PAM-4 payload transmissions, which supports their utility for high throughput data communications. The open eye diagrams exhibit Q-factors of 8 dB.

Ultra-compact mode converter based on longitudinal refractive index modulation

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 ( μ m), a high mode conversion efficiency (>97.6%), and a low modal crosstalk (< −20.0 dB) over a broad wavelength range from 1500 to 1600 nm (∼100 nm). In addition, based on different polygonal slots, other types of mode conversions such as from TE1 to TE3 and from TE2 to TE1 have be realized. Fabrication tolerance of the proposed structure is analyzed. Owing to the high efficiency and compact size, the proposed structure could be applied to on-chip mode division multiplexing systems for high-density integration.

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.