Higher Order Mode Couplers Research Articles

Polarization-Insensitive Lithium Niobate-on-Insulator Interferometer.

The key components of a polarization-independent electro-optic (EO) interferometer, including the beam splitter, mode converter, and directional coupler, are designed based on a lithium niobate (LN) platform on an integrated insulator to ensure high extinction ratios. By elaborately designing the geometric structure of the multimode interference (MMI) coupler, beam splitting of equal proportions and directional coupling of higher-order modes are realized. The most prominent characteristic of the proposed interferometer is polarization insensitivity, which is realized by converting TM polarization into TE polarization using a mode converter, providing conditions for the study of light with different polarizations. At 1550 nm, the visibility of the interferometer is 97.59% and 98.16% for TE and TM, respectively, demonstrating the high performance of the proposed LN polarization-independent interferometer.

Higher-order mode analysis for SOLEIL-type superconducting cavity

A 499.8 MHz SOLEIL-type superconducting cavity was simulated and designed for the first time in this paper. The higher-order mode (HOM) properties of the cavity were investigated. Two kinds of coaxial HOM couplers were designed. Using 4 L-type and 4 T-type HOM couplers, the longitudinal impedance was suppressed to 3 kΩ, and the transverse impedance below 30 kΩ/m. The HOM damping requirements of Hefei Advanced Light Facility (HALF) were satisfied. This paper conducts an in-depth study on the radio frequency (RF) design, multipacting optimization, and thermal analysis of these coaxial couplers. Simulation results indicated that under operating acceleration voltage, the optimized couplers does not exhibit multiplicating or thermal breakdown phenomena. The cavity has the potential to reach a higher acceleration gradient.

Coupled Nanomechanical Graphene Resonators: A Promising Platform for Scalable NEMS Networks.

Arrays of coupled nanoelectromechanical resonators are a promising foundation for implementing large-scale network applications, such as mechanical-based information processing and computing, but their practical realization remains an outstanding challenge. In this work, we demonstrate a scalable platform of suspended graphene resonators, such that neighboring resonators are persistently coupled mechanically. We provide evidence of strong coupling between neighboring resonators using two different tuning methods. Additionally, we provide evidence of inter-resonator coupling of higher-order modes, demonstrating the rich dynamics that can be accessed with this platform. Our results establish this platform as a viable option for realizing large-scale programmable networks, enabling applications such as phononic circuits, tunable waveguides, and reconfigurable metamaterials.

Subwavelength grating-based silicon photonic TE mode division multiplexer for C + L band operation

This paper reports a subwavelength grating (SWG) based multiplexer (MUX) on a silicon photonics platform capable of multiplexing three transverse electric modes. The designed MUX is simulated using a commercial 3D finite-difference time-domain solver and shows broadband operation over the whole C and L optical telecom bands from 1530 nm to 1625 nm wavelength range. The effective indices of the Bloch modes in the SWG waveguides are extracted from the band structure plot. The designed MUX consists of two co-directional coupling regions for fundamental to higher-order mode coupling, with each coupling stage consisting of single-mode and multimode SWG waveguides. The transmission characteristics, viz. transmittance, insertion loss, and return loss, are presented and discussed. The coupling lengths without the tapering regions for TE0–TE1 and TE0–TE2 mode couplings are 14μm and 1.48μm, respectively. The transmittance is >78% with the highest insertion loss and return loss of 1.1 dB and –15 dB, respectively. At 1550 nm, the transmission is >88%, insertion loss is <0.6 dB, and return loss is <−15 dB. A uniform under-etch and over-etch of 5 nm are taken for the fabrication tolerance study, which shows a maximum variation of 0.58 dB for the insertion loss with return loss <−14.6 dB at 1550 nm. Over the whole simulated range, the insertion loss is <1.4 dB, and return loss is <−14.6 dB with ±10 nm change in device dimension. A temperature tolerance study with 50 °C and 100 °C rise in temperature has been done, and the device retains its broadband operation over the simulated range. The maximum increase in insertion loss is 0.1 dB for the TE0–TE2 coupling, while the overall return loss of the device decreases to <−20 dB for the TE0–TE1 coupling.

Numerical study on the nonlinear characteristics of shock induced two-dimensional panel flutter in inviscid flow

For an aeroelastic system of a two-dimensional elastic panel subjected to an impinging inviscid oblique shockwave, the nonlinear flutter characteristics are affected by many factors such as shock impingement location, cavity pressure and initial perturbation. The effects of the above factors on the variation of system bifurcation type and dynamic behaviors are investigated numerically. A low-fidelity computational method coupled with local piston theory and van Karman plate model, and a high-fidelity computational method coupled with Euler equations and finite element model are used for fluid-structure interaction simulations. Two sets of new findings are unveiled. First, either the variation of shock impingement location or cavity pressure can induce the aeroelastic system to transition between a subcritical bifurcation and a supercritical bifurcation. For some cases, the system bifurcation characteristics exhibit strong sensitivity to these two factors. Second, it is found that in addition to the limit cycle oscillation in the form of a combination of the second and third structural modes, multiple stable limit cycle oscillations due to the coupling of higher-order modes can be triggered by suitable initial perturbations. These limit cycle oscillations have high frequencies and amplitudes, which suggest a higher risk of structural fatigue failure.

Influence of disk aberrations on high-power thin-disk laser cavities.

We present a systematic study on the influence of thin-disk aberrations on the performance of thin-disk laser oscillators. To evaluate these effects, we have developed a spatially resolved numerical model supporting arbitrary phase profiles on the intracavity components that estimates the intracavity beam shape and the output power of thin-disk laser oscillators. By combining this model with the experimentally determined phase profile of the thin-disk (measured with interferometry), we can predict the operation mode of high-power thin-disk lasers, including mode degradation, higher-order mode coupling, and stability zone shrinking, all of which are in good agreement with experiment. Our results show that one of the main mechanisms limiting the performance is the small deviation of the disk's phase profile from perfect radial symmetry. This result is an important step to scaling modelocked thin-disk oscillators to the kW-level and will be important in the design of future active multi-pass cavity arrangements.

Impedance minimization of the CERN Super Proton Synchrotron cavities using the generalized coupled S -parameter method

In the context of the Large Hadron Collider (LHC) injector upgrade, components with high contribution to the beam coupling impedance of the injector chain have to be identified and optimized to ensure the delivery of high-intensity proton beams to the LHC. The Super Proton Synchrotron (SPS) is the last accelerator in the LHC injector chain. In the existing design, the longitudinal beam coupling impedance of the SPS cavities limits the increase of the beam intensity in the SPS ring. Since the 200 MHz traveling wave cavities are one of the main contributors to the overall beam coupling budget of the SPS machine, different types of higher-order mode (HOM) couplers are used in the long 33-cell and 44-cell cavities for the damping of various HOMs. The location of the HOM couplers in the cavity, as well as their shape, affects the damping of HOMs. Finding a suitable arrangement of the HOM couplers requires solving a discrete optimization problem. The repetitive calculation of the beam coupling impedance by the conventional time-domain wakefield solvers in the optimization is hindered by the large size of the SPS cavities. The generalized coupled $S$-parameter method is a domain decomposition method for the calculation of the $S$ parameters and beam coupling impedance of large structures. In this paper, this method is employed to calculate the beam coupling impedance of the SPS cavities. Then, an optimization method is proposed to find an optimal arrangement of the HOM couplers in the cavities. The article presents the geometrical details of the SPS cavities, a short description of the generalized coupled $S$-parameter method, and a discrete optimization method applied to the SPS cavities.

Optimal design and HOM damping of a 500 MHz 5-cell copper cavity for SAPS

In order to meet the requirements of the Southern Advanced Photon Source (SAPS), a slot-coupled π-mode 5-cell copper cavity with higher order mode (HOM) damping was proposed and developed. In this study, we present the optimal design of a 5-cell copper cavity and an investigation of HOM impedances. A prototype of 5-cell copper cavity based on this design was manufactured. The RF characteristics of 5-cell copper cavity have been tested. The results show that a flat accelerating electric field is obtained, with a coupling coefficient of 1.7% and a shunt impedance more than 19.5 MΩ/m. To damp these HOMs with high impedance, two inductive HOM couplers and two capacitive HOM couplers were designed. Finally, the impedances of all the HOMs are below the threshold of coupled-bunch instabilities (CBI) for SAPS.

Quantum chemical rovibrational analysis of aminoborane and its isotopologues.

Aminoborane, H2 NBH2 and its isotopologues, H2 N10 BH2 , D2 NBD2 , and D2 N10 BD2 , have been studied by high-level ab initio methods. All calculations rely on multidimensional potential energy surfaces and dipole moment surfaces including high-order mode coupling terms, which have been obtained from electronic structure calculations at the level of explicitly correlated coupled-cluster theory, CCSD(T)-F12, or the distinguishable cluster approximation, DCSD. Subsequent vibrational structure calculations based on second-order vibrational perturbation theory, VPT2, and vibrational configuration interaction theory, VCI, were used to determine rotational constants, centrifugal distortion constants, vibrationally averaged geometrical parameters and (an)harmonic vibrational frequencies. The impact of core-correlation effects is discussed in detail. Rovibrational VCI calculations were used to simulate the gas phase spectra of these species and an in-depth analysis of the ν7 band of aminoborane is provided. Color-coding is used to reveal the identity of the individual progressions of the rovibrational transitions for this particular mode.

Multimode Nested Antiresonant Hollow Core Fiber

A novel centro-symmetric nested antiresonant fiber (CNAF) design is proposed and investigated numerically for low-loss, multimode applications. Conventional single tube-lattice and nested antiresonant hollow core fibers (ARHCFs) inherently support multiple core modes, but the higher order modes are more prone to coupling to lossy cladding modes, thereby dissipating power along the length of the fiber. In the proposed CNAF design, coupling of higher order modes to leaky cladding modes is inhibited by bifurcating the airy region of the cladding capillaries with centrosymmetric nested tubes. The designed fiber is shown to support 5 to 9 spatially distinct core mode groups, having confinement loss less than 0.002 dB/m to 0.01 dB/m respectively. We present detailed numerical investigation into the multimode operation of the proposed fiber and its superiority over conventional asymmetric nested tube designs.

Coherent terahertz radiation with orbital angular momentum by helically microbunched electron beam

A scheme to generate coherent terahertz (THz) radiation carrying orbital angular momentum (OAM) through down-frequency conversion of optical lasers and higher order mode coupling is proposed and analyzed. Based on the laser–electron interaction in the undulator under a linear regime, the scheme utilizes dual-frequency lasers resonating at the harmonics of two consecutive undulators and a dispersion section to impose a THz helical microbunching in the electron beam. By choosing appropriate parameters of lasers and dispersion section, the density modulation can be down-converted to a THz spectral-domain and the central frequency can be extensively tuned together with the helical mode number. Then, the helically microbunched beam can be used as a source of THz OAM radiation in a downstream radiator. We expect that the intense vortex beam in the terahertz band will bring new opportunities for the research of quantum topological materials.

Simulation study of phase-matched THz emission from an axially modulated magnetized plasma

We studied and demonstrated the emission of THz waves by the beating of two CW-laser beams in an axially modulated plasma in the presence of a static magnetic field (applied transversely to the direction of propagation of lasers) from two-dimensional particle-in-cell simulation. The ponderomotive-force-induced nonlinear current drives THz radiation with frequency close to the beat frequency (Δω = ω1 − ω2) that propagates in the forward direction. Inside the plasma, THz radiation consists of a mixture of linear and radial polarization, while outside the plasma, only linear polarization dominates; however, radial polarization is stronger in the case of unmagnetized plasma. When the ripple is used, several modes are excited and the mode excited at Δk = (k1 − k2) − kq that meets the phase matching condition is resonantly enhanced. The THz field amplitude grows resonantly with time and it becomes saturated due to the excited higher-order mode coupling. The saturation level is reached about five times higher than the nonrippled plasma case in the vacuum side.

Development and vertical tests of 650 MHz 2-cell superconducting cavities with higher order mode couplers

650 MHz 2-cell superconducting cavities are proposed for the main ring of the Circular Electron–Positron Collider (CEPC). The cavity is designed with two detachable double-notch higher order mode (HOM) couplers to suppress the fundamental mode and extracted the HOM power. The present paper describes the design, fabrication, surface treatment and vertical tests of the cavities and HOM couplers. The performances of the cavity at 2 K is not affected after the installation of the HOM coupler. The maximum intrinsic quality factor Q0 of the cavity dressed with the HOM coupler reached 3.1×1010 at operation accelerating gradient. The vertical tests results showed that the fundamental mode external quality factor Qe of all HOM couplers is an order of magnitude larger than Q0 of the cavity. The HOM damping results for the 650 MHz 2-cell cavity were also measured at cryogenic temperature and compared with the simulated and measured results at room temperature. Two 650 MHz 2-cell cavities each with two HOM couplers will be assembled in the CEPC 650 MHz test cryomodule after helium vessel dressing.

A wide stopband multilayer substrate integrated waveguide bandpass filter with suppression of higher-order mode coupling

AbstractThis paper presents a compact multilayer substrate integrated waveguide (SIW) bandpass filter with wide stopband. The square SIW cavity resonators in multilayer substrates are horizontally and vertically coupled with magnetic coupling. By properly designing the coupling structure, the couplings between the higher-order resonant modes can be suppressed. Compared with the conventional single-layer SIW filters, the proposed multilayer SIW filter also exhibits a compact size. To verify the design concept, a double-layered SIW filter is fabricated and measured. The measured results agree well with the simulations. The measured results show that the upper stopband of the filter is extended to 2.73 times of the center frequency 8.02 GHz.

Integrated end-to-end instrumentation for the study of whispering gallery modes in dynamic liquid micro-cavities coupled to sub-micron tapered fibers

An integrated table-top facility with the end-to-end instrumentation for the study of high-Q Whispering Gallery Modes in solid and liquid micro-cavities is described, with emphasis on the in situ fabrication of reliable tapered fibers of sub-micron waist sizes for coupling light into time-dependent liquid micro-cavities. The experimental parameters have been chosen to get nearly adiabatic tapers, with their waist size consistent with the theoretical model. The oscillations in the transmitted power during fiber tapering are monitored to check the coupling of higher order modes and identify the point when the fiber is suitable for pure single mode coupling. The fabricated tapered fibers have greater than 85% transmission on the average and very good polarization fidelity. The tapered fibers have been used for efficiently exciting and detecting WGMs of Q∼107 in silica microspheres and of Q up to 7×107 in microdrops of silicone oils. The set-up is suitable for conversion to a transportable system, for sensing applications involving WGMs in liquid microdrops.

Thermal Optimization of Functional Insertion Components (FIC) for Cryogenic Applications

Various functional insertion components (FIC), directly connecting a cold mass and the ambient environment, are irreplaceable and play crucial technical roles in many superconducting cryogenic applications. However, such components also bring a huge heat leak to the cold mass. The heat leak is usually much greater than that through entire evacuated MLI insulation system and solid support structures combined. Therefore, this situation brings unimaginable challenges not only to the refrigeration loads to be met but also critical aspects of the FIC design in satisfaction of highly restrictive, even contradicting technical functions. The FIC must simultaneously minimize the heat leak provide a large amounts of DC current and RF power to/from the cold mass, as well as provide a reliable ultrahigh vacuum thermal isolation break with strong mechanical stability. Reviewed are the following commonly used FICs: 1) various RF input couplers for transmitting MW-RF power; 2) high DC power current leads for energizing various SC magnets; 3) various high order mode (HOM) couplers for damping unwanted RF energy; 4) instrumentation cable/wire to cold mass. Approaches to efficiently minimize the DC current heating, RF surface heating, heat leak through the solid body of FIC, efficient cooling approaches, while reliably providing the technical functions, are briefly summarized and compared for select applications from around the world.

Higher order mode coupler for the circular electron positron collider

A conceptual design report (CDR) for the Circular Electron Positron Collider (CEPC) was published in September 2018. CEPC will use a 650 MHz radiofrequency (RF) system with 240 two-cell cavities. The collider will be a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher order modes (HOMs) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. This study examined the impedance budget and HOM damping requirements for the CEPC ring to keep the beam stable, and a coaxial HOM coupler was considered for extracting the beam-induced HOM power. A double-notch coupler was chosen because of its wide bandwidth for the fundamental mode. Each cavity has two detachable coaxial HOM couplers mounted on the cavity beam pipe with a HOM power handling capacity of 1 kW. This paper presents the design process of the HOM coupler: the RF design, thermal analysis, and mechanical aspects. To validate the design, a prototype of the HOM coupler and a coaxial line test bench were fabricated and tested, and the test results agreed well with the simulation results. The HOM damping results for a real 650 MHz two-cell cavity were also measured and compared with the damping requirements.

HOM damping requirements and measurements for CEPC 650-MHz 2-cell cavity

The circular electron–positron collider (CEPC) will use a 650-MHz RF system with 240 two-cell cavities for the collider. The collider is a double ring with shared cavities for Higgs operation and separate cavities for W and Z operations. The higher-order modes (HOM) excited by the intense beam bunches must be damped to avoid additional cryogenic loss and multi-bunch instabilities. To get the real damping results, two prototypes of HOM coupler have been fabricated and installed on the 650-MHz two-cell cavity. The HOMs have been verified by bead pulling method. A test bench with two 2-cell cavities is used to measure the real damping results and study HOM propagating properties for a cavity string. In this paper, the impedance budget, HOM damping and HOM power requirements for the CEPC collider ring are given. The damping results measured for the fundamental mode and HOMs seem good compared with the simulated results. The absorbing efficiency of the absorber and the extraction power efficiency of HOM couplers were also achieved.

Purification of Selective Laser Melting Additive Manufactured Niobium for Superconducting RF-Applications Transactions on Applied Superconductivity

Superconducting radio frequency (SRF)-cavities in the field of particle accelerators are mostly manufactured from bulk or sheet niobium (Nb). Complex shapes of SRF-components like high order mode-couplers or cavities working at high fields are high in cost and lead-time. The fabrication of these parts via additive manufacturing promises a more effective production and higher flexibility in geometries. At CERN, additive manufacturing with pure Nb by selective laser melting (SLM) is under development for future particle accelerator components. In comparison with conventionally produced Nb SRF-components, made of highly pure Nb, the SLM-manufactured Nb material contains considerable impurities due to raw material processing and the SLM-process conditions itself. Contaminations with light elements, such as oxygen and nitrogen, introduced into the material's structure by the powder and the SLM process, contribute to an important lowering of the residual resistance ratio (RRR). In order to increase the RRR-values for conventionally manufactured SRF-cavities, titanium (Ti) gettering-heat treatments were investigated since the 1980s. This paper introduces the first representative Nb-material realized at CERN by additive manufacturing by SLM and the current studies on the post-treatment of sample material by vacuum firing and Ti-gettering-heat treatments. The samples are analyzed by mass spectroscopy and RRR-measurements in correlation with different process parameters. First trials with Ti-gettering heat treatments demonstrate an increase in the RRR from 7 to above 100, whereas the oxygen content could be reduced from 600 to 17 ppm. These results show that a purification post-treatment of SLM-manufactured Nb-components is feasible and pave the way for functioning SRF-components made by additive manufacturing.

HOM coupler design for 166.6 MHz SC cavity

Higher-order modes (HOMs) may affect beam stability and refrigeration requirements of superconducting cavity such as the 166.6 MHz SC cavity, which is studied at IHEP. Under certain conditions, beam-induced HOMs can accumulate sufficient energy to destabilize the beam or quench the superconducting cavities. In order to limit these effects, we consider the use of the HOM coupler with HOMs extraction on the cutoff tubes of the 166.6 MHz SC cavity. The HOM coupler can deeply suppress the fundamental mode and efficiently extract dangerous high-order modes. In this paper, the HOM coupler design process is presented, which operates at 166.6 MHz. The RF and thermal behavior are discussed. In order to verify the designs, a rapid prototype for the favored coupler was fabricated and characterized on a low-power test stand.