Mode Suppression Research Articles

Parity time symmetry all-optical microwave oscillator using a polarization dependent Sagnac loop

An all-optical microwave oscillator based on parity-time symmetry was proposed and experimentally demonstrated. It mainly consists of a big feedback loop, an active small loop and a polarization-dependent Sagnac loop. The microwave seed signal originates from the beating between the continuous wave and the Stokes wave. The function of microwave envelope detection and feedback modulation are implemented through cross gain modulation of two semiconductor optical amplifiers. By employing the parity time symmetry loops and the Vernier effect, the single mode selectivity capability of the oscillator has been enhanced, and a single mode microwave signal with high quality can be generated. In the experiment, a highly quality microwave signal with a frequency of 10.71 GHz was generated. The single side band phase noise can reach −94.1 dBc/Hz@10 kHz and the side mode suppression ratio of the generated microwave signal reaches 41 dB. Compared to the previously reported all-photonic microwave oscillator, the single side band phase noise is improved by 7 dB at a frequency offset of 10 kHz, and by 20 dB at a frequency offset of 100 Hz. The results indicate that our proposed scheme significantly improves the quality of the generated microwave signals.

SAW velocity reduction and spurious modes suppression on the layer structure

This paper discusses the use of the layered structure for the SAW velocity reduction. First, it is shown that the insertion of SiO2 is crucial between the piezoelectric layer and the base substrate to keep a large electromechanical coupling factor even when the SAW velocity is reduced. It is also shown that the use of a high-velocity substrate such as Si is also crucial for bringing spurious resonances much higher than the main resonance. The impact of Si orientation is also discussed. Next, suppression of transverse mode resonances and lateral leakage is studied. It is shown that the double busbar structure offers Q enhancement through good lateral energy confinement, and its combination with the piston design also gives good transverse mode suppression. Furthermore, discussions are extended to a strong spurious resonance that appears near the anti-resonance.

Ultrahigh Transverse Mode Purity by Enhanced Modal Filtering in Double‐Clad Single‐Ring Hollow‐Core Photonic Crystal Fiber

AbstractMaintaining high fundamental mode purity in low‐loss single‐ring hollow‐core photonic crystal fiber remains a key challenge. Here a double‐clad anti‐resonant‐reflecting design is reported that, through modal filtering, provides high suppression of higher‐order modes while maintaining a low‐loss fundamental mode. The structure consists of a ring of six thin‐walled capillaries fused to the inner wall of a thicker‐walled capillary that is in turn fused at a single point to the silica outer cladding. Greater than 40 dB m−1 suppression of higher order modes over a 6 nm wide window whose central wavelength can be tuned by adjusting the gas pressure is observed. The novel design paves the way to practical applications of such fibers in high‐quality laser beam delivery and low‐noise optical‐signal transmission.

Experimental Study of Transverse Mode Suppression on Wideband Hetero Acoustic Layer Surface Acoustic Wave Resonator.

Transverse mode suppression is a great challenge for high-performance surface acoustic wave (SAW) resonators. Conventional methods work well on narrowband resonators, but their performances on wideband resonator have not been demonstrated. In this article, we give an in-depth study on the transverse mode suppression of wideband resonators using 11° YX-LiNbO3 (LN)/70 °Y90°X -quartz (Qz) hetero acoustic layer structure as a platform. Two groups of design, including new dummy electrode and zigzag shape apodization, are proposed. The measured results show that the shape of the dummy electrode is not the dominant factor to affect the transverse mode. The proposed zigzag shape apodization can effectively suppress the transverse, at the same time maintain the quality ( Q ) factor at the same level with the normal type. Additionally, stronger suppression ability can be realized with a tiny tradeoff of Q -factor.

Use of periodically slotted SiO2 in a layered surface acoustic wave structure for transverse mode suppression

This work demonstrates the applicability of periodically slotted SiO2 (PSS) structures (PSSs) for controlling the urface acoustic wave (SAW) slowness shape and eliminating the transverse modes in layered SAW structures represented by the Incredible High Performance SAW. The use of PSS comprising SiO2 blocks and air gaps provides larger acoustic anisotropy for slowness shape manipulation. The effect of PSSs on transverse mode suppression is revealed. The slowness curves and frequency responses with different structural parameters of PSS are calculated by FEM analysis. Then, an electrode is designed for further spurious resonance suppression. A hammer-head configuration and Si3N4 overlay are applied. It is shown that higher-order transverse modes in the passband were well suppressed without reduction of the electromechanical coupling factor.

Effects of counter-current driven by electron cyclotron waves on neoclassical tearing mode suppression

Through theoretical analysis, we construct a physical model that includes the influence of counter-external driven current opposite to the plasma current direction in the neoclassical tearing mode (NTM). The equation is used with this model to obtain the modified Rutherford equation with co-current and counter-current contributions. Consistent with the reported experimental results, numerical simulations have shown that the localized counter external current can only partially suppress NTM when it is far from the resonant magnetic surface. Under some circumstances, the Ohkawa mechanism dominated current drive (OKCD) by electron cyclotron waves can concurrently create both co-current and counter-current. In this instance, the minimal electron cyclotron wave power that suppresses a particular NTM was calculated by the Rutherford equation. The result is marginally less than when taking co-current alone into consideration. As a result, to suppress NTM using OKCD, one only needs to align the co-current with a greater OKCD peak well with the resonant magnetic surface. The effect of its lower counter-current does not need to be considered because the location of the counter-current deviates greatly from the resonant magnetic surface.

Behaviors of higher-order modes in I.H.P. SAW devices and discussion of their suppression

In this work, behaviors of higher-order modes in an I.H.P. SAW device with a layered LiTaO3/SiO2/SiN/Si substrate are investigated, and techniques for their suppression are discussed. First, variation of the higher-order mode responses with structural parameters are analyzed using two-dimensional FEM. Then, on the basis of the calculated data, structural parameters are determined for higher-order modes suppression while retaining the main mode characteristics. In addition, the effectiveness of the optimized structure is confirmed using measured data from a fabricated one-port resonator and filters. In the measured data, it is shown that higher-order modes in a specific frequency range are successfully suppressed, while the main made characteristics, such as Bode-Q, bandwidth, and TCF, are almost equivalent to those of conventional I.H.P. SAW resonators and filters.

Optofluidic tunable filters using ionic liquid electrolyte capacitors.

Tunable optical filter is a basic component for most optical systems. This study reports a unique design of Fabry-Pérot (FP) tunable filter by using an ionic liquid solution. The tunable filter consists of two neighboring regions: capacitor region and FP region. The former is in the form of electrolyte capacitor and the latter remains transparent as an FP cavity for light transmission. When the capacitor region is applied with a bias voltage, it attracts the ions from the FP region and thus reduces the ion concentration of the FP region, resulting in a change of the refractive index and eventually a shift of transmission peak of the FP cavity. Among four electrolyte solutions studied, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) exhibits the best overall performance, such as low insertion loss (3.2 dB), large side mode suppression ratio (23 dB) and high stability (drift <0.2 nm). Additionally, a wavelength tuning of 0.17 nm/V is achieved over 0-17 V, providing a tunable range of 3 nm. This device features low bias voltage, no mechanical movement, easy fabrication and seamless integration with microfluidics systems, and may find potential applications in spectral analyzers and lab-on-a-chip biosensing systems.

Realization of a Wide Stopband Filter Using the Weakest Electric Field Method and Electromagnetic Hybrid Coupling Method

To enhance filter performance in terms of stopband, we propose a wide-stopband substrate-integrated waveguide filter based on the rejection of higher-order modes. The filter establishes the inner and outer coupling windows at the mode’s weakest electric field to achieve mode suppression. Simultaneously, we employ the electromagnetic hybrid coupling theory to analyze the electromagnetic distribution of the particular modes and determine the dimensions of the coupling circular apertures and slits based on the extracted coupling coefficients to attain suppression of the specific modes. The filter successfully suppresses all higher-order modes having resonant frequencies below that of the TE150 mode, as demonstrated by simulation results. We conducted measurements and found that the filter has a center frequency of 4.78 GHz, a bandwidth of 100 MHz with a -3 dB attenuation, and a stopband of -25 dB that extends up to 16.62 GHz (i.e., 3.48 times the center frequency). Simulation and measurement results demonstrate a good correlation. This method, compared to other SIW filters, is easy to design, has a wider stopband range, can be readily applied to practical microwave communication systems, and has potential applications.

Application of Regularization Methods in the Sky Map Reconstruction of the Tianlai Cylinder Pathfinder Array

The Tianlai cylinder pathfinder is a radio interferometer array to test 21 cm intensity mapping techniques in the post-reionization era. It works in passive drift scan mode to survey the sky visible in the northern hemisphere. To deal with the large instantaneous field of view and the spherical sky, we decompose the drift scan data into m-modes, which are linearly related to the sky intensity. The sky map is reconstructed by solving the linear interferometer equations. Due to incomplete uv coverage of the interferometer baselines, this inverse problem is usually ill-posed, and regularization method is needed for its solution. In this paper, we use simulation to investigate two frequently used regularization methods, the Truncated Singular Value Decomposition (TSVD), and the Tikhonov regularization techniques. Choosing the regularization parameter is very important for its application. We employ the generalized cross validation method and the L-curve method to determine the optimal value. We compare the resulting maps obtained with the different regularization methods, and for the different parameters derived using the different criteria. While both methods can yield good maps for a range of regularization parameters, in the Tikhonov method the suppression of noisy modes are more gradually applied, produce more smooth maps which avoids some visual artefacts in the maps generated with the TSVD method.

Modal analysis and higher order mode suppression of a high impedance surface-based bowtie antenna

Abstract The present work studies the design of a high impedance surface (HIS)-based bowtie antenna in the framework of characteristic mode analysis (CMA) and proposes the method of higher order mode suppression. A triangular-elliptical bowtie antenna operating in the frequency range of 1.6–6 GHz is designed. The radiating and higher order modes of the proposed antenna are identified using CMA, and an HIS structure is used to enhance the desired mode and to suppress the higher order mode in order to get high gain, good front-to-back ratio (FBR), and stable radiation characteristics. The final designed HIS-based bowtie antenna gives stable radiation patterns from 1.7 to 5.5 GHz with a maximum boresight gain of 10.5 dB. Also, gain from 6.5 to 12 dB and FBR from 8 to 18 dB are obtained in the operating bandwidth. The proposed antenna features the advantages of low profile, wideband and high boresight gain making it suitable for ground-penetrating radar applications.

Progress towards edge-localized mode suppression via magnetic perturbations in hydrogen plasmas

The suppression of edge-localized modes (ELMs) by applying resonant magnetic perturbations (RMPs) is well studied in low collisionality deuterium plasmas as a measure to reduce transient divertor heat loads. However, ELM suppression has yet to be demonstrated in non-nuclear fuels such as hydrogen and hydrogen + helium mixtures which are the main ion species to be used in the ITER pre-fusion power operation (PFPO) phase. For the first time, attempts have been made to access ELM suppression with RMPs in ITER-like low collisionality hydrogen plasmas at DIII-D and ASDEX Upgrade. The DIII-D experiments focused on operation with injected power slightly above the L–H power threshold similar to the expected conditions in the ITER PFPO phase with limited external heating power. The RMPs were found to trigger H–L backtransitions, which is shown to be avoided by reducing the L–H power threshold by diluting the plasma with helium. The additional helium combined with a larger measured neutral density of hydrogen inside the separatrix compared to ELM suppressed deuterium plasmas precluded access to a pedestal top density below the known RMP-ELM suppression threshold. At ASDEX Upgrade, RMP-ELM suppression has been achieved when the concentration of 1H in the hydrogen isotope mix is below 40% . While all known access criteria for RMP-ELM suppression were met above this threshold, full ELM suppression was replaced by strong mitigation. The most prominent difference between the hydrogen and deuterium plasmas was a change of turbulence characteristics in the pedestal where Doppler reflectometry measurements suggest a significant reduction of turbulence even at small hydrogen concentrations. In conclusion, these experiments not only identify issues that may prevent access to RMP-ELM suppression in the ITER PFPO phase, but also highlight missing physics in our current understanding of RMP-ELM suppression such as potentially the role of turbulence in the pedestal gradient region.

Robustness of the modified multi-trench fiber design with two gaps.

We investigate the robustness of a modified multi-trench fiber (MTF) design with two gaps numerically. The excellent suppression of high-order modes is demonstrated over a wide range of the gap misalignment and the fundamental mode loss is barely affected even with the 5 dB/m scattering loss in gaps at the modified two-gap MTF for the first time. Therefore, the required fabrication accuracy decreases.

Highly-reflective facet-coated multi-wavelength DFB laser array with exact wavelength spacings.

A distributed feedback (DFB) laser array of twenty wavelengths with highly reflective and anti-reflective (HR-AR) coated facets is both theoretically analyzed and experimentally validated. While the HR facet coating enhances high wall-plug efficiency, it inadvertently introduces a random facet grating phase, thereby compromising the lasing wavelength's predictability and the stability of the single-longitudinal-mode (SLM). In this study, two key advancements are introduced: first, the precisely spaced wavelength is achieved with an error of within ±0.2 nm using the reconstruction-equivalent-chirp (REC) technique; second, the random grating phase on the HR-coated facet is compensated by a controllable distributed phase shift through a two-section laser structure. The SLM stability can be improved while the wavelength can be continuously tuned to the standard wavelength grid. The overall chip size is compact with an area of 4000 × 500 µm2. The proposed laser array has a light power intensity above 13 dBm per wavelength, a high side mode suppression ratio above 50 dB, and low relative intensity noise under -160 dB/Hz. These attributes make it apt for deployment in DWDM-based optical communication systems and as a light source for optical I/O.

Mitigating modal noise in multimode circular fibres by optical agitation using a galvanometer

Abstract Modal noise appears due to the non-uniform and unstable distribution of light intensity among the finite number of modes in multimode fibres. It is an important limiting factor in measuring radial velocity precisely by fibre-fed high-resolution spectrographs. The problem can become particularly severe as the fibre’s core become smaller and the number of modes that can propagate reduces. Thus, mitigating modal noise in relatively small core fibres still remains a challenge. We present here a novel technique to suppress modal noise. Two movable mirrors in the form of a galvanometer re-image the mode-pattern of an input fibre to an output fibre. The mixing of modes coupled to the output fibre can be controlled by the movement of mirrors applying two sinusoidal signals through a voltage generator. We test the technique for four multimode circular fibres: 10 and 50 $\mu$m step-index, 50 $\mu$m graded-index, and a combination of 50 $\mu$m graded-index and 5:1 tapered fibres (GI50t). We present the results of mode suppression both in terms of the direct image of the output fibre and spectrum of white light obtained with the high-resolution spectrograph. We found that the galvanometer mitigated modal noise in all the tested fibres, but was most useful for smaller core fibres. However, there is a trade-off between the modal noise reduction and light-loss. The GI50t provides the best result with about 60 per cent mitigation of modal noise at a cost of about 5 per cent output light-loss. Our solution is easy to use and can be implemented in fibre-fed spectrographs.

Pressure induced emission enhancement (PIEE) in solid-state 2,3,4,5-tetraphenylthiophene: a QM/MM study.

Organic fluorophores exhibit pressure-dependent behaviors that are often irregular and contingent upon the specific system. Elucidating how pressure influences these behaviors is essential for the accurate design of piezochromic materials. Here, we conducted an exhaustive theoretical analysis of the excited-state decay processes of crystalline 2,3,4,5-tetraphenylthiophene (TPT) at high pressure through a combined quantum mechanics and molecular mechanics (QM/MM) method. The study revealed that the fluorescence quantum yield experiences a pronounced initial increase owing to the decrease of nonradiative decay IC rate (k ic). The suppression of low-frequency modes results in the decrease of λ e, and then reduces the electron-vibration couplings, and finally slows down the non-radiative process. Our research provides detailed mechanism analyses on PIEE properties of solid state TPT, aiding the rational design of advanced PIEE materials.

Double Busbar Tilted Structure for Low Temperature Drift and Spurious Modes Suppression in SAW Resonators Using 64° YX-LN Thin Plate

Double Busbar Tilted Structure for Low Temperature Drift and Spurious Modes Suppression in SAW Resonators Using 64° YX-LN Thin Plate

Calibration of toroidal visible bremsstrahlung diagnostics and reconstruction of effective charge profiles in Korea Superconducting Tokamak Advanced Research (KSTAR).

The investigation of impurity behavior in fusion plasmas is a critical issue in fusion plasma research. The effective charge (Zeff) profile is a widely used measure of the impurity levels in fusion plasmas. In this study, the visible bremsstrahlung emissivity profile is reconstructed using toroidal visible bremsstrahlung (TVB) arrays at Korea Superconducting Tokamak Advanced Research (KSTAR). KSTAR TVB arrays have recently been developed and calibrated using a halogen light source and an integrating sphere. The reconstruction algorithm has been developed using the Phillips-Tikhonov method, and the reconstruction accuracy is assessed with test profiles. Electron density and temperature profiles from Thomson scattering diagnostics are fitted for Zeff calculations. Subsequently, the Zeff profiles in the edge localized mode suppression experiment are reconstructed. In addition, line-averaged Zeff values in the 2020 KSTAR campaign are presented, which are mostly distributed from two to four.

Hybrid and gradient design of ultra-thin-ply composite laminates for synergistic suppression of delamination and fiber fracture damage modes

Ply thickness hybridization is a common strategy in the layup structure of composite laminates. The rational allocation of thin-ply and thick-ply to effectively improve the performance of composite materials is a shared goal. Compared to standard laminates, ultra-thin-ply laminates have many advantages, including design flexibility, better damage tolerance, and enhanced in-situ strength. However, the risk of fiber fracture triggered by ultra-thin-ply requires cautious application. Effectively reducing the risk of fiber fracture caused by ultra-thin-ply laminates has become a challenge in the field of composite materials. Therefore, this paper proposes a thick/thin/thick gradient structure to balance delamination and fiber fracture. Short-beam bending test results demonstrate that this novel gradient structure exhibits good bending load-bearing performance and damage resistance, achieving the coexistence of multiple damage couplings and effectively enhancing damage tolerance.

Wavelength-Stable Metal Grating Distributed Feedback Quantum Cascade Laser Emitting at λ ~ 7.2 μm

In this research, we demonstrate a wavelength-stable continuous wave (CW) distributed feedback (DFB) quantum cascade laser (QCL) emitting at 7.2 μm using a surface metal grating approach without epitaxial regrowth. The deep metal grating provides an appropriate DFB coupling coefficient and enhanced thermal extraction, resulting in improved lasing performance and the realization of impressive wavelength stability. Quantitatively, the temperature tuning coefficient of the single-mode emission is only 0.54 nm/°C from 20 °C to 70 °C, and the current tuning coefficient of the single-mode emission is 3.2 nm/A from 1.0 A to 1.6 A. A DFB-QCL with a 2 mm cavity length exhibits a low threshold current of 0.6 A and a power of 1.1 W with a slope efficiency of 1 W/A in the CW mode at 300 K. A single-mode operation with a side mode suppression ratio of 33 dB and a single-lobed far-field without beam steering is obtained in the working temperature range of 20–70 °C The improved wavelength stability using a deep surface metal grating approach promises simplified fabrication, which is meaningful for the commercial applications of QCLs.