Mode Suppression Research Articles

BCB-Based Thin-Film Ka-Band Quarter-Mode SIW Packaged Filters With Ultrawide Stopband and Independently Controlled TZs

In this article, quarter-mode substrate-integrated waveguide (QMSIW) filters featuring ultrawide stopband and flexibly allocated transmission zeros (TZs) are proposed. Eight higher modes with resonant frequencies up to 4.7 times of the fundamental mode are highly suppressed. It is realized by the combination of three approaches without extra circuit areas. First, a slot is cut in the first and last QMSIW resonators to make their resonant frequencies of higher modes different from those of other resonators. Second, avoid exciting the specific higher modes by setting the feed point in the area where the electric field strength of the higher modes is weak. Third, mismatch the higher modes when matching the passband. It is realized by using a stripline impedance transformer with its main body not exceeding the area of QMSIW resonators. Furthermore, except for the TZs derived by the classical cross-coupling matrix, two more independently controlled TZs are generated. The packaged filter is fabricated by using a benzocyclobutene (BCB)-based microelectromechanical systems (MEMS) process, which is suitable for millimeter-wave 3-D system-on-package (SOP) applications. The fabricated thin-film filter is measured with an insertion loss of 2.96 dB at 29.4 GHz and a fractional bandwidth (BW) of 17%. Higher mode suppressions up to 138.3 GHz are achieved.

A reconfigurable balanced loaded‐line phase shifter based on slot line

A reconfigurable balanced phase shifter is designed in this paper, which can achieve stable differential mode (DM) phase shift and high common mode (CM) suppression. The proposed balanced phase shifter is mainly composed of micro strip/slot line transition structure and π-shape slot line. The U-shaped micro strip line and the stepped impedance slot line are employed to form the differential input and output ports, which can make the DM response and the CM one are independent. By switching the PIN diode on or off, a stable 180° DM phase shift difference is achieved between two states. In order to verify the design, a 180° balanced phase shifter with a central frequency of 4.7 GHz was simulated and fabricated. DM insertion loss of the design is 1.1 dB, DM phase deviation range is ±5°, 10 dB DM impedance matching fractional bandwidth is 45%, and phase fractional deviation bandwidth is 42%. Meanwhile, the CM rejection can reach about 35 dB. The simulation and measurement are in good agreement with the theory.

Orthogonal Molecular Assembly: Eliminating Intrinsic Phonon Modes in Organic THz Generators

AbstractThis work reports a series of new organic electro‐optic salt crystals possessing unusual orthogonal dipole‐coupling assembly that act as dimple‐free THz generators. In the new crystals, optically anisotropic aromatic anions introducing highly electronegative fluorinated substituents are orthogonally assembled with optically anisotropic nonlinear optical benzothiazolium cations. These orthogonal cation–anion assembled crystals exhibit strong suppression of intrinsic THz phonon modes along the polar axis, which results in a wide THz absorption‐free range from 1.5 to 4.0 THz without substantial absorption peaks. This is sharply in contrast to previous benchmark crystals with parallel cation–anion assembly; for example, in analogous crystals, several strong absorption peaks appear in this frequency range. This absence of strong absorption peaks is attributed to the orthogonal cation–anion dipole coupling assembly, the formation of fluorinate‐induced strong interionic interactions, and a high packing ability of uniaxial anions. With top‐level macroscopic optical nonlinearity, new orthogonal‐dipole assembled crystals make dimple‐free THz wave generation possible in the range of about 1.5–4.0 THz, which is very beneficial for characterizing signature peaks related to molecular phonon motions of analytes in THz spectroscopy. Therefore, orthogonal molecular assembly of optically highly anisotropic molecules provides an efficient way to substantially reduce and control intrinsic THz phonon modes.

Gate-tunable modulation of the optical properties of multilayer graphene by the reversible intercalation of ionic liquid anions

We demonstrate a substantial modulation of the optical properties of multilayer graphene (∼100 layers) using a simple device consisting of a multilayer graphene/polymer electrolyte membrane/gold film stack. Applying a voltage of 3–4 V drives the intercalation of anion [TFSI]− [ion liquid diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI]] resulting in the reversible modulation of the properties of this optically dense material. Upon intercalation, we observe an abrupt shift of 35 cm−1 in the G band Raman mode, an abrupt increase in FTIR reflectance over the wavelength range from 1.67 to 5 μm (2000–6000 cm−1), and an abrupt increase in luminescent background observed in the Raman spectra of graphene. All of these abrupt changes in the optical properties of this material arise from the intercalation of the TFSI− ion and the associated change in the free carrier density (Δn = 1020 cm−3). Suppression of the 2D band Raman mode observed around 3 V corresponds to Pauli blocking of the double resonance Raman process and indicates a modulation of the Fermi energy of ΔEF = 1.1 eV.

A SiN antenna based on grating–waveguide–grating structure for unidirectional and uniform emission

High-directionality antennas can efficiently avoid blind spots in the field of view. And a uniform emission antenna can improve the near-field distribution of the antenna, increase the effective aperture of the antenna, and thereby reduce the divergence angle. In this manuscript, a SiN antenna of highly upward emission directionality and uniform emission based on the grating–waveguide–grating sandwich structure is designed. For the antenna, it has a flat near-field and a side mode suppression ratio of less than 0.2 from 1540 nm to 1620 nm. The length of the antenna is 500μm and the divergence angle is 0.17° at 1550 nm. The beam steering range is 5.54° from 1540 nm to 1620 nm and the upward directionality is over 80%. The highest upward directionality is about 92% at 1540 nm. These indicate that the proposed antenna can also be applied to LiDAR and so on.

High-Order Modes Suppression in All-Glass Large-Mode Area Rare-Earth Doped Optical Fibers with Modified Cladding

In the present work we have developed and realized a high core-to-cladding diameters ratio active optical fibers operated in a single-mode regime due to specially designed structure containing boron-doped and fluorine-doped rods. The beam quality at the output of the realized fibers was studied by three independent methods: near field investigation, M2 technique and S2 technique. According to the obtained results, the developed approach allows efficient suppression high-order-modes (first of all—LP11 mode) in the core and achieves a diffraction-limited beam at the output of the appropriately bent fiber (suppression of unwanted modes was better than 30 dB). Additionally, it is shown that application of such approach allows for increased bent-resistance of the fundamental mode and realizes polarization-sensitive amplification.

AlN/Al0.8Sc0.2N film S mode lamb wave resonator with spurious mode suppression

High-performance Lamb wave devices have a vast application prospect in modern wireless communication systems. This paper proposes a method of spurious mode suppression by reducing the overlap length of electrodes according to modal shapes. The AlN/Al0.8Sc0.2N composite film is used to achieve a trade-off between electromechanical coupling coefficient (kt2) and Q factor. The finite-element analysis (FEA) is employed to study the performance of the lowest-order symmetric mode (S0) Lamb wave resonators (LWRs), and the FEA result shows a spurious mode in the impedance response. According to the results of modal shape analysis, the typical LWR, the LWR with the circle-shaped arrangement of electrodes, the LWR with the circle-shaped apodization, and the LWR with the arc-shaped apodization are fabricated to suppress the spurious mode and investigate the performance of the LWRs. The measurement results show that the spurious mode is effectively suppressed over a wide frequency range (1.9–2.2 GHz) by the three designed electrode configurations. The LWR with the circle-shaped apodization working at 2.023 GHz has a Qp factor of 955, a moderate kt2 of 2.97%, a figure-of-merit (Q×kt2) of 28.4, and a large value of f×Q×kt2 (5.773 × 1010). Utilizing modal shapes to design the electrode configuration of LWRs can suppress spurious modes without significant performance deterioration.

100 kW ultra high power fiber laser

Based on the self-developed non-photodarkening large mode field gain fiber and the 976 nm wavelength-locked high-power and high-brightness pump source, and using the secondary fiber power combining technology, a high-performance 100kW fiber laser in China was built, realizing high-order mode and non-linear effect suppression. The maximum output power of the laser can reach 101.65 kW, the center wavelength is 1080 ± 5 nm, the spectral bandwidth is (3dB) 5-8 nm, the output fiber core diameter is 400µm, the beam quality BPP is 19.28 mm*mrad, and the laser power instability is ±1.1%. Its laser non-destructive cladding stripping technology, distortion-free taper technology, inclined multi-die beam combining technology and circular inner cladding modification design have all reached the international advanced level.

Quasi-continuous tunable semiconductor laser based on the gain-lever effect with full C-band coverage fabricated by standard lithography.

A quasi-continuous tunable semiconductor laser covered full C-band is demonstrated. The quasi-continuous tuning range of the tunable semiconductor laser is significantly improved by optimizing the length of the phase section using the gain-lever effect, achieving a 36 nm range that covered the whole C-band. In the tuning range, 46 channels with 100 GHz spacing are achieved, and all channels exhibit a side mode suppression ratio above 30 dB. No regrowth or high-precision lithography is involved in the fabrication process of the tunable semiconductor laser, which has the potential to provide a cost-effective light source for dense wavelength division multiplexing systems.

Bandpass filters using grounded stub-loaded microstrip periodic structure for suppression of modes

In this paper, a grounded stub-loaded microstrip periodic structure (SL-MPS) is presented and its transmission characteristics are investigated. A new method of suppression of modes is proposed through adding grounded via holes on the grounded SL-MPS for the design of bandpass filters (BPFs). In order to validate the feasibility of suppression of modes of grounded SL-MPS, two cases of BPFs with different dimensions are designed. For the case of BPF with suppression of adjacent modes, the simulated and measured results of BPFs demonstrate that the fundamental and 2nd high-order modes can be effectively suppressed. For the other case of BPF with suppression of multiple modes, the fundamental, 1st, 3rd and 4th high-order modes can be successfully suppressed. The measurements of these BPFs are reasonably in agreement with their simulations. Observed from the theoretical analysis and experimental results, the proposed method can effectively widen the stopband of BPF by suppressing unwanted modes.

Mode-coupling effects in an optically-injected dual-wavelength laser.

Lasers designed to emit at multiple and controllable modes, or multi-wavelength lasers, have the potential to become key building blocks for future microwave photonic technologies. While many interesting schemes relying on optical injection have been proposed, the nonlinear mode coupling between different modes of a multi-wavelength laser and their dynamical behavior under optical injection remains vastly unexplored. Here, we experimentally and numerically study the effect of optical injection around the suppressed mode of a dual-wavelength laser and the resulting interactions with the dominant mode. We highlight a wavelength shift of the dominant mode triggered by injection locking of the suppressed mode and report a strong impact of the mode suppression ratio on the locking range. Finally, we show numerically that the cross-coupling parameter between the two modes might have a key role in this effect.

Impact of precessing vortex core dynamics on the thermoacoustic instabilities in a swirl-stabilized combustor

Global instabilities in swirling flows can significantly alter the flame and flow dynamics of swirl-stabilized flames, such as those in modern gas turbine engines. In this study, we characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate. This study is performed using experimental data obtained from a model gas turbine combustor employing two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at five bar pressure. The flow split between the two streams is systematically varied to observe the impact of flow structure variation on the system dynamics at both non-reacting and reacting conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame and fuel flow behaviour, respectively. Spectral proper orthogonal decomposition and a complex network analysis are used to identify and characterize the dominant oscillation mechanisms driving the system. In the non-reacting data, a PVC is present in most cases and the amplitude of the oscillation increases with increasing flow through the centre nozzle. In the reacting data, three dominant modes are seen: two thermoacoustic modes and the PVC. Our results show that in the cases where the frequency of the PVC overlaps with either of the thermoacoustic modes, the thermoacoustic modes are suppressed. The complex network analysis coupled with a weakly nonlinear theoretical analysis suggests the mechanisms by which this coupling and suppression of the thermoacoustic mode occur.

Improved SPGD Algorithm for Optical Phased Array Phase Calibration

A chip-level optical beam steerer is an inevitable choice for next-generation light detection and ranging (LiDAR). The research on optical phased array (OPA) is the most intriguing. However, the complexity of control and calibration speed limit the full potential as the number of channels increases. In this paper, an improved stochastic parallel gradient-descent algorithm combined with the Nesterov accelerated gradient method (NSPGD) is presented and applied in a 512-channel OPA. This algorithm can reduce the phase calibration time of large-scale OPA and demonstrates a better convergence performance than traditional SPGD. Compared with the traditional SPGD and hill-climbing (HC) algorithm, optimized convergence performance of NSPGD is shown. The side mode suppression ratio (SMSR) of over 10dB for 512-channel OPA is obtained with the NSPGD algorithm, and the convergence speed is twice that of traditional SPGD. In addition, a temperature-controlled OPA is also studied to stabilize the whole calibration system.

Wall-cooling effects on pressure fluctuations in compressible turbulent boundary layers from subsonic to hypersonic regimes

Pressure fluctuations play an essential role in the transport of turbulent kinetic energy and vibrational loading. This study focuses on examining the effect of wall cooling on pressure fluctuations in compressible turbulent boundary layers by high-fidelity direct numerical simulations. Pressure fluctuations result from the vorticity mode and the acoustic mode that are both closely dependent on compressibility. To demonstrate the effects of wall cooling at various compressibility intensities, three free-stream Mach numbers are investigated, i.e. $M_\infty =0.5$ , 2.0 and 8.0, with real gas effects being absent for $M_\infty =8.0$ due to a low enthalpy inflow. Overall, opposite effects of wall cooling on pressure fluctuations are found between the subsonic/supersonic cases and the hypersonic case. Specifically, the pressure fluctuations normalized by wall shear stress $p^\prime _{rms}/\tau _w$ are suppressed in the subsonic and supersonic cases, while enhanced in the hypersonic case near the wall. Importantly, travelling-wave-like alternating positive and negative structures (APNS), which greatly contribute to pressure fluctuations, are identified within the viscous sublayer and buffer layer in the hypersonic cases. Furthermore, generating mechanisms of pressure fluctuations are explored by extending the decomposition based on the fluctuating pressure equation to compressible turbulent boundary layers. Pressure fluctuations are decomposed into five components, in which rapid pressure, slow pressure and compressible pressure are dominant. The suppression of pressure fluctuations in the subsonic and supersonic cases is due to both rapid pressure and slow pressure being suppressed by wall cooling. In contrast, wall cooling strengthens compressible pressure for all Mach numbers, especially in the hypersonic case, resulting in increased wall pressure fluctuations. Compressible pressure plays a leading role in the hypersonic case, mainly due to the APNS. Essentially, the main effects of wall cooling can be interpreted by the suppression of the vorticity mode and the enhancement of the acoustic mode.

Doppler compensation for cavity-based atom interferometry.

We propose and demonstrate a scheme for Doppler compensated optical cavity enhancement of atom interferometers at significantly increased mode diameters. This overcomes the primary limitations in cavity enhancement for atom interferometry, circumventing the cavity linewidth limit and enabling spatial mode filtering, power enhancement, and a large beam diameter simultaneously. This approach combines a magnified linear cavity with an intracavity Pockels cell. The Pockels cell induces a voltage-controlled birefringence allowing the cavity mode frequencies to follow the Raman lasers as they track gravitationally induced Doppler shifts, removing the dominant limitation of current cavity enhanced systems. A cavity is built to this geometry and shown to simultaneously realise Doppler compensation, a 5.8 ± 0.15 mm1/e2 diameter beam waist and an enhancement factor of >5× at a finesse of 35. Tuneable Gouy phase enables the suppression of higher order spatial modes and the avoidance of regions of instability. Atom interferometers will see increased contrast at extended interferometry times along with power enhancement and the reduction of optical aberrations. This is relevant to power constrained applications in quantum technology, alongside the absolute performance requirements of fundamental science.

Improved Half-Wave Coupled V-Cavity Laser Using Aggregating-Image Multimode Interference Coupler

We theoretically analyze and experimentally demonstrate improved half-wave couplers for widely tunable V-cavity lasers. An overlapping-image 6×6 MMI coupler and an aggregating-image 16×16 MMI coupler are investigated, which give optimal coupling phase and coefficient for achieving high single-mode selectivity in V-cavity lasers while producing a low loss. A single-electrode controlled tuning of 38 channels is achieved with channel spacings of 100 GHz and 150 GHz. By combining two single-electrode tuning segments at different temperatures, a wavelength tuning range of 52 channels from 1515 nm to 1585 nm is obtained, and the side mode suppression ratios (SMSRs) reach 45 dB.

Chiral trabeated metabeam for low-frequency multimode wave mitigation via dual-bandgap mechanism

An elastic wave in a physical beam naturally possesses many wave modes, such as flexural, longitudinal, and torsional. Therefore, suppression of all possible vibration modes has been rarely achieved in beam-shaped periodic systems, especially at low frequencies. Here we present a low-frequency complete bandgap mechanism by overlapping the flexural bandgap with the longitudinal-torsional bandgap. To strengthen the general framework, we enforce an extra degree of freedom (rotational and torsional-spring) on the spring-mass system for the flexural and coupled (longitudinal-torsional) modes. The low rotational stiffness provides a low flexural bandgap, whereas the torsional stiffness yields a coupled-mode bandgap. To meet these prerequisites in physical modeling, a chiral trabeated metabeam is conceived, which allows all wave modes to be suppressed by a complete bandgap. Apart from single-mode mitigation, our work provides a route to implementing a low-frequency complete bandgap in a periodic fashion, potentially enabling the use of chirality in elastic structures.

Drag reduction of a generic transport vehicle model using a fluidic oscillator

Base blowing is performed using a sweeping jet actuator centrally located at the base of a square-back Ahmed body. Drag reduction along with a significant effect in terms of wake RSB mode suppression is reported for such fluid injection. All three scales of fluidic actuation exhibit similar wake bistability suppression characteristics. RSB mode suppression is also corroborated through independent force measurements. For all scales of actuators, the optimal blowing coefficient Cqopt is found to scale with bleed-to-base area ratio as (Sj/S)3/4, whereas the blowing coefficient corresponding to the minimum magnitude of wake asymmetry scales as Cqopt(Sj/S)5/12. The small-scale actuator also presents a more energy-efficient base blowing in terms of the amount of drag reduction achieved per energy consumed.

High wall-plug efficiency and narrow linewidth III-V-on-silicon C-band DFB laser diodes.

We present recent results on compact and power efficient C-band distributed feedback lasers through adhesive bonding of a III-V die onto a silicon-on-insulator circuit. A wall-plug efficiency up to 16% is achieved for bias currents below 40 mA. The laser cavity is 180 µm long and a single facet output power up to 11 mW is measured at 20 °C by incorporating a broadband reflector in the silicon waveguide at one side of the cavity. Single mode operation at 1567 nm with a side mode suppression ratio of around 55 dB is demonstrated. By controlling the phase of the external feedback, the laser linewidth is decreased to 28 kHz. Measurement result shows a low relative intensity noise below -150 dB/Hz at 60 mA up to 6 GHz. We also report 20 and 10 Gbps data transmission at a bias current of 50 mA at 20 °C and 40 °C, respectively.

Inverse design of programmable optical frequency comb using deep learning

Optical frequency comb (OFC) has important applications in measurement, communication, military and other fields. Usually, OFC needs to be designed according to different applications. However, the existing methods to design the operating parameters of the OFC generators are time-consuming, inefficient, and difficult to achieve optimal results. In this paper, a novel method of inversely designing OFC using deep learning, which is real-time and can improve the performance of the generated OFC, is proposed and applied to an OFC generator based on a single dual-drive Mach–Zehnder modulator. In this method, according to the required target OFC, the trained neural network can be used to inversely design the corresponding parameters. Using this inverse design method, the generated OFC not only is highly consistent with the target OFC, but also has the programmability of comb-line number, comb-line power, side mode suppression ratio, and comb spacing. Moreover, the proposed method can be utilized for more complicated OFC generator, and is an inspiration for efficient design of OFC.