Input Coupling Research Articles

Electromagnetic field compensation to attenuate energy leakage in TM020 mode damped cavity

This paper discusses the TM020 mode Higher-order Mode (HOM) damped cavity, which has a higher quality factor and lower characteristic impedance than the conventional TM010 mode. The unique electromagnetic field (EMF) distribution of the TM020 mode allows for an absorber to be placed at the magnetic field wave node, protecting the working mode while suppressing the HOMs. However, the introduction of high power input couplers and other structures can cause an asymmetrical EMF distribution, leading to operating mode leakage.To address this, EMF compensation methods are applied to the high power input coupler to achieve symmetrical EMF distribution, reducing operating mode loss. Overcoupling of the high power input coupler and the resonant cavity can significantly affect local EMF distribution, causing TM020 mode asymmetry. A tab structure at the coupler port are used to compensate for coupler-induced perturbations, attenuating EMF leakage.The paper proposes electromagnetic compensation structure schemes to mitigate working mode EMF leakage. The reliability of the scheme is demonstrated by the analysis of radiofrequency performance and other aspects through simulation software.

Spin Photonics‐Based on a Twinning Hyperbolic Metamaterial

AbstractRouting and multiplexing of the optical surface waves are critical for the ultra‐compact nano‐photonic systems. Given this background, traditional metal‐based systems, despite of the ability of supporting the surface plasmon polariton propagation, are constrained on manipulations due to the optical isotropy, and artificially designed structures are always required at the input or output to help attain the additional selectivity. Here, a spin‐dependent multi‐channel scheme is proposed based on the modulation ability of the hyperbolic metamaterial's anisotropy on Dyakonov Surface Waves. The input coupling, propagation direction, as well as the output coupling of the optical single are well determined by the certain spin feature carried by the light and the orientation of the interface's anisotropy because of the spin‐momentum locking. Specially, by constructing a twinning hyperbolic metamaterial, which contains the structural symmetry and materials symmetry, a photonic chip with 4 routing channels are demonstrated experimentally, where the spin of an incident beam can be maintained during the propagation on‐chip and then delivered back into the free space, offering a new planar scheme for metamaterial‐based spin‐controlled nano‐photonic applications.

Optimization design of the cavity tuning and RF input coupler for an 11 MeV superconducting cyclotron

This study addresses the compact design requirements for a 11 MeV superconducting cyclotron utilized in isotope production by proposing a design and optimization scheme for a radio frequency (RF) input coupling system. The system, comprising a coupler loop, transmission lines, and RF window, has been tailored to accommodate the spatial constraints imposed by the super conducting magnet radius, thereby facilitating efficient RF power transfer to the RF cavity and minimizing input power. Structural optimization of the coupler loop and RF window has achieved S11 parameter of -82.23 dB, significantly reducing input power while maintaining high coupling efficiency. A coaxial coupler suitable for lower frequency ranges has been designed, employing a tapered structure for the transmission line to ensure a smooth transition to reduce impedance discontinuity effects and enhance adjustability. The multi-stage design of the RF window has achieved fine impedance matching, significantly lowering the S11 parameter. In response to frequency drift caused by RF loss and temperature increase, the study utilizes perturbation theory to analyze the perturbation effect of coupling components on the resonant frequency, designing an adjustable structure with both coarse and precise tuning capabilities, realizing tuning precision at the kHz/mm level and a tuning range of several MHz. A conjectural formula has been proposed based on the impact of the tuner's structural parameters on the resonant frequency, accurately calculating the equivalent inductance of the tuner and the resonant frequency of the RF cavity post-tuner installation, with the maximum estimation error of the resonant frequency kept within 0.1%, offering significant guidance for engineering design.

Ultrahigh reflectivity photonic crystal membranes with optimal geometry

Photonic crystal (PhC) structures with subwavelength periods are widely used for diffractive optics, including high reflectivity membranes with nanoscale thickness. Here, we report on a design procedure for 2D PhC silicon nitride membrane mirrors providing optimal crystal geometry using simulation results obtained with rigorous coupled-wave analysis. The Downhill Simplex algorithm, a robust numerical approach to finding local extrema of a function of multiple variables, is used to optimize the period and hole radius of PhCs with both hexagonal and square lattices, as the membrane thickness is varied. Following these design principles, nanofabricated PhC membranes made from silicon nitride have been used as input couplers for an optical cavity, resulting in a maximum cavity finesse of 33 000, corresponding to a reflectivity of 0.999 82. The role played by the spot size of the cavity mode on the PhC was investigated, demonstrating the existence of an optimal spot size that agrees well with predictions. We find that, compared to the square lattice, the hexagonal lattice exhibits a spectrally wider reflective range, less sensitivity to fabrication tolerances, and higher reflectivity for membranes thinner than 200 nm, which may be advantageous in cavity optomechanical experiments. Finally, we find that all of the cavities that we have constructed exhibit well-resolved polarization mode splitting, which we expect is due primarily to a small amount of anisotropic stress in the silicon nitride and PhC asymmetry arising during fabrication.

Resonant Ring with a Gain of 36 for Use with a 1 MW 110 GHz Gyrotron

A 110 GHz quasi-optical ring resonator, designed for use with a 1 MW pulsed gyrotron, has been built and successfully tested using a 100 mW solid-state source. A low reflectance (2.4%) input coupler and a low-loss, four-mirror ring demonstrated a compression ratio, defined as the ratio of output to input power, of 36. The 6 ns output pulses were generated from the 2 m length ring using a silicon laser-driven semiconductor switch (LDSS). The quasi-optical ring resonator was designed with large waist sizes so that input pulses of up to 1 MW will stay under the 35 kV/cm electric field limit for ionization in ambient air. Maximum compression gain was achieved by matching the input coupling fraction to the round trip loss in the ring, achieving close to critical coupling. The experimental output pulse shape obtained after firing the LDSS was modeled using the reflectance, transmittance, and absorptance of the switch vs. time and vs. laser pulse fluence, with good agreement found with theory. The timing for the peak energy efficiency of 32% was found and the main loss mechanism limiting that efficiency was found to be the absorptance in the silicon wafer.

Ultrafast evanescently coupled waveguide MUTC-PDs with high responsivity

Novel evanescently coupled waveguide modified uni-traveling carrier photodiodes (MUTC-PDs) employing a thick multi-layer coupling waveguide are reported. To improve the optical-to-electrical (O/E) conversion efficiency, a thick multi-layer coupling waveguide with a gradually increased refractive index from the bottom layer to the absorption layer is utilized. The refractive index profile facilitates the upward transmission of incident light into the absorption region, thereby enhancing the evanescent coupling efficiency. Meanwhile, the coupling waveguide, with a total thickness of 1.75 µm, expands the mode field diameter, thereby reducing the input coupling loss. Additionally, the top layer of the coupling waveguide also serves as the drift layer. This configuration facilitates efficient light absorption within a short PD length, thus ensuring ultrawide bandwidth and high O/E conversion efficiency simultaneously. Without an additional spot size coupler or anti-reflection coating, the measured responsivity is as high as 0.38 A/W for the PD with an active area of 5 × 6 µm2. Meanwhile, an ultrawide 3-dB bandwidth of 153 GHz has been demonstrated.

Systematic comparison of commercial devices for temporal characterization of few-cycle laser pulses in the 500-1000 nm spectral range

We compare multiple temporal pulse characterization techniques in three different pulse duration regimes from 15 fs to sub-5 fs, as there are no available standards yet for measuring such ultrashort pulses. To accomplish this, a versatile post-compression platform was developed, where the 100 fs near infrared pulses were post-compressed to the sub-two-cycle regime in a hybrid, three-stage configuration. After each stage, the duration of the compressed pulse was measured with the d-scan, TIPTOE and SRSI techniques and the retrieved temporal intensity profiles, spectrum and spectral phases were compared. Spectral homogeneity was also measured with an imaging spectrometer to understand the input coupling conditions of the temporal measurements. Our findings suggest that the different devices give similar results in terms of temporal intensity profile, however they are extremely sensitive to alignment and to beam quality, especially in the case of the shortest pulses. We address specific steps of measurement procedures, which paves the way towards the standardization of pulse characterization in the near future.

Compact high-Q Ka-band sapphire distributed Bragg resonator

In a class of high quality (Q-) factor dielectric resonators with low radiative losses, including popular whispering-gallery mode (WGM) resonators with high azimuthal mode numbers, due to high confinement of modal field in dielectric, the Q-factor is limited by the value of inverse dielectric loss tangent of dielectric material. Metal enclosures necessary for device integration only marginally affect the Q-factor while eliminating the residual radiative loss and allowing the optimization of input and output coupling. While very high Q-factors ∼200000 are available in sapphire WGM resonators in X-band, at millimeter wave frequencies increasing dielectric loss limits the Q-factor to much smaller values, e.g. ∼50000 and ∼25000 for quasi-TE and quasi-TM modes, correspondingly, at 36 GHz. The use of distributed Bragg reflection (DBR) principle allows to push modal energy outside dielectric while also isolating it from Joule losses in metallic enclosure walls. Very high Q∼600000>tgδ has been demonstrated in X-band [C. A. Flory and R. C. Taber, IEEE Trans. Ultrason., Ferroelectr., Freq. Control 44, 486–495 (1997).] at the expense of impractically large dimensions. In this work, we report on the assembly and testing of a compact Ka-band sapphire distributed Bragg reflector cavity characterized with Q-factor seven times larger than one predicted by the material’s dielectric loss at the frequency of interest. An intrinsic Q-factor of ∼200000 is demonstrated at 36 GHz for the lowest order TM-mode of a sapphire DBR. The resonator has 50 cm3 volume, smaller than previously demonstrated DBRs.

Design of highly selective balanced bandpass filter with wide differential-mode stopband and high common-mode suppression

A highly selective and wide stopband bandpass filter (BPF) with multiple transmission poles (TPs) and transmission zeros (TZs) has been proposed. The BPF employs a box-like coupling structure consisting of two half-wavelength resonators and a dual-mode open stub loaded resonator, enabling the generation of two TZs and four TPs. Three controllable TZs are obtained by adding an annular spiral open stub to the terminal of the input and output coupling lines, which significantly improves the selectivity of the BPF. To achieve a wide stopband characteristic, a wide stopband unit with three additional TZs is cascaded within the structure. A balanced BPF is realized by introducing a microstrip-slotline-microstrip balun structure to the input and output ports of the proposed BPF. Experimental results validate the feasibility of the proposed BPF and balanced BPF applied in 2.45 GHz ISM band. The introduction of eight TZs gives the BPF a 20 dB rectangular coefficient of 1.3 with an upper stopband exceeding than 12 GHz. The balanced BPF can achieve 40 dB of high common-mode(CM) suppression in the passband, while differential-mode (DM) transmission is close to the performance of the proposed BPF. The minimum insertion loss for BPF and balanced BPF is 0.8 dB and 1.3 dB, respectively.

Theoretical efficiency limit of diffractive input couplers in augmented reality waveguides.

Considerable efforts have been devoted to augmented reality (AR) displays to enable the immersive user experience in the wearable glasses form factor. Transparent waveguide combiners offer a compact solution to guide light from the microdisplay to the front of eyes while maintaining the see-through optical path to view the real world simultaneously. To deliver a realistic virtual image with low power consumption, the waveguide combiners need to have high efficiency and good image quality. One important limiting factor for the efficiency of diffractive waveguide combiners is the out-coupling problem in the input couplers, where the guided light interacts with the input gratings again and get partially out-coupled. In this study, we introduce a theoretical model to deterministically find the upper bound of the input efficiency of a uniform input grating, constrained only by Lorentz reciprocity and energy conservation. Our model considers the polarization management at the input coupler and can work for arbitrary input polarization state ensemble. Our model also provides the corresponding characteristics of the input coupler, such as the grating diffraction efficiencies and the Jones matrix of the polarization management components, to achieve the optimal input efficiency. Equipped with this theoretical model, we investigate how the upper bound of input efficiency varies with geometric parameters including the waveguide thickness, the projector pupil size, and the projector pupil relief distance. Our study shines light on the fundamental efficiency limit of input couplers in diffractive waveguide combiners and highlights the benefits of polarization control in improving the input efficiency.

Magneto-thermal limitations in superconducting cavities at high radio-frequency fields

The performance of superconducting radio-frequency Nb cavities at high radio-frequency (rf) fields in the absence of field emission can be limited by either a sharp decrease of the quality factor Q0(Bp) above peak surface magnetic fields Bp ∼100 mT or by a quench. We have measured Q0(Bp) at 2 K of several 1.3 GHz single-cell Nb cavities with different grain sizes, and with different ambient magnetic fields and cooldown rates below the critical temperature. Temperature mapping and a novel magnetic field mapping systems were used to find the location of “hot-spots” and regions of trapped magnetic flux. The use of a variable input coupler allowed further exploring the dissipative state. The results showed a remarkable thermal stability in some cavities with up to 200 W of rf power dissipation at 2 K, whereas other cavities quenched at much lower rf power. We observed a narrow distributions of the onset fields of hot-spots which were not affected by thermal cycling or by conditions which favor the formation of Nb hydrides. Furthermore, a poor correlation was found between the location of hot-spots and trapped vortices. We suggest that the totality of our experimental data can be explained by a sharp increase of the residual surface resistance above 120–140 mT due to the field-induced breakdown of a proximity-coupled metallic suboxide layer at the surface.

Control of the PVTOL with Strong Input Coupling

This paper studies the modeling and control of a Planar Vertical Take-Off and Landing (PVTOL) with steerable thruster. A longitudinal model is obtained using Newton’s second law for the PVTOL which evolves in 3 degrees of freedom and has two control inputs. The aerial vehicle is driven by steerable propulsion controlling its evolution in the vertical plane through the thrust and torque control inputs, which drive the vehicle body and generate a rotation. The obtained model is nonlinear and is significantly different with respect to the well-known PVTOL. For this reason, different control algorithms are presented, and the closed-loop behavior is studied for each of them. The proposed control strategies perform a stationary flight at a desired altitude and control the position of the aerial vehicle. The performance of the proposed control algorithms is tested in numerical simulations.

Waveguide holography for 3D augmented reality glasses

Near-eye displays are fundamental technology in the next generation computing platforms for augmented reality and virtual reality. However, there are remaining challenges to deliver immersive and comfortable visual experiences to users, such as compact form factor, solving vergence-accommodation conflict, and achieving a high resolution with a large eyebox. Here we show a compact holographic near-eye display concept that combines the advantages of waveguide displays and holographic displays to overcome the challenges towards true 3D holographic augmented reality glasses. By modeling the coherent light interactions and propagation via the waveguide combiner, we demonstrate controlling the output wavefront using a spatial light modulator located at the input coupler side. The proposed method enables 3D holographic displays via exit-pupil expanding waveguide combiners, providing a large software-steerable eyebox. It also offers additional advantages such as resolution enhancement capability by suppressing phase discontinuities caused by pupil replication process. We build prototypes to verify the concept with experimental results and conclude the paper with discussion.

Few-shot regression with differentiable reference model

Precise control models of industrial manufacturing are essential for high-quality products. In specific industrial fields, there usually exist certain theoretical results and analytical reference models. Due to the diversity of the control input and complex coupling effect on results, the analytical reference model often has strong assumptions and simplified conditions, so that the error can be large. However, it can characterize the overall trend of the real control curve. Deep learning is a promising solution for precise control model, but the huge data demand hinders its application. To this end, we propose a novel few-shot regression framework based on a differentiable reference model, where the gradient of reference model is used as the prior of the function fitter. We implement the framework on a neural network, named Gradient Guided Network, GGN, achieving accurate regression with few samples. Experimental results of sinusoidal regression and roll bend forming problem demonstrate the effectiveness and superior performance compared with meta learning methods. In addition, by combining the trend of the theoretical reference model with real data, our method also achieves better results than the reference model.

Formation control of multi-agent systems with actuator saturation via neural-based sliding mode estimators

In this paper, the formation control problem for second-order multi-agent systems with model uncertainties and actuator saturation is investigated. An estimator-based robust formation controller is developed to ensure boundedness of the system’s formation tracking error. To estimate uncertain factors without prior knowledge of their Lipschitz constants, two new estimator designs that combine the neural-based estimation process and the sliding mode technique are proposed. Finite-time stability is achieved by introducing a new terminal estimation surface. Regarding the formation controller design, we provide a new framework to study the combined effect of input saturation and input coupling. Both an adaptive compensator and an input regulation algorithm are employed to attenuate state fluctuation led by the reverse effect. Comparative simulations regarding a multi-robot system are conducted to demonstrate the effectiveness of the proposed estimators and the estimator-based controller.

Design and test of rack-integrated cavity combiner for China initiative accelerator driven system project.

The next-generation 650MHz solid state power amplifier designed by the Institute of Modern Physics will utilize 24 modules with an output power of 60 kW. The outputs of each of the 12 modules will be combined using a 12-in-1 rectangular cavity combiner integrated into the rack. This cavity combiner, requiring only a single stage to combine power, is characterized by a minimal power loss and a high combining efficiency. The input couplers of the combiner are adjustable to change the number of combination channels. In the event of one amplifier module failure, the corresponding port can be adjusted to decouple, transforming the combiner to an (N-1)-channel combiner with a combining efficiency decay of 0.2%. The prototype of the combiner has been fabricated and tested with a small signal. The combining efficiency is 98.5%. In this paper, we will validate the feasibility of the combiner from the design, simulation, and experiment.

Parallel-coupled bandpass filters with directly coupled input and output resonators

A novel method for modifying input and output coupling in the parallel-coupled bandpass filters, which allows for direct connection of source and load to the outer resonators, is proposed. In the filter modification a coupled-resonator approach is utilized, which allows one for calculating both the impedance of the introduced new quarter-wave section and the necessary coupling modification of the first and second resonators. The proposed filter modification allows for realization of wider bandwidth filters by removing the necessity of narrow gap realization. Further the following gap in the filter is also enhanced, what helps in wide band filter realization. The presented theoretical approach has been confirmed by theoretically calculated responses of three- and seven-resonator filters. The theoretically investigated filter modification has been confirmed with the experiment in which a seven-resonator modified bandpass filter has been fabricated and measured. The presented measurement results confirm the theoretical predictions and prove the proposed design method.

Cross-backstepping control with prescribed performance for input-coupled underactuated systems under arbitrary initial conditions

Underactuated robotic systems with input coupling are widely applied in practice. However, designing controllers for such systems poses significant challenges due to the complexities arising from input coupling. To address this issue, this paper proposes a novel cross-backstepping control strategy based on a composite error transformation. The transformation compresses the system’s initial error into a definable range to deal with the issue of initial value dependency of prescribed performance control, while maintaining the transient and steady state of the system. Based on such a technique, a cross-backstepping controller is proposed whose core idea lies in constructing a virtual term that crosses with coupled states to generate a unified form, which serves as a bridge between multiple coupled states and a single actual control input. Furthermore, a nonlinear disturbance observer is introduced to improve the robustness of the system. The stability of the closed-loop system is proved according to the Lyapunov theorem, and two numerical examples of underactuated robotic systems with coupled inputs are presented to demonstrate the effectiveness of the proposed approach.

High impedance Josephson junction resonators in the transmission line geometry

In this article, we present an experimental study of microwave resonators made out of Josephson junctions. The junctions are embedded in a transmission line geometry so that they increase the inductance per length for the line. By comparing two devices with different input/output coupling strengths, we show that the coupling capacitors, however, add a significant amount to the total capacitance of the resonator. This makes the resonators with high coupling capacitance to act rather as lumped element resonators with inductance from the junctions and capacitance from the end sections. Based on a circuit analysis, we show that the input and output couplings of the resonator are limited to a maximum value of ωrZ0/4Zr, where ωr is the resonance frequency and Z0 and Zr are the characteristic impedances of the input/output lines and the resonator, respectively.

SIMPLIFIED EXPONENTIAL REACHING LAW BASED SLIDING MODE CONTROL OF TWO-LINK PLANAR ROBOT

Robotics has gained attention for its ability to perform industrial tasks like welding, machining, position con- trol, and other essential medical operations. Controllers are continuously being improved in design for simplicity, robustness, and performance accuracy. High-accuracy trajectory tracking is a challenging area in robot control due to nonlinearities and input couplings. To achieve high accuracy and precision, robots need accurate, and fast- processing controllers. This research focuses on applying the exponential reaching law (ERL) to control a two-link planar robot. The Euler-Lagrange approach was used for dynamic modeling, and the MATLAB/Simulink toolbox was used for the simulation. The result was obtained in terms of RMS error for the links, with and without the fric- tional disturbance, in the case of the proportional and derivative (PD) controller, reaching law based sliding mode controller (RSMC), and the simplified exponential reaching law based sliding mode controller (SSMC). RMSE with disturbance for SSMC (0.0753 rad, 0.0817 rad for link 1 and link 2 respectively), RSMC (0.0828 rad, 0.0852 rad) and PD (0.2784 rad, 0.1062 rad), and RMSE values without disturbance for SSMC (0.0743rad, 0.0811rad for link 1 and link 2 respectively), RSMC (0.0752 rad, 0.0815 rad) and PD (0.2579 rad, 0.1021 rad). Based on the result obtained, SSMC was found to be more accurate and robust as a result of having the minimum tracking error minimum change in RMSE respectively, as compared to the other controllers. It is evident that SSMC control scheme has proven to have an improved performance with simplicity and robustness.