Traveling-wave Resonator Research Articles

Record Purcell factors in ultracompact hybrid plasmonic ring resonators.

For integrated optical devices and traveling-wave resonators, realistic use of the superior wave-matter interaction offered by plasmonics is impeded by ohmic loss, which increases rapidly with mode volume reduction. In this work, we report composite hybrid plasmonic waveguides (CHPWs) that are not only capable of guiding subwavelength optical mode with long-range propagation but also unrestricted by stringent requirements in structural, material, or modal symmetry. In these asymmetric CHPWs, the versatility afforded by coupling dissimilar plasmonic modes provides improved fabrication tolerance and more degrees of device design optimization. Experimental realization of CHPWs demonstrates propagation loss and mode area of 0.03 dB/μm and 0.002 μm2, corresponding to the smallest combination among long-range plasmonic structures reported to date. CHPW ring resonators with 2.5-μm radius were realized with record Purcell factor compared with existing plasmonic and dielectric resonators of similar radii.

Микрополосковые удвоители СВЧ с нетрадиционной реализацией

Frequency multipliers are used in electronic devices to generate spectrally pure sinusoidal signals in the frequency range from a few to tens of GHz. The multipliers are used to multiply the frequency of highly stable but more low-frequency devices with the subsequent extraction of the necessary harmonics from the frequency spectrum of the received microwave range. The frequencies selected after multiplication (set) have significantly higher energy, spectral and range characteristics, which allows them to be used as local oscillators and synthesizers in receiving and transmitting systems. The authors of this paper theoretically substantiate and practically demonstrate the possibility of an unconventional implementation of a microstrip multiplier of the microwave range based on a directional traveling wave filter. The proposed implementation does not require the use of active semiconductor elements. The well-known circuit and technological principles for the creation of microstrip microwave multipliers are considered in the paper. The features, problems and shortcomings arising from their implementation are analyzed. The effectiveness of using the balanced circuit for frequency multiplication is confirmed. A list of mandatory requirements and conditions necessary for the implementation of the microwave multipliers is given. It is demonstrated that the features of the microstrip travelling-wave filter are identical to the conditions and requirements for the implementation of balanced multipliers. It is shown and substantiated how an unconventional implementation of a passive microwave multiplier is possible due to the electromagnetic interaction of the input and output nodes of such a filter with an annular travelling-wave resonator. Using the example of modifying a block diagram of a directional filter into a multiplier circuit, the possibility of creating a microwave doubler is confirmed by separating a given frequency from the frequency spectrum of a traveling-wave ring resonator.

Quasioptical Three-Mirror Echelette Traveling-Wave Resonator with Frequency Tuning: Diffraction Theory and Experiment

We develop the theory of a three-mirror echelette traveling-wave resonator with frequency tuning, in which one of the mirrors is an echelette reflective diffraction grating. The theory makes it possible to allow for the diffraction loss related to finite sizes of the mirrors, the loss due to the existence of a mirror lobe at the grating (coupling loss), and the ohmic losses. The possibility of significant rarefaction in the spectrum of the resonator eigenfrequencies is demonstrated. The structure of mode fields allows one to ensure efficient input of the radiation to the resonator. An experiment performed in a frequency range near 140 GHz confirmed the theoretical calculations.

Reconfigurable optomechanical circulator and directional amplifier

Non-reciprocal devices, which allow non-reciprocal signal routing, serve as fundamental elements in photonic and microwave circuits and are crucial in both classical and quantum information processing. The radiation-pressure-induced coupling between light and mechanical motion in travelling-wave resonators has been exploited to break the Lorentz reciprocity, enabling non-reciprocal devices without magnetic materials. Here, we experimentally demonstrate a reconfigurable non-reciprocal device with alternative functions as either a circulator or a directional amplifier via optomechanically induced coherent photon–phonon conversion or gain. The demonstrated device exhibits considerable flexibility and offers exciting opportunities for combining reconfigurability, non-reciprocity and active properties in single photonic devices, which can also be generalized to microwave and acoustic circuits.

Tutorial: Integrated-photonic switching structures

Recent developments in waveguided 2 × 2 and N × M photonic switches are reviewed, including both broadband and narrowband resonant devices for the Si, InP, and AlN platforms. Practical actuation of switches by electro-optical and thermo-optical techniques is discussed. Present datacom-and-computing applications are reviewed, and potential applications are proposed for chip-scale photonic and optoelectronic integrated switching networks. Potential is found in the reconfigurable, programmable “mesh” switches that enable a promising group of applications in new areas beyond those in data centers and cloud servers. Many important matrix switches use gated semiconductor optical amplifiers. The family of broadband, directional-coupler 2 × 2 switches featuring two or three side-coupled waveguides deserves future experimentation, including devices that employ phase-change materials. The newer 2 × 2 resonant switches include standing-wave resonators, different from the micro-ring traveling-wave resonators. The resonant devices comprise nanobeam interferometers, complex-Bragg interferometers, and asymmetric contra-directional couplers. Although the fast, resonant devices offer ultralow switching energy, ∼1 fJ/bit, they have limitations. They require several trade-offs when deployed, but they do have practical application.

Traveling wave-like Fabry–Perot resonator-based add-drop filters

We have proposed and studied a novel channel add-drop filter (ADF) based on a single Fabry-Perot resonator. The resonator consists of two mode-conversion Bragg grating reflectors separated by a wide waveguide that laterally coupled to two narrow waveguides. It behaves like a traveling-wave resonator where fields are coupled to the buses in one direction. Compact and narrowband ADFs are achieved with dropping efficiencies higher than 95%, as shown by the three-dimensional finite-difference time-domain simulations. In addition, the proposed device is applied to realize an eight-channel add-drop multiplexer in the C-band by cascading the ADFs with adjusted channel wavelengths.

Low temperature difference thermoacoustic prime mover with asymmetric multi-stage loop configuration

Environmentally friendly and low-cost technologies to recover low-grade heat source into usable energy can contribute to ease the energy shortage. Thermoacoustic technology is expected as one promising approach in this ascendant field. In this work, the multi-stage looped thermoacoustic prime movers with asymmetric configuration, which can provide travelling-wave resonator and appropriate acoustic field for efficient regenerator, have been proposed and experimentally studied. The presented looped thermoacoustic prime movers can start to oscillate with quite low temperature difference along the regenerator. The lowest onset temperature difference obtained in the experiments is only 17 °C (the corresponding heating temperature is 29 °C), which can be achieved in both three-stage and four-stage looped thermoacoustic prime movers, with CO2 of 1 MPa or 1.5 MPa as the working fluid. An electric generator driven by a three-stage looped thermoacoustic prime mover with low heating temperature was tested to achieve the acoustic to electric conversion.

Patch-Probe Excitation for Ultrahigh Magnetic Field Wide-Bore MRI

In this paper, we present the design of probe excitations for 7- and 10.5-T traveling-wave wide-bore magnetic resonance imaging systems. The probes are 297- and 447-MHz coaxially fed microstrip patches, designed to give a circularly polarized magnetic field when placed in the metallic bore waveguide. Images of a water phantom using the patch probes are obtained and compared with full-wave electromagnetic simulations. Additionally, periodic axial metal strip cylinders are inserted into the bore, resulting in improved field uniformity in a phantom with a simultaneous increase in SNR. The numerical analysis for nonquasi-static fields can be computationally intensive, and the numerical error for finite-element simulations is quantified using modified boundary conditions of the system. When the waveguide effects are taken into account, the mode content in the images compares well to full-wave simulations.

Design of high power RF amplifier for 3 MW/CW transmission line test rig

India is developing 2.5MW RF source at VSWR 2:1 in the frequency range 35–65MHz for ITER project. Eight such RF sources will generate total 20MW of RF power for plasma heating and current drive. A large number of high power transmission line components are required for connecting various stages of RF source. To test these passive transmission line components at high power, a 3MW test facility based on the concept of Traveling Wave Resonator (TWR) is underway. A high power CW RF amplifier is required to feed RF power to 10dB coupler of TWR. To fulfill the requirement a tetrode based amplifier is designed to operate in class B with optimized efficiency, gain and harmonics level. The detail design is carried out to estimate operational parameters derived from the data sheet of selected tetrode, simulation for RF cavities, power supply requirements and thermal management for CW operation.

Coupled-mode-theory framework for nonlinear resonators comprising graphene.

A general framework combining perturbation theory and coupled-mode theory is developed for analyzing nonlinear resonant structures comprising dispersive bulk and sheet materials. To allow for conductive sheet materials, a nonlinear current term is introduced in the formulation in addition to the more common nonlinear polarization. The framework is applied to model bistability in a graphene-based traveling-wave resonator system exhibiting third-order nonlinearity. We show that the complex conductivity of graphene disturbs the equality of electric and magnetic energies on resonance (a condition typically taken for granted), due to the reactive power associated with the imaginary part of graphene's surface conductivity. Furthermore, we demonstrate that the dispersive nature of conductive materials must always be taken into account, since it significantly impacts the nonlinear response. This is explained in terms of the energy stored in the surface current, which is zeroed-out when linear dispersion is neglected. The results obtained with the proposed framework are compared with full-wave nonlinear finite-element simulations with excellent agreement. Very low characteristic power for bistability is obtained, indicating the potential of graphene for nonlinear applications.

Chiral symmetry breaking in a microring optical cavity by engineered dissipation

We propose a method to break the chiral symmetry of light in traveling wave resonators by coupling the optical modes to a lossy channel. Through the engineered dissipation, an indirect dissipative coupling between two oppositely propagating modes can be realized. Combining with reactive coupling, it can break the chiral symmetry of the resonator, allowing light propagating only in one direction. The chiral symmetry breaking is numerically verified by the simulation of an electromagnetic field in a micro-ring cavity, with proper refractive index distributions. This work provokes us to emphasize the dissipation engineering in photonics, and the generalized idea can also be applied to other systems.

Development of data acquisition and control system for ICH & CD transmission line components test facility

India is developing a very high power test bed (∼MW level) based on concept of Travelling Wave Resonator (TWR) for testing of passive transmission line components [3]. Data acquisition and control system is required for safe and reliable operation of TWR. It includes online monitoring of input power and circulating ring power. Acquired RF signal provides information about forward and reflected RF power. For MW power level testing, system is protected against arc and thermal effect during CW operation. National Instrument make hardware and software have been used to acquire signals from RF detector, Arc detectors, IR camera and thermocouples etc. LabVIEW™ based software has been developed for calculation of RF parameters. Graphical user interface is developed for better visualization. Initial testing of the TWR setup with 10 dB coupler provides a power gain of 13.58 dB (22X). For future 3 MW TWR test bed the required power gain is ∼20 dB (100X) since the available source is ∼40 kW. This paper describes the detail design and integrated test results for data acquisition and control system for TWR test bed.

Resonant cavity based time-domain multiplexing techniques for coherently combined fiber laser systems

This paper describes novel time-domain multiplexing techniques that use various resonant cavity configurations for increasing pulse energy extraction per each parallel amplification channel of a coherently combined array. Two different techniques are presented: a so-called N2 coherent array combining technique, applicable to a periodic pulse train, and a coherent pulse stacking amplification (CPSA) technique, applicable to a pulse burst. The first technique is a coherent combining technique, which achieves simultaneous beam combining and time-domain pulse multiplexing/down-counting using traveling-wave Fabry-Perot type resonators. The second technique is purely a time-domain pulse multiplexing technique, used with either a single amplifier or an amplifier array, which uses traveling-wave Gires-Tourmois type resonators. The importance of these techniques is that they can enable stacking of very large number of pulses, thus increasing effective amplified-pulse duration potentially by 102 to 103 times, and reducing fiber array size by the corresponding factor. This could lead to very compact coherently combined arrays even for generating very high pulse energies in the range of 1 to 100 J.

Channel add-drop filter based on dual photonic crystal cavities in push-pull mode.

We demonstrate an add-drop filter based on a dual photonic crystal nanobeam cavity system that emulates the operation of a traveling wave resonator, and, thus, provides separation of the through and drop port transmission from the input port. The device is on a 3×3 mm chip fabricated in an advanced microelectronics silicon-on-insulator complementary metal-oxide semiconductor (SOI CMOS) process (IBM 45 nm SOI) without any foundry process modifications. The filter shows 1 dB of insertion loss in the drop port with a 3 dB bandwidth of 64 GHz, and 16 dB extinction in the through port. To the best of our knowledge, this is the first implementation of a port-separating, add-drop filter based on standing wave cavities coupled to conventional waveguides, and demonstrates a performance that suggests potential for photonic crystal devices within optical immersion lithography-based advanced CMOS electronics-photonics integration.

Multiplexed Millimeter Wave Communication with Dual Orbital Angular Momentum (OAM) Mode Antennas.

Communications using the orbital angular momentum (OAM) of radio waves have attracted much attention in recent years. In this paper, a novel millimeter-wave dual OAM mode antenna is cleverly designed, using which a 60 GHz wireless communication link with two separate OAM channels is experimentally demonstrated. The main body of the dual OAM antenna is a traveling-wave ring resonator using two feeding ports fed by a 90° hybrid coupler. A parabolic reflector is used to focus the beams. All the antenna components are fabricated by 3D printing technique and the electro-less copper plating surface treatment process. The performances of the antenna, such as S-parameters, near-fields, directivity, and isolation between the two OAM modes are measured. Experimental results show that this antenna can radiate two coaxially propagating OAM modes beams simultaneously. The multiplexing and de-multiplexing are easily realized in the antennas themselves. The two OAM mode channels have good isolation of more than 20 dB, thus ensuring the reliable transmission links at the same time.

Development of an interference filter model based on a total-internal-reflection resonator

An analytical model of a traveling-wave optical resonator with total-internal-reflection mirrors is studied. It is shown that the number of reflections from the mirrors can be increased manifold without attenuation of the light wave in comparison with Fabry–Perot resonators. Ways to eliminate edge effects and design solutions to compensate angular errors in manufacturing the resonator are considered.

Broadband Tunable Pre-Bunched Electron Cyclotron Maser for Terahertz Application

The relativistic electron cyclotron maser (ECM) has been successfully applied to generating high-power THz wave. In order to realize the additional advantages of broadband tuning and high efficiency interaction, this paper is devoted to exploring the THz pre-bunched ECM. Other than a conventional open-cavity tunable gyrotron consecutively switching between axial modes to realize frequency tuning, a pre-bunched ECM system operates on the backward traveling-wave resonance to achieve broadband smooth tuning. Especially, an interaction circuit of specified axial profile of beam-wave detuning frequency is built to achieve high efficiency. An optimized 0.1 THz pre-bunched ECM system using an electron beam of 30 kV voltage and 3 A current is predicted to generate broad bandwidth of 10 GHz and efficiency between 10% $\sim$ 25%. The broadband tuning pre-bunched ECM is promising for a new generation of broadband and high-power THz source.

Stabilized Operation of a Microwave Compressor Driven by Relativistic S-Band Magnetron

The stabilized operation of a microwave compressor based on a travelling wave resonator, producing at its output microwave pulses with a power of 1.15 ± 0.05 GW and duration of 12 ± 2 ns, is described. The compressor is pumped by a relativistic S-band magnetron generating microwave pulses with a power up to 250 MW and duration of ~100 ns. To stabilize the frequency of the microwaves generated by the magnetron, a transparent cathode having a limited electron emission surface and double ring-type straps between the anode resonators were applied. It was shown that the application of a phase-shifter between the magnetron and resonator results in the influence of the positive feedback on the magnetron operation beginning within the first 10 ns of the microwaves generation. Finally, the incorporation of symmetrically placed phase-shifters in the compressor's resonator allows the travelling wave phase to be adjusted, and accordingly, reliable ignition of the microwave gas discharge switch.

A traveling-wave ring resonator with Bragg deflectors in a two-stage terahertz free-electron laser

We propose a scheme of a two-stage terahertz free-electron laser (FEL) based on a Bragg traveling-wave ring resonator and parallel ribbon relativistic electron beams. The first beam moves in the field of a planar undulator and generates a pumping wave in the millimeter range. At the second stage, this wave transforms into terahertz radiation in the course of intraresonator backscattering on the second parallel electron beam. The ring resonator consists of four Bragg deflectors, each deflecting radiation by 90° in the vicinity of Bragg resonance, thus closing the feedback ring of the low-frequency pumping generator and simultaneously providing effective transfer of the pumping wave to the scattering stage.

Optical bistability with hybrid silicon-plasmonic disk resonators

Optical bistability with a hybrid silicon-plasmonic configuration consisting of a nonlinear traveling-wave (disk) resonator side-coupled with a bus waveguide is theoretically investigated. The nonlinear response is studied with a theoretical framework combining perturbation theory and temporal coupled-mode theory. For the CW case, a general closed-form expression is derived. The effect of the parameters entering in the expression on the bistability curve is thoroughly investigated, and the physical system is accordingly designed so as to exhibit minimum power threshold for bistability and maximum extinction ratio between bistable states. Finally, the temporal dynamics are assessed. The system can toggle between bistable states in approximately 5 ps and is thus suitable for ultrafast memory and switching applications.