Nonreciprocal Filters Research Articles

Numerical Insight Into the Origin of Nonreciprocity and Performance Enhancement in Nonreciprocal Bandpass Filters Using Evolutionary Algorithm

In this paper, a non-reciprocal filter with enhanced directivity is analyzed methodically, and the filter parameters are optimized using an evolutionary algorithm. The non-uniform spectral dispersion of power among the harmonics in opposite directions is substantiated with the aid of filtering polynomials. These polynomials indicate the origin of at least one transmission zero within the passband in the reverse direction, thereby promoting the non-reciprocal filter response. The filter’s return loss, insertion loss, and isolation characteristics exhibit a trade-off that demands an optimization algorithm to effectuate the judicious choice of circuit parameters. This problem is addressed by designing a 150 MHz lumped element non-reciprocal bandpass filter based on the parameters extracted using an evolutionary algorithm based particle swarm optimization (PSO). The simulated and measured results comply well with the modeling, and the results exhibit maximum directivity of 28.2 dB without degradation in insertion loss (1.1 dB) and return loss (16.2 dB) within the passband. The algorithm can be utilized in designing non-reciprocal filters having different center frequencies and bandwidths.

A Topological Multichannel Add-Drop Filter Based on Gyromagnetic Photonic Crystals.

We theoretically proposed a topological multichannel add-drop filter (ADF) and studied its unique transmission properties. The multichannel ADF was composed of two one-way gyromagnetic photonic crystal (GPC) waveguides, a middle ordinary waveguide, and two square resonators sandwiched between them, which can be regarded as two paralleling four-port nonreciprocal filters. The two square resonators were applied with opposite external magnetic fields (EMFs) to support one-way states propagating clockwise and counterclockwise, respectively. On the basis of the fact that the resonant frequencies can be tuned by the EMFs applied to the square resonators, when the intensities of EMFs were the same, the multichannel ADF behaved as a power splitter with a 50/50 division ratio and high transmittance; otherwise, it functioned as a demultiplexer to separate two different frequencies efficiently. Such a multichannel ADF not only possesses excellent filtering performance but also has strong robustness against various defects due to its topological protection property. Moreover, each output port can be switched dynamically, and each transmission channel can operate independently with little crosstalk. Our results have the potential for developing topological photonic devices in wavelength division multiplexing systems.

Nonreciprocal photonic management for photovoltaic conversion: design and fundamental efficiency limits

Significant progress in the development of nonreciprocal optical components with broken Kirchhoff symmetry paves the way for increasing the photovoltaic (PV) conversion efficiency beyond the Shockley–Queisser limit due to reuse of emitted photons. Recent papers have analyzed the PV converter with several or an infinite number of multijunction cells, in which the cells are coupled via nonreciprocal filters (optical diodes) in such a way that the light emitted by one cell is absorbed by another cell. We proposed and investigated a single cell converter with nonreciprocal external photon recycling, which provided reabsorption and reuse of the emitting light by the same cell. We considered properties of photons in the sunbeam in terms of ergodicity, disorder, energy availability, information entropy, and coherence, and established fundamental limitations imposed by endoreversible thermodynamics on conversion efficiency at maximal power output. Our results show that the nonreciprocal converter with an ideal multijunction cell can approach the Carnot efficiency, whereas operating exactly at the Carnot limit requires an infinite number of photon recycling processes. This requirement resolves the famous thermodynamic paradox of the optical diode because any small dissipation in the cell or optical system enhanced by infinite recycling will stabilize the converter operation below the Carnot limit. We generalized endoreversible thermodynamics to photonic distributions with nonzero chemical potential and derived the limiting efficiency of the nonreciprocal single-junction PV converter. The performance of this converter with available GaAs solar cells was evaluated.

Nonlinearity-Induced Nonreciprocity—Part II

This is the second part of our study of the properties and limitations of nonlinearity-induced nonreciprocal devices. In the first part, we discussed the general working principle of nonreciprocal devices based on resonators filled with Kerr-like nonlinearities, and we showed that strong nonreciprocal responses can be obtained when an asymmetric resonator is excited from only one port with a quasi-monochromatic signal. In this second part, we further investigate their behavior under more complex input signals. Specifically, we discuss simultaneous two-port excitations, which are expected to be detrimental to isolation purposes due to the lack of linearity of these devices. We show, however, that this regime can be used to achieve novel functionalities, such as optically controlled transmission switching. In order to assess the device performance in realistic scenarios, we also investigate how noise and random inputs affect the nonreciprocal response. Finally, we show how increasing the system complexity can help to overcome some device limitations, and lead to novel applications such as bandpass nonreciprocal filters. Overall, this work shows that nonlinearity-induced nonreciprocal devices, with their appealing passive, bias-free operation, offer interesting opportunities for various practical application scenarios and may form the basis for a plethora of new technologies of great relevance for microwave engineering systems, from radars and lidars to classical and quantum signal processing and computing.

Coupling Matrix Representation of Nonreciprocal Filters Based on Time-Modulated Resonators

This paper addresses the analysis and design of non-reciprocal filters based on time modulated resonators. We analytically show that time modulating a resonator leads to a set of harmonic resonators composed of the unmodulated lumped elements plus a frequency invariant element that accounts for differences in the resonant frequencies. We then demonstrate that harmonic resonators of different order are coupled through non-reciprocal admittance inverters whereas harmonic resonators of the same order couple with the admittance inverter coming from the unmodulated filter network. This coupling topology provides useful insights to understand and quickly design non-reciprocal filters and permits their characterization using an asynchronously tuned coupled resonators network together with the coupling matrix formalism. Two designed filters, of orders three and four, are experimentally demonstrated using quarter wavelength resonators implemented in microstrip technology and terminated by a varactor on one side. The varactors are biased using coplanar waveguides integrated in the ground plane of the device. Measured results are found to be in good agreement with numerical results, validating the proposed theory.

Radio Frequency Angular Momentum Biased Quasi-LTI Nonreciprocal Acoustic Filters.

We report on the design and operation of a novel class of nonreciprocal acoustic filters operating in the radio frequency (RF) range. These devices use the spectral characteristics of commercial acoustic filters placed in angular momentum biased networks to achieve large nonreciprocity, low insertion loss (I.L.), and wideband operation. Owing to the high rejection exhibited by acoustic filters, these novel devices can achieve an unprecedented suppression of undesired intermodulation products, thus approaching the spectral purity attained by conventional linear-time-invariant (LTI) filtering components. In addition, a new analytical model suitable to capture the behavior of any angular-momentum-biased nonreciprocal device is presented. This model allows us to identify the main characteristics of the transfer function (poles and zeroes) relative to this new class of nonreciprocal filters, thus enabling new synthesis capabilities through standard numerical methods. Ultimately, the performance of a built 1.1-GHz nonreciprocal acoustic filter prototype is reported. This device relies on a modulation implemented through switched capacitors and shows I.L., isolation, and half-power bandwidth values of 4.5 dB, 28 dB, and 20 MHz, respectively, achieved through the use of a 40-MHz modulation frequency. Moreover, by showing an intermodulation distortion lower than -34 dBc, it approaches the operation of LTI circuits.

Isolating Bandpass Filters Using Time-Modulated Resonators

In this paper, we demonstrate, for the first time, an isolating bandpass filter with low-loss forward transmission and high reverse isolation by modulating its constituent resonators. To understand the operating principle behind the device, we develop a spectral domain analysis method and show that the same-frequency nonreciprocity is a result of the nonreciprocal frequency conversion to the intermodulation (IM) frequencies by the time-varying resonators. With appropriate modulation frequency, modulation depth, and phase delay, the signal power at the IM frequencies is converted back to the RF frequency and adds up constructively to form a low-loss forward passband, whereas they add up destructively in the reverse direction to create the isolation. To validate the theory, a lumped-element three-pole 0.04-dB ripple isolating filter with a center frequency of 200 MHz and a ripple bandwidth of 30 MHz is designed, simulated, and measured. When modulated with a sinusoidal frequency of 30 MHz, a modulation index of 0.25, and an incremental phase difference of 45°, the filter achieves a forward insertion loss of 1.5 dB and a reverse isolation of 20 dB. The measured nonmodulated and modulated results agree very well with the simulations. Such nonreciprocal filters may find applications in wideband simultaneous transmit and receive radio front ends.

Synthetic phonons enable nonreciprocal coupling to arbitrary resonator networks.

Inducing nonreciprocal wave propagation is a fundamental challenge across a wide range of physical systems in electromagnetics, optics, and acoustics. Recent efforts to create nonreciprocal devices have departed from established magneto-optic methods and instead exploited momentum-based techniques such as coherent spatiotemporal modulation of resonators and waveguides. However, to date, the nonreciprocal frequency responses that these devices can achieve have been limited, mainly to either broadband or Lorentzian-shaped transfer functions. We show that nonreciprocal coupling between waveguides and resonator networks enables the creation of devices with customizable nonreciprocal frequency responses. We create nonreciprocal coupling through the action of synthetic phonons, which emulate propagating phonons and can scatter light between guided and resonant modes that differ in both frequency and momentum. We implement nonreciprocal coupling in microstrip circuits and experimentally demonstrate both elementary nonreciprocal functions such as isolation and gyration, as well as reconfigurable, higher-order nonreciprocal filters. Our results suggest nonreciprocal coupling as a platform for a broad class of customizable nonreciprocal systems, adaptable to all wave phenomena.

Generalized approximation techniques for selective linear-phase digital and nonreciprocal lumped filters

The paper presents expressions for low-pass digital filter transfer functions with finite-band approximation to constant amplitude and delay in the passband, together with an arbitrary number of transmission zeros at half the sampling frequency. The corresponding high-pass filter transfer functions can be obtained by a transformation. The available degrees of freedom can be divided arbitrarily between the passband and stopband responses. The functions are of the nonminimum-phase type, and the corresponding nonreciprocal analog (continuous) filters are also covered; these can be realized in standard active RC structures. The zero-bandwidth cases are obtained, either directly or as limiting cases of the finite-band ones. It is also indicated that there is an upper bound on the number of transmission zeros which may be introduced while maintaining the stability of the filter for a given degree. The technique represents the most comprehensive one available for the solution of the combined amplitude and phase approximation problem, and leads to a large family of stable transfer functions.

Technical Memorandum. Eigenvalue analysis of 4-port crosswire yig coupling circuit

A 4-port symmetrical coupling circuit met in the theory of YIG devices, such as nonreciprocal filters, limiters and circulators, is that of a junction of two orthogonal wires or other transmission lines. Although the scattering matrix description of this network has been analysed using Kirchhoff's laws, a more simple derivation may be obtained by the eigenvalue method.

Nonreciprocal YIG filters

Several techniques for achieving nonreciprocity in YIG filters employing stripline or miniature coaxial line construction are presented. Nonreciprocal filters can be designed with either single-ended or double-ended nonreciprocity. In either case, the circuit element possesses isolator and circulator properties as well as that of a ferrimagnetic filter. Experimental results have indicated that nonreciprocity greater than 10 dB over octave bandwidths is possible with passband characteristics similar to those of conventional YIG filters. The insertion loss may be somewhat higher than the latter, depending on the technique of nonreciprocity utilized. Nonreciprocal YIG filters are useful with TWT microwave receiver systems as preselectors and post-selectors and obviate the need for isolators. These filters are also useful as tunable microwave circulators in conjunction with tunnel diode amplifiers and parametric amplifiers. An individual filter may be used as a diplexer, while several filters will provide multiplexer operation.

Solid-State Devices Other than Semiconductors

The solid-state properties of matter are of interest not only to the research scientist but also to the engineer who has translated the findings of basic research into endeavors of great technological significance. The research in ferromagnetism has led to the magnetic amplifier, which for certain applications is superior to amplifiers using vacuum tubes and to the development of new materials such as ferrites, which are now finding use at radio frequencies in pulse transformers, radio frequency inductors, antenna rods, magnetic core delay lines, computer elements, and magnetic recording media. Ferrites and antiferromagnetic materials are now also widely used at microwave frequencies and millimeter wavelengths as nonreciprocal phase shifters, load isolators, filters, ferrite switches, ferrod radiators, modulators and power limiters. Such phenomena as piezoelectricity, ferroelectricity and magnetostriction have resulted in the use of ultrasonic waves for the study of phononelectron and phonon-spin interactions as well for such practical devices as the detection of imperfections and faults in solids, electromechanical transducers, resonators, filters, delay lines, computer memories, ultrasonic soldering, cleaning, drilling and cutting, and strain and acceleration gauges. Dielectric materials have found applications as prisms, polarizers, restrahlen plates and more recently in dielectric wave guides and fiber optics.