Driven by the increasing demand on handing microwave signals with compact device, low power consumption, high efficiency and high reliability, it is highly desired to generate, distribute, and process microwave signals using photonic integrated circuits. Silicon photonics offers a promising platform facilitating ultracompact microwave photonic signal processing assisted by silicon nanophotonic devices. In this paper, we propose, theoretically analyze and experimentally demonstrate a simple scheme to realize ultracompact rejection ratio tunable notch microwave photonic filter (MPF) based on a silicon photonic crystal (PhC) nanocavity with fixed extinction ratio. Using a conventional modulation scheme with only a single phase modulator (PM), the rejection ratio of the presented MPF can be tuned from about 10 dB to beyond 60 dB. Moreover, the central frequency tunable operation in the high rejection ratio region is also demonstrated in the experiment.

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Photonic crystal nanocavity assisted rejection ratio tunable notch microwave photonic filter

The radio-frequency (RF) electromagnetic spectrum, extending from below 1 MHz to above 100 GHz, represents a precious resource. It is used for a wide range of purposes, including communications, radio and television broadcasting, radionavigation, and sensing. Radar represents a fundamentally important use of the electromagnetic (EM) spectrum, in applications which include air traffic control, geophysical monitoring of Earth resources from space, automotive safety, severe weather tracking, and surveillance for defense and security. Nearly all services have a need for greater bandwidth, which means that there will be ever-greater competition for this finite resource. The paper explains the nature of the spectrum congestion problem from a radar perspective, and describes a number of possible approaches to its solution both from technical and regulatory points of view. These include improved transmitter spectral purity, passive radar, and intelligent, cognitive approaches that dynamically optimize spectrum use.

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Radar Spectrum Engineering and Management: Technical and Regulatory Issues

Highly selective and reconfigurable microwave filters are of great importance in radio-frequency signal processing. Microwave photonic (MWP) filters are of particular interest, as they offer flexible reconfiguration and an order of magnitude higher frequency tuning range than electronic filters. However, all MWP filters to date have been limited by trade-offs between key parameters such as tuning range, resolution, and suppression. This problem is exacerbated in the case of integrated MWP filters, blocking the path to compact, high-performance filters. Here we show the first chip-based MWP bandstop filter with ultrahigh suppression, high resolution in the megahertz range, and 0–30 GHz frequency tuning. This record performance was achieved using an ultralow Brillouin gain from a compact photonic chip and a novel approach of optical resonance-assisted RF signal cancellation. The results point to new ways of creating energy-efficient and reconfigurable integrated MWP signal processors for wireless communications and defence applications.

Low-power, chip-based stimulated Brillouin scattering microwave photonic filter with ultrahigh selectivity

This paper describes a synthesis method for symmetric dual-passband microwave filters. The proposed method employs frequency transformation techniques for finding the locations of poles and zeros of a desired filter. This method can be used to design dual-passband filters with prescribed passbands and attenuation at stopbands directly without the need for any optimization processes. To validate the procedure a dual-passband stripline filter is designed and fabricated. The stripline dual-passband filter is designed with passbands at 3.90-3.95 and 4.05-4.10 GHz, and 30-dB attenuation at the stopband. This measured results show a good agreement with the theoretical ones. The frequency transformation for symmetric dual-passband filters is also extended to include asymmetric dual-passband responses. This flexible frequency transformation preserves the attenuation characteristics of the low-pass filter prototype. Examples are shown to discuss the flexibility of this transformation.

A Synthesis Method for Dual-Passband Microwave Filters

A design methodology and equations are described for lumped-element filter prototypes having low-pass, high-pass, bandpass, or bandstop characteristics with theoretically perfect input- and output-match at all frequencies. Such filters are a useful building block in a wide variety of systems in which the highly reactive out-of-band termination presented by a conventional filter is undesirable. The filter topology is first derived from basic principles. The relative merits of several implementations and tunings are then compared via simulation. Finally, measured data on low-pass and bandpass filter examples are presented, which illustrate the practical advantages, as well as showing excellent agreement between measurement and theory.

Theoretical and Experimental Study of a New Class of Reflectionless Filter

Two absorptive bandstop filter circuit topologies are introduced. The preferred topology is analyzed and its advantages over traditional approaches quantified. A corresponding varactor-tuned microstrip absorptive bandstop filter is described and its unique ability to maintain its attenuation while tuning across a broad frequency range using only resonant frequency tuning is demonstrated.

Compact, frequency-agile, absorptive bandstop filters

The lumped-element bridged-T notch filter concept is extended to reciprocal distributed-element microwave filters. A prototype microstrip bridged-T notch filter employing single-mode resonators is described that enhances the effective resonator Q/sub u/ by a factor of 325, while a different microstrip filter topology with a triple-mode resonator is described that enhances the effective resonator Q/sub u/ by a factor of 89. The new notch filters can be either partly reflective or fully absorptive within the stopband. Additional enhanced-Q/sub u/ notch filter topologies, such as a hybrid-coupler notch filter, that include at least two resonances and at least two signal paths are suggested.

Passive enhancement of resonator Q in microwave notch filters

Normal realizations of bandstop resonators with finite unloaded Q suffer from degradation of performance due dissipation loss. In this paper, it is shown theoretically that there exists a class of second-order networks which simultaneously exhibit an ideal bandstop resonance, with infinite stopband attenuation, and a perfect match at all frequencies. Theoretical analysis is backed up with experimental results for three different physical realizations.

Perfectly-matched bandstop filters using lossy resonators

A frequency-agile bandstop filter technology with tunable stopband attenuation and constant absolute bandwidth is described. The technology is demonstrated by a six-resonator planar microstrip filter with simultaneous varactor-diode tuning of stopband attenuation from 30dB to 50dB and of operating frequency from 1.8 GHz to 2.2 GHz, with a stopband bandwidth of 60 MHz and absolute 3dB bandwidth of less than 390 MHz.

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Frequency-agile bandstop filter with tunable attenuation

An absorptive bandstop filter includes at least two frequency-dependent networks, one of which constitutes a bandpass filter, that form at least two forward signal paths between an input port and an output port and whose transmission magnitude and phase characteristics are selected to provide a relative stopband bandwidth that is substantially independent of the maximum attenuation within the stopband and/or in which the maximum attenuation within the stopband is substantially independent of the unloaded quality factor of the resonators. The constituent network characteristics can also be selected to provide low reflection in the stopband as well as in the passband. The absorptive bandstop filter can be electrically tunable and can substantially maintain its attenuation characteristics over a broad frequency tuning range.

Douglas R. Jachowski

Papers

Compact, frequency-agile, absorptive bandstop filters

Passive enhancement of resonator Q in microwave notch filters

Perfectly-matched bandstop filters using lossy resonators

Frequency-agile bandstop filter with tunable attenuation

Narrow-band absorptive bandstop filter with multiple signal paths