Narrow Radio Frequency Research Articles

Time delays between radio and X-ray and between narrow radio bands of Sgr A* flares in the shock oscillation model

ABSTRACT We examine the time delay between radio and X-ray and between narrow radio frequency flares in Sagittarius A* (Sgr A*), from analyses of the synchrotron, bremsstrahlung, and monochromatic luminosity curves. Using the results of 2D relativistic radiation magnetohydrodynamic (MHD) simulations based on the shock oscillation model, we find three types of time delay between the synchrotron and bremsstrahlung emissions: Type A with a time delay of 2–3 h on the shock descending branch, Type B with no time delay, and Type C with an inverse time delay of 0.5–1 h on the shock ascending branch. The time delays in Types A and C are interpreted as a transit time of Alfvén and acoustic waves between both emission dominant regions, respectively. The delay times between 22 and 43 GHz flares and between 8 and 10 GHz flares are ∼13–26 and 13 min, respectively, while the inverse delay also occurs dependently on the shock location branch. These time delays between the narrow radio bands are interpreted as the transit time of the acoustic wave between the frequency-dependent effective radii $R_{\tau _{\rm \nu =1}}$, at which the optical depth τν at the accretion disc surface becomes approximately unity. The shock oscillation model explains well the observed delay times of 0.5–5 h between radio and X-ray, 20–30 min between 22 and 43 GHz, and ∼18 min between 8 and 10 GHz in Sgr A*.

Quantum-Dash Laser-Based Tunable 50/75 GHz mmW Transport System for Future L-Band Networks

Tunable 50/75 GHz millimeter wave (mmW) transmission system has been demonstrated employing self-injection locked quantum-dash laser diode comb source (SIL QDCS) in the challenging 1610 nm region. The generated 50/75 GHz mmW beat-tones exhibited low phase noise of −64 dB/Hz at 1 kHz offset and narrow radio-frequency (RF) linewidths of ~1 kHz at the transmitter end. A 4 GBd (8 Gb/s) quadrature phase-shift keying (QPSK) data stream is successfully transmitted over both mmW carriers and across various channels <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">viz. 2</i> m wireless (WL) link, 11.6 km single-mode-fiber (SMF) – 2 m WL link, and 11.6 km SMF – 6 m free-space-optics (FSO) – 2 m WL link, thus affirming the potential of quantum-dash laser diode (QDLD) in high-frequency mmW applications. This work is a potential step towards realizing next-generation high-frequency tunable mmW optical-RF convergent hybrid networks operation in L-band, and one such possible proposal with a single QDCS is presented.

A Systems Analysis of Energy Usage and Effectiveness of a Counter-Unmanned Aerial System Using a Cyber-Attack Approach

Existing counter-unmanned aerial system (C-UAS) defensive mechanisms rely heavily on radio frequency (RF) jamming techniques that require a large amount of energy to operate. The effects of RF jamming result in undesirable consequences, such as the jamming of other nearby friendly radio devices as well as the increase in RF footprint for local operators. Current cybersecurity analysis of commercial off-the-shelf (COTS) UASs have revealed multiple vulnerabilities that give rise to opportunities to conduct C-UAS operations in the cyber domain. This is achieved by performing cyber-attacks on adversarial UASs through hijacking the device-specific communication’s link on a narrow RF band and without the need for broad-spectrum RF energy bursts during C-UAS operations, which can result in lower energy usage to accomplish the same outcome. This article validates the cyber-attack C-UAS (CyC-UAS) concept through reviewing recent C-UAS operational experimental scenarios and conducting analysis on the collected data. Then, a simulation model of a defense facility is constructed to analyze and validate specific mission scenarios of interest and several proposed concepts of operation. A comparison of the energy requirements between CyC-UAS and existing C-UAS techniques is performed to assess energy efficiency and trade-offs of different C-UAS approaches. In this article, the comparison of energy requirements between the CyC-UAS prototype and existing C-UAS products that utilize RF jamming methods reveals that CyC-UAS achieves significant energy savings while not affecting other telecommunication devices operating at the same frequencies. While both the C-UAS techniques adopt the denial-of-service strategy, the CyC-UAS is able to achieve the same mission by consuming much less energy. Therefore, the CyC-UAS concept shows promise as a new, lower energy, and lower collateral damage approach to defending against UAS.

Surface acoustic wave photonic filters with a single narrow radio-frequency passband in standard silicon on insulator

Integrated microwave photonic filters are becoming increasingly important for signal processing within advanced wireless and cellular networks. Filters with narrow transmission passbands mandate long time delays, which are difficult to accommodate within photonic circuits. Long delays may be obtained through slow moving acoustic waves instead. Input radio-frequency information can be converted from one optical carrier to another via surface acoustic waves and filtered in the process. However, the transfer functions of previously reported devices consisted of multiple periodic passbands, and the selection of a single transmission band was not possible. In this work, we demonstrate surface acoustic wave, silicon-photonic filters of microwave frequency with a single transmission passband. The filter response consists of up to 32 tap coefficients, and the transmission bandwidth is only 7 MHz. The results extend the capabilities of integrated microwave photonics in the standard silicon-on-insulator platform.

Detection of a 20-min time lag observed from Sgr A* between 8 and 10 GHz with the VLA

ABSTRACT We report the detection and analysis of a radio flare observed on 17 April 2014 from Sgr A* at 9 GHz using the VLA in its A-array configuration. This is the first reported simultaneous radio observation of Sgr A* across 16 frequency windows between 8 and 10 GHz. We cross-correlate the lowest and highest spectral windows centred at 8.0 and 9.9 GHz, respectively, and find the 8.0 GHz light-curve lagging $18.37^{+2.17}_{-2.18}$ min behind the 9.9 GHz light curve. This is the first time lag found in Sgr A*’s light curve across a narrow radio frequency bandwidth. We separate the quiescent and flaring components of Sgr A* via flux offsets at each spectral window. The emission is consistent with an adiabatically expanding synchrotron plasma, which we fit to the light curves to characterize the two components. The flaring emission has an equipartition magnetic field strength of 2.2 Gauss, size of 14 Schwarzschild radii, average speed of 12 000 km s−1, and electron energy spectrum index (N(E) ∝ E−p), p = 0.18. The peak flare flux at 10 GHz is approximately 25 per cent of the quiescent emission. This flare is abnormal as the inferred magnetic field strength and size are typically about 10 Gauss and few Schwarzschild radii. The properties of this flare are consistent with a transient warm spot in the accretion flow at a distance of 10–100 Schwarzschild radii from Sgr A*. Our analysis allows for independent characterization of the variable and quiescent components, which is significant for studying temporal variations in these components.

The NANOGrav 12.5 yr Data Set: Observations and Narrowband Timing of 47 Millisecond Pulsars

Abstract We present time-of-arrival (TOA) measurements and timing models of 47 millisecond pulsars observed from 2004 to 2017 at the Arecibo Observatory and the Green Bank Telescope by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). The observing cadence was three to four weeks for most pulsars over most of this time span, with weekly observations of six sources. These data were collected for use in low-frequency gravitational wave searches and for other astrophysical purposes. We detail our observational methods and present a set of TOA measurements, based on “narrowband” analysis, in which many TOAs are calculated within narrow radio-frequency bands for data collected simultaneously across a wide bandwidth. A separate set of “wideband” TOAs will be presented in a companion paper. We detail a number of methodological changes, compared to our previous work, which yield a cleaner and more uniformly processed data set. Our timing models include several new astrometric and binary pulsar measurements, including previously unpublished values for the parallaxes of PSRs J1832−0836 and J2322+2057, the secular derivatives of the projected semimajor orbital axes of PSRs J0613−0200 and J2229+2643, and the first detection of the Shapiro delay in PSR J2145−0750. We report detectable levels of red noise in the time series for 14 pulsars. As a check on timing model reliability, we investigate the stability of astrometric parameters across data sets of different lengths. We also report flux density measurements for all pulsars observed. Searches for stochastic and continuous gravitational waves using these data will be subjects of forthcoming publications.

Escape and trapping of low-frequency gravitationally lensed rays by compact objects within plasma

We consider the gravitational lensing of rays emitted by a compact object (CO) within a distribution of plasma with power-law density $\propto r^{-h}$. For the simplest case of a cloud of spherically symmetric cold non-magnetized plasma, the diverging effect of the plasma and the converging effect of gravitational lensing compete with one another. When $h<2$, the plasma effect dominates over the vacuum Schwarzschild curvature, potentially shifting the radius of the unstable circular photon orbit outside the surface of the CO. When this occurs, we define two relatively narrow radio-frequency bands in which plasma effects are particularly significant. Rays in the escape window have $\omega_{0} < \omega \leq \omega_{+}$ and are free to propagate to infinity from the CO surface. To a distant observer, the visible portion of the CO surface appears to shrink as the observed frequency is reduced, and vanishes entirely at $\omega_{0}$, in excess of the plasma frequency at the CO surface. We define the anomalous propagation window for frequencies $\omega_{-}< \omega \leq \omega_{0}$. Rays emitted from the CO surface within this frequency range are dominated by optical effects from the plasma and curve back to the surface of the CO, effectively cloaking the star from distant observers. We conclude with a study of neutron star (NS) compactness ratios for a variety of nuclear matter equations of state (EoS). For $h=1$, NSs generated from stiff EoS should display significant frequency dependence in the EW, and lower values of $h$ with softer EoS can also show these effects.

Tunable optoelectronic oscillator with an embedded delay-line oscillator for fine steps

Tunable optoelectronic oscillator (OEO) with an embedded delay-line oscillator (EDO) for fine steps is presented. A delay-line oscillator consisting of an amplifier, phase shifter, and electrical delay line is embedded in the general single loop OEO. The oscillation frequency is controlled by the EDO loop phase, which is adjusted by the phase shifter. Without any narrow radio frequency bandpass filter, about 340 MHz tunable range with a fine step of ∼100 kHz is realized by directly tuning the direct current voltage of the phase shifter for 8 GHz oscillation. We also experimentally demonstrate κ-band tunable microwave signal generation within 22.18 to 23.32 GHz. The tunable accuracy and tunable range have a better performance compared to other OEO schemes with a fine frequency tunable step. The phase noise of the generated 22.25 GHz microwave signal is −108.7 dBc∕Hz at 10 kHz offset. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.OE.55.3.031117)

A high‐speed tunable optoelectronic oscillator with a fine step based on semiconductor optical amplifier

ABSTRACTAn optical tunable dual‐loop optoelectronic oscillator (OEO) based on birefringence effects in a semiconductor optical amplifier (SOA) is presented and the theoretical fundamentals of the design are explained. A prototype of the OEO with a frequency of 8.8 GHz is experimentally demonstrated. About 150 MHz tunable range at X‐band with a fine step about 15 MHz is realized by directly tuning the SOA injection current, and the tuning speed can reach to over 1 GHz/µs. The generated radiofrequency (RF) signal exhibits a good stability with a single sideband phase noise lower than −101 dBc/Hz at an offset of 10 kHz within the whole tunable range, without any narrow RF band‐pass filters in the loop. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:464–467, 2015

Current MR Imaging Lipid Detection Techniques for Diagnosis of Lesions in the Abdomen and Pelvis

One application of the unique capability of magnetic resonance (MR) imaging for characterizing soft tissues is in the specific detection of lipid. Adipose tissue may be abundant in the body, but its presence in a lesion can greatly limit differential diagnostic considerations. This article reviews MR imaging fat detection techniques and discusses lesions in the abdomen and pelvis that can be readily diagnosed by using these techniques. Traditional fat detection methods include inversion-recovery and chemically selective fat-suppression pulse sequences, with the former being less sensitive to field heterogeneity and less tissue specific than the latter. Chemical shift-based sequences, which exploit the inherent resonance frequency difference between lipid and water to depict intracytoplasmic fat, have great utility for evaluating hepatic steatosis and lesions such as adrenal and hepatic adenomas, hepatocellular carcinoma, focal lipomatosis of the pancreas, and adrenal cortical carcinoma. The signal from large amounts of fat can be suppressed by using a narrow radiofrequency pulse for selective excitation of fat protons (ie, fat saturation imaging), a technique that increases image contrast resolution and highlights lesions such as contrast-enhancing tissue, edema, and blood products. This technique is especially useful for evaluating renal angiomyolipomas, adrenal myelolipomas, ovarian teratomas, and liposarcomas. MR spectroscopy is a promising method for quantifying absolute liver fat concentration and changes in hepatic triglyceride content during treatment. New and evolving techniques include magnetization transfer and modified Dixon sequences. A solid understanding of these techniques will help improve the interpretation of abdominal and pelvic imaging studies.

Optical pulse generation in single section InAs/GaAs quantum dot edge emitting lasers under continuous wave operation

Observation of sub-picosecond pulses from a single section Fabry Perot InAs/GaAs edge emitting quantum dot (QD) based laser at 1.3 μm under continuous wave operation is reported. After group delay dispersion compensation, pulse durations as short as 770 fs in a 45 GHz repetition rate device have been measured, with 1.9 W of peak power and a narrow radio frequency spectrum of only a few kHz linewidth. The experiments show evidence of an unexplored mode locking regime in the InAs/GaAs quantum dot material system, which still needs theoretical modelling and further analysis.

Generation of a narrow microwave carrier from a bimodal fiber laser

AbstractWe report the heterodyne generation of a narrow radiofrequency carrier by the beat of the signal of a bimodal fiber laser. The bimodal operation of the designed multiple‐cavities‐filter‐based laser is presented, as well as the measured narrow microwave tone, suitable for modern microwave photonics links. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:416–421, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26566

PERFORMANCE OF A QUANTUM DOT PASSIVELY MODE-LOCKED LASER UNDER OPTICAL FEEDBACK AND TEMPERATURE CONTROL

The effect of external optical feedback on a packaged monolithic quantum dot passively mode-locked laser is investigated. The radio-frequency (RF) linewidth narrows from 8 KHz in the free-running situation to a value as low as 170 Hz under relatively low feedback and temperature control. The RF linewidth characterization under resonant feedback at a multiple of the laser cavity length agrees well with the published theory. The narrow RF linewidth with external optical feedback makes the chip-scale quantum dot mode-locked laser an attractive source for ultra low noise applications such as optical clocking and optoelectronic oscillators.

How close is the all-optical transceiver for software-defined radio?

Software-defined radio (SDR) is an emerging technology that offers various advantages over conventional radio designs. The SDR approach uses a common hardware platform, programmed by software modules and firmware, to support many communications standards and operating frequencies. The technology is being promoted by the US Department of Defense to replace tens of thousands of single-protocol, single-purpose radios with a common platform. Military and public safety organisations worldwide are also considering this technology to solve interoperability problems enabling, for example, police and firefighters to easily communicate in the field. In addition, the commercial wireless industry views SDR as a means of reducing development cost and time to market. To fully realize SDR’s promise of an all-encompassing communications device capable of transmitting and receiving over a very wide frequency range requires special attention to the radio frequency (RF) transceiver. Although it is relatively easy to use conventional electronics to build SDRs operating at up to 6GHz, it is increasingly difficult to do so at higher operating frequencies. It needs a hardware platform capable of operating up to about 60GHz to take full advantage of the SDR concept. Ideally, the SDR hardware platform has a continuouslytunable source to act as a local oscillator. It should be possible to modulate this source in any format, perform signal conditioning, and distribute the signal: either by wireless transmission or over an optical network. The prospect of using conventional electronics to develop a commercially-viable source with these properties is limited, so it may be necessary to find another approach. Optical technology can emulate many of the functions normally associated with RF reception and transmission. For example, it is possible to generate microwave signals optically over a very large frequency range—from MHz to THz—using heterodyne techniques such as optical injection locking, optical phaselocked loops, and nonlinear up-conversion. The electrical RF signal is generated by shining two optical inputs, separated by the desired microwave frequency, on to a square-law photodetector that acts as a mixer and generates their beat frequency. In general, when no phase or frequency control is applied, the phase noise of the microwave signal is approximately equal to the sum of the optical linewidths. The key to reducing this typicallyMHz linewidth down to values that are more practical for a communications system is to ensure that the phases of the optical signals are highly correlated. If they are, a very narrow RF linewidth can be achieved, even from large-linewidth optical sources. A carrier is of limited use in a communication system without some form of encoded information. A laser source can be modulated by inserting a baseband optical modulator in the optical path, or by other means. The modulation formats are varied and may act on the frequency, amplitude, or phase of the signal. In general, the bandwidth of the modulator/demodulator is dictated by the format of the protocol or the radio-spectrum channel allocation and is relatively modest, since the high-frequency carrier is already optically generated. This means that digital demodulation can be done directly by the computer, after optical down-conversion, to make it easier to interface with the SDR software. Both the receiver and transmitter need RF filtering for signal conditioning. Techniques for electrical filtering in the optical domain have been examined extensively, but the all-optical microwave filters proposed to date are primarily based on incoherent manipulation of optical carriers with only positive taps. This approach can only create low-pass filters. Many applications, including SDR systems, need bandpass and flat-top filters as well. To overcome this limitation, Yao et. al. recently reported a way to build an all-optical microwave bandpass filter with a simple structure.

FORTE satellite observations of very narrow radiofrequency pulses associated with the initiation of negative cloud‐to‐ground lightning strokes

The FORTE satellite's radiofreqency receiver/recorder system has been used to study extremely narrow (∼100 ns width) radiofrequency pulses accompanying the initiation of negative cloud‐to‐ground strokes. These pulses have been observed from the ground for over two decades. The FORTE space‐based observations are substantially consistent with the prior ground‐based results, at least in regard to pulsewidth, distance‐scaled pulse amplitude, and the pulses' basic association with negative cloud‐to‐ground strokes, relative to either positive cloud‐to‐ground or intracloud strokes. New results from the FORTE observations include (1) information on the radiation pattern (versus elevation angle), (2) the tendency of the underlying fast‐pulse radiation process to occur preferentially in marine, rather than dry‐land, locations, and (3) the high degree of linear polarization of the radiated signal. These three new results were not accessible to ground‐based measurements, which do not sample elevation angles other than zero, whose signal distortion with overland propagation paths (at zero elevation angle) tends to confuse the issue of the (land versus sea) location affinity of the pulse source itself, and whose received signal is a fortiori linearly polarized because of the adjacent ground plane. The narrow radiofrequency pulse that accompanies negative cloud‐to‐ground strokes provides a useful identifier, in the satellite's radiofrequency datastream, of the occurrence of this particular kind of stroke. Additionally, the observations reported here indicate that the radiating element is a single, vertical current stalk, rather than a collection of randomly oriented, mutually incoherent dipoles, such as are believed to be responsible for most very high frequency signals from lightning.

Collision-induced Ramsey resonances in Sm vapor.

Ramsey’s method of separated fields for the observation of narrow radiofrequency (rf) resonances is well known from atomic and molecular beam experiments /l /. In this contribution, we demonstrate the occurrence of similar Ramsey resonances in an atomic vapor due to collisional velocity diffusion of sublevel coherence within an optical Doppler distribution; here the velocity diffusion of sublevel coherence between the excitation and detection processes plays a role similar to the spatial motion of the active atoms in conventional Ramsey experiments. The new phenomenon is observed using coherent resonance Raman processes to optically induce and detect Zeeman coherence in the Sm λ=570.68 nm J=1 - J’=0 transition.

Nonlinear narrow optical resonances induced by laser radiation

THE ATTAINMENT of narrow and frequency stable resonances in the absorption or emission spectra of any substance in various regions of the electromagnetic spectrum is of great importance in physics. Each discovery in this direction increases considerably the accuracy of many physical experiments and gives rise to numerous applications in different fields of science. Two classic examples can be used as illustrations. The decade 194@50 saw the development of the use of atomic and molecular beams for observing narrow resonances in the radiofrequency region. In one example of this (Ramsey ) (181 two spatially separated electromagnetic fields interacting with an atomic beam of cesium were used to measure the frequencies of the transitions between the hyperfine levels of the ground state of Cs. Resonances with a width Av = 50Hz at a frequency v0 = 9.3 GHz (Bechler et ~1.‘~~‘) were obtained. This corresponds to a relative width Av/u, = 5 x 10e9 or a quality factor Q of 2 x 108. Such narrow radiofrequency resonances have provided the basis of atomic frequency standards and for the time scale which has now been internationally adopted. Extremely narrow resonances in a higher frequency region, in nuclear transitions, have been observed by Mossbauer. (176) For example, in a y-ray transition in 67Zn with a quantum energy of 93 keV it is possible to obtain a resonance with a quality factor of 2 x 10”. The narrow resonances of recoilless nuclear transitions in a crystal lattice give at the present time the highest resolution of any physical experiments, of the order of 10-i’. Until recently the resonance width in the optical region of the spectrum has been no better than 10-6. In the last few years a number of effects have been discovered in the resonant nonlinear interaction of a light field with atgmic and molecular gases. These effects have produced a narrowing of the optical resonance width of at least 103-lo4 times. To illustrate this, Fig. 1 gives some values of the relative width for the sharpest resonances of quantum transitions in the microwave, optical and y-ray regions of the spectrum. These discoveries gave impetus to the development of nonlinear optical spectroscopy with superhigh resolution and high precision. The purpose of this paper is to present successively and from one single point of view the resonant effects which occur in the saturated gas absorption of laser radiation, which underlie the methods for obtaining narrow nonlinear resonances in the optical region. As is well known, the motion of the gas particles shifts the spectral line centre of the emission a(w) of an individual particle (atom or molecule) by a value which depends on the component of the atomic velocity u in the direction of observation n:

Physical Sciences: Fine Structure of the Jupiter Radio Bursts

SHORT pulses with a time scale of milliseconds and fractions of a millisecond have been observed in the radio emissions of Jupiter at wave frequencies near 20 MHz since 1956 (refs. 1 and 2). Little is known about these “S” bursts, however, although it has been established3 that their occurrence depends strongly on the orbital position of the satellite Io. Further, Gordon and Warwick4 were able to show from a small set of observations that the bursts may have a relatively narrow radio frequency bandwith (<50 kHz) and that their centre frequency may change rapidly with time at the rate 25–35 MHz s−1, at least near 26 MHz.