Sideband Oscillations Research Articles

Theoretical and experimental characterization of non-Markovian anti-parity-time systems

Open systems with anti-parity-time ({{{{{{{mathcal{APT}}}}}}}}) or {{{{{{{mathcal{PT}}}}}}}} symmetry exhibit a rich phenomenology absent in their Hermitian counterparts. To date all model systems and their diverse realizations across classical and quantum platforms have been local in time, i.e., Markovian. Here we propose a non-Markovian system with anti-{{{{{{{mathcal{PT}}}}}}}}-symmetry where a single time-delay encodes the retention of memory, and experimentally demonstrate its consequences with two time-delay coupled semiconductor lasers. A transcendental characteristic equation with infinitely many eigenvalue pairs sets our model apart. We show that a sequence of amplifying-to-decaying dominant mode transitions is induced by the time delay in our minimal model. The signatures of these transitions quantitatively match results obtained from four, coupled, nonlinear rate equations for laser dynamics, and are experimentally observed as constant-width sideband oscillations in the laser intensity profiles. Our work introduces a paradigmatic non-Hermitian system with memory, paves the way for its realization in classical systems, and may apply to time-delayed feedback-control for quantum systems.

A detailed study on Transient Stability Enhancement of Grid-Forming Inverters

The integration of the weak grids along with the clean energy that comes from the natural resources requires stable interconnection of Voltage-Source Inverters (VSIs) with weak grids. During severe voltage sags, weak grid integrated VSIs may fail synchronization with the grid.These inverters' transient stability under severe grid disruptions is weak and significantly different from that of synchronous machines. To prevent the side band oscillations in the weak grids, the emerging inverters based on grid-forming are preferred over the grid following inverters in the weak grids. These inverters synchronise with the grid using a power-based synchronisation technique in order to prevent the instability brought on by a typical Phase Locked Loop in weak AC grids. This study presents a comprehensive review of the transient stability improvement of Grid-Forming VSIs.

A 237 GHz Traveling Wave Tube for Cloud Radar

In this article, the first 237 GHz traveling wave tube (TWT) is presented as a high-power amplifier for the terahertz (THz) cloud radar. As is common with previous G-band traveling wave tubes developed at Beijing Vacuum Electronics Research Institute, the 237 GHz traveling wave tube employs a 20 kV, 50 mA pencil electron beam focused using periodic permanent magnets (PPMs) to achieve compactness. A folded waveguide (FWG) slow-wave structure (SWS) with modified circular bends is optimized to provide high impedance and eliminate sideband oscillations. Limited by insufficient drive power, this device is not saturated. The measured maximum output power and gain are 8.9 W and 35.7 dB, and the 3 dB gain bandwidth achieves 4 GHz.

Rabi spectroscopy of the clock transition in thulium atoms in a one-dimensional optical lattice

The clock transition at a wavelength of 1.14 μm in thulium atoms in an optical lattice in the Lamb – Dick regime is studied. The capture of thulium atoms makes it possible to suppress completely both the transition broadening due to the first-order Doppler effect and the shift of the transition frequency caused by the recoil effect. The excitation spectra of sideband oscillation frequencies and Rabi oscillations of population between the clock transition levels upon resonance excitation are investigated.

Extensive photometry of the intermediate polar V1033 Cas (IGR J00234+6141)

To measure the spin period of the white dwarf in V1033 Cas with high precision, we performed extensive photometry. Observations were obtained over 34 nights in 2017. The total duration of the observations was 143 h. We found that the spin period of the white dwarf is equal to 563.11633+/-0.00010 s. Using this period, we derived the oscillation ephemeris with a long validity of 100 years. The spin oscillation semi-amplitude was stable and was equal to 95.5+/-1.3 mmag. This is a very large semi-amplitude of the spin oscillation among intermediate polars, which have similar and lesser spin periods. This large semi-amplitude suggests that the system is noticeably inclined. The spin pulse profile was sinusoidal with high accuracy. This may mean that the spin oscillation is produced by a single accretion curtain whereas the second accretion curtain may be obscured by the accretion disc. Despite the large amount of our observations, we did not detect sidebands. The semi-amplitudes of the undetected sideband oscillations do not exceed 10 mmag. The absence of sideband oscillations seems puzzling. We detected the orbital variability of V1033 Cas with a period of 4.0243+/-0.0028 h and with a semi-amplitude of 55+/-4 mmag. The orbital variability semi-amplitude seems large and also suggests that the system is noticeably inclined. Using our oscillation ephemeris and the times of spin pulse maximum obtained in the past, we found that the spin period is very stable. dP/dt is most probably less than -4 X 10^(-12). This contradicts the assumption that the white dwarf in V1033 Cas is not spinning at equilibrium. Our spin period and our oscillation ephemeris can be used for further investigations of the stability of the spin period in V1033 Cas.

Extensive photometry of the intermediate polar V2069 Cyg (RX J2123.7+4217)

To obtain the spin period of the white dwarf in the intermediate polar V2069 Cyg with high precision, we fulfilled extensive photometry. Observations were performed within 32 nights, which have a total duration of 119 hours and cover 15 months. We found the spin period of the white dwarf, which is equal to 743.40650+/-0.00048 s. Using our precise spin period, we derived the oscillation ephemeris with a long validity of 36 years. This ephemeris and the precise spin period can be used for future investigations of spin period changes of the white dwarf in V2069 Cyg. In addition, for the first time we detected the sideband oscillation with a period of 764.5125+/-0.0049 s. The spin and sideband oscillations revealed unstable amplitudes both in a time-scale of days and in a time-scale of years. On average, the semi-amplitude of the spin oscillation varied from 17 mmag in 2014 to 25 mmag in 2015. The semi-amplitude of the sideband oscillation varied from 12 mmag in 2014 to an undetectable level of less than 7 mmag in 2015. In a time-scale of years, the optical spin pulse profile revealed essential changes from an asymmetric double-peaked shape in 2014 to a quasi-sinusoidal shape in 2015. Such drastic changes of the optical spin pulse profile seem untypical of most intermediate polars and, therefore, are of great interest. The pulse profile of the sideband oscillation was quasi-sinusoidal. Moreover, we note that V2069 Cyg possesses strong flickering with a peak-to-peak amplitude of 0.4-0.6 mag.

Dual-parallel Mach-Zehnder modulator based microwave photonic down-conversion link with high dynamic range

With the rapid development of the microwave photonic communication technology, the frequency of the microwave signal is expanded to the Ka waveband, since most of low frequency bands are occupied. However, the current commercial detectors and signal processing modules are limited by bandwidth. Therefore, the traditional method of directly detecting the microwave signals cannot meet the actual demands. It is essential to achieve the microwave photonic down-conversion from the high frequency microwave signal (~10 GHz) to the lower frequency signal (~100 MHz). Meanwhile, the down-conversion low frequency signal can be processed by the existing mature technology and low cost devices. The microwave down-conversion link can effectively avoid leaking the local oscillator, and it possesses many advantages such as high bandwidth and spurious free dynamic range, low loss and low noise. In this paper, a microwave photonic down-conversion system is presented based on the integrated dual-parallel Mach-Zehnder modulator (DPMZM) to increase the spurious-free dynamic range as well as conversion efficient of microwave photonic link. The integrated DPMZM is mainly comprised of two intensity modulators (MZM-a and MZM-b), and a phase shifter. The radio frequency (RF) signal is loaded into DPMZM to modulate the optical signal. The local oscillator is loaded into the MZM-a to produce the 1st local oscillator sideband, and two RF signals are fed to the MZM-b to form the 1st RF signal sideband. The direct current bias of the DPMZM is adjusted to output a high carrier suppressed double sideband (DSB) signal. The erbium-doped fiber amplifier is used to increase the power of light to match the power range of the detector. The RF signal sideband and local oscillator sideband are mixed to produce the beat frequency, and the frequency down-conversion can be achieved. The principle of frequency down-conversion is elaborated by theoretical analysis. The conversion efficiency and spurious free dynamic range are analyzed and simulated. On this basis, the microwave photonic link of frequency down-conversion is built. The performance of the system is tested. The ratio of optical carrier power to sideband power of the DSB signal is 26 dB. The experimental result shows that the conversion efficiency is 7.43 dB and spurious-free dynamic range is 110.85 dB/Hz2/3. The down-conversion method based on the DPMZM can optimize the output spectrum of the sideband. The structure of system is simple and easy to implement, so it is a good option for improving the conversion efficiency and spurious-free dynamic range.

Extensive photometry of the intermediate polar MU Cam: detection of a spin period change

Intermediate polars with known rates of spin period changes are not numerous because such tasks require measurements performed for a long time. To measure a spin period change, MU Cam is a good candidate because it has a spin oscillation with a large amplitude enabling measurements with high precision. Fortunately, in the past the spin period of MU Cam was measured with high precision. To measure the spin period anew, in 2014-2015 we performed extensive photometric observations of MU Cam, spanning a total duration of 208 h within 46 nights. We found that the spin, sideband and orbital periods are equal to 1187.16245+/-0.00047 s, 1276.3424+/-0.0022 s and 4.71942+/-0.00016 h, respectively. Comparing the measured spin period with the spin period of MU Cam in the past, we detected the spin period change with dP/dt=-(2.17+/-0.10) X 10^(-10). This rate of the spin period change was not stable and varied in a time scale of years. During four nights in 2014 April-May MU Cam was fainter than usual by 0.8 mag, and the amplitude of the sideband oscillation was five times larger, denoting significant fraction of disc-overflow accretion. The sideband oscillation showed a double-peaked pulse profile in the normal brightness state. When the star brightness was decreased by 0.8 mag, the sideband oscillation showed a single-peaked pulse profile. In contrast, the spin pulse, which was quasi-sinusoidal, remained remarkably stable both in profile and in amplitude. Moreover, the spin pulse was also remarkably stable in a time scale of years and even decades. MU Cam is of great interest because it represents a distinctive object with a large and unstable rate of the spin period change and exhibits a distinctive behaviour of the pulse profiles.

The soft-photon approximation in infrared-laser-assisted atomic ionization by extreme-ultraviolet attosecond-pulse trains

We use the soft-photon approximation, formulated for finite pulses, to investigate the effects of the dressing pulse duration and intensity on simulated attosecond pump–probe experiments employing trains of attosecond extreme-ultraviolet pulses in conjunction with an IR probe pulse. We illustrate the validity of the approximation by comparing the modelled photoelectron distributions for the helium atom, in the photon energy region close to the N = 2 threshold, to the results from the direct solution of the time-dependent Schrödinger equation for two active electrons. Even in the presence of autoionizing states, the model accurately reproduces most of the background features of the ab initio photoelectron spectrum in the 1s channel. A splitting of the photoelectron harmonic signal along the polarization axis, in particular, is attributed to the finite duration of the probe pulse. Furthermore, we study the dependence of the sideband integrated signal on the pump–probe time delay for increasing IR field strengths. Starting at IR intensities of the order of ∼ 1 TW cm−2, overtones in the sideband oscillations due to the exchange of three or more IR photons start to appear. We derive an analytical expression in the frequency-comb limit of the soft-photon model for the amplitude of all the sideband frequency components and show that these amplitudes oscillate as a function of the intensity of the IR field. In particular, we predict that the amplitude of the fundamental component with frequency 2ωIR, on which the rabitt optical reconstruction technique is based, changes sign periodically.

Electromagnetic effects on geodesic acoustic and beta-induced Alfvén eigenmodes

The local kinetic theory of geodesic acoustic modes and beta-induced Alfvén eigenmodes is developed. The local dispersion relations are derived in two opposite limits: and , where k0 = (m − nq)/qR, m and n are poloidal and toroidal mode numbers, and is the electron thermal velocity. It is shown that the nature of the (m ± 1, n) sideband oscillations depends on the radial modes width. The localized modes are mostly electrostatic, while the meso-scale modes of the radial width larger than c/(ωpiq) have a strong electromagnetic component. It is shown that the dispersion relations are remarkably similar provided the radial mode width of the principal (m, n) harmonic is sufficiently small.

Design of Broadband Highly Linear IQ Modulator Using a 0.5$\mu$m E/D-PHEMT Process for Millimeter-Wave Applications

A broadband highly linear IQ modulator using a 0.5-mum enhancement/depletion-pseudomorphic high-electron mobility transistor process is presented in this letter. An innovative broadside/edge coupler is proposed to apply to the IQ modulator. The chip size is only 1times1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , including radio frequency and baseband PADs. The sideband and local oscillation suppressions of the modulator are better than -33 and -15 dBc, respectively. At a carrier frequency of 60 GHz with a 64 quadrature amplitude modulation (QAM) modulation, the modulator demonstrates an error vector magnitude of within 3%, and an adjacent channel power ratio of better than -40 dBc. To the best of the authors' knowledge, this work demonstrates the best modulation quality with a 64 QAM modulation up to 60 GHz among all the reported reflection-type IQ modulators.

Dynamical evolution of multiple four-wave-mixing processes in an optical fiber

We present unique results of detailed experimental and theoretical investigations of the dynamical evolution of four-wave-mixing spectra in an optical fiber. The experimental measurements probe the evolution of sidebands generated through four-wave mixing as they copropagate with the pumps along the fiber. We find that standard theoretical models are inadequate to predict the experimental results and that it is necessary to modify the approach to modeling the dynamics in two ways. The first modification is to include a pump laser input with multiple longitudinal modes. This reflects the fact that the pump laser fields may actually have internal structure that is not resolved by the spectrometer used and is very small compared to the spacing of the central frequencies of the pump fields. Yet the evolution of the fields is dramatically altered for the sidebands generated by nonlinear processes in the fiber medium. The second is the inclusion of phase noise added along the propagation length; this causes damping of the sideband oscillations. These two modifications lead to excellent agreement of the measurements with numerical predictions of the sideband evolution.

Influence of sideband oscillations on gyrotron efficiency

We report the observation of sideband mode effects on the efficiency of overmoded gyrotron operation. Two cavities were designed and operated which differed in the presence of sideband modes. In one version of the cavity, parasitic backward waves modes were observed and efficiencies were approximately 22% at 40 A beam current. With the use of a multi-mode self-consistent nonlinear code, a modified design was generated which eliminated the sideband modes. Experiments were conducted with this new cavity which produced efficiencies of approximately 33% at 30 A and 27% at 40 A beam current, but with a slightly higher velocity ratio than seen with the earlier cavity. An additional cavity, also with no sideband modes but with a longer cavity length and therefore higher Q obtained powers up to 1.3 MW with an efficiency of 39% at a 40 A beam current. >

Selective reflection in the cholesteric and blue phases of a chiral-racemic mixture of CE6

Abstract The reflection spectrum for visible light is examined for the cholesteric and blue phases of chiral CE6. Pronounced side band oscillations are observed. The Bragg wavelength for total reflection diverges towards the smectic phase with an exponent v = 0·71±0·05. Going from the cholesteric phase to BPI, the lattice parameter increases by (2)1/2. Evidence is given for the existence of a long-lived supercooled blue phase (BPS).

Sideband damping of water waves over a soft bed

The Benjamin &amp; Feir sideband instability of gravity waves over a rigid bed is extended here to the case of wave propagation over a compliant bed. The bed is assumed elastic with inhomogeneous properties, following some vertical stratification profile. The elastic stiffness at the mudline is assumed very small, as is typically the case for upper-stratum gel-like marine mud, so that its effect is assumed comparable in magnitude with that of wave nonlinearity. It is then shown that through the sideband instability, the carrier gravity wave would lose energy to sideband oscillations. These sidebands will be shown to be contaminated with small-amplitude, but very short elastic shear waves. These shear waves will interact with the viscous boundary layer at the bed-water interface, and thus significantly enhance the viscous damping of wave energy in the boundary layer.

Regular and chaotic dynamics of optically pumped molecular lasers.

We present a detailed analysis of the dynamical behavior of an optically pumped molecular laser. This study combines bifurcation and power spectral analysis with numerical investigation of the global features of attractor topology under control parameter variation. The special role of the pump laser in generating physically distinct periodic and chaotic dynamics is emphasized through the complementary use of laser gain and dispersion characteristics. Our main results are the following: (i) instabilities associated with the physically distinct mechanisms of relaxation and pump-induced Rabi sideband oscillations are readily generated; (ii) the topological characteristics of both periodic and chaotic attractors reflect these physically distinct mechanisms, making it possible to discriminate between both types of behavior in heterodyne power spectra; and (iii) the ratio of deenergization to dipole-dephasing rates is central to determining the operating characteristics of the laser.

Convenient and precise method of measuring both stability and homogeneity of magnetic fields

This paper presents a method which can be used to measure both stability and homogeneity of magnetic fields in terms of nuclear side-band oscillation (NSO). By simply monitoring the changes of the NSO frequency ωm, one can calculate the changes of the magnetic field according to the simple formula Δωm=γΔH. The basic principle of NSO and the conditions for stable NSO are elucidated. The measuring accuracy depends on the removal of relaxation oscillation in NSO. The mechanism of relaxation oscillation in NSO is analyzed in detail. The results of measuring the homogeneity distribution of a laboratory magnet are given as well.

Computer Analysis of Microwave and Millimeter-Wave Mixers (Computer Program Descriptions)

The program analyzes the performance of single-diode microwave and millimeter-wave mixers. A Schottky-barrier diode is assumed, whose I-V and CV characteristics are known. The diode mount is taken to be lossless, but may have external loads at any number of sideband and local oscillator (LO) harmonic frequencies.

Theory of Large-Amplitude Oscillations in a plasma

The damping rate of a large-amplitude electron wave in one dimensional plasma is derived by using the first order perturbation. As the unperturbed solution, the stationary state of Bernstein, Greene and Kruskal's theory is employed. It is shown that the damping rate is proportional to the depth of trapping trough and also there exists an upper limit of the depth for stable sideband oscillations.

Conversion of A-60 NMR spectrometers to Fourier transform operation

A description of the instrumental modifications and additions required for the conversion of this class of field-swept, field-modulated 60-MHz NMR spectrometers is given. The rf portions are straightforward adaptations of the single-coil sample circuit to pulsed operation. The standard instrument achieves field-frequency stabilization through use of a NMR sideband oscillator employing a separate sample. While such a configuration is adequate for the normal mode of operation, it does not provide sufficient absolute stability for the pulsed Fourier mode of operation. The requisite stability was achieved by phase locking the sideband oscillation frequency to an external reference by forcing the radio frequency to change in accordance with the phase-lock error signal. Both analytical and control channels are driven by this common rf source, as is the reference signal to the analytical channel synchronous detector.