Symmetric Sidebands Research Articles

The Closed-Loop Sideband Harmonic Suppression for CHB Inverter With Unbalanced Operation

For <i>N</i>-cells cascaded H-bridge (CHB) inverters based on unipolar double frequency phase-shifted pulsewidth modulation (PS-PWM), the output voltage is distorted and its harmonics are difficult to be suppressed under unbalanced conditions (different dc voltages and&#x002F;or modulation indices). This article proposes a closed-loop harmonic suppression scheme based on the PS-PWM, and the scheme is easily implemented in the <i>dq</i> frame to adaptively adjust the displacement angles of the triangular carriers. The harmonic content can be suppressed to its possible minimum value without the limitation of the operational point by the proposed method. The adjustment rules of displacement angles and the harmonic suppression principle are presented in detail. This method can effectively restrain harmonic magnitudes without the limitation of the cells number and does not require complicated trigonometric function calculations or optimization algorithms. Moreover, the principle of symmetrical sideband harmonic suppression is also discussed. The effectiveness of the proposed method is verified by a downscale test platform of the single-phase CHB inverter.

Asymmetric Millimeter-Wave Spectrum in Laser Mode Partition Noise Generated by Fiber Transmission

In this article, we report on the asymmetry of intensity noise spectrum at millimeter-wave (mm-wave) heterodyne carriers. Passively mode-locked laser diode (PMLLD) is employed for a low phase noise mm-wave photonic generation using direct detection. This optical source produces an excess intensity noise known by laser mode partition noise (LMPN) in which its spectrum usually appears as symmetric double side-bands around the beat-note. The propagation over a dispersive medium such as optical fiber can modify the spectral behavior and profile of the LMPN. This article explores the impact of fiber transmission on the distortions of the LMPN in optical and electrical spectrum. Experimental results reveal, for the first time, that the LMPN symmetry is converted to an asymmetric deformation on the generated beating carrier. The mathematical model and numerical simulation are presented to give hints for analyzing this phenomenon. The findings of the simulations and measurements demonstrate the main role of second- and third-order fiber dispersion on the LMPN profile.

Experimental Observation of Modulational Instability in Crossing Surface Gravity Wavetrains

The coupled nonlinear Schrödinger equation (CNLSE) is a wave envelope evolution equation applicable to two crossing, narrow-banded wave systems. Modulational instability (MI), a feature of the nonlinear Schrödinger wave equation, is characterized (to first order) by an exponential growth of sideband components and the formation of distinct wave pulses, often containing extreme waves. Linear stability analysis of the CNLSE shows the effect of crossing angle, θ , on MI, and reveals instabilities between 0 ∘ &lt; θ &lt; 35 ∘ , 46 ∘ &lt; θ &lt; 143 ∘ , and 145 ∘ &lt; θ &lt; 180 ∘ . Herein, the modulational stability of crossing wavetrains seeded with symmetrical sidebands is determined experimentally from tests in a circular wave basin. Experiments were carried out at 12 crossing angles between 0 ∘ ≤ θ ≤ 88 ∘ , and strong unidirectional sideband growth was observed. This growth reduced significantly at angles beyond θ ≈ 20 ∘ , reaching complete stability at θ = 30–40 ∘ . We find satisfactory agreement between numerical predictions (using a time-marching CNLSE solver) and experimental measurements for all crossing angles.

Full-duplex radio-over-fiber system with tunable millimeter-wave signal generation and wavelength reuse for upstream signal.

A full-duplex radio-over-fiber system is proposed, which provides both the generation of a millimeter-wave (mm-wave) signal with tunable frequency multiplication factors (FMFs) and wavelength reuse for uplink data. A dual-driving Mach-Zehnder modulator and a phase modulator are cascaded to form an optical frequency comb. An acousto-optic tunable filter based on a uniform fiber Bragg grating (FBG-AOTF) is employed to select three target optical sidebands. Two symmetrical sidebands are chosen to generate mm waves with tunable FMFs up to 16, which can be adjusted by changing the frequency of the applied acoustic wave. The optical carrier is reused at the base station for uplink connection. FBG-AOTFs driven by two acoustic wave signals are experimentally fabricated and further applied in the proposed scheme. Results of the research indicate that the 2-Gbit/s data can be successfully transmitted over a 25-km single-mode fiber for bidirectional full-duplex channels with power penalty of less than 2.6 dB. The feasibility of the proposed scheme is verified by detailed simulations and partial experiments.

Nanotube mode locked, wavelength-tunable, conventional and dissipative solitons fiber laser

We report the generation of widely wavelength tunable conventional solitons (CSs) and dissipative solitons (DSs) in an erbium-doped fiber laser passively mode-locked by nanotube saturable absorber. The tuning ranges of CSs and DSs are ∼15 and ∼25 nm, respectively. In anomalous dispersion regime, the output CS exhibits symmetrical spectral sidebands with transform-limited pulse duration of ∼1.1 ps. In the contrastive case of normal dispersion regime, the DS has rectangular spectrum profile and large frequency chirp, which presents pulse duration of ∼13.5 ps, and can be compressed to ∼0.4 ps external to the cavity. This fiber laser can provide two distinct types of tunable soliton sources, which is attractive for practical applications in telecommunications.

Switchable linear-cavity nanotube-mode-locking fiber laser emitting picosecond or femtosecond pulses

A switchable linear-cavity nanotube-mode-locking fiber laser is proposed by exploiting fiber Bragg grating for the first time to the author׳s best knowledge. The proposed all-fiber laser can deliver either picosecond or femtosecond pulses by controlling the polarization controllers. The durations of picosecond and femtosecond pulses are 16.4ps and 838fs with the central wavelengths of 1529.5 and 1560nm, respectively. The femtosecond pulse has symmetrical spectrum sidebands, but the picosecond pulse almost has no spectral sidebands. Our work provides a simple, low-cost, dual-scale pulse source for practical applications.

Observation of symmetrical reflection sidebands in a silica suspended-core fiber Bragg grating

We have observed symmetrical sidebands in reflection from Bragg grating written in a silica suspended-core fiber, which are caused by longitudinal periodic refractive index modulation in the Ge-doped suspended-core fiber with a core diameter of approximately 1.3 microm. Our simulation shows that the effective refractive index of the guided mode varied by 0.023% along the fiber with a period of approximately 650 microm. The periodic index variation can lead to amplitude modulation of fiber Bragg gratings, which can be studied by observing the spectra of a fiber Bragg grating written in the Ge-doped core. In addition, we have also characterized the temperature and strain responses of the fiber Bragg gratings, and showed that both responses in the suspended-core fiber are 20 to 25% lower than that of a fiber Bragg grating written on a conventional fiber.

Atomic-ensemble-based quantum memory for sideband modulations

Interaction of a control and a signal field with an ensemble of three-level atoms allows direct mapping of the quantum state of the signal field into long-lived coherences of an atomic ground state. For a vapour of caesium atoms, using electromagnetically induced transparency (EIT) and Zeeman coherences, we compare the case where a tunable single sideband is stored independently of the other one to the case where the two symmetrical sidebands are stored using the same transparency window. We study the conditions in which simultaneous storage of two non-commuting variables carried by light and subsequent read-out is possible. We show that excess noise associated with spontaneous emission and spin relaxation is small, and we evaluate the quantum performance of our memory by measuring the signal transfer coefficient T and the conditional variance V and using the T–V criterion introduced by Hétet et al (2008 Phys. Rev. A 77), Roch et al (1997 Phys. Rev. Lett. 78) and Ralph and Lam (1998 Phys. Rev. Lett. 81) as a state-independent benchmark.

Bandpass Sampling of One RF Signal Over Multiple RF Signals With Contiguous Spectrums

In this letter, we consider a case in which only one radio frequency (RF) signal is of interest over multiple RF signals with contiguous spectrums (i.e., these RF signals are adjacent to each other). The minimum valid bandpass sampling range for a desired RF signal over multiple ones is derived under the assumption of contiguous spectrums. The sampling frequency confined by the derived minimum valid sampling range is significantly reduced compared to the case in which all the RF signals are desired. The minimum valid bandpass sampling frequency can be further reduced when the desired RF signal has symmetric double sidebands.

Reversible Quantum Interface for Tunable Single-Sideband Modulation

Using electromagnetically induced transparency in a cesium vapor, we demonstrate experimentally that the quantum state of a light beam can be mapped into the long-lived Zeeman coherences of an atomic ground state. Two noncommuting variables carried by light are simultaneously stored and subsequently read out, with no noise added. We compare the case where a tunable single sideband is stored independently of the other one to the case where the two symmetrical sidebands are stored using the same electromagnetically induced transparency window.

Optical characterization of a vertical cavity surface emitting laser for a coherent population trapping frequency reference

Characterization of a vertical cavity surface emitting laser source for a frequency reference that uses coherent population trapping with atomic rubidium is reported. The frequency reference requires a low-noise laser that operates in a single transverse mode at 794.7nm. We show that biasing the laser at low output power and modulating it at higher frequency than its low relaxation oscillation frequency are necessary to obtain symmetric modulation sidebands and improve the long-term performance of the frequency reference.

Sidebands to the lower kilohertz quasi-periodic oscillation in 4U 1636-53

In this paper we report on further observations of the third and fourth kilohertz quasi-periodic oscillations (QPOs) in the power spectrum of the low-mass X-ray binary (LMXB) 4U 1636-53. These kilohertz QPOs are sidebands to the lower kilohertz QPO. The upper sideband has a frequency 55.5 +/- 1.7Hz larger than that of the contemporaneously measured lower kilohertz QPO. Such a sideband has now been measured at a significance >6sigma in the power spectra of three neutron-star LMXBs (4U 1636-53, 1728-34 and 1608-52). We also confirm the presence of a sideband at a frequency ~55Hz less than the frequency of the lower kilohertz QPO. The lower sideband is detected at a 3.5sigma level only when the lower kilohertz QPO frequency is between 800 and 850Hz. In that frequency interval, the sidebands are consistent with being symmetric around the lower kilohertz QPO frequency. The upper limit to the rms amplitude of the lower sideband is significantly lower than that of the upper sideband for lower kilohertz QPO frequencies >850Hz. Symmetric sidebands are unique to 4U 1636-53. This might be explained by the fact that lower kilohertz QPO frequencies as high as 800-850Hz are rare for 4U 1728-34 and 1608-52. Finally, we also measured a low-frequency QPO at a frequency of ~43Hz when the lower kilohertz QPO frequency is between 700 and 850Hz. A similar low-frequency QPO is present in the power spectra of the other two systems for which a sideband has been observed. We briefly discuss the possibility that the sideband is caused by Lense-Thirring precession.

Gain without inversion at two symmetrical sidebands of resonance in cold free87Rb atoms: an experimental proposal

We propose an experiment for obtaining about 0.4% inversionless gain at the sidebands of resonance in cold free 87Rb atoms. This gain is of the same order of magnitude as the one already observed near resonance (Kitching and Hollberg 1999 Phys. Rev. A 59 4685).

Large X-Ray Flares from LMC X-4: Discovery of Millihertz Quasi-periodic Oscillations and Quasi-periodic Oscillation–Modulated Pulsations

We report the discovery of millihertz (mHz) quasi-periodic oscillations (QPOs) and QPO-modulated pulsations during large X-ray flares from the high-mass X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing Explorer. The light curves of flares show that, in addition to ~74 mHz coherent pulsations, there exist two more time-varying temporal structures at frequencies of ~0.65-1.35 and ~2-20 mHz. These relatively long term structures appear in the power density spectra as mHz QPOs and also as well-developed sidebands around the coherent pulse frequency, indicating that the amplitudes of the coherent pulsation are modulated by those of the mHz QPOs. One interesting feature is that, while the first flare shows symmetric sidebands around the coherent pulse frequency, the second flare shows significant excess emission in the lower frequency sidebands due to the ~2-20 mHz QPOs. We discuss the origin of the QPOs using a combination of the beat-frequency model and a modified version of the Keplerian-frequency model. According to our discussion, it seems possible to attribute the origin of the ~0.65-1.35 and ~2-20 mHz QPOs to the beating between the rotational frequency of the neutron star and the Keplerian frequency of large accreting clumps near the corotation radius and to the orbital motion of clumps at Keplerian radii of (2-10) × 109 cm, respectively.

Low sideband guided-mode resonant filter

A guided-mode resonant filter with low sideband reflections is proposed. It is shown that, for serious reduction of out-of band reflectance, the combination of waveguide-grating filter design with conventional antireflective stack design methods is not adequate. To achieve symmetrical low sideband reflectances, independent control of various layer thicknesses is necessary. At a given illumination angle with appropriate control of the waveguide thickness, a specific resonant grating filter is designed whose out-of-band reflectance on both sides of the resonant peak is well below 10(-4). Even 50 nm away from the peak, on both sides, the out-of-band reflectance remains below 10(-3). Analysis of the variation in the main manufacturing parameters indicates that such filters can be reliably produced with present-day technologies.

Symmetric sideband correlations and the random phase screen approximation

It is shown that the validity of the random phase screen approximation in coherent wave scattering from random media can be determined from the correlation between symmetric sidebands in the scattered field. Analysis of doublet correlations in microwave reflectometer data taken on the Tokamak Fusion Test Reactor confirms the validity of the model and allows for the numerical back propagation of measurements without the use of imaging optics.

Symmetric sideband correlations and the random phase screen approximation

Abstract It is shown that the validity of the random phase screen approximation in coherent wave scattering from random media can be determined from the correlation between symmetric sidebands in the scattered field. Analysis of doublet correlations in microwave reflectometer data taken on the Tokamak Fusion Test Reactor confirms the validity of the model and allows for the numerical back propagation of measurements without the use of imaging optics.

Organization of response areas in ferret primary auditory cortex.

1. We studied the topographic organization of the response areas obtained from single- and multiunit recordings along the isofrequency planes of the primary auditory cortex in the barbiturate-anesthetized ferret. 2. Using a two-tone stimulus, we determined the excitatory and inhibitory portions of the response areas and then parameterized them in terms of an asymmetry index. The index measures the balance of excitatory and inhibitory influences around the best frequency (BF). 3. The sensitivity of responses to the direction of a frequency-modulated (FM) tone was tested and found to correlate strongly with the asymmetry index of the response areas. Specifically, cells with strong inhibition from frequencies above the BF preferred upward sweeps, and those from frequencies below the BF preferred downward sweeps. 4. Responses to spectrally shaped noise were also consistent with the asymmetry of the response areas. For instance, cells that were strongly inhibited by frequencies higher than the BF responded best to stimuli that contained least spectral energy above the BF, i.e., stimuli with the opposite asymmetry. 5. Columnar organization of the response area types was demonstrated in 66 single units from 16 penetrations. Consistent with this finding, it was also shown that response area asymmetry measured from recordings of a cluster of cells corresponded closely with those measured from its single-unit constituents. Thus, in a local region, most cells exhibited similar response area types and other response features, e.g., FM directional sensitivity. 6. The distribution of the asymmetry index values along the isofrequency planes revealed systematic changes in the symmetry of the response areas. At the center, response areas with narrow and symmetric inhibitory sidebands predominated. These gave way to asymmetric inhibition, with high-frequency inhibition (relative to the BF) becoming more effective caudally and low-frequency inhibition more effective rostrally. These response types tended to cluster along repeated bands that paralleled the tonotopic axis. 7. Response features that correlated with the response area types were also mapped along the isofrequency planes. Thus, in four animals, a map of FM directional sensitivity was shown to be superimposed on the response area map. Similarly, it was demonstrated in six animals that the spectral gradient of the most effective noise stimulus varied systematically along the isofrequency planes. 8. One functional implication of the response area organization is that cortical responses encode the locally averaged gradient of the acoustic spectrum by their differential distribution along the isofrequency planes. This enhances the representation of such features as the symmetry of spectral peaks and edges and the spectral envelope.(ABSTRACT TRUNCATED AT 400 WORDS)

A mechanism responsible for the observation of symmetric lower hybrid sidebands and a low frequency mode in the upper ionosphere

A low‐frequency (∼500 Hz) mode in conjunction with symmetric sidebands, displaced by approximately 500 Hz off the carrier frequencies of 11.90 and 12.65 kHz, has been observed in experiments conducted with the Aureol 3 satellite at altitudes of 1200 km above the Komsomolsk‐on‐Amur Alpha Station (50°N, 135°E) (Sotnikov et al., 1991 and references therein). These observations are examined and interpreted with an alternative mechanism based on the theory of Lee and Kuo [1984]. The proposed mechanism suggests that the injected VLF wave parametrically excites a purely growing mode and both the Stokes and anti‐Stokes sidebands of the lower hybrid waves. The Doppler‐ shifted frequencies of these modes resulting from the satellite motion adequately account for both the sidebands and the low‐frequency mode observed in the Aureol 3 experiments.

Analytic study of the sideband instability in free electron laser oscillators

The steady-state distribution of a bunched electron beam in a resonator with a large-amplitude carrier signal is obtained self-consistently and for arbitrary injected distribution. It is then employed to analyze the stability of operation for free electron laser (FEL) oscillators fed by a continuous electron beam. The dielectric response to small perturbations (sidebands) includes the nonuniformity from the aperiodic beam bunching and the cross-coupling among symmetric upper and lower sidebands caused by the unharmonic (nonlinear) nature of the trapped particle trajectories. The dominant contribution comes from resonances of the sideband frequency detuning δ=‖ωs−ω0‖ with harmonics of the Doppler-upshifted synchrotron (bounce) frequency ωb, δ≂2γ2znωb. The instability is triggered by the gradients, both across, ∂F/∂J, and around, ∂F/∂θ, the bunched beam distribution F(J,θ), where the action J parametrizes the distance from, and θ, the angle around, the center of the trapped particle island. When the wiggler length becomes much longer than the synchrotron period, the contribution from the nonuniformity around θ phase mixes to zero; one then recovers the results for a FEL amplifier [Phys. Fluids 31, 1708 (1988)]. It is also shown that when the nonlinear shift of the carrier frequency is considerable, upper and lower sidebands decouple, because symmetric sideband frequencies correspond to asymmetric wave numbers.