Frequency Sweep Rate Research Articles

Optimal design for spectral narrowing and fast frequency sweep of an interferometer-stabilized laser

Self-heterodyne fiber interferometers have been shown to be capable of stabilizing lasers to ultra-narrow linewidths and present an excellent alternative to high-finesse cavities for frequency stabilization. In addition to suppressing frequency noise, these devices are highly tunable and can be manipulated to produce high-speed frequency sweeps over the entire range of the laser. We present an analytic approach for choosing a delay-line length for both optimal noise suppression and highest in-loop frequency sweep rate. Based on this model, we stabilize an extended cavity diode laser to a fiber Michelson interferometer and demonstrate a linewidth of 700 Hz over millisecond timescales while also allowing for a frequency scan rate of 1 THz/s. We independently measure the maximum deviation from linearity of the sweep to be only 100 kHz.

Circuit model based predistortion technique for linear frequency modulation of voltage controlled oscillator

A voltage controlled oscillator (VCO) is a key component in a frequency modulated continuous wave reflectometer. The linear frequency modulation (LFM) of the VCO has been implemented by predistorting the tuning voltage waveform or by feedback control of the output frequency based on a phase locked loop. The latter has a better linearity but the frequency sweep rate is limited. The former has a relatively poor linearity but has the capability of a fast frequency sweep. In this paper, the predistortion technique is sophisticated to improve the linearity in applications of very fast LFM. The voltage waveform predistorted based on the static characteristics of VCO does not eliminate the nonlinearity completely because VCO’s behavior gets deviated from the static characteristics as the frequency sweep rate increases. The dynamic characteristics of the VCO are obtained by modeling the equivalent circuit inside the VCO as a low pass filter and simulating the circuit response numerically. The calculated voltage waveform is driven by an arbitrary waveform generator, and the output frequency of the VCO is experimentally measured to demonstrate the accuracy of the LFM. The techniques for frequency measurement and frequency linearization are described in detail and the results are presented.

Geometric distortions in FMCW SAR images due to inaccurate knowledge of electronic radar parameters: analysis and correction by means of corner reflectors

In the last years the Frequency Modulated Continuous Wave (FMCW) technology has been playing an ever greater role in the realization of compact, light and cheap Synthetic Aperture Radar (SAR) systems to be mounted onboard small, low altitude platforms such as airplanes, helicopters and drones.To correctly focus FMCW SAR images, it is necessary to accurately know some system parameters, including the frequency sweep rate of the signal transmitted by the radar. It may happen, however, that this frequency sweep rate is not very accurately measured by the radar provider, and thus an incorrect value of this parameter is used during the SAR data focusing procedure. This may produce serious geometric distortion effects in the focused FMCW SAR images. To circumvent these problems, in this work we present a procedure that estimates the frequency sweep rate actually employed by the FMCW radar, thus providing a key information that can be then profitably used to achieve the correct focusing of the SAR data acquired by the radar system at hand. More specifically, we propose an algorithm that exploits on one side the focused SAR images corrupted by the geometric distortion effects induced by the inaccurate knowledge of this radar parameter, and on the other side the very precise in-situ measurements of the positions of a limited number of Corner Reflectors (CRs) properly deployed over the observed scene.The effectiveness of the proposed algorithm has been tested on real data acquired by an airborne X-band FMCW SAR system.

Evaluation of Plasma Properties From Chorus Waves Observed at the Generation Region

AbstractIn this study we present an inversion method which provides thermal plasma population parameters from characteristics of chorus emissions only. Our ultimate goal is to apply this method to ground‐based data in order to derive the lower‐energy boundary condition for many radiation belt models. The first step is to test the chorus inversion method on in situ data of the Van Allen Probes in the generation region. The density and thermal velocity of energetic electrons (few kiloelectron volts to 100 keV) are derived from frequency sweep rate and starting frequencies of chorus emissions through analysis of wave data from the Electric and Magnetic Field Instrument Suite and Integrated Science on board the Van Allen Probes. The nonlinear wave growth theory of Omura and Nunn (2011, https://doi.org/10.1029/2010JA016280) serves as the basis for our inversion method, assuming that the triggering wave is originated by the linear cyclotron instability. We present 16 consecutive rising‐tone emissions recorded in the generation region between 11 and 12 UT on 14 November 2012. The results of the inversion are compared with density and thermal velocities (parallel and perpendicular) of energetic electrons derived from the unidirectional flux data of the Helium, Oxygen, Proton, and Electron instrument, showing a good agreement: The normalized root‐mean‐square deviation between the measured and predicted values are less than ∼15%. We found that the theoretical amplitudes are consistent with the measured ones. The relation between linear and nonlinear wave growth agrees with our basic assumption; namely, linear growth is a preceding process of nonlinear wave growth. We analyze electron distributions at the relativistic resonant energy ranges.

Transient evolution of optical magnetic resonance in rubidium vapor.

We observe the transient evolution of the laser pumped magnetic resonance in a paraffin-coated rubidium vapor cell and analyze the phenomena numerically by using the four-level density matrix. With the increased radio frequency (RF) sweep rate, the traditional Lorentz signal turns to an asymmetric shape at low sweep rate and starts to oscillate at a high sweep rate. The transient oscillation's features, including frequency and the decay time, are studied by suddenly turning on the RF field to the resonance Larmor precession frequency under the different RF field and light field parameters. The experimental result reveals the transient signals' strong dependence on the RF field power and light power. Especially, the transient oscillation frequency primarily depends on the RF field's power and whatever power the laser light is. When the laser power is lower, the transient oscillation frequency is proportional to the RF field's amplitude. These transient evolution signals are also confirmed with the numerical calculations based on the density-matrix equation of motion.

Series expansion for shear stress in large‐amplitude oscillatory shear flow from oldroyd 8‐constant framework

AbstractWhen polymeric liquids undergo large‐amplitude oscillatory shear flow, the shear stress responds as a Fourier series, the higher harmonics of which are caused by the fluid nonlinearity, and the first harmonic of which is a nonlinear function of both the frequency and the shear rate amplitude. The Oldroyd 8‐constant framework for continuum constitutive theory contains a rich diversity of popular special cases for polymeric liquids. The shear stress response for the Oldroyd 8‐constant framework has recently yielded to exact analytical solution. However, in its closed form, Bessel functions appear 24 times, each within infinite summations. In this paper, to simplify the exact solution, we expand it in a Taylor series. We truncate the series after its 16th power of the shear rate amplitude. Our main result reduces to the well known expression for the corotational Jeffreys and corotational Maxwell fluids. Whereas these special cases yielded to the Goddard integral expansion (GIE), the more general Oldroyd 8‐constant framework does not. We use Ewoldt grids to show our main result to be highly accurate, for the corotational Jeffreys and corotational Maxwell fluids. For these two special cases, our solutions agree closely with the exact solutions so long as . We apply our main result for the special case of the Johnson‐Segalman fluid to describe the measured frequency sweep and shear rate amplitude sweep responses of molten atactic polystyrene. For this, we use the Spriggs relations to generalize our main result to multimode, which then agrees closely with the measured responses.

Dynamics of frequency-swept nuclear spin optical pumping in powdered diamond at low magnetic fields

A broad effort is underway to improve the sensitivity of NMR through the use of dynamic nuclear polarization. Nitrogen vacancy (NV) centers in diamond offer an appealing platform because these paramagnetic defects can be optically polarized efficiently at room temperature. However, work thus far has been mainly limited to single crystals, because most polarization transfer protocols are sensitive to misalignment between the NV and magnetic field axes. Here we study the spin dynamics of NV-13C pairs in the simultaneous presence of optical excitation and microwave frequency sweeps at low magnetic fields. We show that a subtle interplay between illumination intensity, frequency sweep rate, and hyperfine coupling strength leads to efficient, sweep-direction-dependent 13C spin polarization over a broad range of orientations of the magnetic field. In particular, our results strongly suggest that finely tuned, moderately coupled nuclear spins are key to the hyperpolarization process, which makes this mechanism distinct from other known dynamic polarization channels. These findings pave the route to applications where powders are intrinsically advantageous, including the hyperpolarization of target fluids in contact with the diamond surface or the use of hyperpolarized particles as contrast agents for in vivo imaging.

Experiment for identifying free span of buried gas pipeline under internal excitation load

Free span is a great danger and an unavoidable problem for buried gas pipeline. In this paper, a method for identifying buried gas pipeline free span based on vibration signal analysis under internal excitation load is experimentally studied. Collected signals are pre-processed by sliding average method to reduce noise and smooth the vibration curve. Two evaluation indicators, frequency response function and response signal energy, are established to evaluate free span detection results. A free span detecting apparatus is developed, some experiments were carried out and their detection results are analyzed to verify the reliability of the two indicators and feasibility of the method. The experimental results show that the free span can be identified by two indicators and the proposed method is feasible. Some parameters, such as excitation force, frequency sweep rate etc., have a serious influence on free span detection results.

A method for an accurate estimation of natural frequencies using swept-sine acoustic excitation

Abstract Early detection of cracks in engineering structures by vibration-based methods requires monitoring systems that can evaluate natural frequencies with high accuracy. This paper introduces a method for estimating the natural frequencies using features extracted from the vibrational response of structures excited to cross through the resonance. To this aim, an open-loop acoustic system able to ensure bidirectional excitation, in swept-sine or short-time mode, with controllable parameters was developed. Analyzing the structural response at the crossing area through the resonance, two particularities on which the method is based have been revealed. The first refers to the symmetry of the frequency shift at peak amplitude from the natural frequency, if positive and negative sweep rates are applied. Therefore, the natural frequency can be found as the average of the frequency values at the resonance. Also, the necessary time to achieve the peak amplitude after passing through the natural frequency is similar, irrespective to the sign of the sweep rate. This accomplishment allows finding the natural frequency from a graphical representation. Frequency estimation made for a test structure by employing the two procedures lead to convergent results. Comparing these results to those achieved by impulsive excitation, a better precision was remarked at the swept-sine excitation. With regard the simplicity, rapidity and stability of the method, the swept-sine excitation has been proved as the most effective.

Analytical Method for Stroboscopically Sampling General Periodic Functions With Arbitrary Frequency Sweep Rates

Parameter sweeps are commonly used to explore the behavior of dynamical systems. This paper derives exact solutions for the instances in time to stroboscopically sample the response of a dynamical system subject to varying input excitations. This work will enable more accurate bifurcation diagrams and Poincaré sections in parameter regimes where numerical approaches may lead to incorrect behavior characterization. The simplest case of a linear frequency sweep is first considered before generalizing the results to include more complex functions with nonlinear sweep rates and arbitrary phase shifts.

On the conditions for nonlinear growth in magnetospheric chorus and triggered emissions

The nonlinear whistler mode instability associated with magnetospheric chorus and VLF triggered emissions continues to be poorly understood. Following up on formulations of other authors, an analytical exploration of the stability of the phenomenon from a new vantage point is given. This exploration derives an additional requirement on the anisotropy of the energetic electron distribution relative to the linear treatment of the instability, and shows that the nonlinear instability is most favorable to increasing growth rate when electrons become initially trapped in the wave potential of a constant frequency wave. These results imply that the initiation of the nonlinear instability at the equator requires a positive frequency sweep rate, while the initiation of the instability by a constant frequency triggering wave must occur at a location downstream of the geomagnetic equator.

Multiscale mapping of frequency sweep rate in mouse auditory cortex

Functional organization is a key feature of the neocortex that often guides studies of sensory processing, development, and plasticity. Tonotopy, which arises from the transduction properties of the cochlea, is the most widely studied organizational feature in auditory cortex; however, in order to process complex sounds, cortical regions are likely specialized for higher order features. Here, motivated by the prevalence of frequency modulations in mouse ultrasonic vocalizations and aided by the use of a multiscale imaging approach, we uncover a functional organization across the extent of auditory cortex for the rate of frequency modulated (FM) sweeps. In particular, using two-photon Ca2+ imaging of layer 2/3 neurons, we identify a tone-insensitive region at the border of AI and AAF. This central sweep region behaves fundamentally differently from nearby neurons in AI and AII, responding preferentially to fast FM sweeps but not to tones or bandlimited noise. Together these findings define a second dimension of organization in the mouse auditory cortex for sweep rate complementary to that of tone frequency.

Observational evidence of the nonlinear wave growth theory of plasmaspheric hiss

AbstractWe test the recently developed nonlinear wave growth theory of plasmaspheric hiss against discrete rising tone elements of hiss emissions observed by the Van Allen Probes. From the phase variation of the waveforms processed by bandpass filters, we calculate the instantaneous frequencies and wave amplitudes. We obtain the theoretical relation between the wave amplitude and frequency sweep rates at the observation point by applying the convective growth rates and dispersion factors to the known relation at the equator. By plotting the theoretical relation over scatterplots of the wave amplitudes and the frequency sweep rates for rising tone elements, we find good agreement between the hiss observations and the nonlinear theory. We also find that the duration periods of the hiss elements are in good agreement with the nonlinear transition time necessary for the formation of a resonant current through coherent nonlinear wave‐particle interactions.

Cluster observations of non–time continuous magnetosonic waves

AbstractEquatorial magnetosonic waves are normally observed as temporally continuous sets of emissions lasting from minutes to hours. Recent observations, however, have shown that this is not always the case. Using Cluster data, this study identifies two distinct forms of these non–temporally continuous emissions. The first, referred to as rising tone emissions, are characterized by the systematic onset of wave activity at increasing proton gyroharmonic frequencies. Sets of harmonic emissions (emission elements) are observed to occur periodically in the region ±10° off the geomagnetic equator. The sweep rate of these emissions maximizes at the geomagnetic equator. In addition, the ellipticity and propagation direction also change systematically as Cluster crosses the geomagnetic equator. It is shown that the observed frequency sweep rate is unlikely to result from the sideband instability related to nonlinear trapping of suprathermal protons in the wave field. The second form of emissions is characterized by the simultaneous onset of activity across a range of harmonic frequencies. These waves are observed at irregular intervals. Their occurrence correlates with changes in the spacecraft potential, a measurement that is used as a proxy for electron density. Thus, these waves appear to be trapped within regions of localized enhancement of the electron density.

A distributed optical fiber sensing system for synchronous vibration and loss measurement

We propose a fully distributed fusion system combining phase-sensitive optical time-domain reflectometry (Φ-OTDR) and OTDR for synchronous vibration and loss measurement by setting an ingenious frequency sweep rate (FSR) of the optical source. The relationships between FSR, probe pulse width and repeat period are given to balance the amplitude fluctuation of OTDR traces, the dead zone probability and the measurable frequency range of vibration events. In the experiment, we achieve synchronous vibration and loss measurement with FSR of 40 MHz/s, the proble pulse width of 100 ns and repeat rate of 0.4 ms. The fluctuation of OTDR trace is less than 0.45 dB when the signal-to-noise ratio (SNR) is over 12 dB for a captured vibration event located at 9.1 km. The proposed method can be used for not only detection but also early warning of damage events in optical communication networks.

Observational evidence of generation mechanisms for very oblique lower band chorus using THEMIS waveform data

AbstractChorus waves are intense coherent whistler mode waves with frequency chirping which play a dual role in both loss and acceleration of radiation belt electrons in the Earth's magnetosphere. Although the generation of parallel chorus waves has been extensively studied by means of theory, simulations, and observations, the generation mechanism of very oblique chorus waves still remains a mystery. In this study, we have analyzed hundreds of very oblique discrete (rising or falling tone) lower band chorus events collected from 7 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) waveform data to investigate their potential generation mechanisms. Comparisons between wave normal angles directly measured onboard THEMIS in the dawn‐day sector at L = 5–9 and inferred from theoretical models on the basis of measured wave characteristics (frequency sweep rate, mean frequency, and amplitude) show that these very oblique waves are more commonly generated through cyclotron resonance with anisotropic electron streams. However, a second generation mechanism via Landau resonance with low‐energy electron beams seems to be also operating on the nightside at L < 6.7 and at all local times at L > 8.5. Moreover, very oblique lower band chorus waves with large frequency chirping rates or small magnetic field amplitudes are more likely excited via cyclotron resonance, while waves with small frequency chirping rates or large magnetic field amplitudes are preferentially generated through Landau resonance. This comprehensive statistical study provides interesting insight into the possible generation mechanisms of very oblique lower band chorus waves in the Earth's magnetosphere.

Implementation and Evaluation of Online System Identification of Electromechanical Systems Using Adaptive Filters

Online system identification method provides useful insight into the electromechanical plant behavior during commissioning as well as health monitoring. In addition to controller tuning, the methods can be applied for identifying mechanical modes or resonances, sensor vibrations, and unmodeled load dynamics. Online system identification using frequency response analysis is considered in this study. The focus of this paper will be on estimating the real-time frequency response of various systems using adaptive filters. For comparative evaluation, a commercial off-the-shelf spectrum analyzer is used. The essential aspects to be addressed for adaptive filter are: understanding the influence of convergence gain on accuracy of frequency response and selecting the frequency sweep rate. In this work, online system identification using frequency response is demonstrated on two setups: 1) magnetic levitation system and 2) permanent-magnet motor drive operating in sensorless mode. The experimental result validates that the method elaborated is simple, easy to implement, and accurate when compared with spectrum analyzer.

Enhancement and quenching of high-intensity focused ultrasound cavitation activity via short frequency sweep gaps

This letter reports on the use of frequency sweeps to probe acoustic cavitation activity generated by high-intensity focused ultrasound (HIFU). Unprecedented enhancement and quenching of HIFU cavitation activity were observed when short frequency sweep gaps were applied in negative and positive directions, respectively. It was revealed that irrespective of the frequency gap, it is the direction and frequency sweep rate that govern the cavitation activity. These effects are related to the response of bubbles generated by the starting frequency to the direction of the frequency sweep, and the influence of the sweep rate on growth and coalescence of bubbles, which in turn affects the active bubble population. These findings are relevant for the use of HIFU in chemical and therapeutic applications, where greater control of cavitation bubble population is critical.

Orthogonal test design to optimize the acoustic vibration method for pear texture measurement

The test parameters for pear texture measurement using a laser Doppler vibrometer (LDV) were optimized. The pear was placed in the middle of the vibration stage and excited by swept sine wave signals with a constant frequency sweeping rate and acceleration amplitude. The response signals from the tops of the pears were measured using an LDV. First, 10 pear samples were tested under different test conditions in the single-factor experiment. The test parameters included the frequency sweeping mode and rate, acceleration amplitude, pear laying style, and LDV detection point. The results show that the frequency–response curves at the same detection point have good repeatability under all tested frequency sweeping rates (regardless of the linear or logarithmic frequency sweeping mode) and acceleration amplitudes. However, the frequency–response curves at different detection points have poor repeatability for each laying style. To find a better combination of test parameters, an L27(313) orthogonal test probing the frequency sweeping rate, acceleration amplitude, pear laying style, and their 2-way interactions was designed according to the results of the single-factor experiments. No significant 2-way interaction is found among the frequency sweeping rate, acceleration amplitude, and pear laying style on the repeatability of the frequency–response curves. The test parameter values required to obtain a better repeatability are as follows: frequency sweeping rate of 23.33Hz/s with the linear frequency sweeping mode, acceleration amplitude of 1g (g=9.8m/s2), and laying style that the pear is placed with its stem upward. After the optimization of the LDV method, a total of 118 pear samples were used to compare the LDV method with the destructive puncture test. The results show that the elasticity index (EI) is well correlated with the stiffness (Stif) of pears and that the correlation is not significantly affected by the pear laying style. In addition, the LDV method is superior to the puncture test in terms of repeatability and sensitivity.

Very oblique whistler generation by low‐energy electron streams

AbstractWhistler mode chorus waves are present throughout the Earth's outer radiation belt as well as at larger distances from our planet. While the generation mechanisms of parallel lower band chorus waves and oblique upper band chorus waves have been identified and checked in various instances, the statistically significant presence in recent satellite observations of very oblique lower band chorus waves near the resonance cone angle remains to be explained. Here we discuss two possible generation mechanisms for such waves. The first one is based on Landau resonance with sporadic very low energy (<4 keV) electron beams either injected from the plasma sheet or produced in situ. The second one relies on cyclotron resonance with low‐energy electron streams, such that their velocity distribution possesses both a significant temperature anisotropy above 3–4 keV and a plateau or heavy tail in parallel velocities at lower energies encompassing simultaneous Landau resonance with the same waves. The corresponding frequency and wave normal angle distributions of the generated very oblique lower band chorus waves, as well as their frequency sweep rate, are evaluated analytically and compared with satellite observations, showing a reasonable agreement.