Fast Sweep Rate Research Articles

Improving the Accuracy and Resolution of Filter- and Frequency-to-Time Mapping-Based Time and Frequency Acquisition Methods by Broadening the Filter Bandwidth

In this article, a method for improving the accuracy and resolution of the filter-and frequency-to-time mapping (FTTM)-based time and frequency acquisition system in the fast sweep scenario is proposed by broadening the filter bandwidth. It is found that the width of the generated pulse via FTTM is mainly determined by the impulse response of the filter when the sweep rate is very fast. Therefore, appropriately increasing the filter bandwidth can significantly reduce the pulsewidth, so as to improve the measurement accuracy and resolution. To verify the validity, short-time Fourier transform (STFT) and microwave frequency measurement (MFM) systems employing the stimulated Brillouin scattering (SBS)-based FTTM are further investigated by inserting a frequency-sweep pump wave for SBS gain bandwidth broadening. The improvement of accuracy and frequency resolution is well confirmed by comparing the measurement results with and without SBS gain spectrum broadening. The MFM accuracy is improved by more than ten times compared with other reported works under a similar fast sweep rate, and the frequency resolution of the STFT is also much improved compared with our former results.

Magnetoresistance relaxation steps originating from dynamic spin-orbital interactions in Ca3Ru2O7

The in-plane magnetoresistance and magnetostriction of ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ single crystals were investigated under pulsed magnetic field. Field-induced resistance steps and structural change were observed for specific field directions and far below the metal-to-insulator transition temperature ${T}_{\mathrm{MI}}=48$ K, which were not observed under static magnetic field. Especially, the resistance steps were only observed in the field-descending branch, indicating the relaxation properties, and no magnetization steps were observed. These results unveil the existence of instantaneous interactions among different degrees of freedom in ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, which are observable under pulsed magnetic field thanks to its fast field sweep rate. Based on the orbital states of ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ and the Kugel-Khomskii model, we propose that the resistance steps and the structural change are related to a dynamic spin-orbital interaction, and this interaction can be strongly suppressed by the magnetocrystalline anisotropy.

Narrow linewidth swept laser source based on cascaded multi-wavelength injection of DFB lasers.

A linearly swept laser source over broadband with a fast sweep rate and narrow linewidth is realized using a novel optoelectronic scheme based on a multi-wavelengths (mutually coherent) injected distributed feedback (DFB) laser. Under the condition of multi-wavelengths injection, the injection-locking and four-wave mixing (FWM) process can occur simultaneously in the DFB laser, inducing a swept laser source with a sweep range of 100 GHz and sweep rate of 10 THz/s. Furthermore, with the phase noise character analyzation of the swept laser source, the phase noise deterioration due to the radio frequency (RF) signal is studied quantitatively. Besides the influence of the RF signal noise, the phase noise deterioration in the FWM process can be suppressed completely with the phase-locked pump beam and signal beam based on the injection-locking principle. This low phase noise swept laser source with sub-kilohertz linewidth could have wide applications in lidar.

A Novel Method of Measuring Instantaneous Frequency of an Ultrafast Frequency Modulated Continuous-Wave Laser.

Ultrafast linear frequency modulated continuous-wave (FMCW) lasers are a special category of CW lasers. The linear FMCW laser is the light source for many sensing applications, especially for light detection and ranging (LiDAR). However, systems for the generation of high quality linear FMCW light are limited and diverse in terms of technical approaches and mechanisms. Due to a lack of characterization methods for linear FMCW lasers, it is difficult to compare and judge the generation systems in the same category. We propose a novel scheme for measuring the mapping relationship between instantaneous frequency and time of a FMCW laser based on a modified coherent optical spectrum analyzer (COSA) and digital signal processing (DSP) method. Our method has the potential to measure the instantaneous frequency of a FMCW laser at an unlimited sweep rate. In this paper, we demonstrate how to use this new method to precisely measure a FMCW laser at a large fast sweep rate of 5000 THz/s by both simulation and experiments. We find experimentally that the uncertainty of this method is less than 100 kHz and can be improved further if a frequency feedback servo system is introduced to stabilize the local CW laser.

Microwave reflectometer for density profile and turbulence measurements on the COMPASS tokamak.

The fast microwave reflectometer system on the COMPASS tokamak consists of an O-mode polarized K-band (18 GHz-26 GHz), Ka-band (26 GHz-40 GHz), and a part of U-band (40 GHz-54 GHz). The plasma density profile from the edge plasma area is measured using a fast sweeping rate up of to 6 µs of the probing wave. The processing of the reflected signal is realized by the heterodyne detection configuration based on the I/Q modulator. Two different methods of dynamic calibration of the required linear sweep frequency, together with static frequency and dispersion calibration, were used. The electron density profile was reconstructed by a spectrogram-based method with four sweeps on average. The system has the capability to measure the mid-plane low-field side electron density profile in the density range from 4 × 1018 m-3 to 3.6 × 1019 m-3. Experimental results obtained on COMPASS discharges are presented to demonstrate the performance of the diagnostics.

A pulsed high magnetic field facility for electric polarization measurements

<sec>Multiferroic materials, which exhibit the coexistence of ferromagnetic, ferroelectric, or ferroelastic orders, are of particular interest for not only fundamental physics but also potential applications. An important physical property of multiferroic materials, especially those with magnetically driven ferroelectricity, is known as a strong magnetoelectric coupling between the magnetic order and the ferroelectric order. The external magnetic fields can directly interact with spins or magnetic moments of the materials and lead the spontaneous ferroelectricity to be suppressed, and in some cases result in field-induced ferroelectricity in a higher field. Depending on the exchange interactions, these ferroelectric phase transitions may take place in a critical magnetic field as high as several tens of tesla. The standard electric-polarization measurement based on a commercial PPMS system is limited by the strength of the static field consequently. As an extremely experimental condition, pulsed magnetic fields can be used to reveal new physical phenomena in multiferroic materials. Due to the short pulse duration and the effect of eddy current, this measurement technique under pulsed high magnetic fields is still a challenge to date although a few laboratories have developed it in recent years.</sec><sec>Wuhan National High Magnetic Field Center (WHMFC) of China is a newly built pulsed-field laboratory. This experimental station is equipped with the many measuring instruments such as for measuring electric transport, magnetization, electron spin resonance, magneto-optics, and high pressure, which were established after the national assessment at the end of 2014. Recently, using a pyroelectric technique we successfully constructed an electric-polarization measurement system based on the large-scaled facility at the WHMFC. The nondestructive magnet driven by discharging a 1.25 MJ capacitor bank can generate a pulsed field up to 60 T. By tuning the charging energy and voltages, the pulse duration time can be modulated from 4.3 ms to 10.8 ms. A helium-3 cryogenic system equipped on this facility can achieve a lowest temperature down to 0.5 K. A high-precision rotation probe is designed and fabricated with angle varying from –5° to 185° for an angular-dependent study. The pyroelectric current is detected by a shunt resistor of 10 kΩ and the electric polarization is derived by integrating the pyroelectric current over the time. The resulting data have a good accuracy and quality which are helpful in detecting weak ferroelectric phase transitions induced by pulsed fields with a fast field sweep rate. In this paper, we introduce this measurement system in detail including the method, principle and its advantages in comparison with those in static fields. Recent study and progress of magnetoelectric multiferroic materials under high magnetic fields are also reported.</sec>

High frequency transient-evoked otoacoustic emission measurements using chirp and click stimuli

Transient-evoked otoacoustic emissions (TEOAEs) at high frequencies are a non-invasive physiological test of basilar membrane mechanics at the basal end, and have clinical potential to detect risk of hearing loss related to outer-hair-cell dysfunction. Using stimuli with constant incident pressure across frequency, TEOAEs were measured in experiment 1 at low frequencies (0.7–8 kHz) and high frequencies (7.1–14.7 kHz) in adults with normal hearing up to 8 kHz and varying hearing levels from 9 to 16 kHz. In combination with click stimuli, chirp stimuli were used with slow, medium and fast sweep rates for which the local frequency increased or decreased with time. Chirp TEOAEs were transformed into equivalent click TEOAEs by inverse filtering out chirp stimulus phase, and analyzed similarly to click TEOAEs. To improve detection above 8 kHz, TEOAEs were measured in experiment 2 with higher-level stimuli and longer averaging times. These changes increased the TEOAE signal-to-noise ratio (SNR) by 10 dB. Slower sweep rates were investigated but the elicited TEOAEs were detected in fewer ears compared to faster rates. Data were acquired in adults and children (age 11–17 y), including children with cystic fibrosis (CF) treated with ototoxic antibiotics. Test-retest measurements revealed satisfactory repeatability of high-frequency TEOAE SNR (median of 1.3 dB) and coherence synchrony measure, despite small test-retest differences related to changes in forward and reverse transmission in the ear canal. The results suggest the potential use of such tests to screen for sensorineural hearing loss, including ototoxic loss. Experiment 2 was a feasibility study to explore TEOAE test parameters that might be used in a full-scale study to screen CF patients for risk of ototoxic hearing loss.

Magnetic flux instability in NbN films exposed to fast field sweep rates

Magneto-optical imaging of dendritic flux instability is reported for NbN films exposed to magnetic fields ramped at a fast rate (0.1–3.2 kT s−1). The results show that as the magnetic ramp rate increases, the temperature and field range of the instability extends significantly. In particular, the lower and upper threshold fields ( and respectively) that bound the field range for dendritic instability are affected. The upper field is found to increase linearly with the applied field sweep rate, a behavior which is discussed in terms of a recent theoretical work (Vestgarden et al 2016 Phys. Rev. B 73 174511). The extended instability range should be taken into account in applications in which the superconducting films are exposed to rapid changes in the magnetic field.

Dispersion-Tuned Swept Lasers for Optical Coherence Tomography

Swept-source optical coherence tomography (SS-OCT) is a powerful tool for fast medical imaging. For the real-time three-dimensional imaging, the wide tuning range over 100 nm and fast sweep rate over 100 kHz are typically required. We have proposed a new wavelength-swept laser for SS-OCT. It is based on a principle called dispersion tuning. Since the cavity contains no mechanical components, such as tunable filters, we could achieve the very high sweep rate. In this paper, we describe the principle of the dispersion-tuned swept laser and present our recent works on the applications to the SS-OCT system.

Effect of electrolyte cation on the charge storage mechanism of manganese dioxide for electrochemical capacitors

The performance of manganese dioxide (γ-MnO2) as an electrode material in electrochemical capacitors is examined in aqueous electrolytes of 0.5 M Li2SO4, Na2SO4, K2SO4 or MgSO4. Performance and mechanistic analysis is conducted using cyclic voltammetry and step potential electrochemical spectroscopy. The choice of cation affects electrode performance, and this is explored in terms of the bare and hydrated ion size, and their interaction with the γ-MnO2 surface and bulk structure. At fast sweep rates charge storage is dominated by double layer formation on the geometric (outer) electrode surface, in which case the smaller hydrated cations are best. At intermediate sweep rates charge storage occurs via double layer charge storage on the porous surface and diffusional (or pseudo-capacitive) processes, and is determined by the affinity of the cation to its hydration shell. Ions with a lower hydration shell affinity are more likely to undergo a solvation shell rearrangement or desolvation in order to access the smaller pores or undergo insertion into the bulk of the electrode. At slow sweep rates, charge storage is via diffusional capacitance. This mechanism is similarly dependent on the ability of an ion to undergo desolvation to be inserted into the bulk of the electrode.

1 μs broadband frequency sweeping reflectometry for plasma density and fluctuation profile measurements.

Frequency swept reflectometry has reached the symbolic value of 1 μs sweeping time; this performance has been made possible, thanks to an improved control of the ramp voltage driving the frequency source. In parallel, the memory depth of the acquisition system has been upgraded and can provide up to 200 000 signals during a plasma discharge. Additional improvements regarding the trigger delay determination of the acquisition and the voltage ramp linearity required by this ultra-fast technique have been set. While this diagnostic is traditionally dedicated to the plasma electron density profile measurement, such a fast sweeping rate can provide the study of fast plasma events and turbulence with unprecedented time and radial resolution from the edge to the core. Experimental results obtained on ASDEX Upgrade plasmas are presented to demonstrate the performances of the diagnostic.

Molecular magnetic hysteresis at 60 kelvin in dysprosocenium.

Lanthanides have been investigated extensively for potential applications in quantum information processing and high-density data storage at the molecular and atomic scale. Experimental achievements include reading and manipulating single nuclear spins, exploiting atomic clock transitions for robust qubits and, most recently, magnetic data storage in single atoms. Single-molecule magnets exhibit magnetic hysteresis of molecular origin-a magnetic memory effect and a prerequisite of data storage-and so far lanthanide examples have exhibited this phenomenon at the highest temperatures. However, in the nearly 25 years since the discovery of single-molecule magnets, hysteresis temperatures have increased from 4 kelvin to only about 14 kelvin using a consistent magnetic field sweep rate of about 20 oersted per second, although higher temperatures have been achieved by using very fast sweep rates (for example, 30 kelvin with 200 oersted per second). Here we report a hexa-tert-butyldysprosocenium complex-[Dy(Cpttt)2][B(C6F5)4], with Cpttt = {C5H2tBu3-1,2,4} and tBu = C(CH3)3-which exhibits magnetic hysteresis at temperatures of up to 60 kelvin at a sweep rate of 22 oersted per second. We observe a clear change in the relaxation dynamics at this temperature, which persists in magnetically diluted samples, suggesting that the origin of the hysteresis is the localized metal-ligand vibrational modes that are unique to dysprosocenium. Ab initio calculations of spin dynamics demonstrate that magnetic relaxation at high temperatures is due to local molecular vibrations. These results indicate that, with judicious molecular design, magnetic data storage in single molecules at temperatures above liquid nitrogen should be possible.

Communication—Excellent Threshold Selector Characteristics of Cu2S-Based Atomic Switch Device

Bipolar-threshold switching was demonstrated using volatile-atomic switches (VAS) for cross-point selector applications. The VAS (Cu/Cu2S/W) exhibits asymmetric current–voltage (I–V) characteristics. An abrupt conductance change occurs by the formation of a Cu filament when a positive voltage is applied to the Cu electrode. Meanwhile, nonlinear I–V characteristics are observed because of the high concentration of vacancy of Cu under a negative voltage. We thus introduce the complementary-VAS configuration for the self-compliance characteristics at both polarities. Furthermore, the increased the switching voltage at a fast sweeping rate allows the VAS to show a strong immunity to electrical disturbance.

Electron Propagation within Redox-Active Microdomains in Thin Films of Ferrocene-Containing Diblock Copolymers.

This paper reports the electrochemical behavior of redox-active microdomains in thin films of ferrocene-containing diblock copolymers, polystyrene-block-poly(2-(acryloyloxy)ethyl ferrocenecarboxylate) (PS-b-PAEFc). PS-b-PAEFc with different PAEFc volume fractions (PS154-b-PAEFc51, PS154-b-PAEFc26, and PS154-b-PAEFc12, where the subscripts represent the polymerization degree of each block; f(PAEFc) = 0.47, 0.30, and 0.17, respectively) was synthesized by sequential atom transfer radical polymerization. PS-b-PAEFc films of controlled thicknesses (20-160 nm) were prepared on gold substrates via spin-coating and characterized by ellipsometry. Microdomains were observed via atomic force microscopy on the surfaces of PS154-b-PAEFc51 and PS154-b-PAEFc26 thin films but not on the surfaces of PS154-b-PAEFc12 thin films. Electrochemical behavior of films was assessed by cyclic voltammetry and chronocoulometry in acetonitrile solution. The redox potential of ferrocene moieties was similar (ca. + 0.29 V vs Fc(+)/Fc) regardless of fPAEFc and film thickness. For PS154-b-PAEFc51 and PS154-b-PAEFc26, thicker films afforded larger faradaic peak currents and exhibited diffusion-controlled voltammograms at faster sweep rates. PS154-b-PAEFc26 produced voltammograms less influenced by solvent-induced swelling than PS154-b-PAEFc51, reflecting the improved morphological stability of PAEFc microdomains by redox-inert PS frameworks. In contrast, PS154-b-PAEFc12 films yielded similar faradaic peak currents regardless of film thickness and exhibited voltammograms indicative of surface-confined species. These observations suggest that PS154-b-PAEFc51 and PS154-b-PAEFc26 films contain continuous PAEFc microdomains extending from the electrode to the surface, in contrast to the PS154-b-PAEFc12 films which contain isolated PAEFc microdomains buried within the PS matrix. Electron propagation took place only through PAEFc microdomains that could electrically communicate with the underlying electrode. Apparent diffusion coefficients within PAEFc microdomains were similar (≈ 2 × 10(-11) cm(2)/s) for PS154-b-PAEFc51 and PS154-b-PAEFc26. The relatively low efficiency in electron propagation was attributable to ineffective electron self-exchange reaction within the PAEFc microdomains and/or limited counterion migration through the acetonitrile-swollen microdomains. These results provide guidance in design of redox-active metalloblock copolymers for various applications, which include electrocatalysis, electrochemical mediation in enzyme sensors, and redox-controlled molecular deposition.

Investigating the mechanism of stress corrosion cracking in near-neutral and high pH environments for API 5L X52 steel

Stress corrosion cracking behaviour of API-5L-X52 steel under cathodic protection in near-neutral and high pH conditions was studied using slow strain rate test method and electrochemical measurements. The slow strain rate test showed ductile and brittle fracture feature at low and high applied potentials, respectively. In order to identify the mechanism contributes in stress corrosion cracking; the electrochemical potentiodynamic polarisation test was done at fast and slow sweep rate. The results revealed that at near-neutral pH condition the anodic dissolution at crack tip was the dominant mechanism. While at high pH medium, the hydrogen based mechanism was dominant.

Interleaved optical coherence tomography

We present a novel and cost-effective technique--interleaved optical coherence tomography (iOCT)--to enhance the imaging speed of swept source OCT systems by acquiring data from multiple lateral positions simultaneously during a single wavelength sweep, using a single detector and a virtually imaged phase array (VIPA) as a multi-band demultiplexer. This technique uses spectral encoding to convert coherence length into higher imaging speed; the speed enhancement factor is independent of the source speed or center wavelength, and the effective A-scan rate scales linearly with sweep speed. The optical configuration requires only a change in the sample arm of a traditional OCT system and preserves the axial resolution and fall-off characteristic of a traditional SS-OCT using the same light source. Using 10 kHz, 20 kHz and 100 kHz sources we provide a first demonstration of image speed enhancement factors of up to 12, 6 and 10, respectively, which yield effective A-scan rates of 120 kHz, 120 kHz and 1 MHz for B-scan imaging, with a sensitivity of up to 82.5 dB. We also show that iOCT can image faster dynamics than traditional OCT B-scan imaging and is capable of 3D biological imaging. The iOCT concept suggests a new route to high-speed OCT imaging for laser developers: that is, by focusing on improving the coherence length and linewidth of existing and emerging sources. Hence, iOCT is a nice complement to ongoing research and commercial efforts to enable faster imaging through development of lasers with faster sweep rates, and offers new hope for existing sources with slow sweep rates and potential for enhancement of coherence length to compete with faster sources to achieve high-speed OCT.

Spectrally encoded confocal microscopy of esophageal tissues at 100 kHz line rate

Spectrally encoded confocal microscopy (SECM) is a reflectance confocal microscopy technology that uses a diffraction grating to illuminate different locations on the sample with distinct wavelengths. SECM can obtain line images without any beam scanning devices, which opens up the possibility of high-speed imaging with relatively simple probe optics. This feature makes SECM a promising technology for rapid endoscopic imaging of internal organs, such as the esophagus, at microscopic resolution. SECM imaging of the esophagus has been previously demonstrated at relatively low line rates (5 kHz). In this paper, we demonstrate SECM imaging of large regions of esophageal tissues at a high line imaging rate of 100 kHz. The SECM system comprises a wavelength-swept source with a fast sweep rate (100 kHz), high output power (80 mW), and a detector unit with a large bandwidth (100 MHz). The sensitivity of the 100-kHz SECM system was measured to be 60 dB and the transverse resolution was 1.6 µm. Excised swine and human esophageal tissues were imaged with the 100-kHz SECM system at a rate of 6.6 mm(2)/sec. Architectural and cellular features of esophageal tissues could be clearly visualized in the SECM images, including papillae, glands, and nuclei. These results demonstrate that large-area SECM imaging of esophageal tissues can be successfully conducted at a high line imaging rate of 100 kHz, which will enable whole-organ SECM imaging in vivo.

A flux-coupled ac/dc magnetizing device

We report on an instrument for applying ac and dc magnetic fields by capturing the flux from a rotating permanent magnet and projecting it between two adjustable pole pieces. This can be an alternative to standard electromagnets for experiments with small samples or in probe stations in which an applied magnetic field is needed locally, with advantages that include a compact form-factor, very low power requirements and dissipation as well as fast field sweep rates. This flux capture instrument (FLUXCAP) can produce fields from -400 to +400 mT, with field resolution less than 1 mT. It generates static magnetic fields as well as ramped fields, with ramping rates as high as 10 T/s. We demonstrate the use of this apparatus for studying the magnetotransport properties of spin-valve nanopillars, a nanoscale device that exhibits giant magnetoresistance.

Fast sweep-rate plastic Faraday force magnetometer with simultaneous sample temperature measurement

We present a design for a magnetometer capable of operating at temperatures down to 50 mK and magnetic fields up to 15 T with integrated sample temperature measurement. Our design is based on the concept of a Faraday force magnetometer with a load-sensing variable capacitor. A plastic body allows for fast sweep rates and sample temperature measurement, and the possibility of regulating the initial capacitance simplifies the initial bridge balancing. Under moderate gradient fields of ~1 T/m our prototype performed with a resolution better than 1 × 10(-5) emu. The magnetometer can be operated either in a dc mode, or in an oscillatory mode which allows the determination of the magnetic susceptibility. We present measurements on Dy(2)Ti(2)O(7) and Sr(3)Ru(2)O(7) as an example of its performance.

Bi-directional electrical characterisation of microbial fuel cell

The electrical performance of microbial fuel cells in steady-state is usually investigated by standard characterisation methods that reveal many important parameters e.g. maximum power. This paper introduces a novel “bi-directional” method to study how the acquisition parameters (i.e. sweep rate and sweep regime) can influence measurements and consequently performance estimations. The investigation exhibited considerable differences (hysteresis) between the forward and backward characterisation regimes, indicating a difficulty to reach steady-state under certain conditions. Moreover, it is found that fast sweep rates (time-step of 2min) can lead to an overestimation of the short-circuit currents, while prolonged operation with high external loads leads to maximum power overestimation and extended conditioning at high currents can result in its underestimation.