Amplitude-modulated Pulses Research Articles

Radiation generation from an array of magnetized anharmonic carbon nanotubes

An investigation into second-harmonic generation from magnetized anharmonic carbon nanotubes (CNTs) mounted on a silica substrate is conducted using a Gaussian laser pulse with modulated amplitude. An intense amplitude-modulated laser pulse incident on the array of CNTs displaces their electrons generating a restoration force. This restoration force is the nonlinear function of displacement of electrons, which ensures the anharmonic behavior of CNTs. Using the paraxial ray approximation, the nonlinear interaction of the incident pulse with CNTs is expressed in terms of a wave equation derived using the perturbative technique. The nonlinear current at second-harmonic frequency arises due to the perturbation of the electron density of CNTs by the ponderomotive force exerted on them by an incident pulse. Numerical outcomes validate the enhanced efficiency of the generated harmonic when considering the phenomenon of self-focusing. With the substantial optical nonlinearities of an anharmonic CNT structure, the plasmon resonance is broadened and high efficiency in generating harmonics is attained. It is seen that the augmenting of the amplitude-modulated parameter of the pulse and the external magnetic field strength enhances system nonlinearity, resulting in increased amplitude of generated second harmonics. Additionally, the effect of the heating rate of CNTs on the efficiency of the generated harmonic is also discussed.

Psychoacoustic and electroencephalographic responses to changes in amplitude modulation depth and frequency in relation to speech recognition in cochlear implantees

Temporal envelope modulations (TEMs) are one of the most important features that cochlear implant (CI) users rely on to understand speech. Electroencephalographic assessment of TEM encoding could help clinicians to predict speech recognition more objectively, even in patients unable to provide active feedback. The acoustic change complex (ACC) and the auditory steady-state response (ASSR) evoked by low-frequency amplitude-modulated pulse trains can be used to assess TEM encoding with electrical stimulation of individual CI electrodes. In this study, we focused on amplitude modulation detection (AMD) and amplitude modulation frequency discrimination (AMFD) with stimulation of a basal versus an apical electrode. In twelve adult CI users, we (a) assessed behavioral AMFD thresholds and (b) recorded cortical auditory evoked potentials (CAEPs), AMD-ACC, AMFD-ACC, and ASSR in a combined 3-stimulus paradigm. We found that the electrophysiological responses were significantly higher for apical than for basal stimulation. Peak amplitudes of AMFD-ACC were small and (therefore) did not correlate with speech-in-noise recognition. We found significant correlations between speech-in-noise recognition and (a) behavioral AMFD thresholds and (b) AMD-ACC peak amplitudes. AMD and AMFD hold potential to develop a clinically applicable tool for assessing TEM encoding to predict speech recognition in CI users.

Transient eddy current analysis for a spiral gradient pulse

Eddy currents are induced in the metallic structures of MRI machines due to the rapid switching of gradient fields generated by gradient coils. Several undesirable effects are associated with the induced eddy currents such as heat, acoustic noise, and MR image distortions. Accurate transient eddy currents numerical computations are required to predict and ameliorate such effects. Spiral gradient waveforms are of importance, particularly for fast MRI acquisition applications. For mathematical convenience, previously published work is mostly concerned with transient eddy currents computations associated with trapezoidal gradient waveforms; where spiral gradient waveforms were not considered. We recently presented preliminary transient eddy currents computations induced by an amplitude-modulated sinusoidal pulse in the scanner’s cryostat. In this work, we present a full computational framework for transient eddy currents induced by a spiral gradient waveform. A mathematical model for transient eddy currents involving the spiral pulse was derived and presented in detail using the circuit equation. Computations were implemented using a tailored multilayer integral method (TMIM) and results were compared to Ansys eddy currents analysis for cross-validation. The transient response of resultant fields generated by both an unshielded transverse coil driven by a spiral waveform was computed showing high agreement between Ansys and TMIM; albeit with high computational efficiency concerning time and memory. For further validation, computations for a shielded transverse coil were performed showing how eddy currents effects are reduced.

Pitch discrimination in electric hearing with inconsistent and consistent amplitude-modulation and inter-pulse rate cues.

Users of cochlear implants (CIs) struggle in situations that require selective hearing to focus on a target source while ignoring other sources. One major reason for that is the limited access to timing cues such as temporal pitch or interaural time differences (ITDs). Various approaches to improve timing-cue sensitivity while maintaining speech understanding have been proposed, among them inserting extra pulses with short inter-pulse intervals (SIPIs) into amplitude-modulated (AM) high-rate pulse trains. Indeed, SIPI rates matching the naturally occurring AM rates improve pitch discrimination. For ITD, however, low SIPI rates are required, potentially mismatching the naturally occurring AM rates and thus creating unknown pitch effects. In this study, we investigated the perceptual contribution of AM and SIPI rate to pitch discrimination in five CI listeners and with two AM depths (0.1 and 0.5). Our results show that the SIPI-rate cue generally dominated the percept for both consistent and inconsistent cues. When tested with inconsistent cues, also the AM rate contributed, however, at the large AM depth only. These findings have implications when aiming at jointly improving temporal-pitch and ITD sensitivity in a future mixed-rate stimulation approach.

Sensitivity to Pulse Rate and Amplitude Modulation in an Animal Model of the Auditory Brainstem Implant (ABI).

The auditory brainstem implant (ABI) is an auditory neuroprosthesis that provides hearing by electrically stimulating the cochlear nucleus (CN) of the brainstem. Our previous study (McInturff et al., 2022) showed that single-pulse stimulation of the dorsal (D)CN subdivision with low levels of current evokes responses that have early latencies, different than the late response patterns observed from stimulation of the ventral (V)CN. How these differing responses encode more complex stimuli, such as pulse trains and amplitude modulated (AM) pulses, has not been explored. Here, we compare responses to pulse train stimulation of the DCN and VCN, and show that VCN responses, measured in the inferior colliculus (IC), have less adaption, higher synchrony, and higher cross-correlation. However, with high-level DCN stimulation, responses become like those to VCN stimulation, supporting our earlier hypothesis that current spreads from electrodes on the DCN to excite neurons located in the VCN. To AM pulses, stimulation of the VCN elicits responses with larger vector strengths and gain values especially in the high-CF portion of the IC. Additional analysis using neural measures of modulation thresholds indicate that these measures are lowest for VCN. Human ABI users with low modulation thresholds, who score best on comprehension tests, may thus have electrode arrays that stimulate the VCN. Overall, the results show that the VCN has superior response characteristics and suggest that it should be the preferred target for ABI electrode arrays in humans.

Using Interleaved Stimulation and EEG to Measure Temporal Smoothing and Growth of the Sustained Neural Response to Cochlear-Implant Stimulation.

Two EEG experiments measured the sustained neural response to amplitude-modulated (AM) high-rate pulse trains presented to a single cochlear-implant (CI) electrode. Stimuli consisted of two interleaved pulse trains with AM rates F1 and F2 close to 80 and 120Hz respectively, and where F2 = 1.5F1. Following Carlyon et al. (J Assoc Res Otolaryngol, 2021), we assume that such stimuli can produce a neural distortion response (NDR) at F0 = F2-F1 Hz if temporal dependencies ("smoothing") in the auditory system are followed by one or more neural nonlinearities. In experiment 1, the rate of each pulse train was 480 pps and the gap between pulses in the F1 and F2 pulse trains ranged from 0 to 984µs. The NDR had a roughly constant amplitude for gaps between 0 and about 200-400µs, and decreased for longer gaps. We argue that this result is consistent with a temporal dependency, such as facilitation, operating at the level of the auditory nerve and/or with co-incidence detection by cochlear-nucleus neurons. Experiment 2 first measured the NDR for stimuli at each listener's most comfortable level ("MCL") and for F0 = 37, 40, and 43Hz. This revealed a group delay of about 42ms, consistent with a thalamic/cortical source. We then showed that the NDR grew steeply with stimulus amplitude and, for most listeners, decreased by more than 12dB between MCL and 75% of the listener's dynamic range. We argue that the NDR is a potentially useful objective estimate of MCL.

ELECTROMAGNETIC METHODS OF SEARCHING AND DEFINITION
 OF HYDROCARBON DEPOSITS

The article studies the characteristics of an anisotropic medium over hydrocarbons with the complex use
 of electromagnetic geo-exploration methods. The simulation of the components of dielectric permittivity tensors
 in the mode of amplitude-modulated, frequency-modulated, amplitude-frequency-modulated and radio pulse signals
 is carried out. The frequencies of the electron plasma and electron cyclotron resonances for the specified
 regimes are established. A study was made of the influence of modes of probing signals on the characteristics
 of an anisotropic medium above deposits and the components of the dielectric tensor. The effect of particle concentration
 variation on the real components of the components of the dielectric permittivity of an anisotropic
 medium above the UVZ is analyzed. Recommendations are given for improving the methods of electrical exploration
 and their application for prospecting geophysics.

High-order singular value decomposition of contrast pulsing sequences for improved clutter suppression

Contrast-enhanced ultrasound is a diagnostic tool used to visualize blood flow in the cardiovascular system. The use of ultrasound contrast agent (microbubbles) in combination with contrast pulsing scheme (CPS) improves the sensitivity and specificity of ultrasound flow imaging by enhancing the signal in the blood compartment. The commonly used CPS are pulse inversion (PI), amplitude modulation (AM), and amplitude-modulated pulse inversion (AMPI). Using differences in phase or amplitude of multiple pulses, the linear tissue clutter signal can be suppressed. However, this process can be degraded by motion or non-linear propagation of the ultrasound wave. These effects cause the cancellation of linear clutter signal to be ineffective We propose using higher-order singular value decomposition (HOSVD) with spatial, temporal, and pulsing dimensions as the input to improve clutter suppression under the conditions of motion and non-linear propagation. We performed systematic in-vitro- experiment emulating these conditions, as well as in-vivo cardiac measurements. The results showed that HOSVD increases the clutter suppression of all the 3 CPS compared to the conventional linear processing. The improvement of clutter reduction could be beneficial to various cardiac evaluation like myocardial perfusion or intra ventricular flow assessment.

ELECTROMAGNETIC METHODS OF SEARCHING AND DEFINITION OF HYDROCARBON DEPOSITS

The article studies the characteristics of an anisotropic medium over hydrocarbons with the complex use of electromagnetic geo-exploration methods. The simulation of the components of dielectric permittivity tensors in the mode of amplitude-modulated, frequency-modulated, amplitude-frequency-modulated and radio pulse signals is carried out. The frequencies of the electron plasma and electron cyclotron resonances for the specified regimes are established. A study was made of the influence of modes of probing signals on the characteristics of an anisotropic medium above deposits and the components of the dielectric tensor. The effect of particle concentration variation on the real components of the components of the dielectric permittivity of an anisotropic medium above the UVZ is analyzed. Recommendations are given for improving the methods of electrical exploration and their application for prospecting geophysics.

Amplification of an Amplitude Modulated Electromagnetic Pulse Through Nonlinear Self-Compression

An electromagnetic short laser pulse is considered as amplitude modulated in time. To study the advancement of amplitude-modulated short laser pulses in a plasma, we preformed numerical modelling using time-dependent generalized nonlinear Schrödinger wave equation. This analysis is limited to one-dimensional geometry, as the transverse profile is considered as uniform. Paraxial like approach is used to derive dimensionless nonlinear pulse-compression parameter equation. The effect of amplitude modulation on nonlinear compression is studied. A significant enhancement in laser intensity is observed due to the amplitude modulation of the electromagnetic pulse. We observe about 50% increase in laser intensity as compared to the unmodulated laser pulse. Our results may be useful in the clean and affordable energy goal of sustainable development goals.

Vestibular Co-stimulation in Adults with a Cochlear Implant.

Background : Vestibular co-stimulation is a side effect of cochlear implant stimulation. The electrical currents delivered by the cochlear implant can spread toward the vestibular system and thus stimulate it. The aim of the study is to evaluate whether it is feasible to functionally restore the balance by modifying the vestibular co-stimulation.Methods: Four adult patients, who had received a commercially available cochlear implant previously, were enrolled. Counterbalanced biphasic pulses were presented as bursts or as an amplitude-modulated biphasic pulse train (modulation frequencies ranging from 1 to 500 Hz) at the participant’s upper comfortable level for electrical stimulation. Subjective sensations and vestibular-mediated eye movements were used for evaluating the possible effects of vestibular co-stimulation.Results: One participant experienced a cyclic tilting of his head in response to an amplitude-modulated biphasic pulse train with a modulation frequency of 2 and 400 Hz. However, during a follow-up visit, the sensation could not be replicated.Conclusion: Subjective vestibular sensations or vestibular-mediated eye movements could not be electrically evoked with a commercially available cochlear implant in 4 adult patients with almost normal vestibular function. Therefore, customized design of the hard-, firm-, and/or software of the commercially available cochlear implant might be necessary in order to electrically restore vestibular performance.

Modular multilevel TMS device with wide output range and ultrabrief pulse capability for sound reduction

Objective. This article presents a novel transcranial magnetic stimulation (TMS) pulse generator with a wide range of pulse shape, amplitude, and width. Approach. Based on a modular multilevel TMS (MM-TMS) topology we had proposed previously, we realized the first such device operating at full TMS energy levels. It consists of ten cascaded H-bridge modules, each implemented with insulated-gate bipolar transistors, enabling both novel high-amplitude ultrabrief pulses as well as pulses with conventional amplitude and duration. The MM-TMS device can output pulses including up to 21 voltage levels with a step size of up to 1100 V, allowing relatively flexible generation of various pulse waveforms and sequences. The circuit further allows charging the energy storage capacitor on each of the ten cascaded modules with a conventional TMS power supply. Main results. The MM-TMS device can output peak coil voltages and currents of 11 kV and 10 kA, respectively, enabling suprathreshold ultrabrief pulses (>8.25 μs active electric field phase). Further, the MM-TMS device can generate a wide range of near-rectangular monophasic and biphasic pulses, as well as more complex staircase-approximated sinusoidal, polyphasic, and amplitude-modulated pulses. At matched estimated stimulation strength, briefer pulses emit less sound, which could enable quieter TMS. Finally, the MM-TMS device can instantaneously increase or decrease the amplitude from one pulse to the next in discrete steps by adding or removing modules in series, which enables rapid pulse sequences and paired-pulse protocols with variable pulse shapes and amplitudes. Significance. The MM-TMS device allows unprecedented control of the pulse characteristics which could enable novel protocols and quieter pulses.

Fully integrated 3-bit all-optical analog to digital converter based on photonic crystal semiconductor optical amplifier

Integrating a photonic crystal based semiconductor optical amplifier (PhC-SOA) composed of InP/InGaAsP/InP heterostructure with eight PhC based channel drop filters (CDFs) based on an InGaAsP platform, we have designed a fully integrated all-optical 3-bit analog to digital converter (AO-ADC). A probe signal after passing through a 9-µm long PhC-SOA in the presence of an amplitude-modulated Gaussian pulse of 1.422 ps width and maximum energy of 37.2 fJ exhibited a 4.8-nm wavelength shift. The eight appropriately designed PhC-CDFs have coded the chirped signal into eight quantization levels. The numerical results show that the proposed structure can convert analog photonic signals into 3-bit digital output with no missing code at 10Gs/s. The proposed AO-ADC structure with a small footprint of 455 μm2, along with its ultra-low power consumption, can pave the way for the further development of the next-generation integrated photonic chips.

Amplitude-modulated, Cosine PE and WP Pulses: Theory and Applicability

The amplitude-modulated, cosine powerexponential (PE) and windowed-power (WP) pulses are discussed, by insisting on their time-domain normalization. Illustrative examples of signatures and their correspondent frequency-domain behavior are given. These examples compellingly demonstrate the possibility to replace non-causal pulses of prevalent use by causal, or even time-windowed, pulses with closely resembling signatures.

Ultrafast amplitude modulation for molecular and hemodynamic ultrasound imaging.

Ultrasound is playing an emerging role in molecular and cellular imaging thanks to new micro- and nanoscale contrast agents and reporter genes. Acoustic methods for the selective in vivo detection of these imaging agents are needed to maximize their impact in biology and medicine. Existing ultrasound pulse sequences use the nonlinearity in contrast agents' response to acoustic pressure to distinguish them from mostly linear tissue scattering. However, such pulse sequences typically scan the sample using focused transmissions, resulting in a limited frame rate and restricted field of view. Meanwhile, existing wide-field scanning techniques based on plane wave transmissions suffer from limited sensitivity or nonlinear artifacts. To overcome these limitations, we introduce an ultrafast nonlinear imaging modality combining amplitude-modulated pulses, multiplane wave transmissions, and selective coherent compounding. This technique achieves contrast imaging sensitivity comparable to much slower gold-standard amplitude modulation sequences and enables the acquisition of larger and deeper fields of view, while providing a much faster imaging framerate of 3.2 kHz. Additionally, it enables simultaneous nonlinear and linear image formation and allows concurrent monitoring of phenomena accessible only at ultrafast framerates, such as blood volume variations. We demonstrate the performance of this ultrafast amplitude modulation technique by imaging gas vesicles, an emerging class of genetically encodable biomolecular contrast agents, in several in vitro and in vivo contexts. These demonstrations include the rapid discrimination of moving contrast agents and the real-time monitoring of phagolysosomal function in the mouse liver.

Using Interleaved Stimulation to Measure the Size and Selectivity of the Sustained Phase-Locked Neural Response to Cochlear Implant Stimulation

We measured the sustained neural response to electrical stimulation by a cochlear implant (CI). To do so, we interleaved two stimuli with frequencies F1 and F2 Hz and recorded a neural distortion response (NDR) at F2-F1 Hz. We show that, because any one time point contains only the F1 or F2 stimulus, the instantaneous nonlinearities typical of electrical artefact should not produce distortion at this frequency. However, if the stimulus is smoothed, such as by charge integration at the nerve membrane, subsequent (neural) nonlinearities can produce a component at F2-F1 Hz. We stimulated a single CI electrode with interleaved sinusoids or interleaved amplitude-modulated pulse trains such that F2 = 1.5F1, and found no evidence for an NDR when F2-F1 was between 90 and 120 Hz. However, interleaved amplitude-modulated pulse trains with F2-F1~40 Hz revealed a substantial NDR with a group delay of about 45 ms, consistent with a thalamic and/or cortical response. The NDR could be measured even from recording electrodes adjacent to the implant and at the highest pulse rates (> 4000 pps) used clinically. We then measured the selectivity of this sustained response by presenting F1 and F2 to different electrodes and at different between-electrode distances. This revealed a broad tuning that, we argue, reflects the overlap between the excitation elicited by the two electrodes. Our results also provide a glimpse of the neural nonlinearity in the auditory system, unaffected by the biomechanical cochlear nonlinearities that accompany acoustic stimulation. Several potential clinical applications of our findings are discussed.

Characterizing Cochlear implant artefact removal from EEG recordings using a real human model

Electroencephography (EEG) recordings from CI listeners are contaminated by electrical artefacts that make it difficult to extract neural responses. Previously, we have removed these artefacts by means of interpolation and spatial filtering. However, the extent to which this method can effectively reduce electrical artefacts has not been fully investigated. Here, we assessed the effectiveness of interpolation and spatial filtering to remove electrical artefacts using recordings from a human head specimen implanted with a CI.•Electrical artefacts were obtained using amplitude-modulated (AM'ed) pulse trains presented at several pulse rates (100-to-902 pps) or using high rate pulse trains (902 pps) in which either a pair of electrodes or AM frequencies alternated periodically at a rate of 1Hz.•By adding auditory change complex (ACC), auditory steady-state response (ASSR), or auditory change following response (AC-FR) template waveforms to the contaminated recordings, we show that interpolation allows for effective artefact removal for pulse rates below 400 pps whilst interpolation and spatial filtering are effective at higher pulse rates, with minimal distortions for ACC and AC-FR, and with a degree of amplitude- and phase-distortions for ASSR.•Recordings from CI listeners agreed with simulations, demonstrating that reliable responses can be recovered.

MRI exploiting frequency-modulated pulses and their nonlinear phase

Frequency-modulated (FM) pulses can provide several advantages over conventional amplitude-modulated pulses in the field of MRI; however, the manner in which spins are manipulated imprints a quadratic phase on the resulting magnetization. Historically this was considered a hindrance and slowed the widespread adoption of FM pulses. This article seeks to provide a historical perspective of the different techniques that researchers have used to exploit the benefits of FM pulses and to compensate for the nonlinear phase created by this class of pulses in MRI. Expanding on existing techniques, a new method of phase compensation is presented that utilizes nonlinear gradients to mitigate the undesirable phase imparted by this class of pulses.

Improving Interaural Time Difference Sensitivity Using Short Inter-pulse Intervals with Amplitude-Modulated Pulse Trains in Bilateral Cochlear Implants.

Interaural time differences (ITDs) at low frequencies are important for sound localization and spatial speech unmasking. These ITD cues are not encoded in commonly used envelope-based stimulation strategies for cochlear implants (CIs) using high pulse rates. However, ITD sensitivity can be improved by adding extra pulses with short inter-pulse intervals (SIPIs) in unmodulated high-rate trains. Here, we investigated whether this improvement also applies to amplitude-modulated (AM) high-rate pulse trains. To this end, we systematically varied the temporal position of SIPI pulses within the envelope cycle (SIPI phase), the fundamental frequency (F0) of AM (125Hz and 250Hz), and AM depth (from 0.1 to 0.9). Stimuli were presented at an interaurally place-matched electrode pair at a reference pulse rate of 1000 pulses/s. Participants performed an ITD-based left/right discrimination task. SIPI insertion resulted in improved ITD sensitivity throughout the range of modulation depths and for both male and female F0s. The improvements were largest for insertion at and around the envelope peak. These results are promising for conveying salient ITD cues at high pulse rates commonly used to encode speech information.

Mathematical model for temperature estimation forecasting of electrically conductive plate elements under action of pulsed electromagnetic radiation of radio-frequency range

A mathematical model for determining the temperature of an electrically conductive plate element under the action of pulsed electromagnetic radiation of the radio-frequency range is proposed. This model allows us to take into account the influence of the process of thermoelastic energy dissipation on the forecasting of the value of temperature in addition to the joule heat. This process is determined by thermal expansion and the action of poendromotive forces arising in the element. This approach allows us to predict a decrease in the error of temperature determination. On this basis, the distributions of temperature in an electrically conductive plate element under the action of an amplitude-modulated radio pulse have been investigated numerically. Thermoelastic energy dissipation is taken into account when using the frequencies of the carrying electromagnetic oscillations beyond the resonant frequencies and of the first resonant frequency of the electromagnetic field for this element. An estimate of the influence of the process of taking into account mechanisms of energy dissipation on the total value of temperature in the element at the specified action and used frequencies is obtained. This has allowed us to increase the accuracy of temperature measurements in this element.