Dual-frequency Pulse Research Articles

Distributed Discharge Detection Based on Improved COTDR Method With Dual Frequency Pulses

Distributed Discharge Detection Based on Improved COTDR Method With Dual Frequency Pulses

Nanoparticle-mediated histotripsy using dual-frequency histotripsy pulsing: Comparison of bubble-cloud characteristics and ablation efficiency

Nanoparticle-mediated histotripsy (NMH) is a targeted ablation method using perfluorocarbon-filled nanoparticles to generate bubble-clouds at pressure levels (9–12 MPa) significantly below the histotripsy intrinsic threshold (>25 MPa). Prior studies have also shown a significant reduction in ablation efficiency compared to conventional histotripsy, likely from reduced bubble expansion and bubble-cloud density. Here, we investigate the bubble-cloud characteristics and ablation efficiency for NMH using dual-frequency pulsing. We hypothesize this method will increase ablation efficiency by increasing the bubble-cloud density and individual bubble expansion. High-speed optical imaging was used to characterize the cavitation threshold, cloud dimensions, and bubble-density of bubble clouds generated in agarose tissue phantoms, with and without perfluorohexane-filled nanocones, exposed to single-cycle dual-frequency pulses using a 500 kHz–3 MHz array transducer (1:1 pressure ratio). Ablation efficiency was investigated using red blood cell phantoms. Results showed dual-frequency NMH predictably produced smaller, denser, and more well-confined bubble-clouds and increased ablation efficiency compared to previous single-frequency studies, with complete ablation of the focal volume observed within 2000 pulses. This study demonstrates the potential of enhancing NMH ablation efficiency with dual-frequency pulsing and highlights the need for further studies to optimize NMH pulsing parameters for future clinical therapy development.

Dual-frequency pulse laser based on acousto-optic modulation.

Non-collinear stimulated Brillouin scattering (SBS) amplification can obtain high peak power Stokes output while ensuring the stability, but the frequency mismatch reduces the energy conversion efficiency of the system. In this paper, a dual-frequency pulse laser based on acousto-optic crystal modulation is designed. The output pulse pair can be used as pump and Stokes light, respectively, which realizes the active frequency matching of the gain medium Brillouin frequency shift during the SBS amplification process and helps to maintain ideal energy conversion efficiency. The dual-frequency laser finally produced a laser pulse pair with a pulse width adjustment range of 100 ps-50 ns, a frequency shift range of 0 GHz-2 GHz, and the polarization extinction ratio (PER) reaches 20.82dB.

Dual-Frequency Chirp Excitation for Passive Cavitation Imaging in the Brain.

One of the main challenges that impede cavitation-mediated imaging in the brain is restricted opening of the blood-brain barrier (BBB) making it difficult to locate cavitating microbubbles (MBs). Passive cavitation imaging (PCI) has received attention due to the possibility of performing real-time monitoring by listening to acoustic cavitation. However, the long excitation pulses associated with PCI degrade its axial resolution. The present study combined a coded excitation technique with a dual-frequency chirp (DFC) excitation method to prevent interference from the nonlinear components of MBs' cavitation. The use of DFC excitation generates a low-frequency (0.4, 0.5, or 0.6 MHz) chirp component as the envelope of the signal-driving MBs' cavitation with a dual-frequency pulse ( ω1 = 1.35 MHz and ω2 = 1.65 MHz, ω1 = 1.3 MHz and ω2 = 1.7 MHz, and ω1 = 1.25 MHz and ω2 = 1.75 MHz). The cavitation of MBs was passively imaged utilizing a chirp component with pulse compression to maintain abundant insonation energy without any reduction in the axial imaging resolution. In vitro experiments showed that the DFC method improved the signal-to-noise ratio by 42.2% and the axial resolution by 4.1-fold compared with using a conventional long-pulse waveform. Furthermore, the cavitating MBs driven by different ultrasound (US) energy (0, 0.3, 0.6, and 0.9 MPa, N = 3 for each group) in the rat brain with an intact skull still could be mapped by DFC. Our successful demonstration of using the DFC method to image cavitation-induced BBB opening affords an alternative tool for assessing cavitation-dependent drug delivery to the brain, with the benefit of real-time and high convenient integration with current US imaging devices.

Parametric array for tissue harmonic imaging

More than 50% of abdominal scans are performed on technically difficult patients due to worldwide increase of obesity. Tissue harmonic imaging has shown some improvements on obese patients but the problem remains unsolved. The use of dual frequency pulses allows the generation of sum and difference frequencies in tissue. We hypothesize that the difference frequency (Δf, parametric array) can survive larger penetration depths and increase SNR while maintaining the spatial characteristics of the primary wave. Our objective was to optimize the parameters for efficient generation of the parametric array and study its spatial characteristics for imaging obese patients. Numerical simulations using KZK equation and measurements in tissue-like media were performed to study parametric arrays. Pulses with two frequencies (f1, f2) were considered where 0.1f1≤Δf≤f1. Pulse inversion was utilized to isolate even harmonics, including Δf. The amplitude of Δf was maximized at Δf = f1. The axial resolution is not seriously compromised by the lower frequency of Δf when using PI. For 0.5 dB/cm attenuation difference between Δf and 2f1, Δf echoes from 20 cm deep were greater by 10 dB. Application of the parametric array on larger technically difficult patients would offer similar benefits as those in sonar applications.

Microbubble response to dual frequency excitation for broadband contrast imaging

Contrast imaging relies on pulsing schemes like pulse inversion (PI), power-modulated pulse inversion (PMPI), and power modulation (PM) to isolate certain harmonic components. These multi-pulse schemes normally employ single frequency pulses (SFP). In this work, we propose dual frequency pulses (DFP) coupled with pulsing schemes (PI, PMPI, and PM) for microbubble imaging. The proposed DFP contain an amount b 1 of a fundamental frequency f and an amount b 2 of its phase-shifted (φ) second harmonic 2f. Microbubbles were excited with a variety of DFP combinations (b 1, b 2, φ) and their nonlinear response was compared to that from conventional SFP. All pulsing schemes benefit in terms of overall signal from the use of DFP by ~4–6 dB. PI benefits the most when coupled with DFP as it extracts an additional amount of nonlinear fundamental signal (~25 dB) something not feasible with SFP. It was observed that microbubble nonlinear response changes when the relative amount of f and 2f in DFP change, allowing the design of specific pulses for different microbubbles. In conclusion, since the use of DFP with pulsing schemes extracts harmonics more efficiently than SFP it has the potential to improve contrast imaging in terms of bubble sensitivity and image resolution.

Dual-rail optical gradient echo memory.

We introduce a scheme for the parallel storage of frequency separated signals in an optical memory and demonstrate that this dual-rail storage is a suitable memory for high fidelity frequency qubits. The two signals are stored simultaneously in the Zeeman-split Raman absorption lines of a cold atom ensemble using gradient echo memory techniques. Analysis of the split-Zeeman storage shows that the memory can be configured to preserve the relative amplitude and phase of the frequency separated signals. In an experimental demonstration dual-frequency pulses are recalled with 35% efficiency, 82% interference fringe visibility, and 6° phase stability. The fidelity of the frequency-qubit memory is limited by frequency-dependent polarisation rotation and ambient magnetic field fluctuations, our analysis describes how these can be addressed in an alternative configuration.

Harmonic generation with a dual frequency pulse.

Nonlinear imaging was implemented in commercial ultrasound systems over the last 15 years offering major advantages in many clinical applications. In this work, pulsing schemes coupled with a dual frequency pulse are presented. The pulsing schemes considered were pulse inversion, power modulation, and power modulated pulse inversion. The pulse contains a fundamental frequency f and a specified amount of its second harmonic 2f. The advantages and limitations of this method were evaluated with both acoustic measurements of harmonic generation and theoretical simulations based on the KZK equation. The use of two frequencies in a pulse results in the generation of the sum and difference frequency components in addition to the other harmonic components. While with single frequency pulses, only power modulation and power modulated pulse inversion contained odd harmonic components, with the dual frequency pulse, pulse inversion now also contains odd harmonic components.

Multi-Group of Dual-Frequency Pulse Waveform Design for Anti-Range Eclipse of High PRF PD Guidance Radar

Range eclipse is a serious problem in high pulse repetition frequency (PRF) pulse doppler (PD) terminal guidance radar. In this paper, we propose a new scheme, which exploits multi-group of dual-frequency pulse waveform, for anti-range eclipse. Each group contains two pulse waveforms with different PRF, which has its own range region of anti-range eclipse and anti-range ambiguity simultaneously. In the scheme, only one pulse group works at a moment, which can avoid range eclipse and effectively measure target distance. According to the changing of target distance, radar switches the pulse group to other group to guarantee target in the range region of no eclipse in the tracking term. Simulation results show that this method can effectively prevent high PRF PD radar seeker from range eclipse.

Investigation of terahertz generation from passively Q-switched dual-frequency laser pulses

When two Nd:YLF crystals share a Cr:YAG crystal functioning as a single passive Q switch, the timing jitter between each pair of dual-frequency pulses generated by the two crystals has been reduced by a factor of 20. Such a reduction in the timing jitter allows us to generate terahertz pulses by focusing such a passively Q-switched laser beam onto a nonlinear crystal. Such a result represents the first step for us to eventually implement a compact terahertz source based on ultracompact microchip lasers.

Nonlinear propagation acoustics of dual-frequency wide-band excitation pulses in a focused ultrasound system

In this article, acoustic propagation effects of dual-frequency wide-band excitation pulses in a focused ultrasound system are demonstrated in vitro. A designed and manufactured dual-frequency band annular array capable of transmitting 0.9/7.5 MHz center frequency wide-band pulses was used for this purpose. The dual-frequency band annular array, has been designed using a bi-layer piezo-electric stack. Water tank measurements demonstrate the function of the array by activating the low- and high-frequency layers individually and simultaneously. The results show that the array works as intended. Activating the low- and high-frequency layers individually, results in less than -50 dB signal level from the high- and low-frequency layers respectively. Activating both layers simultaneously, produce a well defined dual-frequency pulse. The presence of the low-frequency pulse leads to compression, expansion, and a time delay of the high-frequency pulse. There is a phase shift between the low- and high-frequency pulse as it propagates from the array to the focus. This makes the latter described effects also dependent on the array configuration. By varying the low-frequency pressure, a shift of up to 0.5 MHz in center frequency of a 8.0 MHz transmitted high-frequency pulse is observed at the array focus. The results demonstrate the high propagation complexity of dual-frequency pulses.

Transmit beams adapted to reverberation noise suppression using dual-frequency SURF imaging

A method that uses dual-frequency pulse complexes of widely separated frequency bands to suppress noise caused by multiple scattering or multiple reflections in medical ultrasound imaging is presented. The excitation pulse complexes are transmitted to generate a second order ultrasound field (SURF) imaging synthetic transmit beam. This beam has reduced amplitude near the transducer, which illustrates the multiple scattering suppression ability of the imaging method. Field simulations solving a nonlinear wave equation are used to calculate SURF imaging beams, which are compared with beams for pulse inversion (PI) and fundamental imaging. In addition, a combined SURF and PI beam generation is described and compared with the beams mentioned above. A quality ratio, relating the energy within the near-field to that within the imaging region, is defined and used to score the multiple scattering and multiple reflection suppression abilities when imaging with the different beams. The realized combined SURF-PI beam scores highest, followed by SURF, PI (that score equally well), and the fundamental. The amplitude in the imaging region and therefore also the SNR is highest for the fundamental followed by SURF, PI, and SURF-PI. The work hence indicates that when substituting PI for SURF, one may trade increased SNR into use of increased imaging frequencies without loss of multiple scattering and multiple reflection noise suppression.

Microbubble destruction by dual-high-frequency ultrasound excitation

The efficiency of high-frequency destruction of microbubble-based contrast agent is limited by the high pressure threshold, while the difficulty of spatially confining destruction induced by low-frequency excitation to a small sample volume potentially increases the risk of adverse bioeffects. The dual-frequency excitation method involves the simultaneous transmission of 2 high-frequency sinusoids to produce an envelope signal at the difference frequency. The envelope signal provides the low-frequency driving force for oscillating the contrast-agent microbubbles to improve destruction efficiency, while the destruction sample volume remains small due to the high frequency of the carrier signal. Experimental results indicate that dual-frequency excitation consistently results in destruction of contrast-agent microbubbles that is superior to using a tone burst at the carrier frequency. With 1 micros pulse length, the acoustic pressure threshold for 95% microbubble destruction markedly reduces from 2.6 MPa to 0.9 MPa when the dual-frequency pulse having envelope frequency of 3 MHz is utilized instead of the 10-MHz sinusoidal pulse. In addition, the dual-frequency pulse having lower envelope frequency generally provides more efficient microbubble destruction, especially when the excitation waveform is long enough to guarantee sufficient envelope component.

Interaction of dual-frequency pulses in passively mode-locked lasers

We have found, numerically, that three stable pulses of different shapes can exist in systems described by the complex Ginzburg–Landau equation, such as passively mode-locked lasers with a fast saturable absorber. At the same cavity parameter values, however, only two of them can coexist, which two depending on the particular values of the parameters. The region of existence for each pulse is investigated numerically. The interaction between each pair of pulses is studied numerically. Using the interaction plane technique, we have found stable bound states of composite pulses.

Dual-frequency pulses in fiber lasers

We analyze pulse propagation in a soliton fiber laser, modelled by the complex Schrödinger-Ginzburg-Landau equation. It is shown that this equation admits two previously unknown forms of stable stationary solutions. The first of them is a dual-frequency pulse, which propagates as a single unit, but has two symmetric peaks in the spectrum. The second one is a moving pulse, with the asymmetric double-hump spectrum.

Dispersive and nonlinear propagation of dual-frequency pulses in optical fiber

Analytic solutions for limiting forms of the nonlinear Schrödinger equation are presented for dual-frequency input pulses propagating through single-mode, polarization-maintaining fiber. Linear dispersion and nonlinearity are considered separately. The article concludes with mathematica code appropriate for use by undergraduate students to explore dispersive and nonlinear propagation of light in an optical fiber.

On the derivation of Zone I spectra for a pulsed finite-amplitude source operating in a nonviscous nondispersive fluid medium

A mathematical procedure for deriving the complex spectral amplitudes generated by nonlinear interaction of the frequency components of a pulsed finite-amplitude source in a nonviscous nondispersive fluid medium, within a distance prior to shock formation, is established. In order to illustrate the procedure, the nonlinear propagation spectra of a dual frequency pulse and a dual sweep linear FM pulse are derived.