Mechanism Of Pulse Generation Research Articles

Generation of periodic high-power ultrashort pulse sequences in a chain of coupled traveling-wave tubes operating in the regimes of amplification and nonlinear Kompfner suppression

We have studied the dynamics of an electron microwave oscillator with a feedback loop containing two coupled traveling-wave tubes, the first operating in the regime of amplification and the second in the regime of nonlinear Kompfner absorption. It is established that oscillations generated in this system can take the form of a periodic sequence of ultrashort pulses. The proposed mechanism of pulse generation is analogous to the well-known method of passive mode locking.

Dynamic backcalculation with different load-time histories

This paper focused attention to the falling weight deflectometer (FWD) load-time history. For a commonly used device, it studied the pulse generation mechanism and the influence of different load histories on backcalculation results. In this connection, a semi-analytic impact theory was first introduced for realistically simulating FWD pulse generation. Then a newly developed finite-element code was presented for FWD interpretation; the code is capable of addressing dynamics, time-dependent layer properties, and quasi-nonlinear behaviour. Both new developments were demonstrated for an experimental dataset that resulted from operating an FWD with different loading configurations. It was found that backcalculated parameters are sensitive to the FWD pulse features. Consequently, it is recommended that, whenever advanced pavement characterisation is sought, experimental attention should be placed on generating diverse FWD pulse histories. Collectively, the resulting deflection histories will contain pertinent constitutive information for supporting the calibration of more complex pavement models.

A robust and tuneable mid-infrared optical switch enabled by bulk Dirac fermions

Pulsed lasers operating in the mid-infrared (3–20 μm) are important for a wide range of applications in sensing, spectroscopy, imaging and communications. Despite recent advances with mid-infrared gain platforms, the lack of a capable pulse generation mechanism remains a significant technological challenge. Here we show that bulk Dirac fermions in molecular beam epitaxy grown crystalline Cd3As2, a three-dimensional topological Dirac semimetal, constitutes an exceptional ultrafast optical switching mechanism for the mid-infrared. Significantly, we show robust and effective tuning of the scattering channels of Dirac fermions via an element doping approach, where photocarrier relaxation times are found flexibly controlled over an order of magnitude (from 8 ps to 800 fs at 4.5 μm). Our findings reveal the strong impact of Cr doping on ultrafast optical properties in Cd3As2 and open up the long sought parameter space crucial for the development of compact and high-performance mid-infrared ultrafast sources.

Passively Q-switched thulium-doped fiber laser with silver-nanoparticle film as the saturable absorber for operation at 2.0 µm

In this work, a compact thulium-doped fiber laser with a Q-switched output is proposed and demonstrated. A thulium-doped fiber is used for the laser, with a peak absorption of 200 dB m−1 at 790 nm and a cutoff wavelength of 1350 nm as the primary gain medium, and a silver-based saturable absorber as the pulse generation mechanism. The pulses obtained from the proposed laser have repetition rates from 38.3 kHz up to 56.2 kHz, with a pulse width as low as 4.2 µs and pulse energy as high as 67.3 nJ at a maximum pump power of 228.8 mW. The generated pulses are highly stable, showing no changes or fluctuations over operation for a period of 60 min, and further validated with signal-to-noise ratios of 57.0 dB and 59.5 dB in the optical and frequency domains respectively. The proposed laser has high potential for eye-safe applications in manufacturing and medicine.

Generating ultrafast pulses of light from quantum cascade lasers

The generation of ultrashort pulses from quantum cascade lasers (QCLs) has proved to be challenging. It has been suggested that the ultrafast electron dynamics of these devices is the limiting factor for mode locking and, hence, pulse formation. Even so, the clear mode locking of terahertz (THz) QCLs has been demonstrated recently, but the exact mechanism for pulse generation is not fully understood. Here we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic modulation and the generated THz pulse in the QCL. By using the phase-resolved detection of the electric field in QCLs embedded in metal–metal waveguides, we demonstrate that active mode locking requires the phase velocity of the microwave round-trip modulation to equal the group velocity of the THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting short-pulse generation. Mode locking was performed on QCLs employing phonon depopulation active regions, permitting the coherent detection of large gain bandwidths (500 GHz) and the generation of 11 ps pulses centered around 2.6 THz when the above “phase-matching” condition is satisfied. This work brings an enhanced understanding of QCL mode locking and will permit new concepts to be explored to generate shorter and more intense pulses from mid-infrared, as well as THz, QCLs.

Nanosecond Yb-Doped Monolithic Dual-Cavity Laser Oscillator With Large Core Fiber

A high pulse energy Yb-doped all-fiber dual-cavity laser oscillator is reported, based on the pulse generation mechanism of passive Q-switch with large core fiber saturable absorber. Sequential pulses with 49-ns pulse duration and 21-W average power operating at 1080 nm can be achieved in the scheme using the Yb-doped fiber with the core-diameter of 10 μm. The repetition rate varies from ~1 to 113.6 kHz, while the incident pump power increases. The monolithic all-fiber dual-cavity oscillator can achieve the highest pulse energy of 187 μJ. Moreover, using the all-fiber scheme with the core-diameter of uniform 20 μm, the single-pulse energy can be further scaled up to 484 μJ, which is directly the output from the all-fiber oscillator without additional amplifiers.

Robust Pulse Generators in an Excitable Medium with Jump-Type Heterogeneity

Heterogeneity is one of the most important and ubiquitous types of external perturbations. We study a spontaneous pulse-generating mechanism in an excitable medium with jump-type heterogeneity. Such a pulse generator (PG) has attracted considerable interest due to the computational potential of pulse waves in physiological signal processing. We first investigate the conditions for the onset of robust-type PGs, and then we show the global bifurcation structure of heterogeneity-induced patterns, including the complex ordered sequence of pulse-generating manners. We devise numerical frameworks to trace the long-term behavior of PGs as periodic solutions, and we detect the associated terminal homoclinic orbits that are homoclinic to a special type of heterogeneity-induced ordered pattern with a hyperbolic saddle. These numerical approaches assist us in identifying a candidate for the organizing center, and producing a variety of PGs as a codimension-two gluing bifurcation, in which two homoclinic trajectories...

Possible Involvement of Neuropeptide Y in Photo-Induced Suppression of Growth Hormone Pulses

It is well established that growth hormone (GH) is secreted in a pulsatile manner. Although the GH pulse-generating mechanism is not fully understood, we have previously reported that neuropeptide Y (NPY) profiles in the cerebrospinal fluid were negatively correlated with serum GH pulses. In addition, it is known that photic stimulation suppresses GH pulses for a certain period of time. In the present study, to investigate the involvement of NPY in regulating GH pulse generation, NPY gene expression in the arcuate nucleus (ARC) of the hypothalamus in rats was analyzed at around the lights on. First, we confirmed that GH pulses did not occur for around 1.5 hr after the start of the light phase. Then, we analyzed the activity of neurons and expression of NPY mRNA 1 hr before and 0.5 and 2 hr after lights on. Both the activity of neurons, which was evaluated by immunohistochemical detection for phosphorylated-cAMP response element binding protein (pCREB), and NPY mRNA levels in the caudal ARC were higher at 0.5 hr after lights on than the other two time points, while pCREB-positive cell numbers in the rostral ARC remained unchanged throughout the experimental period. In addition, NPY immunoreactivity in the periventricular nucleus (PeVN) was also higher at 0.5 hr after lights on than the other time points. These results suggest that NPY neurons in the caudal ARC projecting to the PeVN play a role in inhibiting GH pulses at the commencement of the light phase.

Pulse-like ruptures induced by low-velocity fault zones

Low-velocity fault zones (LVFZs) are found in most mature faults. They are usually 100–400 m wide and have ~20%–60% wave velocity reductions relative to the country rock. To study the effect of LVFZs on earthquake rupture and the radiated wavefield, we conducted two-dimensional (2-D) simulations of dynamic rupture on faults that bisect a LVFZ, considering a range of velocity reductions and widths. Most earthquakes apparently have slip rise times much shorter than their overall rupture duration. A number of dynamic mechanisms for such pulse-like ruptures have been proposed, including frictional self-healing, fault strength heterogeneities, and bimaterial effects. We find that ruptures in LVFZs with strong enough wave velocity contrast behave as pulses. These pulses are generated by fault locking induced by waves reflected from the boundaries of the LVFZ. This mechanism of pulse generation is robust to variations of initial stress, smoothness of the LVFZ structure, rupture mode, and exclusion of frictional healing. Moreover, we find that LVFZs can generate complex rupture patterns. LVFZs with low-velocity reduction induce multiple rupture fronts involving coexisting pulses and cracks. LVFZs with certain widths can accelerate the transition to supershear rupture speed. These additional effects of LVFZs on dynamic rupture can contribute to the complexity of high-frequency ground motions.

Electrophysiological Mapping of Embryonic Mouse Hearts: Mechanisms for Developmental Pacemaker Switch and Internodal Conduction Pathway

Understanding sinoatrial node (SAN) development could help in developing therapies for SAN dysfunction. However, electrophysiological investigation of SAN development remains difficult because mutant mice with SAN dysfunctions are frequently embryonically lethal. Most research on SAN development is therefore limited to immunocytochemical observations without comparable functional studies. We applied a multielectrode array (MEA) recording system to study SAN development in mouse hearts acutely isolated at embryonic ages (E) 8.5-12.5 days. Physiological heart rates were routinely restored, enabling accurate functional assessment of SAN development. We found that dominant pacemaking activity originated from the left inflow tract (LIFT) region at E8.5, but switched to the right SAN by E12.5. Combining MEA recordings and pharmacological agents, we show that intracellular calcium (Ca(2+))-mediated automaticity develops early and is the major mechanism of pulse generation in the LIFT of E8.5 hearts. Later in development at E12.5, sarcolemmal ion channels develop in the SAN at a time when pacemaker channels are down-regulated in the LIFT, leading to a switch in the dominant pacemaker location. Additionally, low micromolar concentrations of tetrodotoxin (TTX), a sodium channel blocker, minimally affect pacemaker rhythm at E8.5-E12.5, but suppress atrial activation and reveal a TTX-resistant SAN-atrioventricular node (internodal) pathway that mediates internodal conduction in E12.5 hearts. Using a physiological mapping method, we demonstrate that differential mechanistic development of automaticity between the left and right inflow tract regions confers the pacemaker location switch. Moreover, a TTX-resistant pathway mediates preferential internodal conduction in E12.5 mouse hearts.

Use of mass loading to operate a thin film ultrasonic transducer in the 300 kHz–10 MHz frequency range

Thin film ultrasonic transducers have been designed which operate over an important frequency range, 300 kHz to 10 MHz. The transducers were made using piezoelectric aluminium nitride films a few microns thick. The films would have a fundamental thickness mode resonance at 1–3 GHz if fabricated as an unsupported film, however operation at much lower frequencies has been demonstrated when the transducers are fabricated on bulk substrates. This would enables them to be used in ultrasonic non-destructive testing in circumstances where the film can be deposited directly onto the object under test. We have found that the major factors influencing the below-resonance operation of the thin film transducers are the device impedance, the spectrum of the excitation pulse, and any mechanical (mass) loading applied to the back face of the transducer. Results are presented showing that the evolution of device impedance as a function of device area could be predicted using a PSpice model of the thin film transducer. The ability of the transducer to generate longitudinal mode pulses rather than shear wave pulses was found to depend on increasing the mechanical loading at the back face of the transducer. This mechanism for pulse generation was confirmed by Finite Element Modelling using PZFlex.

A Kolsky tension bar technique using a hollow incident tube

Load control of the incident pulse profiles in compression Kolsky bar experiments has been widely used to subject the specimen to optimal testing conditions. Tension Kolsky bars have been used to determine dynamic material behavior since the 1960s with limited capability to shape the loading pulses due to the pulse-generating mechanisms. We developed a modified Kolsky tension bar where a hollow incident tube is used to carry the incident stress waves. The incident tube also acts as a gas gun barrel that houses the striker for impact. The main advantage of this new design is that the striker impacts on an impact cap of the incident tube. Compression pulse shapers can be attached to the impact cap, thus fully utilizing the predictive compression pulse-shaping capability in tension experiments. Using this new testing technique, the dynamic tensile material behavior for Al 6061-T6511 and TRIP 800 (transformation-induced plasticity) steel has been obtained.

Transient pulse formation in jasmonate signaling pathway

The jasmonate (JA) signaling pathway in plants is activated as defense response to a number of stresses like attacks by pests or pathogens and wounding by animals. Some recent experiments provide significant new knowledge on the molecular detail and connectivity of the pathway. The pathway has two major components in the form of feedback loops, one negative and the other positive. We construct a minimal mathematical model, incorporating the feedback loops, to study the dynamics of the JA signaling pathway. The model exhibits transient gene expression activity in the form of JA pulses in agreement with experimental observations. The dependence of the pulse amplitude, duration and peak time on the key parameters of the model is determined computationally. The deterministic and stochastic aspects of the pathway dynamics are investigated using both the full mathematical model and a reduced version of it. We also compare the mechanism of pulse formation with the known mechanisms of pulse generation in some bacterial and viral systems.

Single-mode giga-hertz optical pulses generated by a self-modulation of the relaxation oscillation frequency in a semiconductor laser with subharmonic polarization-rotated optical feedback

In this research, single-mode 3.76GHz optical pulses were generated in a Fabry–Perot type single-mode semiconductor laser with polarization-rotated optical feedback (PROF) at a round-trip feedback distance of 50.8cm, corresponding to a feedback frequency of 0.59GHz. Experimental results and numerical simulations revealed that the pulse generation mechanism involved a self-modulation of the laser’s relaxation oscillation frequency so that the oscillation frequency approximated to an integer multiple of the PROF round-trip feedback frequency. This effect is very different from similar experiments reported by many researchers before, in which the laser’s output was amplitude modulated by the feedback frequency and an ultra-short feedback distance was required to generate giga-hertz optical pulses. Investigations about some characteristics of the self-modulation mechanism will be reported in the paper.

Picosecond pulse generation by internal gain switching in laser diodes

A laser diode structure realizing internal gain switching is suggested for high-power picosecond optical pulse generation. A thin wideband barrier layer is incorporated in the active region of this structure, in order to provide additional control of the carrier distribution and the gain dynamics. The dynamic laser output response to the nanosecond pumping current pulses is modeled and analyzed. The pulse generation mechanism is studied and the possibility to generate optical pulses with power density of 0.416 W/μm2 is demonstrated. This result compares well with that provided by the best modern picosecond laser diodes operating in Q-switching mode.

The causes of bedload pulses in a gravel channel: the implications of bedload grain‐size distributions

AbstractUnsteady bedload transport was measured in two c. 5 m wide anabranches of a gravel‐bed braided stream draining the Haut Glacier d'Arolla, Switzerland, during the 1998 and 1999 melt seasons. Bedload was directly sampled using 152 mm square Helley–Smith type samplers deployed from a portable measuring bridge, and independent transport rate estimates for the coarser size fractions were obtained from the dispersion of magnetically tagged tracer pebbles. Bedload transport time series show pulsing behaviour under both marginal (1998) and partial (1999) transport regimes. There are generally weak correlations between transport rates and shear stresses determined from velocity data recorded at the measuring bridge. Characteristic parameters of the bedload grain‐size distributions (D50, D84) are weakly correlated with transport rates. Analysis of full bedload grain‐size distributions reveals greater structure, with a tendency for transport to become less size selective at higher transport rates. The bedload time series show autoregressive behaviour but are difficult to distinguish by this method. State–space plots, and associated measures of time‐series separation, reveal the structure of the time series more clearly. The measured pulses have distinctly different time‐series characteristics from those modelled using a one‐dimensional sediment routing model in which bed shear stress and grain size are varied randomly. These results suggest a mechanism of pulse generation based on irregular low‐amplitude bedforms, that may be generated in‐channel or may represent the advection of material supplied by bank erosion events. Copyright © 2003 John Wiley & Sons, Ltd.

Neurophysiology of puberty

Puberty is the stage of development during which an individual first becomes capable of reproducing sexually and is characterized by maturation of the genital organs, the development of secondary sexual characteristics, and changes in mood and behavior. Although puberty is a phase of development, an introduction to the neurophysiology of the process is facilitated by a review of the essential features of the central neural control of the ovary and testis in the adult. In sexually mature individuals, the processes of ovulation, spermatogenesis, and sex steroid production are regulated by the pituitary gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). The synthesis and secretion of the pituitary gonadotropins, in turn, are governed by the brain peptide gonadotropin-releasing hormone (GnRH), which is secreted by nerve terminals in the base of the forebrain into a specialized portal circulation subserving the pituitary gland. GnRH is not secreted continuously into the portal vasculature but instead is released as intermittent discharges with a frequency of approximately one pulse every 1 to 3 h. This pulsatile mode of GnRH release by the brain is absolutely essential for sustained gonadotropin secretion, and when GnRH stimulation of the pituitary is continuous, LH and FSH release is greatly reduced. As may be predicted from the foregoing considerations, if pulsatile secretion of GnRH by the brain of men and women is interrupted, as may be the case in highly trained ballerinas and female athletes or severely undernourished individuals, pituitary gonadotropin secretion is impaired and gonadal function arrested. Thus, the GnRH pulse-generating mechanism in the brain occupies a position of primacy in the control of reproduction in the adult. Moreover, the proximal trigger activating the pituitary gonadal axis at puberty is an increase in pulsatile GnRH release in association with a modest increase in the drive to express the gene encoding this neuropeptide. For the foregoing reasons, a consideration of the mechanisms underlying the pulsatile secretion of this brain hormone in the adult provides a point of departure for a discussion of the neurophysiology of puberty.

Respiratory sinus arrhythmia from two coupled pacemakers

We reproduce global features of respiratory sinus arrhythmia (RSA), a prominent source of heart rate variability, from two signals coupled in alternate fashion so dominance periodically switches back and forth between them. We consider two different possibilities for this coupling and illustrate our method with numerical simulations that we contrast with the corresponding results from real data. We interpret our findings within the context of the two-pacemaker model of the heartbeat, an alternative to the single-pacemaker mechanism of pulse generation in the orthodox conduction model.

Short-Term Fasting Selectively Suppresses Leptin Pulse Mass and 24-Hour Rhythmic Leptin Release in Healthy Midluteal Phase Women without Disturbing Leptin Pulse Frequency or Its Entropy Control (Pattern Orderliness)1

Nutritional signals strongly regulate neuroendocrine axes, such as those subserving release of LH, GH, and TSH, presumptively in part via the adipocyte-derived neuroactive peptide leptin. In turn, leptin release is controlled by both acute (fasting) and long-term (adipose store) nutrient status. Here, we investigate the neuroendocrine impact of short-term (2.5-day) fasting on leptin release in healthy young women studied in the steroid-replete midluteal phase of the normal menstrual cycle. Eight women each underwent 24-h blood sampling at 10-min intervals during a randomly ordered 2.5-day fasting vs. fed session in separate menstrual cycles. Pulsatile leptin release was quantified by model-free Cluster analysis, the orderliness of leptin patterns by the approximate entropy statistic, and nyctohemeral leptin rhythmicity by cosinor analysis. Mean (24-h) serum leptin concentrations fell by 4.6-fold during fasting; namely, from 15.2+/-2.3 to 3.4+/-0.6 microg/L (P = 0.0007). Cluster analysis identified 13.9+/-1.1 and 14.3+/-1.1 leptin peaks per 24 h in the fed and fasting states (P = NS), and unchanging leptin interpeak intervals (89+/-5.4 vs. 92+/-5.3 min). Leptin peak area declined by 4.2-fold (155+/-21 vs. 37+/-7 area units, P = 0.004), due to a reduction in incremental leptin pulse amplitude (4.4+/-0.7 vs. 1.0+/-0.13 microg/L, P = 0.0011). The cosine amplitude and mesor (mean) of the 24-h leptin rhythm decreased by 4-fold, whereas the acrophase (timing of the nyctohemeral leptin peak) remained fixed. The approximate entropy of leptin release was stable, thus indicating preserved orderliness of leptin release patterns in fasting. Cross-correlation analysis revealed both positive (fed) and negative (fasting) leptin-GH relationships, but no leptin-LH correlations. In summary, short-term (2.5-day) fasting profoundly suppresses 24-h serum leptin concentrations and pulsatile leptin release in the sex steroid-sufficient midluteal phase of healthy women via mechanisms that selectively attenuate leptin pulse area and incremental amplitude. In contrast, the pulse-generating, nyctohemeral phase-determining, and entropy-control mechanisms that govern 24-h leptin release are not altered by acute nutrient restriction at this menstrual phase. Leptin-GH (but not leptin-LH) showed nutrient-dependent positive (fed) and negative (fasting) cross-correlations. Whether similar neuroendocrine mechanisms supervise altered leptin signaling during short-term nutrient restriction in men, children, or postmenopausal women is not known.

Ultrafast pulse generation in photoconductive switches

Carrier and field dynamics in photoconductive switches are investigated by electrooptic sampling and voltage-dependent reflectivity measurements. We show that the nonuniform field distribution due to the two-dimensional nature of coplanar photoconductive switches, in combination with the large difference in the mobilities of holes and electrons, determine the pronounced polarity dependence. Our measurements indicate that the pulse generation mechanism is a rapid voltage breakdown across the photoconductive switch and not a local field breakdown.