Pulse Merging Research Articles

Temporal Cavity Soliton Interaction in Passively Mode-Locked Semiconductor Lasers

Weak interactions of temporal cavity solitons resulting from gain saturation and recovery in a delay differential model of a long cavity semiconductor laser were studied numerically and analytically using an asymptotic approach. This paper shows that in addition to the usual soliton repulsion leading to a harmonic mode-locking regime, soliton attraction is also possible in a laser with a nonzero linewidth enhancement factor. It is shown numerically that this attraction can lead either to pulse merging or to pulse bound-state formation.

H-shaped pulse generation with tunable leading edge from a Tm-doped mode-locked fiber laser

A novel h-shaped pulse train with a tunable leading-edge (resembling a h-shaped pulse merging with a weak square pulse) is experimentally generated in a Tm-doped mode-locked fiber laser (MLFL) incorporating a 100 m highly nonlinear fiber. Through carefully adjusting the pump power and cavity polarization states, the leading-edge duration of the obtained h-shaped pulse can be widely tuned from zero (corresponding to the typical conventional h-shaped pulse train) to 567.8 ns (equivalent to the cavity roundtrip time). Our results can further facilitate understanding the intrinsic mechanism and characteristics of novel h-shaped pulse dynamics in MLFLs.

Dynamics and deposition of sediment-bearing multi-pulsed flows and geological implication

ABSTRACT Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems.

Scaling Analysis of Multipulsed Turbidity Current Evolution With Application to Turbidite Interpretation

AbstractDeposits of submarine turbidity currents, turbidites, commonly exhibit upward‐fining grain size profiles reflecting deposition under waning flow conditions. However, more complex grading patterns such as multiple cycles of inverse‐to‐normal grading are also seen and interpreted as recording deposition under cycles of waxing and waning flow. Such flows are termed multipulsed turbidity currents, and their deposits pulsed or multipulsed turbidites. Pulsing may arise at flow initiation, or following downstream flow combination. Prior work has shown that individual pulses within multipulsed flows are advected forward and merge, such that complex longitudinal velocity profiles eventually become monotonically varying, although transition length scales in natural settings could not be predicted. Here we detail the first high frequency spatial (vertical, streamwise) and temporal measurements of flow velocity and density distribution in multipulsed gravity current experiments. The data support both a process explanation of pulse merging and a phase‐space analysis of transition length scales; in prototype systems, the point of merging corresponds to the transition in any deposit from multipulsed to normally graded turbidites. The scaling analysis is limited to quasi‐horizontal natural settings in which multipulsed flows are generated by sequences of relatively short sediment failures (<10 km long) that develop progressively up‐dip and predicts pulse merging after only a few tens of kilometers. The model cannot provide quantitative estimation of merging in down‐slope flows generated by axially extensive (>10 km) sequences of breaches or where pulsing arises from combination at confluences of single‐pulsed flows, such flows may be responsible for the pulsing signatures seen in some distal turbidites, >100 km from source.

Transient chaos and associated system-intrinsic switching of spacetime patterns in two synaptically coupled layers of Morris-Lecar neurons.

Spatiotemporal chaos collapses to either a rest state or a propagating pulse solution in a single layer of diffusively coupled, excitable Morris-Lecar neurons. Weak synaptic coupling of two such layers reveals system intrinsic switching of spatiotemporal activity patterns within and between the layers at irregular times. Within a layer, switching sequences include spatiotemporal chaos, erratic and regular pulse propagation, spontaneous network wide neuron activity, and rest state. A momentary substantial reduction in neuron activity in one layer can reinitiate transient spatiotemporal chaos in the other layer, which can induce a swap of spatiotemporal chaos with a pulse state between the layers. Presynaptic input maximizes the distance between propagating pulses, in contrast to pulse merging in the absence of synapses.

Dynamics of vortex-antivortex pairs and rarefaction pulses in liquid light.

We present a numerical study of the cubic-quintic nonlinear Schrödinger equation in two transverse dimensions, relevant for the propagation of light in certain exotic media. A well-known feature of the model is the existence of flat-top bright solitons of fixed intensity, whose dynamics resembles the physics of a liquid. They support traveling wave solutions, consisting of rarefaction pulses and vortex-antivortex pairs. In this work, we demonstrate how the vortex-antivortex pairs can be generated in bright soliton collisions displaying destructive interference followed by a snake instability. We then discuss the collisional dynamics of the dark excitations for different initial conditions. We describe a number of distinct phenomena including vortex exchange modes, quasielastic flyby scattering, solitonlike crossing, fully inelastic collisions, and rarefaction pulse merging.

Avalanche breakdown of p-n-junction in radiotechnics

The paper presents research results of fractal properties of microplasma noise at LED avalanche breakdown in the visible spectrum (λ= 660; 700 nm). The breakdown type of p-n-junctionwas determined as a result of measured current-voltage characteristics at room temperature, at the temperature of 100-105 °C and after cooling down to room temperature. It was shown that the breakdown of avalanche type is realized in the majority of LEDs. It was established that the partial avalanche breakdown mode may be realized in LEDs, when a small current flows in pulses through the device. By increasing the voltage, pulse amplitude increases, closely spaced pulses merge, and time intervals between them are reduced. To interpret experimental results we applied model of processes occurring in microplasma, and noise model of partial and advanced avalanche breakdown (by A.S. Tager). The study revealed previously non-described features of microplasma noise – the fractal nature of microplasma noise. The algorithm for fractal dimension calculating was implemented in MATLAB. The dependence of fractal dimension on the reverse voltage applied to the LEDs was found out. Obtained fractal signal can be applied in optical communication systems for noise free and confidential information transmission.

EMI's TwinGun Concept for a New Light-gas Gun Type Hypervelocity Accelerator

A new two-stage light-gas gun type accelerator was developed at Fraunhofer EMI. The accelerator features two parallel pump tubes that are attached to a single powder chamber. Loading conditions are adjusted to achieve a small delay between the arrival of the two pistons at their respective accelerated reservoir. A merging section combines both gas channels and guides the two pressure pulses generated to a single launch tube breech. The two pressure pulses merge to a single, but elongated pressure pulse that acts on the projectile. Carefully adjusting the loading conditions shall in the future increase the performance compared to standard light-gas gun accelerators. Initial experimental results of the calibre 4 mm prototype show feasibility of the concept.

Velocity Statistics of a Fully Pulsed Round Jet in Streamwise Direction

In this paper, statistical quantities of a fully pulsed round jet along the jet centerline are reported. A range of the Reynolds (1.5 x 104 < Re < 4 x 104) and Strouhal (0.0064 < St < 0.0076) numbers is used to generate the jet. Physically this unsteady jet produces a series of distinct pulses due to the excitations. The mechanically excitations lead to the appearance of pulse dominated and high turbulence steady jet region in which their existence is of a strong dependence on the level of the controlled parameters. After the pulse merging completes the pulsed jet alters to a self-preserving steady jet with a significantly higher turbulence intensity. Under a constant mass flow rate the pulsed jet tends to be more fluctuating at a less intense pulsation thus permitting the endurance of the normalized periodic component and a more rapid velocity decay in the pulse-dominated region.

Collisional lasing on a self-terminating transition 2 1Po1 — 2 1S0 in helium atom

Lasing on a self-terminating transition 2 1Po1 — 2 1S0 (λ = 2.058 μm) in helium atom is studied for a single- and double-pulse operation regimes under electron beam pumping in pure helium and its mixtures with H2, N2, O2, CO2, H2O, NH3, and N2O. In pure helium, the maximal pulse duration is ∼50 ns, which agrees with the calculated value. Recovery of lasing in the second pulse is observed at a time delay between the pulses of longer than 1.25 μs. In adding CO2, N2O, NH3, and H2O, the relaxation rate for population of the metastable state He(2 1S0) increases, which makes the delay, needed for recovering lasing, shorter up to pulse merging in the case of H2O. At the exciting pulse base-level duration of 1.2 μs, in mixtures of helium with NH3 and H2O, laser pulses with a duration of ∼0.8 μs are observed, which testifies that collisional quasi-cw lasing occurs. Mechanisms of collisional lasing are discussed.

Investigation on the Interaction of Secondary Current Pulses with the Main Wavefront

Additional current pulses get injected into the lightning channel from branches and/or from successive reflections within struck Tall Grounded Objects (TGOs). These pulses termed here as current waves/pulses affect the lightning Electro-Magnetic Fields (EMFs). With regard to these, an important question arises on whether or not these pulses get reflected at the main wavefront and there seems to be uncertainty in the pertinent literature. A particular case involving only the first ground end reflection from a struck TGO has been fully dealt in our earlier work. However, the same for subsequent reflections from TGO and that for the current fed by the branches were not dealt and this forms the main goal of present work. As the main objective is to ascertain the status of these secondary current pulses after they have reached the main wavefront, dynamics at the point of injection assumes lesser importance. In view of this, a lumped voltage source is employed for the injection of secondary current pulses at appropriate time instants. The main stroke evolution is emulated by a macroscopic physical model, which was developed in our earlier work. Investigation showed that these secondary pulses merge with the main wavefront without any sign of reflection whatsoever. Analysis showed that the spatial dynamic resistance/conductance profile at the main wavefront is basically responsible for the same with distributed source providing additional support. These findings are in line with the observations made in our earlier work for the first ground end reflected current.

Rationalising MRI, conductance and pressure drop measurements of the trickle-to-pulse transition in trickle beds

Recent MRI data have shown that the transition from trickle to pulsing flow in trickle-bed reactors occurs over a range of liquid velocities at constant gas velocity. The transition is initiated by isolated local pulsing events, which increase in number with increase in liquid velocity until a maximum number exists. Above this liquid velocity, which we have termed the transition point, the individual pulses merge until a single macro-scale pulse is formed and the whole bed demonstrates pulsing flow. In this paper we compare the characterisation of the transition obtained using conductance and pressure drop measurements with that obtained using MRI. Using the insights gained from the 3-D MRI measurements, recorded with a data acquisition time of 280 ms, it is shown that the conductance and pressure drop measurements are sensitive to different stages of the evolution of the hydrodynamic transition, a factor important when using these different measurements in the development and validation of numerical and theoretical models. Conductance measurements identify unambiguously only the onset of the single macro-scale pulse regime, consistent with a determination of the transition point made by visual observation. In contrast, pressure drop measurements are sensitive to both the onset of formation of local pulses and the liquid velocity at which the maximum number of liquid pulses occurs. We also show how a combination of conductance and pressure drop measurements can be used to fully characterise the transition, thereby enabling translation of the insights gained by MRI into a robust measurement strategy for use on larger-scale reactors. Data are reported for a cylindrical column of length 70 cm and inner diameter 43 mm, packed with cylindrical porous γ - Al 2 O 3 packing elements of length and diameter 3 mm. The bed was operated under conditions of co-current downflow of air and water, at ambient temperature and a pressure of 2 barg. Gas and liquid superficial velocities were in the range 25–300 and 0.9– 13.8 mm s - 1 , respectively.

Reshaping ultrashort light pulses in resonant photonic crystals

Ultrashort light pulses propagating through a resonant photonic crystal can be reshaped to shorter duration output through self-induced transparency. The reshaping process is shown to result from the unique transmission behavior of light pulses through the photonic structure. Pulse compression and pulse merging, with which multipeak optical pulses can be evolved to give rise to a single-peak, high-quality transmitted pulse output, are obtained. The pulse propagation in a resonant photonic crystal is compared with that in a bulk atomic medium, which shows that the effect of pulse reshaping is much more powerful in the former than in the latter.

Development of an unbalanced switching scheme for a current source inverter

The development of an unbalanced switching scheme for the power conditioning system of a superconducting magnetic energy storage (SMES) device is described. The pulse-width modulation switching scheme for a current-source inverter, based on the unified switching algorithm, is presented. A device sequencing scheme and pulse merging arrangement is developed which minimises low-order harmonics. The balanced switching scheme is extended to an unbalanced scheme where the positive and negative sequence components of the output AC currents are controlled directly by their respective modulation indices and delay angles. The switching scheme is implemented on an experimental SMES device and its performance is assessed with respect to modulation linearity, harmonic generation, independent control of active and reactive power, and independent control of sequence currents.

Pulse trapping and nonequilibrium spatiotemporal wave mixing in broad-area semiconductor lasers

The trapping and dynamic nonequilibrium spatiospectral mixing of two nonresonant picosecond pulses injected into the active area of a broad-area semiconductor laser are investigated on the basis of spatiotemporal numerical simulations. The spatiotemporal dynamics of the nonequilibrium Wigner distributions of the charge carriers and the interband polarization reveal the spatiospectral wave mixing and spatiospectral nature of self-focusing, propagation-induced hole burning, dynamic nonlinear waveguiding, and interference effects. This interplay of effects is identified as the origin of the pulse merging and of the subsequent formation of a trapped pulse that is reminiscent of a spatial optical soliton.

CONVERGENCE OF TIME EXPONENTIAL DECAYING PULSE TO INTENSITY STEP INPUT PULSE FOR LASER HEATING OF SEMI-INFINITE BODY

The analytical formulation of laser heating mechanism is useful when exploring the laser machining process. The present study covers the analytical formulation of laser pulse heating process for time exponential decaying and intensity step input pulses. The convergence of both solution is introduced in relation to pulse parameters. The analytical expression for the convergence is omitted due to complex nature of the mathematics involved, however, computational method is attempted to identify the pulse parameter for which both results converge. It is found that the surface temperature profiles obtained for both pulses merge when β approaches to zero. © 1997 Elsevier Science Ltd

Bimodality in Size Distributions: The Red Sea Urchin Strongylocentrotus Franciscanus as an Example

We use a model based on the size—structured von Foerster equation to describe how size—dependent growth and mortality rates, pulsed recruitment, and variability in growth affect the shape of a size distribution. The deterministic, equilibrium size distribution with constant recruitment increases with size when the difference between mortality rate and the rate at which growth rate decreases with size is positive (growth dominated), and decreases when it is negative (mortality dominated). Pulsed recruitment causes modes whose relative amplitudes are indicated by the corresponding constant recruitment case. For typical animal growth patterns, the distance between pulses decreases with age. Pulses merge and can be selectively obscured by variability in growth so that their relative amplitudes no longer correspond to the constant recruitment case. We use this information to evaluate why bimodality occurs in size distributions of the red sea urchin, Strongylocentrotus franciscanus, in some habitats, but not others. The mode at larger sizes, which occurs in all habitats, arises because the distribution is mortality dominated and the final sizes of individuals vary. The upper half of a second mode at smaller sizes is caused by higher mortality rates at sizes greater than the peak of that mode. The lower half may be due either to a refuge from predation under the spine canopy of adults or to sampling selectivity.

Charge-density-wave noise propagation in the blue bronzes Rb0.3MoO3 and K0.3MoO3.

A voltage instability in the charge-density-wave conducting state of ${\mathrm{Rb}}_{0.3}$${\mathrm{MoO}}_{3}$ and ${\mathrm{K}}_{0.3}$${\mathrm{MoO}}_{3}$ is investigated. Voltage pulses are created at a well-defined location within the sample and propagate along the crystal. With increasing field the pulse creation frequency increases, and the pulses merge into a narrow-band oscillation. The field and temperature dependence of characteristic quantities is given. The results are interpreted in the framework of a moving charge-density-wave dislocation model by Lee and Rice. The consequences on the origin of narrow-band voltage noise are also discussed.

Optical switching mechanisms in type IIa diamond

The nanosecond optoelectronic switching characteristics of type IIa diamond illuminated by pulsed UV radiation from an excimer laser at λ=193 nm are reported. With sputtered titanium contacts, certain electro-optical conditions produce a secondary current peak which follows the primary photocurrent pulse. When the bias voltage is increased further (with the laser excitation intensity being kept constant), the primary and secondary current pulses merge together, and their combined peak value is observed to have a supralinear increase with respect to bias voltage. The threshold voltages for secondary pulse formation are also shown to decrease with respect to increasing laser excitation intensities. Secondary current pulse formation and decay are shown to be consistent with a model of space-charge-assisted Fowler–Nordheim electron tunneling across the reversed-biased negative Ti/C electrode.

Pulse Statistics Analysis of Room Acoustics

Many sounds of speech and music more nearly resemble pulsed wave trains than abruptly terminated continuous sounds as used in reverberation measurement. It is therefore not surprising to find that two rooms can differ markedly in acoustical quality even if they appear identical under reverberation analysis which ignores details of short transients. This paper introduces a pulse statistics point of view which takes immediate account of the pulse-like nature of common sounds. Fundamentally, the method consists in examining the response of the room to a short pulse. The walls are replaced by an array of image sources (simple images if the walls are hard, or appropriately modified if there is absorption). These image arrays are then considered statistically. From this approach one can derive such classical quantities as reverberation time and mean free path. One can also analyze the detailed nature of discrete reflections including interference effects, and thus obtain an average correlation between room geometry and the character of its pulse response. Idealized experiments in a hard-walled rectangular room are employed to illustrate the essential features of this approach. A point source emits an exponential damped 3600-c.p.s. wave train of about 2 msec. duration. The received signals are recorded logarithmically on an oscillograph and the system is calibrated for quantitative results. Several dozen discrete reflections can be measured and correlated with calculation. The pulses merge into a more or less continuous background after a time that is calculated and confirmed experimentally. Detailed differences arise according to the positions of the source and microphone in the room.