Trailing Edge Of Pulse Research Articles

Hydrazone organics modulation for h-shaped nanosecond pulse generation in 1.3-km ultra-long mode-locked erbium-doped fiber laser

We report an ultra-long erbium-doped fiber laser based on hydrazone organisms that generates a tunable h-shaped pulse at the trailing edge and can readily convert nanosecond pulses into h-shaped pulse. By changing the polarization state in the cavity of the ultra-long laser, the nanosecond pulse can be converted to an h-shaped pulse at a certain power level. At this time, the pump power is increased, ranging from 208.2 mW to 393.2 mW, and the pulse trailing edge can be tuned regularly, with a tuning range from 8.2 ns to 17.9 ns. The experimental results show that the variation of the h-shaped pulse along the width is related to the clamping effect of the peak power and the weak birefringence effect. In conclusion, the investigation of ultra-long laser resonators based on real saturable absorbers is helpful to further explore the anomalous dispersion range of ultra-long round-trip pulses and their potential applications.

Pulse Power Supply With Faster Response and Low Ripple Current Using Inductive Storage and Interleaving Technology

Switched mode pulse power supply is a promising technique for high-power quasi-continuous laser driver. Contrast to lossy linear laser drivers, switched mode laser drivers can achieve higher efficiency. However, many challenges have been proposed, such as fast pulse edge, low current ripple. This paper proposes a multiphase interleaved pulse power supply with energy recovery and inductive storage (MIEF-PPS). The basic concept of the topology is the inclusion of a multiphase converter with pulse forming circuits to the converter system, which decouples the current slew rate and current ripple. Using an inductive storage technology and pulse forming circuits, a shorter pulse current rising time is obtained. The inductor energy is fed back to the input source not discharged to the load, resulting in a fast pulse trailing edge and energy saving. Thus the pulse current response observed when using this proposed technique is found to be much faster when compared to the conventional interleaved buck driver. Moreover, a pre-charge method is proposed to overcome the challenge of digitally controlling the inductive storage. The proposed topology was simulated in MATLAB/Simulink and validated against the experimental results of a laboratory prototype, 360 W dual-interleaved pulsed power supply.

Maximizing energy deposition by shaping few-cycle laser pulses

We experimentally investigate the impact of pulse shape on the dynamics of laser-generated plasma in rare gases. Fast-rising triangular pulses with a slower decay lead to early ionization of the gas and depose energy more efficiently than their temporally reversed counterparts. As a result, in both argon and krypton, the induced shockwave as well as the plasma luminescence are stronger. This is due to an earlier availability of free electrons to undergo inverse Bremsstrahlung on the pulse trailing edge. Our results illustrate the ability of adequately tailored pulse shapes to optimize the energy deposition in gas plasmas.

High-energy sub-phonon lifetime pulse compression by stimulated Brillouin scattering in liquids.

Multi-Joule level stimulated Brillouin scattering (SBS) pulse compression below the acoustic phonon lifetime is demonstrated with a energy-scalable generator-amplifier setup. Single-pass compression of pulses longer than 20τB (τB as phonon lifetime) to as short as 0.5τB with ~100 mJ pulse energy is realized from the generator, by choosing the focusing length to match approximately with the full length at half maximum of the input Gaussian pulses. The interaction length is identified, both experimentally and numerically, as the key parameter in achieving sub-phonon lifetime pulse compression, with its main mechanism being the steepening of the Stokes pulse trailing edge via energy exchange process. After combining the generator with an amplifier that involves only collimated beams and serves as energy booster, the compression of 9 ns, 2 J pulses at 532 nm into 170 ps, 1.3 J per pulse is achieved in water, with very good stability in both pulse energy and duration. This work demonstrates for the first time the efficient high-energy SBS sub-phonon lifetime pulse compression, and paves a way to the reliable generation of sub-200 ps laser pulses with Joule-level energy.

Electrical breakdown of a dielectric on the voltage pulse trailing edge: Investigation in terms of the incubation time concept

The time effects of electrical breakdown at the leading and trailing edges of a voltage pulse applied to an interelectrode gap are studied. The pulsed dielectric strengths of limestone, sandstone, clay, paraffin, and water are determined experimentally at fixed parameters of the voltage pulse and different lengths of the dielectric-filled interelectrode gap. Experimental data are explained in terms of a structure–time approach based on the incubation time criterion. It is found that breakdown occurs when a sufficient power impulse (electric energy) arises within a characteristic time rather than when the electric field reaches a limit.

Breaking of a wake wave excited by a narrow laser pulse in a low-density plasma

The spatial structure of a wake wave excited in a low-density plasma by a laser pulse with a small focal spot radius is studied both analytically and numerically. Numerical study shows that, in a small-amplitude laser field, a wake wave breaks after the formation of an off-axis density maximum, which grows in height away from the pulse to become infinitely high after several periods. Analytical and numerical calculations show that the singularity in the density arises from the intersection of the trajectories of neighboring particles. Numerical simulations demonstrate that, as the laser field amplitude increases, the breaking point of the wake wave rapidly approaches the pulse trailing edge. For weakly nonlinear conditions, an analytic dependence of the coordinate of the breaking point on the amplitude and transverse size of the laser pulse is obtained.

Adjustment of the nanosecond laser pulse duration by using a carbon nanotube suspension

It is shown that the scanning of an optical cell with an aqueous suspension of carbon nanotubes along the optical axis of a focused laser beam leads to a smooth change in the duration of nanosecond laser pulses transmitted through the suspension. Pulses shorten owing to the pulse trailing edge cutoff, when the cell approaches the beam waist. In experiments, pulses of a YAG:Nd3+ laser with passive Q-switching and duration of 16 ns were continuously controlled in the range 16–10 ns.

Controlled Keyhole PAW System with Adjustable Slopes of Pulse-trailing Edge

To make the keyhole status transformation of "open-closure-open" more stable and smooth during keyhole plasma arc welding(PAW),further improvement is made on the previously developed system of pulsed keyhole PAW with square waveform of welding current.The current waveform with two dropping slopes at the pulse trailing-edge is specially designed and employed.Through adjusting the magnitudes of these two dropping slopes,both heat input and plasma force can be controlled more flexibly.Based on the optimization of pulse parameters,the keyhole status transformation from open to closure becomes more smooth.The improved system takes the plasma efflux voltage depicting the keyhole size as the controlled variable,and the pulse current level and its two dropping slopes at the trailing-edge as the controlling variables.The control algorithm PI(proportional and integral) and the strategy of controlled pulse keyholing are employed to ensure the formation of a complete keyhole and maintain the keyhole size close to the preset value.The experimental results show that the system accurately operates in the mode of one keyhole per pulse.Compared to the square waveform,the specially designed current waveform can make the keyhole shift between open and closed states more stable and smooth.Thus,the weld formation quality is fine,and the improved system has significant control effects.

Femtosecond first-order autocorrelation measurement based one-photon induced photocurrent in Si Schottky diodes

Si Schottky diodes promise to provide cheap, reliable, and linear detectors for use in femtosecond and picosecond pulse width measurement. At low pulse excitation density (<100 pJ/cm 2), the pulse width can be deduced from the first-order autocorrelation, and the measured results are in agreement with the theoretical ones. When pulse excitation density is larger than 1 nJ/cm 2, the pulse trailing edge is distorted due to bandgap reduction in semiconductor Si and the pulse width cannot be measured accurately by means of the first-order autocorrelation. The interference fringes presented in the first-order autocorrelation measurement can provide a way of calibrating the delay, and the pulse width can also be obtained by calculating the number of interference fringes.

Superluminal optical pulse propagation in nonlinear coherent media

The propagation of light-pulse with negative group-velocity in a nonlinear medium is studied theoretically. We show that the necessary conditions for these effects to be observable are realized in a three-level $\Lambda$-system interacting with a linearly polarized laser beam in the presence of a static magnetic field. In low power regime, when all other nonlinear processes are negligible, the light-induced Zeeman coherence cancels the resonant absorption of the medium almost completely, but preserves the dispersion anomalous and very high. As a result, a superluminal light pulse propagation can be observed in the sense that the peak of the transmitted pulse exits the medium before the peak of the incident pulse enters. There is no violation of causality and energy conservation. Moreover, the superluminal effects are prominently manifested in the reshaping of pulse, which is caused by the intensity-dependent pulse velocity. Unlike the shock wave formation in a nonlinear medium with normal dispersion, here, the self-steepening of the pulse trailing edge takes place due to the fact that the more intense parts of the pulse travel slower. The predicted effect can be easily observed in the well known schemes employed for studying of nonlinear magneto-optical rotation. The upper bound of sample length is found from the criterion that the pulse self-steepening and group-advance time are observable without pulse distortion caused by the group-velocity dispersion.

Structure of the wake field in plasma channels

An equation is derived that describes the linear response of an underdense inhomogeneous plasma [ω0≫ωp(r), where ω0 and ωp(r) are the laser-carrier and plasma frequencies, respectively] during the propagation of a laser pulse along the axis of a plasma channel with a characteristic width Rch. For a wide channel, i.e., when Rch/λp0&amp;gt;1 (where λp0=2πc/ωp0 is the wavelength of the excited plasma wave and ωp0 is the plasma frequency at the channel axis), the structure of the wake field is studied analytically. It is shown that this structure changes with the distance from the trailing edge of the pulse. As a result, at a certain distance behind the pulse, the fraction of the plasma wave period in which the simultaneous focusing and acceleration of electrons are possible increases by a factor of 2. For a narrow channel (Rch/λp0&amp;lt;1), the structure of the wake field is studied numerically and it is shown that, in this case, the doubling of the phase interval of the wave where the simultaneous focusing and acceleration of electrons are possible also occurs; but, in contrast to a wide channel, a rapid reconstruction of the wake occurs, so that the amplitude of the axial (accelerating) field in the wake decreases while the radial (focusing) field increases with the distance from the pulse trailing edge. The numerical modeling of the laser pulse (90 fs, 2 TW) guiding and the excitation of plasma waves in a narrow plasma channel is carried out and the possibility of reaching GeV energies of accelerated electrons in an experiment is discussed.

A physical model for mode-locking process of colliding pulse mode-locked multiquantum-well semiconductor laser

In this paper, we first present a physical model for mode-locking process of the colliding pulse mode-locked multiquantum-well laser diode (CPM-MQW-LD). The absorption of saturable absorber on pulse leading edge and the scattering of the transient grating generated in absorption and gain regions on pulse trailing edge are considered. As an example, the mode-locking process of a multiquantum-well structure laser and the effect of transient grating on pulse width and peak power are studied. >

Time varyingM 2 inQ-switched lasers

Numerical simulations of the pulse build up in aQ-switched laser reveal a considerable change in the beam spatial profile during the pulse evolution, on a nanosecond time scale. Such changes were measured experimentally, and the measured temporal evolution of the beam profile is shown to be in good agreement with the theoretical results. The time evolution of the spatial profile translates into an evolution of the beam quality parameterM2; for typical experimental conditions, in a Nd:YAG laser utilizing a variable-reflectivity mirror resonator,M2 is very close to a value of 1 at the beginning of the pulse, and increases smoothly throughout the pulse to reach a value of approximately 2 at the pulse trailing edge.

Space–time characteristics of a SPER laser utilizing transitions in Cd, Zn, and In atoms

An experimental investigation was made of the space?time characteristics of lasers utilizing atomic transitions with a segmented plasma source of Cd, Zn, and In metal vapor. It is noted that elongation of the pump pulse trailing edge increased the output energy. Various effects were observed, confirming the recombination mechanism for pumping in an SPER (segmented plasma excitation and recombination) laser utilizing atomic transitions. These included a shift of the lasing range during cooling of the plasma, an increase in the time delay and a shift of the lasing range when the input energy was varied, and quenching of the lasing as a result of additional heating. The feasibility of pulse-periodic operation of an SPER laser was examined. It was established that the limiting pulse repetition frequency may reach 10 kHz.

Dynamic tension in water induced by the reflection of shock waves

Negative pressures were produced in distilled water by reflecting a 70-bar shock pulse off a quasi-free water-Mylar-air interface. The incident pulses, created by symmetric impact of a projectile onto a buffer, had durations of 1–2 μs. A displacement inter-ferometer measured the velocity u of the Mylar. After the pulse's trailing edge reaches the Mylar, u would tend to vanish in the absence of cavitation. If the region of tension cavitates abruptly when a tensile strength S is exceeded, the downward transition in u is terminated. The transition was observed to be terminated when u changed by Δu ≈ −4 m/s. Assuming the flow remains uniaxial, S ≈ −p0 − ρcΔu/2, where ρ, c, and p0 = 1 bar are the water's initial density, sound speed, and pressure [P. L. Marston and G. L. Pullen, Proc. of the 1981 APS Conference on Shock Waves in Condensed Matter, edited by W. J. Nellis (to be published)]. It appears that the Mylar separated from the water during pullback and the data places only a lower limit (28 bars) on S. [Work supported by ONR and the WSU Research and Arts Committee. P. Marston is an Alfred P. Sloan Research Fellow.]