Pulse Evolution Research Articles

The study of morphological evolution, biocorrosion resistance, and bioactivity of pulse electrochemically deposited Hydroxyapatite/ZnO composite on NiTi superelastic alloy

In this work, the effect of zinc oxide nanoparticles on hydroxyapatite deposited on NiTi superelastic alloy was studied. Various concentrations of nanoparticles (200, 400, and 800 mg/L) along with cetrimonium bromide (100 mg/L) as a dispersant agent were added to the coating electrolyte. Morphological observations showed that zinc oxide encouraged hydroxyapatite plate-like crystals to grow in longitudinal and transverse directions, turning into a bone-like structure with greater pores. XRD and FTIR indicated the favorable impact of zinc oxide on the synthesis of pure hydroxyapatite as the most stable calcium phosphate phase. Next, the corrosion evaluations revealed that zinc oxide successfully reduced the corrosion current density of composite coatings down to 1.35 nA/cm2, where pure calcium phosphate coating exhibited 19.05 nA/cm2. Topographic evaluations (AFM) proved a consistent rise in roughness parameters where the roughness average climbed from 208.3 nm for calcium phosphate coating up to 436.45 nm for the composite coating with the highest ZnO concentration. The adhesion strength of composite coatings was above 15 MPa (the critical requirement for orthopedic applications), despite calcium phosphate coating. The wettability results also differed dramatically where calcium phosphate coating exhibited 31.17°, but it dropped to 12.62° for the composite coating formed in the solution containing 400 mg/L ZnO nanoparticles. Ultimately, bioactivity analysis in simulated body fluid revealed that ZnO nanoparticles not only promoted growth but also restricted the size distribution of cauliflower-like apatite grains.

Spatial and temporal evolution of electromagnetic pulses generated at Shenguang-II series laser facilities

The distribution and sources of EMPs produced at Shenguang-II (SG-II) series laser facilities are systematically investigated. The results indicate that the EMP amplitudes in the SG-II ps PW laser are very strong, one order higher than those from the SG-II laser facility. EMPs outside the target chamber decrease exponentially with the distance from the measuring points to the target chamber center at the two laser facilities. Moreover, EMPs can be remarkably reduced when the picosecond laser together with the nanosecond laser is incident to targets compared to the SG-II ps PW laser alone. The resulting conclusions are expected to offer experimental supports for further effective EMPs shielding design and achievement in high-power laser facilities.

The Initial–Final Mass Relation for Hydrogen-deficient White Dwarfs*

Abstract The initial–final mass relation represents the total mass lost by a star during the entirety of its evolution from the zero age main sequence to the white-dwarf cooling track. The semiempirical initial–final mass relation (IFMR) is largely based on observations of DA white dwarfs, the most common spectral type of white dwarf and the simplest atmosphere to model. We present a first derivation of the semiempirical IFMR for hydrogen-deficient (non-DA) white dwarfs in open star clusters. We identify a possible discrepancy between the DA and non-DA IFMRs, with non-DA white dwarfs ≈0.07 M ⊙ less massive at a given initial mass. Such a discrepancy is unexpected based on theoretical models of non-DA formation and observations of field white dwarf mass distributions. If real, the discrepancy is likely due to enhanced mass loss during the final thermal pulse and renewed post-AGB evolution of the star. However, we are dubious that the mass discrepancy is physical and instead is due to the small sample size, to systematic issues in model atmospheres of non-DAs, and to the uncertain evolutionary history of Procyon B (spectral type DQZ). A significantly larger sample size is needed to test these assertions. In addition, we also present Monte Carlo models of the correlated errors for DA and non-DA white dwarfs in the initial–final mass plane. We find the uncertainties in initial–final mass determinations for individual white dwarfs can be significantly asymmetric, but the recovered functional form of the IFMR is grossly unaffected by the correlated errors.

Pulse evolution and multi-pulse state of coherently coupled polarization domain walls in a fiber ring laser.

Pulse evolution and multi-pulse state of coherently coupled polarization domain walls (PDW) is experimentally demonstrated in a novel fiber ring laser. Versatile pulse shapes benefit by wide range moving of PDW in the weakly birefringent fiber. The 8.6 m short-cavity structure is more compact and accessible based on a 976 nm pump with nearly zero negative dispersion (-0.0002 ps2). Besides, multi-pulse patterns such as PDW splitting, harmonic mode-locking, and periodic soliton collision are also observed under larger net negative dispersion (-3.09 ps2) and 151m-longer cavity. This is the first demonstration of coherently coupled PDW in a fiber laser using a bandpass filter and the formation of coherently coupled PDW is ascribed to the BPF's force filtering.

Inverse Design of Equivalent-Graded-Index Photonic-Crystal Fiber Based on Empirical Dispersion Formula

Equivalent-graded-index photonic-crystal fiber (EGI-PCF) is a modified photonic-crystal fiber (PCF) where the cladding is not required to be periodic. A radial gradient of the equivalent refractive index distribution provides extra freedom to efficiently tailor the dispersion of EGI-PCF. However, such additional complexity makes the manual fiber design much more challenging where the time consumption becomes too much to be afforded eventually. In this paper, we for the first time propose the method of computer-aided inverse fiber design (CAFD) based on empirical dispersion formula to circumvent this hurdle. We apply the differential evolution (DE) genetic algorithm to seek for the optimized EGI-PCF design with a target of specific dispersion profile. In the dispersion oriented EGI-PCF design, the use of empirical formula instead of numerical simulation (e.g., finite-element method) is proved to greatly reduce the run-time of CAFD and simplifies the fitness function evaluation as much as possible. We adopt our method and demonstrate a design of EGI-PCF with a near-zero ultra-flattened dispersion in the whole S+C+L band, which merely costs a few seconds of a laptop to return an optimized fiber design. Thus improvement of efficiency is about three orders of magnitude faster than any previous CAFD attempts as far as we learn. In the end, we use our method to explore an EGI-PCF design with a unique dispersion profile of all-normal dispersion-decreasing along the fiber length centered around 1 μm that plays a key role for the self-similar pulse evolution in a fiber laser/amplifier.

Q-switching bifurcation dynamics of passively mode-locked lasers.

We model Q-switched pulses in passively mode-locked lasers using the cubic-quintic complex Ginzburg-Landau equation (CGLE). We show that a wide set of parameters of this equation leads to Q-switched pulses of triangular shape that consist of a periodic sequence of evolving dissipative solitons. Bifurcation diagrams demonstrating various transformations of these pulses are calculated in terms of five major parameters of the CGLE. The diagrams show period doubling transformations as well as the transition to a chaotic evolution of the Q-switched pulses.

Electron bunch charge enhancement in laser wakefield acceleration using a flattened Gaussian laser pulse

We demonstrate the charge enhancement of the electron bunch using a flattened Gaussian laser pulse in laser wakefield accelerations. In laser wakefield accelerations, the change in laser pulse shape affects the diffraction pattern, resulting in laser intensity variation during plasma interaction. We employ a flattened Gaussian laser pulse to excite wakefields for electron acceleration. The central flattened region of the flattened Gaussian laser pulse, having the same energy as the Gaussian pulse, helps to enhance the pulse intensity via self-focusing. Thus, the evolution of flat Gaussian pulse enhances self-injection, which leads a considerable amount of electrons into the accelerating fields. A linear theoretical model is adopted to understand the wakefield generation mechanism using Gaussian and flattened Gaussian pulses. In order to broaden our understanding, quasi-three-dimensional PIC simulations are also performed. Our results show that a flattened Gaussian laser pulse with N=2 (where N is the order of the super Gaussian shape) yields a good quality electron bunch with retaining a relatively higher charge (about 300 pC) with respect to the case of a Gaussian laser pulse. We also show a twofold increase in bunch peak current when a flattened Gaussian pulse is employed. The electron bunches with high charge and high current may be promising candidates to drive next-generation compact free-electron lasers with unique X-ray properties.

Dynamics of Plane Waves in the Fractional Nonlinear Schrödinger Equation with Long-Range Dispersion

We analytically and numerically investigate the stability and dynamics of the plane wave solutions of the fractional nonlinear Schrödinger (NLS) equation, where the long-range dispersion is described by the fractional Laplacian (−Δ)α/2. The linear stability analysis shows that plane wave solutions in the defocusing NLS are always stable if the power α∈[1,2] but unstable for α∈(0,1). In the focusing case, they can be linearly unstable for any α∈(0,2]. We then apply the split-step Fourier spectral (SSFS) method to simulate the nonlinear stage of the plane waves dynamics. In agreement with earlier studies of solitary wave solutions of the fractional focusing NLS, we find that as α∈(1,2] decreases, the solution evolves towards an increasingly localized pulse existing on the background of a “sea” of small-amplitude dispersive waves. Such a highly localized pulse has a broad spectrum, most of whose modes are excited in the nonlinear stage of the pulse evolution and are not predicted by the linear stability analysis. For α≤1, we always find the solution to undergo collapse. We also show, for the first time to our knowledge, that for initial conditions with nonzero group velocities (traveling plane waves), an onset of collapse is delayed compared to that for a standing plane wave initial condition. For defocusing fractional NLS, even though we find traveling plane waves to be linearly unstable for α<1, we have never observed collapse. As a by-product of our numerical studies, we derive a stability condition on the time step of the SSFS to guarantee that this method is free from numerical instabilities.

Observation of Ultrashort Laser Pulse Evolution in a Silicon Photonic Crystal Waveguide

Using the sum frequency generation cross-correlation frequency-resolved optical gating (SFG-XFROG) measurement setup, we observed the soliton evolution of low energy pulse in an Si photonic crystal waveguide, and it exhibited the pulse broadening, blue shift, and evident pulse acceleration. The soliton evolution was also investigated by nonlinear Schrödinger equation (NLSE) modelling simulation, and the simulated results agreed well with the experimental measurements. The effects of waveguide length on the pulse evolution were analyzed; the results showed that the pulse width changed periodically with increasing waveguide length. The results further the understanding of the ultra-fast nonlinear dynamics of solitons in silicon waveguides, and are helpful to soliton-based functional elements on CMOS-compatible platforms.

Ultranarrow bandwidth pulses from a regeneratively mode-locked fiber laser.

We report on the generation of transform-limited nanosecond pulse with an ultranarrow bandwidth from a regeneratively mode-locked erbium-doped fiber laser. A narrow bandwidth fiber Bragg grating is combined with a bulk amplitude electro-optic modulator to shape pulse evolution inside a ring cavity, and regenerative mode locking is applied to produce a stationary shape of pulses in the nanosecond regime (2.05 ns in duration). Spectral characterization via high bandwidth optoelectronic devices shows that optical pulses have an ultranarrow bandwidth of 220 MHz. Numerical simulation reveals that the shape of the narrow spectral filter has a strong effect on the duration and bandwidth of output pulses.

Study of the influence of SESAM parameters on the evolution of mode-locked pulses at different repetition rates

Study of the influence of SESAM parameters on the evolution of mode-locked pulses at different repetition rates

Anomalous reflection and transmission of surface acoustic waves at a crystal edge via coupling to leaky wedge waves

Surface acoustic waves are propagated toward the edge of an anisotropic elastic medium (a silicon crystal), which supports leaky waves with a high degree of localization at the tip of the edge. At an angle of incidence corresponding to phase matching with this leaky wedge wave, a sharp peak in the reflection coefficient of the surface wave was found. This anomalous reflection is associated with efficient excitation of the leaky wedge wave. In laser ultrasound experiments, surface acoustic wave pulses were excited and their reflection from the edge of the sample and their partial conversion into leaky wedge wave pulses was observed by optical probe-beam deflection. The reflection scenario and the pulse shapes of the surface and wedge-localized guided waves, including the evolution of the acoustic pulse traveling along the edge, have been confirmed in detail by numerical simulations.

Evolution of noise-like pulses in mode-locked fiber laser based on straight graded-index multimode fiber structure

Graded-index multimode fiber structure is of interest for implementing all-fiber passively mode-locked laser via nonlinear mode-coupling. We experimentally report the noise-like pulses generation in an all-normal-dispersion Yb-doped fiber laser based on straight graded-index multimode fiber structure. The multimode fiber is precisely cleaved under a microscopic system and fixed in a straight glass tube. Using 108.04 mm-length multimode fiber as saturable absorber, the output noise-like pulses have a central wavelength of 1065.25 nm, fundamental repetition rate of 19.58 MHz, pedestal pulse duration from 2.15 ns to 9.40 ns and peak width from 3.97 ps to 0.43 ps with different pump power. In particular, the optical spectrum evolution of noise-like pulses is associated with the spectral filtering of straight multimode fiber structure. Furthermore, the influence of total cavity dispersion on noise-like pulses are investigated systematically through adding different lengths of HI1060 fiber in the laser cavity. The evolution of noise-like pulses in the easy fabrication and robust all-fiber laser will facilitate potential contribution to the mechanism of noise-like pulses in passively mode-locked fiber laser.

Stochastic heating of electrons due to Raman backscatter radiations in interaction of intense laser pulse with nitrogen atoms

In this paper, simulation study of electron stochastic heating arising from the Raman backscatter radiations during the interaction of the laser pulse with the nitrogen atoms is presented by use of a massively parallel particle-in-cell code. For this purpose, the self-consistent evolutions of the laser pulse via the time–space Fourier transforms of transvers vector potential are investigated at the different times of propagation. It is shown that since the ionization has effect on the emission of the Raman backscattered radiation; it noticeably contributes on the stochastic heating threshold of the electrons. According to our results, it has been found that, when there is the long rise time laser pulse (here 100 fs), the Raman backscattered radiations are seeded by a strong initial noise at the earlier times. Therefore, by considering the ionization, the necessary condition for chaos threshold is met sooner, which, in turn, causes the electron stochastic heating start quickly compared to the case the laser pulse is propagated in the pre-plasma. As a result, in agreement with chaotic nature of the motion, the electrons gain more energy through the stochastic mechanism in the field-ionized plasma.

Mixed Properties of Slow Magnetoacoustic and Entropy Waves in a Plasma with Heating/Cooling Misbalance

The processes of coronal plasma heating and cooling were previously shown to significantly affect the dynamics of slow magnetoacoustic (MA) waves, causing amplification or attenuation, and also dispersion. However, the entropy mode is also excited in such a thermodynamically active plasma and is affected by the heating/cooling misbalance too. This mode is usually associated with the phenomenon of coronal rain and formation of prominences. Unlike adiabatic plasmas, the properties and evolution of slow MA and entropy waves in continuously heated and cooling plasmas get mixed. Different regimes of the misbalance lead to a variety of scenarios for the initial perturbation to evolve. In order to describe properties and evolution of slow MA and entropy waves in various regimes of the misbalance, we obtained an exact analytical solution of the linear evolutionary equation. Using the characteristic timescales and the obtained exact solution, we identified regimes with qualitatively different behaviour of slow MA and entropy modes. For some of those regimes, the spatio-temporal evolution of the initial Gaussian pulse is shown. In particular, it is shown that slow MA modes may have a range of non-propagating harmonics. In this regime, perturbations caused by slow MA and entropy modes in a low- $\beta $ plasma would look identical in observations, as non-propagating disturbances of the plasma density (and temperature) either growing or decaying with time. We also showed that the partition of the initial energy between slow MA and entropy modes depends on the properties of the heating and cooling processes involved. The exact analytical solution obtained could be further applied to the interpretation of observations and results of numerical modelling of slow MA waves in the corona and the formation and evolution of coronal rain.

The effects of laser polarization and wavelength on injection dynamics of a laser wakefield accelerator

Here, we investigate the effects of laser polarization and wavelength on electron injection dynamics in a laser wakefield accelerator. During the ionization process, electrons gain residual momentum and kinetic energy via above threshold ionization, which has a strong dependence on laser polarization. A circularly polarized laser pulse results in a much higher residual momentum and kinetic energy gain for the ionized electrons compared with the linearly polarized case. This residual momentum results in particle injection because of the sensitivity of particle trapping to the initial conditions and enhanced the total injected beam charge in both experiments and particle-in-cell simulations. Due to the strong correlation of above threshold ionization with laser wavelength, in this work we extended the investigation to long wavelength (up to 20 μm) drive pulses using particle-in-cell simulations. Owing to the gain in kinetic energy, it may be expected that the charge trapped would consistently increase for circular polarization with increasing laser wavelength, but this was not observed. Instead, there are oscillations with wavelength in the relative trapped charge between linear and circular polarization cases, which arise because of ionization and heating effects on the plasma. Our studies highlight the complex interplay between several different physical effects, including injection regimes—above threshold ionization assisted injection, wave-breaking injection by carrier-envelope-phase effects and ionization injection—ionization gradient induced laser pulse evolution, and thermal modifications to the wake structure that need considering when extrapolating laser wakefield acceleration to different wavelength regimes.

Hybrid modeling approach for mode-locked laser diodes with cavity dispersion and nonlinearity

Semiconductor-based mode-locked lasers, integrated sources enabling the generation of coherent ultra-short optical pulses, are important for a wide range of applications, including datacom, optical ranging and spectroscopy. As their performance remains largely unpredictable due to the lack of commercial design tools and the poorly understood mode-locking dynamics, significant research has focused on their modeling. In recent years, traveling-wave models have been favored because they can efficiently incorporate the rich semiconductor physics of the laser. However, thus far such models struggle to include nonlinear and dispersive effects of an extended passive laser cavity, which can play an important role for the temporal and spectral pulse evolution and stability. To overcome these challenges, we developed a hybrid modeling strategy by unifying the traveling-wave modeling technique for the semiconductor laser sections with a split-step Fourier method for the extended passive laser cavity. This paper presents the hybrid modeling concept and exemplifies for the first time the significance of the third order nonlinearity and dispersion of the extended cavity for a 2.6 GHz III–V-on-Silicon mode-locked laser. This modeling approach allows to include a wide range of physical phenomena with low computational complexity, enabling the exploration of novel operating regimes such as chip-scale soliton mode-locking.

Evolutions of Q-switched mode-locked square noise-like pulse with different cavity lengths.

A $Q$-switched mode-locked square noise-like pulse (QMLSNLP) is generated in a nonlinear polarization rotation passively mode-locked fiber laser. When the pump power changes from 154 mW to 414 mW, the frequency of the $Q$-switched envelope varies from 21.7 kHz to 38.9 kHz, while the duration of the $Q$-switched envelope decreases from $5.1\,\,{\unicode{x00B5}\rm s}$ to $3.2{\unicode{x00B5}\rm s}$, correspondingly. In the meantime, QMLSNLP keeps the fundamental repetition rate constant, and the pulse duration increases from 3.4 ns to 6.7 ns. By inserting different lengths of single-mode fiber into the ring cavity, the evolutions of QMLSNLP are measured and analyzed. According to the cavity parameters and experimental results, impacts of the cavity length on QMLSNLP are investigated in detail.

Compact, All-PM Fiber Integrated and Alignment-Free Ultrafast Yb:Fiber NALM Laser With Sub-Femtosecond Timing Jitter

We report a simple and compact design of a dispersion compensated mode-locked Yb:fiber oscillator based on a nonlinear amplifying loop mirror (NALM). The fully polarization maintaining (PM) fiber integrated laser features a chirped fiber Bragg grating (CFBG) for dispersion compensation and a fiber integrated compact non-reciprocal phase bias device, which is alignment-free. The main design parameters were determined by numerically simulating the pulse evolution in the oscillator and by analyzing their impact on the laser performance. Experimentally, we achieved an 88 fs compressed pulse duration with sub-fs timing jitter at 54 MHz repetition rate and 51 mW of output power with 5.5 * 10-5 [20 Hz, 1 MHz] integrated relative intensity noise (RIN). Furthermore, we demonstrate tight phase-locking of the laser's carrier-envelope offset frequency (fceo) to a stable radio frequency (RF) reference and of one frequency comb tooth to a stable optical reference at 291 THz.

Propagation of Terahertz Pulses in Photonic Crystal-Based Rib Silicon Waveguides

Propagation of terahertz (THz) waves in silicon waveguides is of special interest for THz applications such as optical integrated circuits. In this paper, a photonic crystal-based rib silicon waveguide with a specific structure is designed for the first time which can be utilized for applications in THz region (including wavelength range of 30-35 μm). The dispersion curve and effective index are simulated over 30–35 μm for different rib heights. In addition, the dispersion coefficients up to the third order as well as the nonlinear parameter are calculated at the center wavelength of 33 μm from which the pulse evolution along the waveguide is simulated via solving the nonlinear Schrodinger equation (NLSE). The split-step Fourier (SSF) method is used to numerically solve the NLSE and then the pulse spectra at the input and output of the waveguide are simulated and compared to each other. Also, the effect of different parameters including rib height, pulse width, chirp and peak power on the THz pulse propagation is investigated. The results show that for lengths less than 1 cm, the pulse propagates almost without distortion, while for longer lengths, the shape and spectrum of the pulse are changed.