Pulse Evolution Research Articles

Dual-Wavelength Self-Similar Pulse Fiber Laser Based On Topological Insulator

Multi wavelength ultrashort pulse is of great significance in the scientific field. The dual-wavelength self-similar (SS) pulse mode-locked fiber laser with topological insulator (TI) is studied in this paper. The study on evolution of dual-wavelength SS pulse is based on Ginzburg-Landau equation (GLE) by split-step Fourier transforms (SSFT). The results have a high theoretical significance and value in practice.

Evolution of ion acoustic solitary waves in pulsar wind

ABSTRACT We have studied the evolution of ion acoustic solitary waves (IASWs) in pulsar wind. The pulsar wind is modelled by considering a weakly relativistic unmagnetized collisionless plasma comprised of relativistic ions and superthermal electrons and positrons. Through fluid simulations, we have demonstrated that the localized ion density perturbations generated in the polar wind plasma can evolve the relativistic IASW pulses. It is found that the concentration of positrons, relativistic factor, superthermality of electrons, and positrons have a significant influence on the dynamical evolution of IASW pulses. Our results may provide insight to understand the evolution of IASW pulses and their role in astrophysical plasmas, especially in the relativistic pulsar winds with supernova outflow, which is responsible for the production of superthermal particles and relativistic ions.

Numerical modeling and experimental investigation of ultrafast pulses generation from all-polarization-maintaining dispersion-managed nonlinear polarization evolution Yb-doped fiber laser.

We investigate an all-fiber all-polarization-maintaining dispersion-managed ultrafast fiber laser mode-locked by nonlinear polarization evolution in polarization-maintaining fibers both numerically and experimentally. We find that the laser can operate in different regions among a wide net dispersion, including dispersion-managed solitons, dispersion-managed dissipative solitons, bound state solitons and noise-like pulses. The laser generates the dispersion-managed soliton pulses with a maximum 3 dB bandwidth of 37.84 nm, which can be further compressed to 161.37 fs. Moreover, pulses generation simulation under different net dispersion condition has been carried out. Nonlinear pulse evolution dynamics in laser cavity has been analyzed through numerical simulation as well. The results are basically consistent with the experimental ones.

Joint Effect of the Dipole–Dipole Interaction and Constant Dipole Moment on the Shape of the Field Pulse without an Envelope

An integrable model is proposed for a two-level medium with a constant dipole moment, which describes the evolution of electromagnetic field pulses without an envelope with account for the dipole–dipole interaction. In the nearest neighbor approximation, this interaction is taken into account in the form of quadratic dispersion. It is found that such a generalization of the reduced Maxwell–Bloch equations preserves the complete integrability. It is shown using the obtained exact soliton solutions as examples that the joint effect of quadratic dispersion and the constant dipole moment has a number of unique features and provides new opportunities for controlling the shape of field pulses. In particular, it is found that the shape and amplitude of a field pulse depend on the signs of the dipole–dipole interaction as well as on the sign of the constant dipole moment.

On the Wiener-Hopf solution of water-wave interaction with a submerged elastic or poroelastic plate.

A solution to the problem of water-wave scattering by a semi-infinite submerged thin elastic plate, which is either porous or non-porous, is presented using the Wiener-Hopf technique. The derivation of the Wiener-Hopf equation is rather different from that which is used traditionally in water-waves problems, and it leads to the required equations directly. It is also shown how the solution can be computed straightforwardly using Cauchy-type integrals, which avoids the need to find the roots of the highly non-trivial dispersion equations. We illustrate the method with some numerical computations, focusing on the evolution of an incident wave pulse which illustrates the existence of two transmitted waves in the submerged plate system. The effect of the porosity is studied, and it is shown to influence the shorter-wavelength pulse much more strongly than the longer-wavelength pulse.

Hinge solitons in three-dimensional second-order topological insulators

Higher-order topological insulators have recently witnessed rapid progress in various fields ranging from condensed matter physics to electric circuits. A well-known higher-order state is the second-order topological insulator in three dimensions with gapless states localized on the hinges. A natural question in the context of nonlinearity is whether solitons can exist on the hinges in a second-order topological insulator. Here we theoretically demonstrate the existence of stable solitons localized on the hinges of a second-order topological insulator in three dimensions when nonlinearity is involved. By means of systematic numerical study, we find that the soliton has strong localization in real space and propagates along the hinge unidirectionally without changing its shape. We further construct an electric network to simulate the second-order topological insulator. When a nonlinear inductor is appropriately involved, we find that the system can support a bright soliton for the voltage distribution demonstrated by stable time evolution of a voltage pulse.

Propagation properties of chirped optical similaritons with dual-power law nonlinearity

Abstract We study the transmission of femtosecond light waves in an inhomogeneous dual-power law nonlinear model under the influence of self-frequency shift effect. Pulse evolution in such system is described by a generalized higher-order nonlinear Schrodinger equation incorporating the contributions of distributed self-frequency shift, dispersion, dual-power law nonlinearity, and gain/loss. As a result, a variety of analytical similaritons are derived for the first time by a similarity transformation connected to the related constant-coefficients model. We found that in the presence of self-frequency shift effect, the optical similaritons acquire a nonlinear chirp which has a power-law dependence on the intensity of the pulse. The evolution behaviors of presented chirped similaritons are also presented in two different soliton control systems.

All-normal dispersion fiber laser with a bandwidth tunable fiber-based spectral filter.

We investigate numerically and experimentally an all-fiber, bandwidth tunable spectral filter comprising birefringent fibers. The spectral bandwidth tunability of the filter is based on the compensation of birefringence in polarization maintaining fibers. This unique filter allows mode-locked operation of a fiber oscillator with the ability to generate distinct laser modes with different output spectral shapes and pulse evolutions.

DRTS method for scattered-photon imaging and the importance of directional information

In this study the capabilities of the Dynamic Radiative Transfer System (DRTS) are shown, both for expressing the temporal and spatial behavior of photon propagation, and most importantly for imaging through optically complex turbid media. A light-beam with linear-shaped source profile was employed and pulse durations ranging from the continuous down to the ultra-fast regime were studied. By further applying simple intensity-normalization processing, clean images of the embedded objects can be extracted. The pulse evolution through the medium can be traced and displayed in time-frame sequence, depicting the gradual image formation and fading. It is demonstrated that illumination from scattered photons in the medium is advantageous, yielding clean full-frame images without need for beam scanning. Additionally, image retrieval is significantly enhanced by registering directionally selected photons, revealing 3-D objects embedded deeper in the medium. In contrast with the accepted notion that early photons are the sole means for image formation, DRTS proves that there exists a capability for image formation using the scattered photons. The forward-problem simulation results can be directly applicable in a variety of scientific and technological fields.

Theoretical Investigation of Pulse Temporal Compression by Graphene-Silicon Hybrid Waveguide

By an improved nonlinear Schrodinger optical transmission equation, we theoretically study the pulse evolution dynamics in two-stage temporal compression by a graphene-silicon hybrid waveguide as a phase modulator. Graphene exhibits obvious saturable absorption (SA) but negligible linearly intensity-dependent absorption (IDA) (or two-photon absorption) when the optical intensity in graphene is below the threshold intensity of IDA of graphene. By the utilization of SA and the repression of IDA, for the temporally compressed pulse, the temporal shape can be achieved to be pedestal-free, and the temporal acceleration can be minimized. Meanwhile, the enhancement of linearly intensity-dependent refraction (or Kerr effect) can contribute to the decrement of the size of device and the input power of pulse. This model will be useful to develop graphene-based all-optical modulators in photonic integrated circuits.

Few-cycle solitons in a dispersive medium with a permanent dipole moment.

An integrable model of a two-level medium with a permanent dipole moment (PDM) is proposed. The model describes the evolution of electromagnetic field pulses beyond the slow envelope approximation. The dipole-dipole interaction is taken into account in the approximation of the nearest neighbors in the form of a quadratic dispersion. It is found that such a generalization of the reduced Maxwell-Bloch equations is completely integrable. Breather solutions are derived. These solutions are used to study the combined influence of the quadratic dispersion and PDM. It is found that the model possess a number of unique features, which give possibilities for controlling the shape of field pulses. In particular, it is found that the shape and amplitude of the field pulse depends on both the signs of the dipole-dipole interaction and the sign and value of the permanent dipole moment.

Resistance to high-rate deformation and fracture of lead at normal and elevated temperatures in the sub-microsecond time range

Using measurements of the evolution of shock compression pulses and the rise time of plastic shock waves, the dependences of the yield stress and dynamic tensile strength (spall strength) of lead were determined depending on the strain rate in a range of 104 to 107 s−1 at room temperature and at 280–310 °C. Similar to other metals with a face-centered cubic structure, the dynamic yield stress of lead under these conditions increased abnormally as the temperature increased. At room temperature, the plastic strain rate in the shock wave did not significantly differ from the initial rate in the elastic precursor wave at the same value of the shear stress, but significantly exceeded it at 280 °C.

An improved spin-down rate for the proposed white dwarf pulsar AR scorpii

ABSTRACT We analyse rapid-cadence, multiwavelength photometry of AR Scorpii from three observatories, covering five observing seasons. We measure the arrival times of the system’s beat pulses and use them to compute an updated ephemeris. The white dwarf spin-down rate is estimated with an uncertainty of only 4 per cent. These results confirm, beyond any doubt, that the white dwarf’s spin period is increasing at the rate consistent with by that of Stiller et al. (2018). We study the evolution of the beat pulse’s colour index across the orbit. The colour of the primary pulse maxima varies significantly across the orbit, with the peaks being bluer after superior conjunction than in the first half of the orbit. Specifically, at orbital phase 0.5, the colour index of the primary pulse shows a very sharp discontinuity towards bluer indices. This supports the Potter & Buckley (2018b) synchrotron emission model where the two emitting poles differ significantly in colour. However, no corresponding jump in the colour of the secondary pulses is seen. Furthermore, our analysis reveals that the arrival times of the pulses can differ by as much as 6 s in simultaneous u and r photometry, depending on the binary orbital phase. If left uncorrected, this wavelength-dependent timing offset could lead to erroneous measurements of the spin-period derivative, particularly with heterogeneous data sets.

Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

Numerical analysis of coupled thermo-hydro-mechanical behavior in single- and multi-layer repository concepts for high-level radioactive waste disposal

A multi-layer repository concept is proposed as an alternative to the single-layer repository concept to improve the disposal density of high-level radioactive waste (HLW) in Korea. Numerical simulations are performed to compare the coupled thermo-hydro-mechanical (THM) behavior in the existing Korean reference HLW disposal system (KRS) based on the single-layer repository concept to that in the new multi-layer repository concept. The applicability of the multi-layer repository concept, such as double- and triple-layer repositories, for the disposal of HLW is analyzed with respect to a maximum temperature criterion of 100 °C, evolution of saturation and pore pressure, and mechanical stability. Maximum temperatures are below 100 °C in the double- and triple-layer repository concepts, as in the single-layer reference, although two peaks in simulated temperatures are observed during the evolution of the thermal pulse, resulting from superposition of decay heat at different vertical levels in the multi-layer repository concept. There is no significant difference in the evolution of pore pressure and saturation among the three concepts. The evolutions of mean effective stress at the bentonite blocks and backfill materials are also not significantly different between the single-, double-, and triple-layer repository concepts, because the hydraulic behavior is quite similar, resulting in almost the same change in swelling pressure. Stress states in the rock mass near the tunnels and deposition holes remain below the Mohr-Coulomb failure envelopes at the repositories of 500 m depth in the single-, double-, and triple-layer repository concept simulations. However, the stress states exceed the Mohr-Coulomb failure envelopes at the repositories below 500 m depth (beneath the top layer) in the double- and triple-layer repository concepts. Also, mechanical analysis shows high potential for spalling failure around the deposition holes and tunnel roof in the double- and triple-layer repository concepts. Subsidence in the double- and triple-layer concepts due to the construction of repositories is much deeper than that in the single-layer concept, and uplift induced by heating the rock mass due to the decay heat in the double- and triple-layer concepts is much higher than in the single-layer concept. These results indicate that multi-layer repositories with larger depth intervals between layers are a feasible alternative to the KRS to safely achieve higher disposal density.

Widely tunable femtosecond soliton generation in a fiber-feedback optical parametric oscillator

Optical parametric oscillators (OPOs) are uniquely versatile—albeit complex—platforms for the generation of tunable coherent light in difficult-to-access spectral regions. A synchronously pumped χ ( 2 ) femtosecond OPO producing sub-100-fs pulses, by means of optical soliton formation in a single-mode fiber-feedback cavity, is presented for the first time. This approach removes the requirement for active stabilization and bulk dispersion compensation elements associated with traditional OPOs, while reducing the physical footprint by a factor of 3. Transform-limited pulses of 80–120 fs are generated for both normal and anomalous dispersion, while a new type of spectral sideband formation is observed and studied. Experimental results are supported by detailed simulations based on nonlinear pulse propagation theory, which serve to enrich the understanding of femtosecond pulse evolution in OPOs perturbed by strong Kerr nonlinearity. The OPO provides a unique platform for the study of intracavity soliton phenomena across a wide wavelength range.

Breathing Dynamics in a Gain-Guided Dissipative Soliton-Similariton Fiber Laser

Dispersion distribution and optical gain dynamics introduce vivid pulse evolution in fiber lasers. Here, we report on a multi-output fiber laser mode-locked by nonlinear polarization rotation (NPR) in net normal-dispersion regime. In particular, the asymmetry settings of the laser parameters can introduce the spectral reshaping and pulse breathing process at different output ports. Both the transformation from gain-guided dissipative solitons (GGDSs) to similaritons and periodic stretching and compression of pulse widths are experimentally observed. Additionally, intriguing pulse fusion and splitting of GGDS molecules are recorded, which further validates the pulse breathing process during propagating in the laser cavity. To the best of our knowledge, we first experimentally investigate the pulse self-evolution based on a multi-output fiber laser in GGDS-similariton regime. All these findings can enrich our understanding towards the breathing dynamics of ultrashort pulses.

Properties of Short GRB Pulses in the Fourth BATSE Catalog: Implications for the Structure and Evolution of the Jetted Outflows

Abstract Considering the shortage of comparative studies on pulse temporal characteristics between single-peaked short gamma-ray bursts (sGRBs) and double-peaked sGRBs, we examine the pulse properties of a sample of 100 BATSE sGRBs using the BATSE Time-Tagged Event data with a 5 ms resolution. A total of 243 individual pulses of the single-peaked, double-peaked, and triple-peaked sGRBs are fitted to find statistical properties such as the pulse asymmetry, amplitude, peak time, and pulse width, etc. For the double-peaked sGRBs, according to the overlapping ratio between two adjacent peaks, we first define two kinds of double-peaked sGRBs as M-loose and M-tight types and find that most of the first pulses are similar to the single-peaked ones. We study the dependence of the FWHM on the photon energy among different energy bands. Interestingly, it is found that a power-law relation with an index of −0.4 does exist between the FWHM and the photon energy for the single- and double-peaked sGRBs. More interestingly, we notice that the power-law relation with a positive index also exists for several special short bursts. In view of the three typical timescales of pulses, namely the angular spreading timescale, the dynamic timescale, and the cooling timescale, we propose that the diverse power-law indexes together with the profile evolution of the GRB pulse can be used as an effective probe to diagnose the structure and evolution of the relativistically jetted outflows.

Motion of dispersive shock edges in nonlinear pulse evolution

Motion of dispersive shock edges in nonlinear pulse evolution

Effect of an input beam’s shape and curvature on the nonlinear effects in graded-index fibers

We present a general framework capable of describing the nonlinear propagation of pulsed optical beams of arbitrary shapes and phase fronts inside a graded-index (GRIN) fiber. The main assumption made is that the spatial self-imaging features of the beam are not affected by the temporal evolution of optical pulses. A propagation kernel known from the work done in the 1970s is used to obtain a distance-dependent nonlinear coefficient that captures all spatial effects within an effective nonlinear Schrödinger equation. We consider three specific beam shapes (Gaussian, circular, and square) to study the impact of the shape, position, and curvature of optical beams on the complex spatiotemporal dynamics specific to GRIN fibers. In particular, we focus on the impact of an input beam’s shape on the modulation-instability sidebands and the generation of multiple dispersive waves when higher-order solitons form inside a GRIN fiber. The results of our numerical analysis indicate that for beam widths chosen to yield the same value of the effective mode area at the input end of the fiber, the nonlinear effects are pronounced considerably when a Gaussian beam is launched into the fiber. We also found that even though the self-imaging period is doubled when an off-centered Gaussian beam is launched into a GRIN fiber, it does not affect the nonlinear evolution because the effective beam area still maintains the same periodicity, as long as the shift in the beam’s center is not so large that it does not remain confined to the fiber’s core.