Velocity Mismatch Research Articles

An in-band diode-end-pumped high-power and high-efficiency ultrashort pulse Nd:YVO4 bulk laser mode-locked by a frequency doubling LBO crystal

Nonlinear mirror (NLM) mode locking has also been verified to be an efficient technology for high-power ultrashort pulse laser generation. In this paper, we report on high-power and high-efficiency NLM mode-locked Nd:YVO4 laser with a 880-nm diode laser for in-band pumping, which has very rarely been investigated in the past. The achieved maximum average output power reached 6.76 W at an absorbed power of 13.82 W corresponding to a slope efficiency of 54.5%. Thanks to the used type I LBO crystal with small group velocity mismatch, the achieved pulse duration was as short as 6.5 ps. The results represent one of the best performances that ever achieved for NLM mode-locked Nd:YVO4 laser. NLM mode locking could be very promising for high-power ultrashort pulse generation in visible and middle infrared wavelengths, at which SESAMs are still not mature and the average output powers of ultrafast laser oscillators are also still limited to great extent.

Smooth and continuous interplanetary trajectory design of spacecraft using iterative patched-conic method

This paper is concerned with the transfer trajectory design among multiple planets using multiple flybys. The proposed trajectory design method consists of a semi-analytical tuning strategy and iterative patched-conic method. The tuning strategy refers to a method realizing the smooth connection between a hyperbolic trajectory near a planet and a trajectory obtained by Lambert's problem between the planet and another one. The proposed method iteratively solves Lambert's problem and the tuning strategy so that the arrival or departure velocity obtained by Lambert's problem coincides with the velocity at the corresponding point in the sphere of influence (SOI), assigned by the tuning strategy. Consequently, a smooth and continuous trajectory with multiple flybys can be obtained, where the SOI of each flyby planet and the flyby time are explicitly taken into account. Moreover, analytical conditions with respect to the orbital elements of the arrival and departure hyperbolas are provided to minimize velocity mismatch at the periapsis of a flyby planet. Through numerical simulation using the Cassini example, it is shown that the resultant trajectory by the proposed method is close to the optimal trajectory obtained by the n-body problem. Thus, a suitable approximate trajectory can be effectively generated without solving the n-body problem.

Methods for online measurement and control of section deviations during hot rolling of wire rod and bars

In the joint project PIREF, the metal forming group of the University of Duisburg-Essen has collaborated with the University of Applied Sciences Ruhr-West Mülheim (Ruhr), the University of Siegen, EMG Automation GmbH and SMS group GmbH to develop sensors, for an online measurement of material velocity and cross section as well as control models for the rolling process of wire rod and bars. The University of Duisburg-Essen provided a metal forming process model for the rolling process to assess the influencing parameters on the rolled section precision. A technique was found to segregate height- from width- influencing parameters from a measured cross-sectional area and actual roll gap. With this measuring technology and with help of the process model, rules for control of the rolling process to achieve close tolerances were obtained. The modelling was accompanied by rolling trials on a laboratory rolling mill at the University of Duisburg-Essen, where a typical Round-OvalRound pass sequence was used for validation of the rolling model concerning lateral spread, inlet and outlet velocity as well as rolling force and torque calculation. The present paper shows how the material flow and the distribution of the velocity in the roll gap can be described. In subsequent rolling of bar and rod in a continuous rolling mill the dimensions can be influenced by application of longitudinal stresses and screwdown. The application of stress can be achieved by an inter-stand velocity mismatch. With the developed models the necessary velocity mismatch can be calculated.

Deterministic generation of parametrically driven dissipative Kerr soliton

Abstract We theoretically study the nature of parametrically driven dissipative Kerr soliton (PD-DKS) in a doubly resonant degenerate micro-optical parametric oscillator (DR-DμOPO) with the cooperation of χ (2) and χ (3) nonlinearities. Lifting the assumption of close-to-zero group velocity mismatch (GVM) that requires extensive dispersion engineering, we show that there is a threshold GVM above which single PD-DKS in DR-DμOPO can be generated deterministically. We find that the exact PD-DKS generation dynamics can be divided into two distinctive regimes depending on the phase matching condition. In both regimes, the perturbative effective third-order nonlinearity resulting from the cascaded quadratic process is responsible for the soliton annihilation and the deterministic single PD-DKS generation. We also develop the experimental design guidelines for accessing such deterministic single PD-DKS state. The working principle can be applied to different material platforms as a competitive ultrashort pulse and broadband frequency comb source architecture at the mid-infrared spectral range.

Stability of moving Bragg solitons in a semilinear coupled system with cubic–quintic nonlinearity

We investigate the photonic bandgap structure, soliton properties and their stability in a semilinear coupled system where one core has cubic–quintic nonlinearity and is equipped with a Bragg grating, while the other one is uniform and linear. The investigation reveals that three distinct bandgaps, namely the upper, lower and central gaps, exist in the model. It is shown that the model supports two disjoint soliton families which are designated as Type 1 and Type 2 solitons. The soliton families exist only in the upper and lower bandgaps and are separated by a boundary at which no soliton solutions exist. A systematic numerical stability analysis is performed. It is found that vast regions in both the upper and lower bandgaps exist where Type 1 solitons are stable. However, Type 2 solitons are found to be always unstable. The effects of system parameters such as group velocity mismatch between the cores, velocity of solitons, and the coupling coefficient on the stability of solitons are analysed.

Classical nonlinear simulation of low-order modes of Lamb waves in plate

For the classical nonlinear research of low-order modes of Lamb waves, this paper firstly introduces the classical nonlinearity derived from the intrinsic nonlinear induced low-order Lamb waves (S0 and A0 modes). Theoretical and numerical calculations are studied in two aspects. The influence of nonlinear effects on the nonlinear effects of classical nonlinear low-order Lamb waves and the cumulative growth effect are analyzed by finite element simulation. The results show that the nonlinear effect produced by the superelastic material model is greater than the geometric nonlinearity, and the linear elastic material model does not produce nonlinear effects. In addition, as the third-order elasticity increases in the material, the amplitude of the second harmonic gradually increases. The second harmonic generated by the A0 mode with phase velocity mismatch is the S0 mode. It can be seen that the group velocity matching is not a necessary condition for generating the second harmonic. Since the phase velocity matching is not satisfied, there is no cumulative growth effect; The higher the S0 mode phase velocity matching degree, the more obvious the cumulative growth effect, and the second harmonic is the S0 mode.

Self-referenced frequency comb from a polarization-maintaining Er: Fiber laser based nonlinear polarization evolution

To the best of our knowledge, this paper presents the first self-referenced polarization-maintaining (PM) Er: nonlinear polarization evolution-based fiber frequency comb. A short PM fiber section segment acts as an artificial saturable absorber and compensates for the group velocity mismatch. A conventional f-2f interferometer allows detecting carrier-envelope offsetting frequency (fceo) after an octave-spanning spectrum generation. A complete description of the PM Er: fiber comb combining a noise study of the repetition rate (fr) and fceo is presented. Accordingly, the in-loop residual phase noise of fceo and fr are 468 mrad and 0.336 mrad from 1 Hz to 3 MHz, respectively. Further, we verify the long-term performance of the comb system by recording the fluctuation of fr and fceo for more than 10 h. We observed that combining fast actuators for phase noise suppression would potentially enable a compact and robust frequency comb system for a range of applications such as comb-based liars, optical clock, and gas sensing.

Valley to charge current conversion in graphene linear defects

The conduction properties of a grain boundary junction with Fermi velocity mismatch are analyzed. We provide a generalization of the Dirac Hamiltonian model taking into account the Fermi velocity gradient at the interface. General boundary conditions for the scattering problem are derived within the framework of the matching matrix method. We show that the scattering properties of the interface, as predicted by the theory, strongly depend on the boundary conditions used. We demonstrate that when the valley degeneracy is broken a charge current is established at the grain boundary interface. These findings provide the working principle of a valley to charge current converter, which is relevant for the emergent field of valleytronics.

Generation of H-shaped pulse rains induced by intracavity Fabry–Perót filtering in a fiber laser

We report the generation of h-shaped pulses and rains in a fiber laser with a Fabry–Perót (F–P) subcavity. An h-shaped pulse state composed of a sharp spike as a leading edge and a long flat-top bunch as a tailing one could be switched to an h-shaped pulse rain state by adjusting the polarization controllers (PCs). F–P filtering was found to play a crucial role in the formation of the h-shaped pulses and rains. The leading and tailing edges of the h-shaped pulse were found to be generated in the primary and secondary F–P filtering profiles, respectively. Furthermore, small discrete pulses could be triggered in the subordinate longer-wavelength F–P filtering profiles by adjusting the PCs. Then, for the first time, h-shaped pulse rains were experimentally observed as many small pulses continuously drifting away from the nanosecond-long h-shaped pulses because of their group velocity mismatch. These results will help contribute to a deep understanding of the dynamics of complex pulse patterns in fiber lasers.

On traveling wave solutions: the decoupled nonlinear Schrödinger equations with inter modal dispersion

In this article, the decoupled nonlinear Schrdingers equations have been considered that describe the model of dual-core fibers with group velocity mismatch, group velocity dispersion, and spatio-temporal dispersion. These equations are analyzed using two different integrations schemes, namely, extended tanh-function and sinecosine schemes. The different kind of traveling wave solutions: solitary, topological, periodic and rational, fall out as by-product of these schemes. Finally, the existence of the solutions for the constraint conditions is also shown.

Theory of charge and spin transport across magnetic tunnel junction with bilayer insulator tunnel barriers

Abstract We theoretically studied the tunneling effect on the transport of charge and spin across the magnetic tunnel junction of two ferromagnetic electrodes with parallel and anti-parallel alignments of the magnetization directions separated by two different insulating layers (FM/I1/I2/FM) using the scattering method under the free electron approximation model in a one-dimensional system of the junctions. The main area of interest is to focus on how to perform either a direct transmission or oscillation behavior for the conductance spectra under the condition of a change in the three different types of model parameters: the insulating thickness of the first (L1) and second (L2) insulator layers, the electron effective mass mismatch, and the potential barrier height. In the calculation process, we try to set up all parameters in the order of real values from the real experiments. It was found that, for the effect of the insulating thickness, the junction can perform a direct transmission when L2 is not thicker than 2L1, besides the oscillation occurs. Generally, for the effect of the effective mass or velocity mismatch, an increase in the electron effective mass of the insulator materials causes the oscillation behavior to increase, which is due to a reduction in the velocity. Interestingly, in this case we can also provide a condition to obtain a direct transmission, and to generate a low or high magnetoresistance of the junction. Finally, for the effect of the potential barrier height of the first (U1) and second (U2) insulator layers, a direct transmission can occur when U2 is not larger than 4U1, as consistent with the experimental result.

Current propagation behaviors and spin filtering effects in three-terminal topological-insulator junctions

Understand the spin and current behavior in the topological-insulator (TI) system is crucial for the design of electronic and spintronic devices. To study these behaviors, some three-terminal hetero-junction TI systems made from the zigzag silicene-like nanoribbons (ZSiNRs) are investigated. Different external fields are applied in the leads and conductor regions, which result in the expected topological edge states. By calculating the local current distribution, we find two important characteristics of topological edge current propagating in these systems. Firstly, the topological edge current prefers to flow into the nearest channel, rather than the other faraway channels. Secondly, the group-velocity mismatch as a scattering engineering has a significant effect on current propagation behavior. In other words, one can manipulate the topological edge current by velocity mismatch. Based on these characteristics, a type of three-terminal spin filter is proposed in this paper. Moreover, some interesting transport phenomena such as space-separated Fano-resonance are also discovered in the TI-junction systems.

Formation control of mobile robot systems incorporating primal-dual neural network and distributed predictive approach

This paper addresses the formation problem for multiple mobile robots with velocity mismatch and system constraints by a distributed model predictive control (DMPC) strategy and a modified virtual structure method. Firstly, a desired virtual structure is employed to generate a set of reference paths for the formation robots. By including approaching angle and path parameter synchronization constraints into cost function, a Nash-based DMPC strategy is presented, where a velocity integral controller is developed to solve the velocity mismatch. Further, consider state and input constraints, the distributed optimization problem is rewritten as a constrained quadratic programming (QP) problem. A PDNN is used to obtain the optimal control input increments, and the stability of the proposed algorithm is analyzed. Moreover, the dynamic formation control is achieved by a modified virtual structure method, and simulation examples are given to verified the effectiveness of the proposed strategies.

Target trajectory estimation by unambiguous phase differences from a single fixed passive sensor

In this Letter, considering the configuration of small antenna array aperture used for special usage, the authors proposed a target trajectory estimation method by unambiguous phase differences with prior knowledge on target velocity from a single fixed passive sensor. First, they developed the maximum likelihood estimator of the target trajectory parameters based on unambiguous phase differences and then deduce the corresponding Cramer–Rao lower bound. Furthermore, in terms of the error in prior knowledge on target velocity, the influence of target velocity mismatch on trajectory parameters estimation is analysed. Finally, they evaluated the performance of the proposed method and validated the correctness of their analysis via numerical simulation.

Quantum analysis of a traveling-wave electro-optic phase modulator in the presence of the phase noise from a radio frequency oscillator and a laser

Quantum analysis has been done for the traveling-wave (TW) electro-optic phase modulator in the presence of the phase noise of a radio frequency (RF) oscillator and a laser source, in which the Hamiltonian for an active section of the phase modulator system is derived. The effect of RF phase noise from the oscillator and a phase velocity mismatch (PVM) between optical photons and the RF field are included in the Hamiltonian. The time evolution of the field operator of the laser in the TW electro-optic phase modulator is derived by the Heisenberg equation of motion. The action of a TW electro-optic phase modulator in the presence of the two distinct weak noises—classical phase noise because of RF and quantum phase noise caused by a laser—has been evaluated and explained in terms of the second-order temporal correlation function and power spectrum. The power spectrum has been written as a function of the semiconductor laser linewidth, an enhancement factor in the laser linewidth, and the RF oscillator linewidth. The results are applied to analyze the basic model of a frequency-coded quantum key distribution (FC-QKD) system. The quantum bit error rate (QBER), the first-order optical intensity correlation function, and the spectrum of intensity fluctuation have been calculated for the partially coherent regime to evaluate the performance of the FC-QKD system.

Reflection and Refraction of an Airy Pulse at a Moving Temporal Boundary

AbstractReflection and refraction of Airy pulses at a moving step refractive‐index boundary are investigated. A comb‐like reflected spectrum and an Airy‐like transmitted spectrum are formed as an Airy pulse crosses a temporal boundary without initial group velocity mismatch or initial central frequency shift. The reflected and refracted pulses are significantly influenced by the location of temporal boundary and time‐dependent refractive index change. The energy of each lobe of the comb‐like reflected spectra can be controlled by changing the value of decay factor of the Airy pulse. In the case of an Airy pulse with an initial launch frequency shift, the reflected and refracted frequency shifts depend on the dispersion relation of the medium. The results can be used in potential applications involving optical manipulation.

Symbiotic quadratic soliton mode-locked non-degenerate optical parametric oscillators.

We analytically and numerically unveil the existence condition of symbiotic solitons in doubly resonant non-degenerate optical parametric oscillators. Resonant signal and idler with terahertz comb bandwidth and femtosecond pulse duration in the mid-infrared are attainable through this symbiotic soliton mode-locking technique. The group velocity mismatches between the three interacting waves are the dominant cause of the symbiotic soliton perturbation, and their effects are numerically investigated in detail. The principle can be applied to commonly used mid-infrared material platforms, making it a competitive ultrashort pulse and broadband comb source architecture.

Temperature gradient engineered to maintain pulse shape and efficiency of a 532 nm picosecond second-harmonic generation

We introduced a theoretical model to preserve the quality and the efficiency of a picosecond second-harmonic generation (SHG) in a 30-mm-long LBO crystal which is surrounded by a thermal oven. It is proposed to arrange a temperature gradient throughout the crystal length for mitigating the undesirable consequences of dispersion and group velocity mismatch. To optimize the pulse shape, a distance parameter is introduced and in addition, the SHG efficiency and pulsewidth are included in a merit function. By setting the temperature gradient as the variable the merit function is optimized. The results show that for temperature gradient of 60 °C, an SHG pulse with mostly Gaussian shape with 80.7% efficiency and with 4.47 ps pulsewidth is attainable. Furthermore, the effect of the chirping on the SHG pulse is investigated. Final results have turned the proposed model into an excellent candidate for obtaining an efficient SHG conversion with an appreciable pulse quality.

Exploring the spectral information content in the LAMOST medium-resolution survey (MRS)

Although high-resolution stellar spectra allow us to derive precise stellar labels (effective temperature, metallicity, surface gravity, elemental abundances, etc.) based on resolved atomic lines and molecular bands, low-resolution spectra have been proved to be competitive in determiningmany stellar labels at comparable precision. It is useful to consider the spectral information content when assessing the capability of a stellar spectrum in deriving precise stellar labels. In this work, we quantify the information content brought by the LAMOST-II medium-resolution spectroscopic survey (MRS) using the gradient spectra as well as the coefficients-of-dependence (CODs). In general, the wavelength coverage of the MRS well constrains the stellar labels but the sensitivities of different stellar labels vary with spectral types and metallicity of the stars of interest. Consequently, this affects the performance of the stellar label determination from the MRS spectra. By applying the SLAM method to the synthetic spectra which mimic the MRS data, we find that the precision of the fundamental stellar parameters Teff, log g and [M/H] are better when combining both the blue and red bands of the MRS. This is especially important for warm stars because the Hα line located in the red part plays a more important role in determining the effective temperature for warm stars. With blue and red parts together, we are able to reach similar performance to the low-resolution spectra except for warm stars. However, at [M/H] ∼ −2.0 dex, the uncertainties of fundamental stellar labels estimated from MRS are substantially larger than that from low-resolution spectra. We also tested the uncertainties of Teff, log g and [M/H] From MRS data induced from the radial velocity mismatch and find that a mismatch of about 1 km s−1, which is typical for LAMOST MRS data, would not significantly affect the stellar label estimates. Finally, reference precision limits are calculated using synthetic gradient spectra, according to which we expect abundances of at least 17 elements to be measured precisely from MRS spectra.

Diode-pumped passively mode-locked Nd:GYSGG laser at 1061 nm with periodically poled LiNbO3 nonlinear mirror

ABSTRACTWe report a nonlinear-mirror mode-locked diode-pumped solid-state Nd:GYSGG laser operating at 1061 nm. The nonlinear mirror comprises a periodically poled LiNbO3(PPLN) crystal and a dichroic mirror. Continuous-wave mode-locked laser with an output power of 450 mw is obtained under a diode pump power of 5.5 W at 808 nm. The repetition rate of the mode-locked laser is 97.1 MHz and the bandwidth of spectral is 1.3 nm. Considering the group velocity mismatch (GVM) of PPLN, we estimate the minimum pulse width is about 9 ps.