Isolated sub-30-attosecond pulse generation using a multicycle two-color chirped laser and a static electric field

We present a theoretical investigation of high-order harmonic generation in a chirped two-color laser field, which is synthesized by a 10-fs/800-nm fundamental chirped pulse and a 10-fs/1760-nm subharmonic pulse. It is shown that a supercontinuum can be produced using the multicycle two-color chirped field. However, the supercontinuum reveals a strong modulation structure, which is not good for the generation of an isolated attosecond pulse. By adding a static electric field to the multicycle two-color chirped field, not only the harmonic cutoff is extended remarkably, but also the quantum paths of the high-order harmonic generation （HHG） are modified significantly. As a result, both the extension of the supercontinuum and the selection of a single quantum path are achieved, producing an isolated 23-as pulse with a bandwidth of about 170.6 eV. Furthermore, the influences of the laser intensities on the supercontinuum and isolated attosecond pulse generation are investigated.

Isolated attosecond pulse generation with few-cycle two-color counter-rotating circularly polarized laser pulses

Most of the schemes for generating isolated attosecond pulses（IAP） are sensitive to the carrier-envelope phase（CEP）of the driving lasers. We propose a scheme for generating IAP using two-color counter-rotating circularly polarized（TCCRCP） laser pulses. The results demonstrate that the dependence of the IAP generation on CEP stability is largely reduced in this scheme. IAP can be generated at most of CEPs. Therefore, the experiment requirements become lower.

Pulse Generation and Compression Techniques for Microwave Electronics and Ultrafast Systems

Ultrabroadband systems and ultrafast electronics require the generation,transmission,and processing of high-quality ultrashort pulses rang-ing from nanoseconds(ns)to picoseconds(ps),which include well-established and emerging applications of time-domain reflectometry,arbitrary wave-form generation,sampling oscilloscopes,frequency synthesis,through-wall radar imaging,indoor communication,radar surveillance,and medical radar detection.Impulse radar advancements in industrial,scientific,and medical(ISM)domains are,for example,driven by ns-scale-defined ultrawideband(UWB)technologies.Nevertheless,the generation of ultrashort ps-scale pulses is highly desired to achieve unprecedented performances in all these ap-plications and future systems.However,due to the variety and applicability of different pulse generation and compression techniques,the selection of optimum or appropriate pulse generators and compressors is difficult for practitioners and users.To this end,this article aims to provide a comprehen-sive overview of ultrashort ns and ps pulse generation and compression techniques.The proposed and developed pulse generators available in the litera-ture and on the market,which are characterized by their corresponding pros and cons,are also explored.The theoretical analysis of pulse generation us-ing a nonlinear transmission line(NLTL)presented in the literature is briefly explained as well.Additionally,a holistic overview of these pulse genera-tors from the perspective of applications is given to describe their utilization in practical systems.All of these techniques are well summarized and com-pared in terms of fundamental pulse parameters,and research gaps in specified areas are highlighted.A thorough discussion of previous research work on various topologies and techniques is presented,and potential future directions for technical advancement are examined.

Noise-like pulse generation from a thulium-doped fiber laser using nonlinear polarization rotation with different net anomalous dispersion

A mode-locked thulium-doped fiber laser(TDFL) based on nonlinear polarization rotation(NPR) with different net anomalous dispersion is demonstrated. When the cavity dispersion is-1.425 ps^2, the noise-like(NL) pulse with coherence spike width of 406 fs and pulse energy of 12.342 nJ is generated at a center wavelength of 2003.2 nm with 3 dB spectral bandwidth of 23.20 nm. In the experimental period of 400 min, the 3 dB spectral bandwidth variation, the output power fluctuation, and the central wavelength shift are less than 0.06 nm, 0.04 d B, and0.4 nm, respectively, indicating that the NPR-based TDFL operating in the NL regime holds good long-term stability.

Q-switched pulse generation from an all-f iber distributed Bragg reflector laser using graphene as saturable absorber

A Q-switched distributed Bragg reflector fiber laser using a graphene passive saturable absorber is proposed in a cavity consisting of a fiber Bragg grating and Faraday rotator mirror as end mirrors, together with a highly doped erbium-doped fiber as a gain source. The laser has a Q-switched threshold of about 28 mW and a tunable repetition rate of 10.4-18.0 kHz with varying pump power. The shortest pulse width obtained from the system is 3.7 its, with a maximum pulse energy and peak power of 22.2 nJ and 3.4 mW, respectively.

A Mutual Pulse Injection-Seeding Scheme for Optical Short Pulse Generation

A mutual pulse injection-seeding scheme is developed to produce wavelength tunable optical short pulse generation. The sidemode suppression ratio obtained is more than 31 dB over the wavelength-tuning rang of 18 nm.

Pulse Generation by Mode-Beating in a Dual-Wavelength Injection-Locked Laser

Generating of repetition-rate tunable optical pulses is achieved by mode-beating within a CW-operated Fabry-Perot laser. The repetition-rate was tunable from 6GHz to 32GHz and may be synchronized by a low frequency (500MHz) optical input.

A comparison study of on-chip short pulse generation circuits based on a coplanar waveguide

A few traditional pulse-forming circuits are implemented in a commercial 0.13 μm digital complementary-metal-oxide-semiconductor （CMOS） technology. These circuits, based on a coplanar waveguide, are analyzed and compared through CadenceTM Spectre simulations. The results show that these traditional pulse-forming-line （PFL） based circuits can be implemented in standard CMOS technology for short pulse generations. Further work is needed to explore the potential of the circuit techniques and to minimize parasitic effects.

High-Quality Ultrashort Pulse Generation Utilizing a Self-Phase Modulation-Based Reshaper

An ultrashort 10-GHz pulse generation scheme was successfully demonstrated using a bulk material InGaAsP electroabsorption modulator to generate the seed pulse. A self-phase modulation-based reshaper was used after the adiabatic soliton compression in a comb-like dispersion profiled fiber. Experiments and simulations confirm that the reshaper effectively removes the pulse pedestal and improves the pulse extinction ratio. As a result, the 10-GHz pulse had no pedestal, a high extinction ratio, and a pulse width of only 1.4 ps.

Single Ultrashort Attosecond Pulse Generation via Combination of Chirped Fundamental Laser and an Ultraviolet Controlling Pulse

We theoretically study the high-order harmonic generation （HHG） from a hydrogen atom in an intense few-cycle chirped fundamental laser in combination with an ultraviolet （uv） controlling pulse. The high-order harmonic spectrum is calculated by solving the time-dependent Schr6dinger equation using the split-operator method. In our calculation, we present the difference of the high-order harmonic spectrum from one-dimensional （1D） model hydrogen atom and three-dimensional （3D） real hydrogen atom. We found that the plateau of the high-order harmonic generation from the 1D ease and 3D case are all extended effectively to Iv -k 35Up due to the presence of the chirped laser pulse and the HHG supercontinuum spectrum is generated by adding an ultraviolet controlling pulse at a proper time, but the efficiency of the HHC for 3D case is more higher at the near cut-off region than the 1D case. Therefore, the generation of the attosecond pulse by synthesizing the harmonics near cut-off region have some slight differences between 1D and 3D simulations. As a real 3D case study, we show that an isolated 18 as pulse with a bandwidth of 232.5 eV is generated directly by optmizing the combination laser fields.

Generation and Shaping of Soliton-Like Pulses along Resonant Tunneling Diodes NMOS Varactors Lattice

The characteristics of N-type accumulation-mode MOS (NMOS) varactors line periodically loaded with resonant tunneling diodes (RTDs) are used for soliton-like pulses generation and shaping. The problem of wide pulse breaking up into multiple pulses rather than a single is solved. Applying perturbative analysis, we show that the dynamics of the nonlinear transmission line (NLTL) is reduced to expanded Korteweg-de Vries (KdV) equation. Moreover, numerical integration of nonlinear differential and difference equations that result from the mathematical analysis of the line is discussed. As results, NLTL can simultaneously sharpen both leading and trailing of pulse edges and one could obtain a rising and sharpening step pulse.

Generation of 47 fs Pulses from an Er:Fiber Amplifier

We demonstrate a self-starting erbium fiber oscillator-amplifier system based on the nonlinear polarization rota- tion mode-locked mechanism. The direct output pulse from the amplifier is 47fs with an average power of 1.22 W and a repetition rate of 50 MHz, corresponding to a pulse energy of 24 nJ. The full width at half-maximum of the spectrum of the output pulses is approximately 93nm at a central wavelength of 1572nm so that the transform- limited pulse duration is as short as 39 fs. Due to the imperfect dispersion compensation, we compress the pulses to 47fs in this experiment.

Generation of 54 Fs Laser Pulses from a Diode Pumped Kerr-Lens Mode-Locked Yb:LSO Laser

A diode pumped Kerr-lens mode-locked femtosecond Yb：LSO laser is experimentally demonstrated for the first time. The 54fs laser pulses at central wavelength of 1052nm with a bandwidth of 22.5nm are obtained at the repetition rate of 113 MHz. To the best of our knowledge, this is the shortest pulse duration ever produced from the Yb-doped orthosilicates lasers family.

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.

Generation of Isolated Attosecond Pulse from Asymmetric Molecular Ions by Introducing Half-Cycle-Like Laser Fields

We propose an efficient method for the generation of an isolated attosecond pulse from the asymmetric molecular ions HeH^2＋ by adding a half-cycle-like field （HCLF） to the fundamental driving laser field. The high-order harmonic generation （HHG） is investigated by numerically sowing the time-dependent Schrodinger equation. By performing the time-frequency distributions and the electronic wave packet probability densities, we find that the optimizing combined field is not only useful for extending the HHG cutoff, but also for simplifying the recombination channels through controlling the electron localization. In addition, by adjusting the intensity of the HCLF, a dominant short quantum path is selected to contribute the HHG spectrum. As a result, a 75-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.

Generation of Linear Isolated Sub-60 Attosecond Pulses by Combining a Circularly Polarized Pulse with an Elliptically Polarized Pulse

The two-color circularly polarized pulses scheme was proposed to generate isolated attosecond pulses in our previous work [Phys. Rev. A 87 （2013） 043406], while the polarization of the attosecond pulse was not investigated. We show a supplementary explanation of this scheme and present another scheme to generate linear isolated attosecond pulses by combining a circularly polarized pulse with an elliptically polarized pulse. High-order harmonic generation and quantum path control are investigated to compare these two schemes. Both schemes can obtain supercontinuum spectra plateau from about 200eV to 550eV, which belong to the water window region. It is found that the latter scheme can clearly eliminate the short quantum path and extend the harmonic plateau. A linear isolated attosecond pulse with a duration of sub-6Oas can be generated by superposing a bandwidth of 70eV.

Energy Scaling of Terahertz Pulses Produced through Difference Frequency Generation

We study the energy scaling of terahertz (THz) emission through difference frequency generation of near-infrared pulses, and demonstrate that Gigawatt few-cycle THz transients at the central frequency of 30 THz are produced from GaSe crystal pumped by two pulses at 1.65 and 1.95 micrometers, with the high quantum yield of 28%. Our analysis indicates that the high yield of DFG originates from the largely reduced group velocity mismatch as the long-wavelength pumping pulses are employed.

Backward Raman amplification in plasmas with chirped wideband pump and seed pulses

Chirped wideband pump and seed pulses are usually considered for backward Raman amplification（BRA） in plasmas to achieve an extremely high-power laser pulse. However, current theoretical models only contain either a chirped pump or a chirped seed. In this paper, modified three-wave coupling equations are proposed for the BRA in the plasmas with both chirped wideband pump and seed. The simulation results can more precisely describe the experiments, such as the Princeton University experiment. The optimized chirp and bandwidth are determined based on the simulation to enhance the output intensity and efficiency.

Generation of an extreme ultraviolet supercontinuum with a multicycle chirped laser and a static electric field

We theoretically present a method for generating an ultrabroad extreme ultraviolet （XUV） supercontinuum by using the combination of a multicycle chirped laser and a static electric field. At a low laser intensity, the spectral cutoff is extended to the 495th order harmonic, and the bandwidth of the supercontinuum spectrum is broadened to 535 eV. At a high laser intensity, the harmonic cutoff is enlarged to the 667th order, and a supercontinuum covering a bandwidth of 1035 eV is generated. In these two cases, the long quantum path is removed, and the short quantum path is selected. Especially for the relatively high laser intensity, an isolated 23-attosecond pulse with a bandwidth of about 170.6 eV is directly obtained. Finally, we also analyze the influences of the chirp parameter and the duration of the chirped pulse as well as the static field strength on the supereontinuum.

Controlling the contributions to high-order harmonic generation from different nuclei of N_2 with an orthogonally polarized two-color laser field

The generation of high-order harmonic and the attosecond pulse of the N2 molecule with an orthogonally polarized two-color laser field are investigated by the strong-field Lewenstein model.We show that the control of contributions to high-order harmonic generation（HHG） from different nuclei is realized by properly selecting the relative phase.When the relative phase is chosen to be φ= 0.4π,the contribution to HHG from one nucleus is much more than that from another.Interference between two nuclei can be suppressed greatly; a supercontinuum spectrum of HHG appears from 40 e V to125 e V.The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical threestep model with a finite initial transverse velocity.By superposing several orders of harmonics,an isolated attosecond pulse with a duration of 80 as can be generated.