Chirped Laser Research Articles

Generation of Chirped Femtosecond Pulses Near 977 nm Using a Mode-Locked All-Fiber Laser

A cladding-pumped all-fiber chirped femtosecond pulse Yb-doped mode-locked laser operating near 977 nm was demonstrated for the first time. Nonlinear polarization evolution was used as the mode-locking mechanism. The laser was self-starting and extremely stable (the stability of the repetition rate of 41.6 MHz was better than that of 15 Hz). Amplification near 977 nm was achieved using a specially designed cladding-pumped Yb-doped fiber. The main feature of the current laser design was its increased intracavity nonlinearity, which resulted in a strong self-phase modulation and significant widening of the generated pulse spectrum. This nonlinearity allowed us to generate pulses compressible down to a duration of 230 fs using an external diffraction grating compressor.

Effects of intense chirped x-ray laser on resonant Auger process through multiple core-excited states

In the context of the recent reported results indicating chirped laser induced suppression of resonant Auger effect the author investigates the effects of chirping on the resonant Auger process from a coherent superposition of core-excited states and direct ionization from the ground state. It is found that for the positive chirping case total electron yield initially increases with increase in chirp rate eventually decreasing with further increase in chirp rate. However, in the case of negative chirping total electron yield always decreases with increase in chirp rate. The results demonstrate the possibility of controlling Rabi oscillations between ground and core-excited states through the variation of chirp rate. It is also found that the variation of chirp rate significantly influences the peak magnitude of the two peaks visible in the electron spectrum. The author also studies the effects of chirping of the laser on the electron spectrum computed for direct ionization from the ground state. The calculated results indicate that chirping of x-ray pulse can modify the signatures of interference between two competing pathways in the electron spectrum.

Direct electron acceleration by chirped laser pulse in a cylindrical plasma channel**Project supported by the National Natural Science Foundation of China (Grant Nos. 11865014, 11765017, 11764039, 11475027, 11274255, and 11305132),the Natural Science Foundation of Gansu Province of China (Grant No. 17JR5RA076), the Scientific Research Project of Gansu Higher Education of China (Grant No. 2016A-005), the Natural Science Foundation of Education

We study the dynamics of single electron in an inhomogeneous cylindrical plasma channel during the direct acceleration by linearly polarized chirped laser pulse. By adjusting the parameters of the chirped laser pulse and the plasma channel, we obtain the energy gain, trajectory, dephasing rate and unstable threshold of electron oscillation in the channel. The influences of the chirped factor and inhomogeneous plasma density distribution on the electron dynamics are discussed in depth. We find that the nonlinearly chirped laser pulse and the inhomogeneous plasma channel have strong coupled influence on the electron dynamics. The electron energy gain can be enhanced, the instability threshold of the electron oscillation can be lowered, and the acceleration length can be shortened by chirped laser, while the inhomogeneity of the plasma channel can reduce the amplitude of the chirped laser.

Remote Detection of Uranium Using Self-Focusing Intense Femtosecond Laser Pulses

Optical measurement techniques can address certain important challenges associated with nuclear safety and security. Detection of uranium over long distances presents one such challenge that is difficult to realize with traditional ionizing radiation detection, but may benefit from the use of techniques based on intense femtosecond laser pulses. When a high-power laser pulse propagates in air, it experiences collapse and confinement into filaments over an extended distance even without external focusing. In our experiments, we varied the initial pulse chirp to optimize the emission signal from the laser-produced uranium plasma at an extended distance. While the ablation efficiency of filaments formed by self-focusing is known to be significantly lower when compared to filaments produced by external focusing, we show that filaments formed by self-focusing can still generate luminous spectroscopic signatures of uranium detectable within seconds over a 10-m range. The intensity of uranium emission varies periodically with laser chirp, which is attributed to the interplay among self-focusing, defocusing, and multi-filament fragmentation along the beam propagation axis. Grouping of multi-filaments incident on target is found to be correlated with the uranium emission intensity. The results show promise towards long-range detection, advancing the diagnostics and analytical capabilities in ultrafast laser-based spectroscopy of high-Z elements.

Optical steering of electron beam in laser plasma accelerators.

Using a Dazzler system and tilting a compressor grating, we provide an effective way of using the laser group delay dispersion to continuously steer the electron beam accelerated by an asymmetric laser wakefield. The deviation angle of the electron beam was the same as that of the angularly chirped laser pulse from its initial optical axis, which is determined by the laser pulse-front-tilt (PFT). This method can be utilized to continuously control over the pointing direction of electron bunches to the requisite trajectories, especially for practical applications in highly sensitive alignment devices such as electron-positron colliders or undulators. Additionally, we investigate the effect of PFT on the properties of the electron beam.

Generation of single isolated attosecond pulse from a monolayer hexagonal boron nitride using a polarization gating technique

An ab initio approach is presented to generate an isolated attosecond pulse (IAP) by gating high-order harmonic emission from a monolayer hexagonal boron nitride (h-BN), where in this case the ellipticity of the fields are varying rapidly with time. The high harmonic generation from a monolayer h-BN lattice is studied when it is exposed to the single circularly- and bicircularly-polarized laser fields. The effect of positive and negative chirped laser pulses on the high harmonics are considered. The role of ellipticity in the HHG to create single IAP from 2D h-BN under counter-rotating bicircularly polarized laser pulses is investigated in mid-infrared wavelength region. In order to generate single IAP, the polarization gating (PG) technique is implemented. By adjusting properly positive chirped parameters and implementing the conventional polarization gating to an input intensity of 20 TW cm−2, an isolated 80 as pulse from the superposition of several harmonics is generated.

Waveform control of currents in graphene by chirped few-cycle lasers

The residual current density in monolayer graphene driven by an intense few-cycle chirped laser pulse is investigated via numerical solution of the time-dependent Schrödinger equation in the light-field-driven regime. Strikingly, it is found that a purely chirped laser pulse breaks the inversion symmetry in graphene, generating a residual directional current, which is absent for a Fourier-transform limited pulse (2017 Nature 550 224) and is attributed to the chirp-dependent Landau–Zener–Stückelberg interference among different quantum pathways in the reciprocal space. Moreover, the directionality of such a current changes with laser chirp rate following a sine-functional way, which possibly provides a novel application in ultrafast photo-electronics based on two-dimensional materials.

Semi-Lagrangian Vlasov simulation of the interaction between femtosecond chirped and double laser pulses with a thin plasma slab

The wakefield generation and electrons acceleration in the interaction of femtosecond chirped and double laser pulses with an underdense thin plasma slab is examined by numerical solution of the relativistic Vlasov-Maxwell system of equations using the backward semi-Lagrangian method (BSL). Unlike particle-in-cell methods, the BSL method is free of numerical noise and provides an accurate representation of the distribution function, even in very low density regions of phase space where acceleration and trapping occur. The effect of density fluctuations in low density regions of phase space are important, since they affect the acceleration processes over the following interactions. Therefore, a detailed structure of the distribution function is needed to investigate this process in these regions, which only can be obtained by Vlasov codes. Excitation of wakefields by chirped, chirp free, and double pulse lasers in an underdense plasma slab are simulated using the BSL code and the results are compared. It is shown that by making use of chirped laser pulses, the mean kinetic energy of plasma electrons can be enhanced up to 40% compared with the chirp free case. The investigation of wakefield generation by double laser pulses with a time repetition equal to one plasma period, exhibits amplification of wakefield amplitude after the entrance of the second laser pulse into plasma slab.

Carrier-envelope-phase–controlled molecular dissociation by ultrashort chirped laser pulses

We demonstrate and characterize that a carrier-envelope-phase (CEP)-controlled ultrashortchirped field is an efficient and robust mechanism to modify the dissociation dynamics of molecularhydrogen. Different dissociation pathways are collectively induced and their interference contributeto the kinetic energy release spectra. Chirping is able to efficiently manipulate the interferencesof different dissociation pathways. We demonstrate a linear relationship between chirp and CEP-dependence, dissociation as well as directional electron localization.

Second-harmonic generation by a chirped laser pulse with the exponential density ramp profile in the presence of a planar magnetostatic wiggler

AbstractSecond-harmonic generation of the relativistic self-focused chirped laser pulse in plasma has been studied with the exponential plasma density ramp profile in the presence of a planar magnetostatic wiggler. It is evident that the exponential plasma density ramp is helpful in enhancing second-harmonic generation as, with the introduction of the exponential plasma density ramp, self-focusing becomes stronger and hence, it leads to enhance the harmonic generation of the second order in the plasma. Also, it is observed that the efficiency of second-harmonic generation enhances significantly with an increase in the value of the chirp parameter. Further, the magnetostatic wiggler helps in enhancing the harmonic generation of the second order. This is due to the fact that dynamics of the oscillating electrons is altered due to the Lorentz force which, in turn, modifies the plasma wave and, hence, results in the efficient second-harmonic generation.

Optimization of temporal gate by two-color chirped lasers for the generation of isolated attosecond pulse in soft X rays

We propose a simple and efficient method to optimize two-color chirped laser pulses by forming a “temporal gate” for the generation of isolated attosecond pulses (IAPs) in soft X rays. We show that the generation process for higher and cutoff harmonics can be effectively limited within the temporal gate, and the harmonic emission interval can be further reduced with the help of phase-matching by only selecting the contribution from short-trajectory electrons. This two-color gating mechanism is verified by increasing the pulse duration, raising the gas pressure, and extending the target cutoff. Compared to the five-color waveform in Phys. Rev. Lett.102, 063003 (2009)PRLTAO0031-900710.1103/PhysRevLett.102.063003, our waveform can be used to generate the IAP in the long-duration laser pulse while the cutoff energy is higher without the reduction of harmonic yields. Our work provides an alternative temporal gating scheme for the generation of IAPs by simultaneously improving the harmonic conversion efficiency, thus making the attosecond soft X rays an intense and highly time-resolved tabletop light source for future applications.

Generation of carrier-envelope phase-controlled isolated attosecond pulses using chirped femtosecond excitation lasers

ABSTRACTWe present an efficient approach for producing a carrier-envelope phase controlled isolated attosecond pulse by an optimized intense driving laser pulse. High-order harmonics are produced by numerically solving the time-dependent Schrödinger equation for the one-dimensional hydrogen atom in an ultrashort laser pulse. We define an efficient cost function to optimize the laser pulse by a genetic algorithm scheme. Our approach produces single attosecond pulses with desired properties, including the carrier-envelope phase, central frequency, and duration. Also, we analyze the time–frequency profiles of the attosecond emissions to gain a deeper insight into the underlying physical mechanism.

Investigation of robust population transfer using quadratically chirped laser interacting with a two-level system

We have proposed and demonstrated a fast and robust method of population transfer between two quantum states using a quadratically chirped laser source. Numerically solving the time-dependent Schrödinger equation, transition probabilities have been obtained and the condition of the adiabatic passage is investigated. In this scheme, the wavelength of a laser source is quadratically swept in time to arbitrarily engineer the transition probabilities. The results show that complete and robust population transfer could be achieved by appropriate adjusting of the system parameters including the laser chirping parameter, the center frequency, and the coupling strength. Using this method, the time of the complete transition could be drastically decreased compared to the incorporation of the traditional linearly chirped laser.

Ultrafast scanning of space with pulsed chirped laser radiation

Ultrafast scanning of space with pulsed chirped laser radiation

Accuracy and stability evaluation of the 85Rb atom gravimeter WAG-H5-1 at the 2017 International Comparison of Absolute Gravimeters

We report on the realization and evaluation of the 85Rb mobile atom absolute gravimeter named WAG-H5-1. With a distinctive Raman laser implementation, two new systematic errors, ac Stark shift imbalance and Raman laser chirp, are found and evaluated. The robustness of the instrument was tested during 1200 km-long transportation for the 10th International Comparison of Absolute Gravimeters at Beijing in October 2017. We acquire the degree of equivalence of 3 Gal after the comparison. Its performance was further assessed by comparing with an absolute gravimeter (FG5X) in Wuhan. With high mobility and environmental adaptability, the gravimeter reaches a sensitivity of 30 Gal (Hz)−1/2, stability of 1 Gal@ 4000 s and accuracy within 10 Gal.

Chirp Compensation of Directly Modulated 3s-DBR Laser for WDM-RoF-Based Mobile Fronthaul

In this letter, we propose and demonstrate through simulations, a chirp compensation scheme with a directly modulated three section distributed Bragg reflector (3s-DBR) laser, for use in a WDM-RoF-based mobile fronthaul network to eliminate the signal distortion caused by the interplay between laser chirp and fiber dispersion, including RF power fading and the composite second-order (CSO) distortion. Employing the phase section characteristics of the 3s-DBR laser, the frequency chirp of the device is reduced with reverse modulation current injection into the phase section. The simulation results show that with the chirp compensation scheme, reduced DBR laser chirp can be achieved, and error vector magnitude (EVM) of a 12 channel 211-MHz bandwidth filtered-OFDM (f-OFDM) signal with 64-QAM after transmission over 20-km standard single mode fiber (SSMF), can still satisfy the 3rd generation partnership project’s (3GPP) requirement.

Femtosecond Laser Pulses Amplification in Crystals

This paper describes techniques for high-energy laser pulse amplification in multi-PW femtosecond laser pulses. Femtosecond laser pulses can be generated and amplified in laser media with a broad emission spectral bandwidth, like Ti:sapphire crystals. By chirped pulse amplification (CPA) techniques, hundred-Joule amplified laser pulses can be obtained. Multi-PW peak-power femtosecond pulses are generated after recompression of amplified chirped laser pulses. The characteristics and problems of large bandwidth laser pulses amplification in Ti:sapphire crystals are discussed. An alternative technique, based on optical parametric chirped pulse amplification (OPCPA) in nonlinear crystals, is presented. Phase-matching conditions for broad bandwidth parametric amplification in nonlinear crystals are inferred. Ultra-broad phase matching bandwidth of more than 100 nm, able to support the amplification of sub-10 fs laser pulses, are demonstrated in nonlinear crystals, such as Beta Barium Borate (BBO), Potassium Dideuterium Phosphate (DKDP), and Lithium Triborate (LBO). The advantages and drawbacks of CPA amplification in laser crystals and OPCPA in nonlinear crystals are discussed. A hybrid amplification method, which combines low-medium energy OPCPA in nonlinear crystals with high energy CPA in large aperture laser crystals, is described. This technique is currently used for the development of 10-PW laser systems, with sub-20 fs pulse duration and more than 1012 intensity contrast of output femtosecond pulses.

Reconstruction of coherent anti‐Stokes Raman scattering signals generated by means of laser pulses with asymmetric amplitude and phase

AbstractTime and frequency asymmetries in ultrashort chirped laser fields might appear as a consequence of dispersive propagation, pulse shaping techniques, or generation of auxiliary light pulses needed in nonlinear optics. Here, we try to find an answer to the question of how to solve analytically coherent anti‐Stokes Raman scattering (CARS) under asymmetric conditions of chirped femtosecond laser pulses. The approach breaks in two parts. One for the field amplitudes and the other for the phases. The former revolves around Gaussian dependences that, besides being rather common in ultrashort laser physics, can be arranged and mixed to reproduce spectrally asymmetric laser amplitudes. The latter is limited to field phases with cubic frequency dependence (i.e., second‐order chirp) whose asymmetry is simulated by adding a linear term to the quadratic phase. Both approximations for amplitudes and phases of the three laser pulses are mandatory to guarantee the solution to the complexity posed by the CARS problem. Comparisons with known experimental and numerical results support the validity of the model.

Experimental Demonstration of a WDM-RoF Based Mobile Fronthaul With f-OFDM Signals by Using Directly Modulated 3s-DBR Laser

Future 5G mobile fronthaul (MFH) network will require high bandwidth and ultralow latency data transmission. Radio-over-fiber (RoF) technology is an efficient way to save bandwidth, which can significantly improve the radio signal transmission capacity over fiber. In this paper, we experimentally demonstrate a WDM-RoF based MFH with a directly modulated three-section distributed Bragg reflector (3s-DBR) laser. To transport the massive multiple-input-multiple-output radio signals, 12 channels of 211 MHz bandwidth filtered orthogonal frequency division multiplexing (f-OFDM) signals with 30 MHz guard-band are generated and transmission, using both 16 and 64-QAM, on different wavelength channels over 20 km standard single mode fiber is investigated. Experimental results show that all channels can meet the error vector magnitude requirements of 3GPP standard for the case of 16QAM; however, when moving to 64QAM, this is not guaranteed for all 12 channels due to signal degradation caused by the laser chirp and fiber dispersion.

A benchtop shock physics laboratory: Ultrafast laser driven shock spectroscopy and interferometry methods.

Common Ti:sapphire chirped pulse amplified laser systems can be readily adapted to be both a generator of adjustable pressure shock waves and a source for multiple probes of the ensuing ultrafast shock dynamics. In this paper, we detail experimental considerations for optimizing the shock generation, interferometric characterization, and spectroscopic probing of shock dynamics with visible and mid-infrared transient absorption. While we have reported results using these techniques elsewhere, here we detail how the spectroscopies are integrated with the shock and interferometry experiment. The interferometric characterization uses information from beams at multiple polarizations and angles of incidence combined with thin film equations and shock dynamics to determine the shock velocity, particle velocity, and shocked refractive index. Visible transient absorption spectroscopy uses a white light supercontinuum in a reflection geometry, synchronized to the shock wave, to time resolve shock-induced changes in visible absorption such as changes to electronic structure or strongly absorbing products and intermediates due to reaction. Mid-infrared transient absorption spectroscopy uses two color filamentation supercontinuum generation combined with a simple thermal imaging microbolometer spectrometer to enable broadband single shot detection of changes in the vibrational spectra. These methods are demonstrated here in the study of shock dynamics at stresses from 5 to 30 GPa in organic materials and from a few GPa to >70 GPa in metals with spatial resolution of a few micrometers and temporal resolution of a few picoseconds. This experiment would be possible to replicate in any ultrafast laser laboratory containing a single bench top commercial chirped pulse amplification laser system.