Chirped Laser Research Articles

Induced modulation of a chirped laser pulse at terahertz frequency with spectral phase shaping

The possibility of using harmonic modulation of the spectral phase to generate multiple replicas of the original short laser pulse or controlled periodic intensity modulation at the terahertz frequency of the stretched chirped laser pulse is shown theoretically and experimentally.

Laser chirping in inverse Compton sources at high electron beam energies and high laser intensities

The onset of nonlinear effects, such as ponderomotive broadening, increases the radiation bandwidth and thereby places a stringent limitation on the laser intensity used in inverse Compton sources. Recently, we have shown that a judicious longitudinal laser frequency modulation (``chirping'') can perfectly compensate for this ponderomotive broadening and restore the narrow band property of scattered radiation in the Thomson regime, when electron recoil during the collision with the laser can be neglected. Consequently, using QED, the laser chirping has been extended to the Compton regime, where electron recoil is properly accounted for. Here we present a new, semiclassical model for computation of scattered spectra in the Compton regime. We also derive a comprehensive generalization of the expressions for chirping prescription for linearly polarized laser pulses in 1D plane-wave approximation with arbitrary shapes and arbitrary scattering angle in the Compton regime. We use these new expressions to show that the higher-order harmonics in sources with high laser fields and high electron beam energies (nonlinear Compton regime) will be nonlinearly redshifted when compared to those with lower beam energies (Thomson regime). The chirping prescription will act to correct ponderomotive broadening in very high harmonics.

Controlling the dynamics of the plasmonic field in the nano-femtosecond scale by chirped femtosecond laser pulse

Arbitrary control of the electromagnetic field in femto-nano scale has attracted significant research attention in nano-photonics. Although the electromagnetic field controlled in femto-nano scale could be realized by illuminating metallic nanoparticles with femtosecond chirped laser pulses, the quantitative relation of the laser chirp and the temporal evolution of the plasmonic field hasn’t yet been fully revealed. Here, active control of the localized plasmonic field is demonstrated by a chirped femtosecond laser pulse in an asymmetric Au nano-cross system within nm-fs scale using the finite differential time domain method. The transferring of the plasmonic field between the two poles of the nanocross is determined by the laser chirp and exhibits linear dependence on the time interval between the corresponding plasmonic resonant frequencies dispersed in the chirped laser pulse. The arrival time and amplitude of the peak field from the plasmonic hot spot are determined by the superposition of the induced field excited by the on-resonant and off-resonant frequency components distributed in the chirped laser pulse. The peak field would arrive behind the resonant frequency component for sufficient oscillation of the localized field. This relative delay between the resonant frequency and field peak is influenced by the temporal distribution of the resonant frequency in the chirped pulse and the response of localized field interpreted by the damped harmonic oscillator model. This result demonstrates that larger near-field enhancement is determined by not only the temporal sequence of the frequency component modulated by chirp but also the temporal distribution of the resonant frequency.

Chirp-dependent ionization of hydrogen atoms in the presence of super-intense laser pulses* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774131 and 91850114).

We perform a theoretical study on dynamic interference in single photon ionization of ground state hydrogen atoms in the presence of a super-intense ultra-fast chirped laser pulse of different chirp types (equal-power and equal-FWHM laser pulses) by numerically solving the time-dependent Schrödinger equation in one dimension. We investigate the influences of peak intensity and chirp parameters on the instantaneous ionization rate and photoelectron yield, respectively. We also compare the photoelectron energy spectra for the ionization by the laser pulses with different chirp types. We find that the difference between the instantaneous ionization rates for the ionization of hydrogen atom driven by two different chirped laser pulses is originated from the difference in variation of vector potentials with time.

Frequency sweep extension using the Kerr effect for static temperature measurement range enhancement in Chirped Pulse φ-OTDR.

In Chirped Pulse φ-OTDR systems used for sensing temperature or strain along an optical fiber, the largest disturbance between two single-shot measurements that can reliably be detected depends on the range of frequencies swept by the chirped pulse. If electrical modulation is used to generate the laser frequency sweep, the achievable sweeping range is limited by the electrical components, leading to a narrow measurement range for static measurements. In this work, we demonstrate the extension of the frequency range of a chirped laser pulse by all-optical means using evenly spaced frequency sidebands generated via the Kerr effect to improve the Chirped Pulse φ-OTDR measurement range. We report chirp extensions by factors up to 13 and apply the effect to achieve a sixfold increase in the measurement range of a Chirped Pulse φ-OTDR system measuring the temperature of a random fiber grating array. The method described in this paper can be applied to other optical systems utilizing chirped laser pulses and allow for variable extension of their chirping range.

Fugitive methane detection using open-path stand-off chirped laser dispersion spectroscopy.

We report an open-path chirped laser dispersion spectrometer capable of detecting the atmospheric methane concentration above the background using both specular and diffusive reflective surfaces via two distinct operation modes in a stand-off detection configuration. The system is integrated with simultaneous ranging functionality, which enables average concentration measurements for varying optical pathlengths. The system was first tested for accuracy and characterized to achieve sensitivity of 2.9ppm-m/Hz1/2 and pathlength precision of 0.2m/Hz1/2 with a controlled release of methane outside the laboratory. The instrument was subsequently field-deployed in the proximity of a natural gas compressor station for fugitive methane detection. The instrument successfully detected methane plumes and narrowed down the location of the plume through multi-path measurement. The field measurements were verified by a co-located reference mobile methane sensor.

Measurement of amplified binary-modulated chirped laser pulses generated by different acousto-optic pulse shaping algorithms

The paper belongs to the field of ultrafast optics. Acousto-optic pulse shaping has been studied experimentally for binary intensity modulation in a chirped pulse amplification laser system. Direct time-domain measurement of modulation rise/fall time with a picosecond streak camera was performed for a Ti:sapphire regenerative amplifier. It was discovered that the sign of the second order dispersion produced by the acousto-optic dispersion delay line affects the modulation rise/fall time. Two algorithms for synthesis of ultrasonic waveforms for feeding the delay line, dispersive Fourier synthesis and the Gerchberg-Saxton algorithm, were compared. Minimum modulation rise/fall time of 3.6 ps for 3 mJ pulses with the linear chirp of 6.2 ps/nm at the wavelength of 795 nm was obtained.

First-principles study of the electronic and magnetic properties of Fe, Co, and Ni dimers adsorbed on polycyclic-aromatic-hydrocarbon molecules as well as the laser chirp effect on the ultrafast spin dynamics

We investigate the geometric, electronic, and magnetic properties of ${\mathrm{TM}}_{2}(\mathrm{PAH})$ complexes [transition metal (TM) = Fe, Co, Ni; $\text{polycyclic-aromatic}\phantom{\rule{4.pt}{0ex}}\text{hydrocarbon}\phantom{\rule{4.pt}{0ex}}\text{(PAH)}={\mathrm{C}}_{10}{\mathrm{H}}_{8}, {\mathrm{C}}_{16}{\mathrm{H}}_{10}, {\mathrm{C}}_{24}{\mathrm{H}}_{12}, {\mathrm{C}}_{32}{\mathrm{H}}_{14}]$ as well as the optically induced ultrafast spin dynamics from a first-principles study. Geometrically, the magnetic dimer of each of the complexes [except for ${\mathrm{Ni}}_{2}({\mathrm{C}}_{16}{\mathrm{H}}_{10}$)] turns out to prefer to adsorb above the hollow site of the outer carbon ring of the PAH molecule. The PAH-size effect and TM-element effect on the energy levels and spin localizations are analyzed. It is found that for the structures with the same PAH molecules, the level bands generally get wider from Fe via Co to Ni, while the number of spin-localized states overall decreases in the same order. For the structures with the same TM species [except for the two complexes ${\mathrm{Ni}}_{2}({\mathrm{C}}_{16}{\mathrm{H}}_{10}$) and ${\mathrm{Fe}}_{2}({\mathrm{C}}_{32}{\mathrm{H}}_{14}$)], the low-lying levels are hardly affected by the size of the PAH molecules. Among all the calculated levels for each complex [except for ${\mathrm{Ni}}_{2}({\mathrm{C}}_{16}{\mathrm{H}}_{10}$)], there are always more states with spin localized on the remote magnetic center than those localized on the near center. Driven by nonchirped subpicosecond laser pulses, a series of ultrafast spin-flip and spin-transfer scenarios on these structures are predicted and analyzed, based on which the laser chirp effect is further explored and some rules of thumb about the chirp tolerance and sensitivity are obtained. The results demonstrated in this paper are believed to enrich our understanding of the size- and structure-dependent electronic and magnetic properties of the carbon-based magnetic molecular structures, and further to promote the relevant experimental realization of the spin dynamics and their potential applications in future molecular spin devices.

Influence of linear absorption and density ramp on self-focusing of the Hermite-Gaussian chirped pulse laser in plasma

An analysis of the self-focusing of Hermite Gaussian chirped pulse laser in the relativistic and ponderomotive regime under the influence of exponential plasma density ramp and linear absorption has been done. Differential equation for beam width parameter is derived by using paraxial ray approximation and then solved numerically. It is observed that self-focusing of the chirped pulse laser is strongly influenced by linear absorption. Also it is seen that with the increase in intensity of laser beam, plasma density ramp and chirped parameter value, self-focusing becomes more effective. Current work may be useful in understanding laser induced fusion, quantum dots etc.

Obtaining an Operating Point Solution of a Traveling Wave Laser Model

This paper presents a method of obtaining an operating point configuration for a laser model based on a traveling wave model (TWM), which can then be used in a circuit-level simulator. The method first finds an approximate distributed single-mode stationary solution, this solution is then iterated using the traveling wave equations to an accurate single-mode solution, and finally a short pre-simulation is used to add harmonic content to create a multi-mode configuration of the laser approximating its behavior at an operating point. The effectiveness of this approximation is tested by initiating transient simulations from this operating point and comparing them to the output of the model started from an off state. The stochastic variation in the operating point for a particular configuration is also well predicted. Included in the formulation are gain compression and dispersion effects, laser chirp due to variation in the effective index of the laser mode, and spontaneous emission. Finally, the use of the three-stage process of finding the operating point in a circuit-level simulator is discussed. Not only does the three-stage method provide a quick, accurate operating point for the circuit simulator, but the ability to provide an orders of magnitude faster estimate for the initial circuit-level operating point is critical to the practicality of its use in the simulator. The first stage of the three-stage method does just this.

Generation of high-intensity ultra-short optical pulses: 2018 Nobel Prize Winners in Physics Gerard Mourou and Donna Strickland

In the early 1980s, French physicist G. Mourou and his Canadian collaborator D. Strickland solved the problem of power drop by dispersing in time and space the processes of amplification and compression: a method of obtaining super-powerful chirped laser pulses (CPA – chirped pulse amplification). The paper presents brief biographical references to Mourou and Strickland. The 2018 Nobel Prize in Physics was awarded “for groundbreaking inventions in the field of laser physics”: Artur Isidorovich Ashkin (Ashkinazi, born 02.09.1922, USA), half of the prize “for the optical tweezers and their application to biological systems”; Gerard Albert Mourou and Donna Theo Strickland (became the third woman to be awarded the Nobel Prize in Physics) (quarterly premium) “for their method of generating high-intensity, ultra-short optical pulses”. Since that time all lasers have been built on a new principle: after the amplifiers place a compressor from diffraction bars. Instead of simply amplifying the pulse, it is first spread out on spectral components spread over time, then they are amplified separately, then again assembled into a single pulse. At each point in time, only a fraction of the pulse is amplified, not the entire pulse, allowing for a much higher peak intensity of laser light flow.

Control of the high-order harmonic generation by sculpting waveforms with chirp in solids

Novel method for controlling the high-order harmonic generation in solids with few cycle nonlinear chirped driver laser pulses is proposed. High-order-harmonic generation (HHG) is simulated by solving the one-dimensional (1D) time-dependent Schrödinger equation. The quantum path of HHG is complex in solid system, multichannel contribution in the plateau region is identified and manipulated in this paper. Our results show that negative chirp can broaden the second plateau of HHG when the time point of the chirped central frequency remains unchanged. The physical mechanism is discussed by the time-frequency, quasi-classical analysis and population in each energy band.

A novel design of double chirped pulse amplification laser systems for fourth-order dispersion control.

A novel design of double chirped pulse amplification laser systems implementing a combination of negatively and positively chirped pulse amplification is proposed for the first time. Without utilizing any extra dispersion compensation element, this design can sufficiently cancel out the second-, third- and especially fourth-order dispersion simultaneously, just by optimizing the parameters of the stretcher and compressor in first chirped pulse amplification stage which applies negatively chirped pulse amplification. The numerical results indicate that near Fourier-transform-limited pulse duration about 20fs can be achieved in high-peak-power femtosecond laser systems up to multi-Petawatt level. This design not only provides a feasible solution for the dispersion control in high-contrast and high-peak-power femtosecond laser systems, but also can avoid the degradation of temporal contrast induced by seed energy loss in the presence of additional dispersion compensation components.

Compact 50 W All-Solid-State Picosecond Laser System at 1 kHz

Compact, stable, high-power and high repetition rate picosecond laser systems are excellent sources for optical parametric chirped pulse amplification systems and laser satellite ranging systems. Compared with the traditional complex high-power amplifier, this article reports a compact high-power picosecond laser system at a repetition rate of 1 kHz based on Nd:YAG bulk crystal. The thermal lens effect limits the regenerative amplifier to directly output higher energy. For this reason, multi-stage traveling-wave amplifiers are usually used to gradually increase the laser pulse energy. So as to achieve a compact structure, a regenerative amplifier that can output higher power at 1 kHz is designed in the laser system. The regenerative amplifier can output the power of 6.5 W at the pump power of 41.5 W; the beam quality of M2 factor was about 1.3. A more flexible thermal depolarization compensation structure is applied in the side-pumped amplifier, which can effectively compensate for thermal lens effect and thermal depolarization at different pump powers. Finally, the laser pulse can achieve an output power higher than 50 W at 1 kHz after passing through an end-pumped traveling-wave amplifier and a side-pumped traveling wave amplifier.

Optomagnonic Barnett effect

Combining the technologies of quantum optics and magnonics, we find that the circularly polarized laser can dynamically realize the quasiequilibrium magnon Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and spins generates the optical Barnett field, and its direction is controllable by switching the laser chirality. We show that the optical Barnett field develops the total magnetization in insulating ferrimagnets with reversing the local magnetization, which leads to the quasiequilibrium magnon BEC. This laser-induced magnon BEC transition through optical Barnett effect, dubbed the optomagnonic Barnett effect, provides an access to coherent magnons in the high frequency regime of the order of terahertz. We also propose a realistic experimental setup to observe the optomagnonic Barnett effect using current device and measurement technologies as well as the laser chirping. The optomagnonic Barnett effect is a key ingredient for the application to ultrafast spin transport.

Generation and characterisation of few-pulse attosecond pulse trains at 100 kHz repetition rate

The development of attosecond pump–probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system.

Improvement in the temporal contrast in the tens of ps range of the multi-PW Apollon laser front-end.

We demonstrate the impact of the optics roughness in Öffner stretchers used in chirped pulse amplification laser chains and how it is possible to improve the temporal contrast ratio in the temporal range of 10-100 ps by adequately choosing the optical quality of the key components. Experimental demonstration has been realized in the front-end source of the multi-petawatt (PW) laser facility Apollon, resulting in an enhancement of the contrast ratio by two to three orders of magnitude.

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.