Sapphire Chirped Pulse Amplification Research Articles

Comparative study on generation of attosecond pulse train and phase information reconstruction

<sec>Attosecond pulses provide higher measurement precision for analyzing ultrafast dynamics in atoms, molecules, and electrons, laying the foundation for studying electronic motion in atomic and molecular systems. The most important method currently is to generate attosecond pulse trains and isolated attosecond pulses through the interaction of femtosecond lasers with gases. The temporal information of attosecond pulses and the dynamic information of electrons can be extracted from spectrograms by using attosecond streak camera or the Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBITT) method based on two-photon transition interference. Although the differences in phase among different high-order harmonics can be directly extracted from the oscillation frequencies of sidebands, the iterative algorithm of attosecond streak camera can provide complete phase information of attosecond pulse trains to better support the study of electron dynamics in atoms.</sec> <sec> <b>Research purpose</b> This work is dedicated to the investigation of the generation, measurement, and characterization of attosecond pulse train (APT), which are essential for probing ultrafast dynamics in atomic, molecular, and electronic systems. The focus is on the generation of APTs through interactions between femtosecond lasers and gases, as well as the extraction of temporal and dynamic information from these pulses by using advanced spectroscopic techniques such as the RABITT method.</sec> <sec> <b>Methods</b> The experimental approach involves the use of a homebuilt femtosecond titanium sapphire regenerative amplifier to produce high-order harmonics, leading to the generation of APTs. The setup includes the homebuilt titanium sapphire chirped pulse amplifier and a collinear attosecond pulse generation and measurement beamline, which are used to conduct RABITT experiments. The process requires the interaction of femtosecond lasers with gas targets to generate high-energy photons in the extreme ultraviolet and soft X-ray spectral ranges. By optimizing the phase-matching conditions within the gas target, strong high-order harmonic signals are observed on an XUV spectrometer. The temporal information of the attosecond pulses is indirectly measured through the photoelectron spectrum produced by the interaction of attosecond pulses with femtosecond lasers. The research also employs the FROG-CRAB algorithm and the Extended Phase Retrieval and Iterative Engine (ePIE) algorithm for temporally reconstructing APTs and attempts to use a genetic algorithm to extract phase information.</sec> <sec> <b>Results</b> The study yields three sets of RABITT spectrograms, which are analyzed by using the RABITT sideband phase method to directly reconstruct APTs. Fourier transform analysis is used to extract phase differences between sidebands, offering insights into the phase differences between corresponding high-order harmonics. This method, however, provides an estimation of the phase in the center of each harmonic order, which does not fully represent the actual pulse shape. The FROG-CRAB algorithm and ePIE algorithm successfully reconstructs the attosecond pulse trains from the RABITT spectrograms, revealing similar temporal pulse train morphologies. In contrast, the genetic algorithm, despite its potential for high constraint optimization, does not yield satisfactory results, possibly due to the sensitivity of the algorithm to discrepancies between theoretical simulations and experimental data.</sec> <sec> <b>Conclusions</b> The research concludes that achieving ideal inversion results for APTs necessitates small time delay steps and a wide scanning range in the experimental data collection process to ensure a rich dataset for inversion. The FROG-CRAB algorithm and ePIE algorithm demonstrate their effective performance in reconstructing APTs, with ePIE showing higher computational efficiency. The genetic algorithm, while offering a high degree of constraint, faces challenges and requires to be further refined. The study underscores the importance of the signal-to-noise ratio in experimental data for the accuracy of inversion results. This work provides significant guidance for future measuring electron dynamics and explaining their evolution patterns, contributing valuable experimental methods and data analysis techniques to the field of attosecond science.</sec>

Investigation and suppression of pre-pulses on nanosecond time scale in the SULF-1PW laser.

It is of crucial significance to investigate and suppress pre-pulses on nanosecond time scale because the intense pre-plasma generated by them may have enough time to expand and, thus, cause fatal impact on laser-matter interactions. In this research, we analyze the potential origins of pre-pulses on nanosecond time scale in a typical Ti:sapphire chirped pulse amplification laser system. Based on the analysis, the initial status of these generated pre-pulses in the SULF-1PW laser is measured and investigated. Then different measures, including fine control on the time synchronization and the replacement for the Ti:sapphire, are adopted in the SULF-1PW laser to suppress these pre-pulses with respective origins, which can promote the energy ratio between the main pulse and these pre-pulses by 2-3 orders of magnitude. This research not only improves the temporal contrast of the SULF-1PW laser on nanosecond time scale but also provides beneficial guidance for the design and construction of similar laser facilities.

Reducing temporal pedestal in a Ti:sapphire chirped-pulse amplification system by using a stretcher based on two concave mirrors.

We demonstrate the temporal pedestal suppression in a Ti:sapphire chirped-pulse amplification laser system. The far-field spectral phase noise can be avoided by using a stretcher based on two concave mirrors in the system, reducing the intensity and the time range of the amplified pulse's pedestal. At the amplified energy of 1.7 J, the contrast measurement showed that the pedestal intensity was at a level of about 10-10 within a 10 ps time window near the main pulse. In the proton acceleration experiment, a 10 nm thickness CH target was irradiated by the high-contrast pulse with the focused intensity of ∼1.4×1020W/cm2, which generated a proton beam with a cutoff energy of 16 MeV.

Efficient generation of UV-enhanced intense supercontinuum in solids: Toward sub-cycle transient

Ultrabroadband generation of white-light continuum spanning from ultraviolet to near-infrared (375–920 nm) is demonstrated by using induced-phase modulation between two-color femtosecond pulses in multiple thin plates. The fundamental wave and its second-harmonic wave from one Ti:sapphire chirped-pulse amplifier are injected into nine 100 μm-thick fused silica plates. When the two pulses are temporally and spatially optimized in the plates, an intense supercontinuum is efficiently achieved by utilizing self-phase modulation and self-steepening together with induced-phase modulation. As a result, using 1 kHz, 1 mJ, and 35 fs pulses at 800 nm as the pump, we demonstrate the generation of 0.6 mJ white-light pulses with an ultra-broad bandwidth supporting 1.6 fs transform-limited pulses corresponding to 0.6-optical-cycle at a central wavelength of 750 nm. Analysis of the calculated spectrum fits the experimental results well, and the pulse is quasi-linearly chirped and compressible. The resulting intense robust supercontinuum could be a promising light source for an ever greater degree of tailored optical waveform coherent control in new parameter spaces.

Terawatt chirped pulse Raman amplified laser for two-color experiments

We report the generation of a two-color terawatt (TW) laser by the insertion of a three-stage barium nitrate Raman shifter and amplifier system into a conventional Ti:sapphire chirped-pulse amplification system. The Raman subsystem produces a pulse that is shifted from 800 to 873 nm and is amplified and compressed to TW scales (140 mJ and 140 fs) and then recombined with the 45-TW 800-nm fundamental pulse.

Status and progress of the J-KAREN-P high intensity laser system at QST

We report on femtosecond petawatt laser pulses at 0.1 Hz that combine both Ti:sapphire chirped-pulse amplification (CPA) and optical parametric CPA (OPCPA) techniques. High temporal contrast of 1012 prior to the main pulse of 10 J output energy has been demonstrated with a cleaned high-energy seeded low gain OPCPA preamplifier. Intensities as high as 1022 W∕cm2 on target have been achieved by focusing a wave-front corrected 0.3 PW laser by adaptive optics and reflective-type optics with an f ∕1.3 off-axis parabolic mirror. We describe the origins of the pre-pulses generated by the post-pulses through non-linear processes and demonstrate the removal of the pre-pulses by switching to optical components with a small wedge angle at our petawatt laser facility. We also briefly introduce some experimental results. Exploration of new regimes in high field science is now possible with the unprecedented laser intensity levels of the J-KAREN-P laser.

High-energy hybrid femtosecond laser system demonstrating 2 × 10 PW capability

Abstract We report on a two-arm hybrid high-power laser system (HPLS) able to deliver 2 × 10 PW femtosecond pulses, developed at the Bucharest-Magurele Extreme Light Infrastructure Nuclear Physics (ELI-NP) Facility. A hybrid front-end (FE) based on a Ti:sapphire chirped pulse amplifier and a picosecond optical parametric chirped pulse amplifier based on beta barium borate (BBO) crystals, with a cross-polarized wave (XPW) filter in between, has been developed. It delivers 10 mJ laser pulses, at 10 Hz repetition rate, with more than 70 nm spectral bandwidth and high-intensity contrast, in the range of 1013:1. The high-energy Ti:sapphire amplifier stages of both arms were seeded from this common FE. The final high-energy amplifier, equipped with a 200 mm diameter Ti:sapphire crystal, has been pumped by six 100 J nanosecond frequency doubled Nd:glass lasers, at 1 pulse/min repetition rate. More than 300 J output pulse energy has been obtained by pumping with only 80% of the whole 600 J available pump energy. The compressor has a transmission efficiency of 74% and an output pulse duration of 22.7 fs was measured, thus demonstrating that the dual-arm HPLS has the capacity to generate 10 PW peak power femtosecond pulses. The reported results represent the cornerstone of the ELI-NP 2 × 10 PW femtosecond laser facility, devoted to fundamental and applied nuclear physics research.

On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses

High-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undesired effects. Here, we present a detailed description of the effect of third-order dispersion (TOD) in the pump pulse on the generation of THz radiation via phase-matching of broadband highly chirped pulse trains. Although the analysis is general, we focus specifically on parameters typical to a Ti:Sapphire chirped-pulse amplification laser system for quasi-phase-matching in periodically-poled lithium niobate (PPLN) in the range of THz frequencies around 0.5 THz. Our analysis provides the tools to optimize the THz generation process for applications requiring high energy and to control it to produce desired THz waveforms in a variety of scenarios.

Broadband mid-infrared pulse via intra-pulse difference frequency generation based on supercontinuum from multiple thin plates**Project supported by the National Key R&D Program of China (Grant No. 2017YFB0405202), the Major Program of the National Natural Science Foundation of China (Grant No. 61690221), the Key Program of the National Natural Science Foundation of China (Grant No. 11434016), and the National Natural Science

We report on the generation of optical pulses with a nearly one octave-spanning spectrum ranging from 1300 nm to 2500 nm at 1 kHz repetition rate, which are based on intra-pulse difference frequency generation (DFG) in β-barium borate crystal (β-BBO) and passively carrier-envelope-phase (CEP) stabilized. The DFG is induced by few-cycle pulses initiated from spectral broadening in multiple thin plates driven by a Ti: sapphire chirped-pulse amplifier. Furthermore, a numerical simulation is developed to estimate the conversion efficiency and output spectrum of the DFG. Our results show that the pulses from the DFG have the potential for seeding intense mid-infrared (MIR) laser generation and amplification to study strong-field physics and attosecond science.

Two-beam coherent combining based on Ti:sapphire chirped-pulse amplification at the repetition of 1 Hz.

In this Letter, we report the demonstration of the coherent beam combining (CBC) of two-beam femtosecond laser pulses based on chirped-pulse amplification (CPA). To the best of our knowledge, this is the first experimental realization of coherent combination pulses from Ti:sapphire CPA systems. In this experiment, the combined laser beams operate at a repetition rate of 1Hz, and a reference laser is introduced to detect the phase error between the sub-beams. Finally, we realize a coherent combining efficiency of 90% which verified the feasibility for CPA CBC. Moreover, the influence of the amplification process on CBC is studied.

High-Power Ultrashort Pulse Lasers to Pump Plasma-Based Soft X-Ray Lasers

High-energy ultrashort pulse optical lasers operating at high repetition rate are required to drive intense compact coherent soft X-ray sources, including soft X-ray lasers, for applications in nanoscience and nanotechnology, material studies, and high field science. This paper reviews two types of high-power solid-state laser systems developed at Colorado State University to pump compact, high repetition rate, and plasma-based soft X-ray lasers. One is a flashlamp-pumped, petawatt-class Ti:sapphire chirped-pulse amplification laser that generates 0.85 PW pulses of 30 fs duration at 3.3 Hz repetition rate. Using a fraction of the output energy from this laser, we have demonstrated gain-saturated, compact, repetitive lasers down to 6.85 nm in Ni-like Gd and have observed gain in shorter wavelength transitions, down to 5.85 nm in Ni-like Dy. The second type of pump laser is an all diode-pumped chirped-pulse amplification system based on cryogenically cooled Yb:YAG active mirror amplifiers that produces 1 J pulses of picosecond duration at 500 Hz repetition rate. These compact diode-pumped laser systems have enabled new advances in high-average-power tabletop soft X-ray lasers that include 100 Hz demonstrations at wavelengths ranging from 10.9 to 18.9 nm, as well long duration operation at these high repetition rates for periods up to one hour. These and other recent results that include the operation of an 18.9 nm laser Ni-like Mo laser at 400 Hz repetition rate open a path to compact kilohertz repetition rate soft X-ray lasers.

Wavefront evolution of the signal beam in Ti:sapphire chirped pulse amplifier**Project supported by the National Natural Science Foundation of China (Grant No. 61775223) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB1603).

We studied the evolution of wavefront aberration (WFA) of a signal beam during amplification in a Ti:sapphire chirped pulse amplification (CPA) system. The results verified that the WFA of the amplified laser beam has little relation with the change of the pump beam energies. Transverse parasitic lasing that might occur in CPA hardly affects the wavefront of the signal beam. Thermal effects were also considered in this study, and the results show that the thermal effect cumulated in multiple amplification processes also has no obvious influence on the wavefront of the signal beam for a single-shot frequency. The results presented in this paper confirmed experimentally that the amplification in a Ti:sapphire CPA system has little impact on the WFA of the signal beam and it is very helpful for wavefront correction of single-shot PW and multi-PW laser systems based on Ti:sapphire.

339 J high-energy Ti:sapphire chirped-pulse amplifier for 10 PW laser facility.

We report on the laser pulse output of 339J centered at 800nm from a chirped-pulse amplification (CPA) Ti:sapphire laser system at the Shanghai Superintense Ultrafast Laser Facility. The experimental results demonstrated that the parasitic lasing as well as the transverse amplified spontaneous emission of the homemade 235-mm-diameter Ti:sapphire final amplifier were suppressed successfully via the temporal dual-pulse pumped scheme and the index-matching liquid cladding technique. The maximum pump-to-signal conversion efficiency of 32.1% was measured for the final amplifier. With a compressor transmission efficiency of 64% and a compressed pulse duration of 21fs obtained for the sample light at a lower energy level, this laser system could potentially generate a compressed laser pulse with a peak power of 10.3PW. The experimental results represent significant progress with respect to the CPA laser.

Improvement of the focusing ability by double deformable mirrors for 10-PW-level Ti: sapphire chirped pulse amplification laser system.

Double deformable mirrors (DMs) with different actuator densities are cascaded to optimize the wavefront aberrations to improve the focus intensity of the Shanghai super-intense ultrafast laser facility (SULF), which plans to generate 10 PW laser pulse. The beam aberrations near the focal spot are corrected from 0.556 um to 0.112 um in RMS by a 300-mm DM with a large stroke installed after the compressor. After then, it is further optimized to 0.041 um using a 130-mm DM with a high spatial resolution working after the main amplifier. The corrected beam is focused to 2.75 × 2.87 um2 at the full width at half maximum (FWHM) with an f/2.5 off-axis parabolic mirror (OAP), which contains approximately 27.69% energy. A peak intensity of 2 × 1022 W/cm2 is achieved at the output of 5.4 PW, and it could exceed 1023 W/cm2 in the SULF 10 PW laser facility using an f/1.8 OAP.

Coherent synthesis of ultrashort pulses based on balanced optical cross-correlator

Coherent synthesis of laser pulses is a major trend in the generation of ultrafast pulse field. There is no good way to compensate for the whole spectrum when the spectrum of ultrashort pulses is wide enough to reach an octave, so it is difficult to realize a sub-cycle pulse in a single-path laser system even if the spectrum range is wide enough. In this paper, 0.8 mJ, 30 fs laser pulses at 1 kHz repetition rate with 790 nm center wavelength from a Ti:sapphire chirped pulse amplifier (CPA) system are focused into hollow fiber with an inner diameter of 250 μm and a length of 1 m to produce an octave-spanning white-light supercontinuum (450-950 nm). Using this supercontinuum, we conduct two sets of comparative experiments. 1) We split the supercontinuum into two pulses with different spectrum ranges (450-750 nm and 650-1000 nm) by a dichroic mirror (HR, 500-700 nm; HT, 700-1000 nm), and we compress the two pulses by the double-chirped mirrors and wedge pairs to generate two few-cycle pulses:the long and short wavelength yielding pulses are 7.9 fs and 6.1 fs, respectively. Then we coherently synthesize two pulses by using another dichroic mirror, and controlling the relative time delay between the two pulses, and thus we synthesize a pulse of 4.1 fs. 2) We directly compress the supercontinuum by the double-chirped mirrors and wedge pairs, and obtain an optimization result of 5.3 fs, of which the pulse duration is wider than that in experiment 1. In these comparative experiments, the advantage of coherent synthesis for shorter pulse duration is preliminarily verified. Besides, the balanced optical cross-correlator technique is used to lock the relative time delay between two pulses. The root-mean-square value of relative time delay drift is less than 80 as in the case with feedback control loop, which ensures the stability of coherent synthesis system. This scheme can be adopted to accurately compensate for the dispersion and obtain the sub-cycle synthesized pulse, which is useful for generating the high harmonic and atto-second pulse.

Generation of sub-two-cycle millijoule infrared pulses in an optical parametric chirped-pulse amplifier and their application to soft x-ray absorption spectroscopy with high-flux high harmonics

An optical parametric chirped-pulse amplifier (OPCPA) based on bismuth triborate (BiB3O6, BIBO) crystals has been developed to deliver 1.5 mJ, 10.1 fs optical pulses around 1.6 μm with a repetition rate of 1 kHz and a stable carrier-envelope phase. The seed and pump pulses of the BIBO-based OPCPA are provided from two Ti:sapphire chirped-pulse amplification (CPA) systems. In both CPA systems, transmission gratings are used in the stretchers and compressors that result in a high throughput and robust operation without causing any thermal problem and optical damage. The seed pulses of the OPCPA are generated by intrapulse frequency mixing of a spectrally broadened continuum, temporally stretched to approximately 5 ps then, and amplified to more than 1.5 mJ. The amplified pulses are compressed in a fused silica block down to 10.1 fs. This BIBO-based OPCPA has been applied to high-flux high harmonic generation beyond the carbon K edge at 284 eV. The high-flux soft-x-ray continuum allows measuring the x-ray absorption near-edge structure of the carbon K edge within 2 min, which is shorter than a typical measurement time using synchrotron-based light sources. This laser-based table-top soft-x-ray source is a promising candidate for ultrafast soft x-ray spectroscopy with femtosecond to attosecond time resolution.

200 J high efficiency Ti:sapphire chirped pulse amplifier pumped by temporal dual-pulse.

We report on an experimental and theoretical study of a large-aperture Ti:Sapphire (Ti:S) amplifier pumped with a novel temporal dual-pulse scheme to suppress the parasitic lasing (PL) and transverse amplified spontaneous emission (TASE) for high-energy chirped-pulse amplification (CPA). The pump energy distribution was optimized and the time delay between each pump pulse was controlled precisely. Both the numerical and experimental results confirm that the temporal dual-pulse pump technique can effectively suppress PL and TASE. The maximum output energy of 202.8 J was obtained from the final 150-mm-diameter Ti:S booster amplifier with a pump energy of 320.0 J, corresponding to a conversion efficiency of 49.3%. The compressed pulse duration of 24.0 fs was measured with a throughput efficiency of 64%, leading to a peak power of 5.4 PW. This novel temporal dual-pulse pump technique has potential applications in a 10 PW CPA laser system.

Spatial chirp in Ti:sapphire multipass amplifier**Project supported by 100 Talents Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 61475169, 61521093, and 11127901), and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

The spatial chirp generated in the Ti:sapphire multipass amplifier is numerically investigated based on the one-dimensional (1D) and two-dimensional (2D) Frantz–Nodvik equations. The simulation indicates that the spatial chirp is induced by the spatially inhomogeneous gain, and it can be almost eliminated by utilization of proper beam profiles and spot sizes of the signal and pump pulses, for example, the pump pulse has a top-hatted beam profile and the signal pulse has a super-Gaussian beam profile with a relatively larger spot size. In this way, a clear understanding of spatial chirp mechanisms in the Ti:sapphire multipass amplifier is proposed, therefore we can effectively almost eliminate the spatial chirp and improve the beam quality of a high-power Ti:sapphire chirped pulse amplifier system.

Parasitic lasing in large aperture Ti:sapphire chirped pulse amplifier

We research some properties of parasitic lasing (PL) in the Ti: sapphire chirped pulse amplifier with the crystal diameter of 100 mm. The evolutionary process from the spontaneous emission to the PL and its influence on amplified output energy, spectrum, and beam profile are experimentally measured. The threshold of PL in the crystal is 22 J, and the output signal can still keep rising with the pump when the pump energy is below 38 J. The PL has no obvious impact on the output spectrum and beam profile besides the energy.

Conical third-harmonic generation of optical vortex through ultrashort laser filamentation in air.

We experimentally generate third-harmonic (TH) vortex beams in air by the filamentation of femtosecond pulses produced in a lab-built Ti:sapphire chirped pulse amplifier. The generated TH beam profile is shown to evolve with increasing pump energy. At a sufficiently high pump energy, we observe a conical TH emission of the fundamental vortex and confirm that the conical radiation follows the conservation law for orbital angular momentum. We further investigate the far-field angularly resolved spectra of the TH wave to analyze the conical emission angle. We theoretically verify that the formation of the conical TH vortex results from the phase-matching between the fundamental and TH waves during the filamentation process.