High-energy femtosecond Yb-doped all-fiber monolithic chirped-pulse amplifier at repetition rate of 1 MHz

A high-energy femtosecond all ytterbium fiber amplifier based on a chirped-pulse amplification（CPA） technique at a repetition rate of 1 MHz seeded by a dispersion-management mode-locked picosecond broadband oscillator is studied.We find that the compressed pulse duration is dependent on the amplified energy,the pulse duration of 804 fs corresponds to the maximum amplified energy of 10.5 μJ,while the shortest pulse duration of 424 fs corresponds to the amplified energy of 6.75 μJ.The measured energy fluctuation is approximately 0.46% root mean square（RMS） over 2 h.The low-cost femtosecond fiber laser source with super-stability will be widely used in industrial micromachines,medical therapy,and scientific studies.

Femtosecond laser user facility for application research on ultrafast science

The advent of chirped-pulse amplification （CPA） has greatly advanced the field of ultrafast and ultra-intense laser technology. CPA has become an indispensable platform for multidisciplinary research, such as physics, chemistry, life sciences, and precision metrology. The femtosecond laser facility at the Synergic Extreme Condition User Facility （SECUF） is a comprehensive experimental platform with an advanced femtosecond laser source for ultrafast scientific research. It will provide an ultrafast scientific research system having a few-cycle pulse duration, wide spectral range, high energy, and high repetition rate for multipurpose applications.

Optical terminal analysis of a multigrating tiled compressor in a PW-class CPA-laser

In the highest-power chirped-pulse amplification lasers, the pulse must be stretched in time, amplified, compressed in a grating compressor and subsequently focused by off-axis parabola to obtain a high peak power. In the optical terminal, the temporal and spatial effects of mismatched multigrating tiled compressor on the far-field pulse are critical factors to be analysed. In this paper, a k-space raytracing model is proposed for the temporal and spatial analyses of possible errors in a four-grating single-pass tiled compressor. The results show that the last grating affects mainly the partial focal spot, while the middle two gratings affect the temporal waveform, and the partial focal spot needs much higher error control than that in the temporal domain in a picosecond pulse compression.

MTW-OPAL:a technology development platform for ultra-intense optical parametric chirped-pulse amplification systems

Optical parametric chirped-pulse amplification implemented using multikilojoule Nd:glass pump lasers is a promising approach for producing ultra-intense pulses(>10^(23)W/cm^(2)).We report on the MTW-OPAL Laser System,an optical parametric amplifier line(OPAL)pumped by the Nd:doped portion of the multi-terawatt(MTW)laser.This midscale prototype was designed to produce 0.5-PW pulses with technologies scalable to tens of petawatts.Technology choices made for MTW-OPAL were guided by the longer-term goal of two full-scale OPALs pumped by the OMEGA EP to produce 2×25-PW beams that would be co-located with kilojoule-nanosecond ultraviolet beams.Several MTWOPAL campaigns that have been completed since“first light”in March 2020 show that the laser design is fundamentally sound,and optimization continues as we prepare for“first-focus”campaigns later this year.

High power Yb-fiber laser amplifier based on nonlinear chirped-pulse amplification at a repetition rate of 1 MHz

We report a high power fiber amplifier based on nonlinear chirped-pulse amplification(NCPA).To manage the nonlinearity,pulse shaping is introduced by self-phase modulation in the fiber stretcher with the help of spectral filtering.The third-order dispersion is compensated for by the nonlinear phase shift in the NCPA.With optimization,the system can output 382 fs pulse duration with 20 W average power at 1 MHz repetition rate.The long-term average power fluctuation is measured to be 0.5%in 24 h,and the beam quality factor(M2)is 1.25.

How the laser beam size conditions the temporal contrast in pulse stretchers of chirped-pulse amplification lasers

In this work,we propose and verify experimentally a model that describes the concomitant influence of the beam size and optical roughness on the temporal contrast of optical pulses passing through a pulse stretcher in chirped-pulse amplification laser systems.We develop an analytical model that is capable of predicting the rising edge caused by the reflection from an optical element in a pulse stretcher,based on the power spectral density of the surface and the spatial beam profile on the surface.In an experimental campaign,we characterize the temporal contrast of a laser pulse that passed through either a folded or an unfolded stretcher design and compare these results with the analytical model.By varying the beam size for both setups,we verify that optical elements in the near-and the far-field act opposed to each with respect to the temporal contrast and that the rising edge caused by a surface benefits from a larger spatial beam size on that surface.

Experimental investigation on pulse-contrast degradation caused by surface reflection in optical parametric chirped-pulse amplification

In this Letter, we experimentally explore the pulse-contrast degradation caused by surface reflection in optical parameter chirped-pulse amplification. Different pump-to-signal conversion efficiencies and post-pulses with different intensities are obtained by changing the seed-pulse or pump-pulse energy and inserting etalons with different reflection coefficients, respectively. The contrast measurements show that the generated first pre-pulse intensity is proportional to the product of the surface reflection intensity ratio and the square of the pump-to-signal conversion efficiency.

Numerical study of spatial chirp distortion in quasi-parametric chirped-pulse amplification

Optical parametric chirped-pulse amplification is inevitably subject to high-order spatial chirp,particularly under the condition of saturated amplification and a Gaussian pump;this corresponds to an irreversible spatiotemporal distortion and consequently degrades the maximum attainable focused intensity.In this paper,we reveal that such spatial chirp distortion can be significantly mitigated in quasi-parametric chirped-pulse amplification(QPCPA)with idler absorption.Simulation results show that the quality of focused intensity in saturated QPCPA is nearly ideal,with a spatiotemporal Strehl ratio higher than 0.98.As the seed bandwidth increases,the idler absorption spectrum may not be uniform,but the Strehl ratio in QPCPA can be still high enough due to stronger idler absorption.

Highly efficient TW multipass Ti:sapphire laser system

An efficient chirped-pulse amplification, Ti:sapphire laser system, has been developed using mainly domestic components. The gain-narrowing effect has been significantly overcome by shaping spectrum of seeding pulses. With a novel aberration-free stretcher and two stage multi-pass amplifiers, pulses with duration of 25 fs and 36-mJ energy have been obtained at 10 Hz repetition rate, using only less than 290 mJ green Nd: YAG pump energy. This corresponds to a 1.4 TW peak power and 32% main amplification efficiency. The energy stability of the laser systems is better than ± 3% .

Technology development for ultraintense all-OPCPA systems

Optical parametric chirped-pulse amplification(OPCPA) [Dubietis et al., Opt. Commun. 88, 437(1992)] implemented by multikilojoule Nd:glass pump lasers is a promising approach to produce ultraintense pulses(>1023 W/cm2).Technologies are being developed to upgrade the OMEGA EP Laser System with the goal to pump an optical parametric amplifier line(EP OPAL) with two of the OMEGA EP beamlines. The resulting ultraintense pulses(1.5 kJ, 20 fs,1024 W/cm2) would be used jointly with picosecond and nanosecond pulses produced by the other two beamlines. A midscale OPAL pumped by the Multi-Terawatt(MTW) laser is being constructed to produce 7.5-J, 15-fs pulses and demonstrate scalable technologies suitable for the upgrade. MTW OPAL will share a target area with the MTW laser(50 J, 1 to 100 ps), enabling several joint-shot configurations. We report on the status of the MTW OPAL system, and the technology development required for this class of all-OPCPA laser system for ultraintense pulses.

Design and experimental demonstration of a high conversion efficiency OPCPA pre-amplifier for petawatt laser facility

We present the design and experiment of a broadband optical parametric chirped-pulse amplifier(OPCPA) which provides high conversion efficiency and good beam quality at 808 nm wavelength. Using a three-dimensional spatial and temporal numerical model, several design considerations necessary to achieve high conversion efficiency, good beam quality and good output stability are discussed. To improve the conversion efficiency and broaden the amplified signal bandwidth simultaneously, the nonlinear crystal length and OPCPA parameters are analyzed and optimized with the concept of dissipating amplified idler between optical parametric amplification(OPA) of two crystals configuration.In the experiment, an amplifier consisting of two OPCPA stages of ‘L’ type configuration was demonstrated by using the optimized parameters. An amplified signal energy of 160 mJ was achieved with a total pump-to-signal efficiency of 35%(43% efficiency for the OPCPA stage 2). The output bandwidth of signal pulse reached 80 nm and the signal pulse was compressed to 24 fs. The energy stability reached 1.67% RMS at 3% pump energy variation. The optimized OPCPA amplifier operates at a repetition rate of 1 Hz and is used as a front-end injection for the main amplifier of SG-II 5PW laser facility.

Ultrahigh temporal contrast performance of the PHELIX petawatt facility

We report on the temporal contrast performance of the PHELIX facility in view of the requirements imposed by solidtarget interaction experiments. The requirement analysis for the nanosecond and picosecond temporal contrast is derived from empirical data and simple theoretical modeling, while the realization shows that using an ultrafast optical parametric amplifier and plasma mirrors enables meeting this specification.

5.4-TW/46-fs 10-Hz Ti:sapphire laser system

We have developed a compact Ti:sapphire laser system which can generate 5.4-Tw pulses at 10-Hz repetition rate. Chirped-pulses are amplified to 450 mj by a regenerative amplifier, a 4-pass amplifier and a 3-pass amplifier. After compression, 250-mJ, 46-fs pulses are obtained.

Ultra-broad band supercontinuum produced by terawatt femtosecond laser

The characteristics of supercontinuum produced by high-intensity femtosecond pulses were investigated under different interaction lengths, various pump intensities, different pump wavelengths at the fundamental 800 nm and its second-harmonic 400 nm. High transfer-efficiency supercontinuum with flat-top in liquid media was produced. As the main nonlinear mechanisms, the effects of self-phase modulation (SPM) and four-photon parametric emission were also investigated.

Enhancement of pre-pulse and picosecond pedestal contrast of the petawatt J-KAREN-P laser

We have experimentally improved the temporal contrast of the petawatt J-KAREN-P laser facility.We have investigated how the generation of pre-pulses by post-pulses changes due to the temporal overlap between the stretched pulse and the post-pulse in a chirped-pulse amplification system.We have shown that the time at which the pre-pulse is generated by the post-pulse and its shape are related to the time difference between the stretched main pulse and the post-pulse.With this investigation,we have found and identified the origins of the pre-pulses and have demonstrated the removal of most pre-pulses by eliminating the post-pulse with wedged optics.We have also demonstrated the impact of stretcher optics on the picosecond pedestal.We have realized orders of magnitude enhancement of the pedestal by improving the optical quality of a key component in the stretcher.