High-harmonic generation (HHG) in gaseous media is a workhorse tool in attosecond science. Its description originates from the microscopic scale of a single atom or molecule interacting with a driving IR-laser pulse. A complete macroscopic picture corresponding to usual experimental realizations aggregates all the microscopic emitters and brings novel physical mechanisms that drive the generation. One of the key mechanisms within the macroscopic scale is the shaping of the driving pulse due to the non-linear response of the medium.
We present a comprehensive numerical model describing and coupling the physics on both scales. The model consists of different modules that provide different levels of approximation to choose an optimal trade-off between accuracy and computational cost. We then use it to address two generation schemes, for which we provide a detailed picture together with experimental realizations. The first scheme uses a long medium homogeneously pre-ionized by an electrical discharge to optimize the phase-matching of the harmonic signal. This scheme allows, in particular, for optimizing HHG in long media where the control is difficult because the driving pulse undergoes strong re-shaping and defocusing due to the non-linear response after the entrance to the medium. The second scheme is based on the driving-pulse-wavefront shaping that is imprinted in the HHG beam and used to control the harmonic-beam divergence. This mechanism is harmonic dependent and may provide a tunable spectral filter of the harmonic spectrum. The proposed scheme is optics-free and dodges unavoidable losses inherent to the use of optics in the XUV region.
The Laser Megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion and plasma physics research. LMJ front-ends are based on fiber laser technology at nanojoule range [1]. Scaling the energy of those fiber seeders to the millijoule range is a way to upgrade LMJ’s front ends architecture and could also be used as seeder for lasers for ELI project for example. However, required performances are so restrictive (optical-signal-to-noise ratio higher than 50 dB, temporally-shaped nanosecond pulses and spatial single-mode top-hat beam output) that such fiber systems are very tricky to build.
High-energy fiber amplifiers
In 2015, we have demonstrated, an all-fiber MOPA prototype able to produce a millijoule seeder, but unfortunately not 100% conform for all LMJ’s performances. A major difficulty was to manage the frequency modulation used to avoid stimulated Brillouin scattering, to amplitude modulation (FM-AM) conversion, this limits the energy at 170µJ.
For upgrading the energy to the millijoule range, it’s necessary to use an amplifier with a larger core fiber. However, this fiber must still be flexible; polarization maintaining and exhibit a strictly single-mode behaviour. We are thus developing a new amplifier architecture based on an Yb-doped tapered fiber: its core diameter is from a narrow input to a wide output (MFD 8 to 26 µm). A S² measurement on a 2,5m long tapered fiber rolled-up on 22 cm diameter confirmed that this original geometry allows obtaining strictly single-mode behaviour. In a 1 kHz repetition rate regime, we already obtain 750 µJ pulses, and we are on the way to mJ, respecting LMJ performances.
Beam delivery
In LMJ architecture the distance between the nanojoule fiber seeder and the amplifier stages is about 16 m. Beam delivery is achieved with a standard PM fiber, such a solution is no longer achievable with hundreds of kilowatt peak powers. An efficient way to minimize nonlinear effects is to use hollow-core (HC) fibers. The comparison between the different fibers will be presented in the conference.
Fiber spatial beam shaping
Spatial beam shaping (top-hat profile) is mandatory to optimize the energy extraction in free-space amplifier. It would be very interesting to obtain a flat-top beam in an all-fiber way. Accordingly, we have design and realize a large mode area single-mode top-hat fiber able to deliver a coherent top-hat beam. This fiber, with larger MFD adapted to mJ pulse, will be implemented to perform the spatial beam shaping from coherent Gaussian profile to coherent top-hat intensity profile in the mJ range. In conclusion, we will present an all-fiber MOPA built to fulfil stringent requirements for large scale laser facility seeding. We have already achieved 750 µJ with 10 ns square pulses. Transport of high peak power pulses over 17 m in a hollow-core fiber has been achieved and points out FM to AM conversion management issues. Moreover, spatial beam shaping is obtained by using specifically designed single-mode fibers. Various optimizations are currently under progress and will be presented.
The Laser megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion research. Its front-ends are based on fiber laser technology and generate highly controlled beams in the nanojoule range. Scaling the energy of those fiber seeders to the millijoule range is a way explored to upgrade LMJ’s architecture.
We report on a fully integrated narrow line-width all-fiber MOPA prototype at 1053 nm designed to meet stringent requirements of large-scale laser facilities seeding. We achieve 750 µJ temporally-shaped pulses of few nanoseconds at 1 kHz. Thanks to its original longitudinal geometry and its wide output core (26µm MFD), the Yb-doped tapered fiber used in the power amplifier stage ensures a single-mode operation and negligible spectro-temporal distortions. The transport of 30 kW peak power pulses (from tapered fiber) in a 17 m long large mode area (39µm) hollow-core (HC) fiber is presented and points out frequency modulation to amplitude modulation conversion management issues. A S² measurement of this fiber allows to attribute this conversion to a slightly multimode behavior (< 13dB of extinction between the fundamental mode and higher order modes). Other HC fibers exhibiting a really single-mode behavior (<20 dB) have been tested and the comparison will be presented in the conference. Finally, fiber spatial beam shaping from coherent Gaussian beam to coherent top-hat intensity profile beam in the mJ range with a specifically designed and fabricated fiber will also be presented.
We report on the first polarization maintaining single-mode fiber that delivers a flat-top intensity profile at 1050 nm. A high quality fundamental flat mode was obtained. We showed that our fiber can be considered as single-mode in practice with low confinement losses. Its birefringence was measured to be 0.6x10-4, and the PER was measured at more than 20 dB even for a 20 m fiber long. Strategies to enhance this birefringence preserving the flat top profile and the singlemode behaviour as well are also discussed.
Compactness, long term stability and no free-space alignment are important advantages of fiber lasers over bulky systems. These fiber lasers have also demonstrated their capability to deliver high-power pulses and are thus suitable for numerous applications. Nevertheless the intensity profile delivered usually has a Gaussian-like shape, which most of the time is sufficient, but it could be interesting, for many applications (laser-biological tissues interactions, heat treatment, industrial laser processing or for seeding large-scale laser facilities like Laser MegaJoule) to obtain a homogeneous intensity profile at the fiber laser output. Moreover several of these applications required a linearly polarized output beam. In order to achieve all these requirements we have developed and realized a new fiber design. This fiber is the first polarization maintaining single-mode fiber delivering a flat top intensity. A high quality flat mode was obtained at 1.05μm through the use of a well-tailored index profile and single-mode behavior was verified by shifting the injection and using the S² imaging. Moreover, boron Stress Applying Parts (SAPs) including in the cladding led to a birefringence of 0.6x10-4 and a measured PER better than 20dB even for a long fiber length (~20 m). Alongside the fabrication, we developed a simulation code, using Comsol Multiphysics®, to take into account the stress dependency induced by the SAPs. Further modeling allows us to present an effectively single-mode fiber design, delivering a top-hat mode profile and exhibiting a polarizing behavior.
In large scale laser facility dedicated to laser-matter interaction including inertial confinement fusion, such as LMJ or NIF, high-energy main amplifier is injected by a laser source in which the beam parameters must be controlled. For many years, the CEA has developed nano-joule pulses all-fiber front end sources, based on the telecommunications fiber optics technologies. Thanks to these technologies, we have been able to precisely control temporal shaping and phase-modulated pulse. Nowadays, fiber lasers are able to deliver very high power beams and high energy pulses for industrial needs (laser marking, welding,…). Therefore, we have currently developed new nanosecond pulses fibered amplifiers able to increase output pulse energy up to the mJ level. These amplifiers are based on flexible fibers and not on rod type. This allows us to achieve a compact source. Nevertheless the intensity profile of theses fibers usually has a Gaussian shape. To be compatible with main amplifier section injection, the Gaussian intensity profile must then be transformed into ‘top-hat’ profile. To reach the goal, we have recently developed an elegant and efficient solution based on a single-mode fiber which directly delivers a spatially coherent ‘top-hat’ beam. In the conference, we will present this mJ-class top-hat all-fiber laser system, the results and the industrial prototype which can be used as a front-end of high-power lasers or as a seeder for other types of lasers.
Solid-Core Photonic BandGap Fibers (SC-PBGF) belongs to the family of microstructured optical fibers whereby the cladding is made of high refractive index inclusions as compared to that of the fiber core. In such fibers, light is confined to the core by an anti-resonant mechanism and several high transmission windows separated by high loss regions compose the transmission spectrum. Guiding mechanism is then identical to the one observed in Hollow-Core PBGF (HC-PBGF) except that a solid core can be exploited. Such PBGFs have proven to be good candidates for single-mode high power delivery and for controlling the spectral extension of supercontinuum generation. Mixing different types of resonators in the cladding or mixing PBG with modifed-Total Internal Reection (m-TIR) mechanism also lead to original and more exible fiber designs. Recent developments in the design and realization of Large Mode Area (LMA) and Highly NonLinear (HNL) fibers are presented, including single-mode ring-structured Bragg _bers, LMA fibers exhibiting a fundamental mode with a flat-top profile, and hybrid fibers for supercontinuum generation or frequency conversion.
We present a new passive air/silica microstructured optical fiber designed to be single mode and which delivers a flat-top
intensity profile at 1 μm. By inclusion of a raised index ring surrounding the central core, the refractive index profile of
the fiber flattens the intensity distribution of the fundamental mode. Experimental results clearly demonstrate the
feasibility of all-fibered top-hat beam delivery systems with one spatial mode suitable for many applications.
We report the first experimental demonstration of an optical fiber supporting a fundamental mode with flattened intensity
profile around 1050 nm. The design has been defined through intensive numerical simulations by paying a special
attention to the constraints imposed by the fabrication process. We show that the fabricated fiber presents a single-mode
behaviour.
Ultra-fast soft x-ray lasers have opened a new area of laser-matter interactions which in most cases differ from the well
understood interaction of UV-vis radiation with solid targets. The photon energy >30eV essentially exceeds the width of
band gap in any known material and excites the electrons from the deep atomic and valence levels directly to the
conduction band. Both thermal and non-thermal phenomena can occur in such a material being caused by electron
thermalization and bond breaking, respectively. We report the first observation of non-thermal single-shot soft x-ray
laser induced desorption occurring below the ablation threshold in a thin layer of poly (methyl methacrylate) - PMMA.
Irradiated by the focused beam from the Free-electron LASer in Hamburg (FLASH) at 21.7nm, the samples have been
investigated by an atomic-force microscope (AFM) enabling the visualization of mild surface modifications caused by
the desorption. A model describing non-thermal desorption and ablation has been developed and used to analyze singleshot
imprints in PMMA. An intermediate regime of materials removal has been found, confirming the model predictions.
We also report below-threshold multiple-shot desorption of PMMA induced by high-order harmonics (HOH) at 32nm as
a proof of an efficient material removal in the desorption regime.
Among X-ray and extreme ultraviolet light sources able to produce shorter and shorter, coherent and intense pulses, High
order harmonics generated in rare gases are currently the unique way to generate attosecond pulses. However, the
manipulation and transport of attosecond pulses require the development of dedicated optics for reaching specific
characteristics in terms of amplitude but also in terms of spectral phase control. We present here a multilayer design for
chirp compensation of attosecond pulses. We also present an application of these multilayers mirrors for attosecond train
pulse holography experiment with high harmonics. This experience took benefit of both temporal and spatial phase
properties of high harmonics. A resolution of 750 nm has been achieved by using a 350 as train pulse for the reference
wave constituted of four consecutive harmonics (λ=28 nm to λ=41 nm). This new method will allow making ultra fast
movies with attosecond resolution of transient phenomena with quasi-3D resolution.
Metrology of XUV beams and more specifically X-ray laser (XRL) beam is of crucial importance for development of applications. We have then developed several new optical systems enabling to measure the x-ray laser optical properties. By use of a Michelson interferometer working as a Fourier-Transform spectrometer, the line shapes of different x-ray lasers have been measured with an unprecedented accuracy (δλ/λ~10-6). Achievement of the first XUV wavefront sensor has enable to measure the beam quality of laser-pumped as well as discharge pumped x-ray lasers. Capillary discharge XRL has demonstrated a very good wavefront allowing to achieve intensity as high 3*1014 Wcm-2 by focusing with a f = 5 cm mirror. The measured sensor accuracy is as good as λ/120 at 13 nm. Commercial developments are under way.
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