Abstract
1 Introduction: Apollon design and status
The Apollon laser, currently under construction at Orme des Merisiers, Saclay, France, will be among the first multi-PW installations in the world devoted to the study of high-intensity laser matter interaction in unprecedented regimes and with peak intensities surpassing
The general design characteristics of this unique laser facility have been already extensively presented in previous works[
The beam is then separated into four beams: the main one (F1) to provide up to 150 J, 10 PW pulses after compression, the secondary 1 PW beam (F2) and two more beams: the remaining uncompressed (F3) beam (with variable energy depending on the energy required for F1) and a 10 TW level beam (F4) used as a probe beam. Compression of the F1, F2 and F4 beams is achieved in three separate gold grating-based compressors. Of particular interest is the main 10 PW compressor employing meter size gratings (
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All four beams can be independently distributed in two separate, ionizing radiation protected experimental areas. The first one is dedicated for high-intensity laser matter interaction experiments using mostly solid targets and fast focusing parabolas (
The building of the Apollon facility has been delivered about one year ago (
2 Temporal aspects of the Apollon laser
Several have been the challenges for the design and realization of the Apollon laser. Among them, maybe the most critical ones are those related to the temporal quality of the delivered pulses. Here, we discuss two aspects: the pulse duration and the pulse contrast. Indeed, we are trying to achieve the shortest pulses ever obtained for PW class systems. The typical design goal for the pulse duration for most of the Ti:sapphire-based PW class systems is around 25–30 fs, often limited in reality around 40–50 fs. In our case, very special care has been given to provide all the conditions to achieve, in a realistic and reliable way, 15 fs pulses. Our main motivation is of course the maximization of the intensity, considering the energy limitations due to the current fabrication capacity of large size Ti:sapphire crystals and gratings. The second very critical parameter is the quality of these pulses, often summarized by the so-called contrast ratio (CR). The CR definition is given in different temporal zones of the pulse, where specific upper limiting values of the ratio between the main peak of the pulse and its background are intended. In Table
The pulse duration and, even more, the CR are subject to numerous constrains, with the Apollon specifications considered among the most challenging and hard to achieve. An exhaustive list of these limiting factors is practically impossible, and in Table
Table 1. Temporal duration and contrast intended values and limiting factors considered for the Apollon laser design.
Most of the limiting factors listed above are directly related to the fundamental design considerations of our laser and their nature is more technological than physical. In other words, it is not a physical law that sets a definite upper limit, but, in a general manner, it is the currently available and commercially accessible technology of components and sources that governs the impact of this class of factors. On the contrary, however, a series of effects, which we group under the title of spatiotemporal coupling, presents a particular interest and a qualitatively different challenge. In these cases the limitation is inherent to a physical process resulting in the coupling of the beam parameters in space and time in a way that is often impossible to fully control. The interest in this case is to correctly quantify the impact, understand the nature of the process and ideally limit it to the minimum possible level.
In the following paragraphs we describe first the basic design considerations of our system and eventually how these considerations can guarantee the required performances against the first category of limiting factors. In Section
3 Design considerations of the Apollon laser: impact on the temporal aspects
The most critical point of our design is the Front End of the Apollon laser. Most of the qualities of the output pulses are defined at this stage, while for the rest of the system our design approach is conservative in the sense that, we try to assure that these initial qualities are preserved throughout the entire laser chain.
The Front End of Apollon combines two powerful technologies to provide large bandwidth, high-contrast pulses to the input of the main amplification section. Crossed polarized wave (XPW) generation and OPCPA, operating in the few picosecond regime, guarantee the generation of sub-10 fs pulses with a contrast estimated to be as high as
To quantify these characteristics the output of the OPCPA stage of the Front End has been compressed using the Apollon stretcher, near the zero dispersion range, as a compressor for the few picosecond (6 ps) output pulses. In Figure
The second design consideration concerns the main amplification section. This sub-system is designed to provide a low ASE noise, bandwidth preserving operation. The goal is to minimize the total ASE contribution below the
The third critical design consideration is related to the beam transport throughout the complete laser chain up the experimental chambers. Two main aspects have been considered: (1) the imaging strategy and the minimization of chromatic aberrations and (2) the mirrors technology choice.
Relay imaging in the Apollon laser is based on an optimized combination of doublet lense-based telescopes (for beam diameter up to 18 mm), followed by 5 telescopes using
Mirror technology plays a crucial role for the temporal aspects of the Apollon laser. A set of highly challenging aspects is required including the high damage threshold, the low group delay dispersion (GDD), the large bandwidth and the large size. This combination restricts greatly the commercially available options which even until recently could not fully meet our requirements. Today a qualified, extensively tested and already partially delivered solution has been provided for the Apollon laser (Reosc SAFRAN) meeting all the critical requirements[
The simulation of their reflectivity impact on the Apollon pulses is summarized in Figure
The final key design consideration is related to the management of the spectral phase. Our approach is based on a two-stage minimization of the residual spectral phase. Initially, this is obtained with the precise optimization of the unmatched stretcher–compressor pair. Taking into account the exact type and quantity of material in the chain as well as the GDD of all the mirror coatings, we estimate that we can achieve a minimum pulse duration around 22 fs (assuming an ideal 15 fs Gaussian input pulse, shown in Figure
4 The impact of the spatiotemporal coupling
Four main spatiotemporal coupling effects have been identified and studied so far in the frame of the Apollon laser: (1) the inhomogeneous amplification in the Ti:sapphire disks due to spatial variations of the gain and the saturation, (2) the impact of the finite gratings size and the diffraction effects inside the 10 PW compressor, (3) the low-order spatial-to-spectral phase coupling in the compressors induced by the flatness imperfections of optics handling spectrally dispersed beams and (4) the high-order spatial-to-spectral phase coupling in the stretcher and the compressors induced by the rugosity of their optics.
4.1 Inhomogeneous amplification
The first effect refers to any kind of input beam spatial variations leading to a spatially varying total accumulated gain and saturation for each sub-section of the output beam. As a first example we studied the effect of a Gaussian input beam profile and its progressive evolution to a super-Gaussian one.
In Figure
We observe indeed that the broadest spectrum is obtained in the center of the output beam, where the gain narrowing and the red-shifting effect are nominally compensated. However, as we progressively reach the outer parts of the beam the corresponding input fluence is reduced leading to higher gain and less saturation. The result is less red-shifted and narrower spectra as we move from the center to the edge of the output beam. For this specific simulation, taking into account the energy content of each ring of the beam, we estimate that the corresponding average FTL duration is about 16 fs. However, this is only an approximation since the important parameter is the delivered intensity in the far-field of such an inhomogeneous beam. This study is ongoing along with dedicated experiments to further investigate the real impact of the effect. In parallel, an alternative optimization of the spectral filtering is scheduled to better balance the effect and provide a more uniform output beam.
The second impact of this radial dependence of the gain and the saturation dynamics is on the spectral phase due to the inhomogeneous atomic phase shift in the Ti:sapphire amplifiers. In a previous study[
4.2 Finite size compressor gratings
The second effect is related to the limited size gratings used in high-intensity compressors. In the Apollon laser, for the 10 PW main compressor the size of the gratings (
The complex character of the effect deserves further investigation and experimental verification to correctly define its real impact before corrective measures are decided. It is however highly challenging to experimentally investigate such an effect since it involves a spatial resolved measurement of the contrast in the coherent range. We are currently developing a test bench configuration where very high quality input pulses will be used as reference to study the relative contribution of the grating size effect in the near, the intermediate and the far-field of the beam.
4.3 Low-order spatial-to-spectral phase coupling
This effect is known since the early steps of CPA systems and it refers to the impact of the quality of the optics inside a stretcher or a compressor at positions where the spectrum is spatially dispersed. By ‘low order’ we refer to low-frequency wavefront aberrations and not the rugosity issues, discussed in the following paragraph. Two problems have been investigated in the frame of the Apollon project: (1) the optimization of the positioning of the 4 gratings for the 10 PW compressor and (2) the correct specification of the optical quality of the retroreflector in the folded 2-grating-based 1 PW compressor.
In the first case, we used MIRO in a parametric study based on the wavefront measurements of all the delivered gratings (data provided by LLNL). The goal was to decide the optimal position of each grating so that the pulse spatiotemporal degradation at the output is minimized. As expected, we found that the highest quality gratings should be placed in the second and third positions where the spatial-to-spectral phase coupling is most critical. Excellent PtV transmitted wavefront errors (TWEs) of
In the second study, our approach has been qualitatively different, since now we used our MIRO simulation to specify the required quality of a critical component such as the retroreflector of the 1 PW compressor. We found that in order to restrict the impact below the 15.2 fs we should specify a TWE of this element lower than
4.4 High-order spatial-to-spectral phase coupling
The qualitative difference of this last effect to the previously described is that now we refer to the rugosity issues of the same optics. The rugosity, as a fast variation in space, couples a fast variation of the spectral phase which in return scatters energy of the pulse in the time domain, before and after the main peak. Therefore, this mechanism is expected to principally affect the contrast rather than the pulse duration. Again, the effect is not new. In a relatively recent study from the OMEGA EP laser group from Rochester[
5 Conclusions
The Apollon 10 PW laser is moving fast to its final construction phase. Currently, we are working on the final alignment and optimization of the third amplification stage and the demonstration of the Apollon laser operation on the 35 J level. Regarding the rest of the system, almost all the critical components of the laser are either already delivered (transport mirrors, amplifier crystals, gratings, etc.) or in the final reception phase (high energy pump sources, deformable mirrors, special beam splitting optics, diagnostics, etc.). Commissioning of a first fully operating version of the Apollon laser on the multi-PW level (
In this work we provide a summary of the main considerations of the Apollon laser regarding one of the most critical characteristics of this system, the pulse temporal quality. The very challenging and strict requirements of the Apollon laser are justified and the key design and technological aspects of the system which guarantee their achievement are presented. A more detailed discussion is also included on the impact of the spatiotemporal coupling effects on both the duration and CR of the compressed pulses reaching the experimental target. To the best of our knowledge it is the first time that the impact of a series of effects such as the inhomogeneous amplification, the finite size gratings in PW class compressors, as well as the wavefront and rugosity limitations of critical optical components is discussed in the frame of a large-scale laser facility and the 15 fs second pulse duration regime.
References
[1] D. Papadopoulos, C. Le Blanc, G. Chériaux, P. Georges, G. Mennerat, J. P. Zou, F. Mathieu, P. AudebertAdvanced Solid-State Lasers Congress.
[4] O. Morice. Opt. Eng., 42, 1530(2003).
[5] D. N. Papadopoulos, P. Ramirez, A. Pellegrina, N. Lebas, C. Leblanc, G. Chériaux, J. P. Zou, G. Mennerat, P. Monot, F. Mathieu, P. Audebert, P. Georges, F. P. DruonAdvanced Solid State Lasers.
[9] D. N. PapadopoulosFrontiers in Optics 2012/Laser Science XXVIII.
[11] A. Ricci, A. Jullien, J. Rousseau, Y. Liu, A. Houard, P. Ramirez, D. Papadopoulos, A. Pellegrina, P. Georges, F. Druon, N. Forget, R. Lopez-Martens. Rev. Sci. Instrum., 84(2013).
[14] F. Giambruno, C. Radier, G. Rey, G. Chériaux. Appl. Opt., 50, 2617(2011).
[15] A. Hervy, L. Gallais, G. Cheriaux, D. Slimane, A. Freneaux, N. Bonod, A. Cotel, R. Clady, M. SentisPacific-rim Laser Damage Conference, PLD’16.
[17] G. Mennerat, F. Giambruno, A. Freneaux, F. Leconte, G. CheriauxResearch in Optical Sciences.
[18] J. Bromage, C. Dorrer, R. K. Jungquist. J. Opt. Soc. Am. B, 29, 1125(2012).
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