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International audience; Yttrium orthoaluminate (YAlO3) is an attractive laser host crystal for doping with thulium (Tm3+) ions. We report on the absorption and stimulated-emission (SE) cross-sections of this orthorhombic (sp. gr. Pnma) Tm:YAlO3 crystal for the principal light polarizations, E || a, b and c. Polarized absorption data lead to the Judd-Ofelt parameters Ω2 = 1.46, Ω4 = 2.82 and Ω6 = 1.09 [10-20 cm2]. In particular, for the 3H4 → 3H5 transition, it is found a stimulated emission cross section of 0.86×10-20 cm2 at 2278 nm corresponding to an emission bandwidth of ~12 nm (for E || b). Continuous-wave laser operation on this 3H4 → 3H5 transition is achieved with an 1.8 at.% Tm:YAlO3 crystal under laser-pumping at 776 nm. The mid-infrared Tm:YAlO3 laser generated 0.96 W at ~2274 nm with a slope efficiency of 61.8% and a linear laser polarization (E || b). Tm:YAlO3 is promising for mode-locked lasers at ~2.3 μm.

We compare for the first time the influence of different Yb:YAG gain media on the performance of a large-area, high average–power laser system with an output energy of up to 6 J. Monocrystalline slabs grown by a new technique without central growth defect are compared with ceramics. Small signal gain, maximum output energy and thermal lensing are compared for ceramic slabs with co-sintered amplified spontaneous emission (ASE) absorber cladding, monocrystalline slab with and without optically bonded ASE absorber cladding, and surface structured monocrystalline slabs. We show that these large monocrystals with optically bonded absorber cladding have similar performance to cladded ceramics, so far the only material for high-energy Yb:YAG lasers.

The orthorhombic Tm

Crytur is a company with long tradition of growing and processing crystals for technical applications, with history reaching back to 1943. Recently we have developed Crystal Improved Growth (CRIG) method for production of large core-free single crystals of YAG. The diameter currently achieved is 140 mm (in case of undoped crystal), and the crystal weight is up to 10 kg. The method was used to grow un-doped YAG crystals, YAG:Ce crystals for large scintillating screens, and Yb:YAG and Nd:YAG for high power solid-state laser systems. Large laser slabs were manufactured from Yb:YAG doped crystals for Diode-Pumped Solid State Laser (DPSSL) system Amos, which operates within Extreme Light Infrastructure in the Czech republic (ELI Beamlines). The dimension of the largest Yb:YAG laser slab produced is 120×120×8 mm, there is no visible stress under crossed polarizers and the wavefront distortion in the clear aperture region is smaller than λ/10 (λ=633 nm) in its Peak-to-Valley value. The edges of the slab are from diffusively bonded Cr:YAG cladding in order to suppress ASE (Amplified Spontaneous Emission). In 2018 the performance of three sets of laser slabs (o55x5 mm) with differently realized ASE suppression was characterized at cryogenic temperatures at HiLASE Centre in terms of small signal gain measurements as well as amplification test under 30 J pumping at 1 Hz and 10 Hz repetition rates. We provide data that show that the crystal slabs have comparable properties to the ceramic slabs (produced by Konoshima company, Japan) currently in use at HiLASE.

The orthorhombic Tm3+:YAlO3 crystal is promising for laser operation at the H43→F43 (1.5 μm) and H43→H53 (2.3 μm) transitions. Stimulated-emission cross-sections for these transitions are determined with polarized light. Peak values of 0.59×10−20 cm2 and 0.80×10−20 cm2 are found at 1438 nm and 2275 nm, respectively, both for E∥b. The cross-relaxation defining the upper-laser level lifetime is quantified. Continuous-wave lasing at the H43→H53 transition is achieved with an a-cut 1.5 at.% Tm:YAlO3 crystal. The laser generated 254 mW at 2273 nm with a slope efficiency of 17.8% and linear polarization (E∥b).

A new crystal growth method CRIG (Crystal Improved Growth) has been developed to grow large core-free YAG single crystals. Yb:YAG crystals were used for production of laser slabs for ELI (Extreme Light Infrastructure) Beamlines project.

Overview of progress in construction and testing of the laser systems of ELI-Beamlines, accomplished since 2015, is presented. Good progress has been achieved in construction of all four lasers based largely on the technology of diode-pumped solid state lasers (DPSSL). The first part of the L1 laser, designed to provide 200 mJ

We have measured and characterized, over a wide range of doping levels, the UV-near-IR (190-2000-nm) absorption properties of Ti:sapphire crystals. We find that the strengths of absorption centered around 400-450 and 268 nm depend on the square of the Ti3+ doping level, suggesting an origin from pairs of Ti3+ ions. In addition, we have identified an absorption feature below 210 nm due to Ti3+, rather than Ti4+ charge-transfer transitions. Finally, our data on 800-nm-peak, near-IR absorption shows a complex lineshape, with a lower limit set by Ti3+ pair absorption. Thus the maximum possible Figure-of-Merit for Ti:sapphire reduces as the doping level increases.

The development of higher-power InGaN-based diode lasers facilitates their application to optical pumping of Ti:sapphire lasers. Recent diode-pumping results highlight some unexpected behavior, specifically with 450-nm-wavelength devices. To better understand this we have measured and characterized, over a wide range of doping levels, the absorption properties of Ti:sapphire crystals. We find significant changes in the spectral shape of the pumping band in Ti:sapphire with increased doping, and explain the results in terms of absorption due to pairs of Ti3+ ions. Our subsequent discussion attempts to explain prior data, and also provides guidance on optimizing designs for InGaN-diode-pumped Ti:sapphire lasers.

ELI-BAMLINES Project has stimulated a new trend in material research in the Czech Republic. Recent development of crystal growth technology, allow producing a core free YAG matrices up to 5 inches. Basic optical properties of Yb:YAG laser crystal elements will be compared with ceramics ones.

A core-free Yb:YAG crystal, prepared by a new CRIG method, up to 125 mm in diameter and a laser slab 100mm x 10mm will be presented. PSD parameter was same or better to published values. Crystal utilization in ELI-BEAMLINES Project will be discussed, also.

We present an overview of the projected and/or implemented laser systems for ELI-Beamlines. The ELI-Beamlines facility will be a high-energy, high repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. The facility will make available high-brightness multi-TW ultrashort laser pulses at kHz repetition rate, PW 10 Hz repetition rate laser pulses, and kilojoule nanosecond laser pulses that will be used for generation of 10 PW, and potentially higher, peak power. These systems will allow meeting user requirements for cutting-edge laser resources for programmatic research in generation and applications of high-intensity X-ray sources, in electron and proton/ion acceleration, and in dense plasma and high-field frontier physics.

Spectroscopy of Pr-doped YA1O 3 (YAP) has been investigated under synchrotron radiation and laser excitations. Fast UV fluorescence was shown to be due to allowed d–f transitions of Pr 3+ . Light yield of Pr-doped YAP has been measured in comparison with Ce-doped YAP. The relatively low scintillation efficiency of Pr: YAP is explained by the very different concentration of the dopant ions in both systems and by the presence in Pr: YAP of Ce trace impurities which can act as trap centers for the fast UV scintillation of Pr 3+ ions.

ELI-Beamlines will be a high-energy, repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. It will be an international facility for both academic and applied research, slated to provide user capability since the beginning of 2016. The main objective of the ELI-Beamlines Project is delivery of ultra-short high-energy pulses for the generation and applications of high-brightness X-ray sources and accelerated particles. The laser system will be delivering pulses with length ranging between 10 and 150 fs and will provide high-energy petawatt and 10-PW peak powers. For high-field physics experiments it will be able to provide focused intensities attaining 1024 Wcm-2, while this value can be upgraded in a later phase without the need to upgrade the building infrastructure. In this paper we describe the overall conception and layout of the designed ELI-Beamlines facility, and review some essential elements of the design.