Conventional laser drivers for Inertial Confinement Fusion used phosphate glass as laser medium. The main disadvantage of the glass host is its low thermal conductivity and only one laser shot every 5 minutes can be obtained in the joule range with Laser Megajoule Preamplifier system. Recently, the disordered Nd:CaF2 crystals with co-doping ions has been studied intensely because of wide emission and absorption spectrum, broadband laser operation around 1μm and very good thermal properties. We demonstrate that co-doping Nd:CaF2 single crystals with Lu3+ non-optically active ions significantly improves their absorption, emission, and fluorescence lifetime properties and their laser performance. Laser operation is obtained around 1.055 μm which make these suitable for laser diver for ICF application at 1053 nm.
Over the past few years, broadband laser emission has been achieved with Nd-doped fluorite-type crystals, around 1053 nm (4F4I transition Nd3+) by co-doping the host with buffer ions, such as Y3+ and Lu3+. These configurations are very interesting because absorption and emission spectra as wide as with glasses can be obtained. They still being single crystals and having thus among other advantages a high thermal conductivity which makes these materials suitable for high-energy high repetition rate pulsed laser amplifiers applications. However, in the case of CaF2, the 1 μm Nd3+ emission is completely quenched because of clustering of Nd3+ ions. The quenching of the 4F3/2 emitting level starts at low dopant concentration (0.1 at%). To address this drawback, the co-doping of the crystal with non-optically-active ions to form clusters around single Nd3+ ion and then isolated them from each other. In this work, we present the distinct absorption and emission spectra, fluorescence and radiative lifetimes as well as the respective concentrations of the two main emitting centers in CaF2: Nd3+ :Lu3+.Absorption spectra around 800nm (4I9/26>F5/2 transition) for different co-doped samples.
Measurements of the thermal conductivity of CaF2 have been realized with a thermal lens technique and a mean value of 4.3 W/m.K has been obtained for a 5%Lu concentration. This value is about ten times higher than the thermal conductivity of glasses, between 0.3 and 0.6 W/mK. For demonstration of lasing performance in our laser ICF driver system, crystals with concentration of 5% Lu have been chosen because it offers the best compromise between spectra wide and thermal conductivity.
Jean-Paul Goossens, "Fluoride crystals for inertial confinement fusion laser drivers (Conference Presentation)," Proc. SPIE 10511, Solid State Lasers XXVII: Technology and Devices, 105110V (Presented at SPIE LASE: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291886.5751367195001.
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