Laser facility such as Megajoule Laser dedicated to laser-matter interaction including inertial fusion need pre-amplification modules (PAM) which must respect a high beam quality. The actual Nd:Phosphate is used in high energy laser system because of its capacity to be produced in big size. The current PAM work at a repetition rate of 1 shot/5 min limited by a low thermal conductivity of the Phosphate glass. However, it would be interesting to increase the shot rate for alignment and diagnostics purposes. Therefore we propose to change this amplification material by some Nd:crystal in the PAM with a higher thermal conductivity and working at 1053 nm to match the power chain wavelength. For long time Nd: CaF2 has been abandoned because of quenching between Nd ions. The lutetium is “buffer” ion used to break Nd clusters and allow high emission cross section. The Nd Lu:CaF2 thermal conductivity is ten times higher than actual Nd:Phosphate and would permit to achieve a repetition rate at 10 Hz. Nevertheless, this material must fulfil the beam specifications to be integrated in the actual amplification chain.
We report a characterization of the thermal induced effects on a parallelepiped rod pumped transversally by laser diodes. The pomp-probe beam configuration contains laser diodes emitting at 797 nm with a fluency of 13 J/cm2 and a probe beam passing through the rod at 1053 nm We study the spatially resolved induced birefringence under a mono-shot pump or variable repetition rates. The experimental setup is composed with a cross-rotating polarizer-analyzer and a camera that measures the intensity signal transmitted by the analyzer. A post numerical analysis consists in fitting the intensity signal transmitted for several polarizer-analyzer angles all over the camera picture. Hence the birefringence can be determined spatially at the end front of the rod. These measures are resolved in time to compare the relaxation behaviour of these two materials.
Then we simulate the experiment setup with COMSOL software that includes the thermal and mechanic multiphysics interaction. The objective is to assess physical effects we cannot determine by measures like the mechanical stress induced at the origin of the birefringence pattern. We numerically solve the thermal equation. The thermal source defined must fit the experimental pump geometry and time mono-shot pulse rate or variable repetition rates. We take in account of the Beer-Lambert’s absorption law and supergaussian profile for geometry and time definition. Then we use Hooke’s law in general case for free-elastic material linked to the thermal distribution to deduce the stress and strain induced in the material. The induced birefringence is directly associated to the piezo-optic tensor and the stress material. The stress tensor COMSOL computes allow reconstructing the Jones matrices throughout the rod and thus the spatial birefringence. The numerical spatially and time resolved birefringence is in good agreement with experimental measures. This numerical model allows us to optimize the spatial geometry of cooling in transverse pumping as in longitudinal pumping in thick disk amplifier.
Diane Stoffel, Sébastien Montant, Jean-Paul Goossens, Simone Normani, Alain Braud, and Patrice Camy, "Laser material Nd:Lu:CaF2 characterization for high-energy and high-repetition rate amplification at 1053 nm (Conference Presentation)," Proc. SPIE 10683, Fiber Lasers and Glass Photonics: Materials through Applications, 106831W (Presented at SPIE Photonics Europe: April 25, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2306895.5788846703001.
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