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.
While CaF<sub>2</sub>:Nd<sup>3+</sup>,Lu<sup>3+</sup> spectroscopic features are now well-known for its broadband laser operation near 1 µm and its good quantum efficiency, this material is appealing for a number of applications such as mode-locking operation. In this paper, we investigate this crystal for dual-wavelength picosecond and femtosecond operations by using a semiconductor saturable absorber mirror (SESAM). In dual-wavelength picosecond operation, synchronous mode-locking is demonstrated at 1054 and 1059 nm when pumping at 797nm and when using a high reflective mirror as an output coupler. Only one pulse train at 93,8MHz was formed and the intensity autocorrelation trace shown a period beat frequency of 1.34 THz. Pumping at 791 nm led to the formation of two asynchronous mode-locked pulses probably because the two emission lines at 1049 nm and 1061 nm were too far to be coupled. Hence by spectral filtering it is possible to make a single train mode locked laser at 1061 nm generating femtosecond pulses. The laser generated modelocked pulses with pulse duration of 435 fs, average power of 10 mW, and central wavelength of 1061 nm. More output power could be obtained by using a more transmissivity for the output coupler however degrading other performances. These results open the way for further investigation on CaF<sub>2</sub>:Nd<sup>3+</sup>,Lu<sup>3+</sup> crystals, with the aim of their implementation as active components in high power femtosecond lasers.
Laser facility such as the Megajoule Laser dedicated to laser-matter interaction including inertial fusion need pre-amplifier modules (PAM) which must respect a high beam quality. The current PAM use Nd:Phosphate material to work at 1053 nm with a repetition rate of 1shot/5min limited by a low thermal diffusion. However, it would be interesting to increase the shot rate for alignment or diagnostic purposes. Therefore, we propose to change this amplification material by crystal Nd:Lu:CaF2 with a thermal diffusion ten times higher in a new amplification architecture scheme in view of achieving a repetition rate of 10Hz. However, this material must fulfill the beam specifications to be integrated in the actual amplification chain. We report here a characterization of the thermal induced effects under a diode pump energy density of 13J/cm2. We begin by studying the spatially resolved induced birefringence with a cross polarizer-analyzer setup and then we measure the wave-front variations along two perpendicular polarizations. As the thermal elevation implies stress and then birefringence, we use an IR camera to study the surface thermal diffusion of the samples. Finally we reconstruct the stress pattern of our samples by simulating the global setup with COMSOL software which includes the thermal and mechanic Multiphysics interaction. This model allows us first to compare with experimental results and then to entirely simulate the mechanical behavior of this new material. These results obtained for the regenerative amplifier would enable us to study a new architecture scheme like disks multi-pass amplifier.