• - high power laser - Ultra-short pulse laser

High-power ultrashort-pulse lasers that operate around 2μm spectral region are of particular interest in expanding access to broadband coherent source options for the mid-infrared spectral region. 2 μm lasers are often called “eye-safe” because light in that wavelength range is strongly absorbed in the eye’s cornea and lens and therefore cannot reach the significantly more sensitive retina. For this reason, this kind of laser is recommended in several applications including those that involve propagation through the atmosphere. Nowadays there are a lot of potential implementations of 2 μm ultra-fast lasers, like free-space optical communications, high-harmonic generation, laser physics, infrared pulsed laser ablation of polymer materials, remote sensing, surgery and laser-plasma acceleration. Due to their "eye-safe" proprieties, 2 μm lasers
offer advantages for free space applications compared other systems that operate at shorter wavelengths. Moreover, the high absorption capability in water makes these lasers very useful for medical applications and for earth measuring (for example for wind velocity sensing in vary applications and for the detection of both water vapour and carbon dioxide concentration.)
Ultra-high-power ultrashort pulsed lasers are currently mainly based on Titanium-Sapphire (Ti:Sa) technology which, however, in turn, requires frequency-doubled Neodymium (Nd) lasers as optical pumping sources. The use of these ultrashort pulse lasers is currently limited by the average power currently available, not exceeding a few tens ofWatts. This limitation is essentially due to the optical pumping technology, based on
flash lamps. Recently, the development of pulsed lasers with diode pumping allows the generation of high energy (100 J per pulse) and high average power (kW) Neodymium sources for pumping titanium-sapphire lasers, although, severe constraints remain in the management of the thermal budget in the amplifiers.
The architecture based on Tm-doped crystals (Tm:YLF, Tm:YAG and Tm:Lu2O3) has recently been proposed for the potential advantages of energy efficiency and scalability at very high average powers with emission around 2 μm. The crystal is pumped directly using stacks of commercially available diodes (with emission around about 800 nm). Pumping takes place in quasi-CW (duty cycle of the diodes close to 100%) and the extraction scheme that is exploited is called "multi-pulse extraction mode".
In the thesis the main blocks of the laser system are considered, including a frontend based on Optical Parametric Amplification (OPA) and a power amplification block based on compact multi-pass amplifiers in edge-pumped disk laser (EPDL) configuration.
Numerical simulations were carried out in preparation of the laboratory system, including an optical model and integration in a thermal model. Optical simulations were carried out with the aim of reducing the pumping energy requirement to optimize the thermal budget and the overall efficiency of the system.
Studies of the pumping configuration aiming at increasing the energy coupling efficiency and optimizing the gain profile were performed. The role of the cross-relaxation process and the effect of multi-pulse extraction on the optical efficiency of the active medium were also investigated to predict performance of the amplifiers. A preliminary study of the latter mechanism still poorly known, has allowed to refine and strengthen the thermal and optical simulations.
A multi-pass scheme is considered with amplification in a Thulium doped ceramic Tm:Lu2O3 medium at 4%, in the configuration of active mirror disk, and with lateral (edge) diode pumping at approximately 800 nm. The doping of each disk is tailored in order to optimize the pump energy absorption and minimize unwanted thermal load.
A three-stage amplification chain is considered, with each stage consisting of a multipass amplifier with a final output energy per pulse > 500 mJ from an input energy of 1 mJ is obtained. This configuration represents the best balance between constructive complexity and the efficiency of the operating point in terms of energy gain.
Along with the initial phase of the laser system model development, this thesis also reports on the first laboratory tests aimed at characterizing the first sample of Tm:Lu2O3 acquired for investigation on materials proprieties and optical and laser performance studies. So, the optical characterization of a Tm:Lu2O3 crystal aimed at measuring the main spectroscopic parameters of Tm:Lu2O3 using our test sample is presented. In
particular, the measurement of absorption cross-section and emission proprieties of the Tm:Lu2O3 ceramic sample as a function of the wavelength in the visible range up to the near infra-red was carried out. The experimental data analysis showed that the expected spectroscopic features of the available sample are in line with the values reported in the literature for both crystal and ceramic materials, and confirm the specifications required for our amplifier design.
A laboratory cavity test setup for the study of a 4%at Tm:Lu2O3 doped ceramic active medium laser proprieties was implemented. With a pump at ∼800 nm, an emission at around 2 μm with a slope emission of about 40% (comparable with the theoretical quantum defect λpump/λlaser) was demonstrated. No clear indication on cross
relaxation effects, but studies on the effects and efficiency of cross-relaxation are still ongoing.