Laser shock surface treatment applications, ie with plasma formation, require pulses of very high peak power and duration of the order of a few tens of nanoseconds. This processing technique are generally performed in free space due to the difficulties to inject such peak power in a single core fiber. We present a innovative method allowing to inject 500 mJ pulses at 1064 nm in a single core multimode fiber. This subsystem allows at once to secure the input interface of the fiber and to reduce the optical losses due to Brillouin scattering.
We present experimental and theoretical investigations of interaction of a femtosecond laser (450 fs pulse at 1025nm)
with dielectric materials (fused silica) for the single-shot laser regime. The aim is to analyze and understand the complex
physical mechanisms of laser energy absorption yielding to damage and /or ablation. We outline the distinction between
the ablation and the damage thresholds for dielectric materials. The evolution of the reflection, transmission and
absorption signals is studied as a function of fluence. The experimental curves are accompanied by a modelling, which
takes into account the photoionization and avalanche ionization depicting absorption of the laser energy by the material.
The incident pulse propagation into the material, the temporal evolution of the electron density, reflection and
transmission illustrate the beginning and the duration of the laser pulse absorption. The magnitude of the absorption
process is energy density sensitive and, with the increase of the deposited fluence, the onset of absorption is moved
temporally to the beginning of the pulse. We show the influence of the effective electron collision frequency on the
calculated values of reflection, transmission and absorption. The results are particularly relevant to high micromachining
industrial processes.
We present a coupled study of laser-induced damage and ablation of fused silica in the femtosecond regime. Both
thresholds are essentially different and investigations under a wide excursion of pulse duration (< 10 fs to 300 fs) and
applied fluence (Fth < F < 10 Fth) provide quantitative knowledge on i) the strength of the so-called "deterministic"
character of femtosecond laser damaging, linked to ionization mechanisms ; ii) the physical characteristics of surface
ablation craters demonstrating that high selectivity and nanometric resolution is achievable.
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