Nicolas Sanner

Programmable focal spot shaping of amplified femtosecond laser pulses

We describe the programmable spatial beam shaping of 100-kHz, 4-microJ amplified femtosecond pulses in a focal plane by wave-front modulation. Phase distributions are determined by a numerical iterative procedure. A nonpixelated optically addressed liquid-crystal light valve is used as a programmable wave-front tailoring device. Top-hat, doughnut, square, and triangle shapes of 20-microm size are obtained in a focal plane. Their suitability for femtosecond laser machining is demonstrated.

Toward determinism in surface damaging of dielectrics using few-cycle laser pulses

We introduce a quantitative measurement of the determinism of laser-induced damaging at the surface of a dielectric material, e.g., fused silica. Using laser pulses ranging from 7 to 300 fs, we demonstrate that laser damage occurrence tends to be dramatically deterministic at 7 fs, which is attributed to the increasing importance of tunneling ionization as the major channel for the generation of free-carriers in the conduction band.

Self-compression and Raman soliton generation in a photonic crystal fiber of 100-fs pulses produced by a diode-pumped Yb-doped oscillator

We present the use of a photonic crystal fiber to straightforwardly compress ultrashort pulses from a diode-pumped ytterbium laser emitting around 1 microm. 75-fs pulse generation and a large 1-1.3-microm tunability for sub-100-fs pulses is reported.

Laser induced damage of sapphire and titanium doped sapphire crystals under femtosecond to nanosecond laser irradiation

The use of large Ti:Sapphire crystals in ultra fast high peak power laser amplifiers makes crucial the problem of crystal laser induced damage. These works aim to quantify the laser induced damage threshold (LIDT) of Sapphire and Ti:Sapphire crystals under femtosecond, picosecond and nanosecond laser pulse irradiations, which are typically encountered in such laser chains. Furthermore, a study of the influence of cryogenic conditions on the LIDT of Ti:Sapphire crystals and of their anti-reflection coating has been performed. The results are important to understand the mechanisms leading to the damage, and to reveal the key parameters which will have to be optimized in future high peak power laser chains.

New methods to control quality and precision of micro-machining with femtosecond lasers

Due to the rapid development of ultrashort lasers, quality of the machining is of prime interest for several applications. For instance deep marking of various materials. In this case, the depth can be controlled, knowing the ablation rate for the corresponding material. The evolution of ablation rates of Al, Cu and Ni are given in relation to the energy density. In metals the effect of thermal diffusion has to be taken into account to control collateral effects and especially the heat affected zone.

About the mechanisms of femtosecond laser damage and ablation of dielectric materials

Industrial implementation of femtosecond laser-based micromachining applications requires the characterization of the material response to different levels of laser exposure, ranging from laser damaging evaluation for laser technology purposes to determination of laser ablation characteristics for micromachining processes. The problem of laser damage and ablation measurement in femtosecond regime has been extensively addressed in the literature, with a variety of ex-situ investigations of the morphological changes incurred by the target [1], and in-situ measurements tracking the properties of the created plasma or any change in the optical properties of the studied sample [2]. All these investigations give useful insights on the mechanisms governing the laser-matter interaction in femtosecond regime and important data for the development of laser-based processes. Nevertheless, they are often difficult to confront due to the variety of experimental setups and diagnostics, as well as measurement procedures. To provide valuable information on the effective ionization mechanisms and to progress towards accuracy and predictability of the material response to femtosecond laser irradiation, the precise experimental knowledge and theoretical analysis of both damage and ablation thresholds is essential. To this aim, we perform a simple experiment to precisely evaluate in the same operating conditions the behaviour of the damage and ablation thresholds under the wide excursion of a single experimental interaction parameter, i.e. the laser pulse duration. The experiments are done at 800 nm in single shot regime to avoid any incubation effects. We then measure and model the damage and ablation thresholds at the surface of a well-known dielectric material, e.g. fused silica, using pulses ranging from 7 fs to 300 fs (see Figure 1). The relevant criteria to numerically assess the damage and ablation thresholds are related to the lattice melting temperature Tm and the electronic cohesion temperature Tec, in accordance with experimental observations.

Absorption of femtosecond laser pulse in fused silica: experiments and modelling

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.

Surface ablation of corneal stroma with few-cycle laser pulses at 800 nm

We report measurements of crater diameter and surface ablation threshold as a function of laser fluence in porcine corneal stroma and fused silica with pulse durations of 7 fs (2.7 optical cycles), 30 fs and 100 fs at 800 nm. For laser pulses with Gaussian radial intensity profile, we show experimentally that the square of the crater diameter is a linear function of the logarithm of the fluence in fused silica, while it is closer to a linear function of the fluence in corneal stroma. Extrapolating these relations to zero diameter indicates that for both media the minimum fluence required for surface ablation is reduced with shorter pulse duration. A simple theoretical model suggests that this effect is due to a more significant contribution of photoionization as the laser pulse duration shortens.

Ultra-short laser interaction with metals and optical multi-layer materials: transport phenomena and damage thresholds

In this paper, we are focused on the understanding the underlying physical mechanisms of femtosecond laser interactions with metallic and multi-layer optical materials. The results of the numerical modeling provide an estimation of damage and/or ablation threshold for different laser parameters (pulse duration, fluence, angle of incidence, polarization) and target material properties (metal, dielectric, or multilayer with variable metal layer thickness). These results are compared with the experimental measurements of the thresholds obtained by using different techniques. In particular, dielectric ionization and ablation mechanisms are analyzed based on the experimental findings.

Self-compression of 1-/spl mu/m femtosecond pulses in a photonic crystal fiber

We demonstrated, for the first time to our knowledge, the use of a photonic crystal fiber to straightforwardly compress and generate tunable 1.03-to-1.3 l-pm, 65-fs pulses from an Yb:bulk laser. This oscillator which generates 1 00-fs pulses employs an original Yb-doped apatite crystal.