Nd:YAG laser

Nd:YAG (neodymium-doped yttrium aluminum garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, Nd(III), typically replaces a small fraction (1%) of the yttrium ions in the host crystal structure of the yttrium aluminum garnet (YAG), since the two ions are of similar size. It is the neodymium ion which provides the lasing activity in the crystal, in the same fashion as red chromium ion in ruby lasers. Laser operation of Nd:YAG was first demonstrated by J.E. Geusic et al. at Bell Laboratories in 1964. Technology Nd:YAG lasers are optically pumped using a flashtube or laser diodes. These are one of the most common types of laser, and are used for many different applications. Nd:YAG lasers typically emit light with a wavelength of 1064 nm, in the infrared. However, there are also transitions near 946, 1120, 1320, and 1440 nm. Nd:YAG lasers operate in both pulsed and conti

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

Industrial laser precision machining is predominately based on pulsed Nd:YAG lasers operated at a wavelength of 1.06 µm in a free running pulse regime. The objective of this thesis work has been to investigate second and third harmonic generation for such "long pulse" Nd:YAG lasers. Visible and UV radiation can be better focused. The absorption coefficient at these wavelengths is higher in most materials. Processing at the doubled or tripled frequency increases machining precision and allows processing of new classes of materials such as, e.g., high reflective metals or ceramics. Frequency doubling and tripling of Q-switched, modelocked, or CW operated Nd:YAG lasers has been the subject of numerous publications. However, the particular problems associated with the free running pulse regime have not yet been addressed. The generation of harmonics requires a laser source that can deliver radiation with high brightness. A system based on a slab crystal with a beam quality factor of M2 ≈ 1.6 and a power of up to 1.5 kW has been used in the experiments. An external electro-optical switch has been implemented in order to precisely control the laser pulse shape and duration. The non-linear coefficient and the angular acceptance have been the main criteria for selecting LBO (LiB3O5), KTP (KTiOPO4), and MgO:LiNbO3 for the frequency conversion experiments. The conversion efficiency has been measured for these crystals as a function of the intensity at the fundamental wavelength. In contrast to simple model predictions, the efficiency does not increase linearly with intensity under long pulse conditions. An intensity dependant phase mismatch is at the origin of the deviations. A heuristic model for the efficiency prediction has been developed. It is based on a linear increase of phase mismatch with intensity and it allows describing the experimental findings with sufficient accuracy. The conversion efficiency for frequency doubling is limited to about 20% at pulse durations of 100 µsec. It decreases with increasing pulse duration. Catastrophic thermal damage is at the origin of the limitation in this pulse regime. Our experiments indicate that the damage is initiated by the frequency doubled light. Temporary color centers are created in the presence of the second harmonic light. The dynamic of the creation and annihilation of these centers has been investigated. It could be shown that finally the absorption of the fundamental harmonic (infrared) radiation at these centers results in irreversible thermal damages. A power of 140 W in the second harmonic (178 MW/cm2 on a waist of 5 µm) and 12 W in the third harmonic (61 MW/cm2 in a waist of 2.5 µm) was obtained with pulses of 200 µs. This allowed to demonstrate the feasibility of material processing with very high reproducibility and definition, which for instance allowed the design and the manufacturing of micro-grippers and several other prototypes in the field of robotics and micro-engineering.

EPFL2001

Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon

Lucas Güniat, Patrik Hoffmann, Marc Leparoux

Powder additive pulsed Nd:YAG laser (wavelength 1064 nm), with the aim of preparing AM of such material. The influence of oxygen content in the initial silicon powders on the continuous tracks morphology was investigated. It was found that the initial oxygen content of the processed powders must be lower than 0.1 wt% to produce a smooth silicon melted track. This result is explained as due to the formation of gaseous manufacturing (AM) of materials is affected by the oxygen present in the raw material, especially if they possess a native oxide. The latter influences the properties of the laser molten track. We carried out a study on the interactions between silicon fine powder (

ELSEVIER2020

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

EPFL2001

Second-harmonic generation with free-running Nd:YAG laser

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon

Second-harmonic generation with free-running Nd:YAG laser

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Impact of oxygen content in powders on the morphology of the laser molten tracks in preparation for additive manufacturing of silicon