Articles

RSS

In precise materials processing, femtosecond technology allows processing with very little damage. For extremely short pulses, the production of heat is initially concentrated in surface layers in the range of nanometers while the surrounding material stays relatively cold. By repetition of single energy pulses with high frequency the processing zone can be enlarged in area and depth. Thereby steel of 1 mm thickness can be pierced. The splitting of a process in many single processing steps allows a flexible, adaptive control of the structure geometry.

Empirical model of nanosecond-laser drilling: the process is characterized by two ablation tools, the laser beam itself (red arrow to the left, red curves) and the laser-induced plasma plume (right part and blue arrows).

Schematic of the experimental arrangement for shadowgraphy and resonance absorption photography (applications of the dicam pro).

Within the femtosecond laser technology framework the objective of the PRIMUS project cluster (precise materials processing with ultrashort pulsed beam sources) is to utilize economically the femtosecond technology for ablative production processes of highest precision. As one of the scientifc goals plasma diagnostics are the matter of research.

Experimental setup for femtosecond laser plasma investigations (ICCD = dicam pro).

In the field of high-precision drilling and micro-machining the required processing times can only be achieved if high pulse energies (µJ – mJ) and high repetition rates (> 10kHz) can be used. Here it is necessary to understand the complex interaction between the material to be processed, the laser radiation, and the material vapor / plasma that is generated during the process itself. For the investigation of the laser-induced plasma plume expansion, the experimental arrangement in figure 3 is used. While the ultrashort laser pulse hits the material perpendicular to the surface, the direction of observation is parallel to the surface. An intensified CCD camera with its short time gating capabilities (5ns) is used to image the experiment. For shadowgraphy it is back-illuminated and the images show intensity variations due to the locally varying refractive index distribution. An illumination wavelength tuned to a resonantly absorbing atomic transition furthermore allows to visualize evaporated material vapor (figure on top).

Result of plasma diagnostics: the free electron density distribution for a plasma plume recorded during the pulse maximum of a 12 ns-laser pulse.

SOURCE: The Cooke Corporation