Numerical Calculation of Laser Beam Path Influenced by High Temperature Gas Above Specimen During Laser Welding

Beam deflection confronting again?  Take care of media bias during laser material processing.

By: Masami Mizutani and Seiji Katayama

Joining and Welding Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, Japan

It has been generally accepted that the laser beams of about 1 mm in wavelength, such as fiber, disk and YAG lasers have smaller interaction to the media such as plume and plasma between the optics system and the specimen than CO2 laser beam of 10.6 mm.

However, we have recently found that weld bead formation is greatly affected by the state of the gas flow, the atmosphere, the ultrafine particles or the plasma plume above the specimen during fiber laser remote welding. The result showed that a laser beam tended to be deflected and focused on the far position without the positive elimination of the media above the specimen.    Meanwhile, mirage effects and heat haze effects are seen as natural phenomena by varied temperature of the air, which suggest that the air possessing the temperature gradient enable the ray to deflect.

Therefore, in order to understand what extent the temperature distribution of the media between the optics and the specimen affects the laser beam path, a theoretical calculation of the laser beam trajectory in the assumptive media, which possesses refractive index gradients led by temperature gradients, was carried out.  As one of the important results showing in the figure, the beam trajectories propagating through a high-temperature media below, show that the beam diameter at the original focal point (200 mm) is broadened and the concentration area of the beams shifts downward.  This result may interpret why the laser beam focused on the far position without the positive elimination of the media above the specimen during fiber laser remote welding, which has been experienced.

It should be emphasized that the trajectory of the laser beam, the wavelength of which is about 1 mm, is deflected to a certain extent, by the media possessing the refractive index gradient led by the thermal gradient, which is presumably attributed to the thermal and plume-jet-induced updraft, and the reheat of ultrafine particles.  Therefore, it is concluded that the beam profile of the laser, which has propagated through the gaseous matter possessing the thermal gradient, is no longer the same as originally intended.

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Computing results of focused beams propagating from entrance to focal point of 200 mm, assumed for original rays without deflection to be focused at, showing differences in beam trajectories between with and without refractive index gradient.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper 1403