Impact of the thermal lens effect in atmospheric pressure DBD-plasma columns on coaxially guided laser beams

The combination of laser and low-temperature plasmas is of growing interest for micro-structuring purposes for a wide range of materials. This combination can be used for a reduction of the ablation threshold as well as an improvement of the machining quality in various laser material processing applications. The plasma involved in such combination, however, leads to a thermally generated influence on the laser beam quality. In this paper, a DBD-based argon plasma as typically used in low-temperature laser plasma hybrid arrangements was investigated by interferometric and beam profile measurements. The radial temperature profile as well as the beam propagation characteristics in terms of focal shift and effective pointing stability was determined. Due to the argon plasma, a temperature increase Delta T by up to 25 K, and thus a thermal lens was observed within the light path of the laser beam featuring an increase in refractive index Delta n by maximum 2.86 x 10(-5). In the given setup, the plasma-induced thermal lens caused a focal shift by up to 4 mm. Further, the lateral focus position was deviated by a maximum of about 30 mu m, which is in the order of magnitude of the beam waist radius.