The coupling of a laser focused into a water microjet is studied. Using a high-power laser, the light guided in the jet is used to process various materials. To explain the observed ablation patterns, the propagation of a low-power and highly coherent laser beam coupled into a laminar water jet is studied. The light of a He-Ne laser (5 mW) is focused into the water jet, which behaves as a multimode waveguide. The distribution of the light intensity in the jet impinging on a glass plate cutting the jet perpendicularly to its propagation direction is recorded for various laser coupling conditions. The influence of the jet diameter, as well as the influence of the depth of focus of the incident beam and its position with respect to the center of the jet is studied. A nearly homogeneous grain size was observed over the whole jet cross section. The characteristic grain size was then compared with predictions from standard multimode fiber theory. Finally, it is confirmed that the structures resulting from material ablation using the laser-microjet technology when coupling Q-switched Nd:YAG (ϭ1064 and 355 nm) are closely related to the predicted light intensity distributions. Furthermore, recommendations are made concerning the coupling conditions for optimizing the laser processing applications.
In this article, an optical method to control the break-up of high-speed liquid jets is proposed. The method consists of focusing the light of a pulsed laser source into the jet behaving as a waveguide. Experiments were performed with the help of a Q-switched frequency doubled Nd:Yag laser (k=532 nm). The jet diameter was 48 lm and jet velocities from 100 to 200 m/s. To study the laser-induced water jet break-up, observations of the jet coupled with the high power laser were performed for variable coupling and jet velocity conditions. Experimentally determined wavelength and growth rate of the lasergenerated disturbance were also compared with the ones predicted by linear stability theory of free jets.
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