**3.3 Laser-induced generation of bubbles microjets**

Note an interesting phenomenon in the experiments on the generation of bubbles in the vicinity of the blackened tip surface of the fiber in the water cell: bubble microjets can be generated at a laser power of less than 3 W (Fig. 11) (Yusupov et al., 2010). The lengths of the microjets (Fig. 11a), which always start in the immediate vicinity of the fiber tip, reach several millimeters, the transverse sizes normally range from 10 to 50 μm, and the sizes of the bubbles that form the jets range from several to ten microns. The lifetime of the microjets ranges from a few fractions of a second to tens of seconds. A microjet that emerges at a certain spot on the tip surface remains attached to this spot and exhibits bending relative to the mean position. Bubble microjets didn't use to be continuous from start to end, the discontinuities used to appear on them, which used to restore quite often. The observations show (Yusupov et al., 2010) that the discontinuities are always related to the hydrodynamic perturbations and are caused by relatively large bubbles that move in the vicinity of the microjet. The appearance of quite a large bubble attached to the fiber tip caused the bubble microjet bending around large bubble (Fig. 11b). Thus, we conclude that two conditions must be satisfied for the generation of the bubble microjets. First, a hot spot must be formed on the tip surface. Second, the neighborhood of such a spot must be free of the centers that provide the generation and detachment of large bubbles. Note that the possibility of bubble microjets in the vicinity of a point heat source is demonstrated in (Taylor & Hnatovsky, 2004).

Fig. 11. Bubble microjets in the vicinity of the tip surface of optical fiber. A part of the blackened fiber tip is sown at the right upper corner.
