Effects of cavitation bubble interaction with temporally separated fs-laser pulses

Abstract

Abstract. We present a time-resolved photographic analysis of the pulse-to-pulse interaction. In particular, we studied the influence of the cavitation bubble induced by a fs-pulse on the optical focusing of the consecutive pulse and its cavitation bubble dynamics in dependence on temporal pulse separation in water. As a first result, by decreasing the temporal separation of laser pulses, there is a diminishment of the laser-induced optical breakdown (LIOB) efficiency in terms of energy conversion, caused by disturbed focusing into persisting gas bubbles at the focal volume. A LIOB at the focal spot is finally suppressed by impinging the expanding or collapsing cavitation bubble of the preceding pulse. These results could be additionally confirmed in porcine gelatin solution with various concentrations. Hence, the interaction between the laser and transparent ophthalmic tissue may be accompanied by a raised central laser energy transmission, which could be observed in case of a temporal pulse overlap. In conclusion, our experimental results are of particular importance for the optimization of the prospective ophthalmic surgical process with future generation fs-lasers.

Interaction Mechanisms of Cavitation Bubbles Induced by Spatially and Temporally Separated fs-Laser Pulses

“…Here, there was no impact of two succeeding pulses' bubbles on each other's dynamics. A high spatial overlap, which means the other extreme, and hence a pulse focusing into an existing cavity led to a decrease in laser energy absorption [27] and, partially, to an axial focus shift (see mechanisms 8 to 11). The mechanisms 4 to 7 in between showed a combination of highly complex interaction effects.…”

“…Time-resolved photography as experimental procedure allows for analyzing very fast dynamic phenomena like oscillating cavitation bubbles with a lifetime of some microseconds; its basic principal is sectioning the process into specific events. The method of time-resolved photography as well as this second optical path of the experimental setup can be found explicitly described in former publications [15] , [27] . Briefly, the light of an external triggered flash lamp ( High-Speed Photo Systeme , Wedel, Germany) was collimated onto the cuvette (Koehler type illumination) and the shadow contrast of the cavitation bubble was imaged on a charged coupled device (CCD) chip of the camera ( Lumenera , Ottawa, Canada) using a long working distance microscope objective (20x, NA = 0.28; Mitutoyo , Kawasaki, Japan).…”

“…A delay generator was utilized for realizing the controlling and timing ( Bergmann Messgeräte Entwicklung KG , Murnau, Germany; for details see [15] , [27] ). Due to the implementation of the polygon scanner in the laser beam path a modification of the input signal was performed.…”

“…It is associated with an asymmetric oscillation behavior as well as a formation of a strong jet perpendicular to the optical axis of the laser [15] . On the other hand an only temporal separation of laser pulses was investigated which leads to a scenario of focusing a subsequent laser pulse into an existing cavitation bubble [26] – [27] . In this case, a further LIOB would be suppressed due to the increased breakdown threshold for water vapor inside the cavity.…”

“…In this case, a further LIOB would be suppressed due to the increased breakdown threshold for water vapor inside the cavity. Hence, there is an increase of laser transmission behind the focal volume [27] . Furthermore, certain US patents deal with two simultaneously generated cavitation bubbles, which are utilized for surface processing (for example [28] ).…”