Impact of a temporal pulse overlap on laser-tissue-interaction of modern ophthalmic laser systems

Abstract

Interaction of subsequent laser pulses becomes important relevant with the use of high-repetition rate fs-laser systems for ophthalmic laser surgery. Therefore, we investigated the interaction of temporally separated laser pulses in water by time-resolved photography. With decreasing temporal separation of pulses the probability of laser-induced optical breakdown (LIOB) is firstly diminished by disturbed focusing into persisting gas bubbles. Finally, LIOB is totally impaired by the expanding or collapsing cavitation of the preceding pulse. Hence, laser-tissue interaction might be accompanied by a raised laser energy transmission. In conclusion, these results are of great interest for the prospective optimization of the ophthalmic surgical process with modern fs-lasers.

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

“…Here, it has to be noticed that the measurements with the "μJewel" laser system were performed with a water-immersion microscope objective of NA ¼ 0.5; the details of this previous setup can be found in Ref. 28,29. In more solid sample media, that means beside water, a remaining mechanical damage can be induced at the focal spot.…”

“…The dynamics of expanding and collapsing cavitation bubbles can be reconstructed and illustrated by lining up single frames of different bubbles at subsequent delays. 28 Image analysis of the individual images was done by the open source software ImageJ. The procedure included an identification of the bubble contour and fitting of an ellipse to this contour to consider the different axial as well as radial bubble diameter.…”

“…This configuration provides a magnified and sharp image on the charge-coupled device (CCD) chip of the camera (Lumenera, Ottawa, Canada). 28 The controlling and timing are realized by a delay generator (Bergmann Messgeräte Entwicklung KG, Murnau, Germany) with a specified jitter ≤50 ns. By changing the delay of the flash lamp spark as well as the camera CCD chip opening in respect to the applied laser pulse, images at different stages during the bubble oscillation are taken.…”