Abstract
During laser-induced, breakdown-based medical procedures in human eyes such as posterior capsulotomy and vitreolysis, shock waves are emitted from the location of the plasma. A part of these spherically expanding transients is reflected from the concave surface of the corneal epithelium and refocused within the eye. Using a simplified experimental model of the eye, the dominant secondary cavitation clusters were detected by high-speed camera shadowgraphy in the refocusing volume, dislocated from the breakdown position and described by an abridged ray theory. Individual microbubbles were detected in the preheated cone of the incoming laser pulse and radially extending cavitation filaments were generated around the location of the breakdown soon after collapse of the initial bubble. The generation of the secondary cavitation structures due to shock wave focusing can be considered an adverse effect, important in ophthalmology.
Highlights
During laser-induced, breakdown-based medical procedures in human eyes such as posterior capsulotomy and vitreolysis, shock waves are emitted from the location of the plasma
Many laser medical procedures in human eyes are based on tightly focusing laser light within transparent ocular tissues to achieve optical breakdown followed by a rapid absorption of light [1]
The therapeutic effect of such ionization is formation of plasma in a localized focal volume that causes disintegration of tissue within this volume [2]. This highly perturbed state of matter is relaxed by emitting a shock wave and by a generation of a cavitation bubble [3]
Summary
Many laser medical procedures in human eyes are based on tightly focusing laser light within transparent ocular tissues to achieve optical breakdown followed by a rapid absorption of light [1]. Experiments supported by a simple ray theory show that the dominant secondary cavitation cloud occurs if the shock wave emitted during the first collapse of the main bubble refocuses in the anterior laser cone. For example, a 10-mJ laser pulse is focused within the vitreous of the eye with a convergence angle of 22° in water, it takes about 170 μs for the initial cavitation bubble to reach its maximum diameter of 3.6 mm (15% of the diameter of a human eye!) [4]. It is well known that when laser breakdown is generated in water close to its free surface, smaller secondary bubbles appear within the laser path after the shock wave reflected from the free surface overpasses the volume irradiated by the laser pulse [18,19]. We present the experimental results accompanied by a discussion of the importance of secondary cavitation in ophthalmology
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