Tissue Dissection with Ultrafast Laser using Extended and Multiple Foci

Ultrashort lasers are typically utilized for tissue dissection by sequential application of tightly focused beam along a scanning pattern. Each pulse creates a small (on the order of 1μm) zone of multiphoton ionization (optical breakdown). At energies exceeding vaporization threshold cavitation bubble is formed around the focal volume. A continuous cut is formed if the rupture zones produced by separate bubbles coalesce. We present an alternative approach, in which an extended zone of tissue is cut by simultaneous application of laser energy in multiple foci. Simultaneous formation of multiple cavitation bubbles results in hydrodynamic interactions that can lead to significant extension of the rupture zone in tissue. Two simultaneously expanding bubbles compress and strain material between them, while simultaneously collapsing bubbles can produce jets towards each other. We calculated and experimentally imaged the flow dynamics of expanding and collapsing bubbles and obtained maps of tissue deformation. With the measured tissue threshold strain, the deformation map allows predicting the rupture zone as a function of maximum bubble size and distance between the bubbles. We also demonstrate an optical system producing 1 mm long dissection with a single laser pulse. A combination of a lens and an axicon produces a line of optical breakdown, with aspect ratio 250:1. The subsequent cavitation bubble has aspect ratio 100:1 at early stage of expansion. We calculated an optimal laser beam intensity profile to create axiallyuniform elongated ionization pattern.