Kagome fiber-based ultrafast laser surgery probe with custom CaF2 miniaturized optics for rapid in vivo tissue dissection (Conference Presentation)

We present the development of an endoscopic ultrafast laser scalpel with improved miniaturized optics as a follow-up to our previous studies. We previously determined that the nonlinear susceptibility of ZnS crystal lenses at high pulse energies can limit the maximum energy reaching the tissue surface. Here, we improve the nonlinear properties of miniaturized optics using CaF2 in lieu of ZnS as a lens material to mitigate the problem of three-photon absorption at the high energies needed for rapid tissue ablation. We built and tested a miniaturized objective consisting of a CaF2 crystal lens pair to focus ultrashort laser pulses delivered through a large air core Kagome fiber to a 3.4 μm diffraction limited spot and scan the beam with a piezo-tube across a 100×100 μm^2 field of view (FOV). Negligible three-photon absorption and high transmission through the probe (>50%) allows for delivery of fluences >8.0 J/cm^2. The entire opto-mechanical system, enclosed within a 5-mm hypodermic tubing, can remove tissue at material removal rates (MRR) >0.5 mm^3/min in excised soft (porcine vocal folds) and hard (bovine rib bone) tissue samples. We found that MRRs could be increased by the optimized combination of piezo- and translation scanning parameters, providing clinically relevant tissue removal speeds. Further, we present the first ultrafast tissue ablation experiments in a live animal model (hamster cheek pouch) using a handheld surgical probe. Towards clinical acceptance, we designed an injection-molded enclosure to seal the 5-mm diameter opto-mechanical assembly and constituent wires into a sterilization-ready ergonomic handheld stylus tool.