Analysis of thulium fiber laser induced bubble dynamics for ablation of kidney stones

The Thulium fiber laser (TFL) is being explored as an alternative to the Holmium : YAG laser for lithotripsy. TFL parameters differ in several fundamental ways from Holmium laser, including smaller fiber delivery, more strongly absorbed wavelength, low pulse energy/high pulse rate operation, and more uniform temporal pulse structure. High speed imaging of laser induced bubbles was performed at 105,000 frames per second and 10 μm spatial resolution to determine influence of these laser parameters on bubble formation and needle hydrophone data was also used to measure pressure transients. The TFL was operated at 1908 nm with pulse energies of 5-65 mJ, and pulse durations of 200-1000 μs, delivered through 105-μm-core and 270-μm-core silica optical fibers. Bubble dynamics using Holmium laser at a wavelength of 2100 nm with pulse energies of 200-1000 mJ and pulse duration of 350 μs was studied, for comparison. A single, 500 μs TFL pulse produced a bubble stream extending 1200 ± 90 μm and 1070 ± 50 μm from fiber tip, with maximum bubble widths averaging 650 ± 20 μm and 870 ± 40 μm (n = 4), for 105 μm and 270 μm fibers, respectively. These observations are consistent with previous studies which reported TFL ablation stallout at working distances beyond 1.0 mm. TFL bubble dimensions were four times smaller than for Holmium laser due to lower peak power and smaller fiber diameter used. The maximum pressure transients measured 0.6 bars at 35 mJ pulse energy for TFL and 7.5 bars at 600 mJ pulse energy for Holmium laser. These fundamental studies of bubble dynamics as a function of specific laser and fiber parameters may assist with optimization of the TFL parameters for safe and efficient lithotripsy in the clinic. Image of bubble formation during fiber optic delivery of Thulium fiber laser energy in saline (35 mJ, 500 μs).