Characterization of a 50-μm-Core Optical Fiber for Potential use in Thulium Fiber Laser Lithotripsy

Our laboratory is currently studying the Thulium fiber laser (TFL) as a potential alternative laser lithotripter to the standard clinical Holmium:YAG laser. We have previously demonstrated efficient coupling of TFL energy into fibers as small as 100-mu m-core-diameter without damage to the proximal end. Although smaller fibers have greater tendency to degrade at the distal tip during lithotripsy, fiber diameters (<= 200 mu m) have been shown to increase saline irrigation rates through the working channel of a flexible ureteroscope, maximize ureteroscope deflection, and reduce stone retropulsion during laser lithotripsy. In this study, a 50-mu m-core-diameter, 85-mu m-outer-diameter fiber is characterized for TFL ablation of human calcium oxalate monohydrate urinary stones, ex vivo. The stone ablation rate was measured to be 70 +/- 22 mu g/s for 35-mJ-pulse-energy, 500-mu s-pulse-duration, and 50-Hz-pulse-rate. The ureteroscope working channel flow rate including the 50-mu m fiber decreased by only 10% with no impairment of ureteroscope deflection. The fiber delivered up to 15.4 +/- 5.9 W under extreme bending (5-mm-radius) conditions. Stone retropulsion and fiber burn-back averaged 201 +/- 336 and 3000 +/- 2600 mu m, respectively, after 2 minutes. With further development, Thulium fiber laser lithotripsy using ultra-small, 50-mu m-core fibers may introduce new integration and miniaturization possibilities and potentially provide an alternative to conventional Holmium:YAG laser lithotripsy using larger fibers.