AbstractIntroductionThe Thulium fiber laser has recently been tested as a potential alternative to the Holmium:YAG laser for lithotripsy. This study explores use of a short taper for expanding the Thulium fiber laser beam at the distal tip of a small‐core fiber.MethodsThulium fiber laser radiation with a wavelength of 1,908 nm, 10 Hz pulse rate, 70 mJ pulse energy, and 1‐millisecond pulse duration was delivered through a 2‐m‐length fiber with 150‐µm‐core‐input‐end, 300‐µm‐core‐output‐end, and 5‐mm‐length taper, in contact with human uric acid (UA) and calcium oxalate monohydrate (COM) stones, ex vivo (n = 10 each). Stone mass loss, stone crater depths, fiber transmission losses, fiber burn‐back, irrigation rates, and deflection through a flexible ureteroscope were measured for the tapered fiber and compared with conventional fibers.ResultsAfter delivery of 1,800 pulses through the tapered fiber, mass loss measured 12.7±2.6 mg for UA and 7.2±0.8 mg COM stones, comparable to conventional 100‐µm‐core fibers (12.6±2.5 mg for UA and 6.8±1.7 mg for COM stones). No transmission losses or burn‐back occurred for the tapered fiber after 36,000 pulses, while a conventional 150‐µm fiber experienced significant tip degradation after only 1,800 pulses. High irrigation rates were measured with the tapered fiber inserted through the working port of a flexible ureteroscope without hindering its deflection, mimicking that of a conventional 150 µm fiber.ConclusionsThe short tapered distal fiber tip allows expansion of the laser beam, resulting in decreased fiber tip damage compared to conventional small‐core fibers, without compromising fiber bending, stone vaporization efficiency, or irrigation rates. Lasers Surg. Med. 42:45–50, 2010. © 2010 Wiley‐Liss, Inc.