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The Thulium fiber laser (TFL) is being studied as an alternative to Holmium:YAG laser for lithotripsy. The near
single mode TFL beam profile enables transmission of higher laser power through smaller optical fibers than
possible during Holmium laser lithotripsy. Current free-space coupling of collimated TFL output beam into a
disposable silica fiber for ureteroscopy is limited by back-reflected light from the fiber input surface, which may
result in laser shutoff or damage, if left unchecked. This study examines whether anti-reflection (AR) coated fibers
may sufficiently reduce back-reflected light to prevent laser shutoff, increase fiber optic transmission, and
potentially increase laser stone ablation rates as well. Fiber optic transmission and stone ablation studies were
conducted comparing uncoated and AR-coated 105- and 200-μm-core fibers. Magnified images of proximal fiber
surfaces were taken before and after each trial to examine for AR-coating damage. TFL wavelength of 1908 nm
was coupled into silica fibers, with incrementally increasing pulse energy (5-35 mJ), fixed 500-μs pulse duration,
and pulse rates of 50-300 Hz. For each pulse rate, 100,000 pulses were also delivered through the fibers to examine
for potential damage. Back-reflection at proximal fiber surface was reduced from 3.25% with uncoated fibers to ~
0.06% with AR coated fibers. For both fiber diameters, output power was stable, and no proximal fiber damage was
observed after delivery of 100,000 pulses at 35 mJ, 300 Hz, and 10.5 W average power. There was no significant
difference in stone ablation rates between fiber diameters (105 vs. 200 μm) or bare or AR-coated fibers. Laser
shutdown was not observed using AR-coated fibers, which reduce back-reflection and improve energy transmission,
but do not improve stone ablation rates.
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