The threshold fluences at which vaporization is initiated at the tip of a multimode fiber that is submerged in water were investigated when free-running and <i>Q</i>-switched thulium laser pulses (λ = 2.01 μm) were applied with different pulse energies. We focused on the quantification of temporal and spatial fluence modulations of the beam profile at the tip of a 400-μm fiber. The spatial and the temporal fluence peaks over the average fluence were measured to as high as 1.5 and 4 in the <i>Q</i>-switched mode, respectively, and 2.5 and 40 in the free-running mode, respectively. The fluence peaks significantly influence the vaporization process. An increase in the threshold fluence with increasing pulse energy was found for the <i>Q</i>-switched mode, but there was a decrease for the free-running mode. Pressure transients of the order of 1 kbar and temperatures higher than 200 °C were calculated for a 30-mJ <i>Q</i>-switched laser pulse at the onset of vaporization. Collecting all the data allowed us to trace the thermodynamic path of rapid heating and vaporization in a phase diagram of water.
© 2000 Optical Society of America
(030.1670) Coherence and statistical optics : Coherent optical effects
(030.6140) Coherence and statistical optics : Speckle
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.6810) Lasers and laser optics : Thermal effects
(170.1020) Medical optics and biotechnology : Ablation of tissue
Ralf Brinkmann and Christoph Hansen, "Beam-Profile Modulation of Thulium Laser Radiation Applied with Multimode Fibers and its Effect on the Threshold Fluence to Vaporize Water," Appl. Opt. 39, 3361-3371 (2000)