Abstract

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 Q-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 Q-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 Q-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 Q-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

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