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High laser intensities are being transmitted through optical fibers in a growing number of applications. Our interest in laser initiation of explosives has led us to examine the transmission of Q-switched, Nd:YAG laser pulses through step-index, multimode, fused-silica fibers for a number of years. A common limiting process is a plasma-forming breakdown occurring at the fiber entrance face. The breakdown threshold at this face depends on the surface characteristics that result from the particular method of end-face preparation. In previous studies we examined entrance-face breakdown thresholds for several different mechanical polishing schedules, and also for several schedules of CO2-laser conditioning following mechanical polishing. In the present study we examined three end-face preparation methods that were base on the recent availability of exceptionally good cleaved surfaces for our fibers of interest. Using test procedures similar to those in past studies, we examined the cleaved fibers directly, fibers with cleaved surfaces that were subsequently flame polished, and fibers with cleaved surfaces that were subsequently conditioned with a CO2 laser. All of these preparation methods resulted in fibers that showed a broader range of entrance-face breakdown conditions than found in past studies, together with a susceptibility to subsurface exit-face damage. By introducing additional cleaning steps with the cleaved surfaces, we were able to reduce the variability in breakdown thresholds observed after subsequent CO2- laser conditioning. A consistent location of exit-face damage sites indicates that subsurface fracturing occurs during the cleaving process, and that the subsequent end-face processing steps were not effective in mitigating damage at these sites. Threshold energies for entrance- face breakdown are also affected by the relation between incident laser energy and the resulting peak local fluence at this surface. Laser characteristics and the design of the laser-to- fiber injection optics determine this relation. The present study utilized an improved injection system consisting of a custom diffractive optical element combined with a lens having a very short focal length. This system produced the lowest value for the ratio of peak-to-average fluence at the entrance face that we have observed, and was very successful in inhibiting internal damage mechanisms along the fiber path.
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