Motivated by interest in optical firing systems for initiating explosives, laser-induced damage thresholds have been investigated in step-index, multimode fibers having pure fused silica cores. A compact, multimode Nd/YAG laser operated at a pulsewidth of 16 ns was used for the experiments. The focusing geometry for introducing the beam into the fiber was chosen to avoid damage along the core/cladding interface as observed in previous studies. Five lots of twenty fibers each were tested, with polishing steps varied between successive lots to produce improved finishes on the fiber end surfaces. Each fiber was subjected to a sequence of progressively increasing energy densities up to a value more than 80 J/cm2. Initial damage was monitored by observing scattered HeNe laser light from the fiber faces using magnified video cameras.
The majority of the fibers damaged initially at the rear fiber face, once a “laser conditioning” process at the front fiber face was completed. In this process, a visible plasma was generated at the front face for one or more laser shots. Rather than produce progressive damage at the front surface, this process apparently improved the surface finish in nearly all cases, resulting in improved resistance to damage. Other modes of damage along the fiber length were observed either at locations of handling stresses or at the location of highest static tensile stress corresponding to the fiber's minimum bend radius. A fraction of the fibers in each lot never damaged, and this fraction increased as the successive lots were tested. The fibers that did not damage in the final lot were subjected to an additional series of laser shots at the maximum input conditions, and the majority still did not damage. These results, together with some insights into the dominant damage processes suggested by previous studies, are encouraging in terms of realizing high damage thresholds in this type of optical fiber. However, several important areas for further study are indicated.