Due to its importance in many optical applications (e.g. lasers, beam shaping, beam guiding), the laser damage behaviour of fused silica has received considerable attention during the past decades. The threshold for damage by ablation amounts to a fluence of 2.5 J/cm2 for single ns pulses. This damage threshold can be lowered by cracks, inclusions or surface impurities leading to a local intensity enhancement. Already the application of low fluences (<10 mJ/cm²) leads to modifications in fused silica like rarefaction and compaction with a related decrease and an increase of the refractive index, respectively. We characterize fused silica samples using laser pulse numbers (e.g. ArF) for micro-channel formation in dependence of fluence and sample length. Micro-channel formation can be observed using different techniques. It can be explained as a cumulative or multi-pulse effect.