The functional lifetime of large-aperture optical components used on a laser such as the National Ignition Facility is an important engineering parameter. To predict the lifetime of fused silica transmissive optics, it is necessary to measure the rate of damage propagation as a function of fluence and understand the effects of the laser parameters. In order to begin such predictions without a large-area flat-top laser beam, damage growth experiments were conducted using a small Gaussian beam. Damage was initiated by producing mechanical flaws on the surface of the optic. Since output surface damage in transmissive topics can propagate at least two orders of magnitude faster than input surface damage, the experiments were focused on damage initiated at the output surface. The experiments showed that if damage was initiated, it could not grow at fluences below a threshold of about 5 and 8 J/cm2 at 355 and 1064 nm, respectively. When damage was able to propagate under laser irradiation, the phenomenon occurred in two stages. Initially, the damage grew both laterally and along the optical axis at a rate varying linearly with fluence. Lateral growth stopped in areas where the fluence was lower than the growth threshold. At this point, the area of damage typically filed the size of the beam and the rate of axial damage propagation significantly decreased. During this stage, laser irradiation drilled at constant rate a channel through the window. During stage I, the damage area grew much faster at 355 nm than at 1064 nm. During stage II, 355 nm light drilled four to five times faster than 1064 nm light. At given fluence at 1064 nm, the drilling rate did not change between 3 ns and 10 ns. Finally, drilling at 1064 nm produced a well-defined cylindrical damaged region while 355 nm light generated less regular clusters of cracks.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.