A novel x-ray laser (XRL) application, aimed at understanding the microscopic effects involved in formation of laser-induced damage in optical materials exposed to high-power sub-ns laser pulses, is presented. Standard fused silica substrates with permanent damage threshold below 20 J/cm2, when irradiated by 438 nm laser pulses, were probed in situ by a neonlike zinc XRL at 21.2 nm. The probing beamline employed a double Lloyd’s mirror x-ray interferometer, used in conjunction with an imaging mirror to achieve magnification of ∼8. In conjunction with an array of in situ optical diagnostics, the main question addressed is whether the damage on the rear surface of the beamsplitter is transient or permanent. The second issue, examined by both the x-ray interferometric microscopy and the optical diagnostics, is whether a local rear-surface modification is associated with nonlinear effects such as self-focusing or filamentation of the damaging laser beam in the bulk.