We report on an extensive study of the defect structure associated with catastrophic failure of GaAs-based 980 nm pump lasers. Electron beam induced current (EBIC) analysis shows that catastrophic optical damage (COD) is characterized by the introduction of high densities of extended defects in the optical cavity of the laser, in the vicinity of the output facet. The heterostructure comprising the active region of the device is interdiffused in a spherical region surrounding the region of highest defect density. In some regions, melting of the laser cavity is observed. A "fast capture" laser degradation analysis demonstrates that the COD damage initiates at the laser facet, and propagates back along the cavity with continued device stressing. COD failure under pulsed operation results in a dramatically altered defect distribution consisting of periodic arrays of dislocation tangles along the laser cavity. Successive pulses following the initial failure event result in the formation of fresh defect "packets" which are separated from the damaged region generated due to the preceding pulse by a volume of relatively defect free material. The periodicity of these defective packets is related to the magnitude of the drive current pulse at the time of failure. Following the description of the defect distribution obtained using EBIC, we employed site-specific transmission electron microscope sectioning methods to form a detailed description of the structural modifications that the device undergoes at the onset of failure.