The defects generated by the catastrophic optical degradation (COD) of high power laser diodes have been examined using cathodoluminescence (CL). Discontinuous dark lines that correspond to different levels of damage have been observed along the ridge. Finite element methods have been applied to solve a physical model for the degradation of the diodes that explicitly considers the thermal and mechanical properties of the laser structure. According to this model, the COD is triggered by a local temperature enhancement that gives rise to thermal stresses leading to the generation of dislocations. Damage is initially localized in the QW, and when it propagates to the waveguide layers the laser ends its life.