High power laser diodes (HPLDs) have been widely used in the defense sector, material processing, and pumping sources, considering their advantages such as high efficiency, long lifetime, small size, and low cost. This study describes the types and emission characteristics of the defects and the related advances in GaAs-based near infrared lasers and GaN-based blue-green lasers. By focusing on the commercial devices and using the condition-variable emission spectra for separated wavebands and the corresponding thermal imaging, the origination and spatial distribution of the emission signals related to the defects are determined. The internal catastrophic optical damage (COD) mechanism is also analyzed. Furthermore, the limitation of the current “external COD” model for interpreting the thermal evolution mechanism of the devices is pointed out.