Abstract: An optical sensor for detecting the presence of laser radiation in locations outside an intended optical path in a high energy laser device. An optical sensor, such as a photodiode, is positioned to receive light through an optical component when it fails to operate properly and laser light burns through the component. The optical sensor preferably includes a diffuser, an optical filter, and electrical circuitry to compare the signal generated by the photodiode with a selected reference signal, and to use the photodiode signal to actuate an alarm indicator and to disable power to the laser source. A thermal detector may be employed as a backup detection device.