A method and apparatus for detecting the instant on explosive is detonated by way of an optical signal. A length of optical fiber has one end wrapped around or positioned near the explosive with the other end coupled to a remote recorder. The end of the fiber at the explosive has been cleaved so as to internally reflect an optical signal propagating therein. A beam of radiation is launched into the fiber at the record where it is reflected from the distal end and redirected back to the recorder where it is detected. Detonation of the explosive destroys the reflective end of the optical fiber causing the radiation to leak from the broken fiber end. The instant the recorder does not receive the reflected radiation is the instant of the explosive detonated.