the self - monitoring link utilizes a grin lens / blazed grating combination to multiplex an optical time domain reflectometry signal with data signals on a fiber optic communication link . an initial average otdr reference signature for the otdr signal on the link is determined and is compared with a continuously determined average otdr signature . the reference signature and the continuously determined signature are compared and a warning or alarm signal is generated when the continuously determined signature differs from the reference signature by a preselectable limit . a microprocessor is provided to adjust the preselectable limit in response to the communication link operational parameters .