Abstract

There is disclosed a condition responsive detection system and method comprising a length of optical fiber including a core-cladding material which has a modal dispersion ratio less than a predetermined number. The fiber is placed in the area to be monitored and is illuminated on one end by a source of radiant power. Electronic circuitry including a photocell are positioned at the receiving end of the fiber and detect when the radiant power transmitted by the fiber is reduced below a nominal level when the cladding material is altered, as for example by a fire or an overheat condition in the area being monitored. The reduction in radiant power received is due to the alteration of the cladding material such that the reflected radiant power is lost through the alteration in the material. The invention further comprises enhancing the sensitivity of the system by modifying the radiant power source in order to direct the emitted radiant power at an angle to the center line of the core thereby reducing the MDR of the fiber to less than a predetermined number. The circuitry at the receiving end of the fiber can detect a gradual reduction in the amount of radiant power received caused, for example, by the alteration of the cladding due to an overheat condition; as well as a relatively instantaneous reduction resulting from, perhaps, a fire. Additional circuitry provides for the detection of an abrupt cessation of transmission due to the breaking of the fiber, loss of power or otherwise.