Abstract: Apparatus and method for self-referencing and multiplexing light intensity modulating fiber optic sensors provides a pulsed or chirped source light beam. The light beam is propagated through an input fiber optic cable into at least a first loop path. In the first loop path, the source light beam is modulated in accordance with a detected phenomenon to provide a modulated light beam. In the first loop path, a coupler splits-off (a) a portion of the source light beam, and (b) a portion of the modulated light beam. The split off portions are directed through an output fiber optic cable to a photo detector which thus receives a pulse train having a modulated pulse and an unmodulated pulse. The output of the photo detector is supplied to a processor which forms a ratio of the modulated and unmodulated pulses and provides an output signal which varies in accordance with the detected phenomenon but is relatively insensitive to optical transmission losses and optical source drift.