Incoherent Optical Frequency Domain Interferometry for Avionics

We present a new approach to fault location in avionics fiber optic networks. A frequency domain reflectometry technique is introduced where the light from a transmitting laser is frequency modulated at a RF frequency and the reflected signal is heterodyne beaten with a portion of the transmitted signal on an optical detector. The detected beat frequency is proportional to the time-of-flight of the transmitted signal reflected from a fault down the fiber. Since the transmitting laser is operated CW and does not require high peak optical power, a significant advantage is obtained, compared to a time-domain reflectometry technique. This advantage becomes very significant in applications requiring built-in test, as for avionics applications, where one must use the existing transmitter circuit with a given power supply. Very low reflections are detected and an optical dynamic range of 73 dB is obtained, which can be further increased