Accurate Optical Fibre Path Length Measurement based on Frequency Modulation of a Laser Diode

摘要

The measurement of optical fibre length is a key parameter for locating faults within telecommunication systems and in quality control of manufactured fibre spools. Typically, the measurements are based on pulse timing techniques, as used in Optical Time Domain Reflectometers (OTDR), with National Standards' bodies providing accuracies approaching 2 parts in 10~(4). In this paper, a technique based on the spectral analysis of an incoherent interferometer with a frequency modulated (FM) source laser is described. The magnitude of the incoherent beat signal will oscillate with variations of the phasing/dephasing of the instantaneous frequencies of the two interfering E-fields. Variations in the driving modulation frequency for a given optical path difference (OPD) results in beat frequency changes which allow accurate optical path length measurements to be made. In fact, using a "Gold" fibre Standard, National Standards' accuracy has been achieved. A cheaper implementation of the approach further outlines the advantages of this technique.