Monitoring the oscillation frequency drift of an optoelectronic oscillator with a vector network analyzer

The classical structure of an optoelectronic oscillator (OEO) is based on a long optical fiber loop creating a delay line with very low insertion loss. The oscillation frequency is proportional to the reciprocal of the propagation delay which depends on temperature by the way of the effective refractive index of the fiber. In case of a long fiber loop, even a small change of the temperature can induce an oscillation frequency drift. In this paper, a system based on a vector network analyzer (VNA) is proposed to monitor the oscillation frequency drift with a minimum of extra optical components. There is a good correlation between the phase evolution determined by the VNA and the OEO frequency drift. Knowing the drift it is possible to correct results obtained from the OEO used as an optical path measurement tool for either distance or refractive index variation detection.