Offline estimation of ring-down time for an experimental Fabry-Perot optical cavity

This paper presents an application of the discrete-time extended Kalman filter (EKF) for offline estimation of the ring-down time constant of an experimental Fabry-Perot optical cavity for the purpose of cavity ring-down spectroscopy (CRDS). The ring-down time corresponds to the time taken by the light inside an optical cavity to decay to 1/e of its initial intensity. The estimation of ring-down time (or decay time) for a cavity is a direct indication of the absorbing species contained in it and can be used to detect improvised explosive devices and concealed explosives. The data for the estimation process is obtained in the form of light intensity at the output of the optical cavity from a CRDS experimental setup. During the process of data collection, the resonant frequency of the optical cavity is held in lock with the input laser frequency with the aid of a proportional-integral (PI) controller in order to allow for maximum constructive interference of light within the cavity.