Nonlinear estimation of ring-down time for a Fabry-Perot optical cavity
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Published on: February 25, 2017
Abhijit G Kallapur1, Toby K Boyson, Ian R Petersen
1School of Engineering and Information Technology, University of New South Wales at the Australian Defence Force Academy, Canberra, ACT, Australia. abhijit.kallapur@gmail.com
Abstract:
This paper discusses the application of a discrete-time extended Kalman filter (EKF) to the problem of estimating the decay time constant for a Fabry-Perot optical cavity for cavity ring-down spectroscopy (CRDS). The data for the estimation process is obtained from a CRDS experimental setup in terms of the light intensity at the output of the cavity. The cavity is held in lock with the input laser frequency by controlling the distance between the mirrors within the cavity by means of a proportional-integral (PI) controller. The cavity is purged with nitrogen and placed under vacuum before chopping the incident light at 25 KHz and recording the light intensity at its output. In spite of beginning the EKF estimation process with uncertainties in the initial value for the decay time constant, its estimates converge well within a small neighborhood of the expected value for the decay time constant of the cavity within a few ring-down cycles. Also, the EKF estimation results for the decay time constant are compared to those obtained using the Levenberg-Marquardt estimation scheme.
