Dynamics of a noncontacting, white light Fabry–Perot interferometric displacement sensor

Abstract

A white light extrinsic Fabry-Perot interferometer is implemented as a noncontacting displacement sensor, providing robust, absolute displacement measurements with micrometer accuracy at a sampling rate of 10 Hz. This paper presents a dynamic model of the sensing cavity between the sensor probe and the nearby target surface using a Fabry-Perot etalon approach obtained from straightforward electromagnetic field formulations. Such a model is important for system characterization, as the dynamically changing cavity length imparts a Doppler shift on any signals circulating within the sensing cavity. Contrary to previously published results, Doppler-induced shifting within the low-finesse sensing cavity is shown to significantly distort the measurement signal as recorded by the sensor. Experimental and simulation results are compared, and the direct effects of cavity dynamics on the measurement signal are analyzed along with their indirect impact on sensor performance. This document has been approved by Los Alamos National Laboratory for unlimited public release (LA-UR 12-00301).

Simultaneous measurement of displacement and velocity using white light extrinsic Fabry-Perot interferometry

“…4 (with the velocity still assumed unknown for the moment). Using the experimental parameters determined in this problem, with f 0 = 1.935×10 14 Hz, ∆f 0 = 9.993×10 12 Hz (corresponding to an 80 nm filter sweep), and ∆t = 40 ms, L b = −0.77v, which is extremely close to the results presented in [4].…”

“…3, a comparison between the actual (independently known) DUT position and the estimated DUT position clearly shows a bias, manifesting itself in an apparent jump in the DUTs estimated position when there is non-zero DUT velocity. The right-hand side of the figure shows a sample cepstrum comparison between the static and dynamic cases for the specific velocity of 5 mm/s; in [4], it was shown that even a DUT velocity of 0.1 mm/s resulted in a relatively large 64 µm bias in the position. Errors of this magnitude can severely limit sensor performance.…”

Understanding the effects of Doppler phenomena in white light Fabry–Perot interferometers for simultaneous position and velocity measurement

“…In spite of these advantages, the authors recently observed that white light EFPI position sensors are highly sensitive to DUT velocity, where any DUT motion was shown in [9] to impart a Doppler shift on the signal circulating within the Fabry-Perot sensing cavity (Fig. 2).…”

“…2). More specifically, in the research performed in [9], the authors utilized a PXIe-4844 optical interrogator (National Instruments Inc.), which consists of a broadband InGaAs source, a swept filter that tunes over the 1510-1590 nm wavelength range in 0.04 s with an absolute wavelength accuracy of 1 pm (which relates to the filter's linearity), wavelength referencing hardware, a broadband photodetector, and circulators, all arranged similarly to the generic diagram shown in Fig. 1.…”

“…A high-precision motorized linear stage with feedback control and an absolute position accuracy of 4.8 μm (ILS200CCL, Newport Corp.) was used for dynamic control of the DUT. Further details of the experimental setup and results are beyond the immediate scope of this paper and may be found in [9]. An example of DUT position, measured using this commercial system and calculated from the peak in the interferogram's PSD estimate, is shown in Fig.…”