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

Low-power (<;5 mW) white light extrinsic Fabry-Perot interferometric sensors offer micron-level absolute position measurement capability for devices under test (DUTs) over large (cm) displacements. Because the position is determined directly by demodulating the interferogram, the sensors are robust to most common sources of power fluctuations but sensitive to DUT velocity, which induces Doppler shifting. This shifting results in interferogram bias errors as large as 50-100 μm for DUT velocities as low as 0.1 mm/s. This paper derives a model for predicting this Doppler-induced bias in the interferogram and exploits the derived relationship with a dual-filter optical architecture to obtain simultaneously the DUT velocity and bias-corrected absolute displacement. This approach is successfully demonstrated with a prototype in lab bench experiment. (Approved for release; LA-UR #12-23426).