Multi-axis modulated compact fiber-based Fabry-Perot interferometric probe

This paper presents design and operation of a multi-axis optical probe for independently measuring the relative displacements of external surfaces. Displacement is measured by mechanically modulating the optical cavity formed by an internal surface and the external surfaces, each of which comprise a Fabry-Perot interferometer. Multiple sensing axes are created using a beam splitter, and these sensing axes are separated by modulating the measurement paths at different frequencies. Two probes have been fabricated and tested; the first uses a single laser source with axes monitored by shuttering; the second probe uses two source wavelengths with beam splitting achieved using dichroic mirrors. Experimental results are presented for mirrors moving independently over distances of 1 to 2 µm with displacement noise less than 10-nm rms that is higher than the noise floor of 2.8 nm for stationary mirrors. Currently, the bandwidth is limited to the modulation frequencies ranging between 200 Hz and 2.43 kHz.