Differential optical shadow sensor for sub-nanometer displacement measurement and its application to drag-free satellites

Abstract

Abstract:We present a method for 3D sub-nanometer displacement measurement using a set of differential optical shadow sensors. It is based on using pairs of collimated beams on opposite sides of an object that are partially blocked by it. Applied to a sphere, our 3-axis sensor module consists of 8 parallel beam-detector sets for redundancy. The sphere blocks half of each beam's power in the nominal centered position, and any displacement can be measured by the differential optical power changes amongst the pairs of detectors. We have experimentally demonstrated a displacement sensitivity of 0.87 nm/ √ Hz at 1 Hz and 0.39 nm/ √ Hz at 10 Hz. We describe the application of the module to the inertial sensor of a drag-free satellite, which can potentially be used for navigation, geodesy and fundamental science experiments as well as ground based applications.

Light-Alignment Controllable Beam Splitter and Vectorial Displacement Sensor in the Stopped-Light Regime of Plasmonic Metasurfaces

“…Alongside with the beam-splitting functionality, we show that the circular structures we designed can also serve as high-resolution vectorial displacement sensors, capable of monitoring simultaneously the magnitude and direction of the displacement between the plasmonic metasurface structure and the illuminating beam centers. This vector-displacement information, along with the microscale size, and the overall reduced complexity, are highly sought-after characteristics for practical applications in the fields of mechanical engineering, space and military missions, robotic arms, and phototherapy. − Although our structures have a 40 μm × 40 μm footprint, same with that of the structure presented in ref , their size can be scaled down to 28 μm × 28 μm (∼441 holes) without changing their functionality . Another advantage of the herein-presented plasmonic devices is that while a part (∼8%) , of the excited SPPs launch on the surrounding smooth metal film, the majority of them couple back into photons enhancing the transmission of the illuminating beam through the nanostructured area; , the transmission of the illuminating beam through the nanostructured area is ∼25%, while the area of the plasmonic metasurface covered with holes is only 3%, and thus allows for nondestructive sensing or sampling, and parallel processing of the beam.…”

High precision six-degree-of-freedom interferometer for test mass readout

“…Capacitive sensing is a widely used method that allows simultaneous motion sensing and control of TM, but has relative poor accuracy when the gap between the TM and the capacitive plate is big and introduces large feedback disturbance noise when the gap is small [7]. Optical measurement methods, including optical levers [8,9], optical shadow sensing [10], gratings sensing [11], homodyne interferometry [12], heterodyne interferometry [13], and differential wavefront sensing (DWS) [14], have all been successfully proven to be effective in achieving high-precision measurement of TM. LISA-pathfinder uses a combination method for TM readout [15], with laser interferometry for the sensitive-axis translation as well as two tilts and capacitive sensing for the other three degrees of freedom.…”