Five degrees of freedom test mass readout via optical levers

Abstract

High precision measurement of all six degrees of freedom of freely floating test masses is necessary for future gravitational space missions as the sensing noise is frequently a limiting factor in the overall performance of the instrument. Femto-meter sensitivity has been demonstrated with LISA Pathfinder which used a complex laser interferometric setup. However, these measurements where restricted to the length changes in one degree of freedom only. When aiming for sensing multiple degrees of freedom, typically capacitive sensing is used, which facilitates a compact setup but does not provide competitive precision. An alternative approach to improve the sensitivity beyond capacitance readout systems and to reduce the complexity of the setup, is to use optical levers. Here, we report on the realization of a test mass sensing system by means of a modulation/demodulation technique in combination with four optical levers detected by quadrant photodiodes. The results of our table-top experiment show that this configuration allows us to extract information on five degrees of freedom of a cubic test mass. With basic off-the-shelf laser diodes we demonstrate an angular resolution of below 600 nrad at frequencies between 10 mHz and 1 Hz (which is better than a conventional autocollimator) while simultaneously measuring the linear motion of the test mass with a precision of better than 300 nm in the same frequency band. Extension of the geometry will enable optical sensing of all six degrees of freedom of the test mass.

Single-Element Dual-Interferometer for Precision Inertial Sensing

“…Precision interferometry with a dynamic range over multiple fringes is the core metrology technique employed in space-based detectors, such as the Laser Interferometer Space Antenna (LISA) [ 1 ], to measure the displacements between the space-craft and free-floating test masses, or in ground-based detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) [ 2 ], to measure ground motion in order to isolate the suspended test masses from vibration. The pinnacle of this technology is applying such measurement to all degrees of freedom (DOF) of one or multiple test masses [ 3 ] whilst providing increased sensitivity over other schemes such as electrostatic readout [ 4 ] or optical levers [ 5 , 6 ]. Such ambitious goals require a drastic reduction of the size and complexity of the optical setup.…”

A multi-axis interferometer based 6-DoF readout system for the gravitational wave detection

“…As a typical ultra-sensitive and high-precision measurement technology, the laser interferometry is usually applied for the single-axis displacement or angle measurement 6,7 . The conventional multi-axis interferometer is based on the special designed multi-axis optical prism.…”

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.…”