Development of an advanced tilt actuator for tilt-to-length coupling investigations

Abstract

A coupling between angular jitter of satellite and length readout, called tilt-to-length coupling, is one of the major noise sources in space-based laser interferometers such as LISA and GRACE Follow-On. With comprehensive knowledge of its characteristics and reduction, experimental investigations are an indispensable gateway to develop future space laser interferometers. A recurring difficulty in tilt-to-length coupling experiments is the motion errors of the actuators used to generate the tilting beam. This motion error then couples into the path length readout and cannot be distinguished from the tilt-to-length coupling under investigation. Within this thesis, an optical testbed named advanced tilt actuator(ATA) was developed in order to provide a tilted beam and reduce the actuator’s parasitic longitudinal displacement, thereby enabling the characterisation of the tilt-to-length coupling. The actuator employed in the ATA can produce a motion in three degrees of freedom for yaw and pitch and a displacement and is used for tilting a beam needed in tilt-to-length experiments. The vertices of retroreflectors attached to the rear of the actuator are traced by dedicated interferometers, such that the actuator’s motion could be measured. Although potential misalignments during the construction of the ATA endeavoured to be minimised through the devised alignment techniques, residual misalignment may degrade the advanced tilt actuator’s performance. Based on analytical and numerical analyses of the various misalignment effects, possible readouts errors that may appear in a real experiment were simulated. For counteracting the readout error due to misalignments, a calibration method enabling suppressing the longitudinal displacement readout error was established, and its validity was verified through numerical simulations. As the purpose of the ATA, an optical breadboard for examining tilt-to-length coupling effects was constructed, aiming to experimentally demonstrate that the imaging system enables mitigating the tilt-to-length coupling. Prior to this experiment, suppressing the ATA’s longitudinal displacement readout error caused by various misalignments was performed through the calibration method. In the main experiment, we measured two path length readouts with and without the imaging system, applying the calibration enabling the suppression of linear and hysteresis error. The best experimental result with the imaging system showed tilt-to-length coupling of about 2μm/rad, which is a significantly better performance than the 25 μm/rad required for LISA. Additionally, the ATA’s angular readouts were compared with differential wavefront sensing signals measured on the optical breadboard, showing the measurement error in the order of a micro-radian within the rotation angle of ± 200 μrad.