Tilt-to-length coupling metrology in the LISA mission
This paper describes a setup aimed at measuring the so-called Tilt-To-Length (TTL) coupling in the optical benches of the LISA mission. The TTL is the coupling of the angular jitter of any optical setup into the optical path length between its input and output pupils. This might be deleterious in laser ranging experiments and must be evaluated for further compensation. The setup is made of two laser beams, one features an angular jitter that mimics the input beam as seen from the jittering bench under test (BUT), the other is aligned to the optical axis of the BUT and provides a phase reference for the jittering beam. The induced phase variations between both beams detected at the BUT's output pupil gives access to the TTL coupling. The ''TTL probe'' must feature a negligible residual TTL coupling which implies a micrometric accuracy in the centering of the setup pupil, the beams and the angular jitter associated pivot point. The setup integrates optical masks as a link between the setup optical reference frame to its mechanical reference frame, together with position memories and servo-loops for the beam's alignment. We show that the stability, the accuracy, and the noise floor of the setup is compliant with the LISA specifications for the TTL mitigation, although it makes use of off-the-shelf components and is operated in a standard environment laboratory.