GRAVITY: beam stabilization and light injection subsystems

TL;DR

The paper details the design and lab testing of the second-generation GRAVITY beam stabilization and light injection system, crucial for achieving micro-arcsecond astrometry with the VLTI.

Contribution

It introduces a novel laser guiding system and fiber coupler units with integrated correction and stabilization features for the VLTI instrument GRAVITY.

Findings

Lab results show effective closed-loop performance of the guiding system.

Prototype fiber couplers demonstrate high optical quality and stability.

Design enables on-axis and off-axis fringe tracking without changing optical paths.

Abstract

We present design results of the 2nd generation VLTI instrument GRAVITY beam stabilization and light injection subsystems. Designed to deliver micro-arcsecond astrometry, GRAVITY requires an unprecedented stability of the VLTI optical train. To meet the astrometric requirements, we have developed a dedicated 'laser guiding system', correcting the longitudinal and lateral pupil position as well as the image jitter. The actuators for the correction are provided by four 'fiber coupler' units located in the GRAVITY cryostat. Each fiber coupler picks the light of one telescope and stabilizes the beam. Furthermore each unit provides field de-rotation, polarization analysis as well as atmospheric piston correction. Using a novel roof prism design offers the possibility of on-axis as well as off-axis fringe tracking without changing the optical path. Finally the stabilized beam is injected with minimized losses into single-mode fibers via parabolic mirrors. We present lab results of the first guiding- as well as the first fiber coupler prototype regarding the closed loop performance and the optical quality. Based on the lab results we discuss the on-sky performance of the system and the implications concerning the sensitivity of GRAVITY.