  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.
