  We present the installed and fully operational beam stabilization and fiber
injection subsystem feeding the 2nd generation VLTI instrument GRAVITY. The
interferometer GRAVITY requires an unprecedented stability of the VLTI optical
train to achieve micro-arcsecond astrometry. For this purpose, GRAVITY contains
four fiber coupler units, one per telescope. Each unit is equipped with
actuators to stabilize the telescope beam in terms of tilt and lateral pupil
displacement, to rotate the field, to adjust the polarization and to compensate
atmospheric piston. A special roof-prism offers the possibility of on-axis as
well as off-axis fringe tracking without changing the optical train. We
describe the assembly, integration and alignment and the resulting optical
quality and performance of the individual units. Finally, we present the
closed-loop performance of the tip-tilt and pupil tracking achieved with the
final systems in the lab.
