  This article describes the operation of the near-infrared wavefront sensing
based Adaptive Optics (AO) system CIAO. The Coud\'e Infrared Adaptive Optics
(CIAO) system is a central auxiliary component of the Very Large Telescope
(VLT) interferometer (VLTI). It enables in particular the observations of the
Galactic Center (GC) using the GRAVITY instrument. GRAVITY is a highly
specialized beam combiner, a device that coherently combines the light of the
four 8-m telescopes and finally records interferometric measurements in the
K-band on 6 baselines simultaneously. CIAO compensates for phase disturbances
caused by atmospheric turbulence, which all four 8 m Unit Telescopes (UT)
experience during observation. Each of the four CIAO units generates an almost
diffraction-limited image quality at its UT, which ensures that maximum flux of
the observed stellar object enters the fibers of the GRAVITY beam combiner. We
present CIAO performance data obtained in the first 3 years of operation as a
function of weather conditions. We describe how CIAO is configured and used for
observations with GRAVITY. In addition, we focus on the outstanding features of
the near-infrared sensitive Saphira detector, which is used for the first time
on Paranal, and show how it works as a wavefront sensor detector.
