  The very large telescope (VLT) interferometer (VLTI) in its current operating
state is equipped with high-order adaptive optics (MACAO) working in the
visible spectrum. A low-order near-infrared wavefront sensor (IRIS) is
available to measure non-common path tilt aberrations downstream the high-order
deformable mirror. For the next generation of VLTI instrumentation, in
particular for the designated GRAVITY instrument, we have examined various
designs of a four channel high-order near-infrared wavefront sensor. Particular
objectives of our study were the specification of the near-infrared detector in
combination with a standard wavefront sensing system. In this paper we present
the preliminary design of a Shack-Hartmann wavefront sensor operating in the
near-infrared wavelength range, which is capable of measuring the wavefronts of
four telescopes simultaneously. We further present results of our design study,
which aimed at providing a first instrumental concept for GRAVITY.
