  Radial velocity instruments require high spectral resolution and extreme
thermo-mecanical stability, even more difficult to achieve in near-infra red
(NIR) where the spectrograph has to be cooled down. For a seeing-limited
spectrograph, the price of high spectral resolution is an increased instrument
volume, proportional to the diameter of the primary mirror. A way to control
the size, cost, and stability of radial velocity spectrographs is to reduce the
beam optical etendue thanks to an Adaptive Optics (AO) system. While AO has
revolutionized the field of high angular resolution and high contrast imaging
during the last 20 years, it has not yet been (successfully) used as a way to
control spectrographs size, especially in the field of radial velocities.
  In this work we present the AO module of the future NIRPS spectrograph for
the ESO 3.6 m telescope, that will be feed with multi-mode fibers. We converge
to an AO system using a Shack-Hartmann wavefront sensor with 14x14
subapertures, able to feed 50% of the energy into a 0.4" fiber in the range of
0.98 to 1.8 $\mu m$ for M-type stars as faint as I=12.
