  We present a novel atom interferometer configuration that combines large
momentum transfer with the enhancement of an optical resonator for the purpose
of measuring gravitational strain in the horizontal directions. Using Bragg
diffraction and taking advantage of the optical gain provided by the resonator,
we achieve momentum transfer up to $8\hbar k$ with mW level optical power in a
cm-sized resonating waist. Importantly, our experiment uses an original
resonator design that allows for a large resonating beam waist and eliminates
the need to trap atoms in cavity modes. We demonstrate inertial sensitivity in
the horizontal direction by measuring the change in tilt of our resonator. This
result paves the way for future hybrid atom/optical gravitational wave
detectors. Furthermore, the versatility of our method extends to a wide range
of measurement geometries and atomic sources, opening up new avenues for the
realization of highly sensitive inertial atom sensors.
