  We implement Bragg spectroscopy in a paraxial fluid of light. Analogues of
short Bragg pulses are imprinted on a photon fluid by wavefront shaping using a
spatial light modulator. We measure the dispersion relation and evidence a
parabolic single-particle regime as well as a linear phonon regime even for
very weakly interacting photons and low sound velocity. Finally, we report a
measurement of the static structure factor, $S(k)$, and we demonstrate the
presence of pair-correlated excitations, revealing indirectly the quantum
depletion in a paraxial fluid of light.
