Ferroelectrics are intrinsically multifunctional materials with a particular efficiency in electro-mechanical coupling which play a central role in many devices in telecommunications and sensors technologies (piezoelectric, electro-optic and acousto-optic devices). Among ferroelectrics, LiNbO3 is currently employed in many acousto-optic devices for light processing but with a limited device bandwidth of tens of MHz (CMOS electronic limits). To envision new opportunities for applications, new ultrafast acousto-optic processes have to be revealed. For that, we will show that it is first necessary to control the spectrum of 10-100 GHz coherent acoustic phonons and this is currently achieved with femtosecond laser action [1-3]. Secondly, we will discuss how these high frequency coherent acoustic phonons can be used in LiNbO3 or BiFeO3 to manipulate the light through an efficient acousto-optic process. In particular we have revealed an original ultrafast light mode conversion process (ordinary-extraordinary) which appears to be intrinsically related to large acousto-optic coefficient of ferroelectrics while no equivalent effect was observed in the canonical birefringent CaCO3 (calcite) . Our results highlight new capabilities in using ferroelectrics in modern photoacoustics and photonics.