In order to explore the acousto-optic (A-O) applications of the ferroelectric single crystal lead zinc niobate and lead titanate, (PZN-PT), the A-O coefficients π were characterized for 0.9Pb(Zn1/3Nb2/3)O3-0.1PbTiO3, a composition near the morphotropic phase boundary. The independent elements of π were determined by means of both the hydrostatic and uniaxial stress measurements using an interferometer method. The results show that the crystal exhibits very large A-O effects, on a peer with TeO2, the best A-O crystal currently available. Considering the fact that this material also possesses very high E-O and piezoelectric responses, it allows the development of a class of unique devices, combining the high A-O, E-O, and piezoelectric effects, which could lead to significant advance in the device concept, size, performance, and power consumption. One example will be discussed here.